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From: "Joseph Zaidan" <jnadiaz@...>
Subject: The energy of the Sun
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Index <http://www-spof.gsfc.nasa.gov/stargaze/Smap.htm>
The Sun
S-2.Solar Layers <http://www-spof.gsfc.nasa.gov/stargaze/Sun2view.htm>
S-3.The Magnetic Sun
<http://www-spof.gsfc.nasa.gov/stargaze/Sun3mag.htm>
S-3A. Interplanetary
<http://www-spof.gsfc.nasa.gov/stargaze/Simfproj.htm>
Magnetic Fields
<http://www-spof.gsfc.nasa.gov/stargaze/Simfproj.htm>
S-4. Colors of Sunlight
<http://www-spof.gsfc.nasa.gov/stargaze/Sun4spec.htm>
S-4A.Color Expts. <http://www-spof.gsfc.nasa.gov/stargaze/Scolors.htm>
S-5.Waves & Photons
<http://www-spof.gsfc.nasa.gov/stargaze/Sun5wave.htm>
Optional: Quantum Physics
Q1.Quantum Physics <http://www-spof.gsfc.nasa.gov/stargaze/Q1.htm>
Q2. Atoms <http://www-spof.gsfc.nasa.gov/stargaze/Q2.htm> (and 6
more)
--------------------------
S-6.The X-ray Sun <http://www-spof.gsfc.nasa.gov/stargaze/Sun6new.htm>
S-7.The Sun's Energy
<http://www-spof.gsfc.nasa.gov/stargaze/Sun7enrg.htm>
S-7A. The Black Hole at
<http://www-spof.gsfc.nasa.gov/stargaze/Sblkhole.htm>
our Galactic Center
<http://www-spof.gsfc.nasa.gov/stargaze/Sblkhole.htm>
LS-7A. Discovery <http://www-spof.gsfc.nasa.gov/stargaze/Ls7adisc.htm>
of Atoms and Nuclei=20
<http://www-spof.gsfc.nasa.gov/stargaze/Ls7adisc.htm>
S-8.Nuclear Power <http://www-spof.gsfc.nasa.gov/stargaze/Snuclear.htm>
S-9.Nuclear Weapons
<http://www-spof.gsfc.nasa.gov/stargaze/Snucweap.htm> Nuclear Physics
Gradually the picture became clearer. Atoms were found to consist of
heavy nuclei, consisting of electrically positive protons and uncharged
neutrons, while around the nuclei swarmed lightweight electrons, with a
negative electric charge. An electron has about 1/1840 the mass of the
proton, which is also the nucleus of hydrogen.
[Please excuse the recitation of dry facts, but to explain how all
this was established would be asking too much from such a brief review.
A few of the benchmarks in the discovery of atoms and nuclei are listed
here <http://www-spof.gsfc.nasa.gov/stargaze/Ls7adisc.htm> .]
Electrons and nuclei were kept together by electric attraction
(negative attracts positive). Furthermore, electrons were sometimes
shared by neighboring atoms or transferred to them (by processes of
quantum physics), and this link between atoms gave our world its many
chemical compounds.
But something else was needed to hold nuclei together, since all
protons carried positive charges and repelled each other. Electric
forces are definitely not the glue that holds nuclei together, they act
in the wrong direction! Besides, binding neutrons to nuclei clearly
requires a non-electrical attraction.
All that suggested a different kind of force, a nuclear force, was
holding nuclei together. That force had to be stronger than the electric
repulsion at short distances, but weaker far away, or else different
nuclei might have tended to clump together, too. In other words, it had
to be a short-range force, like the force between two small
magnets--very hard to separate when stuck together, but once pulled a
short distance apart, the force between them drops almost to zero
(please do not take this analogy too literally!).
Actually two kinds of force are active in the nucleus, known simply
as the "strong force" and the "weak force," or more commonly the "strong
interaction" and the "weak interaction" (because their main effect is in
converting and creating particles). The weak interaction also affects
electrons and other particles, but in the nucleus its main role is to
maintain a balance between protons and neutrons, which except for their
electric charge are very similar particles (diferent kinds of
"nucleons"). Nuclear structure (in light nuclei, at least) favors nuclei
containing equal numbers of protons and neutrons, and although moderate
inequalities can also exist (in "isotopes"), when they get too big, the
weak interaction can convert nucleons of one kind to the other, emitting
an electron (or a positron, its positive counterpart) in the process.
That is known as beta radioactivity and will not be discussed any
further.
The strong nuclear force (the only nuclear force considered from
here on) can bind protons and neutrons into bigger nuclei. Being
positively charged, all these nuclei repel each other, and therefore,
except in the presence of extreme temperatures and pressures--such as
exist in the core of the Sun--two different nuclei are not likely to
combine into one. Their electric repulsion does not allow them to get
close enough for the nuclear force to take over.
