UNIVERSE. - It is the totality of all that exists in space and time; also called "world," which in itself signifies a part of the U.
In the earliest Greek poets, the principal problem was the origin of the world, generated from Chaos (Hesiod) or from Time (Orphics). As the problem became more precise among the first Ionian philosophers, Anaximander posits that the infinite succession of worlds is generated from the infinite, ἀπειρον (H. Diels, Frag. d. Vors., Berlin 1934, 12, A, 14). Infinite worlds arise from atoms ceaselessly in motion within the infinity of space and time, according to atomistic doctrines (ibid., 54, A, 21). Plato teaches of a world made in imitation of the Ideas by the Demiurge; beyond it lie the eternal Ideas, while Aristotle models the material U. according to his metaphysics: unique, eternal, arranged in concentric spheres in motion around the Earth, enclosed within the sphere of the fixed stars (De Caelo), moved by the Unmoved Mover, God, who is external to it (Phys., VIII). According to the school of Epicurus, and the infinite series of times, from the boundless, random whirling of atoms falling in infinite space arise infinite worlds (Epicurus, Ad Erod., 45, 73; Lucretius, De rerum natura, II, 1048 ff.; V, 528); the Stoa, on the other hand, conceives of a U. (τὸ πᾶν) that encompasses the world (τὸ ὅλον) as unique, finite, round, mobile, compact, animated, and, outside the world, the infinite void, incorporeal (Diogenes Laertius, VII, 140). The material U. is unique, according to Plotinus, emanated from the One through the Soul that orders and vivifies it; beyond it lies the empyrean spiritual U. (Enn., III, 2). The Middle Ages, beyond the outermost sphere of the material U., conceived according to the Aristotelian schema but created and ordered by God, admits the empyrean world (St. Thomas, Dante Alighieri). St. Thomas asserts the necessity of the unity and uniqueness of the U., because all bodies must have reciprocal spatial order (Sum. Theol., I, q. 47, a. 3); St. Bonaventure, however, asserts the possibility, at least in the abstract, of the existence of two or more U.s without reciprocal spatial relation (In Sent., I, dist. 37, a. 2, q. 3); the same opinion was held by the nominalists G. Buridan, Albert of Saxony, Marsilius of Inghen, Paul of Venice, etc.
During the Renaissance, N. Cusanus calls the U. the "contraction of the divinity" (De docta ignor., II, 4) and, with W. of Ockham, denies the incorruptible celestial matter of Aristotelianism; God is the center and circumference of the world, which has its center everywhere and its circumference nowhere, because God is everywhere and in no place (ibid., II, 12); thus, the U. is infinite in a privative sense, insofar as it has no limits (ibid., II, 1). Cusanus influenced Giordano Bruno, for whom the U. is infinite, with innumerable worlds (De immenso et innumerabilibus, I, II), is a magnum animal, whose soul is God; and R. Descartes, who calls his mechanistic U. indefinite, but in fact infinite (Princ. Phil., II, 21). Subsequently, the concept of the infinite U. dominates realistic conceptions until the end of the 19th century.
In E. Kant's Critique, the U. or world is the second idea of pure reason, "the mathematical whole of all phenomena and the totality of their synthesis" (Crit. d. rag. pura, Bari 1945, p. 343), and contains, according to H. Maier, four elements: the concept of the real, the system of the laws of being, the system of real individualities, and the system of the origin of individuals (H. Maier, Wahrheit und Wirklichkeit, Tübingen 1926, p. 491).
In the theory of relativity, the U. is the set of all possible quadruples of the values of the coordinates x, y, z, t (that is, of all points) of space-time (O. Minkowski), or the four-dimensional manifold constituted by the sets of physical events (P. Langevin). It is unbounded but finite, curved in four dimensions, as analogously a spherical surface is in three dimensions.
In logic, A. De Morgan (Formal logic or the calculus of inference, London 1847) introduced the concept of the U. of discourse: the set of elements and logical classes considered in a discourse; for example, the winged horse is an object of the U. of discourse of myth and not of zoology.
The first and most natural hypothesis one can make is that this physical space is Euclidean, that is, such that the line representing the shortest path between two points (the geodesic of this space) is given by the straight line, as considered in ordinary geometry, called precisely "Euclidean." In this case, the sum of the internal angles of a triangle is exactly equal to two right angles; from a point, one and only one parallel can be drawn to a given line, etc.; that is, all the properties deduced from Euclid's "fifth postulate" hold in this space. In this case, space is infinite and unbounded; the straight line is also infinite. Euclidean geometry adapts very well to the study of the physical world. Astronomers have used it and continue to use II.
The deductions they make using this geometry fully agree with the results of observations. On the other hand, however, the space of astronomers—that is, that portion of space which astronomers have been able to explore—is a very small thing compared to the entire space constituting the Universe. It is perhaps due to this smallness that it appears Euclidean to us, just as in a gulf or inlet, calm water appears flat, while we know that the water of the entire sea, to which the gulf belongs, is spherical. Therefore, one might say that Euclidean geometry works well on a small scale but may not adapt equally well on a large scale, giving "large" and "small" the meaning just described.
If the entire space is not Euclidean, one must consider that one of the other two non-Euclidean geometries must apply to it: hyperbolic or elliptic. In the first, the sum of the internal angles of a geodesic triangle is less than two right angles; in the second, it is greater. In the first, from a point, infinite geodesics can be drawn parallel to a given geodesic; in the second, from a point, no parallel can be drawn to a given geodesic. Hyperbolic space is infinite, as is the geodesic within it; whereas in elliptic geometry, space is unbounded but finite, as is the geodesic. The question is to determine which of these types of spaces, conceived by mathematicians, corresponds to the physical space in which we are immersed.
In the theory of relativity, Einstein suggested a type of space called "Riemannian" (after the mathematician Riemann), with four dimensions, in which time is also considered as a dimension (a second depth), and he showed that, while in first approximation the geometry in this space coincides with Euclidean geometry—so that it does not contradict the numerous and excellent deductions made with it—in second approximation, it would provide the key to explaining certain phenomena not previously accounted for and to connecting many others that initially appeared unrelated. From a conceptual standpoint, there would be a simple experiment to decide whether this geometry of Einstein is the correct one, that is, whether it truly adapts to the description of the Universe: it would consist in measuring the three internal angles of a cosmic triangle, having its vertices at three points of our Universe, and seeing whether their sum is less than, equal to, or greater than two right angles. But practically, such an experiment (already attempted, incidentally, by Gauss) is perhaps illusory. Indeed, geometric considerations show that the difference between the sum of the three angles of a triangle and two right angles is proportional to the area of the triangle and inversely proportional to the square of the radius of curvature of space. Since this radius of curvature (which would be infinite for Euclidean space) is immensely large, one would have to use triangles with extremely large sides so that the considered difference could acquire values detectable by our measuring instruments (without even considering the difficulty of transporting these instruments to other "fixed" points of the cosmic Universe). Ultimately, it seems that, at least for now, there is no possibility of deciding anything about the nature of the physical space in which we are immersed, that is, about the "container," while astronomers are in a position to say something more certain and positive about the second part of the question, namely the "content" and thus the constitution of the cosmic Universe.
1. The Metagalactic Universe. The Galaxies. — Until a few years ago, astronomers' investigations were limited solely to our own stellar system (the Milky Way, or Galaxy; V. below), but now we also speak of the metagalactic Universe or metagalaxy, that is, what lies outside and beyond, up to a distance of one billion light-years (or perhaps even 2 billion light-years, as seems to emerge from the "new scale of cosmic distances" [1953], which would double all extragalactic distances), where the 5-meter-diameter telescope at Mount Palomar in California reaches. In the metagalaxy, the unit is precisely constituted by the gigantic star systems, similar to our own, which in the telescope or on the photographic plate appear as nebulae (extragalactic nebulae), but are more modernly called galaxies, from the Greek name of the Milky Way, the Galaxy par excellence.
