Down the throat of a dying star

By Phil Plait | June 10, 2009 6:30 am

I love planetary nebulae; they are among my favorite objects in the sky. First, of course, they are beautiful: eerie rings, ellipses, flowing gas, symmetry. Second, they are poetic: the dying gasps of a sun-like star, ejecting septillions of tons of gas that glows and forms these dramatic objects.

And they all have a tale to tell.

One of the best stories belongs to the Helix Nebula. And it’s telling it loud.

Oh yes, you want to click that to embiggen it.

That picture is in fact the Helix: a star that was once not unlike the Sun, now undergoing paroxysms, blasting out a super stellar wind of gas. The naked core of the star is white hot — 120,000 Celsius, 25 times hotter than the Sun! — flooding the gas with ultraviolet light, causing it to fluoresce like a neon sign (in fact, neon can be seen in the spectrum of such nebulae).

This image was taken using the 2.2-metre Max-Planck Society/European Southern Observatory telescope at the La Silla observatory in Chile. This is an incredibly wide field shot; it spans about half the width of the Moon in the sky. The Helix is huge, more than 2 light years (20 trillion km or 12 trillion miles) across, and close to us: only about 700 light years away. That’s practically in our laps on a galactic scale.

Most planetary nebulae are not spherical shells, but elliptical in shape. If the star is spinning rapidly, the gas it throws off will not be spherical but will be oblate or prolate — like a ball being sat on and squished a bit. The Helix is like that, and we’re seeing it roughly down the pole, so it looks pretty round. If we saw it from the side, it would definitely look squashed.

The center of the Helix

The overall shape is interesting, and I could write a thousand words on it, but instead I want you to take a look in the center of the nebula, in the region surrounding the star that is at its heart. It’s the star at the center of the image here. Take a good look; the Sun will look pretty much like this in just a bit more than 7.5 billion years from now. The star that spawned the Helix was more massive than the Sun — we probably won’t make a big planetary like the Helix — so the remaining ember we see here is more massive and hotter than the Sun will be as well.

But even that’s not as interesting to me as the simple fact that in that image, you can see distant galaxies right through the nebula itself! I’ve marked a few in the image, and if you look at the original hi-res image you can see dozens of them scattered behind the nebula.

Nebulae like this look like gigantic solid objects, but in fact a dense one would still be considered a hard vacuum in the laboratory. There might be something like 10,000 or even 100,000 atoms in a cubic centimeter of nebular gas, but compare that to the 10,000,000,000,000,000,000 atoms in air at sea level!

That’s why you can see through them. Across the several light year diameter of the nebula there are enough atoms all together to see the glow of the gas, but it’s still so thin it might as well hardly be there at all.

And still, that gas has much to tell us. The spectrum reveals what gas is in there — hydrogen (red), oxygen (blue as well as green), sulfur (very red) — and also how hot it is, how dense it is, how fast it’s moving… and the shape of the nebula itself tells us how the star lived, and ultimately how it died as well.

All that, from gas so thin it’s barely distinguishable from space itself. And yet, still very, very beautiful.

Do I have to say it?

Science. I love this stuff!

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science

Comments (48)

  1. NewEnglandBob

    Utterly fantastic! An onion of information.

  2. sophia8

    Sing it, brother, sing it!

  3. We recently had a hand of God, now we have an eye! What more proof do we need that god exists!


    OK a real question. If you look about 1 inch (in the embiggened picture) up and left of teh center star you can see what looks like trails. It look slike if a put a stick in a stream and watch how the water behaves just down stream of the stick. You can even see a couple of these streams in the upper left of your close up picture.

    Is there something there that the gases and dust is flowing around? Or is this an artifact of the photography?

    It cant be other stars can it? If this eye is only 2 light years across, we wouldn’t expect other starts to be that close would it?

  4. Fantastic. Thanks for the post.

  5. @Dr. Plait

    flowing gas

    Careful, you might attract plasma/electric universe proponents.

  6. Beautiful! The pic just became my new desktop wallpaper.

  7. I wanted to ask about the same thing as TechSkeptic (#3) above. The ESO page refers to the white area as a “ring of knots”. Is that just caused by faster-moving (more recently expelled) gas (mostly blue oxygen) slamming into clumps of older gas (the red H, N, S)? What would even slow down the gas over time? Internal friction? I assume the “ring of knots” is far enough from the parent star that the gravity has become negligible. Or is the newer gas just ejected that much harder?

    On the high-res TIFF it’s very clear, the “ring of knots” has a distinct texture almost like cottage cheese.

    Anyway, beautiful image, and great post :) I wouldn’t have noticed those distant galaxies without encouragement to look closely!

