Creation by catastrophe

Collision of the Galaxies sounds like a good title for a spectacular disaster movie.

Actually, a lot of things in the universe depend on things smashing together, including galaxies. The object
Arp 299 is two galaxies in collision.

The two, designated NGC 3690 and IC 694, lying about 134 million light years away from us, have been in the process of collision for around 700 million years. The Hubble Space Telescope image can be found at https://en.wikipedia.org/wiki/Arp_299. The image is dotted with many bright, blue stars.

This is interesting because there are only two types of bright, blue star. One kind are young stars that will dim down a bit as they settle down.

The other kind are stars that collected an exceptionally large amount of hydrogen when they formed. These stars shine extremely brightly, run out of fuel soon, collapse and then explode. These explosions are called supernovae.

In either case, these blue stars cannot be very old. This relates to another interesting aspect of this pair of galaxies: the oddly large number of supernova explosions. What has produced these unusual circumstances?

If you look at a nearby spiral galaxy, such as our close neighbour, the Andromeda Galaxy, you will see the spiral arms glowing with little knots of pink, and sparkling with young stars.

If we could go a couple of million light years off into space, our galaxy, the Milky Way, would look much the same. The reason is that the spiral arms of galaxies are loaded with hydrogen gas, the primary ingredient for making stars.

If we look closer, we will see that these clouds are not uniform; some regions are much denser than others. On occasion, something triggers one of these denser regions to collapse, forming one or more stars.

These youngsters are hot and blue, and their high output of ultraviolet radiation makes the surrounding clouds glow pink. This pink, a characteristic of hydrogen, is known as hydrogen-alpha emission.

Therefore, when we look at a distant galaxy, those pink glows mean two things:

• There is hydrogen to glow
• Hot, blue stars to make it glow.

However, the jewel-box of bright, blue stars we see in the Arp 299 pair of galaxies is really unusual. Some major event caused massive collapses of hydrogen clouds, forming showers of new stars. We are pretty sure this outburst of star formation was caused by the two galaxies colliding.

Paradoxically, collisions between galaxies are not totally catastrophic; they trigger the formation of new stars and planets.

When we look at the computer simulations of collisions between galaxies (there are many on the web), they look pretty catastrophic. One can imagine stars and planets being annihilated on a huge scale.

The youtube on this link is a good example of what we believe a collision between two galaxies would look like. However, the situation is nothing like as bad as it looks.

The nearest star to us, after the Sun, lies about four light years away. This distance is fairly typical of the average distance between stars. So the chance of stars in two colliding galaxies passing close to one another is tiny. Even as fragments fly around and the galaxies combine, all the inhabitants of worlds in those galaxies will see is their equivalent of the Milky Way changing shape over millions of years.

However, for the gas clouds between the stars it's a different matter. These will collide and collapse, forming lots of new stars.

We will get a chance to experience this first hand. The Milky Way and the Andromeda Galaxy are racing toward each other at 110 km/s, and will collide in about four billion years. There are computer simulations of this collision on the web.

• The only easily visible planet is Mars, which can be found high in the southwest during the evening.
• The Moon will reach First Quarter on the 20th.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.