Andromeda's majestic spray of billions of hot stars

By Phil Plait | May 18, 2012 11:33 am

Well, what can I say about this devastating and jaw-dropping picture of our nearest spiral neighbor, the Andromeda Galaxy?

[Click to massive chainedmaidenate. Do it!]

Well, I could start with HOLY HALEAKALA!

This image is a collection of 11 separate observations of Andromeda taken by NASA’s GALEX satellite. Launched in 2003, GALEX (which stands for Galaxy Evolution Explorer) scans the sky in ultraviolet light, specifically targeting galaxies. Hot stars produce UV light, and so does the gas it illuminates, so by looking in the ultraviolet astronomers can learn about how galaxies are constructed. In the decade since its launch, GALEX has been phenomenally successful, cataloging hundreds of millions of galaxies, some as far as ten billion light years away!

This image of Andromeda is simply stunning. It’s comprised of two colors: what you see here as blue is higher-energy ultraviolet light, and red is lower energy (closer to the kind of light we see). Right away you can see that objects emitting the higher-energy UV are confined to the spiral arms, and lower-energy emitters are spread out across the galaxy. That’s exactly what I would expect: massive stars, the kind that really blast out UV, don’t live very long. They’re born, live out their short lives, and die (as supernovae) pretty much near the spot where they formed, which is in spiral arms. Lower mass stars live long enough to gradually move away from their nurseries, populating the rest of the galaxy.

Also, star formation at the very center of the galaxy probably occurred long ago and shut down, so we don’t see many or any massive stars there.

One thing I didn’t know is that the arms of Andromeda are more like rings! The galaxy is at such a narrow angle that it’s hard to tell, but if you trace the blue emission, the pattern does look more like a ring than a spiral. This jibes with earlier images in infrared taken by Spitzer Space Telescope (which I’ve inset here) and a huge and incredibly beautiful newer one taken with ESA’s Herschel far-infrared telescope (and OMFSM you want to click that link).

From what I’ve read, it’s not clear why the spiral arms appear to be more ring-like. Which I love. Why? Because Andromeda is the nearest big spiral galaxy in the sky, a mere 2.5 or so million light years away. It’s easily visible to the naked eye from a dark site, and I’ve seen it myself countless times using my own eyes, binoculars, and telescopes ranging from small ones up to Hubble. Yet there it is, in all its huge and obvious splendor and beauty, still able to surprise me. That rocks.

And a note about GALEX: NASA recently handed off its operations to Caltech, a very unusual move. The satellite was put into standby mode in February, and I was worried it would be shut down permanently. However, Caltech signed a three-year agreement with NASA — while NASA still owns the satellite, Caltech will now be in control of GALEX’s science mission, managing and operating it. At the end of the agreement it can be renegotiated if GALEX is still in good operating condition. This is an interesting idea, and I’m not sure how I feel about it. I love that GALEX gets to continue operations, but handing off science missions to private groups makes me a little uneasy. In this particular case I think it’s fine — Caltech is a research institute after all — but the precedent may have unforeseen consequences. We’ll see.

Still and all, it’s good to see new life breathed into an important and wonderful instrument like GALEX. I certainly hope it will continue to produce cutting-edge science for years to come… as well as amazingly beautiful images like this one.

Image credit: NASA/JPL-Caltech


Related Posts:

- The cold arms and hot, hot heart of the fuzzy maiden
- The first spectacular views of the sky from WISE
- A Swift view of Andromeda
- Andromeda’s warm glow
- Andromeda: born out of a massive collision?

Comments (18)

  1. Scott

    (My first post here ever! Visit often, just never speak.) Anyway, I noticed in the photos in this article and some related posts that Andromeda’s arms are asymetrical. This is especially true for those on the inner left of the core but also for the extreme outer bands. After all the years I’ve looked at images of this galaxy, I never really noticed it before. This site always shows me something new.

    And about the shift in GALEX’s control, I agree that the law of unintended consequences may initiate in the future. Hopefully any other such shifts are kept in the research/academia world.

  2. One thing I didn’t know is that the arms of Andromeda are more like rings! The galaxy is at such a narrow angle that it’s hard to tell, but if you trace the blue emission, the pattern does look more like a ring than a spiral.

    Aren’t ring galaxies* thought to be the result of galactic collisions where a smaller galaxy passes through the disk at right angles to it – i.e. going through the face on angle like a dart through a paper plate standing vertically?

