Thread: The sun

How did scientists initially know that the Sun's heat comes from nuclear fusion?

How did scientists initially know the elements involved?

Sorry if it's too much of a simple question... I'm not big on Astronomy & Cosmology.

as for the elements involved : spectroscopy
it just took scientists a long time to realise that the sun didn't just contain some hydrogen but was mostly hydrogen - until the early 1930s mainstream thought was that the sun's composition was not vastly different from the earth's

addition to my earlier post : it was only gradually that people started realising that the sun's energy comes from nuclear fusion

(1) the first indication came from the fact that no known physical process could keep the sun burning for as long as geology said the earth ahd been around
(2) secondly radioactivity was discovered, hinting at what type of energy source could keep the sun shining
(3) then came Einstein E =mc², which showed how much energy could potentially be liberated if only you knew how to convert mass into energy
(4) near the same time a better insight was gained in the structure of the atom
(5) after that came the finding that the alpha particle had a lower mass than 4 protons added together, giving a hint of the H -> He reaction
(6) by the early 1920s (and not the 1930s, as i said earlier) it was already known that the sun and the stars had a hydrogen atmosphere, although it took still another decade to realise that the sun is mostly hydrogen throughout
(7) even in the 1930s, when calculations were made to find out what type of stellar composition was stable, there turned out to be 2 possible solutions : 95% hydrogen and 35% hydrogen, and the scientific communit plonked for the latter
(8 ) proposals of possible chain reactions to explain the fusion of hydrogen in the sun were only made in 1938

in short, there's no clear watershed : the realisation that the sun uses hydrogen fusion came about as the result of various building blocks coming together over a period of nearly 4 decades

Originally Posted by marnixR

addition to my earlier post : it was only gradually that people started realising that the sun's energy comes from nuclear fusion

(1) the first indication came from the fact that no known physical process could keep the sun burning for as long as geology said the earth ahd been around
(2) secondly radioactivity was discovered, hinting at what type of energy source could keep the sun shining
(3) then came Einstein E =mc², which showed how much energy could potentially be liberated if only you knew how to convert mass into energy
(4) near the same time a better insight was gained in the structure of the atom
(5) after that came the finding that the alpha particle had a lower mass than 4 protons added together, giving a hint of the H -> He reaction
(6) by the early 1920s (and not the 1930s, as i said earlier) it was already known that the sun and the stars had a hydrogen atmosphere, although it took still another decade to realise that the sun is mostly hydrogen throughout
(7) even in the 1930s, when calculations were made to find out what type of stellar composition was stable, there turned out to be 2 possible solutions : 95% hydrogen and 35% hydrogen, and the scientific communit plonked for the latter
(8 ) proposals of possible chain reactions to explain the fusion of hydrogen in the sun were only made in 1938

in short, there's no clear watershed : the realisation that the sun uses hydrogen fusion came about as the result of various building blocks coming together over a period of nearly 4 decades

Thanks. 8)

I'm also interested in how the H fusion came about in the first place... what force made this reaction start occurring, or is the Hydrogen some unique isotope of the ones commonly found on Earth?

Again, sorry if I sound uninformed... because I am (haven't done much astronomy)...

the first thing you have to realise is that under normal conditions 2 hydrogen atoms won't fuse, otherwise we'd all have limitless energy from fusion reactors right now !

what it takes is high pressure, and hence only an object the size of a star (if memory serves me right, at least 0.1 times the mass of the sun) will create sufficient pressure for hydrogen atoms to fuse

that's because (1) hydrogen needs to be in a plasma state, i.e. stripped of its electron; and (2) collisions have to happen frequently enough and with sufficient energy to overcome the repulsion between the nuclei

even for the sun the percentage of hydrogen taking part in fusion is minuscule, but because of the size of the sun that still means several millions of tonnes every second

Originally Posted by marnixR

the first thing you have to realise is that under normal conditions 2 hydrogen atoms won't fuse, otherwise we'd all have limitless energy from fusion reactors right now !

what it takes is high pressure, and hence only an object the size of a star (if memory serves me right, at least 0.1 times the mass of the sun) will create sufficient pressure for hydrogen atoms to fuse

that's because (1) hydrogen needs to be in a plasma state, i.e. stripped of its electron; and (2) collisions have to happen frequently enough and with sufficient energy to overcome the repulsion between the nuclei

even for the sun the percentage of hydrogen taking part in fusion is minuscule, but because of the size of the sun that still means several millions of tonnes every second

Cool. Too bad this process can't go on forever.

just for completeness, the fusion reactions in the sun is actually a set of 3 consecutive reactions, called the proton-proton chain

proton + proton -> deuterium + neutrino + positron

deuterium + proton -> helium-3

helium-3 + helium-3 -> helium-4 + 2 protons

the bottleneck reaction is the first one