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.
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.
Immeasurable Heaven
Galaxies in the universe are not uniformly distributed, but are instead clumped together in groups known as clusters. These clusters are also clumped into larger superclusters, and superclusters are clumped into larger groups. If you image a bubble bath, and the way soapy-water bubble mix clumps to the intersection of the bubbles, the distribution of galaxies is somewhat similar.
CANDELS in the Dark
If you look at different galaxies in the universe, you begin to notice that they seem to fall into some broad types. Edwin Hubble was the first to categorize galaxies into different types, and he divided them into ellipticals, spirals and irregulars. He then further broke these categories into subgroups, and arranged them into what is now known as Hubble’s Tuning Fork diagram.
Galaxy Wind
The Sun gives off a great deal of energy. As a result, ions from the upper atmosphere are pushed away from the Sun, producing what is known as the solar wind. With galaxies, a similar thing can occur. The center of many galaxies have the largest concentration of stars, and if that region is particularly active it can produce a “galactic wind” of molecules streaming away from the galactic center.
Galaxy Rangers
In an earlier post I wrote about one of the mysteries of dark matter. While dark matter matches most observations very well, it doesn’t do well in the area of dwarf galaxies. In particular, computer simulations predict that there should be many more dwarf galaxies than we observe. This has been taken to mean that either the simulations are somehow flawed, or dark matter isn’t the complete solution we’ve thought. But now new research has found that dark matter simulations might be right after all.
Missing Mass Mystery
This year data from the Planck satellite released the most precise observations of the cosmic microwave background. The results tell us that about 68.3% of the universe is dark energy, 26.8% is dark matter, and 4.9% is baryonic matter. Baryonic matter is the regular matter that makes up stars and planets and me and you. It is called baryonic because protons and neutrons which form the nuclei of atoms are known as baryons. (There are other, more exotic, baryonic particles, but protons and neutrons are the most common) So all the atoms and molecules we are familiar with are baryonic matter.
Under Pressure
If you’ve ever been in a traffic jam you’ve noticed that while you slowly make your way through it the overall traffic pattern remains the same. Usually when you’re caught in a traffic jam you eventually find the source (construction, minor accident, etc.) but sometimes you enter a traffic jam and go slowly through it without ever seeing a cause. At some time earlier something started it, but now there is just the traffic. The jam itself is now the cause of the jam.