Follow the reluctant adventures in the life of a Welsh astrophysicist sent around the world for some reason, wherein I photograph potatoes and destroy galaxies in the name of science. And don't forget about my website, www.rhysy.net



Tuesday, 2 April 2013

Infographic : Galaxy Size Comparison Chart

EDIT : Several typos were found my myself and others over the last few months. To prevent links from breaking, the old charts are still online. For the latest, correct versions, see my website.

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.

13 comments:

  1. Dear Rhys
    This is fantastically useful! Thank you SO much.
    Do you have a larger size image available of the comparison chart - I'm finding the captions just a little bit difficult to read, even with the chart enlarged as much as possible. It must be admitted that this is probably because of 64-year-old eyes and the need for new specs...
    By the way, along with being in Universe Today, your chart is posted in the 'Starship Asterisk' forum - Apod's forum - at http://asterisk.apod.com/viewtopic.php?f=31&t=31104

    Best wishes
    Margarita

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  2. Thanks Margarita !

    Text too small to read is one of my pet hates (I'm 29 and my specs are OK - it's them not us !). I added zoomable versions of the images to the article :

    http://zoom.it/ptPq
    http://zoom.it/qbvd

    When I get round to making print versions I'll probably tweak the text size a little.

    APOD ! That would be awesome...

    Cheers,

    Rhys

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  3. I seriously need to use this next week as a poster for an outreach event!!! Do you have high enough resolution versions for me to print it myself as a 4 foot by 4 foot (ish) poster? (we have the printer at work!)

    (and thank you for including one of my thesis galaxies at the top left ;) )

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    Replies
    1. Not yet, I'd like to but I don't have time to do it right now - maybe on the weekend.

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    2. Since I loved this depiction so much, I decided to take the largest size image off to Kinko's and see what they could do (there's also a poster special this month!). Just to let you know, the preview they made showed that there were different shades of black for the background of each image (so you could see where the images were pasted on the black background). The guy at Kinko's said he'd work on it and get it done by morning. Whatever he did, the final poster was *amazing*!! It looked just like on the screen (no variations in black). So just wanted to let you know that the local Kinko's guy was able to make it work as a poster (I did 22 inches square since that's a good size for me to take to schools for my galaxy outreach project). Anyway, it's totally fabulous and I have great pictures of it in action with cute kids, too. :) And I totally bragged about the image to all of my coworkers and told them about your blog. Thanks so much! I'll also be using this image in my defense talk on Tuesday.

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    3. Awesome ! Sorry I wasn't able to get a higher-res version done yet, but I'm glad it printed out well as a poster.

      Different shades of black are always a hazard. I try and make the image work on my own monitor, which is good enough for the standards of the poster print we have here (but images never come out quite as nice as the electronic versions).

      Good luck with your defence ! Which galaxy did I include that you're using ?

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  4. Thanks a bunch for doing this, Rhys! I discovered that you'd done this SUPERLATIVE large , LARGE (oh, I can't make this go into huge text - so imagine it!) Zoomable version on your guest post Asterisk. I have copied it to my diddy little Android Nexus (hope that OK?) because zooming is a bit difficult with it and its my usual machine at present.
    It was lovely seeing you drop in on Asterisk - why don't you come again? You are far more august and PROfessional than (most) of us, but if you didn't mind that we would make you so welcome! It's a really good group, who both love astronomy and contains people with a wicked sense of humour...
    Again - many, many thanks,
    And I'll visit again
    Margarita

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  5. Placing this data about stars and galaxies in the form of an infographic is a very nice idea. I love learning and talking about astronomy but sometimes, due to my busy schedule, I don’t have time to read thick books about them. This simple infographic will definitely be very helpful for me. :)

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  6. Están geniales estas imágenes, me encantan.
    http://bit.ly/16N6NBZ

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  7. Rhys, do you know that the zoomit links now are not available?
    Margarita

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  8. Hi there,

    Great chart, by the way! Just thought I'd point out that your conversions from light years to km and miles are off by 1000. Might be good to correct it before too many outreach posters are made!

    Thanks,
    Rob

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    Replies
    1. No they're not! In fact, they USED to be... but that's already fixed. Check the zoomable links.

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    2. Ah yes, all good then! I was just going from an older copy somebody sent me.

      Thanks,
      Rob

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