The face of beauty

By Phil Plait | February 5, 2009 11:26 am

[Edited to add: If you like this article, please Digg it so others see it. Thanks!]

Holy Haleakala!

That’s NGC 4921, a face-on spiral galaxy in the Coma cluster of galaxies, over 300 million light years away. This Hubble image is a mosaic of 80 separate images, and has stunning clarity and depth. Look at all those background galaxies! You really want to click that and get the bigger version. Trust me here. Wow.

The spiral arms of this galaxy are a bit poorly defined, but that adds to the beauty of this in my opinion. That struck me instantly, and the reason struck me as well: spiral galaxies in clusters are odd ducks. Coma is a rich, dense cluster, with thousands of citizens. That makes collisions common, and spiral disks are relatively fragile. When a spiral collides with another galaxy of comparable mass the disk is disrupted. Whether the two galaxies pass through each other or eventually merge, the disk is usually collaterally damaged or destroyed.

So NGC 4921 is a bit of a freak in that it exists at all. But the cluster itself is filled with tenuous gas, like smog in a city. As galaxies pass through it, the pressure from this gas pushes on the gas inside the galaxy, sweeping it out (like driving with your windows open removes bad odors lingering in your car, possibly also due to gas). In general (though not always!) spirals in rich clusters don’t have lots of gas, and so they can’t form stars. In "field" spirals — galaxies that are on their own in deep space, something like ours — images show them to have strings of intensely reddish concentrated gas clouds vigorously forming new stars, but NGC 4921 is devoid of those. Most of its gas has been swept away by the ram pressure of its travels across the cluster. That makes the spiral arms smooth. But since no stars have been formed in this galaxy for a long, long time [Edited to add: whoa, hold on there. Maybe there has been recent star formation; see this comment and my follow-up below], the galaxy is also predominantly yellow-red, because all the hot young blue stars are long gone, exploded as supernovae shortly after they were born millions of years ago.

Wait, yellow-red? But the galaxy looks blue! Yes, in real life this galaxy should look reddish, but this image is false-color: it was taken using a yellow filter and a near-infrared one. So what you see as bluish in the image is actually yellow or red to the eye. What’s seen as red is actually infrared. Weird, isn’t it?

You can also see a ragged ring of dust circling the galaxy’s core. That’s interesting to me; the gas is gone, but dust remains. Either it’s harder to remove than gas, or it’s a relatively new feature created in stars and blown out into the galaxy.

Detail in the image of NGC 4921

And finally, the background galaxies. Wow. I strongly urge you to go to the Space Telescope page and grab one of the really big images, which are seriously huge. Then just scan the background and look at all the weird beasties lurking there. Ragged galaxies, elliptical galaxies, edge-on spirals, galaxy fragments… it’s a menagerie of shapes and color. And what’s that hanging above and to the right of NGC 4921? It looks like either a huge globular cluster of stars, or a dwarf elliptical galaxy. I lean toward the latter due to its size, but from this image alone it’s hard to tell… and this is the deepest image ever taken of this region, so I don’t think we’ll get an easy answer to that question. Even Hubble has limits.

Image Credit: NASA, ESA and K. Cook (Lawrence Livermore National Laboratory, USA)

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (71)

  1. Wow. Just…wow. That, and the foreground stars, and the background galaxies. It’s mind-bogglingly huge and terrifying and beautiful, all at once. And some people not only think that it was all created just for us, but that the creator is very, very concerned what I do with my wee-wee.

  2. Astronomynut

    If you go to the Hubble site, they have an annotated image. They list an area along one of the spirals as having “clusters of recently formed hot young stars”. You say there shouldn’t be any young stars left.

    What gives Phil?

    Beautiful image though.

  3. MZ

    Have we been able to take pictures of galaxies in mid-collision? That would be a sight.

  4. I boggled over that image for about half an hour this morning before I wrote my own blog entry on this gorgeous pic. what blew me away was the detail you could see THROUGH the galaxy of the galaxies beyond it… it just is amazing.

    yeah, there are hot young stars here, but they could well be the last generation of new ones unless something happens to stoke the fires of star formation again…

    but what a lovely diaphanous view! I join Phil in urging you to go check out the embiggened versions at the space telescope site… this is one of those pics that just keeps you coming back for more!

  5. I see a new desktop wallpaper! Also, I wonder how many of those galaxies in this image have civilizations, with their own versions of the Hubble Space Telescope, peering out into the depths of space, and seeing the Milky Way, and it’s beauty, and also wondering if anyone is looking back at them?