The Binding Energy of Nuclei Nature contains nuclei of many
different sizes. In hydrogen they contain just one proton, in heavy
hydrogen ("deuterium") a proton and a neutron; in helium, two protons
and two neutrons, and in carbon, nitrogen and oxygen--6, 7 and 8 of each
particle, respectively. The weight of all these nuclei has been
measured, and an interesting fact was noted: a helium nucleus weighed
less than the sum of the weights of its components. The same held even
more for carbon, nitrogen and oxygen--the carbon nucleus, for instance,
was found to be slightly lighter than three helium nuclei.
The reason for this "mass defect" has to do with Einstein's famous
formula E=3Dmc2, expressing the equivalence of energy and mass. By this
formula, adding energy also increases mass (both weight and inertia),
removing energy, decreases it.
If a combination of particles contains extra energy--for instance,
in a molecule of the explosive TNT--weighing it will reveal some extra
mass (compared to its end products--an unmeasurably small difference,
for TNT). If on the other time we need invest energy to separate it into
its components, the weight will be less than that of the components.
The latter is the case with nuclei such as helium: to break them up
into protons and neutrons, we would have to invest energy. On the other
hand, if a process existed going in the opposite direction, by which
hydrogen atoms could be combined to form helium, a lot of energy would
be released-- namely, E=3Dmc2 per nucleus, where m is the difference
beween the mass of the helium nucleus and the mass of four protons (plus
2 electrons, absorbed to create the neutrons of helium).
As we go on to elements heavier than oxygen, the energy which can be
gained by assembling them from lighter elements decreases, up to iron.
For nuclei heavier than iron, one actually gains energy by breaking them
up into 2 fragments. That, of course, is how energy is extracted by
breaking up uranium nuclei in nuclear power reactors.
The reason the trend reverses after iron is the growing positive
charge of the nuclei. The electric force may be weaker than the nuclear
force, but its range is greater: in an iron nucleus, each proton repels
25 other protons, while (one may argue) the nuclear force only binds
close neighbors.
As nuclei grow bigger still, this disruptive effect becomes steadily
more significant. By the time uranium is reached (92 protons), nuclei
can no longer accomodate their large positive charge, but emit their
excess protons in the process of alpha radioactivity--the emission of
helium nuclei, each containing two protons and two neutrons. (Helium
nuclei are an especially stable combination.) Still heavier nuclei are
not found naturally on Earth.
[Why are helium nuclei especially stable? Some time ago I received
e-mail from a teacher asking for suggestions of ways of demonstrating
fusion to her students, "e.g. using M&Ms." One can do so (although
tennis balls are better, being visible even from the last row of the
classroom!) by showing how 4 balls (or candy pieces) can be stacked in a
pyramid, each touching the other 3. Considering that the nuclear forces
has a short range, this "close packing" gives the tightest binding.
Maybe that is why helium (4 nucleons) is such a stable combination, as
shown by its peak on the curve of binding energy above. No larger number
can be stacked with every single ball touching all the others!]
The Sun's Energy Source It is believed that the Sun is about 5
billion years old, formed when gravity pulled together a vast cloud of
gas and dust, from which the Earth and other planets also arose. The
gravitational pull released energy and heated the early Sun, much in the
way Helmholtz had proposed.
Heat is the motion of atoms and molecules: the higher the
temperature, the greater is their velocity and the more violent are
their collisions. When the temperature at the center of the newly-formed
Sun became great enough for collisions between nuclei to overcome their
electric repulsion, nuclei began to stick together and protons were
combined into helium, with some protons changing in the process to
neutrons (plus positrons, positive electrons, which combine with
electrons and are destroyed). This released nuclear energy and kept up
the high temperature of the Sun's core, and the heat also kept the gas
pressure high, keeping the Sun puffed up and stopping gravity from
pulling it together any more.
That, in greatly simplified terms, is the "nuclear fusion" process
which still takes place inside the Sun. Different nuclear reactions may
predominate at different stages of the Sun's existence, including the
proton-proton reaction and the carbon-nitrogen cycle which involves
heavier nuclei, but whose final product is still the combination of
protons to form helium.
A branch of physics, the study of "controlled nuclear fusion," has
tried since the 1950s to derive useful power from "nuclear fusion"
reactions which combine small nuclei into bigger ones--power to heat
boilers, whose steam could turn turbines and produce electricity.
Unfortunately, no earthly laboratory can match one feature of the solar
powerhouse--the great mass of the Sun, whose weight keeps the hot plasma
compressed and confines the "nuclear furnace" to the Sun's core.
Instead, physicists use strong magnetic fields to confine the plasma,
and for fuel they use heavy forms of hydrogen, which "burn" more easily.
Still, magnetic traps can be rather unstable, and any plasma hot enough
and dense enough to undergo nuclear fusion tends to slip out of them
after a short time. Even with ingenious tricks, the confinement in most
cases lasts only a small fraction of a second.
