The great beyond has a lot going on in it. Other than crazy chunks of frozen rock hurtling past planets at high speeds, there are balls of gas that grow larger until they explode, giant fireballs that die and suck everything around them into the void, and any number of other inexplicable or unknown things. Today though, we’re just going to talk about two: stellar clusters and quasars. I decided to go with stellar clusters (or just star clusters) because I recently had the opportunity to attend an astronomy lesson about stellar clusters at the University of Alberta, where some of the more advanced astronomy students pointed giant telescopes out at the sky and explained what they were pointed at. I actually thought it would be a busier event than it was but in reality it turned out to be just myself, a friend of mine, and my friend’s mom talking to an oddly charismatic astrophysics major about the finer points of stellar density. I decided to talk about quasars because they are similar to star clusters only reversed. I’ll explain as we go. For now, let’s get into it.
Stellar clusters are pretty much exactly what you think they are, collections of stars sitting significantly closer together than normal. These stars are close enough together to form a blob of light that can be indistinguishable without a very powerful telescope, but far enough apart that they haven’t crashed into each other and likely never will. Part of the reason they are so difficult to see is that there are so few of them near enough for us to see them with our current technology. In the Milky Way Galaxy, there are only 150 catalogued star clusters. Of course, each of these clusters is composed of many stars, but compared to the size of our galaxy, 150 is not a lot. The most well known cluster in the Northern Hemisphere is the Hercules cluster or “M13”, which is a globular cluster. Globular clusters are the older of the two types of clusters. They contain tens of thousands of stars at least! Some clusters even have up to a million stars in them, all coexisting in close proximity. These clusters are said to be anywhere from ten to thirteen billion years old. The other type of cluster is called an Open Cluster. Open Clusters are much younger than their Globular cousins, usually ranging from a few million to a few billion years. They contain much more youthful stars, full of radiance and energy. The other notable feature of Open Clusters is that they don’t contain as many stars as Globular Clusters, usually a few hundred as opposed to the few hundred thousand possible with a Globular Cluster. That being said, a few hundred stars all hanging out in the same space cul-de-sac can get pretty crowded.
Now let’s talk about quasars. Let’s show a picture.
Boom, pretty crazy looking stuff. Now that you know what it looks like, let’s talk about how they are formed. To lend some scale to that however, let’s take a step back and build our way up again. First think about the solar system, a group of planets all orbiting around our sun. Put a whole bunch of star systems together and you get a galaxy, like the Milky Way, where our solar system is. Then think about that galaxy moving through space. Next, think about another galaxy moving through space, except this one is moving towards the first galaxy. Next thing you know, the two galaxies collide and form a supermassive black hole. This black hole starts to draw in energy, as black holes tend to do, and this energy forms a quasar: the most luminous object in the known universe. The reason they’re so bright is two-fold. The first aspect to consider is that there is a lot of energy building up in the same area, taking the form of visible light not to mention other unseen energy rays. The second and perhaps more impressive aspect is the sheer size of most quasars. To get an idea of how we know its size, we need to do a really quick crash course on something called the Schwarzschild Radius. So bare with me for a second:
The size of the quasar is determined by the Schwarzschild Radius of the black hole it is formed around. Without getting too deep into the math of it all, the Schwarzschild Radius is the distance at which you need to travel faster than the speed of light to escape the gravitational pull of an object if it were compressed into a tight sphere. Simplified, the quasar’s size is entirely dependent on the threshold at which light cannot escape an object’s mass. black holes are some of the only objects in known space that have a Schwarzschild Radius that is larger than the object itself. The Schwarzschild Radius of the planet Earth is smaller than the planet when it isn’t compressed into the tiniest possible sphere, so that threshold does not affect us folk who live on the outside of the planet; thus we can leave the planet’s atmosphere with space shuttles that travel much slower than light.
Ok, so now that you know what the Schwarzschild Radius is and I’ve typed it enough times to not have to check my spelling on it anymore, let’s move on to how it contributes to the size of a quasar. A quasar can be anywhere from ten to ten thousand times the size of the black hole’s Schwarzschild Radius. That’s right, ten thousand times the size. First of all, let’s remember that this is a supermassive black hole, so it’s… well it’s super massive. Next, the Schwarzschild Radius of a black hole is larger than the black hole itself, so that’s even more super massive. Then, whatever number that is, multiply that by ten thousand. That’s how big this mass of swirling pure energy is. In fact, the brightest known quasar, which also happens to be one of the closest to us (Quasar 3C 273) is bright enough to be seen from the surface of Earth with one of those consumer telescopes you get from Wal-Mart. That’s pretty bright.
Now you might be wondering by now, “How does all that energy coming in become light on such a massive scale?” It’s all about electromagnetic radiation. There’s something called an Accretion Disk that forms around objects in space with a lot of mass. It’s composed of all sorts of space dust and debris that moves in a spiral around an object, heading towards the center of it. As all this space waste gets closer to the massive object it’s spiraling around (in our case, a black hole), it picks up speed, causing friction, which in turn causes the temperature to rise. That fast-moving, superheated space junk makes electromagnetic radiation which the energy around the black hole bounces off of, then voila, you have visible light for days. Or hundreds of thousands of light years, which is about how long it takes for the light from most quasars to reach us.
So there you have it, two variations on stars being in a really close together space. In one version, the stars just live close to each other and coexist rather peacefully. In the other version, there are way more stars and they cause an implosion at the center of two galaxies worth of celestial bodies, thus creating a massive inescapable void hole surrounded by an incalculable amount of pure energy. So there you have it, once again another completed article. It was a bit of a long one so thanks for sticking it out with me here, hopefully you learned something about space. If not, hopefully you were at least entertained. Next time we’ll be back to things closer to home, so stay tuned for the January series.