Our neighbor the Andromeda Galaxy shines overhead this week. Here's how to see it

"In 1976, Janet and I signed up for an 'astronomy cruise' to Bermuda. We were on deck one night where Fred Hess, a planetarium expert, pointed out the star patterns visible to the naked eye. Janet used her binoculars to look at the objects described, and the high point came when we saw the Andromeda Galaxy for the first time. If the cruise had ended at that point, we would have had our money's worth."

 —   Isaac Asimov, from "In Joy Still Felt" (Doubleday & Company, 1980)

There was a good reason why the famed science writer Isaac Asimov and his wife Janet were jubilant on that night at sea. For they had just had their very first look at the most distant object that can be glimpsed with the unaided eye: the Andromeda Galaxy.

And this week, with the bright moon having left our evening sky, you too will have a chance to see this most amazing of deep sky objects, which will be passing almost directly overhead between 7:30 and 8 p.m. local time.

Related: Night sky, November 2023: What you can see tonight [maps]

Where to look

To find the Andromeda Galaxy, first locate the Great Square of Pegasus — a landmark of the autumn sky. Then, focus binoculars on the bright star Alpheratz, which is at the upper left corner of the Square. Then move straight across to the east (left) and get the star Mirach (in Andromeda) in your field of view. From there, move slowly up to a fairly bright star above Mirach and continue to run up in the same direction until you'll find the "little cloud" described by Al-Sufi, more than a millennium ago. 

That will be your stopping place, for you will have found the Andromeda Galaxy. 

If you aren't familiar with these stars or the Pegasus constellation, you could always use a stargazing app to help you find the Andromeda Galaxy  —   but put the phone away as soon as you've located it to let your eyes adjust to the dark night sky to ensure they can take in as much light from this distant city of stars as possible.

Recall that the Asimov's had the advantage of being located on shipboard in the middle of the Atlantic when they made their Andromeda Galaxy sighting; not much concern about light pollution out there! But in order for you to see it requires good eyesight and a dark, crystal-clear night with no street or house lighting nearby. 

With the unaided eye it appears as nothing more than an indefinite, mysterious glow; a diffuse elongated cloud perhaps two or three times the apparent width of the moon. 

The "Little Cloud"

The Andromeda Galaxy was supposedly first noted by the Persian astronomer Abd-al-Rahman Al-Sufi, who described it as a "little cloud" in his "Book of Fixed Stars" in 964 A.D. But it may also have been known to Persian astronomers in what is now Iran as far back as 905 A.D., or even earlier. An expert on star nomenclature, Richard Hinckley Allen, once reported that it also appeared on a Dutch star map from the year 1500.

Galileo's rival, Simon Marius is usually credited with the first telescopic observation of this object in December of 1612. He described the nebula as an indefinite glow "like a candle shining through the horn window of a lanthorn (lantern)." 

A tremendous city of stars

Even today, binoculars and telescopes reveal this "cloud" as little more than a smooth oval blur, which gradually brightens in the center to a star-like nucleus. While it will certainly look larger and brighter than with your eyes alone, there is little to suggest the grandeur of this object as it is often shown in long exposure observatory photographs. It's oval because from our vantage point we're viewing it not far from edgewise, but in fact, it's a nearly circular, flat spiral assemblage of star clouds. 

The light from that "little cloud" is actually the total accumulation of light from approximately one trillion (1,000,000,000,000) stars. It is listed as Messier ("M") 31, in Charles Messier's famous catalog: hazy objects resembling comets, but later proved to be galaxies, nebulae and star clusters. 

Here is the most distant object that can be seen with the unaided eye. M31 has been estimated to be nearly 200,000 light years in diameter or one and a half times as wide as our own Milky Way galaxy. Its bright nucleus is the hazy patch that is visible to the unaided eye. 

Like our own galaxy, M31 has several attendant satellite galaxies. Two of these: M32 and M110 can be picked out with low magnification in a small-to-medium sized telescope, in the same field of view as M31. There are yet two other smaller companions (NGC 147 and 185) which are much fainter and placed much farther away, close to the border of nearby Cassiopeia. 

Starlight that traveled a long way

As you look at the Andromeda Galaxy tonight, you'll be doing something that no one else in the world except a stargazer can do; you will actually be looking back into the distant past. 

There is a very good reason that this patch of light appears so very faint to the naked eye. When you see it tonight, consider that this light has been traveling some 2,500,000 years to reach you, traveling all that time at the tremendous velocity of 671 million miles (1.08 billion km) per hour. The light you are seeing is around 25,000 centuries old and began its journey around the time of the dawn of human consciousness. The light you are now getting is at least 480 times older than the Pyramids; the distance it has traveled is so inconceivable that even to write the number of miles seems all but meaningless. 

When it began its nearly 15-quintillion (15,000,000,000,000,000,000)-mile journey earthward, mastodons and saber-toothed tigers roamed over much of pre-ice-age North America and prehistoric man was struggling for existence in what is now the Olduvai Gorge of East Africa. 

And this is actually just a neighbor of ours. With large observatory telescopes, we have observed galaxies that are over a billion light years away, or more than 400 times farther than Andromeda! 

Then consider those galaxies that the James Webb Space Telescope is currently detecting. Light left some of them well over 13 billion years ago, within just a few hundred million years of the Big Bang. 

Cosmic collisions of the past ... and future

Recent studies indicate that about six billion years ago, the Andromeda Galaxy was breached by another large spiral galaxy. After several billion years, this intruder looped around Andromeda and finally smashed into its core and caused it to expand. Andromeda's satellite galaxy, M32 — a small, compact elliptical galaxy — is thought to be the core of the renegade galaxy that collided with Andromeda. Initially it was itself probably a spiral galaxy, whose arms were stripped off by Andromeda's gravity. 

Interestingly, Andromeda is approaching our own Milky Way galaxy at a rate of 186.411 miles per second (300 km/s) and a galactic collision between the two is now anticipated to occur in about 4.5 billion years. Based on current calculations there is a 50% chance that in such a merged galaxy, our solar system will be swept out three times farther from the galactic core than its current distance. There is also a 12% chance that the solar system will be ejected from the newly merged galaxy sometime during the collision.

This is all a moot point so far as life on Earth is concerned. In about 0.5 to 1.5 billion years the sun's luminosity will have risen by 35% to 40%, likely initiating a runaway greenhouse effect on our planet. As a consequence, the surface of the Earth will have already become far too hot for liquid water to exist, ending all terrestrial life by the time the two galaxies collide. 

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications.