Comments on: The life of a star, in 14 minutes

By: The life of a star, in 14 minutes | Bad Astronomy | Discover Magazine | Science Journalism & science communcation | Scoop.it

Fri, 26 Oct 2012 12:09:07 +0000

[...] Astronomy | stars | 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 whit (Want to learn how stars work? [...]

By: Messier Tidy Upper

Messier Tidy Upper — Mon, 01 Oct 2012 04:45:24 +0000

@ ^ 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.

By: amphiox

amphiox — Mon, 01 Oct 2012 00:50:57 +0000

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.

By: Dean

Dean — Sun, 30 Sep 2012 22:31:04 +0000

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.

By: Neil Haggath

Neil Haggath — Sun, 30 Sep 2012 22:01:26 +0000

#21 lavocat:
You’re welcome.

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

By: davem

davem — Sun, 30 Sep 2012 09:53:24 +0000

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?

By: Messier Tidy Upper

Messier Tidy Upper — Sun, 30 Sep 2012 05:55:24 +0000

@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.

By: Lavocat

Lavocat — Sat, 29 Sep 2012 01:48:22 +0000

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.

By: amphiox

amphiox — Sat, 29 Sep 2012 00:42:35 +0000

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.

By: Neil Haggath

Neil Haggath — Fri, 28 Sep 2012 23:47:42 +0000

#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.