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

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

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1/4 2 units

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

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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.