Hubble snaps a cosmic photobomb

Hubble snaps a cosmic photobomb

Hubble snaps a cosmic photobomb

Bad Astronomy
The entire universe in blog form
Sept. 13 2010 10:40 AM

Hubble snaps a cosmic photobomb

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.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  



[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