stellar ignition

[johnstone]

I'm looking for any information on the *moment* of stellar ignition.

In other words, over what period does Hydrogen fusion have an impact on the structure of the star and beyond. Is it a big bang, or does it take a week, a year, 10000 years, for fusion to really to get going?

The reason I ask is that I recall some TV programme discussing the ignition of the sun as a way of clearing out debris from the inner solar system and thus quickly reducing planetary bombardment.

Any pointers?

[JulianO]

I don't know exactly, but I think it would be a slow thing. You're probably looking at millions of years.

The density has to get high enough, and the heat sufficient (~5 million K) for fusion to start. Most proto stars start to shine from heat given off by gravitational contraction rather than fusion.

When it does start it will be right in the middle of the core, and density will be sufficient to slow the light's escape. In the Sun it takes ~100,000 years for a photon to make it's way from the fusion event out of the sun to be sunlight.

The pressure early on is from winds and light, and only sufficient to move dust and gas around, so it basically flushes away a lot of the protoplanetary material out of the system that hasn't yet had chance to be accreted by planets.

So the T-Tauri phase early on is not sufficient to remove boulders and meteors, just to halt the growth of those existing ones.

[skybadger]

I would suggest that core collapse to ignition must be rapid on the basis that a slow collapse would push away inflowing material by heat diffusion as the temperature increases, achieving LTE. I would guess there is shock mechanism where outward pressure suddenly collapses above a certain core pressure causing in rush and igntion.

Mike

[nameunknown]

Protostars have quite a complex evolution initially generating heat and light by gravitational collapse and by burning lithium before they reach the temperatures needed for hydrogen to fuse (see: http://en.wikipedia.org/wiki/T_Tauri_star). It is suggested that the lithium burning stage lasts for around 100 million years. I would guess that the heavier the star is, the faster it reaches the hydrogen fusion stage. There is a good article on stellar birth in Sci Am this month.

P

[nameunknown]

..or maybe that article was in Sky at Night

P

[101nut]

Protostars have quite a complex evolution initially generating heat and light by gravitational collapse and by burning lithium before they reach the temperatures needed for hydrogen to fuse

Not sure that's quite correct. Hydrogen burning has to take place to generate the lithium in the first place ... A proton (hydrogen) is shown on the page you quote as part of the p-p chain for lithium burning to generate beryllium but that doesn't indicate where the lithium is sourced. I would probably consider collapse to be relatively slow with gravitational pressure being higher than thermal pressure in the proto-star before finally resulting in fusion in the hot core as the density and temperature increases. Slow is a relative term ... 10M years? 100M years ... it's not going to be a week but it will depends on the mass, density and initial energy of the material of the system forming the star and thus the rate of increase of mass on the star itself. On the other hand, a proto star could be theoretically ignited as a result of a shockwave from a nearby stellar event which could be much faster ...

AndyG

[Knight of Clear Skies]

Not sure that's quite correct. Hydrogen burning has to take place to generate the lithium in the first place ...

That's not correct I'm afraid - the lithium is present in the molecular cloud the protostar forms in.

[JulianO]

Small amounts of lithium were made in the big bang - nothing compared to H & He but it's present.

Also molecular clouds don't just collapse. They spin, and have to lose energy to contract, so it takes time as energy generated has to be radiated away to allow the cloud to collapse further - which all takes time as the cloud gets denser and so can't radiate as effectively.

So - its complicated and fairly slow.

[Knight of Clear Skies]

I would suggest that core collapse to ignition must be rapid on the basis that a slow collapse would push away inflowing material by heat diffusion as the temperature increases, achieving LTE. I would guess there is shock mechanism where outward pressure suddenly collapses above a certain core pressure causing in rush and igntion.

I'd expect the process of fusion ramping up to be very gradual. As soon as fusion begins, radiation pressure will begin to counteract the effect of gravity, slowing the collapse. Main sequence stars regulate their energy production in this manner - radiation pressure is balanced against gravity. It takes about 100 million years for a T-Tauri star to collapse and evolve onto the main sequence. As JulianO says, energy and angular momentum has to be lost before the protostar can collapse fully.

On the other hand, a proto star could be theoretically ignited as a result of a shockwave from a nearby stellar event which could be much faster

Are you referring to a shockwave from a nearby supernova? I doubt it would have much affect on a relatively dense and small protostar, as only a tiny fraction of the supernova remnant would impact it.

[skybadger]

Referring to this URL <http://web.njit.edu/.../Lecture13.html>, I can't see how fusion would ever happen if if wasn't relatively sudden. 1e5 years is relatively sudden in my book. Propagation time through the protostellar atmosphere would take the same length of time all over again. The claim here is that the cool dust cloud can effectively radiate as it compresses, but there must be a low-termperature point when this radiative transfer fails since the density is ramping up rapidly..if not by time then by distance.

Also this is useful: <http://burro.astr.cwru.edu/stu/advanced/stars_birth.html>

Mike

[skybadger]

And then I finlly found this <http://neutrino.aqua...n-chapter5.html> - the key bit being a star evolves in distinct phases each with its own timescale. I haven't yet found anything that indicates the time from core formation to fusion though.

1, cloud of slowly spinning multi-light year size gets disturbed and begins to collapse - supernova shock waves or gravitational disturbances might cause this .

2, protostellar phase of 100,000 to 2M years (depending on globule size) of condensation to form a core radiating under gravitational pressure in an now-transparent nebula so these stars are directly visible as gravitational radiators in IR and low vis. Rotational energy of the nebula is low and relatively unimportant in this phase.

3, transition of core of the globule to a star by contined gravitational contraction leading to nuclear fusion first by deuterium burning to Helium and then by Hydrogen fusion in the p-p cycle turning on. Outer nebula is still collapsing and accreting as a disk on the equator od the star or via polar inflows (inside-out collapse model).

4 Movement to <stable> ZAMS is of the order of 30Myr due to high initial variability of output and core size.

5, Accretion is balanced by radiation and the remnants of the nebula begins to be blown away.

Im sure there will be lots of discussion.

Mike