Hubble shines a light on dimmest stars in the universe

Astronomers have captured rare images of the faintest stars in the galaxy - the burnt-out relics of ancient celestial objects that formed many billions of years ago.

The stars were photographed by the Hubble Space Telescope, which took the images by studying the same tiny patch of sky for more than 75 hours, gathering 378 overlapping images. They are the dimmest stars ever seen in a globular cluster - spherical concentrations of hundreds of thousands of stars - and they offer astronomers a valuable insight into the types of stars that existed in the early universe.

The images represent a stunning technological achievement, according to Professor Harvey Richer of the University of British Columbia in Vancouver. "The light from these faint stars is so dim that it is equivalent to that produced by a birthday candle on the Moon," he said. "These stars, which died long ago, were among the first to have formed in the universe. Pinning down their age narrows down the age range of the universe."

The pictures show red dwarfs, which are powered by nuclear fusion, and white dwarfs, when the star has died and fusion has stopped. White dwarfs glow dimly for billions of years. He said many of the stars in this globular cluster, NGC 6397, - one of about 150 in the Milky Way galaxy - have run out of the hydrogen fuel that sustains nuclear reactions of ordinary stars such as the Sun.

The stars in the globular cluster NGC 6397, which is 8,500 light years away, are estimated to be nearly 12 billion years old, two billion years younger than the universe.