    Maybe they even have their own Bad Astronomer, getting them fired up about the wonders of the Universe! :)

  6. Hmmm, I missed that clump of stars. Now that’s very interesting… and I have a new hypothesis: this galaxy recently ate another one. That might explain both the star-forming region and the ring of dust. The text of the press release does mention young stars… but I would need to see an image taken with a blue filter as well. Are they really blue, or do they just look blue in this filter combination? This image has that clump actually being blue, but I’m not sure what filters they used there either/

  7. Me

    I’m going to go hide under my bed until I get the immense scales at work here out of my head.

    I can barely cope with the distances involved in my daily travel, and I live in the UK. This kind of thing literally blows my mind.

  8. Jamie Mueller

    Is it possible that some of the background galaxies are gravity lensed? Just looking at the image in close there are a lot of “pairs” of objects that are eerily similar?

  9. Cheyenne

    Extraordinary picture. “Composite of 80 images”. I used think that they could just point the Hubble somewhere, push a big red button, and then the telescope would go “click”, and a huge polariod like photo would just immediately spit out down in mission control.

    I’m beginning to think that doing cutting edge Astronomy involves some pretty painstaking work by lots of folks….so cool. Really neat to see other galaxies through its arms, like others have said the scale is just- it’s incomprehensible really (to me), and on the embiggened photos that galaxy in the bottom left looks ferocious. Not cute like the main one.

  10. Chris A.

    Amazing. Just amazing.

    But I’m also a tad confused. For years I’ve been compiling a list of all the Messier and NGC objects that Hubble has imaged and released to the public, based on STScI’s archive of news releases. During that time, I have always assumed that any images released from Hubble’s ESO site were also released from STScI. But this image appears not to have been released on this side of the pond. Does that mean I need to search through the ESO site for all the M’s and NGC’s I may have missed over the last 18 years? Ack!

  11. Most awesome and splendorous.

    The universe is cooler than you can make up.

  12. David

    Has anybody said that this image is the result of 80 exposures totalling 100,000 seconds, or roughly 28 hours?…

  13. Thank you so much for posting this. These kinds of images bring tears to my eyes, because I’m that awestruck by this amazing universe we live in. Each speck a galaxy! teh hubblz, it roolz.

  14. BethK

    Words fail me. I see this image and wonder whether it’s a painting while knowing full well that an artist wouldn’t paint it with that many colors, variety, or orientations. How can there be so many galaxies out there in one small section of our sky?! The large galaxy at lower left looks to be a collision of some sort. WOW!

  15. Look, there’s a cute little Cen A clone down at about 5 o’clock.

  16. Naked Bunny With a Whip said:
    “And some people not only think that it was all created just for us, but that the creator is very, very concerned what I do with my wee-wee.”

    I think I might be more concerned than the Creator, with what someone named Naked Bunny With a Whip does with his, umm, “wee wee” as you call it. ;) :)

  17. Philip

    If spiral galaxies are rare in clusters, how come there are so many of them in this picture?

  18. Jumblepudding

    300 million light years away…So, we’re looking at the nearest objects in this photo as they were before the Carboniferous period, right? So the hot young stars aren’t as young or sultry as they appear here. It’s like looking at a girly mag from the 70’s.

  19. ChazInMT

    Funny you wrote “Holy Haleakala” as the expletive here, cause in a week or so, I hope to be on Haleakala seeing, for the first time myself, the southern cross peeking above the horizon. I just found out I was going to Maui last week and didn’t even know Haleakala existed until then. Small world……

    This photo forces me to marvel as well, much as the various deep field images do. What’s amazing is when you look at NGC4921 on Google Sky, it is just a faint shell of what this picture is.

    And then I remind myself, that no matter which “Faint Shell” of a picture you would look at in the heavens with the Hubble, it would yield the same mind blowing, eternally humbling, and awe inspiring result. I did the math, and it turns out that the Hubble would need to take 8.5 Million images of this size in order to give a complete picture of the celestial sphere.

    Holy Haleakala doesn’t even begin to cover it.

    We are so very, very, very, small.

  20. That’s the most beautiful and mind-blowing image I’ve seen in a while, wish I wasn’t looking at it on my 10″ EEE screen :p

  21. Philip: those are all background galaxies, much much farther away, and not in the Coma cluster.

  22. Dee

    OMG – Hubble has done it again -that is unbelievably gorgeous! Words fail me, I can only agree with all the above comments. Look at that and then wonder what we will see after Hubble is upgraded – if it’s possible to get better than that our minds are going to be blown away!

    Go Hubble!

