Type in "asteroid sizes" into Google and you'll quickly find a bunch of images comparing various asteroids, putting them all next to each at the same scale. The same goes for planets and stars. Yet the results for galaxies are useless. Not only do you not get any size comparisons, but scroll down even just a page and you get images of smartphones, for crying out loud.
Well, it's time to correct this disgusting nay, bigoted oversight with the following infographics (which is what I gather is now the cool term for "posters"). These were prompted partly because no similar images exist that I can find, and also by recent claims that the largest spiral galaxy has been discovered.
The images I used were selected purely on an ad-hoc basis. Obviously, the Milky Way had to be there. Since really giant galaxies are many times larger than the Milky Way, and those were the ones I particularly wanted to show, that basically ruled out showing any dwarf galaxies (like the LMC and SMC for example). I tried to get a nice selection of well-known, interesting objects. I was also a little limited in that I needed high-resolution images which completely mapped the full extent of each object (often, because of a small field of view, only the central regions are mapped).
Still, I think the final selection has a decent mix, and I reckon it was a productive use of a Saturday.
|Zoomable version here.
As will be evident from the poster, "my galaxy is bigger than your galaxy" claims should be treated with caution. The latest hoo-ha is about NGC 6872 (very bottom of the poster), which, though indeed enormous, has been stretched by an interaction with another object. Is it really fair to claim it's the largest if it's been stretched ?
Even defining the edge of ordinary galaxies can be tricky. Especially since their various components (gas, stars, dust, dark matter) extend to different distances from their centers. In some cases, truly enormous radio jets extend many times further out than the stars. Should they be included as part of the galaxy ?
To my mind the gigantic (but very faint) Malin 1 has a better claim to the throne than NGC 6872, as its disc hasn't been temporarily stretched by some interaction. How such a large disc formed is a bit of a mystery, but it is at least a true spiral disc, even if it's very faint and not remotely photogenic (it's barely visible with Hubble, for heaven's sake).
But to some extent, all this is just semantics, and it really doesn't matter which galaxy is the largest, any more than it matters who landed on the Moon second or if Pluto is a planet or not. In any case, spirals are puny. The hands-down largest galaxy of any kind is IC 1101. And it is truly, in every way, monstrous.
|Zoomable version here
I don't just mean it's monstrous in that it's staggeringly vast, although that's part of it. I mean monstrous because it probably got so large by eating its neighbours. cD galaxies like this are found at the center of rich galaxy clusters, where there are plenty of smaller galaxies falling in that can be absorbed. That makes the galaxy heavier, which means their gravity can pull in more and more galaxies. It's like a cannibalistic orgy on steroids*.
* So just like every episode of True Blood then.
Perhaps surprisingly, there aren't many pretty pictures of IC 1101. The galaxy's morbid obesity is offset by its great distance from us, although there are a few nice ones with Hubble (see below). So for the chart, I took the image of M87 (another giant elliptical) and scaled it up. This isn't such a cheap way to do it, because both galaxies are pretty smooth structures, and in any case no image exists that's large enough to display at the massive resolution required.
|IC 1101, as you will have guessed, is the big bright one. More images can be found through the Hubble archive and this website.
Another point is that while colliding galaxies might be initially spectacular, eventually they run out of gas and stop forming new stars (during the collision, the gas gets compressed, triggering star formation). Eventually, there's nothing left but a huge ball of old, red stars, the blue (short-lived) ones having died off aeons ago. With no gas and no new stars being formed, over time the random motions of the long-lived red stars make the galaxy nothing more than a titanic stellar swarm. And that's why our cannibalistic juggernaut isn't going to win any beauty contests.