Tuesday, March 04, 2008

Planetary Nebula NGC2371

Planetary Nebula NGC2371. Credit: Hubble Heritage

Probing a glowing bubble of gas and dust encircling a dying star, NASA's Hubble Space Telescope reveals a wealth of previously unseen structures.

The object, called NGC 2371, is a planetary nebula, the glowing remains of a Sun-like star. The remnant star visible at the center of NGC 2371 is the super-hot core of the former red giant, now stripped of its outer layers. Its surface temperature is a scorching 240,000 degrees Fahrenheit. NGC 2371 lies about 4,300 light-years away in the constellation Gemini.

The Hubble image reveals several remarkable features, most notably the prominent pink clouds lying on opposite sides of the central star. This colour indicates that they are relatively cool and dense, compared to the rest of the gas in the nebula.

A planetary nebula is an expanding cloud of gas ejected from a star that is nearing the end of its life. The nebula glows because of ultraviolet radiation from the hot remnant star at its center. In only a few thousand years the nebula will dissipate into space. The central star will then gradually cool down, eventually becoming a white dwarf, the final stage of evolution for nearly all stars.

The Hubble picture of NGC 2371 is a false-colour image, prepared from exposures taken through filters that detect light from sulfur and nitrogen (red), hydrogen (green), and oxygen (blue). These images were taken with Hubble's Wide Field Planetary Camera 2 in November 2007, as part of the Hubble Heritage program.

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Saturday, March 01, 2008

Interstellar Gas Clouds

Simulation of the collapse of an interstellar gas cloud into a massive star. The left side shows the whole cloud, and the right side shows a zoom-in around the massive star in the center. Credit: Mark Krumholz

Two scientists think they have decoded the gassy recipe to create stars as much as 100 times bigger than the sun, perhaps solving the mystery of their formation.

Mark Krumholz Princeton University in New Jersey and his colleague Christopher McKee of the University of California Berkeley used mathematical models to show how small stars can prime superstar formation, and detail their findings in the Feb. 28 issue of the journal Nature.

Gravity tends to break interstellar gas clouds into small pieces, preventing massive star formation, but little stars heating up a gas cloud can smooth it out, forcing gravity to create a huge star.

Star power
Although massive stars are about a million times rarer than the most common stars — those about 80 percent smaller than the sun — they are the movers and shakers of the universe.

"They're very rare, but massive stars are the dominant players in galaxies," Krumholz said. "They're the things that can push around and heat up interstellar gas - essentially where all stars come from."

He also explained that big stars seed the cosmos with elements that are required for life.

"They enrich the universe with metals from their supernovae," he said, noting that only enormous stars are powerful enough to fuse together small atoms and create the heavy materials.

Hot influence
To form a galactic superpower, Krumholz said an interstellar gas cloud needs to be thousands of times more dense than average. Problem is, gravity tends to break dense gas clouds into pieces and thwart massive star formation.

"The challenge isn't getting enough gas, it's getting the cloud into a small enough region and preventing its breakup," he said.

If a few small stars form within the cloud, Krumholz explained, they can heat up the cloud and increase its "column density," or pressure. The heating process prevents gravity from taking control of the cloud, breaking it up and forming only small stars.

"Heating up the gas helps pressure win over gravity's influence, ultimately forcing the gas cloud to collapse in a massive star," Krumholz said.

The new view of star formation highlights the rarity of massive stars — the only kind astronomers on Earth can see in distant galaxies — but leads to the possibility that more stars form in galaxies than previously thought.

"There may be significant parts of galaxies where massive stars can't form, but lower-mass stars like the sun can," Krumholz said.

We estimate the number of stars in a galaxy on the amount of light we see, and if massive stars are missing, then it's possible that we've dramatically underestimated the rate of star formation in distant regions of the universe.
Massive Stars Need Their Smaller Siblings To Grow @ Universe Today
The Dragon - Beijing's New Monster Airport from The Daily Galaxy
X marks the spot in dark matter web from Space New Scientist

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