  23. I look at lovely pictures like this, showing all those other stars, and all the other galaxies and think to myself there’s no way we can be alone. The odds seem to be so against it. Even if only .000001% of all stars support intelligent life, that’s still a crapload.

    Kinda reassuring.

  24. Wow. That’s an amazing shot.

    @MZ, yes, there are lots of photos of galaxies in mid-collision. A Google image search for “colliding galaxies” is a good place to start, as is the APOD index page.

  25. Joe from Denver

    Is it just or me, or perhaps I’ve been reading the BA blog too much, or maybe I’m just spending too much time in Boulder, but I see the outline of a giant space chimpanzee head!

    All hail the Great Space Chimp!

    :-)
    Joe from Denver (I gotta go lay down. Where’s my medicine? :-)

  26. IVAN3MAN

    Phil Plait:

    And finally, the background galaxies. Wow. I strongly urge you to go to the Space Telescope page and grab one of the really big images, which are seriously huge. Then just scan the background and look at all the weird beasties lurking there.

    [Rant]
    I will, eventually, but I’ll have to wait until the early hours of the morning because my so-called broadband connection has slowed down this evening to a crawl like bloody dial-up! I’ll bet that’s due to idiots cluttering up the Internet! Bastards! :evil:
    [/Rant]

  27. That is utterly gorgeous.

    *stares*

  28. BethK

    My 13yo son said it makes him feel small.

    But then he rotated and squashed that bright galaxy at lower left to be in the same orientation as the one at its 8:30 location in the corner. It sure looks like it matches. The big one is the one labelled on the captioned photo as ‘More distant spiral galaxy with very bright nucleus’

    I told him I don’t know of a mechanism by which reflections are rotated and squashed. Do we? Or has he discovered what he’ll study for his astrophysics PhD? Never mind that he wants to be a filmmaker and hence his aptitude with gimp.

  29. Pat Durrell

    That little object looks a lot more like a dwarf elliptical to me (or to confuse matters, likely a tiny dwarf spheroidal galaxy). Even with the great resolution of the Hubble, globular clusters would not be resolved so cleanly at 300 million light years (indeed, the annotated press release picture shows globular clusters as some of the tiny point sources at the lower left). One the other hand, it should not be too much of a surprise…I would expect quite a few of those little dwarf galaxies in Coma :)

  30. I still think the Andromeda Galaxy is the most beautiful ever. Love seeing competitors.

  31. Drew

    Absolutely majestic. Who needs conspiracy theories and hidden moonbases when you can see something like this and its real!!! Phil, do you also do amateur astronomy? Do you have a telescope with which you gaze at the wonders of the universe from your own backyard?

  32. Mike

    I feel very very small, which is a good thing to experience periodically.

    To ChazInMT who is going to Maui soon…try as hard as you can to get to the Big Island. Rent a 4-wheel drive vehicle and, ignoring the prominent warnings on your contract, drive on the Saddle Road to Mauna Kea. Then, drive up to the peak. The final long ascent after the lower station is unpaved, and it might seem hairy, but the first time I did it was in a rented Ford Taurus (nothing was going to stop me from getting to the top!). Drive carefully. Take a warm jacket. Make sure that you can get down before dark (around 2 hours each way). When at the top, you’ve made one of the fastest accessible ascents on the planet….sea level to 13,700 ft in 2 hours. You’re lungs will feel it! And those scopes….wow. If you’re lucky, or plan ahead, there might be a tour going on that you can join. The first time I went up there was one, and I just stepped into it for a bit.

    Seriously, dude, if you’re reading this site, you won’t want to miss it. The views of Haleakala above the clouds are well worth the effort alone.

    The Big Island is where it’s all happening…you can even check out some lava relatively safely….go to:

    http://www.nps.gov/havo/planyourvisit/lava2.htm

    Maui is great too, Haleakala is an amazing landscape. So don’t sweat it if you can’t make it to Hawaii…all I’m saying is, try if you can!

    Mike

  33. Karthik

    > Most of its gas has been swept away by the ram pressure of its travels across the
    > cluster. That makes the spiral arms smooth.
    >
    So, the spiral arms contain mostly gas? Any idea why this was not dispersed due to ram pressure?

  34. CR

    I recently argued with a person who feels that the space program in the US is a waste of time & money, as it has “no benefit” to anyone. I pointed out various technologies (including medical), various employment opportunities, and tried to wrap up with the idea that just exploring, just gathering data might help someone in the future, because we bothered to take the first steps now. Isn’t that worth it?

    In that person’s opinion, no it is not.

