The Hubble Space Telescope might have "photographed" some of the missing mass of the universe. A team of astronomers has compared the 1995 Hubble Deep Field image - a continuous ten-day exposure of a small area of the sky - with an image of the same region taken in 1997. Two very faint objects appear to be moving, and the astronomers argue that they could be white dwarfs. These small, but very dense, stars are leading candidates for the "dark matter" in our galaxy (Astrophysical Journal October 20 524 L95). It is known that over 90% of the mass in our galaxy is dark or invisible because the galaxy rotates much faster than can be explained by the gravitational influence of the stars and gas that we can see.
When the astronomers – Rodrigo Ibata from the European Southern Observatory, Harvey Richer and Douglas Scott from the University of British Columbia in Canada, and Ronald Gilliland from the Space Telescope Science Institute in Baltimore – compared the two Hubble images, they noticed that five objects appeared to have moved, indicating that they are local to our galaxy. Two of them appear to be white dwarfs, while the other three require further observations.
White dwarfs are Earth-sized stars that have the masses similar to the mass of our Sun, which makes them extremely dense and compact. The possible white dwarfs seen by Hubble are very old – around 12 billion years of age. According to theoretical predictions, if all the missing mass in the galactic halo is made up of these white dwarfs, then four such stars should be seen in the Hubble Deep Field.
