# What happens to galaxies when they die?

Stars explode when they die and blast heavy elements into space. Do galaxies do the same thing?

Well, it would be useful to define what a 'dead' galaxy is. Probably the most simple method would be a galaxy that is no longer producing new stars. We might also consider a galaxy that no longer produces significant light in the visual spectrum, or perhaps EMR across the entire spectrum.

Generally, there's unlikely to be a firm line between living and dead, and not nearly as dramatic as larger stars. More akin to watching a camp fire burn itself out. Star formation is largely dependent available gases, but as more and more stars fuse those gases into heavier elements, there is less gas available for star formation. For your average sized galaxy, this will eventually result in running out of gas. Eventually the galaxy will dim and go dark, a process purported to begin at the center of the galaxy, where star formation is heaviest according to research based on Hubble images of giant galaxies. (Tacchella, et al.) The matter ought to (mostly) all still be there and still orbiting the (presumed) SMBH, but with no energy coming from fusion, it's going to be a dark, cold, and barren place. Sounds dead to me.

There are some complicating factors. It's believed that encounters with nearby galaxies can affect available gases. The gravity from a larger galaxy could potentially strip the gases from a smaller one, a fatal blow for the smaller galaxy. Fortunately, it won't suffer much as the death will come (relatively) quickly. This process has been deemed 'strangulation' by a study published in Nature several years months days ago. (Ping, et al.) Note that as the study indicates, the methods of death are proposed solutions - not conclusive understanding of the exact processes that result in a galaxy's death.

S. Tacchella, C. M. Carollo, A. Renzini, N. M. Förster Schreiber, P. Lang, S. Wuyts, G. Cresci, A. Dekel, R. Genzel, S. J. Lilly, C. Mancini, S. Newman, M. Onodera, A. Shapley, L. Tacconi, J. Woo, and G. Zamorani. Evidence for Mature Bulges and an Inside-out Quenching Phase 3 Billion Years After the Big Bang Science 17 April 2015: 348 (6232), 314-317. [DOI:10.1126/science.1261094]

Y. Peng, R. Maiolino & R. Cochrane. Strangulation as the primary mechanism for shutting down star formation in galaxies Nature 521, 192–195 14 May 2015 [DOI:10.1038/nature14439]

Andrea Cattaneo. Astrophysics: The slow death of red galaxies Nature 521, 164–165 14 May 2015 [DOI:10.1038/521164a]

On any practical timescale (ie say 100 billion years) a galaxy could not be said to have died. The majority of stars that ever formed in that galaxy will still be "alive", but they are dim, low-mass stars. However, some of those low-mass stars will have reached the ends of their main sequence lives and, if more massive than $\sim 0.4M_{\odot}$ will go through a luminous, and highly visible, red giant phase.

Mitch Goshorn's definition of a cessation of star formation is a reasonable one, although on that definition many of the gas-poor elliptical galaxies we see today would be "dead". The reason the galaxies "run out" of gas is that is might be blown out by the winds and supernovae caused by higher mass stars; it might be stripped from them in tidal interactions with other galaxies, or in the long-run it is because the star formation process mostly produces stars that have the mass of the Sun or lower. Such stars, once formed, can lock away material and prevent it being recycled. The lifetimes of these stars increase rapidly with decreasing stellar mass - reaching a 100 billion years for a 0.4 solar mass star, which is still a bit more massive than the average star. So the mass spectrum of stars in a galaxy should tilt towards lower masses as it gets older.

Even when these low mass stars reach the ends of their lives they only return about half their gas to the galaxy, the rest is trapped in a cooling white dwarf. So new star formation will gradually expire as the amount of returned gas gets lower and lower.

Note, that the fact that the gas has been inside a star previously does not prevent new star formation. The returned gas consists mostly of unused hydrogen. It is just a question of how much of that gas there is, and whether it can be concentrated sufficiently to form new stars.

Galaxies are not really living objects, so while they can’t die, they can in fact change. The galaxies we see in the night sky have been around for billions of years and remain unchanged for millions of years.

However they do evolve and they can also be disrupted by the gravity from other galaxies. In fact, our own Milky Way is on a collision with one of our neighbours, the Andromeda galaxy. In a few billion years we will pass close enough that both galaxies will be deformed and will eventually merge. At this point a huge amount of star formation will be kicked off and the resulting galaxy will be a large elliptical blob rather than the fine disc like structures we can currently see.

• What is the boundary between living and non living things? Upon what kind of criterion do we judge such a matter? – Michael Lee Jul 14 '15 at 20:58