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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
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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 Galaxies1. Black Holes
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
Milky Way: at centre, many stars are orbiting around a point in space that seems empty
Affects its surroundings by gravitational pull Can pull a star right into it, destroying the star
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 It is matter in the universe that is invisible
It does not interact with light or other radiation
Astronomers think that 90% of the universe is filled with dark matter
Why? Unexpected motion of galaxies
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 This is a distinct grouping of stars Has a concentration of stars in a smallish
region of space Two types:
1. Open Cluster Contains a few hundred to a few thousand stars One of the youngest star groups in a galaxy
2. Globular Cluster 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:
1. Spiral2. Barred Spiral3. Elliptical4. Irregular
Galaxy Shapes1. Spiral & Barred Spiral
Have spiral-shaped arms coming out from the centre Half of all spiral galaxies have a bar across the
centre These are called barred spiral galaxies (E.g.: Milky
Way) A typical spiral galaxy completes a full rotation once
about every 300 million years Most spiral galaxies have hundreds to thousands of
star clusters 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 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
Galaxy Shapes
4. Irregular Galaxies Neither spiral nor elliptical No regular shape May have happened because...
Two galaxies collided Two galaxies got close enough that the
gravity of one drew stars from the other
Galaxy Shapes
Spiral Galaxy Barred Spiral Galaxy