  8. Stone Age Scientist

    It looks like an eye with brownish eyelid. How about Blue-Eyed Nebula for a nickname?

    Yes, I can see galaxies behind the gaseous curtain; galaxies inside the eye and outside, too. Beautiful, just beautiful. I can see a somewhat greenish galaxy near outside the lower left rim of the eye.

    Being here in your blog reminds me of some of the sci-fi shows that I used to watch a few years back. One particular episode of Voyager was when Capt. Janeway and crew were forced to go into suspended animation chambers so as to avert the deadly radiation of the nebula they were travelling in. Only 7 of 9 was left to comandeer the ship. What hallucinations the radiation brought her! One of my favorite episodes.

  9. @Stone Age Scientist

    I believe the nebula already has one nickname of “Eye of God”.

  10. Cool shot!
    Agree/disagree with my Star Trek by the Minute criticisms?

  11. What would even slow down the gas over time? Internal friction? I assume the “ring of knots” is far enough from the parent star that the gravity has become negligible.

  12. Hey! It’s the Eye of God!

    Seriously, awesome that you can see galaxies through the nebula.

  13. DVMKurmes

    This post reminded me of Karine Polwart’s song “Terminal Star”. It’s available on Itunes and is a very good skeptial/science song. Few and far between.

  14. Stone Age Scientist

    Btw, Phil, what will be the ultimate fate of the star in the center of the nebula? You said that the star is currently undergoing stellar paroxyms. So would its upheavals lead to a supernova explosion, or a collapse into a black hole? Why is it that some stars become black holes, while others explode? I have often wondered about this.

  15. Stone Age Scientist

    Thanks for the info, Todd W. @ #9. They should also have dedicated the nebula to Frank Sinatra. :)

  16. Sarah

    @Stone Age Scientist
    The star in the centre is just the core of the progenitor star (now called a white dwarf). It’s not going to undergo a supernova (unless there’s a binary partner from which it can accrete material), and it’s not going to become a black hole. These things generally happen to really massive stars (and before they’ve thrown off their envelopes).
    Whether or not stars become black holes/neutron stars and explode in supernovae or quietly become planetary nebulae and white dwarfs at the end of their lives depend on their mass.

  17. Me likey PNs too.

    Still recall the first time (ahhhh, first times….) I saw the Ring nebula through my wee little 2.4″ refractor! Smokey little wisp floating in space.

    While I love these hi-rezzy shots and tend to drool over the keyboard when I see them, my favorite of favorites is still that mental image of the Ring, floating there yet in the big black emptiness inside my head.

  18. The galaxies in the background were the first thing I noticed! Just imagine some alien astronomer looking at a nebula in his/her own galaxy, and seeing the Milky Way in the distant background!

  19. John Baxter

    Wonderful stuff. Thank you.

    Here, clicking on the image to emgiggen it ensmallinates it instead–clicking on that does the embiggening.

  20. Wait, this thing is pointed right at us? I just read this book that says these things will kill us! PANIC!!!!!!

  21. 9. Todd W. Says:
    @Stone Age Scientist
    I believe the nebula already has one nickname of “Eye of God”.

    Is there a Mote in it?



  22. Stone Age Scientist

    Thank you, Sarah. Now at least my (casual, afterwork) research into stars has a cohesive core from which I can relate one kind of star with another. Simply memorising the kinds of stars and what they’re about isn’t enough. I never understood before that stellar mass governs how a star will behave during its last days. Thank you.

  23. Stone Age Scientist

    To John @ #18,

    Is there a Mote in it?

    I don’t know…. I reckon if there is, then that in itself will be a sort of Paradox, right? Since God doesn’t sleep, his eye can have no occasion to have motes in it.

    Of course, one can also have motes while perfectly awake.

    Hmm, maybe I can find the answer in the book The Mote in God’s Eye. :D

  24. Dan

    “25 times hotter than the Sun”

    Is that really technically correct? I am not a trained scientist, but I didn’t think temperature was a scaled value that could be expressed in multiples. Am I wrong?

    Wouldn’t the numerical value of the star’s temperature (in this case in Celsius) vis-a-vis the sun’s be a much different multiple than the truel difference in actual heat (ie, the kinetic motion of the molecules). I mean using F it could be 30X hotter than the Sun and be equally valid.

    Trying not to be snarky, just trying to make sure I understand this correctly.

  25. Apocalypse Cow

    Not sure where to put this, but I just recently discovered that Wolfram Alpha is a great source for astronomical computation. For instance, here’s what it does with “5777 K ”

  26. DJCinSB

    The first thing I noticed upon following the “embiggen” directive was the number of galaxies apparent in the image; really amazing, but I was sure that was not going to be your point. And then I read the rest of the article, and saw we were on the same track!

