A dying star with the wind in its hair

By Phil Plait | February 13, 2012 7:00 am

I’ve been doing this astronomy thing for a while, OK? I’ve seen galaxies, clusters, stars, planets… so many I’ve lost count. So it’s hard to find something I’ve never seen before, or even heard of before. So when astrophotographer Adam Block sent me a note about the nebula Abell 31, my first reaction was, "Say what now?", and then I clicked on the picture, and my second reaction was "What the what?" Then my third reaction was to soak in the beauty of this gorgeous object, and my fourth reaction was to nod my head slowly, thinking, "Ahh, I see what’s going on here."

Let me share:

See what I mean? What a beauty! [Click to ennebulenate.]

This image was taken with the 0.8 meter Schulman Telescope at Mt. Lemmon in Arizona, and is the result of an astonishing 21 hours of exposure time in various filters! Right away, that was a clue as to why I had never heard of this object: it’s incredibly faint. A quick perusal of amateur astronomers’ sites proved that to be correct; not too many have observed this jewel because it’s barely detectable.

Abell 31 is a planetary nebula, a cloud of gas formed when a dying star expels winds of matter. At first the wind is slow, but then as the star continues to expire the velocity increases. The faster wind catches up with and slams into the slower wind, forming fantastic shapes, usually displaying amazing symmetry.

Abell 31 isn’t symmetric though! One side is sharply defined, and the other diffuse, fuzzy. I was pretty sure right away what I was seeing: this nebula is in motion, moving through space, and interacting with the thin material between the stars. A literature search confirmed this suspicion: it’s moving through space at relatively high speed. In this picture, the direction of motion is toward the bottom of the frame, and the gas leaving the star in that direction is compressed. Gas heading the other way is moving downwind, and relatively untouched. Think of it like blowing on a dandelion; the side toward your mouth gets compressed, while the other side stays fuzzy.

The red gas is hydrogen, and the blue is oxygen. Interestingly, oxygen is probably located throughout the entire nebula, but only in the center is it close enough to the central star to get lit up and glow. The star doing all the work here is a white dwarf, basically the core of a normal star like the Sun, but exposed after all the outer layers of the star have been shed to form the nebula.

Abell 31’s white dwarf central star is tiny, only about 4 times bigger than Earth (or about 0.04 times the size of the Sun), but it’s incredibly hot, blazing away at about 85,000° Centigrade (150,000°F)! It has about half the mass of the Sun, meaning it probably started out life as a star with twice the mass of our star or so, and lost the rest as it aged and blew those winds. Judging from how fast those winds are blowing outward, the star probably started dying in earnest about 130,000 years ago, after a billion or more years of normal life. What used to be a star a couple of million kilometers across is now this lovely object nearly ten light years wide — that’s 100 trillion kilometers from end to end!

At its distance from the Earth of 2000 light years that makes it pretty big in the sky, roughly half the size of the full Moon. With its light spread out so much, no wonder it’s faint. Too bad: it’s a fascinating object, worthy of study. And it’s also fantastically beautiful, worthy of wider recognition.

And I’d love to see a set of Hubble observations of this! I studied planetaries for several years, and I still sometimes get the desire to poke and prod them again. I’ll just have to wait, I suppose, and be satisfied with images as lovely as this one.

Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Related posts:

The green ghost of a distant dead star
Another nearly perfect circle in space!
Tears of a dying star
The knotty halo of the Cat’s Eye

CATEGORIZED UNDER: Astronomy, Pretty pictures, Top Post

Comments (23)

Links to this Post

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  1. A glimpse into Mira’s future given its velocity and comet like – but stellar material – tail maybe?

    Or even, if its moving fast enough, our own Suns?

    Wonderful image and object. :-)

    PS. Mira article by the BA linked to my name here.

  2. Chris

    Wow, this is unknown. There isn’t even a Wikipedia entry! Although there is one in Italian :-)

  3. Ganzy

    Ohh come on, is this a joke? Don’t tell me you can’t see James Randi on the left there sporting a pink tinged beard and lovingly framed in cerise! ūüėČ

  4. Ganzy

    Wow, ennebulenated this is a real beauty! Here’s my new desktop background for the next few weeks. Thanks.

  5. Ganzy, that’s not James Randi. It’s clearly Edgar Winter getting a Free Ride.

  6. Nathan Hevenstone

    Wow. Thank you. Now if I could just find the uncompressed .tif file…

    Anyone? Phil? Please?


  7. Joe Shuster

    Beautiful. Looks like we’re seeing it headed downward about 20-30 degrees to the plane of the image, so the closer (bottom) edge is visible while the farther (upper) edge is blocked by the core nebula. Like looking at a departing jellyfish.

    Redshift measurements could show the real motion vector.

  8. Sorry if this is a dumb question, but how do you make a 21 hour exposure from the ground? is it a composition of many pictures of shorter exposures that eventually sum up to 21 hours? I’m a little confused hehe

  9. davem

    85,000¬į Centigrade (150,000¬įF)!

    Dear BA, could we be a little more scientific in this ‘ere blog? I see that you use kilometres instead of miles; this is good! So can we drop Fahrenheit (meaningless), and Centigrade (old fashioned) and do stuff in Celsius please?

    Oh, and the picture’s awesome…

    Sorry if this is a dumb question, but how do you make a 21 hour exposure from the ground?

    Over several nights? That’d be my guess.

  10. RAF

    That is such a cool image…your explanation was excellent, Phil.

    …and something you had never encountered?….that’s just WOW.

  11. Sean H.

    Very cool, although all I could see is this: http://i.imgur.com/aDWOm.gif
    …And now you are all stuck seeing it too.

  12. Chris A.

    This gets me to wondering what such a nebula would look like if its motion was entirely radial (i.e. coming straight towards us or going directly away from us), and on the flip side, how close this one is to having zero radial velocity given the sharply defined leading edge (which, presumably, would become less sharply defined as its velocity vector got closer to being parallel to the line of sight).

  13. Jay

    Edgar Winter works, but I see either Leon Russel or Billy Gibbons.

  14. √ĺorfinn

    You should ensure that your naming of units is following the definition.
    The change to “degree Celsius” (symbol: ¬įC) occurred in 1948

  15. Mandarb

    My version of pareidolia in this image is Leonardo da Vinci on the left, and maybe a griffin’s or a chicken’s face on the right.

  16. Nigel Depledge

    I second the notion that you should cease referring to degrees Centigrade.

    Either use Celsius or Kelvin.

    But that is such a cool shot.

  17. Nigel Depledge

    Davem (9) said:

    I see that you use kilometres instead of miles; this is good! So can we drop Fahrenheit (meaningless), and Centigrade (old fashioned) and do stuff in Celsius please?

    I don’t mind the use of Fahrenheit as a parenthetical comment following the actual measurement, in the same way I don’t mind the use of miles parenthetically following a comment in km.

    However, I do agree that Centigrade should cease to be used, as I state above.

  18. Old Rockin' Dave

    If they had just observed a little longer, I’m sure this is what they would have seen:

  19. stardustspeck

    I LOVE this image – not least because it matches my current hair colors (black, blue and pink).
    But really, there aren’t many dying stars that show evidence of their motion through space, Mira is one and R Hya is another and both of them are far from Planetary Nebula phase (both are AGB stars). Also the structures on one side are reminiscent of the Dumbbell and other PNe with clumpy structures. Since we don’t yet understand how clumps like the cometary knots in the Helix, form – this might be a really cool object to investigate more deeply.


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