MACS J0717 is a large galactic cluster formed by the collision of four galaxy clusters. It could help us learn more about dark matter , as well as the structure of the universe.
Spirals
Our Milky Way galaxy is a spiral galaxy. So are several galaxies in our neighborhood, like Andromeda, Triangulum, and the Pinwheel galaxy (seen above). About 70 percent of the galaxies in our corner of the universe are spiral galaxies.
Islands in the Sky
In the 1700s, it was clear that the Messier objects such as M-31 are not stars. They are also not comets, as they they don’t move through the sky. Messier actually cataloged these objects so he wouldn’t confuse them with comets, which also look like fuzzy patches in the sky.
Steady State
There are lots of radioactive isotopes out in the galaxy, but one of these isotopes, aluminum-26, tells us something interesting about supernovae. When aluminum-26 decays into magnesium, it gives off light (gamma rays) at a particular wavelength. By measuring the brightness of light, we can determine the amount of aluminum-26 in our galaxy.
Iron Giant
The Perseus cluster is a cluster of nearly 200 galaxies about 240 million light years away. It is the advantage of being a large cluster of galaxies at a relatively close distance, so it is often the focus of investigations on the behavior and evolution of galactic clusters.
More Things in Heaven and Earth
Take a moment to let the profound nature of this image sink in. This image is what we got when we pointed the Hubble telescope at what looked like empty space. Each smudge of light in this image is a young galaxy, from about 500 million years after the big bang. Thousands of galaxies in a patch of sky the size of a grain of sand.
A Biased View
The figure below shows the positions of more than a thousand galaxies in the universe. You might think that tells us things about the history and evolution of the universe, and you’d be right. But it also tells us something about how we observe the universe. Knowing the latter is important, because all measurements have biases. If you don’t account for observation biases, you might mis-interpret your data.
The Universe in a Grain of Sand
Suppose you picked up a grain of sand and held it at arm’s length. If you held it up in the night sky, it would block a tiny fraction of the visible heavens. Now suppose instead of of a sand grain it were a tiny window, through which you could see even the faintest light. Finally, suppose you were to take your tiny window and point it at the darkest patch of night you could find. What would you see?
Twinkle Twinkle Little Galaxy
The brightest quasar in our sky (in the visible spectrum) is one known as 3C 273. One of the things we’ve observed is that its brightness oscillates over time. Basically the quasar “twinkles”, getting slightly brighter and dimmer about 15 times a year.