The Universe in the Classroom

No. 32 - Fall 1995

© 1995, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112.

Star Science in the Autumn Sky

by John R. Percy, University of Toronto
and George Musser, Astronomical Society of the Pac

Be honest: Do the stars all look the same to you? If you look up at the night sky, you can see anything from dozens to thousands of them, depending on where you live. To most people, the stars are just points of light, each one like the other. To astronomers, however, stars are as varied as people. There are as many different kinds of stars in our galaxy as there are people on Earth. They come in all shapes, sizes, colors, and dispositions. And stars, like people, have their life cycles. They are born, grow up, and die (don't pay taxes, though). The study of the stars and their lives, the great People magazine of the skies, is known as astrophysics.

Astrophysics sounds imposing. Say the word at a cocktail party and see how fast the conversation grinds to a halt. Star science, which is what astrophysics is, sounds much more friendly. The following activities will introduce you to star science as you ramble across the autumn sky. Though the stars are distant, you can come to understand their nature by making simple observations and drawing analogies to everyday things on Earth. When you can look at a star and see more than a point of light, the night sky will come alive.

Activity 1. Finding your way with a star map
Activity 2. Exploring the Big Dipper
Activity 3. Parallax
Activity 4. Finding the Hyades and Pleiades
Activity 5. Angles in the sky
Activity 6. Apparent size as a measure of distance
Activity 7. Ranking stars by brightness
Activity 8. Estimating the distance of brighter stars
Activity 9. Temperature and Color
The Stars They Are A-Changin', Part 1
The Stars They Are A-Changin', Part 2
Here Is My Journey's End: Galileo Finally Arrives at Jupiter, Part 1

Here Is My Journey's End: Galileo Finally Arrives at Jupiter, Part 2

Activity 1. Finding your way with a star map

The so-called horizon map (figure 1) shows the appearance of the autumn sky in mid-evening, as seen from the latitude of the United States and southern Canada. Hold the map in front of you so that the direction that you are facing is down. If you are facing north, for instance, hold the map vertically so that the word NORTH is at the bottom. The sky in front of you will correspond to the bottom section of the map. The circular border of the map represents the horizon; the center of the map represents the zenith, the point directly overhead. The dots on the map represent stars; the bigger the dot, the brighter the star.
Night sky
Figure 1
Road map to the autumn sky. The skies are constantly changing, but this map will guide you to the evening skies in late September to early November. It works at northern temperate latitudes. See text for instructions. Figure by John Perkins, courtesy of John Percy.
Click here for a larger version.

Star maps can be confusing at first. How do you match up this dot on the paper with that point of light in the sky? As with a road map, it takes patience and practice. As with a road map, feel free to ask a knowledgeable local -- an amateur astronomer -- for directions. The best way to learn the sky is to recognize two or three key star patterns, and then use these to find other star patterns. The Big Dipper is a good place to start.

Activity 2. Exploring the Big Dipper

Face north. Hold the star map so that NORTH is down. You should be able to see the Big Dipper low in the sky. The Big Dipper consists of a bowl (a skewed rectangle of four stars) and a handle (a row of three stars). The Dipper is part of a larger pattern called Ursa Major, one of the 88 official constellations adopted by astronomers (see figure 2). The constellations are patterns that some people happen to see in the sky. Different cultures see different star patterns; there is nothing special about the 88 patterns we use. You can even make up your own constellations. Instead of dippers, see whether you can spot cars, TVs, or astronomy teachers.

Ursa Major
Figure 2
Ursa Major. On the left is what you actually see in the sky. On the right is a diagram of the official constellation. The Big Dipper that we know and love is the bottom left part of Ursa Major. The splotches labeled M81, M82, and M101 are galaxies; M97 is a nebula. You'd need a telescope to see these objects. Photo and diagram courtesy of O. Richard Norton, Science Graphics, Bend, Ore.

Let's take a closer look around the Big Dipper. Binoculars help, but aren't essential.

  1. Follow the curve or "arc'' of the handle away from the bowl. You will come to a bright star called Arcturus. A useful way to remember this is "arc to Arcturus.'' If you have good night vision, or binoculars, you may notice that Arcturus is slightly orange. The best time of year to look for Arcturus is late summer. On the star chart, it is located behind the church in the constellation Bootes.
  2. Follow the two end stars of the bowl ("pointers'' in figure 1, a and b in figure 2) out of the bowl and up the sky. After you've gone another five times the distance between these two stars, you'll come to a medium-bright star called Polaris, the North Star.
  3. Look at the second star from the end of the handle of the Big Dipper. Can you see anything unusual about it?
  4. Look at the seven bright stars of the Big Dipper. Is any one star noticeably brighter or fainter than the others?
  5. Look at the seven bright stars of the Big Dipper. Is any one star noticeably different in color from the others?

You may have noticed that the second star in the Dipper handle is actually two stars close together. Their closeness is no accident: They are actually moving together through space, at the same distance from us. How do astronomers know this? They measured the distance to these stars using a clever technique called parallax.

Activity 3. Parallax

Hold your index finger upright at arm's length. Close your left eye, and look at your finger with your right eye. Notice its position with respect to the background. Now close your right eye and look at your finger with your left eye. What seems to happen to the position of your finger? Quickly alternate between looking at your finger with your right and left eyes, and you will see the effect of parallax: the apparent change in your finger's position. It happens because you're looking at your finger from slightly different perspectives (see figure 3). Now move your finger halfway to your nose and repeat the process. How does the parallax effect change? [It gets larger.]

Figure 3
Parallax. This method measures distance by noticing how much an object seems to move when you look at it from different perspectives. You can use the technique to judge the distance to your finger (top) or to a star (bottom). Figure by John Percy.

Stars, like fingers, seem to move when you look at them from different positions. In the case of stars, our vantage point changes as Earth orbits the Sun (see figure 3). When astronomers observe a star in January and again in June, they see it from opposite sides of the Sun. The star appears to change in position relative to the more distant background stars. The more distant the star, the smaller the parallax. Using simple geometry, astronomers translate parallax into distance: Distance is inversely proportional to parallax. This is how they know the stars are enormously far away.

Astronomers measure the distances of stars in light-years. A light- year is the distance that light travels in a year, about 10,000,000,000,000 kilometers (6,000,000,000,000 miles). The nearest star is about 4 light-years away -- or, if you like lots of zeroes, 40,000,000,000,000 kilometers (25,000,000,000,000 miles) away. At 55 miles per hour, it would take you over 10 million years to drive there, not counting stops for gas.

By using this technique of parallax, astronomers discovered that the stars -- even those in the same constellation -- are generally at vastly different distances. Two stars that look like neighbors are usually light-years apart. In some cases, though, nearby stars really are close to each other. The Big Dipper is an example. The middle five stars in the Big Dipper are at the same distance, about 65 light-years. They, along with some of the other stars in the Ursa Major region, form a cluster of stars moving together through space. Two other examples are the Hyades and the Pleiades.

Activity 4. Finding the Hyades and Pleiades

Face east. Hold your star map so that EAST is down. Notice the two tight groups of stars low in the east. If they're tough to spot, wait an hour or two until they've risen higher in the sky. The Hyades is grouped around the bright star Aldebaran, an orange star that is not part of the cluster but instead is at a different distance.

There are thousands of star clusters in our galaxy. Some are young (a few million years), others are old (a billion years or more). They are all about the same size, a few light-years in diameter. Their apparent size depends on their actual size and their distances. The same cluster, 10 times further away, will appear 10 times smaller. To measure the size, astronomers determine the angle between the direction to the bottom of the object and the direction to the top. This angle is called the angular diameter.

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