Q & BA Episode 2: Journey to the Center of the Sun

By Phil Plait | February 11, 2007 8:47 pm

Q & BA Episode 2 is now online! It’s called "Journey to the Center of the Sun", and I talk about the density at the center of the Sun, comparing it to water and iron. I also talk about how the Sun makes energy, and marvel at the numbers. At several points marshmallows are involved.

Caveat emptor — I made a mistake in the video. I don’t mean the five hydrogen atoms; I mean something else. Anyone catch it? It wasn’t important enough for me to edit out or anything, but to be honest I’m admitting it (sortof) here. Post a comment if you think you know what it is.

You can also see this video on YouTube, on LibSyn (with much better resolution), or get it through iTunes (you need to follow the instructions I posted here).

Incidentally, after a few requests, I have made audio-only versions of the podcasts too. I’ll post them on LibSyn simultaneously with the vidcast, and they’ll have the same episode number as the vidcast, but with an "a" after them. Here is Episode 1a (galaxies), and Episode 2a (the current one about the Sun).

Don’t forget to keep those questions coming!


Comments (44)

  1. Lauren

    I love the way you present the material. “Go fusion!” I love it. Also I like the way you change the background pictures. New pictures, new book.

    For the hydrogen, shouldn’t it be hydrogen nuclei (or basically protons) that collide to make the helium isotope?

  2. TAW

    It’s not working for me. I click play and it never loads, and when I click on it it says it’s a private video.

  3. TAW

    Oh. Just clicked the LibSyn link and it works (I thought it was just audio)


  4. Ha, never mind for me as well…

  5. Hmmm. I logged out of YouTube, and it says it’s public (I can see it). The file says it’s public. Maybe you need to refresh the browser. FWIW, a friend said it was private to him, but now it works.

  6. You mean that two protons (hydrogen nuclei) and two neutrons make a helium nucleus (alpha particle).

    Of course the weak interaction can easily convert protons into neutrons, since they’re both members of the same isospin couplet, but unless you’re going to tell us about the Standard Model next it’s a bit misleading in this context.

  7. Oh, and YouTube still says it’s private to me, whether logged in or not.

  8. Indeed 4 hydrogen nuclei would go into a beryllium would it not? I guess if you crush the hydrogen enough you could make it into 2 protons, 2 neutrons and 2 electrons, but I would think that 4 hydrogens would desperately try to make a beryllium if you squeezed them :)

  9. snarkophilus

    I really liked that! Thank you!

    Oh, and I think some Penn is rubbing off (minus the profanity). “Yes, we KNOW about evolution. Deal with it.” I almost wet myself when I saw this.

  10. My mistake had nothing to do with the fusion bit. Like I said at the end, the actual steps are more complicated (it uses the proton-proton cycle and a little bit of the CNO cycle); I was simplifyng for the video.

  11. Your little message about how average (but amazingly spectacular) our sun is reminded me of this great Douglas Adams quote:

    “The fact that we live at the bottom of a deep gravity well, on the surface of a gas covered planet going around a nuclear fireball 90 million miles away and think this to be normal is obviously some indication of how skewed our perspective tends to be.”

  12. Kaptain K

    I was taught (mumblety mumble years ago) that 400,000,000 tons of hydrogen is fused to 395,000,000 tons of helium per second.

  13. DrFlimmer

    VERY VERY GOOD JOB!!!!!!!! Thank you Phil! You did ist so great!!! I was really enjoying it and some laughters at all!

    Well, I guess the “mistake” is that you said: “6000 DEGREES Kelvin”, but I think the word degree is wrong in ths place. Not a big mistake, everyone is doing it.

    Can’t wait to see the next chapter next week! :)

  14. Carina

    You know, I actually can understand what you’re saying in that video!

    It takes ALOT of concentration and I have to watch the video two or three times, but that you look into the camera and I can see the mouth movements helps enourmeously. :3

    *is happy*

    (I found the same when I was visited by an Australian and we could only speak English with each other – I couldn’t understand him if I didn’t see him speaking, but if he looked at me, all was fine. Strange, isn’t it?)

