UH Institute for Astromomy

Educational Outreach


Star Collections

Galaxy Components

Clusters

Galaxy Shapes

Distance and Size

Further Reading

Acknowledgments

Galaxies

by Lisa K. Siegmund

Star Collections

Have you ever collected anything? Baseball cards, little stuffed animals, perhaps interesting rocks, or those milk caps? If you have, then you know that a good collection contains many objects. In space, a galaxy can be thought of as a collection of many stars. Some galaxies, like our neighbor the Andromeda Galaxy, are very good collections containing perhaps as many as three hundred billion stars. Other smaller galaxies such as the Small Magellanic Cloud, visible from the southern hemisphere, are more modest collections of about five billion stars. That is roughly the current human population of our Earth.

Our Sun along with its family of planet is a part of our own special galaxy--the Milky Way. More than two hundred billion stars make up the Milky Way, a decent collection by all standards. In fact, all the stars that you can see in the night sky are part of our galaxy.

Galaxy Components

In addition to the number of objects a collection contains, the variety of those objects also makes for greater interest. You wouldn't want a collection of 300 baseball cards all with the same player's picture! Galaxies, too, contain a variety of objects. While their main component is stars, there are many different types of these from young, hot, white-colored stars to the older, cooler, redder-colored variety. Our own Sun is a fairly average star: middle-aged, relatively cool, and yellow-orange in color.

In addition to stars, the other major components of galaxies are gas and dust. These are the building materials from which stars are formed and in many galaxies there are enough of these materials and the right conditions to produce new stars all the time. Planet and their accompanying moons can also be said to be objects in a galaxy, but unfortunately, due to the great distances of the stars from the Earth, we have not been able to see another planetary system like out own. At this point in time, we can only guess at their existence.


Cluster of galaxies.
Clusters

While it is currently impossible to see planet associated with other stars, it is entirely possible to see large numbers of galaxies from Earth with the aid of modern telescopes. In fact, astronomers are very certain that the Universe is filled with them. Some estimates put the number at one hundred billion galaxies but no one knows for sure. Storing these large collections of stars could be a problem were it not for the fact that space is mostly empty. In fact, the chance of one star colliding with another star in space is almost zero!

Interestingly, galaxies themselves appear to exist in loose collections. Astronomers call these collections of galaxies clusters. Our Milky Way is one of two large, spiral galaxies that make up our Local Group, the name given to our cluster of galaxies. Clusters of galaxies also seem to be grouped together in collections called superclusters. It seems reasonable to imagine that superclusters may be the largest collections of all!


The Sombrero Galaxy.
Galaxy Shapes

Galaxies come in several shapes as well as sizes. Our Milky Way is a spiral-shaped galaxy‹rather like a large pinwheel which slowly turns around a central hub, or nucleus. It has several spiral arms. The sun and its planets are located near the inner edge of one of the arms about two-thirds of the way out from the center. Our entire solar system is swept around the nucleus of the galaxy along with billions of other stars once every 200 million years. This amount of time is called a cosmic year. Although not the most common shape, spiral galaxies tend to be extremely bright objects due to the large amount of star formation taking place within them. Truly spectacular sights, they resemble giant whirlpools of light in the dark sea of sky.


The Whirlpool Galaxy.
Distance and Size

A discussion of galaxies, even a very short one, would not be complete without reference to distance and size. Space is a very big place. For example, let us consider the diameter of the Earth in relation to the diameter of the Milky Way. Now, think of the Earth as being only the size of an ordinary green pea. In this case, the size of the Milky Way would still be over three hundred million miles. This is more than three times the actual distance from the Earth to the Sun!

As mentioned earlier, no planet have been observed orbiting other stars because the distances of the stars are so great. The nearest star to Earth is Alpha Centauri located over 25 trillion miles away, but still easily in our galaxy.

In space, numbers get very large and so astronomers use different units of measurement. One of the units they use is a light-year which is the distance light travels in one year, almost six trillion miles. Using this unit of measurement, Alpha Centauri is over four light-years away. That means that it takes over four years for its starlight to reach the Earth.

The Andromeda Galaxy is two million light-years away. The light from its stars takes two million years to arrive on Earth. Oahu was still a lifeless, active volcano when the light we see today left the Andromeda Galaxy and the Big Island had not yet poked its head up from under the water. By the same token, the light arriving today from the most distant galaxies began its journey long before the Earth even existed! Our continuing study of these incredible, distant galaxies will help us understand more about the Universe and our place in it.

Further reading

Couper, Heather and Nigel Henbest. Galaxies and Quasars. Franklin Watts, New York, 1986.

Fanning, A.E. Planets, Stars and Galaxies. Dover Publications, Inc., New York, 1966.

Hirst, Robin and Sally Hirst. My Place in Space. Orchid Books, New York, 1990.

Knight, David C. Galaxies: Islands in Space. William Morrow and Company, New York, 1979.

Simon, Seymour. Galaxies. Morrow Junior Books, New York 1988

Acknowledgments

Lisa K. Siegmund is a visiting elementary school educator from Torrington, Connecticut. David Jewitt acted as advisor on this Bulletin. All images were obtained by Brent Tully.



| IfA Homepage | What's New | Search IfA | Image Gallery |

Return to top of page
Educational Outreach
The University of Hawaii at Manoa is an Equal Opportunity/Affirmative Action Institution. Copyright © 1996 Institute for Astronomy, University of Hawaii. All Rights Reserved. Revised by Wendy Nakano, February 1999.