20.2: Stars

Lesson Objectives

• Define constellation.
• Describe the flow of energy in a star.
• Classify stars based on their properties.

Vocabulary

• asterism
• nuclear fusion reaction
• parallax
• star

Introduction

When you look at the sky on a clear night, you can see dozens, perhaps even hundreds, of tiny points of light. Almost every one of these points of light is a star, a giant ball of glowing gas at a very, very high temperature. Stars differ in size, temperature, and age, but they all appear to be made up of the same elements and to behave according to the same principles

Constellations

People of many different cultures, including the Greeks, identified patterns of stars in the sky. We call these patterns constellations. Figure below shows one of the most easily recognized constellations.

The ancient Greeks thought this group of stars looked like a hunter, so they named it Orion after their mythical hunter. The line of three stars at the center is “Orion’s Belt”.

Why do the patterns in constellations and in groups or clusters of stars, called asterisms, stay the same night after night? Although the stars move across the sky, they stay in the same patterns. This is because the apparent nightly motion of the stars is actually caused by the rotation of Earth on its axis. The patterns also shift in the sky with the seasons as Earth revolves around the Sun. As a result, people in a particular location can see different constellations in the winter than in the summer. For example, in the Northern Hemisphere Orion is a prominent constellation in the winter sky, but not in the summer sky. This is the annual traverse of the constellations.

Apparent Versus Real Distances

Although the stars in a constellation appear close together as we see them in our night sky, they are not at all close together out in space. In the constellation Orion, the stars visible to the naked eye are at distances ranging from just 26 light-years (which is relatively close to Earth) to several thousand light-years away.

How Stars Are Classified

The many different colors of stars reflect the star’s temperature. In Orion (as shown in the Figure above) the bright, red star in the upper left named Betelgeuse (pronounced BET-ul-juice) is not as hot than the blue star in the lower right named Rigel.

Color and Temperature

Think about how the color of a piece of metal changes with temperature. A coil of an electric stove will start out black but with added heat will start to glow a dull red. With more heat the coil turns a brighter red, then orange. At extremely high temperatures the coil will turn yellow-white, or even blue-white (it’s hard to imagine a stove coil getting that hot). A star’s color is also determined by the temperature of the star’s surface. Relatively cool stars are red, warmer stars are orange or yellow, and extremely hot stars are blue or blue-white (Figure below).

A Hertzsprung-Russell diagram shows the brightness and color of main sequence stars. The brightness is indicated by luminosity and is higher up the y-axis. The temperature is given in degrees Kelvin and is higher on the left side of the x-axis. How does our Sun fare in terms of brightness and color compared with other stars?

Classifying Stars by Color

Color is the most common way to classify stars. Table below shows the classification system. The class of a star is given by a letter. Each letter corresponds to a color, and also to a range of temperatures. Note that these letters don’t match the color names; they are left over from an older system that is no longer used.

Classification of Stars By Color and Temperature

Class	Color	Temperature Range	Sample Star
O	Blue	30,000 K or more	Zeta Ophiuchi
B	Blue-white	10,000–30,000 K	Rigel
A	White	7,500–10,000 K	Altair
F	Yellowish-white	6,000–7,500 K	Procyon A
G	Yellow	5,500–6,000 K	Sun
K	Orange	3,500–5,000 K	Epsilon Indi
M	Red	2,000–3,500 K	Betelgeuse, Proxima Centauri

For most stars, surface temperature is also related to size. Bigger stars produce more energy, so their surfaces are hotter. These stars tend toward bluish white. Smaller stars produce less energy. Their surfaces are less hot and so they tend to be yellowish.

Measuring Star Distances

How can you measure the distance of an object that is too far away to measure? Now what if you don’t know the size of the object or the size or distance of any other objects like it? That would be very difficult, but that is the problem facing astronomers when they try to measure the distances to stars.

Parallax

Distances to stars that are relatively close to us can be measured using parallax. Parallax is an apparent shift in position that takes place when the position of the observer changes.

To see an example of parallax, try holding your finger about 1 foot (30 cm) in front of your eyes. Now, while focusing on your finger, close one eye and then the other. Alternate back and forth between eyes, and pay attention to how your finger appears to move. The shift in position of your finger is an example of parallax. Now try moving your finger closer to your eyes, and repeat the experiment. Do you notice any difference? The closer your finger is to your eyes, the greater the position changes because of parallax.

As Figure below shows, astronomers use this same principle to measure the distance to stars. Instead of a finger, they focus on a star, and instead of switching back and forth between eyes, they switch between the biggest possible differences in observing position. To do this, an astronomer first looks at the star from one position and notes where the star is relative to more distant stars. Now where will the astronomer go to make an observation the greatest possible distance from the first observation? In six months, after Earth moves from one side of its orbit around the Sun to the other side, the astronomer looks at the star again. This time parallax causes the star to appear in a different position relative to more distant stars. From the size of this shift, astronomers can calculate the distance to the star.

For more about parallax, visit http://starchild.gsfc.nasa.gov/docs/StarChild/questions/parallax.html.

Parallax is used to measure the distance to stars that are relatively nearby.

A parallax exercise is seen here: http://www.astro.ubc.ca/~scharein/a311/Sim/new-parallax/Parallax.html.

Other Methods

Even with the most precise instruments available, parallax is too small to measure the distance to stars that are more than a few hundred light years away. For these more distant stars, astronomers must use more indirect methods of determining distance. Most of these methods involve determining how bright the star they are looking at really is. For example, if the star has properties similar to the Sun, then it should be about as bright as the Sun. The astronomer compares the observed brightness to the expected brightness.

Lesson Summary

• Constellations and asterisms are apparent patterns of stars in the sky.
• Stars in the same constellation are often not close to each other in space.
• A star generates energy by nuclear fusion reactions in its core.
• The color of a star is determined by its surface temperature.
• Stars are classified by color and temperature: O (blue), B (bluish white), A (white), F (yellowish white), G (yellow), K (orange), and M (red), from hottest to coolest.
• Parallax is an apparent shift in an object’s position when the position of the observer changes. Astronomers use parallax to measure the distance to relatively nearby stars.

Review Questions

1. What distinguishes a nebula and a star?

2. What kind of reactions provide a star with energy?

3. Stars are extremely massive. Why don’t they collapse under the weight of their own gravity?

4. Of what importance are particle accelerators to scientists?

5. Which has a higher surface temperature: a blue star or a red star?

6. What is a light year?

7. Why don’t astronomers use parallax to measure the distance to stars that are very far away?

Further Reading / Supplemental Links

• Myths and history of constellations: http://www.ianridpath.com/startales/contents.htm
• NASA World Book, Stars: http://www.nasa.gov/worldbook/star_worldbook.html
• NASA, parts of a star: http://imagine.gsfc.nasa.gov/docs/science/know_l1/stars.html

Points to Consider

• Although stars may appear to be close together in constellations, they are usually not close together out in space. Can you think of any groups of astronomical objects that are relatively close together in space?
• Most nebulas contain more mass than a single star. If a large nebula collapsed into several different stars, what would the result be like?

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