By Robert Hazen, Ph.D., George Mason University
Edwin Hubble had demonstrated that the nebulae were, in fact, vast, gravitationally bound groups of stars—perhaps tens, perhaps hundreds of billions of stars. These objects are called galaxies. Following Hubble’s discovery, astronomers began an immediate, intensive search for galaxies, all kinds of galaxies, and that search continues even today.
Cepheid Variable According to Hubble
In order to resolve the nebular debate, the American astronomer Edwin Hubble focused on an unusual type of star called Cepheid variable. These are stars that get brighter and dimmer and brighter again, in periods ranging from a few hours to several weeks. These are giant stars—they’re three times or more the size of the Sun—that have just started the helium-burning process in a fitful way.
The star starts helium burning and balloons out, but when it balloons out, it gets cooler, and so it starts contracting again because the fusion stops. Then it starts helium-burning again, and then it starts cooling down again. So there is a period when the star actually gets larger and smaller, putting out more and less energy over these cycles of a few hours, a few days, or a few weeks.
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Determining Distance to Various Objects
In 1912, astronomer Henrietta Swan Leavitt had discovered a remarkable relationship between the period of a Cepheid variable—that is, the time it takes to brighten and dim—and its absolute brightness, that is how much energy it’s putting out. Leavitt was employed as an astronomical computer at the Harvard Astronomical Observatory. Computers were people who looked at glass photographs and measured the positions of stars.
What she found was that the brighter stars have longer periods. Since the absolute brightness of these stars is directly related to their period of brightening and dimming, you can look for variable stars in more distant objects. You can see how long it takes them to get brighter and dimmer, you can see what their relative brightness is to us, and you can determine their absolute distance from these observations.
By measuring the period, one knows the absolute brightness; by measuring the relative brightness, one, therefore, knows the distance. It’s a neat trick in astronomy and one that’s very important in determining distance to various objects that are far away.
Learn more about how stars die.
Discovery of Galaxies
Using the Mount Wilson telescope, the Cepheid variable stars in the galaxy M31, known as Andromeda, which is one of the closest and brightest of the galaxies, turned out to be resolvable for the very first time.
Edwin Hubble found that Andromeda was more than half a million light-years away, far beyond the Milky Way; and since it was one of the very brightest and the largest of the objects in telescopes, so other galaxies must be much farther away than that.
Galaxies come in many sizes and many shapes. The most familiar are the spiral galaxies, which account for probably three-quarters of all the large galaxies that are known in the universe. Spiral galaxies are characterized by beautiful arms that curve about a bright central core.
The Andromeda galaxy is a typical example of a large galaxy, a spiral galaxy. It has more than 100 billion stars in that spiral.
Seen on edge, spiral galaxies reveal a flat disk and a large central bulge. Most of the stars are in the central bulge, but lots of stars are in the arms too. Our own Milky Way galaxy is a typical spiral, and our home is out on one of the arms, about halfway out from the center.
Learn more about the solar system.
Other Types of Galaxies: Elliptical, Dwarf, and Chaotic
There are many other kinds of bright galaxies as well.
Some of them are very uniformly rounded collections of stars that don’t have arms; these are called elliptical galaxies, and ellipticals represent about a fifth of all the known bright galaxies. They often contain hundreds of billions of stars as well. Each of those stars could have a planetary system, and each of those planetary systems might hold life.
There are also smaller irregular and dwarf galaxies. These are relatively faint, but they’re remarkably abundant. They still contain billions of stars, they’re still thousands of light-years across, and they may be the most abundant galaxies in the universe.
There are a relatively small number of galaxies that have chaotic shapes; these result from the collisions of two galaxies. They’re gravitationally linking as two galaxies come in contact with each other. What might surprise you is that even though these galaxies interact, the chances of any two stars actually hitting each other are almost insignificant because the stars are so widely spaced. Thus, two galaxies can interact gravitationally without ever having a stellar collision.
Common Questions about Types of Galaxies and Their Distribution in the Universe
Galaxies are made up of billions of gravitationally bound stars. There are different types of galaxies in the universe, with different shapes and sizes. The most common types of galaxies are spiral and elliptical galaxies.
Spiral galaxies have beautiful arms curved around a central core. They probably make up three-quarters of all known types of galaxies in the universe. The Milky Way galaxy is an example of a spiral galaxy.
Elliptical galaxies are very uniformly rounded collections of stars that don’t have arms. About 20 percent of all known types of galaxies in the universe are elliptical.