    So, this NGC4921 image doesn’t put food on my plate, fill my vehicle’s gas tank, fix my cold virus nor buy textbooks for my local school. Too bad. It’s still worth every penny spent. It is already helping some people get just a little closer to understanding a greater whole, and as part of that puzzle, really does contribute to our future understanding of the world around us. Maybe someday someone will offer their thanks to us for getting things started, maybe not. But the fact is we did it, and we can do so much more, including fixing up our daily problems.

    That’s the part of my argument I never thought to make then: that the two things, space exploration and the rest of the daily world news, need not be mutually exclusive; we could and should have the best of both worlds. Deal with the daily bits of EXISTENCE, and have inspiration (such as this image provides) to LIVE and enjoy that existence. (I suppose I could have argued that this passion for space exploration is similar to enjoying music, or watching a movie, or enjoying artwork… none of that feeds the hungry, either, but life would be boring without it. Plus, space exploration, this beautiful image, is REAL… it’s actually out there, it’s not made up like the other forms of entertainment. I suspect, though, that the other person would have said that it’s much cheaper to make a song or paint a picture. Sigh…)

    ******************

    Anyway, this is an amazing image. The main galaxy is awesome enough. But just look at all those other galalxies. Imagine all the stars contained in just ONE of those, then in all of them. My mind reels, and yet I want to know more, to see what else there is!
    Wow!

  35. Brian Fane

    The thing that really makes my jaw drop when I look at this galaxy is how you can see TROUGH it to other galaxies. Normally, galaxies appear to be solidly packed with dust and stars, but this image really brings home how tenuous they really are.

  36. Brian Fane

    BTW, the Hubblecast HD video podcast has a pretty good episode going over this image; zooming, talking about different features, etc. It’s pretty big (154 MB) for 4 1/2 minutes, but worth the download.

    http://www.spacetelescope.org/videos/hubblecast.html

  37. IVAN3MAN

    CR, that person you referred to fits definition #1.

    Dictionary.com

    philistine noun 1. (sometimes initial capital letter) A person who is lacking in or hostile or smugly indifferent to cultural values, intellectual pursuits, aesthetic refinement, etc., or is contentedly commonplace in ideas and tastes. 2. (initial capital) A native or inhabitant of ancient Philistia.
    adjective 3. (sometimes initial capital letter) Lacking in or hostile to culture. 4.Smugly commonplace or conventional. 5. (initial capital letter) Of or belonging to the ancient Philistines.

  38. If anyone ever wanted to truly try and grasp the scope of infinity, this pic would be a good place to start and then realise that is just cannot be done.

    Utterly amazing!

  39. Cheyenne

    @Jack – Yeah the Andromeda is cool but she’s headed right for us! Duck! I am going to be very upset with her in another 3 billion or so years when she rips apart the Milky Way.

    But along with the above photo and some of Andromeda I think the Sombrero is right up there. I rarely stare at astronomy photos but I did that with the Hi-Res Sombrero ones for about 15 minutes (which is an epic long time for an ADD head like me).

    @Sci-Fi- The scale to me is somehow humbling but sort of inspiring at the same time. That doesn’t make any sense, it’s weird that I get that feeling from that photo. But it’s the way I felt with the Deep Field shots too. Awesome.

  40. Gary Ansorge

    Amazing what can be done with a mere 4 percent of the mass/energy of the universe,,,

    Wonder what it would look like out “there” if we could see all the Dark matter/energy in EM wavelengths???

    Now, if we could just get funding for a really BIG space telescope, like say, 200 meters in diameter???

    Only 8 million such photos to encapsulate the entire visible universe? I predict, if such is ever accomplished, that someone will see a conglomeration of such objects forming a picture of God,,,or maybe just Jerry Garcia,,,

    Gary 7

  41. Winter Solstice Man

    To the person who said how this image is worth every penny even though the galaxy is doing nothing immediate and definite for him – so true, and just imagine, in that galaxy there may be beings looking at an image of our galaxy from one of their telescopes and one of them is arguing about whether this astronomy stuff is doing them any immediate good.

    Hopefully we are the only planet in the Universe with short-sighted beings.

    And it ain’t a galaxy, it’s the Omega Point as seen from deep space! See how all the other galaxies are being pulled into it!

  42. TheWalruss

    Question!

    There are so many awesome galaxies to see in that picture. At first I thought it was because they were in the same cluster, meaning that if one were to live on a planet orbiting a star somewhere nice in one of the galaxies in the cluster, I would see a gazillion galaxies with their spiral arms blazing and elliptical discs spread out in their fuzzy glory.

