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The Boston Globe

Metro

Astronomical icons decorate the eastern sky

Night comes early now, and it will come even earlier after daylight saving time ends Sunday. But the most interesting part of November’s starry sky does not wheel up into view until somewhat later.

Go out about two hours after nightfall (meaning 8:30 Saturday, 7:30 Sunday and after), and you will spot big, bright ­Jupiter glaring low in the east. It is 35 light-minutes from Earth, almost as close as it gets all year.

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To Jupiter’s right, look for the orange firespark of the ­giant star Aldebaran, 65 light-years away. To Jupiter’s left by a bit more is the icechip-white sparkle of El Nath, 130 light-years distant.

Above Aldebaran sparkle the Pleiades, the sky’s most striking star cluster. Since ancient times they have been called the Seven Sisters, though most people can count only six. The Greeks had a legend that one of the sisters faded. But a legend is all it is. Telescopes reveal hundreds of faint Pleiades, but modern astro­physics finds not one of them that could have been brighter in the last few thousand years. The Pleiades cluster is 410 light-years away and about 14 light-years from side to side.

Jupiter and its companions rise higher, into better view, as the evening grows late.

As so often in astronomy, there’s a remarkable human tale here.

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But other astronomical icons are already well up in view as soon as darkness falls. Face northeast and look very high for W-shaped Cassiopeia. The W is standing more or less on end. It looks a lot bigger in the sky than on the drawing here, which is a much-reduced view of the whole panorama, from the horizon to almost overhead.

Below and a bit right of ­Cassiopeia is a lesser-known constellation that is still in the top tier famewise: Perseus, a legendary Greek hero who rescued Queen Cassiopeia’s daughter Andromeda from the sea monster Cetus. Aside from its bright star pattern, Perseus contains two landmarks that anyone getting into astronomy quickly finds out about.

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One is the Perseus Double Cluster. Its position is marked here, but do not expect to see it with your unaided eyes unless you are way out in the country under a dark, natural night sky. From such a place, the Double Cluster looks like a fluffy little enhancement of the glowing Milky Way, which runs through Perseus and Cassiopeia.

Even in light-polluted Greater Boston, however, binoculars show a pair of faint glows at the spot indicated. A good amateur telescope shows grand twin cities of stars, each richer than the Pleiades.

Winking star

Also in Perseus is Algol, the constellation’s second-brightest star, usually. Algol dims down to a third of its normal brightness every 2 days, 20 hours, 57 minutes, and 25 seconds, like clockwork. It takes several hours to fade down, stays at minimum brightness for a couple of hours, then takes several more to rebrighten.

As so often in astronomy and science, there is a remarkable human tale here. Algol was one of the first variable stars discovered, by a stargazing teenager in 18th-century England. Born into a progressive, well-to-do family, John ­Goodricke lost his hearing at age 5 due to scarlet fever. In an era when the deaf were often shunned and shut away, John’s parents educated him well, and he became proficient in science. As a teenager, he not only ­examined the stars carefully enough to track a few of them changing brightness from day to day — something no one in human history had ever done systematically — he also published theories of why they did.

His theory for Algol turned out to be spot on. What is happening is that two stars, one bright and one dim, are closely orbiting each other with that clocklike period of a little over two days. Once per orbit the dim star passes in front of the bright one from our viewpoint, covering most of its face. Algol thus became the prototype of the “eclipsing variable” stars.

Algol’s next dimmings visible in evening hours for our part of the world will be centered on 10:45 p.m. Nov. 19 and 7:34 p.m. Nov. 22. But that’s a spoiler. If you glance up at Algol on a clear night, you have a 1 in 25 chance each time of catching it fainter than half its normal brightness.

How do you judge this? Compare Algol to Gamma Andromedae to its upper right (by roughly a fist-width at arm’s length) as shown here. Normally, the two are alike. When Algol dims, the difference is obvious.

You wonder why no one anywhere, for thousands of years, had ever noticed Algol’s goings-on, or at least never left a ­record. The answer has to be that people just do not pay atten­tion to most of what’s around them.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (Skyand
Telescope.com). His Star Watch column appears the first Saturday of every month.

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