1. STARS  Stars form when clouds of gases (mainly hydrogen) are pulled together by gravitational forces.  Atomic explosions occur inside stars. –They are so hot inside that they emit heat and light.

2. STARS CONTINUED  Stars can have planetary systems made up of planets and satellites.  An enormous cloud of gas and dust, a nebula, surrounds the stars.

3. STARS AND THEIR SIZES  Stars can have different sizes.  Some stars are similar in size to the earth, they're called dwarf- stars.  When stars are bigger than earth, they're called giant-stars.

4. STARS AND THEIR COLORS  The color of stars is variable: –red stars –yellow stars –White Stars –blue stars  Red stars are the coldest and blue stars are the hottest.

5. CONSTELLATIONS V. ASTERISMS  Constellations are imaginary patterns of bright stars.  Asterisms are patterns of stars that are not constellations. –Example: The big dipper is really apart of Ursa Major (the big bear)

6. CONSTELLATIONS CONT.  There are eighty eight official constellations.  Some examples are: Big dipper, Orion, Gemini, Little dipper.

7. 7   The Zodiac is defined by 12 constellations that lie along the annual path of the sun across the sky. 7

8. So What are Magnitudes? Magnitudes are how bright stars appear. Hipparchus, a Greek philosopher, invented a system of magnitudes. He called the brightest stars in the sky (like Sirius), first magnitude stars.

9. More Hipparchus The next brightest group of stars were 2 nd class or magnitude 2 stars, and so forth, down to magnitude 6 stars, which were just barely visible to the naked eye. Hipparchus also estimated that the brightest (mag. 1) stars were 100 times brighter than the faintest (mag. 6) stars. http://www-gap.dcs.st-and.ac.uk/~history/BigPictures/Hipparchus.jpeg

10. What’s this mean to us? Astronomers today have inherited the magnitude system from the ancients. It takes some getting used to, because the scale appears to be backwards from the way we classify most things: The brighter stars get smaller numbers. The fainter stars get larger numbers.

11. Modern Magnitudes Today, we’ve expanded the scale well beyond the 1 to 6 range. For example, the sun appears much brighter than any other star in the sky. It has an (apparent) magnitude of -26.73. The full moon, at its brightest, has an (apparent) magnitude of -12.6 and Venus can be as bright as -4.4. On the other end, the Hubble Space Telescope can see objects of magnitude 30, way too faint for our eyes.