The metagalactic Universe has so far appeared uniformly filled with galaxies, which are nevertheless at remarkably great distances from one another. The average number of stars in a galaxy seems to be on the order of 10 billion, though there are very large dispersions. There are supergiant galaxies, like the Milky Way, which may contain more than 100 billion stars, while there are also dwarf galaxies that have no more than 10 million. The average distances between stars within galaxies are relatively much greater compared to their diameters than the distances between galaxies compared to their sizes. In a normal region of the metagalaxy, the distance between one galaxy and another is on the order of 100 galaxy diameters (in some richer regions, the average distance drops even below 10 diameters), whereas the average distance between star and star in the vicinity of the Sun is greater than 10 million solar diameters (the density of stars in a galaxy is like that of a forest in which the trees are 15 km apart from one another). Newton wrote: "He who ordered the Universe separated the fixed stars with immense distances, fearing that, by the force of gravitation, they should fall upon one another."
While the average distribution of galaxies in space (at least in the portion explored thus far) appears substantially uniform, in some regions of the metagalaxy many galaxies appear closely clustered together, forming true "clusters of galaxies." In the northern part of the constellation Virgo, there is a group of approximately 200 bright galaxies, covering an area of over 10 square degrees in the sky. Even more numerous in the same region are the fainter galaxies: over 80,000 in 360 square degrees of the sky down to stellar magnitude 17.6, and still several thousand fainter ones. North of the Virgo cluster lies another well-defined and diffuse cluster of galaxies, spanning the constellations Coma Berenices to Ursa Major and Canes Venatici. The most extensive galaxy cluster is found in the southern sky, in the constellation Centaurus, and appears to have actual dimensions exceeding 5 million light-years.
The Milky Way is also part of a "local group" of galaxies, which includes the Andromeda Nebula (at a distance of 1.5 million light-years [according to the new scale], the only galaxy barely visible to the naked eye, being a supergiant galaxy) and about ten smaller nebulae. Of the entire group, at least half are irregular (non-spiral) galaxies, and five of them are very bright. Among the latter are the two Magellanic Clouds, which are the closest galaxies to us (100,000 light-years for the Large Magellanic Cloud and 200,000 light-years for the Small Magellanic Cloud), so much so that some astronomers considered them to be satellite galaxies of the Milky Way.
Galaxies appear highly varied in size and structural organization upon observation; however, the inaccessibility of the vast majority of them, and thus the limited knowledge we still have of them, means they are classified into a relatively small number of types: elliptical, spherical, spiral, and irregular. Yet the great majority appear, even in long-exposure photographs taken with the largest instruments, as small indistinct spots; thus, the most distant objects can only be classified based on their apparent dimensions and the presence or absence of a certain central condensation. Fortunately, the few thousand nearest galaxies, observable in greater detail, have allowed for the analysis and a sufficiently detailed classification of these celestial objects. It is then reasonable to assume that they constitute a truthful sample of all galaxies existing within a billion light-years. It is estimated that, at the greatest distance reached so far, the number of photographable galaxies is at least 200 million (within a sphere of 100 million light-years from Earth, at least one million galaxies).
Not all matter in galaxies is condensed into stars; this depends on the evolutionary stage in which the galaxy finds itself, that is, its age. In the Milky Way, it is believed that beyond the 100 billion stars, there is still diffuse matter, in the form of gas and minute cosmic particles, with a total mass perhaps greater than that condensed into stars. But the Milky Way is a giant galaxy; an average galaxy will have a mass 10 billion times that of our Sun (which is an average star). Thus, it can be inferred that the average density of matter in the metagalactic space explored so far is on the order of 10⁻³⁰ to 10⁻³¹ grams per cubic centimeter. This value, of course, has only the significance of an order of magnitude; some astronomers, in fact, consider it too small, assigning instead a value of 10⁻²⁸ g/cm³ to the average density of matter in all space. That is, the universe is, on average, substantially empty—far emptier than can be achieved in terrestrial laboratories.
2. The Milky Way (Galaxy). — As already mentioned, it is the stellar system, one of the galaxies of the universe, to which our Sun belongs. This galactic system appears as an agglomeration of stars and diffuse matter (gas and interstellar dust) of roughly spheroidal shape with strong flattening, which rotates around an axis perpendicular to the galactic plane, defined by the luminous ring of the Milky Way. It appears in the sky as a faintly luminous band along a great circle that forms an angle of approximately 62° with the celestial equator.
Through the telescope, the Milky Way appears composed of a multitude of stars, individually invisible to the naked eye; photography reveals true stellar clouds. This apparent concentration of stars in the median plane of the Milky Way is, however, due solely to a perspective effect, dependent on the Sun's position within the system itself. The Sun is indeed located near the equatorial plane of the system but very far from its center, at a distance of approximately 30,000 light-years. In the sky, the center of the Galaxy projects in the direction of the constellations Scorpius-Sagittarius, where the Milky Way indeed appears richer and more densely populated with stars. The most probable dimensions of the Galaxy are as follows: diameter of approximately 100,000 light-years, thickness at the center approximately 10,000 light-years. In reality, a certain number of stars have been found that should be situated outside this lenticular system, forming a sort of halo; thus, the equatorial diameter of the entire system would exceed 150,000 light-years, and the thickness 120,000, meaning the Galaxy would ultimately be nearly spherical.
Recent research also seems to show that the Galaxy ultimately has a distinctly spiral form, thus highlighting its analogy with other external galaxies, in which the spiral form predominates. These conclusions were recently (1952) supported by the actual discovery of the spiral arms of the Galaxy, a discovery achieved both directly (by Bok and collaborators at the Harvard Observatory) and through the observation of hydrogen emissions from these arms in radio waves (at a wavelength of 21 cm), which, reaching Earth after passing through the ionospheric barrier, have been detected by appropriate radio-receiving equipment.
Surrounding the true galactic system are the globular clusters, peculiar spherical clusters of stars with strong central concentration, consisting of a vast number of stars that often cannot be distinctly separated even with the largest telescopes. Approximately one hundred (exactly 103) globular clusters are known, and there are good reasons to believe that these are all that exist. Globular clusters seem to accompany all galaxies: Hubble and Baade have proven the existence of at least 300 similar objects around the Andromeda Nebula, which is a galaxy similar to the Milky Way (though of much larger dimensions, according to the "new scale"). Of the globular clusters around our Galaxy, the best known is the cluster...
of Hercules (named after the constellation in which it appears projected in the sky), which is also visible to the naked eye as a small luminous speck. Large telescopes have revealed over 40,000 stars outside its extremely dense and unrealizable central nucleus. In the sky, it appears with a diameter of approximately 30°, that is, the apparent diameter with which the Sun and Moon appear to us. However, the two brightest globular clusters are located in the southern celestial hemisphere. Of all these clusters, the two closest are at a distance of 40,000 light-years, and the farthest at 500,000 light-years. Yet they are all concentrated around the Galaxy, so much so that the galactic center coincides perfectly with the centroid (center of mass) of the globular clusters (Shapley).
In addition to these globular clusters, generally external to the Galaxy, there are other star clusters called galactic clusters, both because they are generally within the system (the farthest is at approximately 40,000 light-years, that is, roughly the distance of the nearest globular cluster) and because they are generally close to the equatorial plane of the Galaxy. They are also called open clusters due to their less dense and compact structure compared to globular clusters, from which they also differ in the "quality" of the stars that compose them and in the number of these stars, which are generally only a few dozen or at most a few hundred. These stars also exhibit common physical characteristics and a noticeable uniformity in their movements through space. Over 300 galactic clusters are known; the closest to Earth is that of the Hyades in the constellation Taurus, located at 108 light-years. The most famous is the Pleiades cluster, clearly visible even to the naked eye. The study of galactic clusters is always very interesting, and from it fundamental discoveries have been made at various times, such as that (1950) of the absorption of light in interstellar space, which later led to the discovery of amorphous interstellar matter.
