1. Stars A star is a large, glowing ball of gas in space, which generates energy through nuclear fusion in its core. The closest start to Earth is the sun, which is considered a fairly average star.

2. Stars A light-year is the distance that light travels in a vacuum in one year, which is about 9.5 trillion kilometers. Proxima Centauri Proxima Centauri is the closest star to the sun, and is about 4.3 light years away.

3. Stars Astronomers classify stars by their color, size, and brightness. Other important properties of stars include their chemical composition and mass. Most stars have a chemical makeup that is similar to the sun, with hydrogen and helium together making up 96 to 99.9 percent of the star’s mass.

4. The Hertzsprung-Russell Diagram A star’s placement on an H-R diagram indicates its absolute brightness and surface temperature (or color).

5. The Hertzsprung-Russell Diagram H-R diagrams are used to estimate the sizes of stars and their distances, and to infer how stars change over time. About 90% of all stars are found on the main sequence.

6. The Hertzsprung-Russell Diagram The very bright stars in the upper right of the H- R diagram are called supergiants. Just below the supergiants are the giants – large, bright stars that are smaller and fainter than supergiants.

7. The Hertzsprung-Russell Diagram A white dwarf is the small, dense remains of a low- or medium-mass star.

8. Life Cycles of Stars A nebula is a large cloud of gas and dust spread out over a large volume of space.

9. Life Cycles of Stars A protostar is a contracting cloud of gas and dust with enough mass to form a star. A star is formed when a protostar becomes so dense and hot that nuclear fusion begins. A star’s mass determines the star’s place on the main sequence and how long it will stay there.

10. Life Cycles of Stars The dwindling supply of fuel in a star’s core ultimately leads to the star’s death as a white dwarf, neutron star, or black hole.

11. Life Cycles of Stars Low- and Medium-Mass Stars Red giant: the core begins to run out of hydrogen Planetary nebula: a dying star is surrounded by a glowing cloud of gas White dwarf: the hot, dense core that is left after the gas surrounding the nebula is gone

12. A.Planetary nebulas, such as the Hourglass Nebula, are clouds of gas that surround a collapsing red giant.

13. Life Cycles of Stars High-Mass Stars Red supergiant: the core begins to run out of hydrogen Supernova: the outer layers of the star collapse, causing a violent explosion – Neutron star – Black hole

14. B.The Crab Nebula is the remnant of a supernova explosion that was observed on Earth in A.D. 1054. The supernova was so bright that people could see it in the daytime.

15. Life Cycles of Stars A black hole is an object whose surface gravity is so great that even electromagnetic waves, traveling at the speed of light, cannot escape from it.

16. The Expanding Universe Hubble’s law says that the speed at which a galaxy is moving away is proportional to its distance from us. The farther away from earth a galaxy is, the faster it is moving away from us.

17. The Expanding Universe Astronomers theorize that the universe came into being at a single moment, in an event called the big bang. The big bang theory states that the universe began in an instant, billions of years ago, in an enormous explosion.big bang theory

19. The Expanding Universe The existence of cosmic microwave background radiation and the red shift in the spectra of distant galaxies strongly support the big bang theory. Astronomers estimate the universe is 13.7 billion years old.

20. The Expanding Universe Dark matter is matter that does not give off radiation. Dark matter cannot be seen directly, but its presence can be detected by observing its gravitational effects on visible matter.