Hubble snaps a cosmic photobomb

By Phil Plait | September 13, 2010 10:40 am

So you’re with a friend you haven’t seen in a while, and you’re having a great time. You decide to get a picture to save the memory. The two of you pose together, you snap the picture… only to find some obnoxious person in the background is looking right in the camera and making a face, ruining the shot. You’ve been photobombed.

Now imagine the person photobombing is actually in the foreground, better lit, and eating up half the picture with their arms flung wide right in front of you.

Yeah, galaxies get photobombed too:


[Click to photobombinate.]

Man, if I wanted to study the dwarf galaxy PGC 39058, I’d be pretty ticked at the star HD 106381. What a jerk!

But since I’m not studying the galaxy, I can actually admire the beauty of this shot. PGC 39058 is a collection of a few tens of millions of stars located about 14 million light years away in the constellation of Draco. Like most of its kind, there are some active regions of star formation in it, and the plethora of blue stars makes it clear this galaxy may be small, but it’s still chugging out stars — blue stars are massive and don’t live long, so they must be young.

The star HD 106381 is actually inside our own Milky Way galaxy which is why it looks so bright. This image is like trying to see a firefly outside your living room window past a table lamp in the way. What’s funny is that the star may appear to be fantastically bright in this image, but that’s because it’s being seen in a half-hour exposure by Hubble! In fact, the star is just barely too faint to be seen with the unaided eye; it shines at a magnitude of 6.7, so you’d need binoculars to see it at all.

Which brings up a cool point. The star is in reality a red giant about 650 light years away. It shines with about 200 times the luminosity of the Sun, so it appears to me that we’re looking at a star much like the Sun but far older. It’s reaching the end of its life, so we’re getting a glimpse of what the Sun will look like in about 6 billion years.

Even though it’s far brighter than the Sun, its vast distance (6.5 quadrillion km!) dims it to human-eye invisibility. Yet it’s a veritable beacon compared to PGC 39058. Of course, the galaxy has millions of stars, and so it’s actually far more luminous than the star. But it’s 20,000 times farther away, and because the light from an object fades with the square of its distance, PGC 39058 has been dimmed by a factor of 400 million over that of the star! That does give the star an unfair advantage.

And look beyond the galaxy itself: you’ll see dozens of smaller, fainter galaxies. In reality many of these are far, far larger and more massive than puny PGC 39058, but are so distant that their power is reduced even more. It’s impossible to tell accurately, but I’d wager a few of those galaxies are over a billion light years away (I’ve measured galaxy distances using Hubble before, and some faint fuzzies about the size of the ones here are that distant).

So let your eye wander over the image and find a faint smear of light somewhere in the background. It’s not too much to say that it could be 100 times farther away than PGC 39058, or 200,000 times more distant than that star… which itself is 40 million times farther away from us than the Sun!

I suppose, in that case, you could say that PGC 39058 is photobombing those more distant galaxies. Obnoxiousness is in the eye of the beholder.

One last thing. What are the odds of a bright star being right in front of a galaxy like this? There are roughly 10,000 stars as bright or brighter than this one in the sky. There are certainly fewer than that many galaxies less than 14 million light years away; perhaps a thousand at most (I’m being very rough here). The sky is pretty roomy: more than 40,000 square degrees, where a square degree is about 5 times the area of the full Moon.

So we have roughly 11,000 objects (stars brighter than HD 106381 plus galaxies as close as PGC 39058) spread out over 40,000 square degrees, or about one object per 4 square degrees. That’s pretty empty! That’s an average, of course, but in fact this star and galaxy are located in a region of the sky where stars are actually less densely packed on average. So the chances of this happening in this part of the sky are pretty low, and it’s just pure bad luck that this galaxy got photobombed.

But it does make a pretty picture… and give us a sense of just how big and deep the sky can be.

ESA/Hubble & NASA

CATEGORIZED UNDER: Antiscience, Pretty pictures

Comments (42)

Links to this Post

  1. Enlaces: Semana 13-09-2010 « Campos de Estrellas | August 27, 2011
  1. Agustina Iansilevich

    Hubble got you! 😀

  2. photobombinate?? LMAO… nice one.

  3. Douglas Troy

    Phil, while I lay here at home suffering from a cold, I was taken away from that misery for a moment with your post and picture.

    The Universe is amazing and I wish I could visit all those Galaxies we can see in this picture. One can only try to imagine the incredible things that exist out there beyond our sight.

