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Measuring the Properties of Distant Galaxies

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  • 0:01 The Properties of…
  • 0:39 The Distance to a…
  • 2:33 The Diameter and…
  • 3:20 The Mass of a Distant Galaxy
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will go over a couple of methods astronomers use to find out the properties of a distant galaxy, namely, its distance away from us and its mass.

The Properties of Distant Galaxies

Weighing yourself on a bathroom scale or measuring the distance between you and a friend are both very simple here on Earth. Simple compared to measuring the properties of our own galaxy, the Milky Way, as we learned in a previous lesson. But how in the world do we ascertain the properties of distant galaxies?

There's no scale large enough, no ruler long enough, and no way to even get ourselves to those galaxies. At least we're within the Milky Way! The other galaxies are many light years away. There have to be workarounds for this. These workaround methods for measuring the distance to a galaxy and its mass will be the subject of this lesson.

The Distance to a Faraway Galaxy

When I said that distant galaxies are light years apart, I was being very conservative. They are so far apart, astronomers have to use a special unit to measure their distance away. This unit is the megaparsec (Mpc), which is equivalent to one million pc. An Mpc is about 3.26 million light years, or 2 * 10^19 miles.

Anyways, the way astronomers can determine the distance to a galaxy is by looking for familiar objects called distance indicators, or standard candles. These are objects, such as supernovae and supergiants, whose luminosity is known and are used to help find intergalactic distances.

One well-known distance indicator is a Cepheid variable star, a type of pulsating variable star with a period between 1 and 60 days. A variable star is one whose brightness changes with time. This property allows astronomers to calculate the distance to its galaxy.

Very distant galaxies, those beyond 30 Mpc, cannot rely solely on Cepheids for distance measurements. Instead, astronomers have to calibrate the distance by using nearby galaxies and their standard candles as guideposts for galaxies further out. Another way to find the distance to a galaxy is by using the Hubble law, which is the subject of another lesson.

In any case, there's one very cool thing about the most distant galaxies, those that are 3,000 Mpc or more away. These galaxies are so far away that you see them as they were millions of years ago because it takes light so long to reach Earth from that distance.

The Diameter and Luminosity of a Galaxy

Once an astronomer ascertains the distance to a galaxy, either through the Hubble law or via distance indicators, they can calculate its diameter and luminosity. For example, by measuring the apparent magnitude of a galaxy you can then use the distance to it to find the luminosity.

Such calculations have shown astronomers that galaxies are very different throughout the universe. Irregular galaxies are usually small, only 1%-25% the size of the Milky Way galaxy, and they have low luminosity.

But there are galaxies larger than our own. There are elliptical galaxies five times the size of our galaxy and spiral galaxies over four times larger and ten times more luminous.

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