There exist about 100 million galaxies now, and there were more in earlier times. The classical galaxial type is a flat ellipsoid whose luminosity increases from the rim towards the center (fig.1.4).
Figure 1.4. Messier 81 clearly shows two spiral arms, NGC (New General Catalog) 3077 is an irregular galaxy. II Zw 40 is a compact galaxy. For the same luminosity, it is much smaller than normal galaxies. It apparently suffers an explosion, as suggested by the three escaping filaments. Most galaxies, as the Milky Way, are of the classical type exemplified by M 81.
Some other types of galaxies do exist (e.g. the Seyfert class of galaxies) whose turbulence would prevent the eclosion of life (fig.1.5).
Figure 1.5. NGC 1068 is a galaxy that belongs to the class of Seyfert. The nucleus of the galaxy is the seat of an intense activity that might be due to the presence of antimatter within a galaxy of matter.
Another possible explanation is the presence of a black hole.
Their luminosity is explained by the inclusion of antimatter. If this mass of antimatter is substantial, collision with matter cannot be avoided and annihilation will take place, with production of photons. These would then produce the intense luminosity observed in the galaxies of the class of Seyfert. Another plausible explanation for the luminosity of the Seyfert’ galaxies is the presence of a black hole. In general, however, the inclusion of antimatter or matter would not be substantial. The electromagnetic field developed by galaxies of either sign will impede their collision and mutual annihilation, and these galaxies will bounce back into a swirling motion that would explain their spiraling form.
However, due to gravitational forces, galaxies have the propensity to form clusters. They have many chances (4 to 40 in a billion years) of colliding, and indeed they do. Some galaxies swallow up others. As a result, there exist giant elliptic galaxies such as Centaurus alpha, that are about twice as luminous as their smaller neighbors, and the number of galaxies tends to decrease with time. Our own galaxy, the Milky Way, swallowed up a smaller neighbor galaxy as long ago as 10 billion years. As a result, the galaxy possesses a grand ring of hundred of millions of stars, about twice as far from the center of the galaxy as our sun. It is in near perfect alignment with the Milky Way’s disk, spread uniformly in a circle about 120,000 light-years wide (fig. 1.6).
Figure 1.6. A thick ring of hundreds of millions of stars embraces the galaxy in a circle about 120,000 light-years wide.
Within our own galaxy, matter is about a million times more abundant than in the universe as a whole. One could thus expect a collapse to have occurred about 100 million years after the formation of the galaxy. This was not the case for the reason that the galaxy is spinning rapidly. In addition, it is just barely large enough to make collisions and fatal encounters among stars unlikely. Instead of collapsing, the objects moving at the outer rim of the galaxy have settled in orbits revolving around the inner parts. The same spinning phenomenon is the reason also why the earth did not collapse within the sun.
The visible band of the Milky Way marks an almost perfect great circle in the sky. This indicates that the sun and its planets are close to the central plane of the galaxy. Also, the fainter stars, which are presumably the most distant, are more concentrated towards the galactic equator than the brighter ones. This indicates that the Milky Way is a large galaxy that is highly flattened (fig.1.7). The flatness indicates that the galaxy rotates at a fairly rapid rate; stars in the neighborhood of the sun move in almost circular orbits at a rate close to 450 kilometers per second. It takes the sun 250 million years to complete a circuit. This length of time is a cosmic year.
Figure 1.7. The sun and the earth are in the galactic plane and would be in the emission path of the gravitational waves beamed by a rotating naked singularity within this galactic plane only. A naked singularity is a singularity that emits a signal.
The appearance of the Milky Way should show considerable changes over as little as one cosmic year. Stars are being continuously reshuffled and some groups of stars dissipate while others form afresh. In general, clusters of stars lead only precarious lives because the nucleus of the galaxy exerts strong tidal forces that are highly disruptive. The disruption of a cluster can also be due to near-passages or full impacts occurring between clusters or clouds of interstellar dust. The result of this is that older stars are distributed in a random way as a spherical halo surrounding the galaxy, at rather far distances from the galactic plane. As said supra, this is not the case with the sun.
