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INTERVIEW
WITH DR. FRED HERRERO
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Stellar Evolution: How the Sun came
to be
Hello. My name is Fred Herrero. I am a physicist in
the Detector System Branch at the Goddard Space Flight
Center. I am here to talk to you about Stellar Evolution,
especially how the Sun came to be. We’ll discuss
how stars forms, how they evolve. I would like
to leave you with the message that the brighter they
are the more rapidly they burn up. It’s like
the talker they are the harder they fall, that’s
the idea.
Formation of a star.
How is a star form? For what we know today there are
giant molecular clouds in the space between the start,
interstellar space. In these giant molecular clouds
there will be density enhancements in certain places
in such a way that the molecules will begin to pull
on each other with their own gravitation pull and get
close enough and dense enough that they will form a
sort of a core or a center of attraction for the surrounding
molecules, that core will then begin to pull in more
and more molecules. As these molecules are pulled in
by gravitational attraction, of course, they speed
up just like when you drop a rock from somewhere above
the surface of the Earth and that speeding up mainly
towards the center, will lead to, at some point when
the density is high enough on the center, will lead
to very energetic collisions between molecules which
will randomize that directed central appointed energy
and turn it into what we call heat, heat being a random
thermal motion, random thermal connected energy.
This leads into a development of high pressure and
the higher the temperature, the higher the heating,
the higher the pressure, that pressure begins to balance
the gravitational pull and that balance between gravitational
pull and the thermal energy is, indeed, what leads
to do the star developing its own size and reaching
an equilibrium. Together with this pulling-in there
is, inevitably, some rotation and that leads to other
things that we will like to discuss in another forum
we can probably spend some time leading to formation
of these and things like that.
Let’s talk a little bit about the evolution.
Once this proto start, which is not quite a star yet,
has high enough density, high enough temperature then
the molecules inside of it, actually the nuclei, like
protons mostly and some helium, nuclei. The nuclei
inside would be close enough and moving rapidly enough
that when they encounter each other they will come
close together enough to get fused together so that
you can have protons and protons coming together to
form lutetium and so on until you get to the production
of helium. On the whole process the final mass of the
helium, for example, will weight less than the mass
of all the protons that went into the reaction. So
that we are saying that some of the mass of these reactants
is being converted into energy according to the Einstein
equation of E = mc² that we always hear about.
That provides a source of energy for the star.
In the case of a star like the sun the mass is
sufficient low and the temperature is not that high and
therefore it can produce its energy directly through
a reaction of proton-proton-proton colliding to produce
the helium. In the case if very massive star there is
a far more efficient mechanism involving carbon, nitrogen,
and oxygen and, I wish we had more time to go into that,
but in that process the efficiency is so high, but it
requires very high temperature therefore a very massive
star, but the efficiency is so high that very large amount
of masses burn up rather quickly so we wind up with a
situation that we know today that the Sun has a life
time on the order of 10 billion years while much more
massive stars, stars with a mass of about 20 times the
mass of the sun have a life only of the order of 6 million
years, a factor of 1600, so we can truly say that the
brighter they are the more rapidly they burn up. Thank
you.
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