Photograph by Chandra X-ray Observatory Center

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This composite image shows the central region of the spiral galaxy NGC 4151, nicknamed the Eye of Sauron.

Photograph by Chandra X-ray Observatory Center

'Eye of Sauron' Opens Galactic Vistas to Astronomers

Gigantic black holes provide a new cosmic yardstick for charting far-off stars.

Monster black holes lurking within the core of galaxies offer a new way to measure how far many objects in the universe are from Earth, astronomers report on Wednesday—and with much better accuracy than ever before.

The cosmic-yardstick breakthrough came courtesy of the galaxy NGC 4151, dubbed the Eye of Sauron because it resembles the baleful bad guy in The Lord of the Rings. (See a similar starry object: "First Picture of Alien Planet ... Isn't?")

Previous estimates of the distance to the sinister-looking galaxy had ranged wildly, putting it 13 million to 95 million light-years from Earth. But using a technique that's similar to land surveying, astronomers believe they've pinned down the distance to 62 million light-years.

A new study published this week in the journal Nature details the innovative technique, which, like the film's plot, revolves around a ring. In this case the ring is galactic debris that surrounds the gigantic black hole at the heart of NGC 4151.

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Using a new technique, astronomers have calculated that NGC 4151 is 62 million light-years from Earth.

A Gift

Using the twin 32-foot (10-meter) telescopes at the W. M. Keck Observatory on the summit of Mauna Kea, Hawaii, and interferometry, the study's team of astronomers, led by Sebastian Hönig of the University of Southampton, achieved the same resolution as a single mirror about 280 feet (85 meters) across, the physical distance separating the two observatories. That's a hundred times the resolution of the Hubble Space Telescope.

This gave the astronomers supersharp views of the center of NGC 4151 and the infrared glow of the ring of hot dust around its black hole, a common feature of such "active" galaxies. Then, using the distance from the ring to the black hole as the base of a triangle, the researchers could determine the distance from Earth to NGC 4151.

"In fact, this method, based on simple geometrical principles, gives the most precise distances for remote galaxies," says Hönig in a press statement. "Such distances are key in pinning down the cosmological parameters that characterize our universe or in accurately measuring black hole masses.

"Indeed, NGC 4151 is a key to calibrating various techniques of estimating black hole masses," he says. "Our new distance implies that these masses may have been systematically underestimated by 40 percent."

The new technique will allow astronomers to measure the distance to the roughly 10 percent of galaxies that are active, says astronomer Martin Elvis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, in a commentary accompanying the study. That means they'll be able measure distances to objects much farther away than the supernovas that showed our universe is expanding at an accelerating rate, a discovery that garnered the Nobel Prize in 2006.

"Usually in science you fight so hard to get something to fit or work properly," says study co-author Darach Watson, of the University of Copenhagen. "But every so often—very rarely—something magical happens. It's like a gift, and everything just falls into place. That is what happened here."

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This skychart shows the location of NGC 4151, visible this time of year in the northeast sky in the predawn hours.

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This finder chart shows NGC 4151 as seen through a backyard telescope.

See for Yourself

You too can hunt down this galactic Eye of Sauron. You'll need a medium to large backyard telescope to get a good view, but a scope with a mirror or lens as small as four inches should give you a glimpse, even in suburban locations.

Shining feebly at magnitude 11, this spiral galaxy lies in a rich field of galaxies in the constellation Canes Venatici, which is visible this time of the year from across the entire Northern Hemisphere in the predawn hours. The galaxy lies halfway up the northeast sky just below the Big Dipper and just west of a 7th magnitude star.

Through the eyepiece under high magnification, NGC 4151 looks humble, a faint but distinct oval-shaped patch of light that's less than a third the width of the full moon. Though just 9 arc minutes in our sky, the galaxy is actually some 124,000 light-years across and has a monster black hole at its center-one that's 37.6 million times the mass of our sun.

Happy Hunting!

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