Astro Bob A look at celestial happenings in the Northland and beyond Bob King View my profile Click here to view previous posts or additional resources

Proxima's proximity puts it in first place

Alpha Centauri is only visible from the latitude of southern Florida and points south. This view shows the star as seen from Honolulu. Alpha is just east of the famous Southern Cross; Proxima is about 2 degrees SW of Alpha. Created with Stellarium

After the sun, you'll often hear that the nearest star is Alpha Centauri in the constellation of Centaurus the Centaur. Alpha is 4.4 light years (25.8 trillion miles) from Earth and the third brightest star in the sky, just a hair brighter than Arcturus. A small telescope will split Alpha into two stars -- Alpha Centauri A and B -- that revolve about their common center of gravity over a span of about 80 years.

Proxima is not nearly as distinguised as the mother star Alpha, but a small telescope will show it easily. Proxima's orbit around the galaxy causes it to slowly move westward against the background stars as seen from Earth. Credit: ESO

Truth be told, the Alpha Centauri pair is not the closest star. That honor belongs Proxima Centauri, a diminuitive red dwarf companion of the A-B system. Alpha gets all the press because it's bright and had its distance measured back in the 1830s. It stood as the closest star until Proxima was caught on a 1915 photographic survey seeking stars with unusually large motions across the sky. Closer stars appear to move faster than distant ones for the same reason a car zooming by in front of you appears to cover more ground than one at the same speed seen from a mile away. At 4.2 light years, Proxima is marginally closer to our solar system than Alpha. Marginal or not, that makes it no. 1.

The relative sizes of the sun and Alpha Centauri star system are shown in this diagram. A and B are solar-type stars while Proxima is a red dwarf. Credit: David Benbennick

Alpha Centauri A is slightly larger than the sun and one of the brightest stars, while Proxima is 11th magnitude and requires at least a 3-inch telescope to see. It's almost certain that Proxima orbits the Alpha Centauri pair at a distance of 1.2 trillion miles or more than a thousand times Saturn's distance from the sun. That's so out there that Proxima's located more than two degrees or four full moon diameters from the A-B pair and takes an estimated 500,000 years to complete an orbit around them.

Artist's impression of a red dwarf star. Red dwarfs are reddish-orange in color and small but have plentiful starspots and flare activity. Credit: NASA

Why is the star so dim compared to its more famous sibling? It all comes down to size. Proxima's a very tiny star, only 1.5 times the size of Jupiter, even though it weighs in with a mass 129 times larger. It belongs to the most populous group of stars in the galaxy called the red dwarfs. Red dwarfs burn their fuel in a miserly fashion compared to most other stars. That means they live long lives. Very long lives. When the sun runs out of nuclear fuel in four or five billion years and evolves into a blazing ember called a white dwarf, Proxima will still be humming along burning hydrogen and looking much the same as it does today for another few trillion years.

Given the stability and longevity of red dwarfs, you might think a planet in orbit around one would make great place for life to evolve. Maybe. Problem is that red dwarfs are known for their explosive flares, similar to the sun's flares, but sometimes much more powerful. These could pose severe radiation problems for any planetary inhabitants. Dwarfs also vary in brightness because of both flares and starspots, which grow to enormous sizes and reduce light output up to 40%. Since we depend on the near constancy of the sun's light, all this variability would impose great stress on most life forms.

The sun would be a bright first magnitude star below the W of Cassiopeia as seen from the Alpha Centauri system. Image source: Celestia

No star stands still. Proxima will remain the nearest star for another 32,000 years before our two stars drift apart. Its replacement will be Ross 248, another dim red dwarf, located 10 light years away in Andromeda and headed our way at this very moment.

Tomorrow we'll talk about the upcoming lunar eclipse and International Space Station night-long marathons. Tonight you'll see the station cross the sky twice beginning at 10:12 p.m. (across the south) and again at 11:47 (across the north).

Posted by: rking@duluthnews.com on 6/24/2010 at 10:27 AM | Comments (0) | Permalink | Edit

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