While observing stars in our own Milky Way galaxy with the Hubble Space Telescope, astronomers stumbled upon a rare find: a distant galaxy teeming with clusters of stars too dim for most telescopes to see. Curiously, the light from some of these clusters is redder than expected, an observation astronomers are still struggling to explain.

Globular clusters are tight-knit collections of stars that are among the oldest surviving structures in the universe.

Our own Milky Way has at least 158 such clusters. While taking a snapshot of one, 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.

Bluer and younger

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 evolution of stars doesn't happen real fast late in life – it's sort of like watching the changes between a 60- and a 65-year-old," says astronomer Stephen Zepf of Michigan State University in East Lansing, US, who was not affiliated with the study. "They're not as spectacular as the changes between birth and the first 5 years of life."

Life stages

The team can't account for this unusual redness, but say 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."

These models calculate the rate at which stars pass through different life stages, which are marked by the fusion of different elements inside their nuclear furnaces.

"It's possible there's something about stellar evolution we don't understand," says Zepf. But he adds there may be a simpler explanation for the ultra-red clusters.

Intervening material can absorb blue light, making objects appear redder than they are. Zepf notes the team might have underestimated this effect: "The authors make the best correction possible, but it's still an uncertainty. It's the explanation that requires the least amount of changes."

Glimpse of the past

Globular clusters have been found in one galaxy that lies even farther away. But these are the most distant globular clusters ever used to measure such detailed properties as redness. Finding similar collections around other distant galaxies, however, is unlikely.

That's because such distant clusters are extremely dim – all of the newly observed clusters put together are still roughly 500 million times dimmer than what can be seen with the naked eye.

Detecting those dim globs took 126 orbits of the Hubble Space Telescope – an amount of observing time that is difficult to obtain. Further, it's impossible to know where to point the telescope next.

"It's very interesting, it's just unfortunate," Kalirai told New Scientist. "They give us a glimpse of what has happened in the past, but only a glimpse."

Journal reference: Astrophysical Journal Letters (vol 682, forthcoming)

Comment subject
Comment
No HTML except lower case italic tags or lower case bold tags, please: <i> or <b>
Your name
Your email
	We need your email in case we need to contact you about the comment. We will not use it for any other purpose.

• For exclusive news and expert analysis every week subscribe to New Scientist Print Edition
• For what's in New Scientist magazine this week see contents
• Search all stories
• Contact us about this story
• Sign up for our free newsletter