1. Characteristics of Stars

2. What is a Star? A sphere of super-hot gases Mostly Hydrogen and Helium
1 to 2 percent of a stars mass may consist of heavier elements.

3. The Sun Our solar system’s star.
Light from the Sun takes 8 minutes to reach Earth.
The Sun is 150 million km away from Earth

4. Distance to Stars
Earth is about 150 million km from the Sun. (93million miles)
That is equal to 1AU (astronomical unit)

5. Distance to Stars Since Stars are so far away:
A light year is used to express great distances.
Light travels at 300,000 km/s
1 light year = the distance a ray of light travels in 1 year trillion km.
Horsehead Nebula is 1,500 light years away)

6. Proxima Centauri
The next closest star is 40 trillion km from the sun. (more than 260,000AU)
Light takes 4.2 years to reach Earth.
Most stars are much further away.

7. Distance to Stars
Stars appear to be the same distance from Earth in the night sky.
How do we know that they are not?
Parallax
Is the apparent change in position of an object when you look at it from different positions.

8. Distance to Stars
Only works for near by stars. (less than 1,000 light years away.
Astronomers look at the star when Earth is on one side of the sun and again six months later.

9. Distance to Stars
They measure how much the star appears to move against the background of stars.
The less the star appears to move the further away it is.

10. Characteristics ?

11. Classifying Stars
Size (Mass)
Color & Temperature
Brightness

12. Mass (size) Mass is the total amount of material in a body.
Some stars are five, ten or more times the size of the sun. Others are less massive.

13. Mass (size) 1. Super Giant – HUGE stars.
Betelgeuse
2. Giant Stars- very large stars
Rigel
3. Medium Size Star- Our Sun
4. White dwarf- smaller than the sun, about the size of Earth.
5. Neutron Star- very small, 20km

14. Density of a Star
One teaspoon full of material from Sirius is so dense that on Earth it would weigh a ton.

15. Temperature How can the temperature of a star be determined?
By the color
Red Stars=
Cool
Blue Stars=
Hot

16. Temperatures & Colors 1. Red Stars - 3,000◦C (Betelgeuse)
2. Red-Orange Stars- 5,000 ◦C (Aldebaran)
3. Yellow Stars – 5,500◦C – 6,000 ◦C (Sun)
4. White Stars- 10,000◦C (Sirius)
5. Blue Stars- more than 15,000◦C (Rigel)

17. Brightness (Luminosity)
Depends on size and temperature. Betelgeuse is large but is cool. It’s size makes it appear bright.
Rigel is smaller but is very hot so it shines brightly.
How bright a star appears from Earth depends on far away the star is.

18. Brightness (Luminosity)
Depends on size and temperature.
Betelgeuse is large but is cool. It’s size makes it appear bright.
Rigel is smaller but is very hot so it shines brightly.

19. Brightness (Luminosity)
Stars are far from Earth.
As a result, what problem does this cause?
Cannot accurately determine the actual brightness of a star from earth.
How bright a star appears from Earth depends on far away the star is.

20. Apparent Magnitude
The measure of how bright a star appears to be from earth.
Less bright objects that are closer to you could be seem brighter than objects that are far and actually brighter.
The lower the magnitude the brighter the star.

21. Absolute Magnitude
The brightness a star would appear if it was set at a standard distance from Earth.
Astronomers calculate the stars apparent magnitude and it’s distance from Earth.
Then calculate the brightness if it were a standard distance from Earth.

22. HR- Diagram

23. HR- Diagram Herzsprung and Russell
A graph used to find out if temperature and brightness are related.
Plots absolute magnitude vs. surface temperature.

24. HR- Diagram
More than 90 % of stars are Main Sequence, form a diagonal band.