High School Earth Science/Stars

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. Some of these stars are smaller than our Sun, and some are larger. Except for our own Sun, all stars are so far away that they only look like single points, even through a telescope.

Lesson Objectives

Define constellations.
Describe the flow of energy in a star.
Classify stars based on their properties.
Outline the life cycle of a star.
Use light-years as a unit of distance.

Constellations

Figure 26.1: The constellation Orion is a familiar pattern of stars in the sky.

For centuries, people have seen the same stars you can see in the night sky. People of many different cultures have identified constellations, which are apparent patterns of stars in the sky. Figure 26.1 shows one of the most easily recognized constellations. The ancient Greeks thought this group of stars looked like a hunter from one of their myths, so they named it Orion after him. The line of three stars at the center of the picture is "Orion's Belt".

The patterns in constellations and in groups or clusters of stars, called asterisms, stay the same night after night. However, in a single night, the stars move across the sky, keeping the same patterns. This apparent nightly motion of the stars is actually due to the rotation of Earth on its axis. It isn't the stars that are moving; it is actually Earth spinning that makes the stars seem to move. The patterns shift slightly with the seasons, too, as Earth revolves around the Sun. As a result, you can see different constellations in the winter than in the summer. For example, Orion is a prominent constellation in the winter sky, but not in the summer sky.

Apparent Versus Real Distances

Although the stars in a constellation appear close together as we see them in our night sky, they are usually at very different distances from us, and therefore they are not at all close together out in space. For example, 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. A light-year is the distance that light can travel in one year; it is a large unit of distance used to measure the distance between objects in space.

Energy of Stars

Only a small portion of the light from the Sun reaches Earth; yet that light is enough to keep the entire planet warm and to provide energy for all the living things on Earth. The Sun is a fairly average star. The reason the Sun appears so much bigger and brighter than any of the other stars is that it is very close to us. Some other stars produce much more energy than the Sun. How do stars generate so much energy?

Nuclear Fusion

Stars are made mostly of hydrogen and helium. These are both very lightweight gases. However, there is so much hydrogen and helium in a star that the weight of these gases is enormous. In the center of a star, the pressure is great enough to heat the gases and cause nuclear fusion reactions. In a nuclear fusion reaction, the nuclei, or centers of two atoms join together and create a new atom from two original atoms. In the core of a star, the most common reaction turns two hydrogen atoms into a helium atom. Nuclear fusion reactions require a lot of energy to get started, but once they are started, they produce even more energy.

The energy from nuclear reactions in the core pushes outward, balancing the inward pull of gravity on all the gas in the star. This energy slowly moves outward through the layers of the star until it finally reaches the outer surface of the star. The outer layer of the star glows brightly, sending the energy out into space as electromagnetic radiation, including visible light, heat, ultraviolet light, and radio waves.

Scientists have built machines called accelerators that can propel subatomic particles until they have attained almost the same amount of energy as found in the core of a star. When these particles collide with each other head-on, new particles are created. This process simulates the nuclear fusion that takes place in the cores of stars. It also simulates the conditions that allowed for the first Helium atom to be produced from the collision of two hydrogen atoms when the Universe was only a few minutes old. Two well-known accelerators are SLAC in California, USA and CERN in Switzerland.

How Stars Are Classified

Stars come in many different colors. If you look at the stars in Orion (as shown in Figure 26.1), you will notice that there is a bright, red star in the upper left and a bright, and a blue star in the lower right. The red star is named Betelgeuse (pronounced BET-ul-juice), and the blue star is named Rigel.

Color and Temperature

If you watch a piece of metal, such as a coil of an electric stove as it heats up, you can see how color is related to temperature. When you first turn on the heat, the coil looks black, but you can feel the heat with your hand held several inches from the coil. As the coil gets hotter, it starts to glow a dull red. As it gets hotter still, it becomes a brighter red, then orange. If it gets extremely hot, it might look yellow-white, or even blue-white. Like a coil on a stove, a star's color is 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.

The most common way of classifying stars is by color. Table 26.1 shows how this classification system works. 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.

Table 26.1: 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. Figure 26.2 shows a typical star of each class, with the colors about the same as you would see in the sky.

