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Galaxies demand a
stellar recount


Posted: August 21, 2009

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New data from NASA's Galaxy Evolution Explorer have turned the idea that stars of certain sizes form in certain quantities on its head.

Until now, the Universe was thought to create stars in specific quantities such that the proportion of small to large stars was fixed. For example, for every star at least 20 times the mass of the Sun there should be 500 stars with the Sun's mass or less.

Now, the Galaxy Evolution Explorer (GALEX) has found examples of small stars existing in even bigger groups than previously thought, with some 2,000 low-mass stars forming for each massive star. The small stars were always there, but masked by their massive, bright companions.

By combining data from GALEX and the Cerro Tololo Inter-American Observatory, astronomers learnt that not all galaxies make stars of different sizes in the same quantities. In these galaxies, NGC 1566 (left) is rich in O stars (appearing as white or pink) compared to smaller B stars (blue), whereas NGC 6902 (right) has a weaker population of O stars compared to B stars. NGC 1566 is 68 million light years away and NGC 6902 is 33 million light years away. Image: NASA/JPL-Caltech/JHU.

"What this paper is showing is that some of the standard assumptions that we've had – that the brightest stars tell you about the whole population of stars – this doesn't seem to work, at least not in a constant way," says Gerhardt Meurer of Johns Hopkins University and principal investigator on the study. The paper is published in the Astrophysical Journal.

Since it is a well known fact that many stars' light is drowned out by brighter stars, astronomers have a method for estimating the total number of stars in a region, known as the 'stellar initial mass function' equation. This assumes a relationship between the amount of light emitted by the brightest stars in that neighbourhood and the number of total stars residing there, regardless of their location in the Universe.

"We tried to understand properties of galaxies and their mass by looking at the light we can see," says Meurer. But it turns out that this assumption may have been leading astronomers astray. The problem can be likened to trying to estimate the population of Earth by observing the light emitted at night – crowded cities emit significantly more light than third world regions where electricity supplies are limited or non-existent, even though those areas might be just as populated.

Using ultraviolet images obtained using GALEX and filtered red-light images from ground-based telescopes at the Cerro Tololo Inter-American Observatory in Chile, Meurer and his team found that many galaxies do not form a lot of massive stars, yet still host numerous small stars.

The ultraviolet images are sensitive to small stars three times or more massive than the Sun, while the filtered optical images are only sensitive to the largest stars with 20 or more times the mass of the Sun.

The results are particularly important where stars are spread out over a large area, meaning that there could be around four times as many stars in these regions than previously estimated. "Especially in these galaxies that seem small and piddling, there can be a lot more mass in lower mass stars than we had previously expected from what we could see from the brightest, youngest stars," says Meurer. "But we can now reduce these errors using satellites like the Galaxy Evolution Explorer."

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Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!


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