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