The life of a star, in 14 minutes

By Phil Plait | September 27, 2012 10:55 am

A popular style of do-it-yourself video is what I think of as the "stop-motion whiteboard drawing", where someone films someone else drawing on a whiteboard, explaining some concept or another. It’s surprisingly engaging, and a lot of otherwise complex topics can be better understood this way.

Case in point: how do stars work? How are they born, live out their lives, and die? The overall story isn’t conceptually difficult, but there are some important details (like how massive the star is) and it can be easy to lose the thread. But if you watch this video, Life of a Star, your understanding will be a whole lot better:

This gives you a pretty good overview of how things work, and I’d certainly recommend it for any Astronomy 101 students who want a quick review, or sciencey-type folks who just enjoy learning about the Universe. Which, admit it, is you. There’s just enough info there to make sense of stellar life cycles, and if you want details, well, there’s Google. Or my book. Either way, if you want more fun stuff about star formation, evolution, and eventual demise, you can find it – and this video is a great start.

Tip o’ the hydrogen burning shell to Tony Silva via a chain of links starting at Giovanni Picogna.


Related Posts:

- More M95 supernova news: progenitor found!
- Betelgeuse’s sandy gift
- Does this cluster make my mass look fat?
- The evolution of creationist astronomy (which is a followup to this)

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (28)

  1. Ian C.

    Shame about the sound quality, but great other than that

  2. DrFlimmer

    Or my book.

    You wrote a book???

  3. Yigit U.

    Your book is so good, it’s the only book I still keep on my Kindle after reading.

    @DrFlimmer, look to the right on this page and you’ll see it.

  4. Almir Germano

    Take a look at this: http://www.youtube.com/watch?v=LZa5H-HyN14
    It is in Brazilian Portuguese, but the idea is just the same.
    It was done with financial support from CNPq, the brazilian government scientific agency.
    The authors: Jane Gregorio-Hetem (Institute of Astronomy, Geophysics and Atmosferic Science of Sao Paulo University – USP) e Annibal Hetem Jr. (ABC Federal University – UFABC)
    Cool! “The Origin of the Metal”!
    Very Cartoon Network like, isn’t it? Indeed the artist is here http://www.marlontenorio.com

  5. Cindy

    Good to bookmark for my Astro students to review.

    However, I thought it was the massive neutrino release when the entire core turns into neutrons via reverse beta-decay that triggered the supernova rather than the bounce of infalling material.

  6. Arek W.

    Everyone says, that fusion stops at Iron.

    But here:
    http://www.badastronomy.com/bitesize/sn87a_explosion.html
    you say, that Iron also fuses:
    “When enough iron builds up in the core, the pressure becomes great enough that it starts to fuse. This robs energy from the star. Worse, the fusion of iron eats up copious amounts of electrons, and electrons help hold up the star too. ”

    As I understand this, in the first scenario – when fusion stops at Iron, core simply stops producing heat and then star collapses.

    In second scenario – when Iron continues to fuse – it uses electrons and heat which causes star to collapse.
    “The iron core collapses, since the heat and electrons holding it up get used to fuse the iron.”

    Am I right? Is there any difference? What is real story?

  7. Matt B.

    0:57 “Each atom is gravitionally attracted to each other”? That sounds ungrammatical. It would have been better to say “to every other” or at least emphasize the word “other”.

    1:10 “Saracun-shaped”? What is that word?

    And I still haven’t figured out why so many people pronounce “fission” as if it has only one “s”. It should be /fish-un/, not /fizh-un/.

    3:10 The Manhattan Project did not invent the hydrogen bomb. (The basic idea arose then, but the project did not produce a hydrogen bomb design.)

    13:14 I’ve never heard of a neutron star having an “enormous electric field”. It doesn’t even sound likely, given how difficult it would be to maintain the separation of charge, especially when the only material to work with is neutrons.

  8. Lavocat

    Excellent lecture.

    Okay, I’ll bite … WHY can’t iron atoms be further fused?

    I remember taking a class in nucleonics @ MIT on this topic but, for the life of me, I can’t remember why fusion stops @ iron.

  9. Rick

    My understanding of the iron-fusing issue has always been that it takes more energy to make the fusion happen than you get out of the reaction. Meaning that it can’t be a self-sustaining process–it’s essentially an energy sink, I guess would be one way to put it.

    If that understanding is wrong, somebody please correct me.

  10. Tim Gaede

    I guess the briefest answer as to why you can’t get exothermal fusion beyond iron is that the iron-56 nucleus is the most tightly bound (the binding energy per nucleon is highest).

