Classification tree tweaks

Some of you may have noticed that on Thursday we made a couple of small changes to the flow of questions that are asked for each object in Galaxy Zoo: Hubble. Both of these changes relate to the set of additional questions which we introduced during the switch from Galaxy Zoo 2 to Galaxy Zoo: Hubble. As you will have certainly noticed, the new Hubble Space Telescope images contain many more galaxies with a clumpy appearance. This type of galaxy was very rare in the Sloan Digital Sky Survey images and doesn’t really fit into the classification tree we used for Galaxy Zoo 2. To obtain useful classifications for these objects in Galaxy Zoo: Hubble we therefore decided to add another branch of questions to the “classification tree”.

Clumpy questionsDuring the first month or so of Galaxy Zoo: Hubble we have received a great deal of very useful feedback, particularly on the forum. In particular, two features of the new classification tree appeared to cause a fair bit of consternation amongst some of the Zooites. After considering your comments, and much deliberation, we decided to make a few changes.

Both points of contention related to the question asked after an answer had been clicked for ‘How many clumps are there?’. If the answer was anything except ‘one’, then we then asked ‘Do the clumps appear in a straight line, a chain, a cluster or a spiral pattern?’. Now, that’s a hard enough question to answer when there is only three clumps, but doesn’t make much sense at all when there are just two. We were trying to keep things simple but, to be perfectly honest, this wasn’t very sensible on our part. We have now changed the tree so that if the answer given is ‘two’, the question about how they are arranged is skipped.

The second issue was more interesting, because the frustration it caused told us something about the appearance of the clumpy galaxies which we hadn’t properly appreciated when planning the questions. New astrophysical insight before we’ve even collected enough clicks to start analysing! If the answer to ‘How many clumps are there?’ was ‘one’, the classification tree went back to the branch for ‘Smooth’ galaxies and asked ‘How rounded is it?’. Our thinking here was that a galaxy that was mostly just one clump would probably be an elliptical or maybe a bulge within a smooth disk galaxy.

It seems we both underestimated the discriminatory power of the Galaxy Zoo participants and how clearly different clumpy galaxies are from other types, even when there is only one clump. After having seen a few clumpy galaxies, it seems that many Zooites come to recognise that there are subtle features that set them apart from other types of galaxies. This suggests that single-clump galaxies really are a clearly different type of galaxy to the ellipticals and disks that are more common nearby. For single clump galaxies we now carry on asking the usual clumpy galaxy questions, skipping those that don’t make sense for only one clump.

Don’t worry – all your previous classifications of one (and two) clump galaxies are still safely stored away and will be very useful in helping us catalogue the subtle differences between the appearances of all these objects. Thank you, and keep clicking!

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14 responses to “Classification tree tweaks”

  1. Gwydion Williams says :

    Nicely done. Every computer system needs ‘tweaks’ after feedback from users.

  2. Gwydion Williams says :

    A separate issue, a bit more on the science of the ‘clumps’ would be useful. If I’ve understood correctly, these are features of relatively young galaxies, those that are only a few billion years old rather than the 13-billion of our immediate galactic neighbourhood. I’d like to know more, even if it is just a list of alternative notions or a confession of ignorance.

  3. Thomas says :

    This is good news, thanks for sorting out these issues, Steven and all.

  4. Jo says :

    Thank you that’s brilliant. So glad this has been sorted.

  5. Jules says :

    Thanks team! 🙂

  6. George G. Kountouris says :

    I like thank you too for these changes. I like to propose an additional change in classification branch button ” IS THERE ANYTHING ODD” In special answer button “other” by adding question button as :
    – Active nucleus (ejecting matter in oposit directions)
    – Massive starborn (more luminus areas)
    Thanks again!

  7. firejuggler says :

    I tend to class ‘peas’ as one clump galaxy…am I right?
    And then, I get get asked if it is symetrical… one clump can’t be symetrical…

  8. Steven says :

    Gwydion: Stay tuned – more about clumpy galaxies coming very soon

    George: it is actually extremely rare that it is possible to spot an active nucleus in these images, so not worth a button. Massive star formation regions are often picked up as galaxies being ‘clumpy’, whereas less massive ones are probably very common in spiral galaxies. Not sure a button would provide much information we could use. Note that we can identify starbursts from their colours and spectra.

    firejuggler: I would say a pea is a one-clump galaxy (though not all galaxies that are one-clump are peas). One clump can be symmetrical. If it is round and apparently centred in an underlying galaxy, if any, then it is symmetrical. Otherwise it is not. Perhaps the button images are a bit misleading.

  9. George G. Kountouris says :

    Thanks Steven for your reply. I understood and i agree.

  10. kalyan 4m india says :

    i cant see clearly y becoz our eys r not telescopes. ok thank u 4 gods(scientists)

  11. Annie Starjalopy says :

    Still having some difficulty differentiating between a galaxy on-edge with no bulge and a smooth cigar-shaped galaxy ( often a few pixels and an elongated smudge ). Any suggestions? How do we inquire about unusual phenomena noted in classifications? Thanks. This is fun and hopefully helpful.

  12. Steven says :


    It can be difficult to tell the difference between an edge-on disk and a cigar-shaped elliptical galaxy. The clearest clue is probably at the ends: disks will have fairly sharp ends whereas ellipticals are more rounded. If the galaxy is small though, it can be impossible to tell; just go with your best guess and move on to the next galaxy.

    If you want to ask about unusual objects, the best place to go is the forum:

  13. Annie Starjalopy says :

    Thanks for the guideline. It will certainly help.

  14. Gert says :

    Wonerfdul explanation of facts available here.

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