Initially, in primeval times at Z = 4 or more, clouds of dust had formed, wherefrom stars evolved. These primitive stars produced some heavy elements by fusion reactions but these stars were rapidly destroyed. Young hot stars now form within galaxies and are about 10 to 25 million years old. Their age is thus between a quarter and a half of 1% of the age of the sun and earth. These young stars are found in association with the interstellar gas and cosmic dust present in the spiral arm structure of the galaxies. Star birth takes place more precisely along the inside of the spiral arm, close to the central plane of the galaxy, were there exists a thin layer of interstellar gas and dust. It is dust that reddens the setting sun.
The principal constituent of the interstellar medium is hydrogen. If the concentration of hydrogen is set at 10,000 atoms, then the concentration of helium is 1,200 atoms, nitrogen is 1 or 2 atoms, carbon 2 atoms, oxygen 3 or 4, neon and sulfur one, and there are also traces of heavier atoms such as iron and chlorine. Besides atoms, there are also molecules present, among them ammonia (NH3), carbon monoxide (CO), water (H2O), formaldehyde (CH2O) and methanol (CH3O). About 30 different types of molecules are found all in all.
This diversity shows that the formation of galaxies, stars and heavy elements are a complex story. Nowadays, due to gravitational forces, the molecules are found mostly in regions of space where cosmic dust is prevalent. The stellar dust itself is composed of particles made of a mixture of graphite, silica and iron. These dust particles have a diameter of about 0.0005 millimeters. The clouds of cosmic dust grains screen out UV light. In addition, they are extremely cold: no more than about 5°K (-268.2°C). This set of conditions will allow cold gaseous atoms to adhere to the dust grains and concentrate there. Protected in this way from UV light, molecules can be formed without being immediately destroyed. In the course of time, the clouds of dust sweep up a considerable amount of matter from the surrounding interstellar medium. These clouds of dust will evolve further, under the impulse of gravity.
Gravitation is a universal force in nature. It ultimately leads to black holes as fundamental objects. Super massive black holes are believed to form the nuclei of galaxies. A rapidly rotating Black Hole has been derived theoretically by Kerr in 1963 as an exact solution to the theory of general relativity but has not yet been observed. A minor fraction of its rotational energy is released in association with gamma-ray bursts- the most energetic explosions in the universe- and a major fraction of the energy is released as gravitational radiation. Gravitational forces originating from the center of the galaxy, where a black hole by some means irradiates a gravitational beam, periodically concentrate the dust and gas to 5 to 10 times its original density and provokes in this way the formation of cool clouds of dust with a mass about 20 times that of the sun. These formations are the nebulas. The collapse of these nebulas can then occur under the force of their own gravitation beyond a critical stage. Protostars are formed in this way.
The concentration of matter occurring during the collapse of a cold nebula results in a higher temperature. It is first observable in the infrared, to become rich in UV light later on. True stars are thus formed. When the nebula collapses into many different units, clusters of stars will be formed that will disperse later. Since it is most likely that the protostar, with its cloud of surrounding dust, is rotating at a high speed, it will lose the angular momentum stored in the rotation by forming shells around itself, that in turn break up into planets.
About 90% of the matter of the universe is invisible. Presumably, it consists of neutrinos that were produced in the first few minutes that followed the "Big Bang" and these neutrinos are now condensed as a dark halo around the galaxies. Another sizeable amount of matter present under elementary form is the matter condensed in Black Holes located at the center of galaxies. Another possibility is that the invisible matter that surrounds galaxies consists of small dead stars. There exist many stars like the sun, which have a diameter of about 1,120,000 kilometers. Stars of such a volume are rather small. Stars of a smaller volume, i.e. only a volume of below 75 to 50 Jupiter masses (MJ), are too small for gravity to ignite their nuclear furnace. Their surface temperature reaches barely 2000°C (the sun’s surface temperature is about 5800°C) and they are called brown stars. The first brown dwarf was discovered in 1995. Now hundreds have been identified, extending from 75 MJ down to 7 to 8 MJ in such a way that the upper tail of planet formation (because some planets have been detected with masses superior to 7MJ) and the lower tail of star formation overlap. Without a long-lived central energy source, these failed stars disappear from view within a few hundred million years. More than 80% of stars are less massive than the sun, and brown stars are probably almost as common as stars. The remaining matter consists essentially of hydrogen, photons and helium. They are condensed into stars where the heat (at least 16 million degrees K in the core of stars) maintains them under the form of ionized plasma consisting essentially of free electrons, protons and helium nuclei.