Measuring the Distance to the Stars
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
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.
How do we find the distances to stars?
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Which star is closest to earth?
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
• As the earth goes around the sun closer stars seem to move a lot against the background of stars.
Background Stars
Earth January Earth JulySun
A B C D E F G
Far Star
Near Star
• As the earth goes around the sun farther stars do not seem to move much against the background of stars.
Background Stars
Earth January Earth JulySun
A B C D E F G
Far Star
Near Star
Background Stars
Earth January Earth JulySun
A B C D E F G
Far Star
Near Star
•As the earth goes around the sun closer stars seem to move more than far stars against the background of stars.
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Which star is closest to earth?
Reading Stellar Parallax
• Pg. 467-468
• R&C 2,3 Pg. 470
• How do we find the distance to the distant stars?
• How do we find the distance to the distant stars?• Unit of brightness distance to star
• How do we find the distance to the distant stars?• Unit of brightness distance to star
1 1 unit
• How do we find the distance to the distant stars?• Unit of brightness distance to star
1 1 unit
1/4 2 units
• How do we find the distance to the distant stars?• Unit of brightness distance to star
1 1 unit
1/4 2 units
1/9 3 units
• How do we find the distance to the distant stars?• Unit of brightness distance to star
1 1 unit
1/4 2 units
1/9 3 units
1/X2 (inverse square) X units
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.
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