Stars change over their life cycles.

1. Stars change over their life cycles.

2. We classify stars by their characteristics. • All stars are huge balls of glowing gas that produce or have produced energy by fusion, just like our Sun.

3. We classify stars by their characteristics. To describe distance between stars, astronomers use a method of measurement called a light year. A light-year is the distance light travels in one year. Light travels about 6 trillion miles in one year (~6,000,000,000,000).

4. We classify stars by their characteristics. • Besides the Sun, the closest star to Earth is 4 light years away (24,000,000,000,000 miles).

5. Brightness • The amount of light a star gives off and its distance from Earth determines how bright it appears to an observer. • To determine true brightness, astronomers must know its distance from Earth.

6. Distance • One method of measurement is parallax. • Parallax is the apparent shift in the position of an object when viewed from different locations. Stars appear to shift when astronomers view them from different locations. • To measure parallax, astronomers plot the star’s position in the sky from opposite sides of Earth’s orbit around the Sun. Then they use the apparent shift in position and the diameter of Earth’s orbit to calculate the star’s distance.

7. Size • It is hard to tell how large stars are by just looking at them in the sky. The Sun, our closest star, is much bigger than it appears in our sky. A jet plane that can fly once around the equator of Earth in 2 days would take 7 months to fly around the equator of the Sun. • Giant and supergiant stars are much larger than the Sun. Betelgeuse is 600 times the diameter of the Sun. This is one of the brightest stars in the sky even though it is 522 light years away. • White dwarfs are much smaller than the Sun. They are about 100 times smaller than the Sun (about the size of Earth). These can only be seen with a telescope.

8. Color and Temperature

9. Color and Temperature • The differences in color are due to differences in temperature. • Stars are classified by color and surface temperature. The cooler stars are red while the hottest stars are blue-white. Our Sun is yellow and is about 6000 degrees C. • Astronomers can spread a star’s light into a spectrum to learn about the star’s composition. By studying the spectrum, astronomers can learn what gases are present in the star.

10. Stars have life cycles. • The life cycle of a star varies depending on the mass of the star. • Nebula – a cloud of dust and gas where stars form • Gravity begins pulling on gas and dust which forms a hot, dense sphere. If the sphere is hot enough for fusion, a star will form in the center. • When a star dies, its matter does not disappear. The matter can either form a new nebula or move to an existing one.

11. Main Sequence – the stage in which stars produce energy through the fusion of hydrogen into helium • LOWER MASS STARS • Use fuel slowly which means it can remain in main sequence stage for billions of years. • Our Sun has been in the main sequence stage for 4.6 billion years and will continue to be for 5 billion years. • When it runs out of hydrogen, it expands into a giant star. The helium fuses into carbon. It will shed all of its layers and become a white dwarf. • A white dwarf is the dead core of a giant star. No fusion is occurring but it can remain hot for billions of years.

12. HIGHER MASS STARS • Use fuel more rapidly • Will expand to become a supergiant star where the core will produce heavier and heavier elements through fusion. Eventually an iron core will develop causing fusion to stop and gravity causes the core to collapse. The star will erupt in an explosion called a supernova. • A supernova can give off as much light as a galaxy. • The matter that shoots off can form a new star or planet.

13. Neutron Stars and Black Holes • Neutron Star – the collapsed core of a supergiant star may form an extremely dense body. • Measure about 12 miles in diameter but have a lot of mass. • Do not radiate much visible light but a lot of x-rays. • Pulsars – neutron stars that emit radio waves as they spin. • Black hole – when a supernova leaves behind a core with a mass more than three times that of the Sun that collapses forming an invisible object. • Very strong radiation that does not allow anything to escape.

14. Star Systems • Many stars are close to each other and held together by the force of gravity between them. • Binary star system: 2 stars orbiting each other • Multiple star system: more than 2 stars • Some stars are too close to each other to be seen as individual stars. Astronomers can determine this by studying the star’s spectrum.