The beginning of the end for a star

By Phil Plait | March 29, 2011 7:00 am

Every now and again I’ll see an image of an astronomical object and think, what the heck?

CRL 618 is definitely one such object!

This Hubble image threw me for a sec: it looks like a planetary nebula, but where’s the central star? What are those long fingers of matter? So I started going through the scientific literature and found some good explanations. And I learned something!

CRL 618 is a star announcing it’s on its way to becoming a planetary nebula.

When stars like the Sun die, they expand hugely and cool off, becoming red giants. They then emit a solar wind that is slow (in astronomical terms at least) and dense. After a few thousand years, as the star loses more and more mass from its outer layers, the deeper, hotter part is exposed. The wind emitted speeds up, slams into the slower wind, compressing it, and ultraviolet from the exposed stellar core lights it up. The result is a beautiful planetary nebula, like the famous Helix Nebula.

That’s all well and good, but this model of interacting winds has a problem explaining some of the features commonly seen in planetary nebulae (or PNe for short). In this picture of IC4593, for example, there are knots and long linear features. How did those get there?

CRL 618 is, perhaps, telling us how! Here’s the (probable) scoop: deep in its heart lies not one star, but two. One of them is a red giant, and the other is probably a lower mass star, and they orbit each other as a close binary system. Because they’re rapidly spinning around each other, a lot of the dust and junk blown off by the red giant heads off in the plane of the stars’ orbits. That’s why the center of the nebula is dark! The stars are there, just hidden beneath all that junk.

Also, a lot of that material forms a smaller disk, called an accretion disk, just around the second star. This (and magnetic fields probably play a role too) helps focus material along the poles of the system, blowing it up and out — there’s less matter up that way, so its easier for material to flow up and down. That’s what causes those long fingers of material seen in the Hubble image of CRL 618. There are probably episodic pulses of energy that fling out matter, which is why there’s more than one finger. And in between those episodes the poles of the system have moved, like a top wobbles as it spins (called precession).

Most likely these fingers will continue to grow, and as they hit the previous emitted gas they’ll form those knots and other weird features seen in PNe like IC4593. The details of all this are fiercesome to determine, of course. For example, each individual finger has waves inside, as if there were more than one pulse blasting out matter to form each finger, and each pulse piles up more material in front of it like a snow plow. Why does that happen? How does matter get accelerated to such velocities (hundreds of kilometers per second!)? How does this process start, and how does it shut down?

Because shut down it does. This stage in a star’s life may only last a few hundred years — as little as only 0.000001% of a star’s entire life. That means we don’t get very long to look, and it’s so brief that we only see a few dozen proto-planetary nebulae like this.

However, keep in mind the scale here. These features may be young and small compared to the entire nebula, but they’re still hundreds of billions of kilometers long!

I find planetary nebulae endlessly fascinating. They are so ethereally beautiful, so complicated, so intricate… and they’re showing us in some ways what the Sun might look like in six billion years (give or take). And while it’s possible the Sun won’t have enough oomph to be able to light up its gas the way most planetary nebulae do, the forces at work are literally universal, and what we learn observing these gorgeous dying stars informs us on how our own star will age and eventually die.

Image credit: ESA/Hubble & NASA

Related posts:

Warm, dusty rings glow around a weird binary star
A delicately violent celestial shell game
Down the throat of a dying star
Night FLIERs

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (30)

  1. astronomynut

    It’s interesting that the waves don’t seem to be confined to the jets. They appear to be spherical and appear in both jets (at least on the bottom pair of jets) like tree rings.

  2. Nigel Depledge

    Another good one, Phil. And thus do we learn: by looking, wondering and looking again.

  3. Interested_Nonexpert

    BA and fellow readers, a question:
    When a lower mass star (like CRL 618) dies and sheds its outer layers into a planetary nebula, how does the mass composition compare with that from the material ejected in the core-collapse supernovae (CCSNe), associated with the death of higher mass stars?
    Naively, it seems that planetary nebulae, because they are largely composed of the outer layers of (lower mass) stars, their composition should mostly be of lighter elements (H, He, etc.); whereas CCSNe eject even deep layers, from heavier mass (longer fusing) stars, and so much higher fractions of heavy elements.
    Thanks in advance for your feedback.

  4. Isn’t a red giant and a low mass star in a binary system usually the recipe for an IA Supernova? Or is the lower mass star not a dwarf and that’s why this isn’t the case? Instead of pulling in the gas from the red giant it’s pushing it away? There’s always a new situation with stars that I knew nothing about.

  5. I wonder if the dark streak on the bottom lobe is the underside of a structure matching the bright reddish arc in the upper lobe. If there we less junk mucking up the view, would those twin features form an hour glass shape, which is fairly common in planetaries?

  6. Perhaps the lower part of the image is darker, because we are seeing it through the dust and other “dark” material in the disk? And the blueish haze in the upper part of the image is light reflected off the disk, which is behind the jets? And the bright red region near the center is reflected light from the star(s), just peeking out of the disk.

