Spectacular and sparkling, but what is it?

By Phil Plait | May 23, 2011 7:19 am

Globular clusters are among the most spectacular of objects in the entire night sky. Compact balls of hundreds of thousands of stars, well over a hundred orbit our galaxy at various distances. When viewed by Hubble, the result is nothing less than jaw-dropping:

[Click to embiggen, and please do; I had to crop the image to get it to fit and the full-size version is even more spectacular!]

This view of Terzan 5, as it’s called, is gorgeous! The thing is… Terzan 5 may not really be a globular cluster. Sure, it’s a cluster, and it’s globular, but it may not be what we usually think of as a globular cluster.

When I read the press release for the picture, the name Terzan 5 looked familiar. So I searched my blog, and found I’ve written about this object before. That post was about a ground-based Very Large Telescope picture of the cluster, seen here. The picture looks odd because Terzan 5 lies in a very crowded region of the Milky Way, lousy with dust. That interstellar junk tends to scatter away blue light, making objects look redder. The dust blankets across Terzan 5, but is thicker in one half than the other, making that side redder than the other.

Terzan 5 itself is also unusually dense, with stars packed in it more tightly than is usual for a globular cluster. Not only that, but studies have shown the stars in the cluster appear to fall into two different age groups; one significantly older than the other. That’s weird. In most clusters, the stars are all the same age, indicating the cluster formed all at once. Terzan 5 isn’t like that, so maybe it has a different birth story.

Astronomers think that the cluster may actually be the remains of a dwarf galaxy, one much smaller than ours, that got eaten by the Milky Way. Billions of years ago the two collided, and the gravity of our bigger galaxy stripped away many of the stars in Terzan 5. What was left over was this roughly spherically-shaped ball. The stars are different ages because some galaxies tend to form stars in bursts, making lots of stars, then not many, then lots of stars again as time goes on. That fits with what we see here.

So Terzan 5 is basically the undigested bits left over after a galactic cannibalism event.

The Hubble picture itself is pretty nifty, too. This is a little complicated, so bear with me. The image is false color: it’s a combination of two observations, one using a yellow filter (colored blue in the final shot here) and the other in the near infrared (colored red). The astronomers also simulated a third exposure by combining and processing the two observations to mimic what an observation through a green(ish) filter would have produced. By combining all three images, they get the results above. I don’t think creating that third quasi-green image produces much science, but it does make a pleasing picture.

It’s different than the VLT image; the striking difference in color across the VLT picture isn’t as obvious as in the Hubble picture. But if you look carefully you can see the streak of red stars going through the Hubble shot. The filters used and the way they were combined change the way we see the resulting image. And it’s more than just a pretty picture: in this case it helps understand how the stars are arranged in the cluster, and that provides clues to what happened to it in the past.

It’s not completely clear that Terzan 5 was once a galactic snack, it still may just be a very odd globular cluster. But by observing it using multiple telescopes in multiple ways we learn more about it, and hopefully will be able to unravel what happened all those eons ago to make this strange and lovely object.

Image credit:ESA/Hubble & NASA; ESO/F. Ferraro

Related posts:

The Milky Way bulges with cannibalized corpses
Scattered jewels in the core of a cluster
A buzzing beehive and a dying star

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (17)

  1. Melissa

    Looks like star soup. Mmmm…star soup.

  2. Michael Sternberg

    I have a hard time matching the two images. Are they meant to be similar in field of view and orientation? They are both optical (or nearly so), so we’d expect roughly similar stellar magnitudes, right?

    Always wondered – do astronomers develop an intuition for connecting dots?

  3. Chris A.

    Just looking at it, I’d wager that its isophotes are nowhere close to circular, compared to your average globular. It looks distinctly flattened at the top, unless that’s more of the non-uniform extinction.

    Also, haven’t they identified at least three epochs of star formation in Omega Cen? Perhaps the paradigm of globular cluster star formation occurring in one go is beginning to fall apart.

  4. dave cortesi

    What’s wrong with it being a merger between two globulars? They must occasionally interact, mustn’t they? Which would account for two age-cohorts in one cluster.

  5. Messier Tidy Upper

    Wonderful image. Superluminous! (Beyond mere brilliance) :-)

    Seems a bit like Omega Centauri which is similarly suspected of being a former dwarf galaxy in its own right. (Edit : As #3 Chris A has noted already I see.)

    Oh & because someone here has to say it :

    “My God its’ full of stars!”
    -Dave Bowman, ‘Space Odyssey : 2001’, Arthur C. Clarke & Stanley Kubrick

    @4. dave cortesi : I think the idea is that all the Milky Way’s globular clusters formed at much the same time all at once and should therefore have stellar populations of stars that are all about the same age which clashes with the varying ages of the stars in Terzan 5.

  6. Michael Sternberg

    Chris: Not sure how isophotes are usually done, but if I apply a Gaussian Blur (r = 50) and then a posterization (n = 10 … 15 levels, say), the cluster comes out only slightly egg-shaped, thick end on the left, thin on the right.


    It’s almost like I want to feel that region of distorted space-time in which that cluster is located!

  8. chris j.

    if terzan 5 is a galactic remnant with sufficient gravity to avoid being stripped by the milky way entirely, then shouldn’t there be streamers like the canis major dwarf or sag deg?

    Messier Tidy Upper @5:
    i’ve often read how the stars within a single globular tend to be the same age, but i’ve never thought to ask before: are all of the milky way’s globulars the same age?

  9. HvP

    I guess I’ve been so used to the older Messier and Herschel catalogs, and the names assigned from orbiting platforms, that the prefix “Terzan” sounds very odd.

    No disrespect to Mr. Agop Terzan, but the designation Terzan 5 sounds like something a Star Trek writer would have come up with. Actually, that’s pretty cool. Who wouldn’t want to have something this interesting named after them? And to be able to give it such a cool name at that :)

  10. Thea

    You said the colour contrast between the two groups of stars was more obvious in the Hubble image than in the VLT image. To me, however, the colour difference is more obvious in the VLT picture. I suppose it’s different to different people.

  11. Matt B.

    The second picture is rotated 90 degrees counterclockwise compared to the first. You can tell by paying attention to a small trapezoid of stars near the center and a pair near that (half-way to the left edge in the second picture).

  12. Whomever1

    So, globular cluster or dwarf galaxy remnant–would there be a black hole in the center of it?

  13. chris j.

    Georg, those stars all appear to be the same age.

  14. Grimbold

    @4- I think a merger between two globulars is definitely plausible. That would explain the two different populations of stars. It could also explain the flattened appearance of Terzan 5; mergers between two objects moving relative to each other impart angular momentum to the merged object, making it rotate.

  15. Do we have any idea of the general distance between any two stars in the cluster?

  16. Hrm … I wonder if a LOT of globular clusters aren’t the remains of galactic cannibalism.

    I remember hearing that most elliptical galaxies (which would include dwarf ellipticals) undergo one, and ONLY one, period of star formation when the galaxy first comes together. If THAT kind of galaxy collided with the Milky Way, got stripped of most of its stars, and ended up as a globe-shaped mass, it would be indistinguishable from Globular Clusters that formed the “normal” way.

    … unless galactic cannibalism IS the “normal” way.


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