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    Star Watch

    3 points of light in night sky make for summer wonder

    Maybe you noticed it on fireworks night. High overhead after dusk, a bit toward the southwest, shines a bright star tinted like a drop of ginger ale. That’s Arcturus, the nearest orange giant. Don’t confuse it with Vega, very high toward the east, equally bright but icy blue-white. The colors are subtle but there if you look.

    Very far below Arcturus, two more bright points await your view: the planet Saturn and, to its lower right, pale blue-white Spica. They form a long, very tall triangle. To astronomers, this bright summer threesome symbolizes three cutting-edge trends on the forefront of astronomy.

    Saturn is the second largest planet in our solar system and the farthest that is easily visible. Many people remember when there were only a handful of planets in the known universe: those circling the sun. Now the tally of well-confirmed planets orbiting other stars stands at 723. Another 3,500 are very probably real.


    Numbers like that give a vastly better perspective on the nature of the planet population across the universe – because we can tell a lot about their sizes, masses, orbits, temperatures, and in a few cases, their likely composition and even atmospheric chemistry.

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    We’ve recently learned, finally and for all time, that most stars have planets, and that planets with tolerable temperatures — possibly allowing rains and oceans — number in the tens of billions in our galaxy alone. We’ve learned that planets come in almost every conceivable size and mass, every temperature from red hot to sub-Plutonian cold, and with densities from superheavy cannonballs to rocky terrestrials to waterglobes to, in one case, the “styrofoam planet.”

    Many solar systems, we have discovered, went through periods of chaos — resulting in wild, loopy orbits for the remaining worlds that did not smash each other apart, or merge, or get flung away to drift loose between the stars. It is a pinball game out there, nature does not care what happens to its worlds, and we’re lucky this stuff did not happen to us.

    Expect more discoveries.

    Glories of the early universe

    Arcturus has been a familiar star of spring and summer since ancient times. But it may be the most alien thing you can easily lay eyes on.


    Arcturus is an oddball, speeding across our stellar neighborhood like an out-of-stater racing 70 miles per hour down the block. Astronomers recently found 52 other, farther stars that share Arcturus’s speed and direction. They’re a big, sparse swarm flying in parallel. The swarm’s speed and size identify it as a tattered remnant from an alien dwarf galaxy that fell into our Milky Way and has mostly merged with it. Arcturus came from another galaxy.

    And it is old — about 7 billion years old, compared with 4.6 billion for the sun and solar system. Arcturus is a messenger from an earlier era, displaying today the heavy-element-poor chemistry of the early universe when the star formed.

    How stars and galaxies took shape in the early universe is another hot area of astronomy right now. Big new telescopes, working ever deeper into the infrared spectrum, have unveiled an era just a few billion years after the Big Bang when a gigantic burst of star formation lit up coalescing galaxies like fireworks. New stars still form today, but in nowhere near such numbers or in such spectacular spitting cauldrons.

    Burn bright, burn fast

    Another way giant new infrared telescopes are changing astronomy is by peering into the workings of star formation close at hand, in our own galaxy.

    Spica is an example of a young, massive star burning so hot and fast that it will die quickly, cosmically speaking. It is only 10 million years old and has just a few million to go. Stars even younger than this, right back to newborns, can be identified around the sky.


    But the actual birth processes — which include the birth of planets — are hidden from sight inside smoky, black gas cocoons. New infrared technologies are seeing the enormously complex and variable processes going on inside. This, too, is an exciting field that’s changing fast. Spica, a young star now shining free and clear of its birth cocoon, is a reminder.

    Whole-sky maps

    Easy-to-use maps of stars and constellations across the entire evening sky are available at

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