The universe came into existence in a big bang about 13.8 billion years ago, but it took another 300,000 years for the universe to cool down enough for the first atoms of hydrogen and helium to form.
Some areas cooled slightly quicker and became slightly denser than others. These density fluctuations provided the primordial seeds of galaxies.
Surrounded by halos of dark matter, these primordial seeds attracted hot hydrogen and helium gas.
Eventually the gas cooled and began collapsing under its own gravity, forming stars and black holes. Bound together by gravity, they eventually become galaxies.
The first galaxies
Galaxies in the first few hundred million years after the big bang look very different to galaxies today. They’re usually far less massive and far less ordered in appearance, with astronomers often describing them as ‘train wrecks.’
As galaxies evolve, they convert more and more of their gas into stars, giving them a bluish hue. Galaxies making lots of new stars at once are called starburst galaxies.
One of the ways galaxies grow is by merging or colliding with other galaxies. When a small galaxy gets sucked into a bigger galaxy it’s called galactic cannibalism.
Collisions between galaxies can trigger starburst, but it can also push gas out of a galaxy in a process called quenching which can stop new stars forming.
Right now the Milky Way is cannibalising a smaller galaxy called the Sagittarius Dwarf Spheroidal Galaxy.
And in about 3.75 billion years from now the Milky Way itself will collide with the much bigger Andromeda Galaxy, forming a giant new elliptical galaxy.
Another way galaxies can change their appearance is through gravitational tidal interactions with other galaxies – this results in stars smeared over large areas and can disrupt a galaxy’s spiral arms.
Our Milky Way galaxy has gravitationally disrupted a nearby galaxy called the Large Magellanic Cloud, which was a barred spiral galaxy.
The Large Magellanic Cloud lost its barred spiral shape after getting too close to the Milky Way and another neighbouring galaxy called the Small Magellanic Cloud.
The gravity from these close encounters pulled and tugged the Large Magellanic Cloud all out of shape, giving it the appearance of an irregular galaxy.
These interactions often also trigger starburst, but can also push gas out of a galaxy.
Galaxies also form stars through a process called accretion which happens when a cloud of cold gas and dust starts to gravitationally collapse.
At the heart of every galaxy is a black hole
Most if not all galaxies have supermassive black holes millions to billions of times the mass of our Sun in their centres.
Scientists are still trying to work out which came first, the galaxy or the supermassive black hole.
If the galaxy came first, then the supermassive black hole formed in the gravitationally densest part of that galaxy which is the centre. Or did the supermassive black hole form first and then attract matter around it which eventually formed a galaxy.
Types of galaxies
Galaxies come in four main types: ellipticals, spirals, lenticulars, and irregulars. Each of these main groups can be further divided into several sub-types. There are also some very rare and unique galaxy types that don’t appear to fit into the main groups at all.
Our home galaxy – the Milky Way
Our home galaxy is the Milky Way, a barred spiral galaxy estimated to be between 100,000 and 180,000 light years wide and containing between 200 billion and 400 billion stars.
Our solar system is about 27,000 light-years out from the galactic centre which is the home of a supermassive black hole called Sagittarius A* over 4 million times the mass of the Sun.
The solar system is located on the inner edge of a minor spiral arm called the Orion-Cygnus Arm, which is about 10,000 light years long.
Andromeda – a spiral galaxy
This is the Andromeda galaxy M31, the largest galaxy in our local group which includes about 45 galaxies.
Andromeda is a spiral galaxy spaning some 260,000 light-years and containing over a trillion stars. It’s located 2.5 million light-years away, making it the nearest big galaxy to the Milky Way.
Spiral galaxies have flat disk like shapes with spiral arms containing lots of molecular gas and dust from which new stars are made. The spiral arms radiate out from a central bulge or core, full of older stars.
Spiral galaxies are also surrounded by faint halos of very old stars, and tightly packed balls of stars called globular clusters.
Close up images of Andromeda’s core show what appears to be a double nucleus consisting of two concentrations separated by about 4.9 light-years.
One of these concentrations is thought to be the galaxy’s central supermassive black hole while the other is a dense concentration of stars in an eccentric orbit around the black hole.
The Sombrero spiral galaxy
This edge on view shows the Sombrero spiral galaxy M104 which is located about 28 million light years away. M104 is about 50,000 light-years across and has an active supermassive black hole at its centre.
Dust lanes and stars can be seen spread out along its disk, and its central bulge is unusually large stretching far above and below the disk.
The barred spiral galaxy NGC 1300
NGC 1300 is a barred spiral galaxy. Two thirds of all spiral galaxies are barred spirals (including our own galaxy the Milky Way). The arms of barred spirals don’t spiral all the way into the galactic centre, but connect to a straight bar of stars containing the nucleus at its centre. This bar is thought to funnel gas inwards from the spiral arms allowing new stars to be created in the galaxy’s central bulge.
