Chapter 27, section 3
Constellations →
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Star Groups
 Constellations are patterns of stars in the sky, named after ancient
gods,
objects and animals
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Galaxies →
Three types of
galaxies →
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There are 88 constellations, which divides the night sky into sections
Sometimes galaxies are part of constellations (ex.- Andromeda)
although most are stars within our galaxy
Planets were never used in constellations because they do not maintain
fixed positions in the sky compared to other planets or stars
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Galaxies are made of stars and nebulae bound together by gravity
A typical galaxy has 100 billion stars and is 100,000 light years across
Astronomers believe there are between 50 billion to 1 trillion galaxies
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in the universe
The closest galaxies (Large and Small Magellanic Cloud galaxies) are
150,000 light years away (150,000 x 9.5 trillion km)
Galaxy types are based on shape (scientists believe the shape evolves)
1) Irregular galaxyo Shape- everything that is not a spiral or elliptical galaxy
o Made of young stars, dust and gas
o Smallest type of galaxies (some have only a few million stars)
o About 10% of all galaxies are irregular
2) Spiral galaxyo Shape- a nucleus of bright stars surrounded by flattened arms
that spiral around the nucleus
o Made of middle-aged stars, dust and gas
o Some spiral galaxies have a bar running through them and are called
barred spiral galaxies
o About 30% of all galaxies are spiral
3) Elliptical galaxyo Shape- round/elliptical, have a nucleus but without arms like a spiral
o Made of older stars, with little dust and gas
o Largest galaxies in the universe are elliptical in shape
o About 60% of all galaxies are elliptical
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The Milky Way →
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The Milky Way is a spiral galaxy made of hundreds of billions of stars,
100,000 light years across and 10,000 light years thick
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Named after the ‘milky’ band seen in the sky, the band is seen because
we are looking though on the flattened arms
Every star in the Milky Way orbits around the nucleus. It will take the
sun 225 million years to complete this orbit one time
Our sun is located in one of the arms of the Milky Way
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Galaxy Mystery Sheet
Directions: Next to each number write the correct type of galaxy (spiral, elliptical, or
irregular).
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
14)
15)
16)