The texture of the universe


By Tom Burns - Stargazing



Back before I retired from Perkins Observatory, I was talking one fine day about the universe to a group of fourth-graders at one of our daytime programs. The universe, I intoned, is made of galaxies, enormous collections of hundreds of billions of stars separated from each other by brain-melting distances.

“Yeah,” said one of the students, “but what’s the universe like?”

“Hmmm, er, ah,” I replied in utter confusion. The question is, after all, a perfectly natural one. Of any part of the universe — the surface of the sun, a rock, a chicken leg — we naturally ask the same question. When we ask it about a chicken leg, we want to know its flavor and texture. How then do we do the same thing about the universe?

So I replied in my most profound, sonorous voice, “The universe is (a drumroll, please) lumpy.” Lumpy. Right.

On the level of our solar system, the point seems obvious. The sun is the main lump, because it takes up more than 99 percent of our solar system. Around it, at vast distances from one another compared to their diameters, are tiny lumps we have come to call planets.

Even an area of supposed relative density like the asteroid belt is made up mostly of empty space. You could travel again and again through the asteroid belt and never encounter an asteroid. Even the asteroid belt is really made up mostly of a whole lot of nothing.

Our sun is a star, of course, and there are perhaps 300 billion such stars in our Milky Way galaxy, the texture of which constitutes the next level of lumpiness. That seems like a whole lot of stars, but they are spread out over a flattened, spiral disk 100,000 light years wide. One light year is about six trillion miles for those of you who like really big, meaningless numbers.

For those of you who don’t, consider the distance from our sun to the nearest star, Proxima Centauri. At about 4.25 light years, it seems pretty close. At 25 trillion miles, it seems incomprehensibly far away.

Perhaps a scale model would help.

The sun is about 900,000 miles wide. Earth is about 8,000 miles wide. Astronomers like to say that you could put Earth inside the sun a million times.

There they go with those big, meaningless numbers again, so let’s try this: If the sun were the size of a volleyball, Earth would be the size of a small pinhead.

Now let’s tackle that 25 trillion miles from our sun, a volleyball, to Proxima Centauri, another volleyball. In the scale mode, our two volleyballs would have to be about 5,000 miles away from each other. If we placed one of them in central Ohio, the other one would have to be in Hawai’i with practically nothing in between.

Furthermore, our sun and Proxima are relatively close to each other by galactic standards. On average, the stars of the Milky Way are seven light years apart, over 40 trillion miles away from each other. Even in the densest parts of globular clusters, among the densest parts of our Milky Way, the stars average about one light year apart. Even in the most compact parts of the Milky Way, we still measure the distance from stars in trillions of miles.

Some stars are pretty empty themselves. If you were inside a red-supergiant star like Betelgeuse in the constellation Orion, you would be very hot at about 6,000 degrees Fahrenheit. Other than that, it would seem like you were in what amounts to empty space. Stars like Betelgeuse have been described as “red-hot vacuums.” However, compared to the emptiness and coldness of space, they are still lumps.

On the other hand, some of the starry lumps are very dense. When the sun dies in five or six billion years, it will collapse to a tiny, dead star called a white dwarf. A single teaspoon of that white dwarf will weigh about seven tons.

From the standpoint of tiny Earth and its even tinier inhabitants, the next order of lumpiness, the galaxies, is vast beyond comprehension. We are part of one of those lumps, of course, a lump within a lump. The Milky Way is all around us, both as the individual stars we see and the milky stream of unresolved stars that stretch just after dark from northeast to southwest.

From the standpoint of the whole universe, 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.

Of course, if we can find a galaxy relatively close to us, it will look much better — like a big fuzzy patch. In that regard, the Andromeda Galaxy is November’s choicest target.

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 second set of two stars. (You are now looking at stars in the constellation Andromeda, which is attached to Pegasus.) The two stars in Andromeda point upward to the Andromeda Galaxy, which is easily visible in binoculars — or to your unaided eye from a dark, rural location.

The distribution of such galaxies is not uniform. Astronomers find clumps of galaxies tied together by loose bonds of gravitation. They travel together like celestial wayfarers.

The Milky Way travels in a clump called the Local Group, a sparse collection of 30 or so galaxies.

The Local Group galaxies are scattered like dust in a darkened room over a three-dimensional void 5 million light years, or 30 quintillion miles, on a side.

I know. I know. These distances are numbers are again meaninglessly huge, but the Local Group still constitutes a lump.

The galaxies of our Local Lump, if I may call it that, 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 lump. Both are spiral galaxies, pinwheel-shaped collections of perhaps 300 billion stars each. The rest of the Local Lump consists of smaller spirals, egg-shaped galaxies called ellipticals, and non-uniform blobs called irregular galaxies.

And the local group is itself part of a larger lump, We hover more or less out on the edge of a supercluster of thousands of galaxies called the Coma-Virgo Cluster.

And the galactic clusters and superclusters themselves are arranged in great streamers of clusters that curve around into bubbles of millions of galaxies. Astronomers often compare the general structure to the head of foam on top of a glass of beer, which shows us, I suppose, where astronomers’ heads are at after work. It also shows us what is inside those bubbles: great voids of practically nothing.

Like I said — lumpy.

When we marvel at the night sky, we are marveling at the lumps. The brightest galaxy we see is the Milky Way. We are, after all, in it. But we can only see parts of it at a time because it stretches around us in all directions.

That level of lumpiness is much easier to see with our own two eyes if we look beyond the Milky Way to our nearest galactic neighbor, the Andromeda Galaxy. It fits nicely in a low-power binocular field.

However, its true glory is lost to us. The Andromeda Galaxy is 2.5 million light years away. The light we see took over two million years to get from the galaxy to our eyeballs.

In effect, we can never know what the universe is like now. As far as our perception of the cosmos is concerned, “now” does not exist. “Now” is separated from us by an unconquerable void — not just of space but also of time.

Even so, take a look at the Andromeda Galaxy if you want to know in the deepest and most secret recesses of your heart what the universe is like.

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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.