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Lecture 2:

Listen to part of a lecture in an astronomy class.

So we pretty much know how the biggest stars die, when a massive star reaches the end of its existence, it explodes, sending out a huge blast of energy, which then fades to invisibility within a few weeks or months.

When a giant star explodes like this, it’s called a supernova.To the naked eye, a supernova appears to be a bright new star, but it’s not a new star. It’s a dying star.

However, the shock waves from a supernova excite nearby clouds of hydrogen gas, causing them to compress, which does form new stars, and that process emits a distinctive type of radiation.

Now of particular interest to us today is that the same type of radiation has been observed in places where galaxies have collided. And in those same areas, we’ve also observed a high rate of new star formation. So not surprisingly, we’ve had a theory that colliding galaxies caused the new star formation, but there was no direct evidence, that is, until recently when some European astronomers found clouds of excited hydrogen gas that could only have been caused by shock waves from a pair of colliding galaxies.

You know, we used to think of galaxies as isolated systems, like islands in the sea of space. Now we know that they are always moving, usually in clusters. I mean, think about all the gravitational forces involved. We are talking about huge systems of stars and gas and dust. And since the average separation between galaxies is only about 20 times their diameter, it isn’t surprising that they often meet, we call it an encounter, and they meet in a variety of ways.

Mergers, for example, they are the most extreme kinds of galaxy encounters. A merger is when two galaxies meet and they don’t have enough momentum to keep going on their separate ways, so they merge, becoming one galaxy. Usually a larger galaxy will absorb, sort of swallow a smaller one, this can trigger huge areas of star formation as enormous clouds of gas from the two galaxies collide and collapse into very active star-forming regions.

Collisions are less violent than mergers. That is, they create fewer or relatively less active areas of star formation. The galaxies pass through each other, each going its separate way after they collide. The collision sends out a ripple of energy into space, cloud and gas and dust in front of it.

Your textbook has a great picture of a galaxy collision taken by the Hubble Space Telescope. See the Cartwheel Galaxy on the left? It looks like the ripples in a pond after a stone has been tossed in. That’s because one of the small galaxies on the right pass through it. And that bright ring around it, that’s where colliding and compressing gases forming stars.

But which galaxy collided with it? The one on the upper-right has been stripped of gas and dust, which is one clear sign that there’s been a collision, but the one on the bottom is showing a lot of new star formation, which is evidence of a collision.

So we are just not sure.

Finally, we have interactions. In this type of encounter, the galaxies exchange gas and dust, but don’t actually collide. Many astronomers believe that the trail of gas and dust that stretches from our Milky Way Galaxy to one nearby is the result of an interaction when the gravitational forces of the Milky Way pulled some dust and gas off the other galaxy.

What about stars? Do they ever collide? Actually, almost never.Stars are very small compared to the size of a galaxy, and they are pretty far apart from each other. Small stars, well, that term is relative, isn’t it? It’s hard to imagine the massive distance we are talking about here. Even encounter seems like an odd term as these events take a long time. We are talking billions of years, which is why most of what we theorize about comes from computer simulations. Otherwise all we have seen is sort of freeze frame of whateve stage the encounter is in, a single picture out of a long sequence of events.

But I mentioned some new evidence. That’s come from a pair of galaxies called the Antennae.

The Antennae are at an early stage of encounter but will probably be a merger and excited hydrogen gas has been found in the regions where the two galaxies overlap.

We know because that distinctive radiation signature is present, and we know that there aren’t enough supernovae to explain the radiation. The only other possibility is that we are seeing the result of shock waves produced by the collision of these two galaxies.

ologists who would love to use this technology.