    Where the heck is The Doctor when you need him; forget my cold, I’m talking about going out there … I mean … wow … just … wow.

  4. Eugine

    The probability of two objects sharing the same square degree of sky is actually much higher than 1 in 4. Let’s not forget the so-called “birthday problem” –

    I think even adjusting for these specific conditions – one galaxy, photobombed by one star – we’ll get a higher probability than 1 in 4.

  5. Charlie

    Just curious. With the rotation of the galaxy, narrowness of this image, and so on, how fast will this star move out of the way? A week, a year, a millennium?

  6. MT-LA

    Never heard the term photobombing before. In college, we called that “poaching”…and we were expert poachers.

    Wait until the last possible second, jump into frame with some buffoonery in mind, and quickly flee the scene before the shot is reviewed. There is no honor among us poachers, and clearly Mr. HD 106381 is as unrepentant as the rest of us. However, we generally tried to stick to one rule: reserve poaching for digital cameras. Wasting a frame in a roll of film is just a sin.

    HD 106381 poached this picture 650 years ago – that’s some planning! Luckily for the Hubble telescope, a digital camera was installed because its impossible to still find a Fotomat in space.

  7. I wonder if that’s on par with the LMC in mass… Good thing HD 106381 wasn’t directly in front of the thing, then we wouldn’t know about it at all. Hope the folks at STScI swing the coronagraph into place and take this image again!

  8. RickK

    Hubble is just too awesome. Even the most mundane view through the eye of Hubble is an explosive, mind-expanding event.

  9. See that bright line going straigth from left to right – that’s were NASA is hiding the Martian spacecrafts! That other vertical line is something completely different, though, but I don’t know what yet /Hoagland
    Jokes aside, Thanks Phil, for showcasing yet another astounding space shot.
    Cheers, Regner

  10. John Bluecalx

    Not sure I understand the maths of degree areas, but 40k square degrees being 5 times the size of the Moon in our sky, that means our sky is over 200,000 times larger than the Moon as we see it? Absolutely incredible!

    I tried to make an image with the correct scale for Moon vs Sky and I crashed Paint.NET :(

  11. mike burkhart

    This is incredable. For some resion it reminds me of the trip into the Monolith in 2001 one of the many images Bowman sees looks like this. You know every time I see that part of 2001 I always see some thing new that I don’t rember seeing before. (I heard that when it was in theaters many smoked pot before that part , let me asure you I don’t smoke pot or do any illeagel durgs so I don’t want any one comenting that that is the reson I miss images in that part)

  12. Chris

    @5 Charlie
    I don’t know how big the photo is. But the galaxy size is (1.9, 0.8) arc minute and the proper motion of the star is (-33.25, 7.01) milliarcseconds/year. Look at the SIMBAD catalog. So to move one galactic diameter (33.25/1000/60/1.9)^-1 = 3428 years. Fortunately it is moving “away” from the galaxy, so we probably don’t need to wait that long but it looks like we’re stuck with it for a while.

  13. RickJ

    Then there’s NGC 404 at about the same distance photo bombed by Beta Andromeda, a second magnitude star. Being a LINER galaxy and that close it would be a good target for study if not for Beta being so close. Yes the angular separation is greater but the magnitude difference more than makes up for it. Link to red DSS 1 image with 48″ Palomar Schmidt Telescope.

    Without atmosphere Hubble could do better of course. PGC 39058 is blue while the star red which makes things easier than here where both the star and galaxy are red. I should be able to get an image of PGC 39058, I’ve added it to my list to try next spring. But so far all attempts with NGC 404 have failed. But then I’m not using Hubble!

    I find many dozen papers on far more distant galaxies complaining of photo bomb stars ruining their data. It’s unfortunately, quite common once you move further out. I’ve had many galaxy photos ruined by such photo bomb stars. But to live far out of our galaxy where this wouldn’t be a problem would sure make campfire stargazing boring with nothing but floating campfire embers to look at.


  14. Navneeth

    Phil, why is this post tagged Antiscience?

  15. And let’s not forget dwarf galaxy Leo I

    Which is a mere 12 arc minutes away from the 22nd brightest star in the whole sky, Regulus.

  16. Nice picture!

    My first whimsical thought when I saw it were this: how much of that white disk is the actual star, and how much of it is white pixels “outside” of the star’s disk which turn white due to over exposure? Is it just a “point” of light, smaller than a pixel in the image or does the star at that distance and magnification actually have some visible disk?