Interstellar matter is found diffused almost generally throughout the Galaxy, but particularly near its equatorial plane, where it forms a denser and noticeable layer several hundred light-years thick. This matter consists of gas and dust. The existence of interstellar gas is demonstrated by the spectra of distant stars, which exhibit characteristic absorption lines, known as "interstellar lines" or "stationary lines," since they show a Doppler shift different from that of other lines due to the star itself, the radial motion of interstellar gas being generally different from that of the stars. Using such methods, calcium, sodium, potassium, titanium, and even molecular compounds formed by carbon, hydrogen, and nitrogen have been identified. However, the most abundant element in interstellar matter (as well as in matter already condensed into stars) is hydrogen, which was only recently detected (1939), as its absorption lines all fall in the ultraviolet regions of the spectrum, which our atmosphere completely absorbs, and also because interstellar hydrogen is generally found stripped of its single electron due to the ultraviolet radiation from the hottest stars scattered throughout the Galaxy. However, if a hydrogen nucleus encounters an electron, recombination will be accompanied by the emission of radiation, which can be observed. Furthermore, a significant portion of the so-called galactic radio waves received by special "radio telescopes" (v. below) also originates from interstellar hydrogen. O. Struve provides the following relative abundances for cosmic matter: in stars, 6 atoms of calcium, nitrogen, and oxygen, compared to 500 atoms of helium and 5,000 of hydrogen; in interstellar matter, in a million cubic inches, 10 million atoms of hydrogen, 100 of calcium, 60 of sodium, 4 of potassium, and 2 of titanium.
As for the presence of interstellar dust (smoke), evidence is provided by the general and selective absorption it exerts on the light of distant celestial objects. Although measurements are extremely difficult and delicate, in regions where interstellar matter is more abundant (the galactic plane), it is found that on average 50% of light is absorbed after traveling 3,000 light-years. Moreover, selective absorption (reddening of distant stars) depends on the size of the absorbing particles, which has led to attributing them a diameter on the order of a tenth of a micron. According to some astronomers, these particles would consist of metallic corpuscles, such as iron. The average density of interstellar matter is extremely low, on the order of 10⁻²⁴ gr/cm³. A volume as large as Earth would weigh barely a kilogram. In total, one atom of interstellar matter per cubic centimeter. But since the distances between stars are immensely vast, ultimately, the total mass of matter diffused in the Galaxy would be roughly equal to the mass of all the stars. Within it, the mass of interstellar dust (which would be found only in limited areas) would be slightly greater than the mass of gases.
Often, these interstellar gases are gathered and condensed in the form of nebulae, which appear dark if not excited by any star and thus constitute vast clouds of absorbing matter; they are nothing more than the numerous "black holes" that appear here and there in the luminous face of the Milky Way. Over 1,500 dark nebulae have been counted so far, including the Coalsack in the southern sky, near the Southern Cross, which is only about 50 light-years away, and the dark clouds in the constellation Ophiuchus, also very close, as well as the dark nebulae of Cygnus at 250 light-years.
Often, these gaseous clouds become luminous because the gas is excited by nearby bright stars and shines by luminescence. These are the galactic nebulae or diffuse nebulae, which appear as more or less vast and dense luminous patches. The most important is the Orion Nebula, visible even to the naked eye in the homonymous constellation. The spectrum of diffuse nebulae generally consists of a number of bright lines belonging to hydrogen, helium, carbon, and nitrogen.
The origin of some lines remained unknown and mysterious for a long time, so much so that they were attributed to a hypothetical element, "nebulium," which is not found on Earth. Later, it was discovered (Bowen, 1929) that these mysterious lines are due to oxygen ionized once or twice, specifically the so-called "forbidden lines," which are not obtained under normal laboratory conditions because they require the absolute absence of collisions with matter atoms and thus an extremely low gas density. Some diffuse nebulae, such as the one surrounding the stars of the Pleiades, instead exhibit an absorption spectrum similar to that of stars. In this case, they are "reflection nebulae" because they simply reflect the light of a central star, which is not hot enough to emit ultraviolet radiation that would cause the gas to luminesce.
Beyond diffuse nebulae, the Galaxy also contains planetary nebulae (of which the characteristic example is the Ring Nebula in Lyra), so named because they appear as small luminous disks, elliptical or nearly circular, very similar to the appearance of a planet. Around a hundred of these objects are known, all of faint luminosity. At their center, there is generally a very hot star. The origin of these singular celestial objects remains mysterious; some believe they are the remnants of ancient novae, the current gases having been expelled by the star at the time of its explosion.
3. The Stars
However, the fundamental constituents of the Galaxy are the stars, which are colossal gaseous globes produced by the concentration and condensation of cosmic gas, and within which all the energy they radiate into space is generated.From the perspective of appearance (from our Earth), stars (like all other galactic and extragalactic objects) participate in the diurnal motion of the celestial sphere but do not exhibit noticeable shifts in their respective positions or in the configurations they present in the sky (constellations). As such, they were called stellae fixae by the ancients to distinguish them from planets or "wandering stars" (the ancients included the Sun among these, though it too is a star).
In the sky, stars are further distinguished from planets by their generally more vivid and brilliant light, greater scintillation, and by the fact that—no matter the magnification of the telescope—they do not present an apparent disk or a measurable diameter (naturally excluding the inevitable diffraction disk). This is due to their immense distance (the nearest star, Proxima Centauri, is 4.3 light-years away, with a light-year being the distance light travels in one year, approximately 10 trillion kilometers, or exactly 9,461 × 10¹² km). Even the strongest magnifications can only amplify the light they emit. In reality, stars are not fixed in space but possess, in addition to apparent motions (diurnal rotation and annual revolution), small angular movements (called proper stellar motions; the record is held by "Barnard’s Arrow Star," whose proper motion is 10.2 arcseconds per year), which, given the great stellar distances, correspond to sometimes extremely high spatial velocities.
The apparent brightness of stars is measured in stellar magnitudes, denoted by numbers on a descending scale starting from the brightest (–2, –1, 0, 1, 2, 3...). The light ratio between one magnitude and the next is approximately two and a half times (precisely 2.512). To the naked eye (for keen vision on a dark night), stars up to the 6th magnitude are visible. With the largest telescope (the 5-meter Palomar) and long-exposure photography, magnitudes up to the 25th have been reached.
Regarding the nomenclature of stars, only the brightest (about a hundred) have received proper names (generally of Arabic, Egyptian, or Eastern origin), while others are designated by letters of the Greek alphabet followed by the name of the constellation to which they belong, e.g., α Orionis (Sirius). This applies only to stars visible to the naked eye (about 5,500 across both celestial hemispheres); for all others, the stellar catalog in which they are listed and their respective order number are indicated, or their celestial coordinates (right ascension and declination, referred to a given equinox) are provided, generally with the addition of the symbol of the constellation containing them.
All knowledge of the physical characteristics of stars depends on the results of spectral analysis, which, inaugurated by G. Donati (1860) with direct-vision ocular prisms, shortly after G. R. Kirchhoff’s discovery of Fraunhofer lines had opened the way to the chemical analysis of celestial bodies (1859), received decisive impetus from Fr. A. Secchi S.J., to whom the first spectral classification of stars is due. Stellar spectra generally present (like the Sun) a bright continuous background crossed by dark lines, which arise from the absorption of light by gases contained in the star’s "atmosphere," while the continuous spectrum is generated by the deepest layer accessible to our direct investigation, called, as in the Sun, the "photosphere." From the earliest examinations by Fr. Secchi, it was expected that each star would have a different spectrum, but it was soon realized that all spectra could be classified into a few "types" or "spectral classes," showing progressive variations in certain characteristics, these being directly linked to the physical properties (color, temperature, etc.) of the stars. The spectral classes are arbitrarily designated by capital letters, in the order O, B, A, F, G, K, M, with each class further subdivided by numbers from 0 to 9.
