Lesson Objectives

  • Define constellation.
  • 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.


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 principle.


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.

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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.

Star Power

The Sun is Earth’s major source of energy, yet the planet only receives a small portion of its energy and the Sun is just an ordinary star. Many stars produce much more energy than the Sun. The energy source for all stars is nuclear fusion.

Nuclear Fusion

Stars are made mostly of hydrogen and helium, which are packed so densely in a star that in the star’s center the pressure is great enough to initiate nuclear fusion reactions. In a nuclear fusion reaction, the nuclei of two atoms combine to create a new atom. Most commonly, in the core of a star, two hydrogen atoms fuse to become a helium atom. Although nuclear fusion reactions require a lot of energy to get started, once they are going they produce enormous amounts of energy (Figure below).

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A thermonuclear bomb is an uncontrolled fusion reaction in which enormous amounts of energy are released.

In a star, the energy from fusion reactions in the core pushes outward to balance the inward pull of gravity. This energy moves outward through the layers of the star until it finally reaches the star’s outer surface. 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 (Figure below).

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A diagram of a star like the Sun.

In particle accelerators, subatomic particles are propelled until they have attained almost the same amount of energy as found in the core of a star (Figure below). When these particles collide head-on, new particles are created. This process simulates the nuclear fusion that takes place in the cores of stars. The process also simulates the conditions that allowed for the first helium atom to be produced from the collision of two hydrogen atoms in the first few minutes of the universe.

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The SLAC National Accelerator Lab in California can propel particles a straight 2 mi (3.2 km).

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The CERN Particle Accelerator presented in this video is the world's largest and most powerful particle accelerator and can boost subatomic particles to energy levels that simulate conditions in the stars and in the early history of the universe before stars formed (2e): (6:16).

How Stars Are Classified

The many different colors of stars reflect the star’s temperature. In Orion (as shown 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).

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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.

For most stars, surface temperature is also related to size. Bigger stars produce more energy, so their surfaces are hotter. Figure below shows a typical star of each class, with the colors about the same as you would see in the sky.

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Typical stars by class, color, and size. For most stars, size is related to class and to color. The colors are approximately as they appear in the sky.

Lifetime of Stars

Stars have a life cycle that is expressed similarly to the life cycle of a living creature: they are born, grow, change over time, and eventually die. Most stars change in size, color, and class at least once in their lifetime. What astronomers know about the life cycles of stars is because of data gathered from visual, radio, and X-ray telescopes.

Star Formation

As discussed in the Solar System chapter, stars are born in clouds of gas and dust called nebulas, like the one shown in Figure below.
For more on star formation, check out and

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The Pillars of Creation within the Eagle Nebula are where gas and dust come together as a stellar nursery.

The Main Sequence

For most of a star’s life, nuclear fusion in the core produces helium from hydrogen. A star in this stage is a main sequence star. This term comes from the Hertzsprung-Russell diagram shown above. For stars on the main sequence, temperature is directly related to brightness. A star is on the main sequence as long as it is able to balance the inward force of gravity with the outward force of nuclear fusion in its core. The more massive a star, the more it must burn hydrogen fuel to prevent gravitational collapse. Because they burn more fuel, more massive stars have higher temperatures. Massive stars also run out of hydrogen sooner than smaller stars do.

Our Sun has been a main sequence star for about 5 billion years and will continue on the main sequence for about 5 billion more years (Figure below). Very large stars may be on the main sequence for only 10 million years. Very small stars may last tens to hundreds of billions of years.

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Our Sun is a medium-sized star in about the middle of its main sequence life.

The fate of the Sun and inner planets is explored in this video:

Red Giants and White Dwarfs

As a star begins to use up its hydrogen, it fuses helium atoms together into heavier atoms such as carbon. A blue giant star has exhausted its hydrogen fuel and is a transitional phase. When the light elements are mostly used up the star can no longer resist gravity and it starts to collapse inward. The outer layers of the star grow outward and cool. The larger, cooler star turns red in color and so is called a red giant.

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, such as the Sun, stops fusion completely. Gravitational collapse shrinks the star's core to a white, glowing object about the size of Earth, called a white dwarf (Figure below). A white dwarf will ultimately fade out.

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Sirius, the brightest star in the sky, is actually a binary star system. Sirius A is on the main sequence. Sirius B, the tiny dot on the lower left, is a white dwarf.

Supergiants and Supernovas

A star that runs out of helium will end its life much more dramatically. When very massive stars leave the main sequence, they become red supergiants (Figure below).

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The red star Betelgeuse in Orion is a red supergiant.

Unlike a red giant, when all the helium in a red supergiant is gone, fusion continues. Lighter atoms fuse into heavier atoms up to iron atoms. Creating elements heavier than iron through fusion uses more energy than it produces so stars do not ordinarily form any heavier elements. When there are no more elements for the star to fuse, the core succumbs to gravity and collapses, creating a violent explosion called a supernova (Figure ). A supernova explosion contains so much energy that atoms can fuse together to produce heavier elements such as gold, silver, and uranium. A supernova can shine as brightly as an entire galaxy for a short time (Figure below). All elements with an atomic number greater than that of lithium were formed by nuclear fusion in stars.

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This false-color image of the supernova remnant SN 1604 was observed as a supernova in the Milky Way galaxy. At its peak it was brighter than all other stars and planets, except Venus, in the night sky.

An animation of the Crab Supernova is seen here:
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This video looks at the origin of the universe, star formation, and the formation of the chemical elements in supernovas (2c): (8:30).

Neutron Stars and Black Holes

After a supernova explosion, 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 becomes a neutron star (Figure below). A neutron star is made almost entirely of neutrons, relatively large particles that have no electrical charge.

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After a supernova, the remaining core may end up as a neutron star. A neutron star is more massive than the Sun, but only a few kilometers in diameter.
If the core remaining after a supernova is more than about five times the mass of the Sun, the core collapses into a black hole. Black holes are so dense that not even light can escape their gravity. With no light, a black hole cannot be observed directly. But a black hole can be identified by the effect that it has on objects around it, and by radiation that leaks out around its edges.

How to make a black hole:
A video about black holes is seen on

A Star's Life Cycle video from Discovery Channel describes how stars are born, age and die (2f): (3:11).
A video of neutron stars is available at: (4:24).

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.


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

A parallax exercise is seen here:

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.
  • Stars form from nebulas. Gravity causes stars to collapse until nuclear fusion begins.
  • 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, and end up as neutron stars or black holes.
  • 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. Which has a higher surface temperature: a blue star or a red star?
4. List the seven main classes of stars, from hottest to coolest.
5. What is the primary reaction that occurs in the core of a main sequence star?
6. What kind of star will the Sun be after it leaves the main sequence?
7. 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?
8. Explain how parallax can be used to measure the distances to stars.
9. Why don’t astronomers use parallax to measure the distance to stars that are very far away?

Further Reading / Supplemental Links


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

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?

Other Resources

Khan Academy: Scale of Distance to Closest Stars