There are times when I just want to post a pretty astronomical image, something that delights the eye and gives a sense of wonder at the sky.
The problem with this – or “problem”, should I say – is that there is no such thing as just A beautiful photo. Either way, they end up leading to some interesting cosmic insight.
But this time it’s more. This particular pretty image may hold the key to the formation of one of the critical components of our Milky Way galaxy.
So first, the feast for the eyes:
Wow! It’s Liller 1, a cluster of stars. It is generically called a globular cluster – they are roughly spherical collections of hundreds of thousands of stars orbiting around their center of gravity. The Milky Way has about 160 such clusters, although some galaxies have many, many more. Regular readers know I like globulars and I’ve written about them extensively. They are beautiful and many of them are easily spotted in small telescopes, so they are one of my favorites for many reasons.
I knew straight away that Liller 1 was weird. It’s so red! And the stars around it are so blue, so something must be happening. In situations like this, the first thing I do is check the filters used to see if the colors might be a bit off, messing up the way we look at things. In this Hubble image (and you should click on it; the full-size image of Liller 1 is jaw-dropping) it turns out that the colors are indeed skewed, but, ironically, that doesn’t have any importance.
In this image, what you see in blue is actually red light. And what you see as red is actually near infrared light, just outside of what our eyes can detect. So no, it’s not displayed in natural color – more or less what you would see with the naked eye – but in fact it shows the cluster to be very, very red. Why?
In this case, it’s the location. Lille 1 is about 26,000 light years from us and very close to the galactic center, probably only a few thousand light years from it. The Milky Way’s core is laden with clouds of dust — made up of tiny grains of rocky matter and soot — that scatter blue light. Only red light can reach us, so any object in it will look much redder than it actually is. In this case, the stars in the Milky Way that we see blue in the image are actually red stars, but the stars in Liller 1 are exceptionally red due to dust.
Still, it gave me pause. Globulars orbit the galactic center, and most are seen far away from it in the sky. A few are relatively close to the center as they plunge through the galaxy in their orbits, but having one so close to the exact center struck me as odd.
And it’s weird! This is because Liller 1 is almost certainly not a globular cluster. These are the remains of what was once a much larger object that the Milky Way ate.
The Milky Way is a spiral galaxy, with a flat disk of stars, gas and dust. In the center is what we call the bulge, usually an oblate spheroid of older, redder stars, though in different galaxies it has different shapes. Ours is diamond-shaped, like a Tic Tac. How the bulge formed is unclear, but one hypothesis is that it formed when large structures fell to the galactic center and were stripped of their stars. Some of these structures could have been huge clusters of stars and gas in the disk of the Milky Way, and others could have been dwarf galaxies on their own, containing a few billion stars.
Could Lille 1 be one of these dwarf galaxies? The evidence strongly shows it! Globulars tend to be very old, with a single population of old stars, usually over 12 billion years old. Some, however, have second or even third populations of younger stars, perhaps 10 billion years old. But by looking at the stars of Lille 1, astronomers discovered that there was a population of stars 12 billion years old and a second that was only 1 to 2 billion years old! That’s a huge discrepancy, and it shows that somehow he may have created stars much more recently than globulars.
But how? Globulars are well known for not containing gas – they don’t have enough gravity to hold onto them when massive stars explode and expel the gas – so they can’t create stars. Liller 1, however, managed to hold onto their gas for eons. It’s probably more massive than your typical globular, with as much as 2.5 million times the mass of the Sun. But in the past, he may have had some billion solar masses of stars inside, again strongly implying that it looked more like a small galaxy than a large cluster.
And it’s not the only one. Terzan 5 is another globular-like cluster in the Milky Way’s core and also contains two distinct star populations; in fact, astronomers were suspicious of Liller 1 because they had previously examined Terzan 5 and come to the conclusion that it was also what they call a bulging fossil fragment.
Both of these objects are difficult to see due to obscuration from dust, but imply that there may be more within the core, partially hidden from view. If these are truly surviving structures from the birth of our galaxy and the building of its bulge, they are essential pieces of the puzzle of how our Milky Way came to be. If we can find more, then solving this puzzle becomes more doable.
And it’s like I said: in astronomy, there isn’t just a pretty picture. There is always a much, much bigger picture of which he is a part.