    Binding energy graph

    If you flip the linked graph along the horizontal axis, you get potential energy per nucleon. Iron-56 would then be at the bottom of a valley; uranium is elevated above iron and has some potential nuclear energy; hydrogen has much more potential nuclear energy.

  11. In fact, iron atoms CAN fuse into heavier elements. This is how all the heavier elements we find in the Earth’s crust (gold, uranium, etc.) were first formed.

    However, unlike with lighter elements, iron fusion is ENDOTHERMIC. It takes more energy to squeeze the iron nuclei together than the fusion process releases. It is this absorption of energy when iron fusion occurs, in fact, that causes the core of a supernova to shrink so rapidly. Iron fusion COOLS OFF the core, and the radiation pressure that was suppoting the star against its own weight literally gets the rug torn out from under it!

    EDIT: What really bugged me about the explanations in this video, though, was the description of red giants. The outer layers collapse, which heats the star, which pushes the outer layers FARTHER AWAY than they were in the first place?! That’s poppycock. It would also be a blatant violation of the conservation of energy. No, there was one crucial detail missing in this explanation: When the outer layers collapse, the layer IMMEDIATELY ABOVE THE CORE gets hot enough that it can undergo nuclear fusion itself. It is this “hydrogen-burning shell” that provides the radiation pressure that expands the star to red-giant size.

  12. Matt B @ #4 asked: “1:10 “Saracun-shaped”? What is that word?”

    “…becomes spherical in shape…”

  13. @ Arek W. #6
    That was always a confusing point for me as well. I have heard it as “when the start begins to produce iron” and “when iron starts to fuse”, it seems to be one of those not-quite-clear- points for many people. My understanding is what Tracer (#11) says, except that it is a bit more complicated because elements don’t just fuse with like elements. In other words, by the time there is iron in the core, there is carbon and silicon and oxygen, sodium, magnesium, calcium and so on, in varying amounts, and different combinations can fuse to produce different end products. Any endothermic reactions, essentially those producing elements heavier than iron, are quickly shut down until there is enough of a buildup of iron that the conditions are right for a runaway endothermic reaction can occur.
    I think…

  14. This reminds me of Vi Hart’s style of explaining mathematics: https://www.youtube.com/user/Vihart?feature=CAoQwRs%3D

  15. ctj

    once a star accumulates an iron core, is there enough time for iron fusion before core collapse? i’ve always thought that heavy elements are formed by neutron capture during the supernova explosion. perhaps some fraction of the iron and other near-iron elements fuse in the star’s last moments, but presumably very few of those atoms escape the core collapse.

    of course, the whole model of stellar fusion in an essentially pure H or He core is highly simplified. it’s easy to forget that there is at least one whole earth mass of uranium in the sun, and it would be silly to completely discount the effects of these heavy elements on the nuclear reactions in the core (although certain effects, such as uranium decay, can be largely ignored because the energy released is such a tiny, tiny fraction of the energy from fusion). but like the bohr atom, it seems we are stuck with it (and unlike the bohr atom, it actually describes things reasonably well).

  16. Ian

    Matt B “13:14 I’ve never heard of a neutron star having an “enormous electric field”. It doesn’t even sound likely, given how difficult it would be to maintain the separation of charge, especially when the only material to work with is neutrons.”

    What about a Magnetar?

  17. amphiox

    What do the models say about the smallest red dwarfs?

    Do they go red giant like medium stars (and if so what would these dinky red giants look like?), or are there any so small that core collapse cannot start up He fusion. Would these just sort of peter out and cool down into something like a heavy brown dwarf? Would being entirely convective mean it gets to mix all its hydrogen and eventually fuse all of it into helium? Are they even massive enough for the core to become degenerate like white dwarfs at all?

    Of course, since these red dwarfs live for trillions of years on the main sequence, it’ll take a long, long time to find out if the models are correct….

  18. Matt B.

    @12 Peter B – Thanks. I’ve played it again with the volume way up, and I was finally able to tell. I guess my mind just wasn’t ready for the way it was phrased.

    @16 Ian – A magnetar has an enormous magnetic field, hence the name.

    13:10 A typical neutron star’s rotation period is less than a second, not the several seconds mentioned in the video, though apparently magnetars in particular do take more than a second.

    The artist also misspelled “occurring” at 11:45, just to be totally nitpicky.