    But that’s not really the case – those spiral galaxies are *behind* the cluster in which the main galaxy in the image resides, according to Phil. This makes sense – when taking pictures at “normal” scales with, say, a telephoto lens, the background is comparatively less small than what you’re focusing on, than if you were right up at your object, because the field of view is so narrow. Much more so, of course, on the scale of hundreds of thousands of light-years.

    So my question is: is there a place in the Universe where I could crane my neck, look up, and see a bunch of galaxies as the colorful swirls that we love so much? In other words – how dense are galaxy clusters? The Milky Way is a free-floating galaxy, right? Except for our little dance with Andromeda, of course.

  43. quasidog

    @ The Walruss

    The Local Group (our cluster) is really the only naked eye cluster you can see … there are 35 galaxies in all but a lot of them to the naked eye are barely visible anyway. There are some cool little maps on the following wiki link that give an idea of what is there, mainly little satellite galaxies.

    http://en.wikipedia.org/wiki/Local_Group

    On the next link’ there is a listing for naked eye groups a bit down the page. Thought I would just post these for reference anyway.

    http://en.wikipedia.org/wiki/List_of_galaxy_clusters

  44. ChazInMT

    The more I look at it zoomed in close up, the more I think I have found the single most amazing galaxy there could be. If you notice that smallish galaxy at the very bottom left of the frame, (the 2nd largest object in the picture), it has an incredible barred spiral structure. The amazing crazy thing about the Galaxy is that the core is insanely bright. How could it be that bright? Boggling indeed.

  45. quasidog

    @ ChazInMT

    I have one theory here …. coincidence. The object right in the center giving off those ‘star’ like rays (which I believe are caused by diffraction of light off the secondary mirror arms) ….is actually a star.

    It could just be that a star just happens to be right in the center of the shot, between Hubble and the galaxy itself. If that is the case its an amazing alignment.

    Might be wrong, it could just be an extremely active galactic nuclei but my guess is that there is a a very bright start in between.

    Just a guess. :)

  46. ChazInMT

    I thought the same thing maybe too, but the odd’s of the bright area being a foreground star are about the same as martians coming over to my house and having a kegger….I mean that sucker is smack in the middle, it has to be the “Nucleus of Untold Peril”! In fact….that’s what I’m naming it….The NUP!

  47. BethK

    On the annotated photo, they note that the spiral at lower left has a bright center. My son thinks it’s the same one as a bit to its left but just rotated a bit and squashed.

    See the annotated image at http://www.spacetelescope.org/images/html/heic0901b.html

  48. quasidog

    I don’t think the odds would be that big. I mean … as far as we know its the first one you have found right? The sky is full of lots of really cool coincidences, like when a galaxy cluster is right in front of a quasar and then by gravitational lensing makes a few images of it surrounding the cluster. Given that there are millions of visible stars in the sky, and so many more galaxies beyond that, I would assume eventually you are going to get some sort of alignment. I still reckon its just a chance alignment. It would be really cool if someone that knows about this actual galaxy could comment on it … but what are the chances ? heehe :) Ill try doing some googling.

  49. MadScientist

    I remember staring at photographic plates (which became extinct over 10 years ago but which have been declining since the invention of the Vidicon tube and later the CCD) – these plates were typically looking at a fairly large section of the sky and I used to look around and spot the galaxies. The galaxies in those plates always appeared much smaller than the galaxies seen in the background of this image – that’s just amazing. I’m also amazed by those bright blobs with radial lines (due to diffraction around the support structure of the secondary mirror) – since they have a size > 1 pixel they must be bright distant galaxies (or star clusters – I wouldn’t know). It’s just amazing what modern instruments can show us; what astronomer from 100 years ago would ever have imagined seeing these things.

  50. quasidog

    @MadScientist … I believe those blobs with the star like lines (mostly at the top of the image) are just foreground stars from our own Milky-way .. right? It usually a dead giveaway that they are just foreground stars, in normal circumstances probably extremely faint or invisible to the naked eye.

  51. Bein'Silly

    Uh, Phil did you say blue … ?

    {Checks screen, zooms up again, cuts & pastes}

    “But the galaxy looks blue! Yes, in real life this galaxy should look reddish, but this image is false-color: it was taken using a yellow filter and a near-infrared one. So what you see as bluish in the image is actually yellow or red to the eye. What’s seen as red is actually infrared. Weird, isn’t it?”

    Yup. He said ‘blue.’

    Which is indeed very weird because it looks very white to me. ;-)

    Perhaps I need my eyes checked but that galaxy NCG whatever it was looks like cappuccino froth or that white icing on a doughnut as I see it.