In general, the spectral class of a star depends essentially on its surface temperature. Thus, while ancient astronomers believed that the relative intensity of spectral lines depended on the abundance of elements, and that there were therefore helium stars, hydrogen stars, or stars with metallic components, it is now known that spectral variations indicate variations in temperature—and, to a lesser degree, variations in pressure—rather than a real difference in chemical composition. This depends on the fact that, for example, hydrogen lines appear intense only when the hydrogen atoms producing them are in physical conditions that allow for intense absorption, and this occurs only when the temperature is sufficiently high. In general, the photospheric temperature of stars decreases progressively from class O to class M, while the number of absorption lines increases.
Some stars exhibit, superimposed on the continuous spectrum, bright lines or "emission lines" in place of the usual dark Fraunhofer lines. Generally, these are hydrogen lines, and such lines are found both among the hottest and the coldest stars, while they are absolutely exceptional among intermediate ones.
Spectral analysis of stars allows investigation not only of their qualitative structure but also of their quantitative composition, that is, the assessment of the relative abundances of different chemical elements. While the results are not yet definitive, two important facts appear to be conclusively established: 1) in stars, as in the Sun, only the chemical elements known on Earth are found: for those among these that have not yet been directly detected, their absence is explained by considering that they are either very rare elements or heavy elements (and thus probably located in the central regions of the stars), or else elements whose spectral lines fall in still inaccessible regions of the spectrum; 2) hydrogen is by far the predominant element in stars, constituting in the majority of them at least 80% of all atoms. Furthermore, there seems to be no reason to think that the chemical composition of stars varies from one to another, and thus it should be analogous to that fairly well established for our Sun, which—given its great proximity—is the best-studied and best-known star. However, some deviation does seem to exist, but it is limited to a few very hot stars (of the spectral class O) or to cold stars (class M, and the other special classes N, R, S characterized by spectral bands similar to those of class M). In conclusion, it would be erroneous to deduce from the examination of a stellar spectrum that a given element is predominant in a star’s spectrum solely because its spectral lines are very intense; nevertheless, the intensity of these lines depends, at least in part, on the abundance of the corresponding element. Here, briefly, are the main characteristics of the different spectral classes, beginning with class B, since the first class O comprises only a small number of stars even hotter than B.
Class B: blue stars, with surface temperatures on the order of 30,000°; the spectral lines, few in number, belong to hydrogen and helium. Metallic lines are weak or absent because, due to the high temperature, metals become ionized and could produce intense lines only in the far ultraviolet regions, which are unobservable. Examples: Rigel (β Orionis), the three stars of Orion’s Belt, Spica (α Virginis).
Class A: white stars, with surface temperatures from 16,000° to 20,000°; the spectrum is characterized by the lines of hydrogen, which appear broad and diffuse. Examples: Sirius (α Canis Maioris), Vega (α Lyrae), Castor (α Geminorum).
Class F: white-yellowish stars, with temperatures on the order of 10,000°. The hydrogen lines are less intense than in A and metallic lines begin to appear; the most intense are the two ultraviolet lines of ionized calcium, which, according to the Fraunhofer nomenclature used in solar spectroscopy, are designated by the letters H and K. Examples: Procyon (α Canis minoris), Canopus (α Argus).
Class G: yellow stars with temperatures close to 7000°; the Sun belongs to this class (temperature ca. 6000°). The spectrum is characterized by very intense metallic lines: some, such as H and K, are distinctly more intense than the hydrogen lines. Examples: the Sun, Capella (α Aurigae).
Class K: yellow-orange stars, with temperatures on the order of 4000° and a spectrum similar to that of G; however, the lines of neutral metals increase in intensity relative to those of ionized metals. Furthermore, some spectral “bands,” typical of molecules not dissociated into atoms, begin to appear. Examples: α Bootis, Aldebaran (α Tauri).
Class M: red stars with temperatures of ca. 3000°. The spectrum has a “fluted” appearance (the “columnar” aspect of Fr. Secchi), due to the presence of numerous absorption bands. The violet end of the spectrum is very weak. Examples: Betelgeuse (γ Orionis), Antares (α Scorpii).
The intrinsic luminosity of stars varies; it is expressed by their absolute magnitude, that is, the stellar magnitude each star would have if placed at a given distance, equal for all, arbitrarily chosen as 10 parsecs (the parsec, abbreviation of parallax equal to one second [of arc], is a unit of measurement for great celestial distances, equal to that of a star whose parallax [the angle under which the semi-major axis of Earth’s orbit would be seen from the star] is one second of arc: one parsec is therefore equal to 3.26 light-years, or approximately 30 trillion kilometers). The Sun thus has an absolute magnitude of -4.85, which means that, if it were at a distance of 10 parsecs, it would appear only as a rather faint star, barely visible to the naked eye. Many other stars are intrinsically more luminous than the Sun; the record is held by a star that is over a billion times more luminous than the Sun, and inversely by a star with an absolute magnitude of +19, that is, approximately a million times less luminous than the Sun.
However, the luminosities of stars are not randomly distributed; indeed, by relating these luminosities (or the corresponding absolute magnitudes) to their respective spectral classes, one obtains a singular characterization defined by the so-called “Hertzsprung-Russell diagram,” in which the majority of stars are distributed along a rather narrow strip that crosses the figure almost diagonally, and which is called the “main sequence.” A relatively small number of stars are then distributed along a second, nearly horizontal strip between spectral classes F and M. This second strip connects with the first near the point corresponding to the star Sirius of class A0. Since these stars are intrinsically more luminous than those of the same spectral class belonging to the main sequence, they are called giant stars, while the corresponding stars of the main sequence (at least those of classes F to M) are called dwarf stars. The Sun, which belongs to the main sequence and is located nearly at the center of the diagram, must therefore be considered a yellow dwarf star. The difference between giants and dwarfs is particularly notable for the cooler stars: thus, the luminosity ratio between dwarf stars of class M0, such as Proxima Centauri, and the red giants of the same class reaches a million. This Hertzsprung-Russell diagram is essentially verified for stars located in the regions of the Galaxy where the Sun is found, that is, in the regions of the “nebular arms.” These are called stellar population I (Baade), to distinguish them from another category of stars (population II), which predominate instead in the central regions (nucleus) of the Galaxy, in globular clusters, etc., and which present a “spectrum-luminosity diagram” different from the classic Hertzsprung-Russell one.
Finally, some stars exhibit characteristics that correspond neither to the main sequence nor to the giant branch of the diagram. These are either stars even more luminous than giants (40-50 times more), which are called supergiants (e.g., Antares of class M0 and Rigel of class B8); or else (quite numerous and widespread) very peculiar stars, which have a white color and extremely dense matter concentrated in a small volume, hence called white dwarfs (e.g., the companion of Sirius). The mass of white dwarfs is on the order of that of the Sun, while their volume is rather similar to that of a planet. Their density is therefore extremely high, reaching up to 100,000 times that of water. Such hyperdense matter, constituted
composed of atomic nuclei and electrons tightly packed together, exhibits characteristic properties quite different from ordinary stellar matter. This matter (called "degenerate matter" or also "Fermi gas," after the physicist who first studied it) proves compressible despite its great density, and the pressure within it is no longer given, as in ordinary stellar matter, by the equation of perfect gases, but is instead largely independent of temperature. Moreover, this matter does not radiate energy; the weak radiation we receive from these stars is due to a thin outer envelope of normal matter surrounding the inner core, from which derives the faint luminosity of these stars, which has hitherto made their discovery rather difficult. However, statistical research conducted in recent times, assuming that the percentage of white dwarfs observed in the vicinity of our Sun (where discovery is naturally easier) should reasonably extend to the entire Galaxy, would yield an enormous number for this singular class of stars, on the order of several billion. This means there would be approximately 8-10 white dwarfs for every hundred stars (Luyten, 1952).