  17. Richard Drumm @ #7: Sorry, no coronagraph on the Wide Field Channel of ACS. There was one on the High Resolution Channel, but that channel wasn’t repaired. We tried, but because its power supply was shorted rather than open, the new power supply is also shorted. We figured there was a 50/50 shot that it would be open and the HRC would be repaired along with the WFC. But no joy there. Still, the Wide Field Channel was the one most people were using, so that was the primary goal of the repair.

  18. Ted

    “the light from an object fades with the square of its distance”

    Can one of you egg head brainbox boffins explain that in idiot please. Have not heard that before but it sounds useful to know.

  19. Pete Jackson

    Wonderful photo and discussion, Phil.

    The photo doesn’t have a scale, but I would estimate that the star and the center of the galaxy are about 30 arcseconds apart, or 1/120 of a degree. The 10,000 stars in the sky are, on average, about two degrees apart. Shoot a galaxy into this matrix, and it should ‘land’, on average, within a degree or so of a star. The probability of a random galaxy shot into the matrix being closer than d degrees to a star would go as d**2 (d squared). Shoot 10,000 galaxies into this matrix and one of them should then land within about 1/100 of a degree of a star. So, having one encounter this close seems within the normal bounds of expectation.

  20. HvP


    “the light from an object fades with the square of its distance”

    You might want to look up the Inverse-square Law. It basically just means that if energy is radiating away from a point source then it has to spread out as it gets further away. Since it acts like a sphere expanding from the center it must get less and less “dense” the further it gets from the center.

    The surface area of a sphere is 4πr², or “four times pi times the radius of a circle squared”.

  21. MadScientist

    The star is just providing a lesson in optics. We can see that the secondary mirror is supported by 4 vanes rather than 3 (3 vanes does not yield the pattern which people might imagine it would) and we can also see that the aperture of the telescope is circular (though that’s hardly surprising).

  22. Brian

    Ted@18: Just think of the light coming off of a star as a sphere that gets bigger and bigger as it moves outward. The surface area of a sphere is 4 pi times r squared. So the same amount of light is spread out over a surface that increases with the square of the distance. Make sense?

  23. Regner Trampedach

    Oh, man! – not a single laugh at my joke @ 9? I didn’t think it was that lame :-(
    Cheers, Regner

  24. 24601

    I’m afraid I don’t get it. Didn’t anyone know the star was there before they directed Hubble to take a 30-minute-long exposure? Seeing as it has a designation, it’s been spotted at least once before, so somebody must have known its location. It’s an interesting picture, yes, but one that speaks to me of failures in planning.

  25. 24601

    @14 Navneeth, my guess is it’s because the star is preventing a full study of the galaxy

  26. gregg

    Looks like hubble needs a little cleaning of it’s lenses
    too much glare.

    Send up the space shuttle again.

  27. Dan M

    Thanks, Phil, for this and many similar posts. Combining amazing photos like these with straightforward explanations induces awe, helps explain the physics behind that awe, and makes it easier for non-astronomers like me to introduce these topics to others. Much appreciated.

  28. Would parallax help us here? If they took this same picture 6 months later, would the star then be out of the way of the galaxy?

  29. @ Regner Trampedach:

    Careful, or the nutters will descend in droves, boo-hooing because you’re making fun of ’em.

    That being said, Ha.

  30. RickJ

    “And let’s not forget dwarf galaxy Leo I”

    It isn’t all that obscured by Regulus. I found it very easy to image compared to NGC 404. My post on it at the Baut forum is at:

    Other than keeping Regulus out of the image nothing was needed to avoid its influence. A light leak letting in light of two LED’s were far more of a problem.


  31. ggremlin

    It’s a great picture, but did someone forget to consult they start charts? Miss that big red star in the way?

  32. Messier Tidy Upper

    How far away is that star?

    Proper motion will carry it away very gradually taking it out of the field of vision there.

    Of course, the proper motion of *other* stars may then bring them into that field of view replacing it!

    This process will take time natch depending on how close or distant the star is and its exact motion.

    I wonder if anybody’s got images of galaxies with foreground stars compared over decades or longer showing this effect and enabling us to get a more detailed less obscured view of galaxies affected by blocking foreground stars?

