Transcript Galaxies - science1d

GALAXIES
Our Solar System – a Speck
in the Milky Way
 Our galaxy is about 100 000 ly in diameter
 What’s a ly again?
 It is 2000 ly thick at its widest point near the
core
 Our solar system is very tiny compared to the
whole Milky Way
 A beam of light from the Sun would take 100
000 years to cross the entire Milky Way
 But it takes only 5 hrs to get to Neptune! :o
Our Milky Way
•Looks like two fried eggs
stuck back to back (from
the side)
•Looks like a giant
whirlpool (from the top)
•Our Sun takes about 220
million years to travel once
around the centre of the
galaxy
•This means that in
millions of years, people
will see different stars at
night
•If our solar system was a
single bean, the Milky Way
would be a bit larger than
the area of Lake Superior
Andromeda Galaxy
•One of the nearest
galaxies to us
•You can see it from
Britain
•It is 2 million ly away
Oooo-aaaah!
•The most distant galaxies
are 15 million ly away
•When light left them, the
Earth did not exist!!
Properties of Galaxies
 All galaxies contain stars, planets and dust
 More dust means more stars
 Stars form from dust and gases present in nebulae
 Older galaxies have less dust because it has all
been used up in star-making
Properties of Galaxies
1. Black Holes
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It is a region of space where gravity is so strong that
nothing (not even light) can escape
It’s believed that each galaxy contains at least one
supermassive black hold at the centre
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Milky Way: at centre, many stars are orbiting around a
point in space that seems empty
Affects its surroundings by gravitational pull
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Can pull a star right into it, destroying the star
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This will increase the mass of the black hole
Milky Way: ours has a mass of 3 million stars like our Sun
When two galaxies collide, the black holes will merge
Properties of Galaxies
2. Dark Matter
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It is matter in the universe that is invisible
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It does not interact with light or other radiation
Astronomers think that 90% of the universe is
filled with dark matter
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Why?
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Unexpected motion of galaxies
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Milky Way: stars revolve around centre so fast, they should
be flung off but aren’t
Most of the gravity in the universe is thought to be
produced by dark matter
Properties of Galaxies
3. Star Clusters
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This is a distinct grouping of stars
Has a concentration of stars in a smallish region of
space
Two types:
1. Open Cluster
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Contains a few hundred to a few thousand stars
One of the youngest star groups in a galaxy
2. Globular Cluster
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Contains hundreds of thousands of stars, drawn together
by the stars’ gravity
One of the oldest star groups in a galaxy
Properties of Galaxies
Open Cluster
Globular Cluster
Galaxy Shapes
 Galaxies are commonly classified according
to four main shapes:
Spiral
2. Barred Spiral
3. Elliptical
4. Irregular
1.
Galaxy Shapes
1. Spiral & Barred Spiral

Have spiral-shaped arms coming out from the centre
Half of all spiral galaxies have a bar across the centre
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A typical spiral galaxy completes a full rotation once
about every 300 million years
Most spiral galaxies have hundreds to thousands of
star clusters
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These are called barred spiral galaxies (E.g.: Milky Way)
Due to the dust and gases between stars (breeding
ground for new stars)
At the core, has a “bulge” with mostly old stars (not
enough dust an gases here)
Galaxy Shapes
3. Elliptical Galaxies
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Looks like a flattened sphere
Can look almost spherical
Can look football-shaped
Can look long and cylindrical like a pencil
Happens when spiral galaxies merge
These are the largest galaxies
Don’t have a lot of dust  fewer young stars