Stargazing: Training your eyes and heart to see, Part 2


By Tom Burns - Stargazing



As you may recall, last week we looked at what a beginner might do when trying to learn to use a new telescope or a set of astronomical binoculars.

We discussed how to find it in the constellation Gemini. Then we examined what it looks like in those instruments.

The next step might be to go to a book or the Internet and find out a bit about the star cluster.

M35 is the 35th object in Charles Messier’s catalog of fuzzy things. Messier was searching for comets and got annoyed with all the patches of light that were not comets but looked like them. He created a list of these “non-stellar” objects.

The credit for M35’s discovery goes to Philippe Loys de Cheseaux in 1745-46. Using a small telescope, he saw a hazy patch that really did look like a comet. By 1864, telescope technology had improved enough that Messier resolved it into a “cluster of very small stars.” Today, even a small, department-store telescope reveals 10 or so bright stars woven into a milky cloud of light.

M35 is several hundred stars crowded in a region about 30 light years (180 trillion miles) wide. It is perhaps 2,800 light years distant. The cluster was born only 80 million years ago, or so the Internet says, a snap of the fingers compared to our 5-billion-year-old sun.

Most of the stars are hot and blue, meaning they are newly formed from the enormous hydrogen cloud that gave them birth.

A few of the stars are yellow or orange. They are giant stars that have already reached maturity and beyond. Having lived a quick and violent life, they will soon perish.

And now, armed with knowledge, we return to our direct observation.

In the telescope, the star cluster looked at first like a ball of fuzz. We now know that the hazy patch consists of unresolved stars just beyond your eye’s ability to resolve them. Look slightly to the side of the patch and catch it out of the corner of your eye. (Astronomers call this technique “averted vision.”)

Slowly, as your eye adapts, more and more faint stars will pop out of the haze. You might see as many as 50 stars in one of the most stunning views the sky has to offer.

Further examination will reveal even more. As your reading suggested, you’ll notice that most of the stars have a slightly bluish cast. A few look distinctly red or orange. That simple piece of information allows us to estimate the age of the cluster.

Young stars exist in clusters because they are born in enormous clouds of gas and dust called nebulae. Stars are balls of exploding hydrogen that condense by gravity from a given cloud. The stars in a cluster often formed within a few million years of one another as the denser parts of the cloud collapsed into spinning balls.

A nebula starts out tens of trillions of miles wide. The stars in it might look close to one another, but they are far enough apart not to be bound by one another’s gravity. Eventually, the gravity of the other stars of the Milky Way will draw them apart. For now, they still are young enough still to be grouped together.

But how young? To find out, we must ferret out the evidence that our observations provide. Really young clusters still have vestiges of the gas that formed the stars surrounding them like a cocoon. Here, it is helpful to look at an image of M35 taken with a large telescope.

No such gas is visible in M35, so it must be getting on in years. However, stars early in their lives tend to burn with a hot, blue flame, and blue stars dominate M35. Still, a few reddish stars as visible, and that’s a sign that some of its stars have reached old age.

We conclude that M35 is young, but not in its infancy. Very large stars reach old age prematurely in a few tens of millions of years and cool to a rather tepid red. The number of red stars in M35 indicates that it must be about 80 million years old. A snap of the cosmic fingers ago, as dinosaurs ruled our planet, the gas of a nebula coalesced into this star cluster.

Learning to see is more than a trick of the eye. When you know at last what you are seeing, when you learn the meaning hidden in the light of the stars, only then will you be transported to distant worlds.

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By Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.

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