Astronomers see faintest stars with Hubble Telescope

An international team of astronomers has captured rare images of the faintest stars in the galaxy, the burnt-out relics of ancient heavenly objects that formed many billions of years ago.

The team led by Harvey Richer of the University of British Columbia of Canada used the NASA’s Hubble Space Telescope to take the photographs. Hubble took the images by studying the same tiny patch of sky for more than 75 hours, gathering 378 overlapping images.

The team then compared those high-resolution photos pixel-by-pixel to identify the dimmest stars in the globular star cluster NGC 6397.

NGC 6397, located 7,200 light-years away in the southern constellation Ara, is a concentrations of hundreds of thousands of stars and is one of the nearest globular star clusters. It comprises nearly 400,000 stars and has undergone a "core collapse," which makes its central area very dense. Globular clusters, so named for their spheroidal appearance, hold clues about the onset of star formation in our galaxy. A recent work suggests NGC 6397 was born just a few hundred million years after the Milky Way's first stars formed.

According to the latest issue of journal Science, the work will bring scientists closer to revealing the formation time of one of the earliest generations of stars in the universe.

The team investigated two distinct stellar populations, red dwarfs and white dwarfs, in NGC 6397. The images depict red dwarfs, which are powered by nuclear fusion, and white dwarfs, when the star has died and fusion has ceased.

Approximately 8 percent of the mass of the sun, the lowest mass red dwarfs are the least massive stars in the galaxy still capable of burning hydrogen in their cores and supporting stable nuclear reactions. On the other hand, white dwarfs glow dimly for billions of years. And, according to the astronomers many of the stars in NGC 6397 have run out of the hydrogen fuel that sustains nuclear reactions of ordinary stars such as the Sun.

Through the process of measuring the temperatures of white dwarfs, much like checking the temperature of smoldering coals in a campfire to estimate how long ago it was burning, astronomers are able to determine the star's age.

Asserting that images represent a stunning technological achievement, Professor Harvey Richer of the University of British Columbia in Vancouver and the lead investigator of the project says, "The light from these faint stars is so dim that it is equivalent to that produced by a birthday candle on the Moon," adding that "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 stars in the globular cluster NGC 6397 are estimated to be approximately 12 billion years old, two billion years younger than the universe.