1. Stars

2. What is a star?
The objects that heat and light the planets in a system
A star is a ball of plasma held together by its own gravity
Nuclear reactions occur in stars (H  He)
Energy from the nuclear reactions is released as electromagnetic radiation

3. Characteristics of Stars
DISTANCE
Measured in light-years
The distance which a ray of light would travel in one year
About 6,000,000,000,000 (6 trillion) miles
186,000 miles per second

4. Characteristics of Stars
Magnitude (brightness)
A measure of brightness of celestial objects
Smaller values represent brighter objects than larger values
Apparent magnitude
How bright a star appears to be from Earth
Absolute magnitude (luminosity)
How bright a star actually is

5. Characteristics of Stars
Temperature & Color
The color of a star indicates the T of the star
Stars are classified by T
Decreasing T (bright to dim)
O, B, A, F, G, K, M [Oh Be A Fine Girl, Kiss Me ]

6. Hertzsprung-Russell Diagram

7. Main Sequence Stars
A major grouping of stars that forms a narrow band from the upper left to the lower right when plotted according to luminosity and surface temperature on the Hertzsprung-Russell diagram

8. Types of Stars Classification
Temperature
Color O
20, ,000 K
Blue B
10,000 – 30,000 K
Blue-white A
7,500 – 10,000 K
White F
6,000 – 7,500 K
Yellow-white G
5,000 – 6,000 K
Yellow K
3,500 – 5,000 K
Orange M
2,000 – 3,500 K
Red

10. Life Cycle of Stars

11. Life Cycle of Stars
Begin their lives as clouds of dust and gas called nebulae
Gravity may cause the nebula to contract
Matter in the gas cloud will begin to condense into a dense region called a protostar
The protostar continues to condense, it heats up. Eventually, it reaches a critical mass and nuclear fusion begins.
Begins the main sequence phase of the star
Most of its life is in this phase

12. Life Cycle of Stars Life span of a star depends on its size.
Very large, massive stars burn their fuel much faster than smaller stars
Their main sequence may last only a few hundred thousand years
Smaller stars will live on for billions of years because they burn their fuel much more slowly
Eventually, the star's fuel will begin to run out.

13. Life Cycle of Stars It will expand into what is known as a red giant
Massive stars will become red supergiants
This phase will last until the star exhausts its remaining fuel
At this point the star will collapse

14. Life Cycle of Stars
Most average stars will blow away their outer atmospheres to form a planetary nebula
Their cores will remain behind and burn as a white dwarf until they cool down
What will be left is a dark ball of matter known as a black dwarf

15. Life Cycle of Stars
If the star is massive enough, the collapse will trigger a violent explosion known as a supernova
If the remaining mass of the star is about 1.4 times that of our Sun, the core is unable to support itself and it will collapse further to become a neutron star

16. Life Cycle of Stars
The matter inside the star will be compressed so tightly that its atoms are compacted into a dense shell of neutrons. If the remaining mass of the star is more than about three times that of the Sun, it will collapse so completely that it will literally disappear from the universe. What is left behind is an intense region of gravity called a black hole

17. Life Cycle of Stars