From knowledge of the absolute magnitude of stars and their apparent luminosity, one can easily deduce the evaluation of stellar diameters (often measurable directly by stellar interferometers). Enormous diameters have thus been found for red giants and small diameters for red dwarfs. Among the former, Capella of class G (like the Sun), Arcturus of class K, and Aldebaran of class K5 have diameters respectively 16, 22, and 35 times greater than that of the Sun.
Even larger are the supergiants: Antares has a diameter approximately 400 times that of the Sun (and thus a volume 60 million times greater), such that the Earth's orbit around the Sun would comfortably fit within II. The record is held by the star ε Aurigae, which has a diameter 2000 times greater than that of the Sun, and thus larger than the orbits of Jupiter and Saturn. Red dwarf stars, on the other hand, generally have dimensions smaller than the Sun, such as Proxima Centauri, which has a diameter 30 times smaller. However, the smallest stars are the white dwarfs: the companion of Sirius has a diameter only a few times that of the Earth. Dwarf stars, yellow or red, are the most numerous, but giants are far more luminous than dwarfs and form the majority of stars visible to the naked eye.
The mass of stars can be determined directly only for a very small number of them (about 500), specifically only for those "double stars" (v. below) whose orbital elements are well known.
From these observed values, however, it has been possible to derive (Eddington, 1924) a general law: the absolute magnitude of stars varies regularly as a function of their mass (mass-luminosity relation). This law has allowed the evaluation of the masses of the majority of stars (with the exception of white dwarfs), finding that they have a mass of the same order of magnitude as the Sun, specifically between 1/5 and 5 times the mass of the Sun. The record is held by the star of Plaskett (discovered in 1921), which would have a mass 139 times greater than that of the Sun. It follows, therefore, that red giant stars, which have such a large volume, have a very low density. Antares has a density 50,000 times less than that of ordinary air, equivalent to a "good vacuum" obtained in our laboratories.
Although spectral analysis allows us to know only the composition of the surface layers of stars, research in theoretical astrophysics has made it possible to obtain fairly satisfactory and conclusive information about the internal constitution of stars, since stellar matter possesses relatively simple properties. This matter, given the almost complete ionization of its atoms, which have lost nearly all the electrons surrounding the nucleus, behaves like a gas even when its density is equal to or greater than that of water. In stellar matter, almost completely ionized, the space occupied by atoms is very small compared to usual conditions, so this matter must prove compressible up to densities much higher than those of ordinary liquids and solids and thus still retain the properties of gases. Furthermore, due to this highly advanced ionization, stellar matter exhibits the singular property of having a density almost independent of its composition. Indeed, the mean atomic mass of an ionized gas is approximately 2, regardless of the nature of the atoms composing II. For example, iron has an atom whose atomic mass is 56, being formed by a nucleus and 26 electrons. When completely ionized, it gives rise to 27 particles, whose mean atomic mass is precisely 56/27=2.1. Only hydrogen and helium yield rather low values upon ionization, but their effect is compensated, overall, by the incomplete ionization of heavy elements. On the other hand, in the internal equilibrium of stars, radiation pressure intervenes, which increases with the fourth power of temperature. Such pressure therefore contributes significantly to supporting the weight of the outer gaseous layers of the star.
In conclusion, it has been possible to determine and calculate the temperature, pressure, and density that must exist in the central regions of stars. In the case of the Sun, for example, there is a central temperature on the order of 20 million degrees, a density 80 times that of water (the Sun's mean density is only 1.41), and a pressure of 100 billion kg/cm². Such central temperatures are roughly of the same order for all stars of the main sequence (from 15 to 30 million degrees), while they are distinctly lower (1-5 million degrees) for red giants and supergiants.
4. Double and Variable Stars
Many stars (it is thought as many as one in three or four), when examined with a telescope or by spectroscopic or photometric methods, prove to be composed of two nearby stars, e.g., Castor, Antares, Rigel. Sometimes the proximity is only apparent, depending on the fact that the two stars happen to be aligned with the Earth ("optical doubles"); but in the majority of cases, the two stars form a real physical system, being connected by Newtonian attraction, as are the Earth and the Moon or the Sun and the planets. They then form a double star or a binary system; similarly, there are ternary, quaternary systems, etc. Double stars that can be seen and studied directly, either with a visual telescope or on a photographic plate, are called visual double stars. Many double stars, however, are so closely aligned that no telescope can separate them, while their duplicity is revealed only by the spectroscope, which shows the superposition of two distinct spectra, and they are thus called spectroscopic doubles. Finally, a third category of double stars is constituted by binaries or photometric doubles, or better still eclipsing doubles, since the orbit of the double appears edge-on or nearly so to us, so that one of the components periodically obscures the other during the revolution that the satellite star makes around the primary. The best-known of these is Algol (β Persei), whose light variations (thus eclipsing binaries are also false "variables") were discovered in 1783, but whose name, meaning "the demon" in Arabic, would indicate that they were known long before.Historically, the first known (visual) double star is Mizar (η Ursae Maioris), whose duplicity was discovered by Fr. Riccioli around the middle of the 17th century. Sirius, the brightest star in the entire sky, is also a double star, whose companion is a white dwarf.
Stars whose brightness is subject to continuous variations are called variable stars or simply variables. The first known variable was Mira Ceti (or Ceti), discovered in 1596 by Fabricius. Currently, many thousands of variables are known, and their number continues to grow. In addition to eclipsing variables, which are in fact double stars, "physical" variables—in which the variation in brightness is intrinsic and real to the star itself—
accompanied by corresponding variations in temperature and spectral class, can be divided into two fundamental groups: regular or periodic variables and irregular variables. The most important are the former, in which light variations occur regularly according to determined periods. Based on the duration of this period, they are further subdivided into three major classes: short-period (approx. half a day), less short-period (approx. one week), and long-period (approx. 280 days). Those with a short period are also called "cluster variables" because they are particularly numerous in globular star clusters, although they are also found in other regions of the sky. None of them is visible to the naked eye. The most important, however, are those of the second class, also called Cepheids (from the name of the typical star of the group, δ Cephei, whose stellar magnitude varies regularly by 0.7 units in 5.37 days), whose variations in brightness are due to actual stellar pulsations, that is, periodic expansions and contractions of the star's atmosphere (or at least of its outermost layers), the cause of which is still unknown. These pulsations are accompanied by regular variations in the star's temperature, which produce the observed variations in brightness. Simultaneously, corresponding variations in the spectrum and regular shifts in the spectral lines occur. Cepheids are particularly important because there exists a law of proportionality between the variations in their apparent luminosity and the period (the law of Miss H. Leavitt, discovered in 1910). And since this law translates into a relationship between absolute magnitude and period, it allows the determination, from the photometric variations of Cepheids alone, of their distance. Since such stars are also found and can be observed in distant celestial objects, such as globular clusters and extragalactic nebulae (external galaxies), the study of Cepheids provides significant information on the distances of such celestial objects. Precisely from a more accurate determination of the "zero point" of the cosmic distance scale, based on the observation of Cepheids in the Andromeda Nebula, the necessity of the "new scale" emerged (Baade, 1952-1953), which would double all distances and dimensions of extragalactic objects.
Among the long-period regular variables, which are generally red giant stars of spectral class M, is Mira Ceti, the first variable discovered, which has a light variation period of approx. 330 days with luminous intensity variations in a ratio of 1 to 250. These variables, too, are perhaps pulsating stars, although the theory of pulsations has not yet succeeded in explaining all their characteristics. It is noteworthy that almost all variable stars (physical) are giant stars.
