Q&A 61: Could Jupiter Be a Gravity Lens? and More...

Would an interview with Dr. Karl while in Australia be possible.

As for why the star doesn't start to push gas away, limiting all stars to the minimum-mass-to-start-fusion, it seems that there simply isn't enough outward pressure at this stage to overcome the massive pull of gravity. The gas is forming into an accretion disk too which means the protostar would need to apply substantial pressure solely against the plane of the disk whereas energy released in any other direction has no significant impact. So I think the answer at this stage is simply that gravity wins against the outward pressure for gas near enough to the star. At temperatures above 15,000,000 K the star can switch to a carbon-nitrogen-oxygen cycle which produces dramatically more energy output and finally does have the energy to change the situation. The bigger the star, the more CNO fusion is possible and somewhere above 150-200 solar masses this pressure will either start pushing gas away, or if the star is substantial enough it may even split the star. Ultimately the answer is some combination of the size of the original gas cloud and the breakdown of the elements which will (at a minimum) change the mass:volume ratio along with a whole bunch of other less obvious ways. (As always, this is the internet therefore I am wrong in many places. I would appreciate anyone who can correct me so that we all learn from my mistakes).

Hi Fraser, following up on the question about star size. I understood that question a little differently. I think the question is why are there stars of different sizes? If hydrogen begins to fuse at some constant point then the original size of the gas cloud should be irrelevant. As long as it meets the minimum size to reach the fusion point. At that point, the star should ignite and blow out all the non-gravitationally attached gas. That would mean most stars should be the same size - give or take.

If I’m not mistaken (and it’s been a while), the movement propagates through the stick at the speed of sound (in the appropriate medium), so it would take many many light years for the movement to reach the switch. The return would of course be 1 year. In wood this would be maybe 3300-4000m/s (depending on the type of wood), so optimistically 75000 years out and 1 year back. If we wanted to complete the experiment quicker, maybe Beryllium which is about 12800m/s? That drops it to a little over 23400 years. (plus a rounding error of 1 year back — my error bars are too large to even consider the return trip).