    So doesn’t this imply that this proess could also have occurred in Andromeda’s case with perhaps one of larger (or smaller?) companions such as Messier 110 and Messier 32 or another such? Do we have any other evidence for such a collison and can we discern which one of M31′s companions was most probably involved?

    Great image thanks GALEX. :-)

    PS. I wonder, if we saw Andromeda face on, would we maybe nickname M31 the “Dartboard galaxy!” ;-)

    ++++++

    * Such as Hoag’s Object and the Cartwheel galaxy – an APOD image of the latter now linked to my name here.

  3. So doesn’t this imply that this process could also have occurred in Andromeda’s case with perhaps a collision with one of larger (or smaller?) companions such as Messier 110 and Messier 32 or another satellite galaxy?

    Or maybe not? This ‘A Swift Tour of M31′ Youtube clip from NASA :

    http://www.youtube.com/watch?v=HWxBTHVhc3I

    claims the Andromeda galaxies ring structure is due to tidal interactions instead.

    Although this clip using Spitzer’s images :

    http://www.youtube.com/watch?v=8LwWdfxUuuY

    does suggest an M31 – M32 collision occurred and left its mark.

    In adidtion this one :

    http://www.youtube.com/watch?v=6eZm3LHlyrs

    notes a past encounter with Messier 33 the Triangulum Pinwheel galaxy could have also perhaps have created the disturbance we see here too!

    PS. Oh & the Astronomy Picture Of the Day image of ring galaxy Hoag’s Object is now linked to my name for this comment too.

    Hope these are informative and enjoyable for y’all here. :-)

  4. þorfinn

    what is the artefact at [5735,3825] with top left as [0,0]

  5. Peter B

    What’s the angular size of Andromeda? And how obvious is it to the naked eye?

  6. TMB

    If anyone has specific questions about the M31-M32 interaction in the Spitzer paper, those are my simulations (I’m 2nd author on that paper), so ask away. :-) =

  7. Jeff

    Wow there’s so much to look at in this picture. Makes me wish I took stellar astronomy. What star is that giant blue dot at the bottom left of the picture? It looks more luminescent then Andromeda’s galactic center.

  8. @ ^ Jeff :

    From what I can work out using the chart in ‘Collins Guide to Stars and Planets’ by Ian Ridpath & Wil Tirion (Collins, 2007.) that star is v or Nu Andromedae.

    Appearances of course can be misleading given that any star in our galaxy is millions of times closer than the core of M31! ;-)

    Incidentally, an unaided eye starhop to M31 involves looking at the great square of pegasus then downwards (upwards in the Northern hemisphere) from its corner star Alpha Andromedae – Sirrah or Alpheratz – then to Mirach (Beta And – second star down the “horses hind leg”) then across to Upsilon Andromedae – a star hosting three exoplanets one of the first exoplanetary systems ever found – then across to the faint smudge that is one of if not *the* furthest object youc an see without optical aid.

    @6. TMB :

    If anyone has specific questions about the M31-M32 interaction in the Spitzer paper, those are my simulations (I’m 2nd author on that paper), so ask away.

    Cool! Congratulations. :-)

    But, um, which paper are you referring to there, sorry?

    Questions ~wise : Could an interaction with M32 be resposnible for the ring structure of M31?

    @ 5. Peter B asked : “What’s the angular size of Andromeda? And how obvious is it to the naked eye?”

    Well, its wikipedia page – linked to my name here – observes :

    At an apparent magnitude of 3.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,[14] making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope.

    Whilst the astropixels-dot-com site notes :

    its [Andromeda galaxy's'] angular diameter is 178 x 63 arc-minutes.

  9. See source :

    http://astropixels.com/galaxies/M31-01.html

    Kaler’s stars website confirms Nu Andromedae as the most probable star youre thinking of too see :

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

    Plus click on my name for Jim Kaler’s page on it. Turns out to be a B5 dwarf and F8 main sequence type close binary star located about 700 light years away.

    Although I’m afraid I’ve messed up on the location of Upsilon Andromedae confusing it with Mu Andromedae instead as Upsilon Andromedae turns out to be further down the “Horses hind leg” / chained damsel in distress. See :

    http://stars.astro.illinois.edu/sow/upsand.html

    For info and photographic finderchart for that ‘un.
    D’oh! My apologies. :-(

  10. @6. TMB : Do we know what M32′s future is as well – will it be merging with the Andromeda galaxy and, if so, when?

