Objects in the Universe: A Primer

There are billions of galaxies in the Universe; galaxies are made up of billions of stars; many stars have planets orbiting around them.

This, very short summary of what makes up the Universe may be correct, but it also misses out a lot of fascinating details. The following, then, is a slightly more detailed inventory of the Universe, emphasizing objects that are important to X-ray astronomy.

Components of the Universe

• The observable Universe is about 13.7 billion light years in radius. On the largest scales, the Universe looks the same in every direction. It is thought to be made up of 73% "dark energy," 23% "dark matter," and about 4% normal matter. For an explanation of what dark energy and dark matter are, and how scientists have measured these numbers, see the Wilkinson Microwave Anisotropy Probe pages.

• In the entire observable Universe, there are perhaps 100 billion galaxies. Each galaxy is a collection of stars, gas, dust, and dark matter that are kept together by their own collective gravity. There are about 200 billion stars in our own Galaxy, the Milky Way; some galaxies are bigger, many are smaller. All large galaxies probably have a supermassive black hole at the center (see, for example, this news release from the Hubble Space Telescope.

• Some galaxies have a bright "nucleus," a central point that is as bright as all the rest of the galaxy put together. Some nuclei are so bright it's hard to see the underlying galaxy. These are the so-called "Active Galactic Nuclei," or AGN. It's likely these are supermassive black holes that are accreting matter at a furious rate. For more, read, for example, AGN pages on Imagine the Universe.

• Many galaxies belong to "Groups" and "Clusters" (see, for example, explanation at seds.org). It turns out that such Groups and Galaxies are filled with X-ray emitting gas, which in turn is held in place by the gravity of a vast amount of dark matter.

Components of a Galaxy

• Our own Galaxy, the Milky Way, is thought to contain about 200 billion stars. Some stars are much brighter than the Sun, although most stars are dimmer.
  • Most stars shine from the energy of nuclear fusion happening at the center.
  • Stars that have exhausted their fuel become white dwarfs, neutron stars, or black holes. You can explore the life cycle of stars.
  • Some stars come in pairs - "binary star systems," or "binaries" for short. Interesting things can happen if one star in a binary is a white dwarf, a neutron star, or a black hole!

• Galaxies also contain a lot of gas and dust. Some gas clouds can be hot enough to emit X-rays, like "supernova remnants" - these are gas expelled violently by a dying star. Read about supernovae here.

For a visual tour of the Universe, and to get a good impression of how immense the Universe really is, the try the Cosmic Distance Scale.