1. Measuring the Distance to the Stars

2. What is a Light Year?
A light year is the distance light travels in one year.
If the speed of light is 300,000 Km/sec. then how many Km is a light year?
300,000Km/sec. x 60sec/min x 60min/hr x 24hr/day x 365 days/year
One light year is 9,500,000,000,000 Km
9.5 trillion kilometers

3. How close is the nearest star?
Our sun is 8 light minutes away.
Proxima Centauri is 4.2 LY away.
4.2 x 9.5 x1012 = 40 trillion Km
In a scale model 1 cm = 1 light min.
Our sun would be 8 cm away.
Proxima Centauri would be 23 km away.

4. How do we find the distances to stars?

5. Which star is closest to earth?1 2 3 4 5 6
January 1 2 3 4 5 6
July
(6 months
Later) 1 2 3 4 5 6
January

6. How do we find the distances to stars?
Parallax:
The closer an object is, the more it appears to move against the background.
Parallax activity

7. As the earth goes around the sun closer stars seem to move a lot against the background of stars.
Background Stars
A B C D E F G
Far Star
Near Star
Earth January
Earth July
Sun

8. As the earth goes around the sun farther stars do not seem to move much against the background of stars.
Background Stars
A B C D E F G
Far Star
Near Star
Earth January
Earth July
Sun

9. As the earth goes around the sun closer stars seem to move more than far stars against the background of stars.
Background Stars
A B C D E F G
Far Star
Near Star
Earth January
Sun
Earth July

10. Which star is closest to earth?1 2 3 4 5 6
January 1 2 3 4 5 6
July
(6 months
Later) 1 2 3 4 5 6
January

11. Reading Stellar ParallaxPg
R&C 2,3 Pg. 470

12. How do we find the distance to the distant stars?

13. How do we find the distance to the distant stars?
Unit of brightness distance to star

14. How do we find the distance to the distant stars?
Unit of brightness distance to star
unit

15. How do we find the distance to the distant stars?
Unit of brightness distance to star
unit
1/ units

16. How do we find the distance to the distant stars?
Unit of brightness distance to star
unit
1/ units
1/ units

17. How do we find the distance to the distant stars?
Unit of brightness distance to star
unit
1/ units
1/ units
1/X2 (inverse square) X units

18. An example of how to use star brightness to find the distance to stars.
The center of the galaxy is about 100 times dimmer than the stars we can see with our eyes.
The stars we can see are about 3000 LY away. So, how far is the center of the galaxy?
The center of the galaxy is about 30,000 LY away from earth.