Science NewsScience News
Dwarfs' new colour may solve big mystery
The Universe
TERENCE DICKINSON
For 30 years, astronomers have searched for something they call the "dark matter." Now they think they have uncovered a big chunk of it in the form of trillions of tiny white dwarf stars, the fizzled remnants of ordinary stars like the sun.
The dark matter problem is one of the prime mysteries in astronomy. If this new discovery is confirmed, there will be "enormous rethinking" of the theories of how galaxies formed and how the universe evolved from the big bang, says Harvey Richer, University of British Columbia astronomer and a member of the international team of researchers who used the Hubble Space Telescope to uncover the white dwarfs.
The dark matter became a big problem in the early 1970s when astronomers noticed that stars in the outer part of the Milky Way Galaxy are orbiting the galaxy's hub at much higher velocity than anyone expected. Estimates of the galas mass-its total cargo of stars showed that the
Milky Way had nowhere near enough collective gravity to make the outer stars move at such a speedy clip. Something was missing. That is, something was there and astronomers couldn't see it.
Evidence from ongoing research has suggested that trill-ions of dim objects less massive than the sun, but more massive than Jupiter might account for the galactic dark matter. Objects in this range include white dwarf stars (about half the sun's mass), red dwarf stars (one-fifth the sun's mass), and brown dwarf stars (less than one tenth the sun's mass). Of these, the white dwarfs seemed to be the best prospects, but not enough of them had been found to make a case.
The breakthrough came not at the telescope but on a computer screen. While working at the Canadian Institute of Theoretical Astrophysics, theoretical astronomer Brad Hansen recalculated the temperature of 12 billion-year-old white dwarfs. He found that their surfaces should be hotter an anyone had predicted and they should look blue rather than red as, everyone had thought.
Armed this new prediction, Richer and his colleagues used the Hubble Space Telescope to rephotograph the famous Hubble Deep Field that shows the faintest galaxies ever seen.
Then they compared the new and old pictures taken two years apart. Sure enough, two faint blue objects had moved slightly. Visible motion over a period of just two years is a sure sign that the objects are relatively close, say, within a few thousand light-years of Earth.
That fact plus the blue colour strongly suggests that the newfound objects are white dwarfs in our galaxy, not somewhere out among the distant galaxies that were the ;target of the original Deep Field image.
Three other faint blue objects in the Deep Field are also suspects. Moreover, other astronomers are starting to uncover faint blue objects in other Hubble pictures. Extrapolating from this small sample, the researchers a say there are about 5 trillion white dwarfs in our galaxy, enough to account for most of the Milky Ways dark matter.
Terence Dickinson is the editor of SkyNews magazine and the author of several guidebooks for backyard astronomers.
|
|
 |