    Hmm … I’m feeling hungry now! ;-)
    Its a sign from the heavens – remember extra cream! ;-)

    Quickly checks again .. before heading to Doughnut King. Yes, looks definightly milky and not-so much blue. Am I wrong people?

    Still blue or white, its an awesome image BA.
    THX for sharing it with us. :-D

  52. @BA “As galaxies pass through it, the pressure from this gas pushes on the gas inside the galaxy, sweeping it out (like driving with your windows open removes bad odors lingering in your car, possibly also due to gas). In general (though not always!) spirals in rich clusters don’t have lots of gas, and so they can’t form stars.”

    The original gas inside the galaxy is gravitationally bound to it. It therefore must take energy to dislodge it. Are there any research reports containing what the density of the gas outside the galaxy is compared to the density of the gas inside it? Also, what is the velocity of NGC4921 compared to the outside gas? I would imagine that there must have been a strong shockwave produced by the interaction of the incoming gas and the native gas in the galaxy.

  53. Based on this blog posting I would imagine that NGC4921 must be a low luminosity galaxy. Any numbers for what its overall luminosity is?

    The luminosity of each star goes as: L(M) = M^3.5

    But low-luminosity stars are much more frequent than high-luminosity stars:
    N(M) = k * M^(-2.5)

    The total luminosity of the galaxy should be something like:

    L-galaxy = integral of L(M) * N(M) * dM

    = integral of M^3.5 * k * M^(-2.5) * dM

    = integral of k * M * dM

    = 0.5*k * (M-max^2 – M-min^2)

    So the total luminosity of the galaxy should be proportional to the square of the maximum mass of stars contained in it.

    A galaxy like our Milky Way has spectral type O0 main sequence stars with a mass of 16.8 solar masses. Of course these stars only last 8.6 million years.

    Depending on when NGC4921 got its gas removed it will have less massive stars than the Milky Way. For example, if NGC4921 got its gas removed 600 million years ago then the most massive stars it has would be spectral class A0V with a mass of 3.04 solar masses (they would just now be nearing the end of their lifetime). Thus, the luminosity would be 3% of the Milky Way’s luminosity (or about 3.0E35 watts).

  54. Spectroscope

    Tom Marking:

    Based on this blog posting I would imagine that NGC4921 must be a low luminosity galaxy. Any numbers for what its overall luminosity is?

    The luminosity of each star goes as: L(M) = M^3.5

    But low-luminosity stars are much more frequent than high-luminosity stars:
    N(M) = k * M^(-2.5)

    The total luminosity of the galaxy should be something like:

    L-galaxy = integral of L(M) * N(M) * dM

    = integral of M^3.5 * k * M^(-2.5) * dM

    = integral of k * M * dM

    = 0.5*k * (M-max^2 – M-min^2)

    So the total luminosity of the galaxy should be proportional to the square of the maximum mass of stars contained in it.

    A galaxy like our Milky Way has spectral type O0 main sequence stars with a mass of 16.8 solar masses. Of course these stars only last 8.6 million years.

    Depending on when NGC4921 got its gas removed it will have less massive stars than the Milky Way. For example, if NGC4921 got its gas removed 600 million years ago then the most massive stars it has would be spectral class A0V with a mass of 3.04 solar masses (they would just now be nearing the end of their lifetime). Thus, the luminosity would be 3% of the Milky Way’s luminosity (or about 3.0E35 watts).

    Ok problem one:

    I think the most massive stars inour galaxy areactually O3 – Idon’t know that we’ve ever found stars of O0 at all …

    & problem two:

    Stars of class AO V (main-sequence dwarf) may be the most massive but would they be brightest or would there be some orange or red giant stars that are brighter despite being less massive because of their greater surface area?

    & problem three

    Is this taking into account evolving sub-dwarf (metal-poor) stars?

    Not meaning to knock you, Tom Marking – just some hopefully constructive feedback to consider.

  55. @Spectroscope “Ok problem one: I think the most massive stars inour galaxy areactually O3 – Idon’t know that we’ve ever found stars of O0 at all”

    Regardless of what the brightest stars that have actually been found in the Milky Way galaxy the principle is the same.

    “problem two: Stars of class AO V (main-sequence dwarf) may be the most massive but would they be brightest or would there be some orange or red giant stars that are brighter despite being less massive because of their greater surface area?”

    The luminosity function L = M^3.5 only applies to main sequence stars. However, the overwhelming majority of stars are on the main sequence. Also, the red giants that evolve from more massive stars tend to be more luminous than the red giants that evolve from less massive stars. Thus, from the standpoint of comparing the overall luminosities of two galaxies the comparison between red giants in one galaxy versus those in the other galaxy, is roughly similar to the comparison between the main sequence stars.

