December 13, 2012

BACKYARD NATURALIST: Winter nights bring out the shine in planets, nearby stars

By DANA WILDE

On a clear night in a dark sky, your eye can pick up about 2,000 individual stars. The astronomers estimate that altogether about 6,000 stars can be seen from Earth, just looking up with human eyes. Binoculars reveal a lot more individuals, and then there are the washes of light that are actually groups of stars, including millions upon millions in that swath that in late summer runs roughly northeast to southwest, the Milky Way. It’s the concentration of stars visible in one arm of our galaxy, from our vantage point. 

The brightest star we see from Earth is Sirius. It’s up there late evenings in the southeast now at Orion’s heels, shining at magnitude minus 1.46. Magnitude is a measure of brightness; the lower the number, the brighter the star or planet.

Jupiter, which right now is a hop, skip and a jump from Sirius across Orion, is shining at minus 2.8. Early in the morning lately you can catch Venus rising in the east almost like a flare at minus 3.9, and close by is Saturn at 0.6. The sun, just for comparison, shines at magnitude minus 26 — you can’t look directly at it. Neptune, which is the farthest planet, shines at 7.9 and requires a lens to spot.

The planets grow slowly brighter and dimmer according to their orbits, but the stars, practically speaking, do not vary in their distances from us, so they remain more or less steady in their brightness. Sirius is only about 8.6 light-years away, so it seems very bright. When compared to other stars if they were all the same distance from us, Sirius would have an absolute magnitude of 1.4. The sun at that distance (32.6 light-years) would shine at the absolute magnitude of 4.8. Betelgeuse, the reddish star at the top of Orion, is to us about magnitude 0.5 — pretty bright, but nowhere near the brightest even though it’s a super-giant star because it’s around 570 light-years away. Its absolute magnitude is minus 5.3, much brighter than the sun or Sirius.

On winter nights here in the great white Northeast, the visible stars seem to blaze up there. The cold freezes the moisture out of the air and turns it almost crystal clear. The 6,000 visible stars burning like jewels are magnified not only by the atmosphere but by your imagination, if it’s situated right. If you look long enough, you might start to wonder what’s going on around these other stars, if it’s anything like what’s going on down here on Earth. What would our sun look like in the sky of the planet orbiting Alpha Centauri B, our nearest neighbor, only 4.3 light-years away?

That planet’s night sky might look roughly similar to ours, with a similar number of stars with similar brightness. In our somewhat rural stellar neighborhood, there’s about one star per 300 cubic light-years.

But what the universe looks like depends on where you are. There are other territories, some of them less like rural Maine and more like cities, where the stars are so near that the night sky would be a dazzling overflow of individual lights as bright as Sirius, Jupiter and Venus, and even brighter.

In globular clusters, which are roughly spherical groups of stars orbiting the galaxy’s central halo, a few hundred thousand to a million or so stars are packed together in areas about 10 to 30 light-years across. There are about two stars per cubic light-year near a globular cluster’s center. So from the vantage point of a planet inside a globular cluster, hundreds of stars as close and closer than Sirius, Alpha Centauri, Canopus, Arcturus and Vega — the brightest stars we see — would be visible in the night sky.

On a planet near the center of a globular cluster, the sky would be completely covered in stars in all directions. Stars shining in the daytime. Out toward the edge of the cluster, a thousand stars brighter than Jupiter and Venus throw their light over snow-filled fields on a winter night. Dazzling light, collectively as bright as the moon, pouring down out of the sky and scintillating the snow and objects so distinctly in the planet evening that they’d seem to lose their actual substance, and become things of the imagination.

It all depends on where you’re situated. From some unknown planet in globular cluster M13 in Hercules, you can’t see our sun. It’s just part of a wash of stars, invisible because its thousands of night-sky stars are as bright as Venus, obscuring the view to the farther reaches. You’d have to imagine us, up there.

Dana Wilde lives in Troy. His writings on space and time are collected in “Nebulae: A Backyard Cosmography,” available from Booklocker.com.

Staff photo by Andy Molloy Dana Wilde

click image to enlarge

Globular cluster M13 is seen in the constellation Hercules.

Photo courtesy of Anthony Ayiomamitis / www.perseus.gr

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