The main idea behind galaxy evolution is that star clusters merged to form larger clusters, which merged into small irregular galaxies. These irregular galaxies merged into spiral galaxies and then into larger spiral galaxies, with most of the stars falling into the disk and bulge of the galaxy.
This theory is borne out through observation. The diagram above shows images of galaxies from the Hubble Space Telescope, grouped by their distance away from us. Since the light takes more time to get to us from farther away, this gives us a history of how galaxies evolved over time.
Some small irregular galaxies still exist, such as these satellite galaxies orbiting our Milky Way galaxy. This photograph of the early morning sky in Australia shows the Large Magellenic Cloud (center) and the Small Magellenic Cloud (upper right). These small irregular galaxies are orbiting the Milky Way at distances of about 180,000 light years and 210,000 light years, respectively. In contrast, the bright star in the lower left, Alpha Carinae, lies in the Milky Way galaxy and is about 310 light years away from us.
The spiral galaxy NGC 1532 is distorted by the tidal pull of its small companion galaxy. Spiral galaxies grow larger and more massive by absorbing small satellite galaxies. Over time, the disruptions are smoothed out and the galaxies settle into a steadier state.
Not all collisions are between a large galaxy and a small satellite galaxy. These two galaxies are in the process of colliding. During the collision, large streams of stars are pulled between the two galaxies, and some stars can be flung off into intergalactic space.
What happens when two large spiral galaxies collide? The Milky Way and our nearest large neighbor, the Andromeda galaxy are on a collision course. This simulation shows what might happen when we collide with Andromeda, some 4.3 billion years from now.
The Antenna galaxy is the name we give this galaxy. It has obviously just undergone a merger between two similarly sized galaxies. The individual stars in the galaxies do not often run into each other, but large clouds of dust and gas do collide, triggering star formation. This newly formed galaxy spans about 500,000 light years.
This image is a close-up of the Antenna galaxy, showing the details of the center of the galaxy, where the disks of two spiral galaxies have merged. This highly irregular structure shows a great deal of dark dust, as well as large regions of bright star formation and young, blue stars. Of course, red stars can be young stars as well, but since blue stars are usually very massive, short-lived stars, the appearance of blue stars indicates recent star formation.
In time, this galaxy will settle down into a more ordered state. A collision like this takes a billion years to complete.
galaxy merger video source
If a galaxy collision takes a billion years to complete, how can we possibly understand it? We obviously can't watch one for a billion years to take data. What we can do is look at very many colliding galaxies to get an idea of how the process takes place over time.
The above video shows a galaxy collision simulation, done using the laws of physics, paused and rotated at various parts of the simulation to compare with actual images of colliding galaxies.
The ring galaxy was likely formed from a collision where a small galaxy punched through the center of a larger galaxy. The collision caused the dust and gas to condense, with shock waves spreading out through it like a large ripple on a pond, triggering new star formation in a ring-like pattern.
Spiral galaxies are rotating, flattened disks. We can see that the details of galaxy collisions can have various consequences,, and that no two galaxy collisions are alike. How can very large elliptical galaxies be formed? Elliptical galaxies do not possess an overall rotation like spirals, so they do not flatten into disks. If two spiral galaxies were to collide in such a way that their spins canceled out through the collision, an elliptical galaxy would result.
A galaxy that has grown larger and larger through mergers, in some sense, contains the information of all of those mergers from its history, in its dynamical behavior. It makes sense that if there were very many mergers in a galaxy's history, over time the rotations of all of the contributing galaxies would cancel out to form a large elliptical galaxy.
This image shows the relative sizes of the Milky Way and Andromeda galaxies, compared a large elliptical galaxy M87, and the largest elliptical galaxy we have found to date, galaxy IC 1101. It is estimated that IC 1101 contains about 100 trillion stars, which makes it equal to a thousand Milky Way galaxies.
It's not clear what formed these concentric shells around galaxy NGC474. It may be that a satellite galaxy merged with it causing the unusual pattern, or that tidal disruption with the nearby galaxy is eliciting the shells.
The bright stars in this, and other pictures of galaxies, are stars in the foreground, in our own galaxy.
These images are representative of an extensive collection of merging galaxy images from the Hubble Space Telescope, showing various stages of behavior.
1. When galaxies get near enough to start the merging process, there is a small distortion of the dust and gas, as the gravitational pull from the galaxies begin to attract matter from each other.
2.These small distortions develop into tidal tails as the galaxies interact further, tugging long streamers of gas and dust between them.
3. The tidal tails are significant features of the interacting pair and can persist for a very long time.
4. As the galaxies get closer, the stars, dust and gas of the main disks become more distorted. Shock waves can develop, extending through the dust and gas between the stars.
5. The churning of the dust and gas spurs intense star formation in the merging galaxies. Emission nebulae develop with high luminosity.
6. Long filaments of stars can stretch for many parsecs, and huge dust lanes appear in dark contrast against the intense star-forming regions. Although the galaxies are greatly disrupted as they merge into one structure, it is believed that actual collisions between individual stars is rare.
Mergers like these take billions of years to occur, before the resulting larger galaxy settles down into a more ordered spiral structure, or into an elliptical structure.
These two galaxies are in the act of merging. They are gravitationally attracted to each other, spiraling closer and closer together. A bridge of dust, gas and stars extends some 75,000 light years between them. A burst of star formation shows up as bright blue stars. Over the course of billions of years, these two galaxies will eventually join together into one larger galaxy.