Life Cycles in Galaxies
One of the greatest astronomical discoveries of the last century was that our own Milky Way is but 1 of 100 billion galaxies sprinkled throughout an almost inconceivably vast extent of the observable universe. Each galaxy like the Milky Way consists of billions of stars, myriad clouds of gas, and—lurking in the very center—a supermassive black hole. These components are surrounded by a large halo of dark matter particles that provide the gravitational “glue” to bind the galaxy together, but which are otherwise invisible.
When first discovered, galaxies were called “island universes” and were thought to reside in majestic isolation. Today we know that galaxies are part of a complex network of interactions with the cosmos that has governed their lives over billions of years. Most gas clouds inside a galaxy eventually collapse to form new stars, but some clouds near the galaxy center are instead captured and eaten by the massive black hole. The life-sustaining nuclear reactions inside stars create new chemical elements like oxygen, carbon, and iron. As they die, stars expel these chemical elements back to the galaxy, providing the raw material to form new stars, planets, and even life. As the gas inside a galaxy is used up in this way, it is replenished by gas flowing in through the halo of the galaxy from a primordial repository of gas in the vast spaces between the galaxies themselves.
However, this flow of gas is not one-way. When massive stars die, they explode violently and heat the surrounding gas to temperatures of millions of degrees. Some galaxies go through episodes in which the rate of such explosions is so high that the galaxy’s gas supply may be blasted completely away. Intermittent powerful eruptions of the massive black hole may do the same. It is these cycles of matter and energy in and out of galaxies that determine how they were born and how they have grown. Understanding stars, black holes, and gas inside and out is a central goal in astrophysics for the next decade.