prepublication copy - The Department of Astronomy & Astrophysics ...
prepublication copy - The Department of Astronomy & Astrophysics ...
prepublication copy - The Department of Astronomy & Astrophysics ...
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BOX 2-4 Lifecycles in Galaxies<br />
One <strong>of</strong> the greatest astronomical discoveries <strong>of</strong> the last century was that our own Milky Way is but<br />
one <strong>of</strong> 100 billion galaxies sprinkled throughout an almost inconceivably vast extent <strong>of</strong> the observable<br />
universe. Each galaxy like the Milky Way consists <strong>of</strong> billions <strong>of</strong> stars, myriad clouds <strong>of</strong> gas, and – lurking<br />
in the very center – a supermassive black hole. <strong>The</strong>se components are surrounded by a large halo <strong>of</strong> dark<br />
matter particles that provide the gravitational “glue” to bind the galaxy together, but which are otherwise<br />
invisible.<br />
When first discovered, galaxies were called “island universes” and were thought to reside in majestic<br />
isolation. Today we know that galaxies are part <strong>of</strong> a complex network <strong>of</strong> interactions with the cosmos that<br />
has governed their lives over billions <strong>of</strong> years. Most gas clouds inside a galaxy eventually collapse to<br />
form new stars, but some clouds near the galaxy center are instead captured and eaten by the massive<br />
black hole. <strong>The</strong> life-sustaining nuclear reactions inside stars create new chemical elements like Oxygen,<br />
Carbon, and Iron. As they die, stars expel these chemical elements back to the galaxy, providing the raw<br />
material to form new stars, planets, and even life. As the gas inside a galaxy is used up in this way, it is<br />
replenished by gas flowing in through the halo <strong>of</strong> the galaxy from a primordial repository <strong>of</strong> gas in the<br />
vast spaces between the galaxies themselves.<br />
However, this flow <strong>of</strong> gas is not one way. When massive stars die, they explode violently and heat<br />
the surrounding gas to temperatures <strong>of</strong> millions <strong>of</strong> degrees. Some galaxies go through episodes in which<br />
the rate <strong>of</strong> such explosions is so high that the galaxy’s gas supply may be blasted completely away.<br />
Intermittent powerful eruptions <strong>of</strong> the massive black hole may do the same. It is these cycles <strong>of</strong> matter and<br />
energy in and out <strong>of</strong> galaxies that determine how they were born and how they have grown. Understanding<br />
stars, black holes, and gas inside and out is a central goal in astrophysics for the next decade.<br />
FIGURE 2‐4‐1 Left: Image <strong>of</strong> the center <strong>of</strong> our galaxy made at X‐ray wavelengths using Chandra X‐ray Observatory,<br />
optical wavelengths using Hubble Space Telescope and infrared wavelengths using Spitzer Space Telescope. <strong>The</strong><br />
four million solar mass black hole in the Galactic nucleus is located in the bright region to the lower right. Credit:<br />
Credit: NASA, ESA, Spitzer Science Center, Chandra X‐ray Center, and Space Telescope Science Institute. Right<br />
shows the three images on the left combined and annotated. Credit: NASA, ESA, Spitzer Science Center, Chandra X‐<br />
ray Center, and Space Telescope Science Institute.<br />
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION<br />
2-20