If you ever get out away from city lights and just stare up at the night sky, you might start noticing more than just the pinpricks of lights of individual stars. You may see smudges of light across the sky, and you may need to use your averted vision (where you glance carefully off to the side of the object instead of looking right at it) to see some of the farthest objects from us. Or, you might take a look at that giant smudge slashed across the sky and marvel at it, wondering what exactly you’re seeing.
As many schoolchildren delight in knowing, we live in the Milky Way Galaxy. It’s so-named because the ancients believed the band of glowing light was actually milk, spilled in the heavens by the gods.
We’ve known for a while that the Milky Way is one of three kinds of galaxies, called a spiral galaxy. I’m sorry to say that one of the best ways to describe it is to think about flushing a toilet, and watching the water swirl around the center. The waves of water you see are the spiral arms, where most of the stars and solar systems exist, and in that center is the supermassive black hole whose gravity we rotate around (and where things disappear into, hopefully never to return).
We live about ⅔ of the way out from the center of our galaxy, on a spiral arm. That bright band of light and dark in the night sky is what we see when we look toward the center of our galaxy, at the billions of stars contained within it. These stars are so far away, on the opposite side of our galaxy, that we can’t quite make them out as individual points of light. But there are so many of them that we still see the glow they make in combination.
Other galaxies that we can see with just our eyes similarly suffuse the area with a glow that signals the presence of billions of stars too far away to make out individually. About 72 percent of the galaxies we’ve studied are spiral, like ours, but there are two other types of galaxies out there that we’ve observed.
Elliptical galaxies are, well, elliptical in shape. They can be fairly round, or much more oval depending on the galaxy. The largest galaxies we know of are of this type, and they also tend to be the oldest, we believe. They usually have older stars and less gas to form new stars, and may have been spiral galaxies at an earlier part of their evolution, before being pulled apart by galaxy collisions. In fact, this may be the future for our own beautiful spiral — we’re scheduled to collide with our nearest neighbor galaxy, the Andromeda Galaxy, in just a few billion more years.
The final type of galaxy is an irregular galaxy, and any type of galaxy that doesn’t fit into one of the previous two categories is kind of shunted into this one. They don’t really have a distinct form, usually because other nearby galaxies have asserted their gravitational influence on them. About 20 percent of galaxies we’ve studied are irregular, including the Large and Small Magellanic Clouds visible in the southern hemisphere.
Note that not all of the smaller smudges you see in the night sky are galaxies. They can also be nebulae, where stars are born or die, or star clusters (which are exactly as they sound), groups of densely packed stars clustered together. But it’s hard to mistake that big, beautiful Milky Way as anything other than the neighboring stars of our galaxy.
LOOKING UP THIS WEEK: Mars is now alone in the western evening sky. In the predawn mornings, look for a bright Venus with Saturn above it, rising as Venus sinks toward the sun. Jupiter will remain above both planets, brighter than Saturn, but outshined by Venus in that southeast sky before sunrise. The moon is currently a waning gibbous and will be third quarter on Wednesday.
Marisa Stoller can be reached at mcorley@norcaldesigncenter.com or on Twitter @MarisaStoller.
