This month I’m revisiting an old — very old — favorite: the Andromeda Galaxy, or M31 in French astronomer Charles Messier’s catalogue of 110 nebulae and star clusters. He was a comet hunter and made the list to keep track of objects that looked like comets but weren’t.

M31 is a barred spiral galaxy like our own that is just barely visible to the naked eye as a faintly glowing smudge larger than the full moon. From mid-northern latitudes, like Connecticut, you can see the galaxy for at least part of every night of the year.

This 9-billion-year-old galaxy is visible even in places with moderate light pollution, even though it’s 2.5 million light years away. Its 1 trillion estimated stars help with that. It’s also about 260,000 light years across, meaning it takes more than a quarter of a million years for light from one end of M31 to reach the other.

If possible, I recommend downloading a free sky map app like Google Sky for your phone, or a free program like Stellarium for your computer, to help you figure out where you should be looking.

Except for the Large and Small Magellanic Clouds, which can only be seen from the Southern Hemisphere, the Andromeda galaxy is the brightest galaxy we can see, and it’s the most distant thing you can see with your own two eyes.

A fairly simple way to find this galaxy is to look for the W-shaped constellation Casseopeia in the northeast. The right side of the W forms an arrow pointing directly to the Andromeda Galaxy, which lies about the length of Casseopeia away.

Our galaxy is actually on a collision course with M31. They’re traveling toward each other at about 75 miles a second. In 4 billion years or so, Andromeda and the Milky Way will collide.

The center of M31 has 26 possible black holes, and many more have been discovered by NASA’s Chandra X-ray Observatory. Like our own galaxy, there’s also a supermassive black hole at M31’s center.

bymeljohnson@gmail.com

June 15: The moon arrives at the farthest possible point in its orbit from Earth, called apogee.

June 18: The moon and Saturn form a pair in the sky, as do Mercury and Aldebaran, an orange giant star just 65 light years away.

June 20: Full moon and June Solstice. This is the first day of summer in the Northern Hemisphere and the first day of winter in the Southern Hemisphere.