  33. Brian

    I have new wallpaper. :)

  34. If you have a statistical sample of galaxies to get through, so this kind of superposition is bound to happen, it really helps that the HST CCDs are set up so that they are very difficult to damage with stars. In fact, because of internal reflections, the results are often much better if you let the star on the chip rather than just off of it. (Which reminds me of a very amusing narrowband plate of NGC 404 I took years ago, a trail of sausages from Beta Andromedae crossing the field). Now, if you have one galaxy photobombing another, then you’ve got something…

  35. Messier Tidy Upper

    @15. Bart Declercq Says:

    And let’s not forget dwarf galaxy Leo I

    Which is a mere 12 arc minutes away from the 22nd brightest star in the whole sky, Regulus.

    There’s a good Astronomy Picture of the day shot of that here :

    Regulus is also the closest B type star to our Sun at a distance of 77 light years and rotates so quickly that it is shaped, as Ken Croswell writes, like a pumpkin – albeit one composed of blue-hot plasma :

    Linked from that article you can also find another of Ken Croswell’s articles – ‘Stellar Theft Sends Guilty Star Into a Spin’, July 12, 2008, New Scientist page 16 – explaining that the Regulan rapid rotation is probably due to an as-yet unconfirmed (?) white dwarf component of a system already containing more distant orange & red dwarf companions.

  36. Messier Tidy Upper

    The star is in reality a red giant about 650 light years away. It shines with about 200 times the luminosity of the Sun, so it appears to me that we’re looking at a star much like the Sun but far older.

    Would that luminosity figure be bolometric (incl. all wavelengths) or just visual?

    Is HD 106381, like so many red giants, a variable?

    Interesting to reflect that the view of that galaxy would be almost the same as seen from HD 106381 – except for not having HD 106381 in the way at night! 😉

    I wonder too if our own Sun is in the way of anything as seen from alien skies? 😉

    @ 14. Navneeth Says: Phil, why is this post tagged Antiscience?

    Perhaps because the star is being anti-the scientists trying to observe the galaxy? 😉

    I guess you could call HD 106381 an obstacle to science – although it also offers a scientific target of its own right when it comes to studying red giant stars.

    @ 6. MT-LA :

    We call it being a bonehead here in Oz. 😉

    @ 9 & 23. Regner Trampedach Says:

    Oh, man! – not a single laugh at my joke @ 9? I didn’t think it was that lame. Cheers, Regner

    Well you raised a smile from me with that. I liked it. :-)

  37. MarcusBailius

    Mike Burkhart (reply 11):

    Just a thought – but parts of that sequence are recoloured footage of the same sequences Kubrick used in Dr Strangelove, as the B52 was heading towards its doom…

    ‘Scuse me while I contemplate how to get a pun out of “Dr Strangelove, or How I Learned to Stop Worrying and Love the Photobomb”…

  38. Messier Tidy Upper

    For those studying the galaxy PGC 39058, the star HD 106381 is being a stellar “Person from Porlock” :

    But then for those studying the edge-on spiral in the lower left-hand corner enmeshed within PGC 39058 then that dwarf galaxy is being the “Person from Porlock” .. But then again for those studying *another* galaxy hidden behind *that* edge on spiral in the lower left hand corner that edge on spiral emmeshed within PGC 39058 is the “Person from Porlock” and.. so on, ad infinitum.

    PS. I wonder if people studying the *star* find it annoying that the light from that dwarf galaxy keeps getting in the way and confusing things? 😉


    “Big fleas have little fleas,
    Upon their backs to bite ’em,
    And little fleas have lesser fleas,
    and so, ad infinitum.”

  39. Jon Hanford


    “…(Which reminds me of a very amusing narrowband plate of NGC 404 I took years ago, a trail of sausages from Beta Andromedae crossing the field).”

    I had a similar experience photographing NGC 404 with a broadband IIa-O plate many years ago and that emulsion’s blue sensitivity didn’t help the situation wrt Beta Andromedae. If only GALEX imagery was available at the time (making NGC 404 an easy, intriguing target):

    “Now, if you have one galaxy photobombing another, then you’ve got something…”

    Like your namesake (NGC 3314), Bill? :)

  40. @Jon Hanford:

    “Like your namesake (NGC 3314), Bill?”

    Now you’re playing on a mania. Not just that one pair – Galaxy Zoo has delivered another 2000 or so useful for study of the foreground dust, and that’s before they turned their energies to Hubble Zoo. I promise not to write a paper on each one.

  41. kuhnigget @ 29 and Messier Tidy Upper @ 36: I am redeemed – thanks, mates!
    Cheers, Regner


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