A buzzing beehive and a dying star

By Phil Plait | February 15, 2011 7:00 am

When I was younger, it was pretty common on clear nights to see me at the end of my driveway with my telescope. And one of my favorite targets to observe was (and still are) globular clusters: hundreds of thousands, and sometimes millions of stars all bound together in a tight ball due to their gravity. And one of the best of those is the fabulous M15… and when it’s seen by Hubble, well, it’s simply spectacular:

Holy wow! Click to englobulenate – I had to shrink the image a lot to get it to fit here, so as gorgeous as this is it’s a shadow of the higher-res version… or the ginormous full-res one!

M15 is relatively nearby as globulars go, about 35,000 light years. Over 150 of these objects orbit our galaxy, and so some are quite far away. Not only is it close, but M15 is also fairly densely-populated, its stars orbiting each other like bees around a beehive, making it a pretty easy target for amateur astronomers. It was one of the first things I’d go after once it got dark in the autumn, and it would appear as a fuzzy ball in my 25 cm ‘scope. Of course, when you aim the 2.4 meter mirror of Hubble at it, well. You can see for yourself.

This false-color image is a combination of two pictures; one taken in visible light (colored blue; in reality the filter used let through yellow and red light), and the other in near-infrared (colored red). That selects out redder stars; the brightest ones are red giants, stars nearing the ends of their lives, and the fainter ones are lower-mass stars that are still busily fusing hydrogen into helium in their cores.

If you look to the left and a bit below the cluster’s center, though, a blue glow sticks out among all the red. If you do grab the seriously super high-res version of the image, you get a much better look at it. I’ve zoomed in on it here. It’s clearly not a star; the blue halo is much larger than any star image, and you can see the rim on the left hand side is bright. What gives?

This is a planetary nebula, the shell of gas expelled by a star as it really starts to give up the ghost. Many of those red giant stars in the cluster are blowing off a slow, dense, wind of material, far too faint to see. But after a few thousand years, so much material is lost by a red giant that the hot core is exposed, and a faster wind starts to blow. This catches up to and slams into the red giant wind, compressing it. The ultraviolet light from the core of the star lights up the gas, causing it to glow.

We see hundreds of these nebulae in the sky, but they don’t last long, just a few thousand years, and not too many are seen in globular clusters. This one, named Pease 1, was actually the first such detected in a globular. It’s a challenging object using just a telescope and your eye, but shows up readily in images. And this Hubble image is the finest I’ve seen! You can actually see some structure in the gas, which is a remarkable achievement given its distance of 350,000,000,000,000,000 kilometers! That’s 210 quadrillion miles, if you like your units that way.

Eventually that gas will diffuse and merge with the ethereally thin stuff between the stars in that cluster, and the star will fade. Such is the fate of all the stars in the cluster, actually, so take a good look now and appreciate what you can see. In a couple of hundred billion years it’ll be gone.


Related posts:

- A distant sparkling eruption of diamonds
- Warm dusty rings glow around a weird binary star (more info on planetary nebulae)
- It’s full of stars!
- Alien clusters invade our galaxy
- Vampires and thrillseekers rejuvenate dead stars

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (26)

Links to this Post

  1. GET TO DA CHOPRAAAAA | Global Posts | February 17, 2011
  1. John O'Meara

    There’s something that makes me feel dirty looking at Halpha in blue….

    I kid, I kid.

  2. So when you say the stars are in a “tight ball” what does that mean distance-wise? How close together are the stars (on average)?

  3. Messier Tidy Upper

    Superluminous image & superb write up. Thankyou. :-)

  4. Yep, just a tad past Enif and there she is! Good ol’ M15. Another part of her story is the black hole she harbors. Probably all globs have them. Holds them together for a good long time, but eventually they interact with themselves and fly apart. Use a large scope and lots of power to get the best view of targets like this one and enjoy!!!

  5. MLP

    “Spectacular” is simply too weak a word to describe it. But English wasn’t designed to describe the universe, so “spectacular” will have to do. Though “awe-inspiring” might describe my own reaction a touch better…

  6. TMB

    @1: Yes, the stars are phyiscally close together. A typical globular cluster might have 200,000 stars and a radius of 1 pc, so that’s a mean density of around 50,000 stars per cubic pc, or a mean star-to-star distance of 0.03 pc (for reference, the nearest star to us is around 1 pc away). And that’s the *mean* – as you can see, it’s much more dense in the center!

