Stargazing: Milky Way travels with the Local Group


Tom Burns - Stargazing



At the largest scale we know for certain, the cosmos is composed of hundreds of billions of galaxies. For the most part those “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.

From the standpoint of the whole Universe, they are tiny islands of stars lost in a vast ocean of space. A single telescope field can show scores of very distant galaxies, each one a tiny fuzzy patch. Over the decades, the estimated number of galaxies has increased from billions to hundreds of billions to trillions.

We live in one of them, of course. The Milky Way is all around us, both as the stars we see with our unaided eyes and the milky stream of light that stretches just after dark from northeast to southwest like the backbone of God. It is the chair beneath you and the room around you. It is you.

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 universe is lumpy.

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, which 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 6 trillion miles.) Five million light years is — scientifically speaking and precisely in miles — really, really big.

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

The other galaxies are smaller spirals, egg-shaped galaxies called ellipticals, and non-uniformly shaped blobs called irregular galaxies. Many of the smaller, irregular galaxies huddle around the larger ones as so-called satellite galaxies.

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. Much of it is hidden by the ground below our feet at night and the brightness of the sun by day.

In Ohio, we are also cursed by our northern latitude. We must be far below the equator to see the Milky Way’s main satellite galaxies.

The Large and Small Magellanic Clouds, as they are called, are tiny indeed. The Large Cloud is perhaps 1/100 the mass of its larger brother, the Milky Way, and each galaxy has perhaps only few hundred million stars in it. They are irregular in their shape, torn and twisted as they are by the enormous shifting and gravitational force of the Milky Way as it spins slowly nearby.

If you want to see a whole galaxy all at once, then 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. M31, as it is also called is easily visible in binoculars or to your unaided eye from a dark, rural location as a faint streak of light in its namesake constellation.

Of course, if you want to avoid all the trouble of finding it, you can come to one of our Friday-night programs at Perkins Observatory. We’ll show you.

In a telescope, you can begin to see some detail. Nearby, for example, are Andromeda’s two main satellite galaxies, M32 and M110.

Note the similarities. Both the Milky Way and the Andromeda galaxies are spirals with two main satellite galaxies. You are seeing the galaxy approximately the way your own galaxy would look if you were on a planet orbiting a star in it and looking toward the Milky Way.

Andromeda is tilted with respect to our Milky-Way vantage at about a 45-degree angle. Its round spiral is thus reduced to a fattened streak of light.

The center of the galaxy is about 2.5 million light years distant. One light year is equivalent to about six trillion miles. Those of you who like big, meaningless numbers can multiple it out of you wish.

Not counting the Milky Way’s puny satellite galaxies, that makes Andromeda the closest large galaxy to our Milky Way.

Because of its great distance and because its light took 2.5 million years to get to you, you are seeing the galaxy the way it looked 2.5 million years ago. We can never see the universe the way it looks now. From the standpoint of our perception, now is an illusion.

But wait. There’s more. Because of its tilt, the far edge of its spiral is considerably farther away than the close edge. That means that you are seeing the close edge a bit less than 100,000 years before you see the far edge. We can’t even see Andromeda the way it looked at some moment in the past. It is as if you looked at a human and saw her head as she looked at 30 years old and her feet the way they looked at 31.

The light you see is forever in your past. 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 rest 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 void 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.

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Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.