1. Stars and Galaxies Traveling Beyond the Earth Chapter 21

2. Constellations Constellations are patterns of stars in the night sky, commonly named after animals and characters in Greek mythology Currently there are 88 that are named If you look in the direction of north, there are constellations that appear to move in a circle around the north star, Polaris. These are circumpolar constellations. They are visible year-round in each hemisphere

3. Starlight, Starbright The stars that are closer to us appear to be brighter Stars shine because of fusion in its core. Two hydrogen atoms fuse into one helium atom. This is why a star shines for billions of years!

4. Brightness, continued Two types of brightness: absolute magnitude and apparent magnitude Apparent magnitude is the brightness we see here on Earth Absolute magnitude is the actual light the star gives off. The more negative the number, the brighter the star is! To figure out how far it is to a star, astronomers look at it from two different positions, called parallax

5. I Can See Your True Colors Not all stars are the same color because different elements burn different colors. Some are red, some are blue, etc. Color tells the temperature of the star Hot stars are bluish/white and cooler stars are reddish/orange Astronomers call this a star’s spectral class. Spectral classes are O, B, A, F, G, K, and M

6. Hertzsprung-Russell Diagram Stars are ranked on the H-R Diagram based on its temperature and absolute magnitude Most stars fall in the middle of the diagram, called the main sequence Very few stars are white dwarfs or giants/supergiants

7. A Star’s Life: Normal Main Sequence Star Here is the life of a normal sized star: 1.Nebula : starts to contract 2.Protostar : temperature increases, fusion begins 3.Main sequence star : using up H fuel 4.Giant : no more H, core contracts, outer layers start to expand 5.White dwarf : outer layers escape into space, hot dense core is left 6.Black dwarf : dead star

8. Life of a Massive Main Sequence Star Here is the life of a massive main sequence star: 1.Nebula 2.Protostar 3.Massive main sequence star 4.Supergiant: core is much hotter than a giant, heavy elements form which causes fusion to end 5.Supernova: the star explodes 6.Neutron star or Black hole: neutron star is the collapsed core of the supernova; a black hole forms when the remaining core is 3 times more massive than the Sun

9. Answer Questions on the Life of a StarLife of a Star 1.Gravity gathers hydrogen gas and particles of dust, and the clouds grow more dense. The hydrogen heats up to a fantastic 18 million degrees. Nuclear fusion turns hydrogen to helium and the fire ignites. 2.T or F. Gravity is always at work, trying to crunch the star down. 3.Stellar age is determined by the size of the star. 4.Smaller stars burn cool and dim. Medium stars, like the Sun, will burn steadily for some 10 billion years. 5.When a medium star runs out of hydrogen fuel, gravity grips the core, the star grows hotter, and the outer layers puff up and engulf nearby planets. 6.A white dwarf is a dying star. 7.A bigger star than the Sun lives hot and dies young. 8.Large stars go out with a bang called a supernova. 9.What is left after the supernova is much more dense than a white dwarf, called a neutron star. 10.Some remains of a supernova are so dense, they become black holes. 11.T or F. Supernovas are common. 12.We may only experience one exploding star in a century.

10. To the Milky Way, and Beyond! Our galaxy, the Milky Way is part of a larger group of galaxies called the Local Group There are about 30 galaxies part of the Local Group There are three types: elliptical, spiral, and irregular The Andromeda galaxy is an example of a spiral galaxy with huge spiral arms. Astronomers believe the Milky Way resembles this.

11. Galaxies, continued Elliptical galaxies are the most common Irregular are all the galaxies that are NOT spiral or elliptical, and are not nearly as common Elliptical can be football-shaped Irregular

12. Where Are We in the Solar System?