    Cheers. :-)

  11. TMB

    There’s definitely lots going on… we argued (Gordon et al. 2006) that M32 can’t be responsible for the overall ring structure, but that it is very consistent with causing a “split” in the ring that you can see near M32. Previous studies that didn’t realize that it was a ring thought that those were manifestations of spiral arms, but that doesn’t match the observations as well as a ring with a big hole that’s exactly what you’d expect if M32 plowed through M31 about 40 million years ago.

    If that is correct, then M32 should merge with M31 pretty quickly… maybe 2-3 more orbits, or a billion years. The thing is, M32 is a pretty funny galaxy – it’s possibly the most compact galaxy we know in the entire universe (of course, part of that is a selection effect – more distant ones might be mistaken for stars), which would make it more resilient to being tidally destroyed by M31. But on the flip side, it could well be that it got so strange because of its interaction with M31.

    As for where the ring comes from in the first place… one intriguing possibility is the Giant Southern Stream, which is the tidal debris of something that once ran through the center of M31. We don’t really know what it was (i.e. whether it still exists or if it got completely destroyed), when it happened, how massive it was, etc… but we know from examples like the Pinwheel Galaxy that when a small galaxy plows through the center of a big one, you can easily get a ring!

  12. Messier Tidy Upper

    @ ^ TMB : Cheers for that – much appreciated. :-)

  13. Nigel Depledge

    The BA said:

    It’s comprised of two colors: . . .

    This is a surprisingly common mistake.

    Either it is composed of two colours, or it comprises two colours. “Comprise” is like “contain” – the verb has no auxiliary.

  14. Messier Tidy Upper

    @ ^ Nigel Depledge : Whaa..? Sorry but isn’t “contained” a perfectly valid word? I’m not sure I get what you’re saying there. Its certainly new to me, I’ve always thought “comprised” was fine grammatically.

    @11. TMB :

    The thing is, M32 is a pretty funny galaxy – it’s possibly the most compact galaxy we know in the entire universe (of course, part of that is a selection effect – more distant ones might be mistaken for stars),

    Wouldn’t that be detected by spectroscopic analysis? When astronomers do stellar surveys isn’t it routine that they’ll take a spectrum and classify it so once they do sufficently detailed surveys shouldn’t M32-like compact dwarf gaaxies be discovered this way by having non-stellar galactic spectra?

    Or are they too faint to be routinely picked up in this way?

  15. Nigel Depledge

    MTU (14) said:

    @ ^ Nigel Depledge : Whaa..? Sorry but isn’t “contained” a perfectly valid word? I’m not sure I get what you’re saying there. Its certainly new to me, I’ve always thought “comprised” was fine grammatically.

    I’m not sure what you thought I said here. Reading my post #13 back, I can’t see any ambiguity.

    What is wrong is “comprised of“. The of is meaningless.

    Thus, BA should have said something more like:
    It is composed of two colours . . .
    or
    It comprises two colours . . . .

    I mention the word “contain” because “comprise” behaves the same. You would never use the phrase “contained of” would you?

    So, for example, you might say that a six-pack contains 6 bottles of beer. You can equally say that a six-pack comprises 6 bottles of beer. Or you could say that a six-pack is composed of 6 bottles of beer. But one should never say that a six-pack is comprised of 6 bottles of beer. This last is ungrammatical.

    As an aside : I just realised that “contained” has its own issues, because sometimes people use a preposition with “contain / contained” (such as “contained in”), which is similarly superfluous (although there are circumstances where “contained by” is perfectly grammatical).

  16. TMB

    @14: Yes, indeed, if you take a medium resolution spectrum it would be obvious. But here’s the thing… to take a spectrum, you need to know where to point. And in order to decide where to point, you need to take an image. So if you take an image and see an object that looks like a star, you’re very unlikely to take a spectrum of it if you want to take spectra of galaxies! Hence, really compact galaxies tend to be very underrepresented in surveys.

  17. Messier Tidy Upper

    @^ TMB : Fair enough. Thanks.

    @ 15. Nigel Depledge :Ah, I get it now. Not the comprised but the ‘of’. Cheers. :-)

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