Figure 26.2: Typical stars by class, color, and size. For most stars, size is related to class and to color. This image shows a typical star of each class. The colors are approximately the same as you would see in the sky.

Lifetime of Stars

As a way of describing the stages in a star's development, we could say that stars are born, grow, change over time, and eventually die. Most stars change in size, color, and class at least once during this journey.

Formation of Stars

Stars are born in clouds of gas and dust called nebulas, like the one shown in Figure 26.3. In Figure 26.1, the fuzzy area beneath the central three stars across the constellation Orion, often called Orion's sword, contains another nebula called the Orion nebula.

Figure 26.3: The Eagle Nebula and the Pillars of Creation. The pillars of gas and dust shown here are in the Eagle Nebula.

The Main Sequence

For most of a star's life, the nuclear fusion in the core combines hydrogen atoms to form helium atoms. A star in this stage is said to be a main sequence star, or to be on the main sequence. This term comes from the Hertzsprung-Russell diagram, that plots a star's surface temperature against its true brightness or magnitude. For stars on the main sequence, the hotter they are, the brighter they are. The length of time a star is on the main sequence depends on how long a star is able to balance the inward force of gravity with the outward force provided by the nuclear fusion going on in its core. More massive stars have higher pressure in the core, so they have to burn more of their hydrogen "fuel" to prevent gravitational collapse. Because of this, more massive stars have higher temperatures, and also run out of hydrogen sooner than smaller stars do.

Our Sun, which is a medium-sized star, has been a main sequence star for about 5 billion years. It will continue to shine without changing for about 5 billion more years. Very large stars may be on the main sequence for "only" 10 million years or so. Very small stars may be main sequence stars for tens to hundreds of billions of years.

Red Giants and White Dwarfs

As a star begins to use up its hydrogen, it then begins to fuse helium atoms together into heavier atoms like carbon. Eventually, stars contain fewer light elements to fuse. The star can no longer hold up against gravity and it starts to collapse inward. Meanwhile, the outer layers spread out and cool. The star becomes larger, but cooler on the surface and red in color. Stars in this stage are called red giants.

Eventually, a red giant burns up all of the helium in its core. What happens next depends on how massive the star is. A typical star like the Sun, stops fusion completely at this point. Gravitational collapse shrinks the star’s core to a white, glowing object about the size of Earth. A star at this point is called a white dwarf. Eventually, a white dwarf cools down and its light fades out.

Supergiants and Supernovas

A star that has much more mass than the Sun will end its life in a more dramatic way. When very massive stars leave the main sequence, they become red supergiants. The red star Betelgeuse in Orion is a red supergiant.

Unlike red giants, when all the helium in a red supergiant is gone, fusion does not stop. The star continues fusing atoms into heavier atoms, until eventually its nuclear fusion reactions produce iron atoms. Producing elements heavier than iron through fusion takes more energy than it produces. Therefore, stars will ordinarily not form any elements heavier than iron. When a star exhausts the elements that it is fusing together, the core succumbs to gravity and collapses violently, creating a violent explosion called a supernova. A supernova explosion contains so much energy that some of this energy can actually fuse heavy atoms together, producing heavier elements such as gold, silver, and uranium. A supernova can shine as brightly as an entire galaxy for a short time, as shown in Figure 26.4.

Figure 26.4: When very large stars stop nuclear fusion, they explode as supernovas. A bright supernova, like the one in the bottom left of the figure, can shine as brightly as an entire galaxy for a short time.

Neutron Stars and Black Holes

After a large star explodes in a supernova, the leftover material in the core is extremely dense. If the core is less than about four times the mass of the Sun, the star will be a neutron star, as shown in Figure 26.5. A neutron star is made almost entirely of neutrons. Even though it is more massive than the sun, it is only a few kilometers in diameter.

Figure 26.5: After a supernova, the remaining core may end up as a neutron star.

If the core remaining after a supernova is more than about 5 times the mass of the Sun, the core will collapse so far that it becomes a black hole. Black holes are so dense that not even light can escape their gravity. For that reason, black holes cannot be observed directly. But we can identify a black hole by the effect that it has on objects around it, and by radiation that leaks out around its edges.

