What is milky way Galaxy?

 The Milky Way

Almost every star we can see in the night sky belongs to our galaxy, the Milky

Way. The Galaxy acquired this unusual name from the Romans who referred

to the hazy band that stretches across the sky as the via lactia, or “milky

road”. This name has stuck across many languages, such as French (voie

lactee) and spanish (via lactea).

Note that we use a capital G for Galaxy if we are talking about the Milky Way.The Structure of the Milky Way

The Structure of the Milky Way

The Milky Way appears as a light fuzzy band across the night sky, but we

also see individual stars scattered in all directions. This gives us a clue to

the shape of the galaxy. The Milky Way is a typical spiral or

disk galaxy. It consists of a flattened

disk, a central bulge and a diffuse

halo. The disk consists of spiral arms

in which most of the stars are located.

Our sun is located in one of the spiral

arms approximately two-thirds from

the centre of the galaxy (8kpc). There

are also globular clusters distributed

around the Galaxy.

In addition to the stars, the spiral arms

contain dust, so that certain directions

that we looked are blocked due to high

interstellar extinction.

This dust means we can only see about

1kpc in the visible.

Components in the Milky Way

The disk: contains most of the stars (in open clusters and associations) and

is formed into spiral arms. The stars in the disk are mostly young. Whilst the

majority of these stars are a few solar masses, the hot, young O and B type

stars contribute most of the light. There is also dust in the disk which can be

seen as dark lanes across the milky way in dark skies. The disk is about 100

pc thick and 25 kpc in diameter.

The bulge: also referred to as the spherical component, the bulge is the

thickened central region of the galaxy. The bulge contains older stars, but is

hard to observe because of the large extinction due to dust that observations

towards the Galactic centre suffer. It is about 1.5 kpc in radius and 0.7 kpc

in height.

Globular clusters: scattered throughout the halo, these are bound clusters

that contain thousands or millions of stars and are about 20 to 30 pc in

diameter. Globular clusters are old – about 11 billion years.

The halo: contains a very old population of stars sparsely scattered out of

the plane of the disk. There is almost no gas and dust and no new star

formation is ongoing. The halo extends out to about 5 kpc.

The Rotation of the Galaxy

We have seen (e.g. in stellar evolution) that in order for any object to be in

equilibrium, the various internal forces must be balanced. Since all the stars

and clusters in the MW have their own gravity, there must be a force to

counteract this. Indeed, it is by rotating that the Galaxy sets up an opposing

force.

At the position of the sun (at a Galactocentric radius of about 8.5kpc), the

rotation speed is about 220 km/s. We call a cosmic year the time taken for the

sun to rotate once around the Galaxy. How long is a cosmic year (given that

the orbit is almost circular)?

First we need to convert all our units to the same thing, let's choose km.

So 8.5 kpc is 8500 pc and we know that 1pc=3.1x1016m. Therefore 1 pc

in km is 1000 times less than this (1000 m in a km), so 1 pc=3.1x1013km.

So now we know that the radius of the sun's orbit is 8500 x 3.1x1013km = 2.64x1017

km.

The circumference of a circle is 2πr, i.e. the circumference of the sun's

orbit is 2 x 3.142 x 2.64x1017 = 1.66x1018 km.

Finally, the time taken to travel this distance is 1.66x1018 / 220 = 7.5x 1015s. I

leave it for you to show that this is about 240 million years.

The Rotation of Other Stars

Stars in the Galactic disk all rotate in

approximately circular orbits, but

stars in the halo (and globular

clusters) have highly elliptical orbits.

Some halo stars move very fast and

it seems that these high velocity stars

are receiving an extra “kick” as the

travel across the disk.

Stars in the very centre of the MW

also move very fast, this is because

there is a black hole at the centre of

our galaxy and its large

gravitational effect causes stars to

swing by very fast. Stars farther

from the centre don't feel the pull of

the black hole so strongly. Only

very recently, astronomers have

been able to look into the centre of

the Galaxy and trace the orbits of

these stars.