MadSci Network: Astronomy |
Hi Richard,
The great majority of stars lie on the main sequence. The excellent
hyperphysics website includes an HR diagram with the main
sequence marked. The sequence defines those stars that fuse hydrogen to
form helium. Given the preponderence of hydrogen (73% by mass) and helium
(25% by mass) in the universe - everything else is only about 2% by mass -
it's no wonder that most stars are made of these elements.
A star's position along the narrow sequence depends solely on the
mass of the star. More massive stars are positioned at the hotter,
brighter and short-lived end of the sequence, while low mass stars are
redder, cooler and have longer lifetimes. Stars that don't primarily fuse
hydrogen alone (such as old stars like the red giants and white dwarfs) lie
away from the main sequence.
As a star ages, its core (where high temperatures and pressures allows
fusion to take place) becomes hydrogen-depleted: we can see this on the
main sequence as stars move slowly across the narrow sequence (not along it
- few star lose enough mass during their lives to substantially travel
along the sequence).
For stars like the sun, the increasingly helium-rich core means that
hydrogen fusion eventually takes place in a shell of purer hydrogen
surrounding the core. The helium core eventually undergoes a helium
flash when conditions reach temperatures and pressures where helium can
be fused into heavier elements such as carbon.
Only the heaviest and most short-lived stars contain enough mass for
conditions in the core to reach the stage at which iron can be fused. These
massive stars can eventually resemble onions, where a core of iron is
surrounded by layers of lighter-fusing elements, up to a hydrogen-fusing
shell. The production of iron is the end of the line for normal
energy-producing fusion - elements heavier than iron require energy to be
formed and are a net drain on the star. In the universe, these heavier
elements (such as silver and gold) are the by-products of supernovae, in
which really massive stars explode at the end of their lives.
For all stars, only about 10% of the hydrogen available to fuse is actually
used during the lifetime of the star. Some is blown off in solar winds,
some in expanding shells of gas that mark the end of many stars' lives on
the main sequence, while some never reacts in the inner core of the star
(this is a complicated topic: you might wish to Google for resources
detailing and modelling stellar convection if you want to follow
this up.)
To answer your question: assuming that a massive star had depleted its
initial core fuel-stock of hydrogen and had begun the iron-core stage of
development, and it was to receive an massive influx of hydrogen (perhaps
from a younger, less evolved main sequence star impacting a growing red
giant in a close binary system), what would happen?
We can see that the red giant would increase in mass, which might make us
think that the influx would simply produce a new, rejuvenated, but
metal-rich star "further up" the main sequence. However, any addition of
new matter would release a load of potential energy into the red giant, and
while we might think that this might prolong the life of the star, the
shock to an iron core of a red giant might not be survivable. That said,
assuming the star survived receiving a mass of hydrogen, the convective
zones around a star (as alluded to earlier) might or might not transfer
this fresh hydrogen to depths where new fusion could occur.
As you can see, it's a complicated question, and one with several potential
outcomes, partly depending on the size of the star, its age and the means
by which new material is added to it. The addition of new material to a
star is not well-modelled by astrophysics - current programs modelling
"normal" stars throughout their lives often have problems with short-term
events (such as the onset of the helium flash, and the processes that lead
to supernovae), so in this instance it's not possible to say precisely what
might occur.
Regards,
Andy Goddard
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