The Sun

[Robthevegetable]

I have recently been watching Wonders of the Solar System again and I just wanted to double check something from the first episode.

The Professor was explaining how the collapsing, spinning cloud of Hydrogen formed the Sun. But the way he explains it is that the Sun suddenly ignited, lighting up the Solar System. I was wondering, did the Sun suddenly ignite when it hit a certain mass (when the core was hot/dense enough to begin to fuse Hydrogen atoms under pressure), or did it gradually get hotter and hotter as more and more Hydrogen was pulled into the Sun as its gravity increased, gradually expelling more and more light and heat?

[JB80]

I'm not sure if this helps at all.

Hydrogen fusion

For a more massive protostar, the core temperature will eventually reach 10 million kelvins, initiating the proton-proton chain reaction and allowing hydrogen to fuse, first to deuterium and then to helium.

The onset of nuclear fusion leads relatively quickly to a hydrostatic equilibrium in which energy released by the core exerts a "radiation pressure" balancing the weight of the star's matter, preventing further gravitational collapse. The star thus evolves rapidly to a stable state, beginning the main sequence phase of its evolution.

Stellar evolution - Wikipedia, the free encyclopedia

If i understand right, the giant cloud of spinning hydrogen and other elements first gathers enough mass to collapse in on itself and form a protostar. As the protostar evolves it reaches a critical temperature which sets of the Hydrogen fusion that turns the Sun ito what we see today.

Roughly.

*waits for the inevitable corrections*

[Robthevegetable]

That makes sense... I was aware of the balancing act between the inward gravity and the outward expulsion of energy, but never thought about it in the context of a stars birth!

Like you say, no doubt there will be some more conclusive answers... But that is an excellent baseline!

[JulianO]

Thats how I understand it to. It heats up until there is enough thermal energy to bring protons close enough together so they fuse. As the cloud of gas gets denser it becomes harder and harder to radiate away energy, because it keeps bumping into gas rather than zooming out into space. You could look up Hayashi tracks for more info.

From that moment on its in hydrostatic equilibrium. If the centre heats up a bit then the star expands a little and cools down. If it begins to cool then it contracts and adds more heat. So its quite well balanced.

[Robthevegetable]

Fair enough, well answered both of you! I thought it was gonna be more complicated than that, but I feel a bit of a tool after reading your answers (and actually thinking about it a little)!

[Ags]

Of course even in the absence of nuclear fusion a protostar is still very hot, purely from the heat released as it contracts.