Galaxy Lesson

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Galaxies & Light Years
Measuring Distances

• Light Year – The distance

light travels in one year

• Light travels at about

186,000 miles/sec or 671
million (6.71×108)
miles/hour

• It takes 8.4 minutes for light

from the Sun to travel to the
Earth

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Galaxies & Light Years
Measuring Distances

• The closest star to the Sun

is Proxima Centauri. It’s
4.24 light years away

• It takes 5.5 hours for the

Sun’s light to reach Pluto

By ESA/Hubble, Proxima Centauri, seen

through Hubble telescope, 2013

4.24
Light
Years

Proxima Centauri

Pluto

Sun

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Galaxies & Light Years

A galaxy is a system of millions or billions of stars, made of gas and dust, held together by

gravitational attraction. Galaxies are different sizes and shapes.

100,000 Light Years

200,000,000,000 stars

How many stars are in
the Milky Way?

How far is it across our
own galaxy?

By NASA/Penn State University

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Galaxies & Light Years

Spiral/Disk Galaxy

• Spiral or disk shaped
• Has ‘arms’
• Dense center

• We live in a spiral galaxy

called the Milky Way

• Our Sun is located about 2/3

of the way out on one of the
arms

The Sun

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Elliptical Galaxy

• Oval-shaped
• Most abundant galaxies in

the universe

• Most are composed of

older, low-mass stars

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Galaxies & Light Years
Irregular Galaxy

• No defined shape
• No clear bulge or disk
• The leftovers of two

colliding galaxies