London, July 9 : The Hubble Space Telescope has found a distant galaxy teeming with clusters of stars, with light from some of them redder than expected, an observation that astronomers are still struggling to explain.
According to a report in New Scientist, Hubble discovered this star cluster while observing stars in our own Milky Way galaxy.
Globular clusters are tight-knit collections of stars that are among the oldest surviving structures in the universe.
While taking a snapshot of one such cluster, Jason Kalirai of the University of California, Santa Cruz, in the US, and colleagues serendipitously captured a rare gem in the background: a distant elliptical galaxy brimming with its own collection of the clusters.
The galaxy and its clusters lie 1.2 billion light years away.
"If it was any farther, we wouldn't have seen it. The globular clusters would have been too faint," Kalirai told New Scientist.
The team found 195 clusters in the galaxy, though it is expected to harbour thousands more that are too dim to observe.
Although it is impossible to date the clusters, they likely formed around the same time as those in the Milky Way, which are estimated to be more than 12 billion years old.
So, since the light from the distant clusters has taken 1.2 billion years to reach Earth, the observation was thought to offer a unique look at an earlier stage in the lives of stars.
Astronomers expect clusters to be bluer the farther back in time they look. That's because younger clusters should contain more blue stars, which tend to be hotter and more massive.
As clusters age, these stars are the first to exhaust their fuel, leaving behind longer-lived, reddish stars.
But the team found the opposite trend: some of the brightest clusters seemed to be 20% redder than stellar models predict.
This is odd, because astronomers don't expect to see a huge difference in the stars' appearance over the last billion years or so.
The research team can't account for this unusual redness, but said that it may have to do with the stars' chemical makeup.
"It could just be that this is one of a strange population of globular clusters that's much more metal-rich than what we would have expected," Kalirai told New Scientist. "But it could also be that the stellar models are incorrect," he added.
According to Stephen Zepf of Michigan State University in East Lansing, US, there may be a simpler explanation for the ultra-red clusters.
"Intervening material can absorb blue light, making objects appear redder than they are," he said.