Sagittarius is constellation lovers? cup of tea

You don't have to be much of an astronomer to spot Jupiter this summer. Just look south after dark, and there it is. It's the brightest point of light in the sky, airplanes excluded. Light pollution? You can see Jupiter from City Hall Plaza, if you shield your eyes from the glare of the city.

That's what you might expect from the solar system's biggest planet. Aside from the sun, Jupiter contains more mass than everything else in the solar system put together. It's 11 times wider than Earth and contains 318 times as much stuff.

In fact, if things had gone differently back when the solar system was coming together, Jupiter now could be turning night almost into day. Astronomers have called it "the little sun that couldn't." It's a gas ball of mostly hydrogen and helium, like the sun, but it falls far short of the sun for heft. It's got only a thousandth of the sun's mass.

That matters. Jupiter's center, though very dense and hot, fails to reach the pressure and heat needed to ignite nuclear fusion and turn a gas ball into a shining star. So Jupiter just sits there, visible only by reflected sunlight, with a temperature at its cloud tops of a sorry -260 degrees Fahrenheit.

In fact it's so cold there that most of Jupiter's clouds are frozen anhydrous ammonia. That's why, when you glance up at it, Jupiter looks basically white. What you're looking at is a substance never seen on Earth, except maybe in a really nasty tank-car wreck.

If you get away from the city and its overarching dome of light pollution, more comes into view. This year, Jupiter lies in front of the constellation Sagittarius, whose brightest stars form (to modern eyes) not a Greek centaur shooting an arrow, but a stick-figure cartoon teapot.

You can see the Teapot from as close to the city as Newton or Medford, if you've got sharp eyes and look from a dark, shadowy corner away from nearby lights. Take some time and look carefully. The Teapot is about as big as your fist held out at arm's length.

Much smaller and dimmer is the little Teaspoon, barely above Jupiter. This isn't a constellation but an asterism - an informal star pattern that didn't make the cut when, in 1930, astronomers divided up the sky into 88 official constellations with borders defined as precisely as those of the 50 states. Everything else got subsumed into one or another of the 88. So the Teaspoon is part of Sagittarius.

Even harder to see is the little Saucer below the Teapot. It's so far south that it barely gets above New England's southern horizon, where it's liable to be lost in light pollution and summer haze. But with binoculars and an open view to the south, maybe you can pick it out. It's big enough to more or less fill a typical binocular's field of view.

The Saucer is a constellation of its own: Corona Australis, the Southern Crown, called that since ancient times. Supposedly, this is the crown of myrtle leaves that Dionysius placed in the heavens after retrieving his dead mother from the underworld. Greek gods were always doing things like that. To me, Corona Australis goes better as part of the summer tea set.

Amateur astronomers, whether casual skygazers or total telescopic fanatics, often make up asterisms of their own. Does that triangle-and-bar star group look a bit like Aunt Hilda's hat? In a family of skywatchers, the asterism of Aunt Hilda's Hat could pass down through generations. After all, it's not going anywhere. Centuries from now, it will still be right there for anyone who looks, come wars, revolutions, and cycles of history unimagined.

But to make up your own star patterns, you need to see a lot of stars. The cause is far from hopeless even around Greater Boston, but you'll probably do better here using binoculars to ferret out interesting little patterns that don't quite make it to naked-eye visibility.

If, however, you travel out to the dark wilderness this vacation season, do plan to spend some time learning constellations that you never normally see - and perhaps making up one or two of your own. Pass one on, and just possibly it will be what your six-times-great-grandchildren will know you for.

It worked for the ancient Greeks.

Easy-to-use maps of stars and constellations across the entire evening sky are available at SkyandTelescope.com/howto/basics.

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