  7. Grand Lunar

    I envy you Phil that you were able to be at the end of your driveway and see the stars.

    Where I grew up, the driveway was too flooded with light. The backyard was the only place good enough for observations. *sigh*

    Anyway, beautiful picture!

  8. Chris

    Ah, Phil. You need another zero. 350,000,000,000,000,000 kilometers is closer to 35,000 light years.

  9. chris j.

    if we accept that pease 1 is in M15, approx how big is it? using 1″ and M15′s estimated distance of 33,600 l.y., i came up with a diameter of about 0.05 parsec. if the average distance to its stellar neighbors is roughly 0.03 parsecs as TMB says, there’s a strong chance pease 1 has interacted with one or more stars during its 80-year observational history. perhaps that’s why the rim facing away from the center of the cluster is noticeably brighter.

  10. eyesoars

    I was hoping it was a ‘blue straggler’. Do we know of any historical instances of star star collisions? How would/could these compare with supernovae? Could we tell the difference?

  11. Fab! Another imaging target to go after!
    And I agree wit John O’Meara (#1) seeing a planetary in bright blue is kinda weird.

  12. Quiet Desperation

    Poor little feller… :-(

  13. Pete Jackson

    “In a couple of hundred billion years it’ll be gone.” Surely the M dwarf stars in the cluster will last longer than that. With masses of about a tenth that of the sun, and luminosities as low as a millionth that of the sun, they should last at least tens of thousands times longer than the sun, or many trillions of years.

  14. Joseph G

    Nice!! Superluminous, indeed.

    I do have a question for y’all, though: I keep seeing references to infrared images captured by the Hubble (apparently infrared capability was added on one of the maintenance missions). But I thought infrared telescopes had to be cold? The Spitzer IR telescope ended its service life when it’s liquid helium coolant was exhausted, and the upcoming James Webb telescope has that impressive layered sun-shade to keep it cool. Hubble doesn’t have anything like that, though, right? I don’t get it.
    Edit: That wasn’t a question, was it. Ok, question: How can the Hubble take IR images without a fancy cooling system?

    Oh, question number 2: Are globular clusters anything like dwarf galaxies? Do they form independently, or are they simply what’s left of dense star-forming regions within a galaxy?

  15. Joseph G

    @#6 TMB: Wow, imagine what the sky would look like from a world in that cluster. And all the proper motion of those stars! If there’s intelligent life in there, I’ll bet their astronomers are extremely busy and their astrocartographers are extremely frustrated :D

  16. CB

    @ Joseph G:

    Edit: That wasn’t a question, was it. Ok, question: How can the Hubble take IR images without a fancy cooling system?

    The HST only goes a little ways into the infrared, which is why a dedicated IR telescope like Spitzer had to be launched. It’s only at deeper infrared wavelengths that the fancy cooling is needed.

    The difference as far as cooling requirements is all about black body radiation. Bodies emit radiation, and the quantity and wavelengths depend on its temperature, with the peak wavelength moving to shorter and shorter wavelengths as the temperature increases. The longer the IR wavelengths you want to observe, the more the observatory’s own radiation will drown out cosmic sources unless it’s cooled.

    So for Spitzer, this is a big problem, while for Hubble it’s not because Hubble isn’t trying to observe at those wavelengths. Also, this is why WISE was able to continue making observations using a small subset of instruments even after it ran out of coolant.

    This is my understanding of the reason; hopefully it’s accurate and helpful. :)

  17. KC

    Re: star collisions, see: http://tinyurl.com/4zzexr7

    Short gamma-ray bursts are thought to be the result of neutron star collisions. I believe some type Ia supernova are thought to be the collision of two white dwarfs.

  18. reidh

    What did this (or any other) cluster look like 70,000,000 years ago, and what will it look like 35,000,000 years from now? Can you tell?

