An international team of astronomers has uncovered the faintest stars ever seen in any globular star cluster, bringing scientists closer to revealing the formation time of one of the earliest generations of stars in the Universe.
Using NASA’s Hubble Space Telescope, the team took hundreds of high-resolution photos and compared the images pixel-by-pixel to identify the dimmest stars in the globular star cluster NGC 6397.
“The light from these faint stars is so dim that it is equivalent to that produced by a birthday candle on the Moon, as seen from Earth,” says led by University Of British Columbia professor Harvey Richer, lead investigator of the project, which was chosen over several thousand other proposals to gain almost five days access to Hubble.
The team surveyed two distinct stellar populations -- red dwarfs and white dwarfs -- in NGC 6397. Located in the southern constellation Ara, approximately 8,500 light-years away, NGC 6397 is the second closest globular star cluster to Earth.
At approximately eight per cent the mass of the Sun, the lowest mass red dwarfs are the least massive stars in the Universe still capable of burning hydrogen in their cores and supporting stable nuclear reactions.
White dwarfs are the burnt out remnants of more massive stars that died long ago. By 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. This information provides important clues to the age of the globular cluster, which formed in the early Universe.
Analysing this relic population of white dwarfs is also the only way to calculate the original number of high-mass stars in the cluster.
“These stars, which died long ago, were among the first to have formed in the Universe,” says Richer, the world’s leading expert in using white dwarfs as a tool for dating globular clusters. “Pinning down their age narrows down the age range of the Universe.”
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