Friday, September 29, 2006

Really Big Stars

Artist illustration of
massive star formation.
Image credit: NRAO
Click to enlarge

Astronomers think they’ve got a handle on how Sun-sized stars come together. But the formation of the largest stars - more than 10 times the mass of the Sun - still puzzle astronomers. New observations on a 20 solar mass star have revealed that these giant stars maintain a torus of material around themselves. They can continuously feed from this “doughnut” of material, while powerful jets of radiation pour from their poles. The material can continue gathering onto the star while avoiding this radiation, which would normally blast it back into space.

Astronomers using the National Science Foundation’s Very Large Array (VLA) radio telescope have discovered key evidence that may help them figure out how very massive stars can form.
“We think we know how stars like the Sun are formed, but there are major problems in determining how a star 10 times more massive than the Sun can accumulate that much mass.

The new observations with the VLA have provided important clues to resolving that mystery,” said Maria Teresa Beltran, of the University of Barcelona in Spain.
Beltran and other astronomers from Italy and Hawaii studied a young, massive star called G24 A1 about 25,000 light-years from Earth. This object is about 20 times more massive than the Sun. The scientists reported their findings in the September 28 issue of the journal Nature.
Stars form when giant interstellar clouds of gas and dust collapse gravitationally, compacting the material into what becomes the star. While astronomers believe they understand this process reasonably well for smaller stars, the theoretical framework ran into a hitch with larger stars.

“When a star gets up to about eight times the mass of the Sun, it pours out enough light and other radiation to stop the further infall of material,” Beltran explained. “We know there are many stars bigger than that, so the question is, how do they get that much mass?”
One idea is that infalling matter forms a disk whirling around the star. With most of the radiation escaping without hitting the disk, material can continue to fall into the star from the disk. According to this model, some material will be flung outward along the rotation axis of the disk into powerful outflows.
“If this model is correct, there should be material falling inward, rushing outward and rotating around the star all at the same time,” Beltran said. “In fact, that’s exactly what we saw in G24 A1. It’s the first time all three types of motion have been seen in a single young massive star,” she added.

The scientists traced motions in gas around the young star by studying radio waves emitted by ammonia molecules at a frequency near 23 GHz. The Doppler shift in the frequency of the radio waves gave them the information on the motions of the gas. This technique allowed them to detect gas falling inward toward a large “doughnut,” or torus, surrounding the disk presumed to be orbiting the young star.
“Our detection of gas falling inward toward the star is an important milestone,” Beltran said.

The infall of the gas is consistent with the idea of material accreting onto the star in a non-spherical manner, such as in a disk. This supports that idea, which is one of several proposed ways for massive stars to accumulate their great bulk. Others include collisions of smaller stars.
“Our findings suggest that the disk model is a plausible way to make stars up to 20 times the mass of the Sun. We’ll continue to study G24 A1 and other objects to improve our understanding,” Beltran said.

Beltran worked with Riccardo Cesaroni and Leonardo Testi of the Astrophysical Observatory of Arcetri of INAF in Firenze, Italy, Claudio Codella and Luca Olmi of the Institute of Radioastronomy of INAF in Firenze, Italy, and Ray Furuya of the Japanese Subaru Telescope in Hawaii.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Original Source: NRAO News Release

Stars being born
in the outer

reaches of space

PPARC akari

Company detects a big opportunity
With PPARC funding, British company e2v has developed sensors to help astronomers see deeper into space. This has made it a world leader in fields as diverse as space science, surveillance, defence and medicine. If you’re doing basic scientific research, then one thing’s certain – you’ll always be trying to push limits and explore further than before. Take astronomy, for example. To explore the outer reaches of the universe, you need to detect tiny amounts of light, X-rays and other forms of radiation. And the more sensitive and accurate your detectors, the more you’ll be able to see. Staring into space British company e2v technologies specialises in the design and manufacture of highly sensitive detectors. It first started working on charge-coupled devices in1975. These are circuits that generate electronic signals based on the amounts of light or other radiation they receive. By using regular arrays of sensors, CCDs allow you to take digital images. And, as every budding digital photographer knows, the more sensors – or pixels – a CCD offers, the more detail you can see in your picture. One of the earliest uses of CCDs, however, was in astronomy. Although e2v’s first component only had 220,000 pixels – a trifle compared to today’s digital cameras – astronomers quickly spotted its potential. The devices were quickly put to work on PPARC’s research programmes, but scientists soon wanted more. To create much larger and more sensitive CCD arrays, e2v found a way to ‘stitch’ together the components made on the same silicon wafer. But that was just the start. As a result of e2v’s later work, pictures of the galaxies are now much clearer. Scientists can also see ‘fainter’ images and detect other types of radiation, such as X-rays and ultraviolet light. Our understanding of the universe and how it works has improved enormously.

About e2v
e2v technologies is a leading supplier of high-power electronic components and state-of-the-art sensors. In 2006, it won a Queen’s Award for Enterprise for its innovative L3Vision™ sensors and cameras. The company employs 1,200 people in the UK, where its production facilities are based. Seventy two percent of what it makes is exported. Sales totalled £112 million in the year to March 31 2006. A third of these revenues come from applications related to PPARC.

The Particle Physics and Astronomy Research Council (PPARC) funds research in four broad areas of science: particle physics, astronomy, cosmology and space science. By making it possible for British scientists to explore fundamental questions about the origin of the universe and the structure of matter, it generates ideas and discoveries that have a much broader impact on everyday lives. The UK economy also benefits as ideas turn into innovative new companies and businesses exploit the world-leading skills they create as they meet PPARC’s challenge.

To find out more, visit
The Particle Physics and Astronomy Research Council
Polaris House, North Star Avenue, Swindon, Wiltshire SN2 1SZ, UK
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