  19. Messier Tidy Upper

    For those who want to see Messier 15 for themselves there’s a photographic finder chart here :

    http://stars.astro.illinois.edu/sow/peg-t.html

    for it via Kaler’s marvellous website which also has a page on M15 here :

    http://stars.astro.illinois.edu/sow/m15-p.html

    [I was about to post those links last night when the power went out - and stayed out. :-( ]

    @11. eyesoars :

    I was hoping it was a ‘blue straggler’. Do we know of any historical instances of star star collisions? How would/could these compare with supernovae? Could we tell the difference.

    We think that at least some type Ia supernova (there are a couple of models both of which may be right in different cases) are indeed produced by star-star collisions – specifically two white dwarfs that orbit closely and end up spiralling into collision with each other.

    Tycho’s supernova :

    http://en.wikipedia.org/wiki/Tycho%27s_Supernova

    which was seen in 1572 by the great astronomer Tycho Brahe & revolutionised our understanding of the sky was one notable example. :-)

  20. Messier Tidy Upper
  21. Messier Tidy Upper

    @19. reidh asked :

    What did this (or any other) cluster look like 70,000,000 years ago, and what will it look like 35,000,000 years from now? Can you tell?

    Well, some globular clusters (“globs”) – Omega Centauri being a notable example – may have been dwarf galaxies in their own right in the very distant past – and boasted more stars which were subsequently stripped away. It seems that Kapteyn’s star which is now a mere 13 light years from us was a past member of Omega Centauri which is now 15,800 light years distant.

    Globular clusters are made of stars which evolve over time so logically enough, globs would have been full of brighter early spectral class (types O-B-A) stars which have since perished and will, logically, dim as they become increasingly dominated by fainter mid to late spectral class stars (F-G-K-M) plus “stellar corpses” such as neutron stars and white dwarfs. Although as their stars evolve there will always be a fair number of red and orange giants dominating their light output. (No supergiants though with a perhaps the odd exception for blue stragglers since those high-mass short-lived stars burnt out earlier.)

    Over billennia these ornage and red giants will, I think, gradually get fainter because as less massive stars end up producing smaller and dimmer giants. (this might be wwrong but) At some point, the lowest mass ornage or red dwarf stars cease to become giants at all instead they circulate all their fuel internally and consume it rather than having distinct layers so ending up simply “fading out” into degenerate black dwarfs rather than ballonning into gianst although this willonlyu occur after trillions of years have passed. I’m not qite sure where that cut-off point is. (Trivia fact via Kaler’s 100 Greatest Stars book no star wiwitha spectral type less massive than G8 has yet died!)

    Globular clusters also undergo processes of core collapse – where they become very compact in the centre as most massive stars wind up there and lighter ones get thrown to their outskirts – and erosion as passes near and through the galactic spiral arms strip away stars from the outer regions, effectively “evapourating” them out into the Galactic halo region. (As again, Katpteyn’s star is an example.)

    I gather we have detected young very bright globulars even still forming ones in other galaxies and that even many of the milky Way’s globs are different in ages, but I could be mistaken. I am pretty sure that insome of the larger globulars – Omega Centauri for one – there have been at leats two different star forming bursts.

    All of which I guess doesn’t quite answer your *specific* question there but I hope its interesting & helpful anyhow. There’s always wikipedia and the various astronomy magazine sand websites if you want more & perhaps get a more specific answer too! :-)

  22. TMB

    @Joseph G: As MTU says, there are a few things we call GCs that are almost certainly the nuclei of former dwarf galaxies. But most GCs are quite different. The key difference is that dwarf galaxies form in a dark matter potential well, while GCs form from dense gas knots that aren’t associated with dark matter overdensities. (or more precisely: what we observe is that, at a given luminosity, dwarf galaxies are much more extended – by a factor of like 100 – than GCs. This implies that the dwarf galaxies have dark matter, since they otherwise would have long ago become unbound and no longer be identified as a coherent object).

    And yes, the sky must look spectacular from inside a GC! I’m sure people must have simulated the sky from a hypothetical GC planet before, but I don’t think I’ve ever seen it.

  23. Joseph G

    @ CB and TMB: Sorry I didn’t get back to you earlier.
    Thanks much, I feel enlightened :)

  24. DHMO

    @16 Joseph G: For an interesting take on the view from inside a GC, read Isaac Azimov’s “Nightfall”. Very good story.

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