    “problem three: Is this taking into account evolving sub-dwarf (metal-poor) stars?”

    Unless this is a very ancient galaxy whose stars were born before there was much enrichment of the interstellar medium with metals there should be relatively few of these stars.

  56. StevoR

    Jack Ruttan said on Feb 5th, 2009 at 5:17 pm :

    “I still think the Andromeda Galaxy is the most beautiful ever. Love seeing competitors.”

    Well my choice for the winner of the “most beautiful galaxy ever award” goes to spiral galaxy NGC 2997 in Antlia.

    M51, the whirlpool galaxy isn’t too far behind it & then in third place overall but first visually – as seen with the unaided eye rather than photos or telescope – its got to be the Large Magellanic Cloud! :-)

    Perhaps the BA could set up a vote and post images of some likely contenders on this blog? One for the suggestion box Phil Plait! :-D

    quasidog said on Feb 6th, 2009 at 5:18 pm
    @ ChazInMT

    “I have one theory here …. coincidence. The object right in the center giving off those ’star’ like rays (which I believe are caused by diffraction of light off the secondary mirror arms) ….is actually a star. It could just be that a star just happens to be right in the center of the shot, between Hubble and the galaxy itself. If that is the case its an amazing alignment.”

    One way to test this idea would be to use a spectroscope & see what the spectrum revals whether its a particular star type for a single or binary star or a blend of a whole lot of them indicating an AGN – wouldn’t that work? Has that been done? Anyone know?

    @ Bein’Silly : I agree it looks kinda milk-white to me, like frothed egg white or something in colour? Then again, it is a false colour image.

  57. Spectroscope

    @Tom Marking: Okay. Thanks. :-)

  58. Considering two spiral galaxies seen from an inclination angle of almost 90 degrees:

    NGC 4921:
    distance = 320 million light-years
    apparent magnitude = +13.04
    apparent dimensions = 2.5′ x 2.2′
    real dimensions = 233,000 x 205,000 light-years

    M51 (Whirlpool galaxy):
    distance = 23 million light-years
    apparent magnitude = +9.0
    apparent dimensions = 11.2′ x 6.9′
    real dimensions = 75,000 x 46,000 light-years

    If M51 was located 320 million light-years away it would be 194 times dimmer than it is. It would have an apparent magnitude of +14.72 which is 1.68 magnitudes dimmer than NGC 4921 (i.e., 4.69 times dimmer than NGC 4921). But M51 has 0.072 times the area of NGC 4921 so we would expect it to be 13.8 times dimmer than NGC 4921 assuming the two have the same luminosity per unit area. So the luminosity per unit area for M51 is 2.95 times what it is for NGC 4921.

    Based on that number we can estimate when NGC 4921 had its gas removed. Let’s assume that the most luminous main sequence stars in M51 are spectral class O3V: luminosity = 16,300 suns, mass = 16.0 solar masses, lifetime = 10 million years. The most luminous main sequence stars in NGC 4921 should have a mass of 9.3 solar masses: spectral class = B1V: luminosity = 2,070 suns, lifetime = 43 million years.

    So NGC 4921 had its gas removed a relatively short time ago (~40 million years) in terms of galactic time scales. This result has some uncertainty due to the use of visual only magnitudes for the two galaxies in question instead of the total multi-band luminosity.

    Having such a recent loss of its gas points to some recent catastrophic event in the history of the galaxy. If the cause was the interaction with the extragalactic medium then one would have expected NGC 4921 to have lost its gas billions of years ago. So I’m a bit suspicious of that explanation.

  59. StevoR

    A few days ago now Spectroscope said :

    “… I think the most massive stars in our galaxy are actually O3 – I don’t know that we’ve ever found stars of O0 at all …”

    Actually spectroscope, I think Tom Marking got it right first time – there are such things as O0 stars. Any O type hypergiant would be an O0 star -the O being the spectral class & the zero being the designation for hypergiant status (one better than bright supergiant which is Ia.)

    That would apply whether the O type hypergiant’s decimalised sub-spectral type (if that’s the term) was 000 ie. O type & hypergiant and the brightest O star decimalised, O01 O07 or OO09.5! Although technically there may need to be spaces there somewhere or slightly different ordering – eg. O0 O,
    O1 0, O7 0 etc ..

    I think I did see the idea somewhere that the hottest most massive main
    sequence or “dwarf” type stars known in our Galaxy were O3 V stars (V =
    dwarf / main-sequence*) but then I wonder :

    1. What about the very first generation of supermassive stars that were apparently many times larger and more massive, hotter and brighter again than today’s “metals” (elements above Helium) contaminated stars? The super
    metal-poor, pure Hydrogen & helium Population III behemoths???