5. Novae and Supernovae. — Novae (formerly also called "temporary stars") are particular stars that suddenly undergo an exceptional increase in brightness, sometimes becoming visible even to the naked eye (hence the name "new stars" given to them by ancient astronomers). After this rapid explosion, the nova slowly returns to its initial conditions as a telescopic star. The variations in stellar magnitude undergone by the star during the explosion phase sometimes reach 13 magnitudes, which would correspond to an intrinsic increase in luminosity on the order of 50-100 thousand times its original value. The cause of the phenomenon is not yet precisely known, but it would seem natural to think of an instability in the star that produces a sort of explosion with ejection and expulsion of the outermost layers of its atmosphere. This would seem to be proven by the spectral variations that always accompany the light variations of the nova. Moreover, in some of them, the explosion phenomenon repeats periodically (recurrent novae).
On the other hand, the phenomenon of novae seems much more frequent than the appearance of the most spectacular novae would suggest. Over a hundred new stars have been recorded (generally through photography) in the last fifty years; but certainly many others remain unknown. Thus, in conclusion, it is thought that at least 20-30 novae explode in our Galaxy every year. It should also be noted that almost all observed novae seem to prefer the regions of the galactic plane. Given the high frequency of the phenomenon, some astronomers have been led to conclude that almost all stars in our galactic system must have passed through the nova stage in the course of the last billion years, or, if the phenomenon cannot affect all stars, those observed must have suffered it repeatedly, and therefore the majority of novae would be recurrent.
Many times, the explosion phenomenon becomes more conspicuous and exceptional, the star reaching in a short time a luminosity even a hundred million times greater than the Sun, and thus as luminous as an entire galaxy. So much so that the phenomenon often becomes observable even in other galaxies. We then speak of supernovae, and indeed modern astronomers tend to classify supernovae in a decisively different way from ordinary novae, considering them as a completely separate and distinct class, showing markedly different spectral and luminosity characteristics. Certainly, a supernova must have been the famous star of Tycho Brahe, which appeared suddenly in 1572 in the constellation of Cassiopeia and became visible even in broad daylight. Another supernova must have been the star that appeared in October 1604 (this time observed by Galileo), and finally also the famous star observed by the Chinese in 1054, which gave rise to the current gaseous nebula of the Crab. This nebula has long attracted the attention and curiosity of astronomers, as it appeared to be of a nature distinctly different from that of any other known nebula, especially since it showed rapid expansion, developing at a speed of over 1000 km/sec. In conclusion, three supernovae certainly appeared in our Galaxy over the course of the last millennium; and this percentage roughly corresponds to that established for the average frequency of their appearances in external galaxies: two supernovae every hundred years. So far, several dozen of these mysterious and singular stars have been recorded (in addition to the three galactic supernovae), which represent, according to H. N. Russell, "the most terrifying phenomenon known to the human mind." The energy released by these stars is indeed on the order of 10⁴⁸-10⁴⁹ erg, that is, on the order of magnitude of the total energy contained in an ordinary star.
6. The Origin of Stellar Energy. — The Sun and the stars continuously diffuse into space an enormous flux of energy, the origin and sources of which remained entirely unknown for a long time. An initial hypothesis was that it derived from chemical reactions, such as combustion: however, if the Sun were composed solely of coal and burned in an oxygen atmosphere, it would have been consumed in a few thousand years. The meteoritic hypothesis was soon abandoned as well, according to which the Sun’s energy would have been supplied by the fall of meteorites onto the Sun itself. Finally, the hypothesis of gravitational contraction met a similar fate, according to which solar heat would be produced by the compression of gaseous masses resulting from the contraction of the Sun due to gravitational attraction: it was easily calculated that such contraction would suffice to provide the energy radiated by the Sun for only about twenty million years. This would not be a sufficient period at all, since the age of the Earth is 3,350 million years (Holmes, 1948), and geologists believe that at least a billion years must have been necessary for the evolution of life on our planet.
The problem of the origin of stellar energy could only be resolved around 1940, when it was recognized that it derives from nuclear reactions occurring in their interiors. By now, such reactions can be produced in our laboratories by bombarding atoms and atomic nuclei with appropriate devices and projectiles. In the interiors of stars, the energy with which particles must be endowed to penetrate atomic nuclei is provided by the extremely high temperature of the central regions of the celestial bodies. Thus, thermonuclear reactions capable of releasing energies thousands of times greater than chemical reactions occur spontaneously within them. In fact, such nuclear reactions are accompanied by relatively significant mass variations, and since, according to Einstein’s formula: E = c²M, there is equivalence between mass and energy, the proportionality coefficient being the square of the speed of light. Therefore, the value of the energy obtained from the transformation of matter is absolutely enormous; one gram corresponds to over 25 million kW.
Among all the nuclear reactions known to physicists, however, it is believed that only a small number can be triggered within the Sun and the stars, particularly the reactions involving the combination of protons or hydrogen nuclei with the nuclei of light elements. In the majority of stars—that is, those belonging to the main sequence of the Russell diagram (like the Sun)—such thermonuclear reactions, numbering six, form what is called the carbon-nitrogen cycle, or the Bethe-Weizsäcker cycle (named after the two physicists who identified it). In this cycle, four hydrogen nuclei, combining successively with carbon and nitrogen nuclei, are ultimately converted into a helium nucleus. Now, the atomic weight of hydrogen is 1.00813, while the atomic weight of helium is 4.00386; after deducting the external electrons, the corresponding nuclear masses are 1.00758 and 4.00276, so that in the reaction a fraction of the mass, equal to 4.00758 - 4.00276 = 0.02866, or 1/141 of the initial mass, is no longer found in the helium nucleus formed by the union of the four hydrogen nuclei. This "mass defect" must therefore appear, according to Einstein’s principle of equivalence, in the form of energy released by the reaction. As for the carbon and nitrogen nuclei, they are regenerated at the end of the reaction itself, behaving like catalysts in ordinary chemical reactions. In conclusion, the thermonuclear reactions occurring within the stars lead to a slow transmutation of hydrogen into helium. It is these reactions that sustain the radiation of the stars and, in particular, produce the heat and light necessary for life. In reality, nuclear reactions do not directly produce light but only gamma rays. These are then absorbed by stellar matter and re-emitted in the form of longer-wavelength radiation. Before reaching the surface of the stars, the radiation is absorbed and re-emitted a great number of times. Ultimately, the emitted radiation is sensibly in equilibrium with the surface temperature.
7. Stellar Evolution. — Knowing the nuclear reactions that occur within stars, one can examine their effects and, in a certain way, predict how they may be successively triggered, giving rise to the evolutionary processes that characterize the "life" of the stars. Indeed, the new knowledge of physics has profoundly modified traditional ideas about stellar evolution. In the case of the Sun, and thus also for the majority of normal stars (those distributed along the main sequence of the Russell diagram), it was previously believed that it was slowly and progressively cooling, since contraction, which was considered the sole source of energy, had to gradually slow down. It is now established, however, that gravitational contraction is important only in the initial phase of the life of stars (the contraction phase), allowing the triggering of the first thermonuclear reactions by raising the temperature at the center of the stars themselves. Once the sub-atomic or nuclear phase has begun, however, thermonuclear reactions (particularly that of the carbon-nitrogen cycle) suffice to produce all the radiated energy, transforming hydrogen into helium. Now, the reserves of hydrogen contained in normal stars are so great that they allow, for example, the Sun to continue living for at least another 10 billion years. Moreover, instead of a slow decline, it is predicted that the slow transmutation of hydrogen into helium—by progressively increasing the opacity of stellar matter—should progressively increase the central temperature and thus the radiation. Therefore, the Sun is rather headed toward a phase of heating before definitively beginning its final decline. In a few billion years, the Sun will probably be a hundred times more luminous. Its intrinsic luminosity will be comparable to that of Sirius. The increase in the Sun’s temperature will entail an increase in the Earth’s temperature, which is thought to reach at least 1,300°, eliminating every trace of life on II. Thus, life is destined to disappear from the Earth not due to cold, as previously believed, but due to heat (note: the "fire" of the Scriptures).
