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SYFY WIRE Bad Astronomy

What is the Milky Way’s most distant globular cluster?

By Phil Plait
A Hubble image of PSO J174.0675-10.8774 goes much deeper and has higher resolution than ground-based images, showing more stars. Credit: NASA/ESA/Hubble and Judy Schmidt

I love globular clusters. These huge balls of thousands or even millions of stars are fascinating scientifically — most are very old, as old as the galaxy itself, up to 13 billion years. Most formed all at once (though some show evidence of a second wave of star formation later in life), which makes studying them somewhat easier; if all the stars are the same age that removes a variable in understanding them.

The bigger and closer ones are also just breathtaking through a telescope, looking like stellar beehives; they have dense cores with stars packed closely together, then fade away outward in all directions. The brightest are staples of amateur astronomical viewing. I’m not sure how many I’ve seen myself, but it’s probably a couple of dozen.

However, the Milky Way has at least 160 of them orbiting its center, and probably many more faint enough to have avoided detection. I’ve written about them many times, and as I was researching one for an article recently, a question popped up in my head that was obvious in retrospect but had never occurred to me before: What is the most distant known Milky Way globular cluster?

Most are many tens of thousands of light years away, orbiting out in the galaxy’s halo outside the main disk of stars, gas, and dust… but some are much farther. Which one currently holds the record?

So I did a journal search, and boom! I immediately got my answer. Allow me to introduce you to PSO J174.0675-10.8774:

A natural-color image of PSO J174.0675-10.8774, the most distant known Milky Way globular cluster, taken using the Wide Field Imager on the MPG/ESO 2.2m telescope. Credit: Laevens et al.

As you can see, it’s a bit ratty, but the shape is about right for a globular. It was discovered independently by two different teams of astronomers in 2014 — more on that below — and had avoided detection for so long because it’s so far away: It’s a a whopping 470,000 light years distant! That’s a long way; the Milky Way’s disk is only 100,000 light years across, and that’s about a sixth of the distance to the Andromeda Galaxy. So it’s out in intergalactic space. Yowza.

As globulars go it’s faint, low mass (the initial estimates are has about 7,000 times the mass of the Sun, which is quite petit for a globular), but oddly large in size. The diameter is hard to measure because globulars don’t really have an edge to them, but it’s very roughly 150 light years across, about twice the size of a normal globular.

Weirdly, though, the age is a lot younger than you’d expect: It’s only about 8 billion years old, far younger than the Milky Way! What gives?

It turns out there are different flavors of globulars. Some formed with the Milky Way and are very old, but some are younger and bigger, like this one. Those latter are thought to have formed with dwarf galaxies that orbited the Milky Way far in the past, but then got too close and were eaten by our galaxy, merging their stars with ours. In some cases globular clusters orbiting the snack-size dwarf galaxy avoid getting digested and wind up orbiting the Milky Way on their own. This may be an example of that.

… but there’s some controversy. The team of astronomers that actually found it first (by just a few months!) concluded it was a dwarf galaxy, and not a globular cluster. They rely on a seeing what are called blue loop stars, stars which have aged so much they’re starting to die; such stars go from red to blue, then turn red again over millions of years (hence the name). These stars would be considerably younger than the other stars in the cluster, so the astronomers interpret them as meaning there have been multiple episodes of star birth. That’s more like a galaxy than a globular.

But the plot thickens! Yet another team showed that two of the three blue loop stars seen are actually foreground stars in the Milky Way, so they have nothing to do with the cluster. That does leave one blue star that might be in the cluster, but its membership is unclear, and basing a big conclusion on a single star is an iffy proposition.

A Hubble image of PSO J174.0675-10.8774 goes much deeper and has higher resolution than ground-based images, showing more stars. Credit: NASA/ESA/Hubble and Judy Schmidt

I figured a pretty quick Hubble observation of the cluster would clear this up. So I searched, and bingo! Astronomers observed it in 2014 with the space telescope, and they found that … it’s a globular cluster.  The blue stars seen are likely to be blue stragglers, which are common in globulars. If two low mass red stars physically collide and merge they can be rejuvenated, turning blue and looking very young. Collisions like this do in fact happen because of the close proximity of stars in globulars, so this explanation makes sense.

[As an aside, the name of this cluster is a mess. Some people call it Laevens 1, after the lead investigator of one team that found it, but they acknowledge the other team found it first. That team calls it Crater, after the constellation it’s in, which is confusing at best. PSO J174.0675-10.8774 is just a nightmare. I’d actually prefer Belokurov 1, after the lead of the team that found it, but I guess we’ll just have to see how this settles out.]

The numbers for the cluster from Hubble are similar to the first team’s, though they find it be more massive (about 10,000 solar masses) and slightly brighter (which isn’t surprising, Hubble can see fainter stars which add to the total mass and brightness). They also find it to be 8 billion years old, so it seems like this really did form with a dwarf galaxy which subsequently got eaten.

Well, that’s science for you. Find an object, get a mystery, take more observations, mystery solved.

Except, not so much. There are tons more questions! Why did the globular survive its host galaxy’s cannibalism event? Can we find stars from that galaxy in our own, or is that impossible? What does this mean for galaxy evolution (that is, how galaxies grow and change over time)? And of course, are there more of these beasties out there, and of so, how do they compare to this one?

That’s science for you. Answer one question, get ten more. That’s one of my favorite things about it.

My very hearty thanks to Judy Schmidt, aka @SpaceGeck, who is amazing at taking raw Hubble data and creating color images from them. I contacted her about this cluster, figuring she’d get a kick out of it, and she grabbed the observations from Hubble and produced the image used here the very same day! She’s aces.