Watch out for Scorpius’ stinger


A couple of years back, I participated in a public star party in the parking lot of the Visitor’s Center of Glacier National Park in Montana. I asked the leader what they would have done if it had been cloudy that night. (I always gave a talk, but the Glacier observing site was ill-suited to do so.) He shrugged, and said, “There’s always tomorrow.”

I said, “Where I’m from if it’s clear, you go out. Where I live, ‘tomorrow’ might not come until next month.”

My recent observations of Comet Neowise serve as an example. The comet was beautiful from our observing site atop the Delaware Dam. “Let’s observe it again tomorrow,” said my spousal unit. As of this writing, a clear-sky tomorrow never came, and the comet is fading fast.

In fact, I have had many opportunities to talk with amateur stargazers from all over the world. I haven’t found a more dedicated lot than the ones who live in central Ohio, especially the members of the Columbus Astronomical Society.

Out west, the weather is at least predictable. They have their wet and dry seasons. Here in central Ohio, the weather forecast this time of year seems to change by the hour.

We live under some of the cloudiest skies in North America. We must battle the behemoth glow of Columbus street lights. Why is our dedication greater than those who live in, say, the nearly cloudless and lightless skies of Arizona or Montana?

Given the difficulties, it’s a pleasure to report that many astronomical objects are still visible through the central Ohio light haze. Two of the best are the clusters of stars near the stinger of the constellation Scorpius, the Scorpion.

Right now, the Scorpion sits close to the southern horizon just after dark at around 10:30 p.m. It even looks a bit like a scorpion, a tilted, elongated, capital-letter “J.” The Scorpion’s stinger curves to the far left.

Up and to the left of the stinger are the clusters in question, M6 and M7. You won’t see them with the unaided eye although the lower cluster, M7, is visible without optical aid from dark, rural skies.

In binoculars, M7 resolves easily into about 20 stars spread pretty much across the field of view. M6 looks like a little fuzzy patch just up and to the right from M7. You’ll need a telescope to resolve it into individual stars.

If you can fit both clusters into the field of your binoculars, you’re in for a treat. The contrast between the resolved stars of M7 and the unresolved haze of M6 is quite lovely.

The difference in appearance is almost purely a matter of the distance to the two clusters. Both contain about 80 stars, but M7 is much closer to us at 800 light-years away. At nearly twice the distance, M6 appears much smaller and more difficult to resolve into individual stars.

The clusters may look close together, but that illusion is created by our celestial vantage. They are actually separated along our line of sight by an unimaginable gulf — over four quadrillion miles.

Open clusters like M6 and M7 are made up mostly of brand-new, baby stars that recently formed together out of the same gigantic cloud of hydrogen. The stars in any given cluster are thus brothers and sisters formed from the same cosmic mother. If we measure their births against the vast age of the universe, they were all born at approximately the same time, just a few million or tens of millions of years apart. Stars are born together in litters, like puppies.

The open clusters of our Milky Way galaxy emerge from giant clouds of dust and gas called nebulae.

Our galaxy is, like many others, a pinwheel-shaped collection of hundreds of billions of stars. Their spiral arms curve around a dense central hub because the whole flattened disc of our galaxy is spinning.

The nebulae are mostly strewn throughout the spiral arms of our galaxy. The central hub has long since mostly converted its gas clouds into stars.

The process of star formation is long and somewhat tedious — but inexorable. The galaxy was initially a spinning cloud of dust and gas. Sections of the primordial gas and dust were a bit denser than the rest of the spinning cloud. Those denser parts had more gravitational pull, so they coalesced into the clumps of gas we have come to call nebulae.

Denser clumps of the larger cloud clumped into smaller, spinning clouds. Those smaller clouds collapsed still further into spinning balls of gas.

Deep in the cores of those spinning balls, temperature and pressure conspire to ignite the hydrogen gas in a thermonuclear conflagration that makes our hydrogen bombs look pretty pathetic by comparison. A star is born.

In a typical nebula, hundreds to thousands of young stars can be born that way. The hottest of those stars ionize the nebula’s remaining gas and dust, which causes the gas cloud to glow with its own light. The nebula has become an “emission nebula.”

A typical example of such emission nebulae is the Great Nebula in the constellation Orion, which is easily visible in binoculars from our Central-Ohio sky.

After millions of years, the hottest stars of the new cluster blow most of the leftover gas and dust into space. What remains is a group of jewel-like baby stars held together loosely by gravity.

However, the clouds are so large and the stars in them so far apart that their filial proximity is a temporary condition. The loose gravitational pull holding them into a cluster is insufficient to keep the family together. Over hundreds of millions of years, they will inevitably drift apart.

As our galaxy spins, the stars, star clusters, and nebulae wander somewhat randomly. As they gravitationally interact with each other, their motion becomes even more random.

Over many tens of millions of years, a typical open cluster will thus encounter other stars and nebulae. Such gravitational interactions mess with the gravity that holds the cluster together. Over time, the cluster’s member stars are ejected into the spiral arms of the galaxy.

There, they continue to revolve about the galactic center by themselves or in loose stellar associations called moving groups. The sun was probably part of a cluster shortly after it formed (4.6 billion years ago) but that cluster has long ago broken up. The sun now travels alone. We don’t (yet) know of any other stars that seem to have come from the same cluster.

By contrast, some of the stars of the familiar Big Dipper and the larger constellation surrounding it are part of the Ursa Major Moving Group. They share the same age and direction of motion, so we can tell that they used to be in the same cluster. That cluster has broken up, but some of the stars in it continue to travel together through space.

The sun and its planets are at present within the confines of the Ursa Major Moving Group, but we are moving in a different direction. Our wandering solar system is “jest passin’ through,” as my pappy used to say.

The fact that M6 and M7 are still together at all suggests that they are relative newcomers to our galaxy. M6 is a scant 100 million years old. Our middle-aged sun is about 5 billion years old.

You can see them for yourselves, brave Central Ohioans, but it won’t be easy. Montanans can look at the stars practically any time they want. The stars are common as dirt to them.

You must struggle a little to see the splendors of the universe. That is why you are such dedicated stargazers.

By Tom Burns


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

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