8. Radio Sources and Cosmic Radio Noise. – The use of appropriate radio-receiving devices, known as radiotelescopes, has revealed the emission, by celestial bodies (particularly the Sun) and by diffuse cosmic matter, of electromagnetic radiations at radioelectric frequencies that manage to reach our Earth after overcoming the barrier constituted by the terrestrial ionosphere. Furthermore, the existence of radio-emitting sources of limited dimensions and not corresponding to any known celestial body has also been revealed. The first discovery is by Rey, Parsons, and Philips (1946), subsequently confirmed by the observations of the Australians Bolton and Stanley (1948) and of Ryle and Smith (1948). In these initial experiments, it was found that the angular diameter of the two most intense observed sources could not exceed 6 arcminutes. Subsequent observations, obtained with special interferometric radiotelescopes, while on the one hand increasing the number of these singular dark bodies that emit only in radiofrequency, have progressively further limited the effective diameter of the sources, which were initially given the name of radiostars. However, since current radiotelescopes do not allow us to affirm whether or not these are dark bodies without apparent diameter, the less committing denomination of cosmic radio sources is preferred, especially since for some of them it has been possible to ascertain the coincidence with particular celestial objects (such as the famous Crab Nebula) or small gaseous nebulae. It should also be noted that the Hertzian radiation reaching Earth from these radio sources is generally much more intense (on average 100 million times greater) than that which reaches us from the Sun (the so-called radio-Sun), in which—given its proximity—such radiation is particularly intense. Furthermore, it is still under discussion whether radio sources are galactic or extragalactic bodies (sometimes it has indeed been possible to ascertain that some of the weakest radio sources coincide in position with some extragalactic nebulae); many, however, lean toward the view that they are predominantly new bodies distributed within our Galaxy, in which radio emissions are more intense than those of the Sun, while their visual emission is much smaller. It is also presumed that such bodies are widely distributed both in our Galaxy and in external galaxies.
Galactic radio noise, on the other hand, is due to radiofrequency emissions from interstellar matter, and in particular from the vast hydrogen clouds whose great diffusion throughout space has been ascertained. The definitive confirmation of this great diffusion came in 1952 with the discovery of the 21 cm wavelength radiation made by H. Ewen and E. M. Purcell (and subsequently by others). To fully understand how this was possible, it is necessary to recall that the outer electron of the hydrogen atom can move only over a certain number of orbits around the central nucleus, and that it can also jump from one orbit to another. Each of these jumps corresponds to an absorption or emission of radiation (ultraviolet, visible, infrared, or even Hertzian). Some of these radiations are rather complex, appearing to be composed of two or three (or even more) radiations extremely close to one another. The complexity of these radiations must be justified by the fact that, instead of single quantum orbits, there must actually be groups of orbits of very similar dimensions. Indeed, theoretical physicists have shown that even the "normal" orbit, that is, the one closest to the nucleus, must be double, and the "transition" of the electron between these two orbits must be revealed precisely by a radiation of 21 cm wavelength (Van Hulst). However, this transition is one of the so-called "forbidden" transitions, since under usual laboratory conditions they are generally hindered by some other event, such as a collision with another atom. But in interstellar space, where the density of matter is extremely low, such perturbing events are exceptionally rare; hence, the electron eventually makes the forbidden jump.
9. The Age of the Universe. – This expression is inaccurate, since astronomers prefer to speak rather of the cosmic time scale, a more appropriate term that implies an attempt to establish reference points in the past, rather than fixing an absolute beginning of cosmic time. Moreover, the possibilities of success in resolving the problem of the time scale are intimately connected with the ideas we are forming, with ever greater precision, about the very origin of the cosmic world.
The beginning must be traced back to a sufficiently simple creative act; probably that of elementary particles (protons, electrons, perhaps neutrons) and the "fields" that interact among them, that is, the laws governing these particles. The primordial matter of the Universe should therefore have consisted of a gigantic nebula of hydrogen, cold and extremely rarefied (the matter inanis et vacua of the Mosaic Genesis: et tenebrae erant super faciem abyssi), in which partial condensations then formed, giving rise to galaxies, and thus, through successive evolutions, to the stars that today populate the Universe.
Now, hydrogen still constitutes the most widespread element in the Universe, not only in matter already condensed into stars (in some of which it represents up to 70 percent), but—as has been seen—also in amorphous interstellar matter, where it reaches concentrations of 80 percent. This primitive hydrogen is slowly transforming into helium and other elements, so the abundance of hydrogen we still observe is good testimony to the "young age" of the Universe. This view is further supported by the considerations of H. Shapley and other equally authoritative astronomers, according to whom the development of stellar systems, that is, of galaxies, must be understood as inverted with respect to the scheme previously accepted by astronomers. That is, galaxies are born as spiral nebulae, and then, progressively losing the arms that wrap around the central nucleus, they slowly transform into elliptical and spherical galaxies (this fact has also been demonstrated theoretically by G. Armellini). Our Galaxy itself is a spiral nebula, of which attempts are being made to outline the arms (an important contribution was made in 1952 by the study of the 21 cm wavelength radioelectric emissions of interstellar hydrogen), in one of which the Sun resides, in an eccentric position and far from the central nucleus. Now, observations have clearly shown the enormous numerical preponderance of spiral galaxies, which represent over 80 percent of all galaxies. Such abundance should therefore indicate a relatively recent stage in the evolution of these stellar systems, and thus the "young age" of the entire Universe.
But it is also possible to quantify these concepts more precisely. In the first decades of this century—when the problem of the age of the Universe began to be posed in a concrete way—astronomers seemed to agree that our galactic system, and also all other galaxies, must have existed, more or less in their current form, for a sufficiently long time, at least for several
The universe is the ensemble of all that exists in space and time. Until this century—when the problem of the age of the universe began to be posed in concrete terms—astronomers seemed to agree that our galactic system, and even all other galaxies, must have existed, more or less in their current form, for a sufficiently long time, at least several trillion (10¹²) years. The reasons supporting these views were provided—beyond a supposed apparent equipartition of kinetic energy among stars—primarily by the incorrect application of Einstein’s relation E = c²M, which quantitatively links matter and energy. This relation led, for example, to the conclusion that the Sun, with a mass of 10³³ grams, could have maintained its currently observed luminosity (10³³ erg/sec) for at least 10²¹ seconds, that is, for a period of quadrillions (10¹⁵) of years. Subsequent developments in nuclear physics, however, showed that the release of energy does not involve the entire mass of the star; thus, results on the order of billions of years (10⁹–10¹⁰) were obtained.
On the other hand, stellar dynamics teaches that the mass of a star cannot exceed a certain limit, otherwise the radiation pressure becomes so strong as to disrupt equilibrium. In particular, calculations show—and observations confirm—that a mass slightly exceeding one hundred times the mass of the Sun must be considered dangerous and already close to disruption. From this, it follows that the age of the Sun could not date back more than 8 trillion years (8×10¹²). Taking an average of the values obtained, J. Jeans believed that the age of the stars was on the order of 5 trillion years (5×10¹²). This value, moreover, seemed to be corroborated by the knowledge of the time regarding the distribution of stellar velocities and the evolution of galactic clusters. Thus, by the end of the 1920s, Jeans summarized the prevailing ideas of astronomers in the view that the history of the universe could be traced back at least a thousand times the established age of the solar system and the Earth, an age which, primarily based on the results of radioactive elements in terrestrial rocks and meteorites, is reasonably considered to be on the order of three or four billion years (3,350 million years: Holmes, 1948).