An elliptical galaxy
Elliptical galaxies such as ESO 325-G004 are among the largest known galaxies in the universe.
Unlike spiral galaxies, which have well defined structures, ellipticals tend to have roughly spherical or oval shapes, with lots of old stars in random orbits around the galactic centre. Current theories suggest they may be formed through collisions between smaller galaxies.
Elliptical galaxies contain very little gas, and so have very low rates of new star formation, which is why they often have a yellowish or reddish hue.
Located about 450 million light-years away, ESO 325-G004 is so massive its gravitational pull has caused other galaxies to orbit around it, forming a galaxy cluster known as Abell S07 40.
The Spindle Galaxy NGC 5866 is a lenticular galaxy which is an intermediate between elliptical and spiral galaxies.
Lenticular galaxies have disk like structures which often contain very little gas and so don’t have a lot of new star formation taking place. However, they do contain lots of dust with the dust density increasing towards the galactic centre. NGC-5866 is about 60,000 light-years wide and about 50 million light-years away.
NGC 2787 is another example of a lenticular galaxy, this time viewed from a flatter angle that lets you see its disk like structure, including its giant dust lanes. NGC 2787 is located about 24 million light-years away.
Messy or irregular galaxies
NGC 1427A is a messy or irregular galaxy, located some 62 million light-years away, which is being drawn at a speed of almost 600 kilometres per second into a large group of thousands of galaxies called the Fornax cluster.
Unlike spiral or elliptical galaxies, irregular galaxies have no distinct regular shape, and are often chaotic in appearance with neither a central bulge or spiral arm structure.
About a quarter of all galaxies are irregular galaxies, and many are thought to have originated as spiral or elliptical galaxies, but were deformed by gravitational encounters with other galaxies.
Irregular galaxies often still contain abundant amounts of gas and dust, allowing them to form lots of new stars.
The Large Magellanic Cloud
At a distance of just 163,000 light-years, the Large Magellanic Cloud is the third closest galaxy to the Milky Way. The Large Magellanic Cloud is considered to be a dwarf irregular galaxy that was once a barred spiral galaxy which was disrupted following a close encounter with the Milky Way and the Small Magellanic Cloud.
The Small Magellanic Cloud
The Small Magellanic Cloud is also considered to be a dwarf irregular galaxy that was once a barred spiral which was disrupted following a close encounter with the Milky Way and Large Magellanic Cloud.
Located 197,000 light years away, the Small Magellanic Cloud has a diameter of about 7000 light-years and contains several hundred million stars.
Hoag’s Object is a rare type of galaxy called a ring galaxy. Only a few have ever been found. Ring galaxies comprise a circle of hot young blue stars, surrounding a core of older stars. The gap between the ring and the core may contain fainter stars.
Hoag’s Object is about 125,000 light years in diameter and is located about 600 million light years away.
This is the galaxy pair called Arp 116 which comprises a giant elliptical galaxy called Messier 60, and a smaller spiral galaxy named NGC 4647.
The pair, located about 63 million light-years away, are just starting to interact with each other and may eventually collide.
M60 is about 120,000 light-years in diameter, while the smaller NGC 4647 is about 90,000 light years wide, making it similar in size to our own Milky Way.
Like most elliptical galaxies M60 has a reddish hue because it’s composed of mostly older stars, while the blue colour of NGC 4647 is a clear sign that lots of hot young stars are being actively formed in the galaxy’s spiral arms.
These two spiral galaxies NGC 2207 on the left and IC 2163 on the right, are colliding.
Powerful gravitational forces generated by the stars, gas, and dark matter in each galaxy, is slowly pulling the other galaxy apart, producing tides of matter, sheets of shocked gas, dark dust lanes, and bursts of new star formation. Tidal streams of stars can also be seen flowing between the pair.
The space between stars is so vast, that while galaxies collide, the stars in them usually don’t collide.
Eventually over hundreds of millions of years, these two galaxies will form a new single galaxy.
Antennae Galaxies – collided galaxies
These are the Antennae Galaxies NGC 4038 and NGC 4039 which have collided and are now locked in a deadly embrace.
Both were originally spiral galaxies, but over the past few hundred million years, they’ve been engaged in a gravitational waltz which has distorted both galaxies and ripped stars into a streaming arc between the two.
Far-flung stars and streamers of gas stretching out deep into space, (out of frame) create long antennae like tidal tails, which give the galaxies their name.
The pair are between 45 million and 65 million light years away.
Written by Stuart Gary.