    Also, are the bright ends of the jets simply more reflected light, or is there actually something there, giving off light? (It reminds me of contrails.)

  7. Rules For

    That has to be a white dwarf (a dead star) in a binary with a red giant, to cause a Type Ia supernova.

  8. Capt Tommy

    Oxnar busy munching on the fried *ln*to*oo, reach over to adjust the churnthup, but he instead pushed the riiipip. The sudden silence caused his eye bulbs to vibrate. SooSooPa turned in his carapace.

    He had just enough time to pose – Fizzbain!

    The above is from the Space Opera “Lobsters In Space”

  9. Jon Hanford

    This proto-PNe is somewhat reminiscent of bipolar outflows seen from very young stars (e.g. Herbig-Haro objects). Typical ages for HH objects are similarly only a few thousand years.

    Interesting that stars can exhibit similar short-timescale phenomenon (bipolar jets, light-obscuring disks) both when very young and near the end of their lives.

  10. Oli

    After this star has become a white dwarf, could it then go Ia supernova when its partner starts swelling up? If yes, how would the supernova’d material interact with the nebula (seems to me like the supernova material would move much faster than the nebula material)?

  11. DrFlimmer

    Why does that happen? How does matter get accelerated to such velocities (hundreds of kilometers per second!)? How does this process start, and how does it shut down?

    Questions not only relevant to this particular object, but to all objects that contain accretion disks (and then mostly jets). Interesting objects, and nobody really knows how they are formed and how they work.

    Very interesting :)

  12. Joseph G

    Lies! All comforting, sweet lies! Oh, how I wish it were a “planetary nebula”. But I know a hatching Cthulhu egg when I see it!
    We’re doooooomed!

  13. Jeff

    yes, it is sobering that all our planetary wonders like Venus, Mercury, Jupiter, Earth, etc. will all probably get turned to dust/vapor and entrained in a planetary nebula in 6 billion years. So much for arrogant mankind and all the to-do about nothing they think up. I’m a grumpy old prof. and fed up with human arrogance and even all the technology, I much prefer old mother nature. Right now as I’m writing this, I’m looking out my office window, and what do I see ? a football stadium with 30,000 seats they are building. Now, it’s pretty amazing what us human builders can build, and all those big cranes are impressive, but it doesn’t impress me like nature does. Human constructs come and go, but this universe is eternal.

  14. HvP

    Wow, quite an image. Any chance that this is a target for a long-IR platform to see deeper into that dust cloud?

    This is certainly more interesting than my first thought when I saw the title of the post, “The beginning of the end for a star” – Charlie Sheen?

  15. Anon

    I see boobs. My breasts are strange.

  16. @ Interested_Nonexpert: Something else to keep in mind is that low mass stars can’t fuse all of the elements that high mass ones can, for lack of high enough temperatures and pressures. High mass stars that die as supernovae can fuse elements as heavy as iron, while low mass ones that die as planetary nebulae usually only fuse elements as heavy as oxygen.

    Something interesting about PNe is that they provide us with a “snapshot” of what their formation region was like when they were born. If a planetary nebula has an element like iron in it, the iron must have been in the interstellar medium at the time it the progenitor star was forming since we know the progenitor star never would have gotten hot enough to fuse it.

  17. Michael

    Let’s name the star “Bieber.”

  18. Whomever1

    Here’s a very tangential question this post brings up for me: Is there any tendency for the stars in a galaxy to rotate in the plane of the galaxy? Of course, I don’t even know how well we can measure that, but it’s a question I’ve had for awhile.

  19. Nemo

    As a pedant, I must point out that the word is “fearsome”, not “fiercesome”. (And don’t try to tell me that “fiercesome” is perfectly cromulent.)

  20. Messier Tidy Upper

    Great image and write-up. :-)

    @9. Whomever1 Says:

    Here’s a very tangential question this post brings up for me: Is there any tendency for the stars in a galaxy to rotate in the plane of the galaxy? Of course, I don’t even know how well we can measure that, but it’s a question I’ve had for awhile.

    That depends which stellar population that star belongs to :

    1) Spiral arm stars – a) thin and b) thick disk populations do indeed orbit in the plane of the galaxy in the spiral arms. Our Sun belongs to this class as do almost all the stars we see in the night sky. The young more massive blue-white stars are (almost all?) members of the thin disk that formed recently and probably won’t last a full orbit of our Galaxy. The sun-like stars (or older ones) and many younger red dwarfs that have been around longer have been dispersed into slightly more eccentric – more elliptical and more inclined – orbits through gravitational encounters with other stars, dust clouds etc … but are still largely orbiting in the Galactic plane and arms. These are also termed population I stars and include the open clusters and star forming HII regions.

    2) Halo stars orbit in the Galctic halo in extremely eccentric and highly inclined orbits and include the stars in the Globular Clusters. These tend to be much older and very metal poor Population II stars. Kapteyn’s Star is one example and possibly also Arcturus although that may be just a member of the thick disk stellar population.

