Disks Bulges And Dissipation

A recurring feature of quantitative classification, and of our understanding of galaxy structure, is the distinction between bulge and disk components. This is widely understood to reflect the different processes by which these systems took shape. Exponential disks seem to result from dissipative processes, in which components can lose energy (such as to collisions between gas clouds, in which some orbital energy is given up to internal heat). The conservation of angular momentum in this situation leads naturally to a thin disk, which is dynamically cold—that is, it consists of material in circular orbits with a very small local dispersion in velocities. The same general considerations apply to forming rings around giant planets or accretion disks around compact objects. This fits with the basic history of the disk of our own Galaxy as inferred from the ages, motions, and makeup of its stars.

In contrast, elliptical galaxies and the central bulges of early-type spirals (collectively known as spheroids) seem to have resulted from a formation process in which dissipation was not important. Such nondissipative collapse would happen if the stars had already formed (leaving little cold gas for further generations of stars) before the final collapse to the configuration we see. The transition from an extended collection of stars to the compact configurations we actually see has to take place by a process known as violent relaxation. In classical dynamics the redistribution of energy among some group of interacting particles is known as relaxation, and the short time available for this process in galaxies (at least in terms of the systems' dynamical timescale) leads to this initially oxymoronic label.

Within galaxies, we see an interplay between stars and interstellar material so complex that it can fairly be called galactic ecology. It is the broad workings of this interplay that lets us associate stellar composition with time, and link the properties of gas in galaxies to their evolution. Stars form from interstellar gas, most immediately from molecular hydrogen with a salting of heavier elements. Stars then return the favor by ejecting gas back into their surroundings, through such diverse avenues as stellar winds, nova outbursts, planetary nebulae, and supernova explosions. In some of these, the returned material was once deep enough inside a star to have been a product of nuclear fusion, so that the interstellar medium becomes enriched in heavier elements with time. Which elements are enriched depends on the stellar source, with supernovae of different types giving differing relative amounts of oxygen, sulfur, and iron, for example. Even a single episode of star formation will give a changing chemical makeup in the interstellar gas as stars with different lifetimes return their portion to the surroundings. In turn, the enriched interstellar gas forms new stars, some of which are massive enough to further enrich the gas. The Sun is often described, loosely, as a "third-generation star'' in this sense—not that there have been three distinct episodes of star formation, but that we would expect the solar abundance (of about 1% of its mass in elements heavier than helium) to be reached only after two rounds of such recycling.

Stars are excellent historical probes of the kind of material they formed from. Although nuclear processes in the stellar cores change the mix of elements there, the atmosphere—what we measure in spectroscopic analyses—does not reflect these changes until late in a star's red-giant phase, so that the exterior composition reflects the original makeup. For long-lived stars, this gives us a chemical window into the interstellar medium long past. As we shall see later, the compositions of various kinds of stars are a powerful way to excavate galactic history, and to connect what we see in our neighborhood with the early Universe as revealed by observations of galaxies at high redshifts.

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