1. Galaxies The Life and Death of the Stars

2. A galaxy is a cluster of stars, gas, and dust that are held together by gravity. There are three main types of galaxies:  Irregular  Elliptical  Spiral

3. Types of Galaxies

4. Irregular Some galaxies do not have definable, regular shapes and are known as irregular galaxies. They contain young stars, dust, and gas.

5. Elliptical Elliptical galaxies look like flattened balls. These galaxies contain billions of stars, but have little gas and dust between the stars. With the lack of gas and dust, new stars cannot form, and so they contain only old stars.

6. Spiral Spiral galaxies consist of a flat, rotating disk with stars, gas, dust and a central concentration of stars, known as the bulge. The bulge is surrounded by a much fainter halo of stars. Contains middle aged stars.

7. The center of a spiral galaxy is reddish because of the red giants. The arms are bluish because of the many young, bright blue stars.

8. Barred Spiral Our Milky Way galaxy has recently (in the 1990s) been confirmed to be a type of spiral galaxy known as a barred spiral galaxy... They have two bar-shaped clusters of stars that stretch out from the center.

9. Lifetimes of Stars

10. Before they can tell how old a star is, astronomers must determine its mass. Medium mass stars, such as the Sun, exist for about 10 billion years. These are known as main sequence stars.

11. Stars with more mass have shorter lives than those with less mass. Small stars use up their fuel more slowly than large stars, so they have much longer lives, about 200 billion years.

12. When a star runs out of fuel, it will become a white dwarf, neutron star, or black hole.

13. Main Sequence Stars The stage in which stars spend the majority of their lifetime. Small and medium stars first become red giants. Their outer layers expand during this phase. Eventually, the outer parts grow bigger still and drift out into space.

14. The blue, white, hot core is left behind causing a white dwarf. When there is no more energy, it becomes a black dwarf. It is the remaining burnt-out cinder left, as the star goes out.

15. A dying giant, or supergiant star, can suddenly explode. Within hours, the star blazes millions of times brighter. The explosion is called a supernova

16. Neutron Stars After a star explodes, some material from the star is left behind. This material may become part of a planetary nebula. The core will compress and form a neutron star. Neutron stars are even smaller and more dense than white dwarfs.

17. Black Holes The most massive stars may have more than 40 times the mass of the Sun. One might have more than five times the mass of the Sun left, after it becomes a supernova. The gravity of this mass is so strong that the gas is pulled inward, packing it into a smaller and smaller space. These massive stars become black holes when they die.

18. Red Giants This is a large star that is reddish or orange in color. Red giants are very large, reaching sizes of over 100 times the star's original size.

19. This phase in a star's life has its supply of hydrogen has been used and helium is being fused into carbon. This causes the star to collapse, raising the temperature in the core. The outer surface of the star expands and cools, giving it a reddish color.

20. White Dwarf The collapsed core left when a red giant loses its outer layers is called a white dwarf. It is made of pure carbon that glows white hot with leftover heat from the spent fuel. It will drift in space while it slowly cools.

21. It is the size of Earth, but very dense. A teaspoon of the material would weigh as much as an elephant.

22. Black Dwarf A black dwarf is a white dwarf star that has cooled completely and does not glow. It will drift in space as a frozen lump of carbon. The star is considered “dead”.

23. Massive Stars

24. A star with a much greater mass will form, live, and die more quickly than a main sequence star. Massive stars follow a similar life cycle as small and medium stars do, until they reach their main sequence stage.

25. This occurs because the gravity squeezes the star's core and creates greater pressures, resulting in a faster fusion rate.

26. Red Supergiant A red supergiant glows red because its outer layers have expanded, producing the same amount of energy over a larger space. The star becomes cooler. Red stars are cooler than blue or white stars. A supergiant has the pressure needed to fuse carbon into iron.

27. This fusion process takes energy, rather than giving it off. As energy is lost, the star no longer has an outward pressure equal to gravity pushing in. Gravity wins, and the core collapses in a violent explosion.

28. Supernova A supernova is an explosion of a massive star at the end of its life; the star may briefly equal an entire galaxy in brightness. At this point, the mass of the star will determine which way it continues in the life cycle.

29. Neutron Star or Black Hole If the star is at least 1.5 but less than 9 times larger than the Sun, the core left after the supernova will collapse into a neutron star. This is a star composed only of neutrons. If the star is at least 9 or more times larger than the Sun, the core will continue to collapse into a black hole, an extremely dense area with a strong gravitational pull that light can not escape.