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From there, astronomers learned that there were whole galaxies in the universe separate from the Milky Way. The primary way astronomers estimate stars in a galaxy is by determining the galaxy's mass. The mass is estimated by looking at how the galaxy rotates, as well as its spectrum using spectroscopy.
All galaxies are moving away from each other, and their light is shifted to the red end of the spectrum because this stretches out the light's wavelengths. This is called " redshift. Astronomers must also know what the inclination or orientation of the galaxy is before making an estimate, which is sometimes simply an "educated guess," Kornreich said. A technique called "long-slit spectroscopy" is best for performing this type of work.
Here, an elongated object such as a galaxy is viewed through an elongated slit, and the light is refracted using a device such as a prism. This breaks out the colors of the stars into the colors of the rainbow. Some of those colors will be missing, displaying the same "patterns" of missing portions as certain elements of the periodic table. This lets astronomers figure out what elements are in the stars.
Each type of star has a unique chemical fingerprint that would show up in telescopes. Any kind of telescope can do this sort of spectroscopy work. Kornreich often uses the inch telescope at the Palomar Observatory at the California Institute of Technology, but he added that almost any telescope of sufficient size would be adequate.
The ideal would be using a telescope in orbit because scattering occurs in Earth's atmosphere from light pollution and also from natural events — even something as simple as a sunset.
The Hubble Space Telescope is one observatory known for this sort of work, Kornreich added. For the Universe, the galaxies are our small representative volumes, and there are something like 10 11 to 10 12 stars in our Galaxy, and there are perhaps something like 10 11 or 10 12 galaxies.
With this simple calculation you get something like 10 22 to 10 24 stars in the Universe. This is only a rough number, as obviously not all galaxies are the same, just like on a beach the depth of sand will not be the same in different places. No one would try to count stars individually, instead we measure integrated quantities like the number and luminosity of galaxies.
ESA's infrared space observatory Herschel has made an important contribution by 'counting' galaxies in the infrared, and measuring their luminosity in this range — something never before attempted. Knowing how fast stars form can bring more certainty to calculations. Herschel has also charted the formation rate of stars throughout cosmic history. If you can estimate the rate at which stars have formed, you will be able to estimate how many stars there are in the Universe today.
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