Hopping through the Sky

Hopping through the Sky
by Dave Adalian

Anyone who’s ever looked up at night has noticed a problem facing astronomers: there are a lot of stars up there, and it’s easy to get lost in space.

That’s one of the reasons we have constellations, to help us fix in our minds the location of the wonders of the night. Still, knowing where to find a constellation only gives us the general location of whole groups of stars, some of them very large. How does anyone find anything among all those points of light?

They hop to it.

Star-hopping is an age-old and easy technique. Start by finding a bright star or well known constellation and connect the dots to find the target.

The best known star-hop is the Arc to Arcturus. It starts with one of the better known constellations in the sky, Ursa Major, the Big Bear.

The most recognized star shape in the sky, the Big Dipper, lies in the Ursa Major. Find it by looking due north just after sundown. As spring wraps up, the Big Dipper pours its contents onto the North Star and its tail points up and toward the east.

Follow the arc along the three bright handle stars about the same distance as the length of the Dipper to come to the first stop, the orange giant star Arcturus.

Arcturus is the brightest star in cone-shaped Boötes (boo-OH-tes), the Herdsman, and the fourth brightest seen from Earth. It’s 20 times larger than the Sun and shines 115 times brighter from 37 light years away. Arcturus is so bright in the infrared we can measure its heat here on Earth--the same as a candle flame five miles away.

Arcturus is one of the fastest stars, traveling at 90 miles a second toward Virgo, the second stop on this star-hop. If you “speed” along the arc past Arcturus, you’ll soon come to first-magnitude Spica, Virgo’s brightest star. Like Arcturus, Spica is brighter than the Sun, 13,400 times so, and masses 11 times more, but it sits 260 light years away.

Continue the arc again to reach our final stop, Corvus the Crow, an irregular box of four stars that is normally not a big attraction. Currently, however, it’s pointing its beak towards Jupiter, which sits directly above the Crow.

Astronomers have an alliterative phrase to remember this popular hop: “Arc to Arcturus, speed on to Spica and continue on to Corvus.”

Enjoy the ride!

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This column appeared originally in the Visalia (Calif.) Times-Delta on May 26, 2005.