  15. JB

    I’ll go with ions as well…. :)
    great summary

  16. Grand Lunar

    Is the mistake using the term “degrees Kelvin”?
    As I read, the term degrees is not used when expressing temperature on the Kelvin scale.

    Very cool video, BTW. I eagerly anticipate future episodes!

  17. Damian

    Neat. I hope this turns into a long-running series.

    It’d be fun to see an episode on Drake’s Equation, since the verbal explanation process seems to help people with the concept of really huge numbers. If this isn’t the place for episode suggestions, sorry. :)

  18. Hydro

    I’ll never view marshmallows or trash cans in the same way ever again…. Keep up the GREAT work! :)

  19. James A. Brown

    Super video. You know, your verbal tics of over-pronouncing MILLions and BILLions is similar to what spawned the urban legend of Carl Sagan uttering “BEELyuns and BEELyuns” during his COSMOS mini-series. He swore up and down that he never uttered that phrase during the episodes, but of course nobody believed him.

    I also liked the dig about Americans and the metric system. Science goes down easier with a sprinkling of humor added.

  20. Ah yes, degrees Kelvin is not technically correct, but I think the punishment for doing that is to be darned to heck for 15 minutes.

  21. Melusine

    I second Lauren’s comments, though I can’t really see the book in the background. I like the added graphics, too, plus the humor. These are fun and help with the huge numbers and size comparisons.

    Don’t feel bad, a NASA page does it, too:

    But of course, SOHO doesn’t:
    The Sun’s visible surface the photosphere is “only” about 5,800 K (10,000 degrees F).

  22. The problem is, no one can really explain how this corona exists. Even if the temperature in the core of the Sun does reach 15 million degrees, it drops to a mere 5000 degrees at the surface.

    That’s from the NASA site and it’s not really wrong. 1K = 1 degree C, and at even 5000 degrees C, it’s probably fair (to 1 significant figure) to say that 5000K = 5000 degrees, with the “Celsius” implied. Certainly at 15 million K it’s very fair.

    I always wondered why Kelvins never really caught on for the weather. I would know that when the weatherman said it will be 250 tomorrow morning to find my scarf and hat, and if he says it will be 300 tomorrow I’ll get to complain about global warming, er, find my shorts. The US could even switch to the Rankine scale.

  23. Gary Ansorge

    The original bombs droped on Hiroshima had about 20 kg of fissionable material. The conversion rate to energy is about one part per 5000 of the mass converted to energy which equals about 4 gms of mass converted to energy. You indicated the conversion as one gm equal to 20,000 tons of TNT. I believe you’re underestimating the conversion rate.

    Of course, my figures are based on really old data, from the days when I was really interested in making things go boom,,,

    Gary 7

  24. Jeff

    I loved the part “Yes, we KNOW about evolution”. I nearly crapped myself with laughter. These little video podcasts are great and I hope they go on for a long time. Happy Darwin Day everyone!

  25. Dr. Flimmer had it first– I said "degrees Kelvin", when actually Kelvins are the units of temperature for the absolute scale. I think it’s a silly thing anyway, since we say degrees Celsius and degrees Fahrenheit.

    Gary, I was talking about converting all the mass into energy in the latter part of the video. So 1 gram of matter times c2 = 9 x 1020 ergs. There are 4 x 1022 ergs/megaton, so there you go.

  26. Great video. Thank you for posting to LibSyn. I can finally view these at work (during lunch hour, of course).

  27. But you also said 4 hydrogen attoms to make a helium nucleus. It is actually two hydrogen atoms. Did you not mean 4 nucleons (2 protons + 2 Neutrons)

    Plus you referred to the Sun as gas, would not the better descriptor be plasma?

