By Robert Hazen, Ph.D., George Mason University
In describing galaxies, distance is one of the most important measurements astronomers can make. However, even the closest galaxies are much too far away to be measured by parallax, that is, by triangulation. Thus, a lot of the effort in modern astronomy is devoted to developing improved methods for determining distances to galaxies.
Cepheid Variable Technique
The Earth, swinging side to side in the Sun’s orbit, covers a distance, a diameter, of about 16 light-minutes. It takes eight minutes for light to get from the Sun to the Earth, so the total distance across, six months apart, is about 16 light-minutes. The closest distant galaxy is hundreds of thousands of light-years away. You just don’t get enough of an angle to do triangulation on distant galaxies.
The technique Edwin Hubble used was the Cepheid variable technique, that is, a star of known brightness, a standard candle. This method, with improvements, is still used for galaxies up to tens of millions of light-years away.
The high-resolution photographs from the Hubble Space Telescope are particularly useful in this regard; the Hubble Space Telescope images allow you to resolve Cepheid variable stars in some galaxies that are 50 or 60 million light-years away.
There are other distinctive kinds of stars, bright stars, including the very brightest red giant stars and what are called the brightest main sequence stars, or supergiant stars. These stars also can be used as standard candles.
Learn more about the life cycle of stars.
Standard Candle: Type I Supernova
There are standard candles called type I supernova. These are very interesting kinds of supernova, exploding stars. Let’s consider the way this supernova occurs.
You have two stars that are orbiting each other in a binary system. Imagine that the larger star is almost large enough to explode as a supernova star, but not quite.
It keeps sucking material up from its companion star. More and more hydrogen and helium get pulled from the smaller star to the larger one until all of a sudden you reach that critical mass when the star is just large enough, where the temperature and the pressure inside the star get just high enough, and suddenly the star can go all the way to iron-56.
When it goes to iron-56, the star burns out, collapses gravitationally, and becomes a supernova. Every one of these type I supernovae have the same mass and the same energy, so they become a standard candle; that standard candle is roughly 50 million times the Sun’s energy output.
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Using Entire Galaxies as Standard Candle
Other approximate distance scales rely on things like the size and brightness of entire galaxies. If there is a cluster of galaxies, you can make an assumption about how bright the brightest galaxy is in a cluster—something like Andromeda or the Milky Way, which are large galaxies—and use that as a standard candle itself.
It requires a lot more assumptions, but still, it’s not a bad way to get an approximate distance for something that’s billions of light-years away. There’s an important caveat to all this: we have no way of knowing how much intervening dust there is. If there’s no dust, then we get a true measure of the distance to an object because the brightness we see is the actual, apparent brightness of that object.
However, if there’s dust in the way, then an object seems fainter than it really is, and so it will seem farther away than it actually is. Infrared radiation is not nearly as strongly absorbed by dust as visible light, and so one way of getting around this problem is to measure the brightness of an object both in visible light and in infrared and see the ratio of those two.
Learn more about the nebular hypothesis.
Red Shift Method
There is a different method of determining distance as well, and this is the redshift method, using the Doppler effect. The redshift is the change in wavelength. When an object is moving away from you, it spreads out the wavelengths—for example, the wavelengths of light—so what might be a hydrogen line, at its normal position, is shifted towards the red end of the spectrum.
What Hubble noticed, in an astonishing discovery, was that the more distant a galaxy is, the greater its redshift. He saw a very simple relationship between redshift and distance, according to Cepheid variables. So you calibrate the redshift by looking at the shift of hydrogen lines or some other absorption lines; you calibrate the distance by looking at Cepheid variables—at least for stars that are out to 50 million light-years or so—and he saw this relationship.
In general, the more distant galaxies are moving away much faster. This led to something called Hubble’s Law, a law that states that the velocity of recession of a galaxy is directly proportional to its distance.
Common Questions about Methods for Determining Distances to Galaxies
The cepheid variable is a technique that Hubble used for determining distances to galaxies. According to this technique, a star with a specific brightness can be used as a standard candle.
A type I supernova occurs when two stars orbit each other in a binary system. The larger star begins to suck hydrogen and helium from the smaller star, and when it’s large enough, it goes all the way to iron-56. When it reaches this stage, it explodes and becomes a supernova. Every type I supernova has the same energy and mass and is therefore used as a standard candle for determining distances to galaxies.
Red shift is a method to determine the distances to galaxies. The change in wavelength is called redshift. When something goes farther, its wavelengths expand and spread out. Therefore, this demonstrates a simple relationship between distance and redshift and is a useful technique for determining the distance to galaxies.