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Space Forum / Astronomy / August 2006Is there a fine line between a planet and a star?
IE: Is it possible to have an on/off star? ...Or does the mass have to surpass the necessary requirements, such that once fusion commences, it would be guaranteed to be ongoing. ...And 2): would the critical mass ensure that the ignition would initially commence with the very central atom within the mass, or can fusion commence within a plus or minus gravitational pressure range? ...Jon > IE: Is it possible to have an on/off star? Potentially yes if the mass is too low to initiate full main sequence powering fusion between the most common isotope of hydrogen, but justenough (around 10^6 K) to slowly burn off the low mass elements like deuterium, lithium, beryllium and boron. If the core temperature gets to 10^7 K or higher then it will almost certainly become a main sequence star, but if the mass is insufficent it may only manage to burn off the easy fuels without ever fully igniting the main sequence hydrogen burning reaction. I don't know if any stars in this category have been observed. They would be rather small and dim even during their brief active stage. >...Or does the mass have to > surpass the necessary requirements, such that once fusion commences, it > would be guaranteed to be ongoing. ...And 2): would the critical mass > ensure that the ignition would initially commence with the very central > atom within the mass, or can fusion commence within a plus or minus > gravitational pressure range? ...Jon It is a soft start. Once the pressure and temperature are almost enough some lucky collisions will result in fusion. Reaction rates then increase rapidly with temperature. Regards, Martin Brown |||newspam|||@nezumi.demon.co.uk (Martin Brown) wrote: >Once the pressure and temperature are > almost enough some lucky collisions will > result in fusion. Reaction rates then increase > rapidly with temperature. I have read where it can take 100,000 years for light generated in the sun's centre, to reach the surface. ...Would this imply that a newly formed sun would basically look like an extremely large Jupiter.... And then, over thousands of years, would gradually light up? ...Jon > I have read where it can take 100,000 years for light generated in the > sun's centre, to reach the surface. ...Would this imply that a newly > formed sun would basically look like an extremely large Jupiter.... And > then, over thousands of years, would gradually light up? ...Jon No, a newly formed sun would be glowing already from the heat released due to gravitational contraction; it would be bright even before fusion kicked in. Remember, it's the pressure and heat at the core due to gravitational contraction that inititally gets the temperature up high enough for fusion to begin. Of course the sun will get much brighter once the fusion process kicks in, and that will take some time to show at what will be the surface of the sun -- there will still be infalling material and some of this will be blasted away by the newly ignited sun, thus defining its surface. > No, a newly formed sun would be glowing already from the > heat released due to gravitational contraction; it would > be bright even before fusion kicked in. It's worth mentioning that newly formed stars are surrounded by lots of dust, so while the surface is bright, it can't be seen in visible light unless you happen to be right beside it. (Don't try this at home! :-) ) Newly formed stars show up just fine in infrared light, though. > Remember, it's the pressure and heat at the core due to > gravitational contraction that inititally gets the temperature up > high enough for fusion to begin. Right. > Of course the sun will get much brighter once the fusion > process kicks in, Not so. All that happens is that the contraction stops. The nuclear reactions act as a thermostat, keeping the core temperature where it was when the reactions started. If the core got hotter, its pressure would go up, and it would expand and cool off. > ...there will still be infalling material... Yes, as the mass grows, additional central pressure is needed to support the additional mass. The resulting contraction causes the core to get hotter, but not much because the nuclear reaction rates are such a strong function of temperature. > ... and some of this will be blasted away by the newly ignited sun, > thus defining its surface. The question of what stops the infall is a topic of active research. Despite several good suggestions, I think it's fair to say no one knows the answer. Part of the answer seems to involve collimated jets; something like the solar wind may also play a role.  SignatureSteve Willner Phone 617-495-7123 swillner@cfa.harvard.edu Cambridge, MA 02138 USA (Please email your reply if you want to be sure I see it; include a valid Reply-To address to receive an acknowledgement. Commercial email may be sent to your ISP.) |
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