Local group of galaxies


By Tom Burns - Stargazing



The best way to get a sense of humanity’s place in the universe is simply to go outside and look at the sky in late summer and early autumn. Of course, the universe is somewhat obscured by the glow of city lights. Why not take a late afternoon drive out to the country to watch the leaves change and stay there until the universe appears in all its glory? Don’t forget to take along binoculars or a small telescope for an even better view.

At the largest scale we know of, the cosmos is composed of trillions of galaxies. These “island universes” are shaped like eggs in some cases and like flat pinwheels in others. They contain hundreds of billions of stars each. From the standpoint of Earth and its puny inhabitants, they are vast beyond comprehension.

In fact, the farthest object that a typical unaided human eye can spy is a galaxy. You can see it for yourself if you live outside the city limits or are willing to drive far enough away from Columbus to escape the glow of the streetlights.

From the standpoint of the whole universe, even such enormities as galaxies are tiny islands of stars lost in the vast cosmic ocean of space. A single telescope field can show scores of very distant galaxies, each one a tiny fuzzy patch.

We live in one of them. The Milky Way is all around you as every star you see with your unaided eyes, as the milky stream of light that gives your galaxy its name, as the ground beneath your feet, and as you.

As twilight turns to darkness, look up with the binoculars you were born with, your own two eyes. The sky above glows with a strangely convoluted band of silvery light stretching from the southwest to the top of the sky and then to the northeast.

You are looking at the distant stars of your own Milky Way galaxy, our cosmic neighborhood. Our sun is only one of the Milky Way’s 300-400 billion suns, many of which are contained in the milky band you see. A simple set of binoculars is all it takes to make that band explode into countless stars.

The Milky Way is shaped much like an Olympic discus, wider at the center and taped at its edges. This cosmic discus is 150,000 light-years from side to side. (One light-year is equal to about 6 trillion miles.)

Turn on a flashlight at one end, and the light takes 150,000 years to get to the other side. Turn on a flashlight and aim it at the moon, and the light takes a hair more than a second to travel the distance there. Compare one second to 150,000 years and you’ll begin to get a sense of the Milky Way’s enormity.

You live near the edge of the Milky Way. As you stare into it, your vision of the larger universe is obscured by the glow of countless stars and clouds of dust and gas out of which stars are born. To get a sense of the lumpy texture of the rest of the universe, you must look away from the densest part of the Milky Way.

Yes, that’s right. The universe is lumpy, or to put it more precisely, the distribution of galaxies in the cosmos is not uniform. Astronomers find clumps of galaxies tied together by loose bonds of gravitation and traveling together like celestial pilgrims.

The Milky Way travels in a clump called the Local Group, a sparse collection of 30 or so galaxies. It is a poor cluster indeed compared to the richness of more distant groupings. They can contain hundreds, even thousands of galaxies. Their galaxies are sometimes so close together that gravitational wars abound and larger galaxies steal stars from their smaller, weaker compatriots.

The galaxies in the Local Group have some room to move. They are scattered like dust over a three-dimensional void perhaps 5 million light-years wide. (One light-year is about six trillion miles. Five million light-years is — scientifically speaking and precisely in miles — really, really big.)

The galaxies of our group vary in size and the number of stars they possess. Two large galaxies, our own Milky Way and the even larger Andromeda Galaxy, dominate the group. Both are spiral galaxies, shaped like children’s pinwheels of perhaps 300 billion stars each.

The other galaxies consist of smaller spiral galaxies, egg-shaped galaxies called ellipticals, and non-uniformly shaped blobs called irregular galaxies. The smaller galaxies are usually satellites of the larger ones. In other words, they are gravitationally bound to Andromeda and the Milky Way. As a result, they are often distorted —torn apart by the larger galaxies’ powerful gravity.

The brightest galaxy we see is, of course, the Milky Way. We’re in it, after all. But we can only see parts of it at a time because it stretches around us in all directions.

If you want to see a whole galaxy all at once, then the other massive member of the Local Group, the Andromeda galaxy, is the target of choice, and now is the best time to see it.

Look toward the east just after dark toward the constellation Andromeda. Look first toward the southeast and about half way up the sky for the Great Square of the constellation Pegasus. Find the leftmost star of the square and sweep to the left into the constellation Andromeda. The galaxy is easily visible to your unaided eye from a dark, rural location. After you’ve seen it with your naked eyes, check it out in binoculars or a small telescope at the lowest possible magnification.

Granted, the experience is, ahem, subtle. Around 964 CE, the Persian astronomer Abd al-Rahman, the first writer to describe the Andromeda Galaxy, described it as a “nebulous smear.” In 1611, Simon Marius, the first person to see it in a telescope, referred to it as “the light of a candle shining through a horn.”

The galaxy’s oval shape is caused by the angle at which we are looking. From the top, it would look round. From the side, it would look like a toothpick with a bulge at the center. From somewhere in between it looks vaguely cigar-shaped.

You are seeing an object 2.5 million light-years away. (One light-year is equal to about 6 trillion miles.) It took a mind-numbing 2.5 million years for the light of M31 to travel down to your eyeballs. In effect, you’re seeing the galaxy the way it looked 2.5 million years ago.

The true glory of this spinning pinwheel of billions of stars is forever beyond your ken. You are not experiencing the galaxy, but its ghost, its dim specter. The “real” galaxy is separated from you by an unconquerable void of space and time.

Take a look anyway. You are looking at the farthest thing you can see with your unaided eye. You are looking directly into the great nothing and seeing the texture of the universe. In one magnificent and futile effort, you are allowing your reach to exceed your grasp. And that, among all the qualities that define us as a species, is what makes us uniquely and gloriously human.

It may be hard to believe that a “faint fuzzy” like the Andromeda Galaxy is made up of around a trillion stars, but at these distances, looks can be deceiving.

In fact, Andromeda looks a bit like your own Milky Way Galaxy would look if you were sitting on some planet in Andromeda and looking back in our direction. Actually, the Milky Way would be a tad smaller and fainter. M31 is at least 200,000 light-years wide, compared to only 150,000 light-years for our galaxy.

The two galaxies have about the same mass, the accumulated amount of stellar material and gas they contain. Despite that similarity, M31 contains as many as a trillion stars, 2–3 times the number in our Milky Way. Into the enormous gulf between the two could be placed end-to-end over 12 Andromeda galaxies and more than 16 Milky Ways.

If the distance and size of M31 boggle your mind, consider the trillions of other galaxies that populate the universe. Except for a few tiny dwarf galaxies that barely qualify as galaxies, M31 is the closest galaxy to us. Those other trillions of galaxies are much farther away. Mind still boggling? Excellent! An occasional boggle is good for the soul.

Stare upward at these things just once in your life. Stare long enough and you will know humanity’s place in the universe. Compared even to the smallest star, you are a tiny, fragile creation.

Your body is small, but your mind measures out the universe as quick as thought. Perhaps you will come to know that you were given eyes to see these things. You were given a mind to share in the vastness of the universe.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.