Stars and Galaxies

1. Stars and Galaxies The Evolution of Stars

2. Classifying Stars: The H-R Diagram • One of the most useful ways to classify and describe stars was discovered by EjnarHertzsprung and Henry Russell in the early 1900s. • They noticed that in general, stars with higher temperatures also have brighter absolute magnitudes. • They developed a graph to show this relationship, now called the Hertzsprung-Russell diagram, or simply the H-R diagram for short.

3. The relationships among a star’s color, temperature, and brightness are shown in this H-R diagram. • Stars in the upper left are hot, bright stars, and stars in the lower right are cool and faint. • Our Sun is a “main sequence” star about halfway through its 10 billion year lifetime, and falls about in the middle of this diagram.

4. Here’s a simpler version. Where’s our Sun?

5. Classifying Stars: The H-R Diagram • The Main Sequence • Forms a diagonal band on the H-R diagram • In the upper left are stars that are hot, blue, and bright. • In the lower right are stars that are cool, red, and dim. • The Sun is an average yellow star in the middle of the main sequence. • About 90% of all stars fall in the main sequence. • Dwarfs, Giants, and Supergiants • The 10% of stars that don’t fall in the main sequence. • They represent stars that are at later stages of their life cycle. • White Dwarfs: hot, but not very bright: they’re small. • Red Supergiants: cool, but extremely bright: they’re huge.

6. How Do Stars Shine? • Generating Energy: • In the 1930s, scientists discovered reactions between the nuclei of atoms: nuclear fissionand nuclear fusion. • Scientists hypothesized that temperatures in the center of the Sun must be high enough to cause hydrogen to fuse to form helium. • This nuclear reaction would release tremendous amounts of energy. • In 1905, Albert Einstein had discovered the relationship between mass and energy: E =mc2 • Nuclear Fusion: • Fusion occurs in the cores of stars. • Here, temperatures are high enough (10 million K) for fusion to occur. • As hydrogen nuclei move so fast that they collide and fuse, a small amount of mass is “lost” and converted to a large amount of energy.

7. Fission or Fusion? • Nuclear Fission: • A large atomic nucleus is split, forming lighter elements. • This converts a small amount of mass into tremendous energy. • This is nuclear energy: used in nuclear power plants and in nuclear weapons (atomic bombs). • Nuclear Fusion: • Atomic nuclei are fused together to form a heavier element. • This converts a small amount of mass into tremendous energy. Fusion occurs in the cores of stars. • This is not yet used to produce energy in a power plant. However, this process is used in thermonuclear weapons (hydrogen bombs).

8. Evolution of Stars • The Birth of a Star: • Stars begin as a large cloud of gas and dust called a nebula. • As the nebula contracts due to gravity, temperatures in the center of the nebula increase. • When temperatures reach 10 million K, fusion begins. • The Main Sequence: • While the star continues to fuse or “burn” hydrogen, it is a “main sequence” star. • The Sun is about midway through its 10 billion year life span, gradually fusing hydrogen into helium in its core. • Stars more massive than the Sun can use up their hydrogen in as little as 1 million years. Stars faint and less massive can last for billions of years on the main sequence. • What happens when the hydrogen runs out?

9. Red Giants and White Dwarfs • Red Giants and Supergiants: • No longer a main sequence star, the Sun will become a red giant in about 5 billion years. • As its core contracts and heats to 100 million degrees, helium nuclei will fuse into carbon. • Its outer layers will expand as the sun swells to about 2 AU in diameter. • Becoming a White Dwarf: • A star like the Sun will use up its helium and its core will contract even more. • The Sun’s outer layers will escape to space leaving behind a hot, dense core: a “white dwarf.” • A white dwarf is only about the size of the Earth.

10. White Dwarfs and Planetary Nebulas • A star like the Sun may lose half its mass in forming a white dwarf and “planetary nebula” like the Helix Nebula shown here. • In this way, stars return much of their material to space, to form new stars from new nebulas.

11. Evolution of Massive Stars • Supergiants: • Stars more than 10 times the mass of the Sun do not become white dwarfs, and will fuse all their hydrogen into helium in only a few million years. • In stages, they will then produce heavier and heavier elements as their cores contract and heat to 1 billion K or more. • The star becomes a “red supergiant.” The star Betelguesein the constellation Orion is an example.

12. Evolution of Massive Stars • Supernovas: • When a red supergiant begins to produce iron in its core, the energy produced by fusion can no longer balance the collapse due to gravity. • The core of the star collapses violently, sending shock waves outward. • The outer portion of the star explodes, producing a supernova explosion. • All other elements in the universe (up to uranium) are produced by fusion in this shock wave. • This is one of the most violent events in the universe: in one second, a supernova explosion can release more energy than the Sun produces during its entire 10 billion year lifetime. Supernova 1987a Crab Nebula