The dusty depths of a spectacular spiral galaxy

By Phil Plait | August 1, 2011 7:00 am

In something I’m considering making a tradition here at BA Central, here is your Monday morning jaw-dropping spiral galaxy: NGC 634 as seen by Hubble:

[Click to galactinate.]

Isn’t that something? This galaxy is a gorgeous nearly edge-on spiral, about 120,000 light years across or so — slightly bigger than the Milky Way — and 220 million light years away. The press release (at the link above) for this spiral talks about a supernova that blew up in this galaxy back in 2008, and I was going to write about that, but then something else tickled my brain.

Look at the picture. The disk of the galaxy, like in most spirals, is ribboned with dark dust lanes, huge clouds of complex organic molecules expelled by stars being born and stars dying. It’s pretty common to see them, but what struck me is the asymmetry of the lanes: they are darker on the bottom than at the top. The overwhelming impression is that we’re looking down on the spiral, so the dust lanes are more obvious on the near side than the far side.

This cannot be a physical effect of having dust only on one side of the galaxy. If it were, then random chance would make it pretty unlikely to have it on the side tipped toward us. Plus, I realized that I’ve seen this before! For example, in this image here of NGC 7049 (click to embiggen) you see the exact same thing. It’s also clear in images of M 64, where the feature is so obvious it gave the object its nickname of the Black Eyed Galaxy.

After mulling it over for a moment, I suddenly realized what it was: each of these galaxies must have an extended halo of stars surrounding the central bulge and disk. If the galaxy were just a disk these features would be very difficult to explain, but if there are a few billion stars that exist above and below the plane of the disk, they would "fill in" the dark lanes, making them less dark. And that explains why we see the dust better on the galaxy’s near side: we’re not looking through as many stars. It’s like we’re looking through a fog; stuff nearby is clearer than stuff farther away because we’re looking through less mist. Except in this case, the fog isn’t obscuring anything. It’s stars, adding their light to the overall picture.

That’s pretty cool, and something I never thought of before. I don’t expect this to be a very common feature of spirals, since most angled or nearly edge-on galaxies don’t show this effect very strongly. That might make an interesting project for an undergrad. You could take a sample of a few hundred spirals at various tilts and then compare the brightness of the dust lanes on one side versus the other. You should be able to get a census of the stars in the halo as well. Having spectra would help too.

I’m not soliciting research here; for me a grant is when I go to the ATM. But if you’re an undergrad in astronomy, ask your professor. Maybe this has been done already… and if so, I’d be interested in reading about it! It’s a lot of fun to get a flash of insight on something, and even better to find out it’s attackable by science.

Images credit: ESA/Hubble & NASA

Related posts:

The Triangulum galaxy writ large
The heat of the Pinwheel
A galaxy choked with dust
The belch of a gassy galaxy

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science

Comments (28)

  1. Mejilan

    I’m only recently come to this awesome blog, but I think I should really start saving some of these stellar (heh) images as potential desktop background fodder!
    As always, thank you for sharing, and the insight!

  2. OtherRob

    Yes, yes, please. More spiral galaxy pics. Spiral Monday — has a good ring to it. :)

  3. Kirk

    STOP IT!!!
    I’m getting tired of changing my desktop background already!!!
    One beautiful pic after another, I think I need some more computers.

  4. It’s like a Monday Metazoan, only ultra, magna, galactazoan.

  5. Perhaps, “click to jawdroppinate”?

  6. that lead pic is really something. Those foreground stars are a nice touch too. lovin that NGC634

  7. Sam H

    I remember NGC 7049 – the so called “Crown of Thorns” galaxy that has particular significance among some Evangelical fundamentalists (including those at my old school), who hold it in regard alongside the core of M51 and (especially) the molecule Laminin. But either way, very beautiful and awesome :)

    As well: I managed to go to my special stargazing site on a desolate country road 80 km south of my city with my 8-inch dob Saturday night – JUST AMAZING! Even though the collimation wasn’t perfect I managed to see the nebulae in Sagittarius (i could see the Lagoon Nebula with my frickin’ NAKED EYE), the Veil Nebula, a few globulars including M13, M57, the double cluster in Perseus (which I could also see with my naked eye – such a pleasant surprise, I thought it was only visible in winter!!), and MANY GALAXIES – M31, M33, M81, and even M51 (and I swear I could catch a hint of its spiral structure!!) And finally, a mystery star cluster that rose in the northeast later in the night I figured out were the Pleaides only as we left – and months ahead of season!! Just…win 😀 /end joyful rant

  8. Douglas Watts

    The rough spiral galaxy to the top center left of the NGC 7049 halo is undergoing some massive blue star formation, well it was, like 2 billion years ago. Like Phil, I am a total sucker for these Hubble images of distant galaxies, if only because of the guilty pleasure derived from examining even more distant galaxies behind them !!! It’s like fried dough at the county fair.

  9. Julie Marton

    I love the idea of starting my week off with spiral galaxies at Bad Astronomy. : )

  10. CWorthington

    Hmm. Your idea makes sense. I hope someone picks it up for research. A pity I can’t do the math of physics or I would go into astronomy. I should ask the astronomy profs at TXState about this.. hmmm…

  11. icewings

    Kirk – If you have Windows 7 you can tell your Display Settings to change the background picture every so often. I’m not sure if earlier versions of Windows have that feature.

