Russell's Teapot issue 4

Colliding Galaxies
Based on a lecture at Gresham College by Carolin Crawford
First, ‘Ordinary’ Galaxies:
These have a massive range of both size, mass, and colour. They can vary from a few thousand
to half a million light years wide. This being from dwarf galaxies all the way up to giant galaxies.
They can vary from mainly reds, yellows, and white stars to more blue coloured stars. This visually
represents the age of the stars; red stars being older, and blue stars being younger and more recently
formed. The two main types of ‘ordinary’ galaxies are elliptical galaxies and spiral galaxies.
• Elliptical galaxies:
This name represents 60% of all known galaxies in our universe and is therefore the most common.
They are symmetrical and consist of red, yellow, and white stars which are older stars.
• Spiral galaxies:
Of all the galaxies in our universe, spiral galaxies take up 30% of them. Spiral galaxies have a large
ball of stars in the centre, with tails of stars reaching all the way around. They are flat, apart from
the ball in the middle, and some people say they often look like two fried eggs placed against each
other. In comparison to the elliptical galaxies, they have a larger range of colours, varying from
reds and yellows in the middle, to younger, bluer stars on the outside.
Both ‘ordinary’ galaxies are symmetrical.
‘Unordinary’ galaxies have a one in a million chance of occurring. This is when the shape of the
galaxy doesn’t conform to the pre-defined shape of either spiral or elliptical. These shapes are
random and have more bluer stars as they have more active star formations. These types of galaxy
shapes occur when two galaxies get so close to each other that they are affected by each other’s
gravitational pull. Professor Carolin Crawford explained this process in three stages:
1. The First Pass. This is when the galaxies first come into range of each other, and we start to see
mild asymmetries between them. As simulations show, this is when the tidal tails begin to develop.
2. The Mice. This is the name for the second stage of colliding galaxies, after the first encounter.
This is when the tidal tails are ‘thrown outwards,’ stretching the galaxy, and increasing its length.
3. The Final Collision. The final stage is the completion of the collision, where are new galaxy
starts to form. This new galaxy is bigger and holds the mass of both smaller galaxies put together.
These new galaxies are much bluer in colour as there is a higher rate of star formation. The spaces
between the stars in each galaxy are called gas clouds, so during the collision they are pressed
against each other and compressed. Under high pressure, this leads to the formation of newer,
bluer stars. This is called high starburst activity.
Scientists have predicted that this simulation will come true with our galaxy, The Milky Way, and
out ‘twin’ galaxy, Andromeda. Due to their remarkably similar sizes and masses, both galaxies
will, eventually, be equally affected by each other’s gravitational pull, although this is only supposed
to happen in eight billion years, give or take.
By Anna Clery