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DARK SUN : One of our nearest neighbours

A dim object less than 10 light years from Earth appears to be the closest brown dwarf yet found. The "star" is so cold that any residents on an orbiting planet would see a dark sun in their starry "daytime" sky.

The discovery suggests that brown dwarfs are common and that the objects could exist even closer to Earth.

Brown dwarfs have so little mass that they never get hot enough to sustain the nuclear fusion reactions that power stars like the sun. Still, they do shine, because they glow from the heat of their formation, then cool and fade.

Philip Lucas of the University of Hertfordshire in Hatfield, UK, and his colleagues discovered the brown dwarf, named UGPS 0722-05, from the infrared radiation it gives off. It is only about 9.6 light years from Earth, a bit more than twice as far as Proxima Centauri, our nearest star after the sun.

At that distance, it is the seventh closest star or star system to the sun. Not since 1947 have astronomers uncovered a new star so close to Earth.
Parallax view

"Great stuff!" says Todd Henry, a nearby-star researcher at Georgia State University in Atlanta, who was not part of the team. "This discovery is as cool as its temperature."

Lucas and his colleagues caution that their estimated distance is preliminary. It is based on parallax, which offers a reliable method of deducing a star's distance from Earth: if an observer on Earth measures the star's position in the sky and then looks at it again months later, the star will appear to have moved slightly because it is being viewed from a different point in our planet's orbit around the sun. Knowing the dimensions of Earth's orbit, astronomers can calculate the star's distance from the amount of its apparent movement.

But at the moment, Lucas and his colleagues don't have good enough parallax measurements to be sure of the brown dwarf's precise distance and could be a light year or so out. In just a few weeks, however, new parallax observations should pin the distance down.

If the current distance estimate is right, the brown dwarf is closer than any other known. The previous record-holder is a pair of brown dwarfs around the star Epsilon Indi, 11.8 light years from Earth.
Record breaker

The new brown dwarf breaks two other records as well. It's the coldest brown dwarf ever seen, with a temperature of just 130 to 230 °C. And it's the dimmest: it emits only 0.000026 per cent as much energy as our sun, and this energy emerges at infrared rather than visible wavelengths. It would take 3.8 million of these brown dwarfs to equal the sun's power. It is about the size of Jupiter, but its mass is 5 to 30 times greater.

The object's feeble nature explains why it has only now been spotted, despite its proximity. It was found after surveying only a few per cent of the sky, which implies that many more brown dwarfs are lurking nearby undetected.


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