Star Watch

Night sky to show off the season’s brilliant constellation

By Alan M. MacRobert
Globe Correspondent / August 6, 2011

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The gibbous moon rides low across the southern sky these evenings, and for the next few days it is crossing the brightest constellation of the season: Scorpius, “the Orion of summer.’’

Like Orion in winter, Scorpius displays a bright, orange-red supergiant star amid a scattering of blue-white points that together make a distinct, easily recognized pattern.

In Orion’s case, it is a stick figure of a hunter, or at least his broad shoulders and legs with a canted belt on his narrow waist. The red supergiant there is Betelgeuse, marking one shoulder.

Scorpius is a curving scorpion shape, with reddish Antares for its heart. The scorpion used to be seen with big, dim claws extending to its right, but someone in ancient times decided to lop them off and make them into the constellation Libra instead.

Scorpius and Orion both stalk across the southern sky during their respective seasons, but Scorpius does so lower down. From New England’s latitudes, you are doing well ever to see the scorpion’s lower parts and stinger tail. Right after dark in early August is as good a chance as you get.

The parallels between Orion and Scorpius are not just a fluke. Both bunches of stars mark regions of space where big bursts of star formation took place in the cosmically recent past. As a result, both areas are scattered with massive young stars of the sort that burn furiously, shining with tens of thousands of times the brightness of our sun. Such super-luminaries burn themselves out in a matter of only some millions to tens of millions of years.

Although the moon helps guide your way to Scorpius for the next few evenings, as shown here, the moon’s bright glare will tend to wash the stars from good view, especially if the air is laden with summer haze. So spot what bright stars here you can, remember them, and come back to fill in the rest later in the week once the moon gets out of the way.

A star that spins and flings Scorpius is a curving scorpion shape, with reddish Antares for its heart. The middle star of the three is brighter than the other two right now, and in fact it’s second only to Antares in this whole region of the sky. And thereby hangs a tale.

The star is Delta Scorpii, also named Dschubba, from a corrupted Arabic phrase meaning “the scorpion’s forehead.’’ From time immemorial, as far as anyone knows, it always shone as a very close match in brightness to the star above it, Beta Scorpii. Then in July 2000 it changed. In the course of a few weeks it brightened substantially, until it was plainly the brighter of the two for anyone who looked up and made the comparison. It remained extra bright, with fluctuations, until 2005, then faded back to normal. Then last year it woke up again, and as of just a couple of days ago it was shining about the brightest it ever has.

What’s going on here?

Delta Scorpii is one of the young, hot, blue-white power stars in the region. Many stars of this type also turn out to be fast spinners, as astronomers discovered many decades ago by analyzing their spectra. When a star rotates, the light from the side of it that is approaching us appears slightly blueshifted, and the light from the side turning away from us is slightly redshifted. The net result is a special, distinctive type of blurring of all the spectral features in the star’s light.

In recent years, astronomers have been able to measure the actual shapes of some stars’ distant faces. And just as physics predicts, the fast spinners turn out to be oblong, rather than perfectly round. Imagine a spinning water droplet. Centrifugal force will make it an ellipse, rather than a sphere.

In some cases, a young, massive star spins so fast that, calculations show, it should be right on the edge of flying apart. If such a star is unstable internally, changing temperature and expanding and shrinking a bit, its equator will occasionally expand outward past the limit and start flying off. That, apparently, is what is happening to Delta Scorpii. But this seems to be only part of the story; other parts remain obscure.

We can be glad our sun does not do things like that.

Amateur astronomers track lots of such variable stars. In Cambridge near Fresh Pond is the world headquarters of the American Association of Variable Star Observers, which is celebrating its 100th anniversary this year. It was founded in 1911 at Harvard, with help from a popular astronomy writer, to encourage and collect careful, precise estimates of what variable stars all over the sky were doing. By Thursday afternoon its online archive contained 20,575,374 brightness measurements of thousands of variable stars over the decades, mostly made by amateurs. This data set is unique in the world, and professional researchers everywhere routinely draw on it.