Measuring Star Distances

The Sun is much closer to Earth than any other star. Light from the Sun takes about 8 minutes to reach Earth. Light from the next nearest star, Proxima Centauri, takes more than 4 years to reach Earth. Traveling to Proxima Centauri in spacecraft similar to those we have today would take tens of thousands of years.

Light-years

Because astronomical distances are so large, it helps to use units of distance that are large as well. A light-year is defined the distance that light travels in one year. One light-year is 9,500,000,000,000 (9.5 trillion) kilometers, or 5,900,000,000,000 (5.9 trillion) miles. Proxima Centauri is 4.22 light-years away, which means that its light takes 4.22 years to reach us.

One light-year is approximately equal to 60,000 AU and 4.22 light-years is almost 267,000 AU. Recalling that Neptune, the farthest planet from the Sun, orbits roughly 30 AU from the Sun, we can realize that the distance from the Earth to stars other than our own Sun is much greater than the distance from the Earth to other planets within our own solar system.

Parallax

Figure 26.6: Parallax is used to measure the distance to stars that are relatively nearby.

So how do astronomers measure the distance to stars? 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 or 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 due to parallax.

As Figure 26.6 shows, astronomers use this same principle to measure the distance to stars. However, instead of a finger, they focus on a star. And instead of switching back and forth between eyes, they use the biggest possible difference in observing position. To do that, they first look at the star from one position, and they note where the star appears to be relative to more distant stars. Then, they wait 6 months; during this time, Earth moves from one side of its orbit around the Sun to the other side. When they look at the star again, parallax will cause the star to appear in a different position relative to more distant stars. From the size of this shift, they can calculate the distance to the star.

Other Methods

For stars that are more than a few hundred light years away, parallax is too small to measure, even with the most precise instruments available. For these more distant stars, astronomers use more indirect methods of determining distance. Most of these other 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. Then, they can compare the observed brightness to the expected brightness. This is like asking, "How far away would the Sun have to be to appear this dim?"

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. The most common system uses the letters O (blue), B (bluish white), A (white), F (yellowish white), G (yellow), K (orange), and M (red), from hottest to coolest.
Stars form from clouds of gas and dust called nebulas. Stars collapse until nuclear fusion starts in the core.
Stars spend most of their lives on the main sequence, fusing hydrogen into helium.
Typical, Sun-like stars expand into red giants, then fade out as white dwarfs.
Very large stars expand into red supergiants, explode in supernovas, then end up as neutron stars or black holes.
Astronomical distances can be measured in light-years. A light year is the distance that light travels in one year. 1 light-year = 9.5 trillion kilometers (5.9 trillion miles).
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

What distinguishes a nebula and a star?
What kind of reactions provide a star with energy?
Which has a higher surface temperature: a blue star or a red star?
List the seven main classes of stars, from hottest to coolest.
What is the primary reaction that occurs in the core of a star, when the star is on the main sequence?
What kind of star will the Sun be after it leaves the main sequence?
Suppose a large star explodes in a supernova, leaving a core that is 10 times the mass of the Sun. What would happen to the core of the star?
What is the definition of a light-year?
Why don't astronomers use parallax to measure the distance to stars that are very far away?

Vocabulary

asterism: A group or cluster of stars that appear close together in the sky.
black hole: The super dense core left after a supergiant explodes as a supernova.
constellation: An apparent pattern of stars in the night sky.
light-year: The distance that light travels in one year; 9.5 trillion kilometers.
main sequence star: A star that is fusing hydrogen atoms to helium; a star in the main portion of its "life".
nebula: An interstellar cloud of gas and dust.
neutron star: The remnant of a massive star after it explodes as a supernova.
nuclear fusion reaction: When nuclei of two atoms fuse together, giving off tremendous amounts of energy.
parallax: A method used by astronomers to calculate the distance to nearby stars, using the apparent shift relative to distant stars.
red giant: Stage in a star's development when the inner helium core contracts while the outer layers of hydrogen expand.
supernova: A tremendous explosion that occurs when the core of a star is mostly iron.
star: A glowing sphere of gases that produces light through nuclear fusion reactions.

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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