  28. Plus you referred to the Sun as gas, would not the better descriptor be plasma?

    It would be neither. The Sun is the physical manifestation of the Holy Light of God. At least, that’s what the voices tell me. It appears hot because Satan tricks us into believing that as a test of faith. Scientists would have you believe it is a star, but stars only come out at night, whereas God chooses to shine His light during the day. Ok, enough of that bollocks :)

    Here’s a serious, and related question to the whole sun thing…

    If my understanding is correct:

    – Stars like the sun fuse hydrogen into helium into heavier elements, and fusing those etc. until the fuel is fused into iron, at which point they more-or-less blow off their outer layers and contract into a white dwarf.

    – Stars larger than 1.5 x our Sun, detonate as a galactic WMD leaving behind a neutron star or a black hole and a pretty nebula for telescopes.

    So the question is: where do elements further up than iron on the periodic table come from in any quantity? The degenerate matter of an old stellar core doesn’t seem to be it. I’ve always wondered this and it wasn’t really covered back when I was in university (I studied physics, not astronomy), and references that I find seem to do a bit of hand waving and say that the heavy elements just sort of appear from these explosions – an unsatisfying answer.

    Presumably a supernova would have enough energy to fuse some of the stellar iron into, say, tellurium, but if that’s the case wouldn’t it be detectable in objects like the crab nebula? I never really read/hear about such things.

    So, for example, how did the gold that we love to make into jewelry and fight over actually get here? Or am I missing the stellar forest for the stellar trees on this one?

  29. Lauren

    To Sticks:
    In the Sun, when hydrogen switches over to helium, some of the hydrogen nuclei (protons) become neutrons. The whole process basically goes through three steps:
    1. Two protons collide, and one becomes a neutron.
    2. A third proton the hits that, so there are two protons and one neutron.
    3. Two of the things in #2 collide, making helium, and it kicks out two of the protons. Overall the reaction uses four protons/hydrogen nuclei, two of which turn into neutrons (also note that there are some other things that are emitted, like gamma rays).

    To Evolving Squid:
    I believe that when a supernova occurs, all the heavier elements are then created. I’m not sure what to say about detecting the tellurium in places like the Crab Nebula. Perhaps some of the elements are present in smaller quantities, so in comparison it’s like there is none of that element.

  30. Cindy

    Evolving Squid,

    Stars like the Sun can only fuse up to about Carbon (smaller ones don’t even manage to fuse He into Carbon). Stars about 2 to 8 times the mass of the Sun can fuse Carbon into Oxygen but don’t get much further. Interestingly enough, some of the fusion reactions involving carbon release neutrons, so you can build up some elements by slow absorption of neutrons.

    Stars greater than 8 times the mass of the Sun can fuse up to iron which triggers the supernova. It’s the iron core that collapses and forms either a black hole or a neutron star. In the supernova explosion, you get large amounts of neutrons being released (though most are trapped in the core). That rapid flux of neutrons allows the much heavier elements to be created. Actually most of the iron in the Earth’s core is not from a core of a massive star but from the decay of cobalt from the supernova. Since Phil’s Ph.D. thesis was on a supernova, he could probably explain it better than me (mine was on a white-dwarf/ normal star binaries called cataclysmic variables).

    Mercury magazine (from the Astronomical Society of the Pacific) had a good article about the origin of the elements on the periodic table. Volume 34, No. 2, pages 19 – 25. It’s the March/April 2005 issue.

    I actually just did a presentation for my school’s parents weekend on this topic recently which is why it’s on the tip of my tongue.

  31. wwitzke

    The fusion reaction in young stars (like ours) is a proton-proton chain reaction. You start with two protons (i.e. hydrogen atoms sans electrons) fusing into deuterium, and releasing a couple of other items, including a neutron.

    Next, you fuse the deuterium with another proton (that is, hydrogen nucleus) to produce the nucleus of an isotope of helium, 3He (imagine the 3 is superscript) and, again, a few other things.