    I had to tell it to change pictures every 5 minutes, to rotate through all the great BA pix I’ve saved over the past year or so :-)

  12. Bill Morehouse

    The NGC 634 galaxy is about 250 Million light years away from earth, correct? So when we say a super nova blew up in the galaxy in 2008, we are really saying that’s when the light information reached earth? Just want to be sure I understand this. It would seem the actual super nova explosion would have happened ~250 Million years prior to its appearance at the earth. Thank you everyone for the interesting input.

  13. Kirk:

    STOP IT!!!
    I’m getting tired of changing my desktop background already!!!

    <aol>Me too!</aol>

    This weekend, we took our kids to a drive-in movie. (Yes, they still exist. This one is in Hyde Park, NY.) It was about an hour’s drive from our house, but our kids had never been to one, and they were showing the final Harry Potter movie.

    Anyway, they show two movies, back-to-back. In between, I got out of the car to get some snacks, and looked up at the sky. It’s amazing how many more stars were visible from there, far away from big cities, in a cloud-free moonless sky.

  14. @kirk –

    For a number of years now I have downloaded astropix from Phil’s site as well as APOD (Google it). I now have 1209 (shortly to become 1211 with the two in this post) and almost 2GB of space used by them. My screen saver is used to randomly show the photos. They are so beautiful that I have set up an older machine in my office to be on screensaver all the time! It’s a Mac, so the graphic images are shown in all their resplendence!

    Phil –

    Good thought! Maybe YOU should fund the grant. (Would you still be able to come to Dragon*Con?)

    @icewing –

    Using the OS to change your desktop every five minutes is a processor-consuming task. If you find your PC to be slow sometimes, you might want to either have it change pictures less often, or try adding memory to offset some of the work the OS has to perform.

  15. Mejilan

    @ Kirk (3) and icewings (11):
    For Win XP, there are a number of tools available to rotate through numerous desktop wallpaper images. A semi-legitimate program is called “Digital Photography Winter Fun Pack 2003” and is available directly from Microsoft for free. I don’t think it allows for a random or shuffle mode, which is why ultimately I went with a third-party program.

  16. Len Bonacci

    I have another theory — From our point of view, the view of the far side of the galaxy is over 100 000 years older than the view of the near side. The near side has had 100 000 more years for dust to build up. Sounds reasonable — my house gets dusty over a span of only days!

    But your theory is nice, too.

  17. Thomas

    A fog of stars… that thought is enough for now. Thanks, Phil.

  18. Bill Loehr

    You can see the halo of stars below NGC 634 as well. Quite astounding to take in the fact that the haze is made up of stars.

    But what also intrigues me is how the left edge of the galaxy seems to be bent up –all those “blue” stars there. Any thoughts on that Phil? Is it an optical illusion of some kind? Is there an nearby galaxy pulling on the edge?

  19. Antonio Ceci

    Another amazing screensaver for my laptop!!!! Every day at 3.00 pm (Rome time) I open your webpage to read someting amazing about science and space… Every time you surprise me with something very very special!
    Thank you Phil!

  20. John Sandlin

    In Windows 7 the wall paper can auto change.

    I have another thought on the lack of visible dust on the farther side. Don’t the disks have a slight change in thickness so that near the core the disk is thicker (north to south) and have more stars than toward the edges. This should help hide dust lanes on the far side and show them better on the near, depending on the degree of tilt. And the halo idea would certainly be a contributing factor as you’re looking through a denser star pattern to see the disk on the far side than the near.

    It’d be like standing in front of a crowd of people arranged tall in back to short in front for the near side, and behind a similarly arranged crowd for the far side. Everyone is visible from the front. Only the tallest are visible from behind.

  21. John Sandlin

    Here’s an image that demonstrates, sort of, what I mean. I made a 3D galaxy-like disk, a central bright ball, rings that would be stars, rings that would be dust, and a faint halo of stars. Depending on the angle, the farther dark rings (and star rings, for that matter) are obscured.

  22. Messier Tidy Upper

    Stunningly, superluminously magnificent. I love that image. Good thinking and probable explanation for the darkness of the nearer side dust lanes there too. :-)

    In something I’m considering making a tradition here at BA Central, here is your Monday morning jaw-dropping spiral galaxy: NGC 634 as seen by Hubble

    I love that idea too. Please make it so! 😀

    Isn’t that something?

    Yes, it is a whole galaxy rich in glowing stars, dark dust lanes and, probably, invisible other exoplanets in their trillions among other wonders all set upon an exceptional star field of the Black speckled with stars and some neat background galaxies too. :-)

  23. Anchor

    I’m frankly amazed that Phil’s ‘explanation’ hasn’t been blazingly obvious to everyone long before HST was even a glimmer in our eye. The effect has been well known for almost as long as we’ve realized what ‘spiral nebulae’ were – vast collections of stars. I have known it for over 45 years and have used it as a means of quickly determining which edge of a galaxy obliquely oriented to our line-of-sight in an image was closer to us: obviously the far side dark clouds are typically washed out by the intervening halo stars which are more densely populated near the core. (Yes, John Sandlin #21 & 22 is quite right). Every semi-serious student of galaxies I know is familiar with this elementary aspect of galaxy geometry, that they are diffuse distributions of stars attended by extensive haloes. Glad to discover that Phil Plaitt “suddenly realized” it now too “after mulling it over for a moment”.


  24. bk_2

    There’s another factor in effect here. The dust lanes on the far side of the galaxy are illuminated on the near side by the brilliant galaxy center. They reflect a lot of that starlight, so they are less dark than the near clouds, which are in sillouhette against the brilliance behind.

    I think this was mentioned before in the context of NGC 7049, but I can’t find a reference yet.

  25. cesar

    awesome!! thank god for smart people


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