Winter star clusters


Winter, I have been told by my non-astronomical friends, is a time for locking yourself in your house and binge watching old Full House reruns.

Well, I say unto ye, brother and sister stargazers, that winter is the best time to observe the heavens. Of course, you don’t want to lug along a lot of heavy equipment. (There’s nothing worse than unloading a half-ton telescope with a cold wind in your face and a sheet of ice beneath your feet.)

So grab your binoculars or a small telescope, put on your long, woolen undies, and come with me as we explore the two best of the open clusters in the winter sky.

Open clusters are aggregations of stars that are gravitationally bound together by their proximity to each other. Visually, they range from a loose collection of a few stars to dense collections of hundreds of stars.

Among the most beautiful of those clusters is the Hyades. To find it, look high in the southeast just after dark for the V-shaped head of Taurus, the Bull. The brightest star in the “V” is the reddish Aldebaran, which is a foreground star and hence not technically part of the cluster. Binoculars will reveal dozens of stars spread in and around the “V,” making for a spectacular view.

The stars of the Hyades look so spread out, because the cluster is among the closest to planet Earth at a mere 900 trillion miles away.

Above the Hyades is the most famous of all star clusters, the Pleiades. The “Seven Sisters,” as the cluster is sometimes called, is visible to the unaided eye as a small, dipper-shaped collection of six stars. (Why six stars have been called the Seven Sisters for thousands of years is lost in the inky depths of history.)

In binoculars, the Pleiades explodes into dozens of loosely packed stars, though not so loose as the Hyades. At 2.5 or so quadrillion miles away, it is almost three times most distant than the Bull’s head.

Open clusters occur because the gas that formed into our Milky Way is not spread out uniformly. Great clouds of hydrogen gas, which are often hundreds of light years in diameter, populate the slowly spinning whirlpool of stars that constitutes our Milky Way.

These clouds, called nebulas, are not very dense, but they are a step up, however slightly, from the emptiness that surrounds them. In the clouds themselves are lumpy. Denser portions of the gas swirl into balls, collecting more hydrogen as they go.

Eventually, and we’re talking many millions of years here, the ball gets dense enough to begin the hydrogen-bomb reaction that causes a star to erupt with intense heat and light.

Many stars can form from a single nebulous cloud. The stars are close enough to each other that they stay together, bound by gravity, for hundreds of millions of years.

One clear night long ago at one of our public programs at Perkins Observatory, I listened in on a father and son who had stepped away from the rest of the group to check out a few constellations. The son pointed out Taurus and its V-shaped head.

His small finger pushed up at the sky as he taught his father to find Aldebaran, the Bull’s red, bloodshot eye.

“You mean the orange dot?” the father asked.

I suppose that from our limited human perspective, which is so far from the tumultuous action of the stars, those heaving hydrogen bombs are indeed tiny dots.

However, appearances can be deceiving. At 65 light years distant, it is nearly four million times farther from us than our own star, the sun.

Having fused its hydrogen fuel into helium for billions of years, it has reached the end of its life. During its halcyon days, its total amount of material was probably less than three times that of the sun, putting it in the sun’s stellar ballpark.

If we want to know what the sun will look like in 5 billion years, we should look in Aldebaran’s direction.

Aldebaran’s mass may be similar to the sun’s, but it is currently 40 times wider at perhaps 40 million miles in diameter. If we removed the sun from its place in our solar system and replaced it with Aldebaran, that behemoth would extend nearly to the orbit of Mercury.

In effect, it has become a ghost of its formerly sun-like self. Over 100 million years ago, Aldebaran swelled from a sun-sized star to a much larger size, a change that took only a few million years. Since it didn’t gain any material when it swelled, its gases became exceedingly thin and spread out — a red-hot vacuum that is barely there at all. The star still produced about the same amount of energy, but that output was spread out over a much larger surface area.

Deep in its center, its rate of hydrogen burning increased significantly, causing it to swell even further over the next 100 million years. Although its energy output increased, its larger size caused its surface temperature to decrease even more. As a result, its temperature decreased from a yellow-hot, 10,000-degree fury to a relatively sedate 6,000 degrees.

Over the next 100 million years, Aldebaran will shrink and expand several times until the fuel at its core is exhausted. It will then collapse to a tiny white dwarf, an extremely dense ball of dead star no wider than our puny planet Earth.

So what, you say? The history of Aldebaran is the history of our sun. As we look at this little dot, we are looking at the future of our own speck-like Earth in the far distant future as it orbits the dot-like star we call the sun.

From the standpoint of stellar evolution, Aldebaran may be typical, but from the standpoint of human experience, it is indeed an odd bird. Present in the Hyades from our human perspective, it is not a part of the star cluster. They are separate but also inextricably linked.

In fact, Aldebaran is also associated with the nearby Pleiades in the oddest of ways.

Much has been made over the years about the number of naked-eye stars in the Pleiades. The “Seven Sisters,” as the cluster is often called, has only six naked-eye stars in it.

An old Arab story tells of Aldebaran, the Brightest star in Taurus, who was rejected by one of the sisters because he was so poor. He left the sky to pursue his fortune and became wealthy after long and arduous labor. He returned to the sky driving before him his herd of camels, which we see as the V-shaped cluster of stars called the Hyades.

However, the beautiful sister left the sky, no doubt frightened or infuriated by Aldebaran’s unwanted attention. The missing sister remains in hiding, but Aldebaran and his camels are still there, hoping against hope for her return.

The point is that every star has its story. In a real sense, the father was right. Aldebaran is really nothing but an orange dot. As with individual humans, its story is a tiny one compared to the overall history of the universe. Like specks of dust floating in a summer breeze, stars drift all-too-briefly into the light before they are lost again in darkness. For that ephemeral 10-billion or 100-million moment, they shine with indescribable glory.

Still, if I could travel back in time, I would ask the father, “If stars are only dots, then what in heaven’s name are we?”

You have plenty of time to check those glorious star clusters. They will shine in more-or-less their current configuration for millions of years. Still, I hope you will feel a sense of urgency every clear night. Odds are you won’t last as long as they will.

In this universe of ours, nothing lasts forever. As the millennia pass, these celestial jewel boxes will spin with the rest of the stars in our Milky Way around the galactic hub. As they go, their weak gravitational attraction will finally do them in, and they will slowly drift apart. But other clusters, forming as you read these lines, will take their place. Should our species survive until then, humans in a distant time will see a different galaxy, but the galaxy will remain, ever faithful, ever waiting, for humans to look up and take full measure of its undying beauty.

By Tom Burns


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

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