Around 1930, it became evident that a complete re-examination of the problem of the cosmic time scale was necessary. The initial impetus was given by the development of the hypothesis of the expansion of the universe, formulated by Lemaître and De Sitter in those very years. Indeed, assuming the Riemannian nature of space, determining its radius of curvature based on the theory of relativity, and "ascertaining," according to Lemaître, that this radius of curvature is not a fixed and immutable quantity but expands at an increasing rate, it appeared natural to place the origin of the universe at the epoch when the distance between galaxies was minimal. Now, the theory of the expansion of the universe indicates that the "catastrophe" should have occurred two or three billion years ago; thus, one was led to place the origin of stars and stellar systems at the time of this catastrophe. That is, there was a tendency to shift from the "long" time scale (on the order of trillions of years, 10¹²–10¹³) to a "short" scale (on the order of billions of years, 10⁹).
Currently, the expansion theory has lost much of its initial favor among astronomers, due in part to the numerous doubts that have arisen regarding the interpretation of the redshift phenomenon in the spectral lines of extragalactic nebulae (the primary experimental support for expansion), which, if interpreted as a Doppler effect, would indicate a recession velocity of galaxies on the order of tens of thousands of km per second. Moreover, these velocities appear (at least within the depth of the universe explored so far) to be proportional to the distances of the galaxies themselves. Even E. Hubble (the discoverer, along with Slipher and Humason, of the phenomenon) never accepted such an interpretation for what he named the "Hubble effect"; he even suggested that it might originate from some new and still unknown physical principle. The thinking of contemporary astronomers can be summarized in the recent statement by H. Shapley: "That the expansion hypothesis is correct and refers to all the space we have so far explored is probable, but it has not been possible to demonstrate it conclusively" (Scientific Monthly, 67 [1948], p. 243).
Instead, evidence in favor of shortening the cosmic time scale has been increasing, particularly based on new knowledge about the dynamics of stellar systems, and primarily galactic rotation and the statistics of binary systems. Theoretical considerations and experimental evidence have shown that all spiral nebulae—that is, external galaxies—possess a rotational motion around an axis perpendicular to the plane of their nebular arms. This rotation has also been confirmed (Oort-Lindblad) for our Galaxy; however, it does not occur as a rigid body, that is, with the same angular velocity for all stars, but in a "differential" manner, such that stars closer to the galactic center complete their rotation in a shorter time than those farther away. Our Sun, with its planetary system, and the stars near it, take approximately 250 million years to complete one rotation, a period referred to as the cosmic year.
This differential rotation of the Galaxy produces a process of disintegration and dissolution in galactic star clusters, which must therefore be celestial objects with a rather short average lifespan, on the order of a dozen cosmic years. Open galactic clusters are subject to disintegration due to galactic tidal forces, while denser ones (such as the Pleiades) tend to break apart mainly due to internal causes (gradual depletion of the cluster through the expulsion of its members as a result of their interactions). Ultimately, ten or fifteen cosmic years appear to be a significant interval in the development of these clusters. Therefore, the presence of several hundred of them in our galactic system should be a very convincing testimony in favor of the "short" cosmic time scale for galactic development.
The second important corroboration is provided by double stars, which are extremely numerous in the sky. Chandrasekhar has given a fundamental formula for the problem of the stability of "wide" binary systems, a formula that links the semi-major axis of the orbit described by the satellite star with the probable "dissolution time" of the binary system. According to this formula, a binary system with a semi-major axis equal to a thousand astronomical units will dissolve, due to tidal forces caused by stellar "encounters" ("encounter" does not necessarily imply contact or collision, but only a close approach with considerable dynamic effects), over the course of 700 billion years; whereas a binary system with a semi-major axis ten times greater, that is, 10,000 astronomical units (a "wide" binary system), will dissolve in just two billion years (the astronomical unit is the average Earth-Sun distance). On the other hand, it is possible to determine the theoretical and observed frequency function of double stars, from which it would appear that the dissolution of binary systems has only just begun; whereas under the assumption of the "long" cosmic time scale, that is, trillions of years, one should not find such a large percentage of "wide" double stars, as current statistics show.
BIBL.:
J. Jeans, Astronomy and Cosmogony, Cambridge 1928; A. Holmes, The Age of the Earth, London 1937; G. Lemaître, L'hypothèse de l'atome primitif, Bruxelles 1946; E. Hubble, The Realm of the Nebulae, New Haven 1936; H. Shapley, Galaxies, Cambridge (Mass.) 1943; S. Chandrasekhar, Principles of Stellar Dynamics, Chicago 1942; inoltre V. COSMOLOGIA; RELATIVITÀ; STELLE.
P. G.
In conclusion, the results achieved by astronomers can be summarized as follows: the cosmic Universe is very young and — even setting aside the findings on the expansion of the Universe, still sub indice — its ancient history should not exceed a few billion years, at most 10 billion (10·10²). Slightly younger, perhaps, would be the normal stars, such as our Sun (much younger, however, are the red supergiants, some of which should still be "being born" even now), and nearly coeval is the entire planetary system, of which the Earth is part. For the latter, a "geological time scale" can also be specified, amounting to two billion years, to which the solidification of the Earth's crust should date back. Still for our Earth, there is then a "biological time scale," on the order of a billion years, to which the appearance of life dates back, and finally an "anthropological time scale" of one million years, to which the appearance of man dates back.
10. Modern astronomy and the Mosaic Genesis. — The enormous progress that, following physics, astronomy has been making since 1939 not only renders the vision of the cosmic Universe increasingly clear and precise but also makes it possible to attempt to outline ever more concretely the processes of its evolution and even the problems of its origin. For which, a scientific way of "seeing" the necessity of an initial creation, to which it is possible to "anchor" the chain of processes that have led from the uniform and disorganized Universe of the "beginning" to the differentiated and organized Universe we presently have before our eyes, is also beginning to emerge. This first creative act must have concerned the creation of matter, and with it of space and time, that is, the environmental platform that constitutes the "container" of the Universe.
Indeed, one of the greatest successes of modern astronomy perhaps lies in having finally made possible the interpretation of the first page of the Mosaic Genesis, showing a perfect agreement (G. Armellini calls it "surprising") between the Mosaic words and the conclusions reached by modern astronomical cosmogony. Naturally, the Mosaic account has a distinctly anthropic character, describing in popular language what a man would have seen had he witnessed the unfolding of events while observing them from Earth. An important observation (Gialanella) may find its place here. In the Mosaic Genesis, two distinct terms have been employed, one creavit (in Hebrew bāra) and the other fiat (in Hebrew jēkā). The first, creavit, has been used only three times, to indicate: 1) the "creation" of the Earth and the Heavens, that is, of the original matter (protons and electrons or, if one prefers, rather neutrons, from which electrons and protons then derived by natural "decay," or, if one prefers still, hydrogen); 2) the "creation" of living beings; 3) the "creation" of man. In all other passages, however, the fiat is used: "fiat lux, fiat firmamentum," etc. Now, it is possible to "reconstruct," for example, solely through the application of the laws of physics, light from the "formless and void" matter of the original creation, as a result of the first thermonuclear reactions that occurred inside stars due to the increased temperature and high density reached following gravitational contraction. "Fiat lux": light came into being, but naturally, without any need for direct creation.
The laws of physics alone, however, are no longer sufficient to explain the origin of life, and thus a new bāra, the second, is required, that is, a second intervention of creative power. Nor is it possible to pass, by physical and natural laws alone, from purely animal life — even through all the complex evolutionary forms — to conscious and rational life, that is, to man. And here, the third marvelous bāra was necessary. — Vedi tav. LXXXIV.