The answer for astronomical galaxies, as for biological life, is probably both. And much like human life, wherein genes are estimated to influence well less than half of human behavior, environmental effects probably dominate changes among the galaxies too. Intrinsic Changes Star formation apparently proceeds at different rates in spiral and elliptical galaxies; after all, spirals currently contain large amounts of interstellar gas and dust, whereas ellipticals contain little.
This fact is clear for two reasons: Dusty regions are associated with spiral and not elliptical galaxies, and radio radiation from atomic hydrogen in spiral galaxies is strong whereas that from ellipticals is often weak or absent implying that loose hydrogen gas is missing in the ellipticals.
Astronomers suspect that early in the "life" of an elliptical galaxy, the star-formation rate was very high. The most massive stars soon exploded since they use their fuel more rapidly, as discussed in the next STELLAR EPOCH , and the ensuing conflagration from many such explosions drove the remaining interstellar gas from the galaxy, thus eliminating the material needed for further star formation.
We can envision such an outflow of gas as forming a "galactic wind," in analogy with the solar and stellar winds directly observed from the Sun and stars. Stellar explosions of this sort supernovae occur frequently enough even today to keep ellipticals swept clean of interstellar matter. By contrast, in spiral galaxies, stars might not have initially exploded frequently enough to cause a catastrophic purging of interstellar space, so a sufficient amount of interstellar matter remains today to support active star formation.
Thus the differing reservoirs of interstellar matter in spirals and ellipticals conceivably result from the different initial formation rates of massive stars, which later explode. Why the rates of star formation might have differed is an unsolved problem of galactic evolution that can be addressed only by observing galaxies as they were long ago. Since "looking out is looking back," such observations are possible in principle by studying galaxies at great distances.
In practice, however, these observations are difficult because of the faintness of those remote galaxies. Nonetheless, x-ray astronomy seems to offer a way to test these ideas.
The existence of galactic winds is quite consistent with current x-ray observations of rich clusters, especially those clusters in which most of the galaxies are ellipticals. In many such cases, astronomers have recently found hot, x-ray-emitting intracluster gas whose total mass and chemical composition agree with the expected accumulations of galactic winds from the various member galaxies of the cluster.
Removal of loose gas from the galaxies is further aided by any intracluster gas; such intracluster gas can purge matter from the galaxies as they move through it.
Environmental Changes Astronomers do have ample evidence that galaxies change in response to external, environmental factors, long after the first preglactic fragments originated.
As already noted, given the size, scale, and groupings of galaxies, collisions and interactions among them are commonplace events. This is especially true for the dark-matter halos surrounding many spiral galaxies, including our own, and probably those around all galaxies.
Computer simulations performed during the past decade show that these dark halos are strongly involved in, and influenced by, such galaxy interactions. As galaxies orbit or encounter one another, halo material from one galaxy can become stripped by tidal forces exerted by the other. In this way, even smaller galaxies can severely distort larger ones, depending upon the angle and proximity of interaction and the energy transferred between them.
In some cases, over the course of a hundred million years—a span of cosmic time that powerful computers can model in minutes—the simulations illustrate how close encounters between galaxies can cause spiral arms to appear where none existed before. If so, then even our home Milky Way plausibly got its arms by interacting with another galaxy at some time in the past. Perhaps the culprits were systems as small as the Magellanic Clouds now orbiting in the halo of the Milky Way, or the Sagittarius dwarf galaxy now being torn apart and subsumed by our Galaxy on its far side opposite the Sun.
Previous, long-ago encounters with a larger, comparable galactic system, such as the nearby spiral galaxy, Andromeda, is another possibility. Andromeda does currently have a component of motion toward us, meaning that our two giant galactic systems are destined for a close encounter that could cause both to become tidally disrupted and eventually more elliptical.
Examples abound. Consider two spherical galaxies, one a little smaller than the other, though each having a mass comparable to our Milky Way Galaxy. Now let the two galaxies experience a close encounter. As depicted in the various frames of Figure 2.
This figure is a computer-generated reenactment of the environmental changes produced exclusively by gravity. Note how the interaction causes the larger galaxy to sprout spiral arms where there were none before. The entire event transpired over several hundred million years—the kind of accelerated evolution that desktop computers can model in an afternoon. Shown there are two galaxies having sizes, shapes, and velocities matching very closely those objects in the computer simulation.
The magnificent spiral galaxy is M51, popularly known as the Whirlpool Galaxy. Its smaller companion is probably an irregular galaxy which, having drifted past M51 millions of years ago, managed to disturb it greatly.
This computer-generated encounter might conceivably be a valid model for the interaction of M51 and its companion. No one claims that the computer model accurately depicts a close encounter that did occur; nor does anyone suggest that M51 became a spiral galaxy specifically because of such a gravitational rearrangement.
Still, the computer rendition does demonstrate a plausible way that these two galaxies might have interacted millions of years ago, and how spiral arms might generally be created or enhanced by such interactions. The MM82 system is another example of a galactic interaction that probably rearranged much matter in at least one of these galaxies.
Shown in Figure 2. Research into how the population of galaxies as a whole has evolved has provided some important insights into galaxy evolution. One of the most important results, the Butcher-Oemler effect , shows that on average, galaxies were bluer in the past than they are today.
This indicates that the rate of star formation in the Universe has declined in recent times, and that the rate of galaxy evolution is slower today than in the past. Galaxies go on to merge with one another to create even bigger galaxies. Our Milky Way, for example, will collide with the Andromeda galaxy in the far future. Nonetheless, there are still open questions about how galaxies form. Some astronomers believe that galaxies usually form from smaller clusters of about one million stars, known as globular clusters.
Other astronomers believe that galaxies formed first and later birthed globular clusters. When it comes to spiral galaxies, it is believed that the stellar halo, bulge, and disks formed at different times and through different mechanisms. Usually, spiral galaxies contain a central bulge surrounded by a flat, rotating disk of stars.
The bulge located in the center is made up of older, dimmer stars, and is thought to usually contain a supermassive black hole. Around two-thirds of spiral galaxies also contain a bar structure through their center, the same as our Milky Way. The disk of stars orbiting the bulge tends to separate into arms that circle the galaxy.
These spiral arms contain a wealth of gas and dust, thus many young stars are birthed in these regions. These young stars shine very brightly before their quick demise. The motion of the spiral arms still remains disputed.
A theory suggests that these galaxy arms could be the result of density waves traveling through the outer disk. Encounters between galaxies may cause such waves as the mass of the smaller galaxy could affect the structure of the larger galaxy as they unite. Their size usually varies greatly, from 5 up to kiloparsecs across. The same can be said regarding their mass which typically is between 10 9 and 10 12 solar masses, and luminosities ranging from 10 8 to 10 11 time that of the Sun.
The vast majority of spiral galaxies rotate in the sense that the arms trail the direction of the spin. Measurements of the rotation curves revealed that the orbital speed of the material in the disk does not fall off as expected if most of the mass is concentrated near the center. Because of this, the visible portion of spiral galaxies is regarded as having only a small fraction of the total mass of the galaxy.
Thus it is concluded that spiral galaxies are surrounded by an extensive halo consisting mostly of dark matter. Spiral galaxies can be classified according to the tightness of their spiral, the lumpiness of their arms, and the overall size of their central bulge. The relative amounts of gas and dust contained within these galaxies can portray these differences. There are currently 3 classifications of classical spiral galaxies and another 3 for barred spiral galaxies:.
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