    2. What about stars in other galaxies perhaps also far less metal-rich and thus more like the population III first stellar generation -could there be O2, O1 & even O0 in such galaxies?

    &

    3. What about the most massive hypergiants in our own galaxy today – did they perhaps begin their brilliant careers as O0 V stars? Eg. Eta Carinae (100 Plus solar masses) the Pistol Star (ditto) Plasketts Stars – were they once those very earliest and shortest lived O type ultimates before they quickly exhausted their core hydrogen?

    —-

    * For those here that don’t already know the luminosity classes convention in astronomy is :

    0 = hypergiant, I = Supergiant (split into Ia –bright supergiant, Ib less bright supergiant & even Iab =intermediate supergiant), II =Bright giant, III = giant,
    IV= subgiant, V dwarf or main-sequence, VI = Sub-dwarf (metal poor stars) & I think also VII =white dwarf

    —-

    “…about 40 supernovae are exploding somewhere in the universe every second. However, light from most of these events won’t reach Earth for billions of years , if ever.”
    – Page 73, “Ask Astro” in ‘Astronomy ‘magazine October 2008.

    The blue supergiant star Rigel A emits more light in a minute than the Sun does in a month and is the most radiant star within a 1,000 light year radius from the Sun.
    – Ken Croswell, “The Blue Witch” in ‘Sky &Telescope’ magazine May-June 2007.

    “The Ramans do everything in threes.”
    – Arthur C. Clarke, ‘Rendezvous with Rama’, Final page (252), Pan
    Books Ltd, 1973.

    —-

    NB. Spell-checked in word & no typos seen – at least as of now.

  60. StevoR-Correcting

    Almost right but not quite. If it ain’t typos and spelling errors its missing words ie:

    2. What about stars in other galaxies perhaps also far less metal-rich and thus more like the population III first stellar generation -could there be O2, O1 & even O0 type stars in such galaxies?

    Also the formatting looks odd for some reason. Sigh. :-(

    Editing capability here please Bad Astronomer.
    ..Please, please, please!!!

  61. If anyone wants to know more about the hypergiant (Luminosity class 0) stars :

    From Wikipedia page on hypergiants – click on my name for the link :

    “The word “hypergiant” is commonly used as a loose term for the most massive stars found, even though there are more precise definitions. In 1956, the astronomers Feast and Thackeray used the term super-supergiant (later changed into hypergiant) for stars with an absolute magnitude greater than MV = -7. In 1971, Keenan suggested that the term would only be used for supergiants showing at least one broad emission component in Hα, indicating an extended stellar atmosphere or a relatively large mass loss rate. The Keenan criterion is the one most commonly used by scientists today. [1] This means that a hypergiant doesn’t necessarily have to be more massive than a similar supergiant. Still, the most massive stars are considered to be hypergiants, and can have masses ranging up to 100-150 solar masses.”

    The hypergiants wiki-page also contains a link to the first ever generation of stars(population III) from where :

    “Population III or metal-free stars (they contained metals at the very end of their lifetimes – they are said to be metal-free because the metals exist in the core and are unobservable) are a hypothetical population of extremely massive and hot stars with virtually no metal content, except for a small quantity of metals formed in the Big Bang, such as Lithium-7. These stars are believed to have been formed in the early universe. They have not yet been observed directly, but indirect evidence for their existence has been found in a gravitationally lensed galaxy in the very distant universe.[6] They are also thought to be components of faint blue galaxies. Their existence is necessary to account for the fact that heavy elements, which could not have been created in the Big Bang, are observed in quasar emission spectra, as well as the existence of faint blue galaxies.[7] It is believed that these stars triggered a period of reionization.
    Current theory is divided on whether the first stars were very massive or not. One theory, which seems to be borne out by computer models of star formation, is that with no heavy elements from the Big Bang, it was easy to form stars with much more total mass than the ones visible today. Typical masses for Population III stars would be expected to be about several hundred solar masses, which is much larger than the current stars. Analysis of data on low-metallicity Population II stars, which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 10 to 100 solar masses instead. This also explains why there have been no low-mass stars with zero metallicity observed. Confirmation of these theories awaits the launch of NASA’s James Webb Space Telescope. New spectroscopic surveys, such as SEGUE or SDSS-II, may also locate Population III stars.”

    If anyone’s interested. I know this is now a very old thread but thought, hey, why not add this info anyway! ;-)

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