Chapter 8, Astronomy Lesson 4, Stars

Objectives Define some of the properties of stars. Compare the evolutionary paths of star types.

Main Idea Stars vary in their size, their brightness, and their distance from Earth.

Vocabulary star constellation parallax light-year nebula supernova black hole

What are stars? A star is a large, hot ball of gases, held together by gravity, that gives off its own light. A constellation is a group of stars that appear to form a pattern.

As earth revolves, different constellations can be seen, like Orion, which is a winter constellation in the Northern Hemisphere. Constellations are classified by the seasons they appear in.

Finding the Big Dipper in Ursa Major, the Great Bear, can help you find Polaris, the North Star. If you are unsure of directions, the North Star can help you. Because of our perspective, the stars in the sky form pictures, as we look at them from Earth. If we looked at our stars far away from our solar system, the pictures would not look the same.

Finding the Distance to a Star Viewed from different points in Earth’s orbit, some stars seem to change position slightly compared to stars farther away. The apparent shift in an objects position when viewed from two locations is called parallax. Astronomers use parallax to find the distance of a star from Earth.

When astronomers measure the distance from Earth to a star, even a kilometer far is too small. They use a measurement of distance called a light year, the distance that light travels in one year. One light year is more than 9 trillion kilometers (6 trillion miles). The nearest star, Alpha Centauri, is about 4.3 light years away from Earth. The light we see when we look at Alpha Centauri left that star about 4.3 years ago.

Quick Check Compare and Contrast How are the constellations alike? How are they different? The constellations all suggest patterns and consists of clusters of stars. The constellations have different numbers of stars and are positioned at varying distances from Earth.

Critical Thinking It takes sunlight about 8 minutes to reach Earth Critical Thinking It takes sunlight about 8 minutes to reach Earth. Is the Sun more than or less than a light year away? Explain. The Sun is far less than a light year away. The reason is that a light year is the distance that light travels in a year. Light from the Sun takes 8 minutes to reach Earth, which is much less than a year.

What are some properties of stars? Some stars are brighter than others. Stars appear less bright the farther they are from Earth. Sirius seems brighter than Rigel. However, Rigel is brighter than Sirius.

Color is one of a star’s properties, it tells you the stars temperature. Red and orange colors indicate cooler stars. Yellow indicates hotter stars. Blue – white indicates the hottest stars.

Stars come in different sizes. Red supergiants are the biggest. Our sun is average. A white dwarf is the smallest.

Brightness and Temperature Astronomers, Ejnar Hertzsprung and Henry Norris Russell looked for similarities between a star’s brightness and temperature. They found that stars appear in groups.

To read an H-R diagram start near the lower left corner. The stars near here are very dim but very hot.

In the lower right corner are dim, very cool stars.

Diagonally up and to the left from the lower right corner is a long band. Most stars here are main–sequence stars.

On the top of the scale of absolute magnitude are supergiants. Giants are dimmer than supergiants. The giants are just below supergiants in the chart.

Quick Check Compare and Contrast How are absolute magnitude and apparent magnitude similar? Both types of magnitude are a measurement of brightness. Absolute magnitude describes the actual brightness of a star, not just how it appears from Earth. Apparent magnitude describes how a star looks to us on Earth.

Critical Thinking What are three properties that all stars have and how do they relate to one another? All stars have magnitude, temperature, and size. Size and temperature both affect absolute magnitude.

How do stars develop? Stars are born, they mature, grow older, and finally they die. The main factor that affects how a star goes through this cycle of development is the star’s mass.

Protostars Every star begins as a nebula, a huge cloud of gas and dust in space. As a cloud shrinks, it heats up and becomes a protostar or a young star. Protostars continue to gain mass because of its gravitational pull.

Main-Sequence Stars Eventually the center of a protostar reaches a temperature of millions of degrees Celsius and a nuclear reaction starts. Hydrogen atoms fuse and forms helium atoms releasing large amounts of energy. Stars spend most of their time as main- sequence stars.

Red Giants and Supergiants As the star expands, its surface becomes cooler and its color becomes redder. The star becomes a red giant or a supergiant, depending on its mass.

Final Stages The final stages of a star’s life depends on its mass. Stars up to ten times the mass of the Sun become red giants. Nuclear reactions give off energy causing the star to release huge clouds of gas. A layer of gases, called a planetary nebula, forms around the star. Its surface becomes white-hot, forming a white dwarf.

A white dwarf is so hot that it gives off enough radiation to make its surrounding shell of gas and dust glow. When the shell glows brightly enough, we see it as a planetary nebula. A white dwarf may take billions of years to cool off. When it cools off, it will become a black dwarf.

Stars with masses greater than ten times the mass of the Sun follow a different path. These large stars become supergiants. They use up energy at a fantastic rate, giving off very large amounts of energy. In a relatively short amount of time, the star can no longer fuse atoms and give off energy at the same rate.

When a supergiant can no longer produce enough energy, it collapses and then explodes. This is called a supernova.

The next stage depends on the star’s mass. What remains of a supernova becomes a neutron star, made of neutrons. As they rotate they blink like a lighthouse beacon. This is a pulsar.

If a star is very massive, the supernova does not become a neutron star. Instead, the core collapses and it becomes a tiny but very massive object called a black hole.

Black holes are detected by the effect they have on other objects. Often, gasses form a nearby companion that is pulled in by the intense gravity of the black hole. When the gasses approach they emit x- rays.

Quick Check Compare and Contrast Compare the development of a less-massive star with that of a more-massive star. A less-massive star: begins as a nebula, becomes a protostar, a main-sequence star, a red giant, and finally, a white dwarf. A more-massive star: begins as a nebula, becomes a protostar, a main-sequence star, a very massive star, a supergiant, a supernova, and finally, either a neutron star (pulsar) or a black hole.

Critical Thinking Why will the Sun not become a black hole someday? The Sun does not have enough mass to become a black hole.

What kind of star is the Sun? The Sun is a main-sequence star in the middle of the H-R diagram. The Sun has been shining for 5 billion years and will continue for another 5 billion, until it becomes a red giant.

The Sun contains 99.9% of the solar system’s mass. It is 92% hydrogen. Its hydrogen is being changed into helium by nuclear reactions.

You should never look at the Sun directly because its brightness can harm your eyes.

Quick Check Compare and Contrast How is the Sun like other stars? Like other stars the Sun is very hot and gaseous. Nuclear reactions occur in its core and large amounts of energy are released.

Critical Thinking Do you think the Sun is less massive or more massive than other stars? Why? Because of its position on the main sequence, it is probably average. The Sun is probably neither less massive nor more massive and is most likely an average-size star.