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# Populations of Stars

The color-color diagram you made in Explore 7 and 8 showed stars far above the plane of the Milky Way. But when you look up in the night sky, every star you see is in the Milky Way. Stars in the planes of galaxies are different from stars above and below the planes. Astronomers call the stars in the galactic planes "Population I" stars and stars outside galactic planes "Population II" stars. Population II stars are older and redder than Population I stars. If you repeated your diagram from Explore 7 and 8 with Population I stars instead, you would see more stars lying along the main trend.

 Question 13. SDSS was designed primarily to look at other galaxies, so most of the survey is far above or below the plane of the Milky Way. But SDSS is still able to see quite a few Population I stars. Where in the survey is the best place to look for these stars?

# Planets and Interstellar Dust

Planets in our solar system and dust between stars both radiate as nearly perfect thermal sources. However, both are very cold compared to stars - planets have temperatures of about 300 K (27 C) and dust grains have temperatures of about 30 K ( -243 C). Because they are so cold, planets and dust grains emit very little thermal radiation. The peak wavelength of this radiation is far into the infrared wavelength range.

# Galaxies

Galaxies are collections of millions or billions of stars, along with surrounding gas and dust. Because the stars in a galaxy may be thermal or non-thermal sources, and because they have a wide range of temperatures, galaxies can not be thought of as thermal sources with a single temperature.

If most of the stars in a galaxy are blue, the galaxy will appear generally blue. If most of the stars are red, the galaxy will appear generally red. The general color of a galaxy can tell astronomers something about what types of stars the galaxy contains.

However, the interpretation of galaxy color is complicated by two factors. First, interstellar dust in other galaxies absorbs light, and the dust absorbs light more easily at shorter wavelengths. Because interstellar dust absorbs short-wavelength light from galaxies, the light that reaches Earth is redder than the light the galaxies emitted.

Second, the color of a galaxy is changed by how fast it is moving. When a galaxy moves away from Earth at high speed, the wavelengths of the light waves it emits are stretched, appearing more red (see the Hubble diagram project for more information). Galaxies moving away quickly have more "redshift," and thus appear more red. Because redshift increases with greater distance, astronomers can use galaxy color to make a three-dimensional map of the universe.

# Quasars

 SDSS image of a quasar

Quasars are the size of our solar system, but they may emit as much light as a hundred galaxies. They are thought to be hot gas spiraling into a massive black hole at the center of a galaxy. The gravity from the black hole heats the gas, and the gas emits both thermal and non-thermal radiation.

The image of a quasar looks just like the image of a star; they were given the name "quasar" because they are "quasi-stellar objects." However, the spectra of quasars look very different from the spectra of stars. The hot gas in quasars emits in X-ray wavelengths as it falls into the black hole, and quasars are so distant that their spectra are highly redshifted. Because quasar spectra are so different from stellar spectra, quasar colors also look different. If you look at the colors of all pointlike objects in the sky, you can figure out which are stars and which are more likely quasars.