    After this, it gets complicated, but the simplest looking step has two of these light helium isotope nuclei combining into 1 regular helium nucleus plus two protons (i.e. hydrogen nuclei), plus some energy. So, what made that one helium nucleus? You needed three hydrogen for the first helium isotope, and three hydrogen for the second helium isotope, and then when you put the two helium isotopes together, you got two hydrogen back. That means that it takes FOUR hydrogen nuclei to make one helium nucleus, at least in this particular chain.

    The other chains are quite a bit more complicated.

    Hope this helps,


  32. wwitzke

    Er, there’s another possibility that has the 3He combining with one hydrogen proton to make a regular helium isotope, which is even simpler. Oops :-) That’s still four hydrogen nuclei to make one helium nucleus.


  33. Jens Peter Bork

    Well, I noticed not a mistake, but a glitch – you didn’t tell the questioner how we actually know that the center of the Sun has a density of 150 g/cc. You went on right to the “impossible young” Sun bit.

  34. George

    Very cool for such a hot topic. I can only think of one way to add more color to it, heliochromologically speaking, of course. ūüėČ [Next time, tell them it ain’t yellow, or interview a solar astronomer while at his unfiltered telescopic projected solar disk (eg McMath-Pierce) which appears…[b]white[/b], no hint of yellow.]

    Melusine: There are many temperatures used for the sun, suprisingly. The effective temp. – integrating the entire irradiance – is about 5777K. A Planck curve overlay will be around 5850K and is higher due to the absorption bands diminishing the total energy which yield the eff. temp.

    Due to the opacity issue of the solar atmosphere, the temp. across the disk will appear different. The central region, where we can look deeper into the interior, is around 6390K; the limb is around 5000K due to the inability to see deeper.

    Adding the Random Walk next time would be nice because I just saw Naked Science’s sun on TV and they, I think it was them, said it was a million years and made kind of a big deal about it.

  35. Melusine

    George, my references were to the use of “degrees” not whether the temperature is 5800 K or 6000 K.

  36. Greg

    With a density of 150 g/cc, why do we say the sun is a ball of gas instead of a ball of liquid (or more generally, just a ball of fluid)? Is it really that, despite this density, the fluid is still compressible?

  37. Bryan D.

    I love that face you make you you say “and for the rest of the World” :)

    Don’t you like Imperial mesurement?? :)

  38. schowster

    I had a professor in college who said that when you are explaining differences in temperature scales you leave out the “degrees”.

    So the difference between 270 K and 300 K = 30 Kelvin
    And the difference between 25 C and 50 C = 25 Celsius

    But if we go outside and want to know how warm it is, it would be “298 degrees Kelvin”, or “25 degrees Celsius”.

    He knew it wasn’t a consensus among the scientific community, but that’s the way he liked to differentiate and I’ve taken a liking to it.

  39. Cameron

    Plasma is a superheated, ionized form of gas. It is kind of a “sub-state” of matter.

  40. BC

    Just in case anyone didn’t figure it out: a Hydrogen nucleus is composed of 1 proton. A Helium nucleus is 2 protons + 2 neutrons. Take 4 hydrogen, convert 2 protons to 2 neutrons than add the other 2 protons, ending up with the 2 protons + 2 neutrons in a single helium nucleus.

  41. ZB

    I was very strongly reminded of Carl Sagan with both of these shows. Yay! I hope you keep them coming.

  42. Jeremy

    One minor complaint about this clip. A couple years ago I heard a seminar about acoustic waves in the sun. The numbers being thrown around for the speed of sound were actually very slow. I had to stop the speaker and ask why sound was so slow when everything I had heard about the sun was that it was so incredibly dense. It turned out that most of the sun is actually very low density (I don’t remember exactly, but I want to say 1/20th of the earth’s atmosphere.) Big numbers are always cool to throw around, but you run the risk of giving the wrong overall picture if you focus on the extremes.

  43. Luke

    I am sure you know this, but when speaking about Kelvin you don’t use the term degrees


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