  19. #6 Arek, #8 Lavocat:
    As someone has stated, the iron-56 nucleus is the most stable nucleus there is, in terms of bindig energy. Fusion of nuclei as far as iron is exothermic, i.e. the energy released is greater than that required to trigger the reaction. Fusion of nuclei to anything heavier than iron is endothermic, i.e. requires a constant input of energy to sustain it.
    So… while a star is on the main sequence, it’s a balancing act between gravitational collapse and the energy released by the exothermic fusion reaction, which enables it to resist the collapse. In the core of a massive star, successively heavier nuclei are produced, as shown in the video, until iron is produced. At that point, further fusion suddenly becomes endothermic instead of exothermic – so the core is no longer releasing energy to resist gravitational collapse; that’s why the core collapse happens, which triggers a supernova.
    The core collapse, and the subsequent implosion, raise the temperature even further, such that the endothermic fusion of iron to heavier nuclei can occur for a short time. The explosion which follows releases those heavier elements into the interstellar medium.
    A supernova is the only natural process which can produce heavier nuclei than iron. All elements heavier than helium are produced in the cores of stars, and all those heavier than iron are produced in supernovae.

  20. amphiox

    All elements heavier than helium are produced in the cores of stars, and all those heavier than iron are produced in supernovae.

    IIRC, a small amount of lithium is hypothesized to have been produced in the early moments of the big bang.

  21. Lavocat

    Thank you, #19 Neil Haggath, for an excellent explanation. I was looking for my old notes and seem to recall iron fusion occurring once the star had gone down an irreversible track towards supernova. Hard to believe that within seconds, iron fusion creates the heavier elements and then BLAMMO.

  22. @17. amphiox asked :

    What do the models say about the smallest red dwarfs?
    Do they go red giant like medium stars (and if so what would these dinky red giants look like?), or are there any so small that core collapse cannot start up He fusion. Would these just sort of peter out and cool down into something like a heavy brown dwarf? Would being entirely convective mean it gets to mix all its hydrogen and eventually fuse all of it into helium? Are they even massive enough for the core to become degenerate like white dwarfs at all? Of course, since these red dwarfs live for trillions of years on the main sequence, it’ll take a long, long time to find out if the models are correct….

    Found an online paper from 2004 by Fred C. Adams, Gregory Laughlin and Genevieve J. M. Graves titled Red Dwarfs and the End of the Main Sequence (linked to my name here) which notes :

    … focusing on the long term development of red dwarf stars. We show that these diminutive stellar objects remain convective over most of their lives, they continue to burn hydrogen for trillions of years, and they do not experience red giant phases in their old age. Instead, red dwarfs turn into blue dwarfs and finally white dwarfs.

    Which is potentially confusing considering a “blue dwarf” is usually an O-B type main sequence star!

    Not sure if that is /was the last word or only theory /model or whether further papers have confirmed / modified or refuted that but thought it may be interesting in this context.

    My understanding was that red dwarfs are fully convective throughout (vaguely recall reading an item on that somewhere) and probably lack sufficient mass to fuse hydrogen to helium at their cores and become red giants but I could be mistaken.

  23. davem

    If stars start as clouds of hydrogen atoms attracted together by gravity, then surely, old ‘dead’ stars could be re-activated by attracting fresh hydrogen fuel? Does that happen?

  24. #21 lavocat:
    You’re welcome.

    #20 amphiox:
    You are of course correct about the lithium. I stand corrected.

  25. Dean

    A bit of a nitpick. The pressure doesn’t go up because the temperture climbs so high. The temperature is so high because this is what is required to keep the star from collapsing on itself, and to keep it in hydrostatic equilibrium.

    What is actually happening is that there is so much mass in the star, that the pressure in the core gets extremely high, and this forces the hydrogen atoms so close together that the protons can tunnel through the Coulomb barrier (due to the repelling of electrostatic charge). and thus the nuclear strong force takes over, fusing the nuclei together.

    The energy released from the fusion is what supplies the kinetic energy to drive the temperature up to 15 million degrees.

  26. amphiox

    re MTU @22;

    Thanks. That paper is mondo cool.

    re davem @23;

    Well one not uncommon scenario for attracting fresh hydrogen fuel is a dead star in binary orbit around another star, and siphoning hydrogen gas from it. And in that scenario the dead star doesn’t reactivate, but instead other wild and interesting things happen, like novas, Type Ia supernovas and the like.

  27. @ ^ amphiox : No worries. :-)

    FWIW. I’ve also found another older book which alternatively claims :

    After 30 billion years on the main-sequence, the core of the red dwarf is filled with the helium manufactured from hydrogen. lacking the pressure to fuse helium the star cools, dimming and contracting nto an inert ball of gas known as a black dwarf.”

    - Page 62, ‘Stars’ editors of Time-Life, 1988.

    Which applied to a red dwarf of one-third solar mass. Again no red giant phase, perhaps an older understanding.

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