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Transcript
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 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: 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 A typical spiral galaxy completes a full rotation once about every 300 million years Most spiral galaxies have hundreds to thousands of star clusters 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 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