Is 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

wrote:
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

(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

wrote in message ...

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.

In article ,
"Greg Neill" writes:
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.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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