Expert: Tom Whiting - 6/19/2005

Question
As a student I have two question and I am asking you.
These two questions came to my mind:


1.Why we would not expect to find life in any planets circling around O5 class star?
2. Why do 5 solar mass stars form planetary nebulas,thus shedding their mass and allowing them to run into white dwarfs but 10 solar mass stars do not?

Thank you so much
Regards


Answer
Hi Sammy,
Any O ar A spectral class massive star does not live
long enough, so using our own standards of organic
evolution requiring hundreds of millions of years....
They consume their useable hydrogen fuel at an enormous
rate, living less than 25-100 million years (a very short time
frame on the astronomical scale), and then go supernova.
Not near enough time for organic evolution to occur, unless you buy into the "instantaneous Creationist" theory of the Christian Fundamentalists.

5 or less solar mass stars can eject enough mass during
their lives, especially the red giant phase, to get their
total mass below the Chandrasekar limit of 1.4 solar masses,
so they can proceed normally from the Red Giant, thru
the planetary nebula stage, on the way to the white dwarf
stage.
10 or more solar masses cannot eject enough mass, thus
they typically end up as a supernova.
(at least that is the current thinking).

What you probably don't understand, the bigger  (more
massive) the star, the much faster it uses up it's available
supply of hydrogen fuel during the hydrogen fusion
phase on the Main Sequence stage of it's life.  That's why
the most massive huge stars only live 10-25 million years,
whereas the little red dwarf stars have life spans measured
in trillions of years...that's trillion with a "T".
(Solar-types have lifespans like our sun, of about 10 billion
years, and also go the red giant, thru planetary neb. to
a white dwarf stages).
Also the more massive stars, 10 solar masses and above,
have the ability, eventually,  to form an iron nuclei core, triggering a supernova reaction in the star, whereas the less massive stars can't pull off that trick as they cannot....(read that...are not massive enough) ... form iron
nuclei down in their cores....so they go the normal route...
red giant thru a planetary nebula, to a white dwarf stage,
and do not explode.
Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA