1. Classifying Stars Brightness

2. Some stars are so bright that you can see them even in a lighted city, while others are so dim that you can only see them through a telescope. A star’s brightness depends on three things: how big the star is (size) how hot the star is (temperature) how far away from Earth the star is (distance)

3. Brightness cont. The bigger and hotter the star, the more light it gives off. However, brightness also depends on distance. A star that is nearer to Earth looks brighter than one that gives off the same amount of light, but is farther away.

4. Magnitude Absolute magnitude is the actual amount of light that a star gives off. Apparent magnitude is the amount of a star’s light that is observed on Earth. If two stars are the same distance from Earth, the one with the greatest absolute magnitude will be the brightest. If one star is farther away than the other, the one that is closer will appear the brightest even if it puts off less light or has less absolute magnitude.

5. Magnitude cont. For example: Rigel has an absolute magnitude greater than that of Sirius (it puts off more light), but it looks dimmer from Earth because it is 100 times farther away. Due to its apparent magnitude, Sirius is the brightest star in the night sky.

6. Classifying Stars Life Cycle

7. Nebula A nebula is the earliest stage of a star’s formation. It forms when the interstellar medium (clouds of gas and dust) begins to collapse due to gravity. Nuclear fusion of hydrogen occurs and more substances or elements are created, fusing to form a core and then a main sequence star.

8. Main Sequence Star A star next becomes a main sequence star. This is the stage that stars spend the most time in. In fact, 90% of all stars are in this stage. Our sun is a main sequence star. The cores of these stars shrink and they become cooler and dimmer.

9. Giants As a main sequence star begins to die, it moves into the next stage. As a star cools and expands, helium fuses to form carbon, becoming a red giant. If it expands to a size that is more than 700 times as large as the sun, it is called a red super giant.

10. Red Giants Red giants then form planetary nebula as their outer layers drift into space. This is where planets form from. These then turn into white dwarfs which are small, hot stars that consist of a hot, dense core that contracts under the force of gravity. White dwarfs cool becoming black dwarfs.

11. Red Super Giants The cores of red super giants collapse producing an explosion and forming supernovas. Stars about three times the size of our sun then become neutron stars where only neutrons exist in their cores. Stars larger than three times the size of our sun become black holes. Black holes are so dense that nothing, including light, can escape them.