The image above shows a pair of colliding spiral galaxies known as Arp 274. What’s interesting is not that they happen to be colliding, but that the two galaxies are spiraling in opposite directions. The one on the left spirals in a clockwise direction, while the one on the right spins in a counterclockwise direction. Sometimes we’ll refer to the left galaxy as left-handed, while the right one is right-handed. The reason is that if you hold hands up with your thumbs pointing at yourself, you’ll see the fingers on your left hand curve clockwise, and the fingers on your right hand curve counterclockwise.
I’ll Fly Away
A couple days ago I wrote about how one of the properties of a galaxy is that its stars are gravitationally bound. This means that the stars don’t have enough speed to escape the gravitational attraction of the galaxy, and so a star by itself can’t escape the galaxy. But this doesn’t mean that stars can never escape their galaxy. In fact, given enough time, many of them will.
Red Light District
There’s a lot of gas and dust in the universe. Some of it has coalesced into dark nebulae, such as bok globules that almost look like holes in the starry night. We can observe these by the background light they absorb. Some clouds of dust are close enough to a star that light reflects off them, creating reflection nebulae such as the one near T Tauri. But sometimes a cloud of gas and dust is near a hot star, but too diffuse to scatter light much. In this case it can produce a faint nebula known as an emission nebula.
Bound
Yesterday I talked about just how small a star can be, so today let’s explore just how small a galaxy can be. Our Milky Way galaxy is about 100,000 light years across, and contains about 200 billion stars. The largest known galaxy (IC 1101) is about 6 million light years across, and has a mass of about 100 trillion solar masses. The smallest galaxy? It has about a thousand stars.
Movin’ Right Along
The motion of a star relative to us can be determined by measuring two quantities, radial motion and proper motion. Radial motion is the motion of a star along our line of sight. That is, motion directly toward us or away from us. Proper motion is the change in angular position of the star, from which we can calculate the motion perpendicular to the line of sight (known as transverse motion. You can see how this works by imagining someone walking through a room. If we know the rate at which the person is walking toward us or away from us (radial motion) and their movement relative to the far wall (proper motion), then we can use a bit of simple geometry to calculate their path through the room.
Sound It Out
Imagine a stadium filled with people. With everyone is in their seats, waiting for the game to begin, there is an undercurrent of noise. A few words between friends, the scuffle of shoes, the creak of a chair. All of these little sounds fill the stadium with a background of white noise. A similar “white noise” occurs with galaxies in our universe, and it helps us understand dark energy.
A Million Star Sky
On a dark night with good viewing conditions, you might be able to see about 4,000 stars with the naked eye. Imagine, then, if you could see a million stars with the naked eye. For such a sky, our galaxy would need to be much more densely packed. While we won’t see such a sky anytime soon, it could exist on a planet in the most densely packed galaxy yet discovered. It is known as M60-UCD1, and it is a rather curious galaxy.
Island Universe
The visible universe is vast. It is 93 billion light years across, and contains more than 100 billion galaxies. The average galaxy contains about 100 billion stars, and untold numbers of planets. Yet a century ago there was serious doubt among many astronomers that the universe was much more than 100,000 light years across. Arguments about whether the universe was small or large became known as the Great Debate.
Calling the Question
A couple days ago I wrote about a rather large cluster of quasars, and how it seemed to be larger than we’d expect for the universe as we know it. The post gathered the attention of an anonymous commenter, who pointed out an opposing view regarding this research. The rebuttal to the cluster research was published in MNRAS, and makes a rather simple claim: not all patterns are real. In other words, if you look deep enough for a pattern in your data, you are bound to find one, even if it is really just noise.