The Life Cycle of Stars

1. The Life Cycle of Stars Pgs. 105 - 109

2. Stars Age • Just like people, stars are born, grow old and can die. • Stars can last much longer than people though. • Stars can live for billions of years. • As they age, they lose material. • This can be gradual or a big explosion. • When material is lost, it is returned to space, where it came from.

3. The Diagram that Did it • In 1911, Danish astronomer EjnarHertzsrung compared the temperature and brightness of stars on a graph. • Later, American astronomer Henry Norris Russell made some similar graphs. • The results of both astronomers were the same. • The combination of their work is now called the Hertzsprung-Russell Diagram (H-R diagram). • This graph shows the relationship between a star’s surface temperature and its absolute magnitude. • The chart is a tool for studying stars over time.

5. The H-R Diagram • Look at the diagram. • The temperature is given along the bottom. • Absolute magnitude or brightness is along the left side. • Hot stars are on the left and cool stars are on the right. • The bright stars are at the top and dim stars are at the bottom. • Bright stars are one million times brighter than the sun and dim stars 1/10,000 times dimmer than the sun.

6. The H-R Diagram Continued • On the diagram there is a band of stars going from the top left to the bottom right corner. • This pattern is called the main sequence. • A star spends most of its life as a main sequence star and then turns into one of the other types of stars. • Blue Stars are massive stars in the main sequence that quickly burn up their hydrogen and turn into giants. • Red dwarf stars are low mass stars in the main sequence that remain in the main sequence for a long time.

7. Stars of the H-R Diagram • White dwarf stars are small hot stars that are the left over centers of old stars near the end of their lives. (The sun will eventually become a white dwarf.) • Red Giants are stars that use up their hydrogen and the center of the star shrinks inward while the outer part expands. • All stars begin as a ball of gas and dust in space until gravity pulls the gas together into a sphere. • Each star will start on the main sequence and progress through stages until the end of its life.

8. When Stars Get Old • Stars stay on the main sequence for a long time but don’t stay there forever. • Average stars (like the sun) turn into red giants and the white dwarfs. • More massive stars leave the main sequence in a spectacular manner. • They may explode.

10. Supernovas • Massive blue stars use up their hydrogen faster than average stars. • They do not last as long as other stars and may explode at the end of their life giving off a large flash of light called a supernova. • A supernova is basically the death of a large star by explosion. • A supernova can be brighter than an entire galaxy for days. • The explosion creates heavy elements like silver, gold and lead.

11. The 1987 super nova explosion actually blew 169,000 years ago.

12. Neutrons and Pulsars • After a supernova the leftover materials in the center are squeezed together to form a star of about two solar masses, but the star is only 20km in diameter. • This is a neutron star. It is extremely dense. • If a neutron star is spinning, it is called a pulsar. • Pulsars send out beams of radiation that also spin around like the light of a lighthouse.

14. Black Holes • Sometime the leftovers of a supernova are so massive that they collapse to form a black hole. • A black hole is an object with more than three solar masses squeezed into a ball only 10km across. • The gravity of a black hole is so great that light cannot escape it. • Black holes do not gobble up stars, but if dust or other smaller objects get near, the black hole with pull it in. • Black holes can be seen using the X-rays that solar dust gives off.