  27. For those “correcting” me about the age of the Sun when it turns into a white dwarf, my number of 7.5 billion years is correct. I wrote a chapter on this in my last book, and while the Sun will turn into a red giant in 4 billion years or so, it won’t totally lose its outer envelope and be just a white dwarf until it’s over 12 billion years old.

  28. Dan (#24): Kelvin is an absolute scale, so something at 200K is twice as hot as something at 100K. At the temperatures quoted, Celsius is close enough to Kelvin to make my statement correct.

  29. Chris A.


    It’s perfectly acceptable to speak of multiples of temperature (or any other measured quantity, for that matter) provided you are using an absolute (as opposed to relative) scale. Astronomers use Kelvin, making such comparisons legitimate (whereas, doing the same in Celsius or Fahrenheit would be a no-no–not that that stops people from doing it ALL THE TIME–can you tell it’s one of my pet peeves?).

  30. But, since the earth is only 6 ky old, these things are just painted in the sky, right? ; – )

  31. Jeff

    This should put the silly creationists to rest. Here, we see a star like the sun evolving, and the sun will do that. Do the creationists seriously think the sun will get to this point in the near future, how ignorant is that, as dumb as the idea that all the strata and fossil record were laid down in the flood. This speaks to billions of years of evolution.

  32. Torbjörn Larsson, OM

    I love this stuff too. Today there are especially good reasons why my heart is embiggened.

    First, I learned there is a paper out that announces a potential detection of an exoplanet in another galaxy (M31). It’s done by microlensing which has an awesome resolution – this detection is at 6 Jupiter masses but they seem to think that the theoretical limit for the technique is 0.1 Earth mass (albeit with awful statistics). Of course, extraordinary claims need extraordinary evidence, but it is heartening all the same. [Tip o’ the ole gravitational field at UT.]

    Second, today The Extrasolar Planets Encyclopaedia ticked up ~ 400 planets (i.e. 353).

    Third, as an avid abiogenesis fan, I’ll add vague memories of a last week’s paper on biosphere longevity. IIRC they modeled how stellar warming can be offset for several Gy by biology and plate tectonics conspiring to, in BA parlance, thinnating [surely biology can’t be mundanely “thinning”?!] the atmosphere until bacteria remains to perpetuate the effect for eons more.

    It may be that this our Death From The Skies is postponed to 2.5 Gy in the future. Break out the champagne! Or in Phil’s case, perhaps prepare for having to make another best selling edition.

  33. Per Bojsen

    Hi Phil, I’m curious about the comet looking features in the nebula. If my understanding
    is correct, these are due to the stellar winds of stars that push against the wind from the
    central star creating bow shocks, etc. But if the nebula is 2 light years across that seems
    to imply the stars are much denser in that neighborhood of the galaxy than where we
    are. Right?

  34. Jeff,

    All the creationists will see is the a big eye looking at them.

  35. IVAN3MAN

    @ #31. Jeff,

    Only two things are infinite: the Universe and human stupidity; but I’m not sure about the Universe.

    — Albert Einstein.

  36. davem

    If you go from the centre star towards the top right of the nebula, another 75% of the distance gets you to another faint ‘shock wave’ What’s that about? It’s only on the top right, nowhere else that I can see.

  37. Wow, I’ve seen this image dozens of times before but never noticed the galaxies that you can see through the nebula

  38. 30. Chris A. Says: “It’s perfectly acceptable to speak of multiples of temperature provided you are using an absolute (as opposed to relative) scale. Astronomers use Kelvin, making such comparisons legitimate (whereas, doing the same in Celsius or Fahrenheit would be a no-no–not that that stops people from doing it ALL THE TIME–can you tell it’s one of my pet peeves?).”

    You are absolutely correct, however in this case, when talking in the hundreds of millions of degrees, those first little 273 don’t amount to much.

    – Jack

  39. 34. Per Bojsen Says: “I’m curious about the comet looking features in the nebula. If my understanding is correct, these are due to the stellar winds of stars that push against the wind from the central star creating bow shocks, etc. But if the nebula is 2 light years across that seems to imply the stars are much denser in that neighborhood of the galaxy than where we
    are. Right?”

    While waiting for the BA to answer, I’ve got to comment that those look exactly (to a first approximation) like the structures in the iris that make it expand and contract (I don’t know their proper name). It makes the illusion of an eye all the more vivid! Interstellar pareidolia!

    – Jack

  40. Crux Australis

    Looking at the embiggened photo (and this may be just me — Edwin Hubble I ain’t) it looks like the galaxies are a little yellower than the other stars. Is that right? If so, how come?