    3) Bulge stars -orbit inside the central bulge of our galaxy, these are like the Halo stars but packed in much more densely and more “metal” rich. The central bulge is almost, perhaps , a bit like an elliptical galaxy of its own! The Bulge stars, I gather, are in inclined (tilted out-of plane) and elliptical orbits.

    That’s my understanding of this summarised in a few words – I could be mistaken. For more I would strongly recommend reading ken Croswell’s book on this topic (& more) The Alchemy of the Heavens which is very readable and informative and one of my fave astronomy books. :-)

  21. Messier Tidy Upper

    See also :

    for more on the stellar populations incl. this :

    diagram of stellar distributions /populations and see :

    for the Alchemy of the Heavens book. Hope these help. :-)

  22. Monu

    No question is stupid, right?

    “a lot of the dust and junk blown off by the red giant heads off in the plane of the stars’ orbits. That’s why the center of the nebula is dark!”

    I don’t get why the centre is dark then. If material is blown out into the plane of the star’s orbit then shouldn’t there be planes of lit-up matter((perpendicular to the fingers))?

  23. Gibby


    While what you’ve written is correct – I think you may have misunderstood the question. @19 writes ‘Is there any tendancy for stars in a galaxy to /ROTATE/ in the plane of the galaxy’

    Not orbiting like the spiral arms, but rotating on their own axis.

    Do stars have a tendancy to allign their axial rotation to the rotation of the galactic arms as a whole?

    I’m only mentionning this as I’d really like to know the answer, as well!

  24. Joseph G

    @MTU re Whomever1: I could be wrong, but I believe he might have been talking about the axis of rotation of the various stars in the galaxy – are they oriented more or less randomly, or is there any tendency for any population of stars to form with an axis of rotation nearly parallel to that of the galaxy (similar to the way that most of the planets in the solar system share a roughly similar spin orientation)?
    This is something I’ve wondered about myself. I know that the solar ecliptic seems pretty random compared to the galactic disc, but I wonder if certain populations of protostars are more influenced by galactic rotation then others? Or is it simply too big a change in scale to make a difference (like the myth about the Coriolis force having anything to do with the way water goes down your sink drain)?

    Edit: Oops, Gibby beat me to it :)

  25. Messier Tidy Upper

    @ ^ Jospeh G. & 24. Gibby : Ah! I see now.

    Well, I’m pretty sure the orientation of individual stellar axis’ (axises? axi?) is random – but I could be wrong about that.

    I do know for sure that we see some stars pole on – Vega is one example – whilst the transit method of finding exoplanets shows we see others edge on.

    There’s no reason to suppose, from what I gather, that star formation would favour any particular axial alignment ie. no reason stars would rotate aligned to the galactic plane.

    As aways though, I could be mistaken and will urge anyone knowing better to please correct me if I’ve got this wrong.

  26. Messier Tidy Upper

    Thinking of our Sun’s future – this :

    is pretty amazing! :-)

    It shows :

    1) Just how wonderfully sensitive Kepler is,

    2) How different red giants are even amongst themselves and how vastly they range in size


    3) That we can work out so much across hundreds of lightyears from the shivers inside red hot plasma all the way to the cores of alien suns. :-)

  27. Joseph G

    @26 MTU: I see. That’s what I’d always assumed – that the formation of protostellar discs from gas clouds are driven by more or less random distribution, and the odd collision with another gas cloud or supernova shockwave to trigger collapse.

    @27 MTU: Daaang that is cool!!! :) Of course, I’m still flabbergasted by the way that spectroscopy lets scientists determine the chemical composition of celestial bodies, and they’ve been doing that for over 100 years :) We nekkid apes are clever sometimes, I tellya what.

  28. Anchor

    Phil, I hate to disagree again, but I think Ken B is correct. Another interpretation that is consistent with what we see in the image of CRL 618: the APPARENT ‘waves’ seen in the lower jet complex (and certainly NOT seen in the upper one) may instead be caused by thick bands of obscuring dust in the dust disk encircling the system, which would be obliquely angled to our line of sight, rather like seeing the banded rings of Saturn at some angle, but substantially more diffuse. (And that disk must be HUGE to partially obscure that lower jet!). I just can’t buy the idea that one jet complex would exhibit evidence of variable jet activity and the other one doesn’t exhibit anything remotely like that.

    In some other planetary nebulae that appear to exhibit similarly CONCENTRIC structures, nested shells of dust may also better explain those features in an analogous way. It’s a simpler interpretation: while it may indeed be true that jet generation can be variable, there is no need to invoke variable activity as the sole explanation for apparent discontinuities, let alone suggest that such discontinuities may be read as a record of jet variability. It seems to me that CRL 618 is a splendid example of a case that clearly REFUTES the interpretation that variable activity is responsible for the apparent ‘waves’ seen ONLY in the lower jet.


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