- Artist’s conception of the spiral structure of the Milky Way with two major stellar arms and a central bar. “Using infrared images from NASA’s Spitzer Space Telescope, scientists have discovered that the Milky Way’s elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms. Courtesy Spitzer Space Telescope/R. Hurt/Wikipedia.
Welcome to my Exploring Galaxies page that accompanies the November 10, 2011 edition of 365 Days of Astronomy. Galaxies are gravitationally bound systems of stars, nebulae, and an interstellar medium that contains gas and dust. We live in a galaxy called the Milky Way. Astronomers have defined our galaxy as a barred spiral, looking roughly like the image here.It has an estimated 200 to 400 billion stars, plus countless planets, nebulae, and clouds of interstellar gas and dust. There’s also a starry-rich core that also contains a supermassive black hole (at least one), and a swarm of globular clusters that seem to buzz around the core like bees near a hive.
- The elliptical galaxy M89. Courtesy NOAO.
Galaxies also come in other shapes and sizes, such as ellipticals and irregulars. Ellipticals have no spiral arms, many have black holes at their hearts, and are quite tightly packed with stars.
Irregular galaxies look like undefined shreds, masses of stars without any regular shape, and no spiral arms.
- The dwarf irregular galaxy NGC 4214, as seen by Hubble Space Telescope. Courtesy STScI.
The best-known examples of irregulars are the Large and Small Magellanic Clouds, visible to people who live in the Southern Hemisphere. They lie more than 170,000 light-years away and are actually being swallowed up by the Milky Way. They aren’t the only little galaxies being cannabalized like this. Several other “dwarf” galaxies are being pulled into our own galaxy by its tremendous gravitational attraction.
- A simulation of structure in the universe. The dark voids are filled with dark matter. Courtesy Harvard CfA.
Galaxies exist in clusters, and those clusters are grouped into superclusters. When you look at the distribution of galaxies in the cosmos, the whole thing looks like a giant web, a network of light. The dark areas are voids, but instead of being empty, they are actually thought to be filled with dark matter.
To learn more about the formation and evolution of galaxies and large-scale structure in the universe, visit the following pages:
If you’d like to try seeing a few naked-eye galaxies, the easiest ones to spot are our own Milky Way (from the inside), the Andromeda Galaxy, and the Large and Small Magellanic Clouds (southern hemisphere). The Milky Way is best viewed during the summer, but you can see it other times of the year. In November, for example, it stretches directly overhead through the constellations Cassiopeia and Cygnus. It will look like a faint cloud of light stretching from the east-northeast to the west-southwest.
While you’re looking at the Milky Way, look for the Andromeda Galaxy. It is a faint smudge of light lying about halfway between the W-shaped Cassiopeia and the Great Square of Pegasus. To help you find both the Milky Way and Andromeda, download the ever-useful star charts from Skymaps.com (PDF download).
If you’re in the southern hemisphere, the Large and Small Magellanic Clouds easy to spot in the southern part of the sky. To help you find them, Skymaps.com also has a southern-hemisphere edition of their charts.
If you have a telescope, then you can scan the sky for other galaxies too faint to see with the naked eye. Most good starcharts will have a few of them listed, so it’s a matter of guiding your telescope to the right place. Galaxy-hunting is a fascinating activity, and if you stick with it, you’ll find yourself rewarded with views of galaxies of nearly every shape and size. And, while you’re at it, you’ll be observing some very crucial parts of the history of the universe and the development of structure in the cosmos!