  41. Chris A.

    39.Jack Haggerty says: “…when talking in the hundreds of millions of degrees, those first little 273 don’t amount to much.”

    True (although Phil was talking about 120,000 degrees C, not 120,000,000). That being said, 120,000 C is more like 21 times the Sun’s surface temperature (5778 K, or 5505 C), not 25.

  42. I looked at that pic again and found dozens of galaxies outside of the nebula too. They’re everywhere!

  43. 41. “it looks like the galaxies are a little yellower than the other stars. Is that right?”

    The integrated light from an older population of stars in a galaxy, will be yellowish, due to the few but very bright (red/orange) giants and the yellow to red contribution of the low-mass main sequence (dwarf) stars (like the Sun). Star formation adds a blue color, since massive stars are blue and bright – but they burn their fuel very fast and disappear (as supernovae) in the blink of a cosmic eye (millions of years). Blue therefore means a young galaxy or a galaxy that is merging with another galaxy, inducing star formation.
    Some of the foreground stars are also over-exposed (saturated) which makes them white. Dust (locally, in our galaxy) or in the distant galaxy will also redden the light.

    3. and others:

    The knots are simply higher density clumps in the interstellar gas. When the last part of the envelope was blown off the red giant (of which the central white dwarf is the core) that strong wind swept up most of the interstellar gas, but didn’t completely erode away the densest knots. The “tail” of each knot is gas being blown off the knot. These are the cold, dense molecular clouds that, if dense and large enough, can form stars. These won’t.

    37. “…gets you to another faint ’shock wave’ What’s that about?”

    The red giant goes through many thermal pulses before it sheds the last bit of its envelope, to reveal the hot white dwarf within (formerly known as its core). What we see in the picture is part of the shell of the last pulse. Click on my name for a wikipedia page on this part of stellar evolution.

    Cheers, Regner Trampedach (at Mt. Stromlo Obs.)

    P.S. Phil – thank you so much for the editing feature! – and for a great blog!

  44. I agree that the shape seen is that of the classic planetary nebula (PN) (see my website and that we are looking nearly along its longitudinal (rotation) axis. I have trouble seeing a surviving source object. That may be one of the nearly central bright spots, or it has been totally expanded out of existence. In my web site, I risk the suggestion of a start-to-finish dynamic model of these PN objects, which risked suggestion needs improvement.

  45. Andrew MW

    thanks for the pic and the fascinating explanation, Phil.

    New background wallpaper for me…!

  46. Regner Trampedach

    At 45. Angelo Campanella, just a few comments on the thesis on your comment and your web-site.
    1) The bright white dot at the centre of the Helix, is the white dwarf, that was once the core of the star, which envelope is now the planetary nebula (PN).
    2) Planetary nebulae don’t have jets, but about 4/5 of them do have bi-polar structure. The term jet is reserved for something with much higher velocities.
    3) The central object is a white dwarf – if it was a neutron star or a black hole, we would be looking at a supernova remnant.
    4) The progenitors of PN are medium mass stars, 0.8-8 solar masses, which end their nuclear fusion at carbon and oxygen (from helium). Their gravity is too weak to hold on to the envelope which is shed during the thermal pulsing phase – basically by a very strong stellar wind – not an explosion.
    5) More massive stars can reach higher densities and temperatures in their core, and fuse elements all the way to the iron-peak, at which point no more energy can be gained, and the core collapses resulting in a supernovae.
    6) The nebula of PNs are illuminated by the UV photons from the white dwarf.
    7) The composition of the white dwarf and of the nebula, both support the above scenario.
    8) The bipolar shapes are most likely caused by binarity of the progenitor, and maybe also by magnetic fields of the progenitor. Neither of these explanations are mysterious since we know most stars are members of multiple systems (binary or more) and have magnetic fields.
    9) White dwarfs are so-called compact objects with about one solar mass crammed into an object the size of the Earth. The pressure resisting gravitational collapse, is not the usual thermal pressure, but a quantum mechanical pressure, arising from Pauli’s exclusion principle and the high densities. With the high densities the lowest energy phase-space states are occupied, and Pauli’s discovered that only one electron can occupy each state. Phase-space consists of space and velocity, so in order for more electrons to gather in the same space, they gain higher velocities – this means higher pressure – the pressure of degenerate electrons.
    Okay – I think that is enough for now. Have a great weekend.


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!


See More

Collapse bottom bar

Login to your Account

E-mail address:
Remember me
Forgot your password?
No problem. Click here to have it e-mailed to you.

Not Registered Yet?

Register now for FREE. Registration only takes a few minutes to complete. Register now »