What give galaxies their different shapes?
A multitude of variables from external gravitational influences to intergalactic collisions .
Galaxies come in a variety of shapes and sizes. We will first talk about what gives some galaxies their spiral shape.
The most notable scientist who studied the formation spiral galaxies was Bertil Lindblad. He observed the spiral arms seen in spiral galaxies. He quickly realised that spiral arms couldn't be sustained and there must be some sort of mechanism that allowed the spiral arm to be stable. To sustain these spirals we'd have to ignore the laws of physics as the stars at the tips of the spiral arms would have to travel faster than the stars near the centre, but of course, in reality, objects move faster when they are closer to the point they are orbiting. The angular rotational speed was so great and the distance of the spiral arms with to the centre of the galaxy varied, the spiral arms would've compressed more and more each time the galaxy rotated like taffy in a taffy machine. This is called the winding problem. This problem still has not been solved, but there are two leading hypotheses:
- The spiral arms are created from density waves in the galactic disc - one theory is that the spiral arms are created from density waves travelling through the galactic disc. Stars pass in and out of these waves filling in what we see as spiral arms. Stars accelerate when they move towards a density wave when it is headed for them and slow down as they move
- SSPSF (Stochastic Self-Propagating Star-Formation) model - this theory suggests that in the initial star formation in a galaxy causes a shockwave which causes more star formation. So the idea is that the galaxy's rotation forms these newborn stars into the spiral arms we see, and this cycle repeats.
Courtesy Astronomy Online: This shows how the windup problem affects the shape of a galaxy.
The shape of Elliptical galaxies ranges from extremely flat to more spherical. The shape of Elliptical galaxies are created from collisions between numerous galaxies. After all these collisions in the course of a few hundred million to a few billion years the stars settle to form the featureless elliptical galaxies.
Courtesy NASA: Visit this link to see how elliptical galaxies form. At the end, we see how the collision between the Milky Way and Andromeda forms an Elliptical galaxy.
Irregular galaxies are formed from violent collisions between two or more galaxies which redistributes interstellar matter, dust and stars e.t.c. to form a chaotic mess. This chaotic mess eventually settles to form an elliptical galaxy. Here's a picture of an Irregular galaxy if you were wondering what it looks like.
Courtesy NASA, ESA and HST