The Sun today still consists mostly of hydrogen. The fuel supply
which has seen it through its first 5 billion years should be good for
about as long in the future.
The Evolution of Stars Apart from the 5 planets, every star we see at
night is a sun: some are bigger than ours, some smaller, some are at an
earlier stage of their developments, some at a later one, and some have
evolved altogether differently, for a variety of reasons. The telescope
allows astronomers to observe and compare stars of different size, at
different stages of evolution. Their smooth spectra tell about their
temperatures, their spectral lines reveal some of their composition, and
based on these a general theory of "stellar evolution" has been
formulated, which also applies to our own Sun, a typical "main sequence"
star.
All such stars burn hydrogen to produce helium, where "burn" refers
to nuclear processes, not to the (completely inadequate) chemical
process of fire. Big stars burn rapidly and brightly, like the candle in
Edna St. Vincent Millay's poem
My candle burns at both ends;
It will not last the night;
But ah, my foes, and oh, my friends
It gives a lovely light!
(Another take on this rhyme, below
<http://www-spof.gsfc.nasa.gov/stargaze/Sun7enrg.htm#icecream> )
Small stars last longer and many are dim; but whatever a star's
size, ultimately it runs out of hydrogen. It can still release energy by
"burning" heavier nuclei and combining them into bigger ones, up to
iron: theory suggests this does happen, but it provides much less energy
and does not greatly extend the star's lifetime. When all the fuel is
gone, gravity again becomes the dominant source of energy, and the star
again begins collapsing inwards.
The Earth keeps its size because its gravity is not strong enough to
crush the minerals of which it consists. Not so with a star massive
enough to sustain nuclear burning. A small star may crush all its atoms
together, creating a "white dwarf"--e.g. of half the mass of the Sun,
but only as big as the Earth. Some energy release continues (hence
"white") but ultimately, the star probably becomes a dark cinder.
This may be the fate of our Sun, too. In the final transition
strange changes occur--the star becomes a "red giant," diffuse and
enormously large, and later much of the material is blown to space where
it forms a "planetary" nebula, but there is no explosion. See "The
Complexity of Stellar Death" by Yervant Terzian, "Science" vol. 256 p.
425-6, 15 October 1999. Supernovas Stars several times the size of
our Sun have enough gravity to crush together not just atoms but even
nuclei, compressing all their matter to a sphere perhaps 15 kilometers
across. After their collapse they become "neutron stars" consisting only
of neutrons (the protons all switching form), giant nuclei as dense as
the ones in atoms. A huge amount of energy is liberated in that final
collapse which is quite rapid, blowing off the top layers of the
collapsing star and also producing elements heavier than iron.
[You may wonder why that collapse is so rapid, considering that
Helmholtz and Kelvin--cited at the start of this section--found the
gravitational energy of the Sun was sufficient to keep it hot for tens
of millions of years. The answer was given by George Gamov and the
Brazilian physicist Mario Schenberg in 1941: enormous energy is indeed
generated, but the extreme temperature produces nuclear processes which
generate neutrinos and these remove energy very, very quickly. The
neutrino is an almost massless uncharged particle with very weak
interactions, able to move unimpeded through thick layers of
material--even the entire solid Earth, even the Sun. Neutrinos escaping
through the outer layers of the supernova carry away its energy, in what
Gamov facetiously named "the Urca process," comparing it to the rapid
way in which a gambler's money disappeared at the roulette tables of the
Casino da Urca in Rio de Janeiro. The process was dramatically confirmed
by the supernova of 1987 (picture above shows it some years later; a
larger picture, with added links, here
<http://antwrp.gsfc.nasa.gov/apod/ap020331.html> ) whose observation
coincided with a burst of 11 neutrinos, detected by the sensitive
Kamiokande underground observatory in Japan, and by 8 registered
independently on a detector in Ohio] That catastrophic event is known
as a supernova explosion (technically, a "type 2 supernova"). Tycho
Brahe was fortunate to have seen one that occured in our galaxy,
outshining Venus and visible even in the daytime. The Chinese observed
one in the year 1054, in the Crab constellation of the zodiac, and still
another occured in Kepler's lifetime. Since then, however, none seemed
to have occured close to Earth. The most notable event of this type was
observed (quite extensively) in 1987 in the Large Magellanic Cloud, a
small galaxy neighboring ours (see image above; the inner cloud is the
one produced in the explosion, the rings seem older). For more about
supernovas, see here <http://www.kingsu.ab.ca/~brian/astro/a200l21a.htm>
The material blown off by a supernova explosion ultimately scatters
throughout space, and some of it is incorporated in clouds of dust and
gas which later form new suns and planets. All elements on Earth heavier
than helium (except, possibly, a small amount of lithium) must have
arrived that way: products of nuclear burning in some pre-solar star,
released or created in the explosion accompanying its final collapse.
Our bodies are made of star stuff--carbon, oxygen, nitrogen and the rest
have all been produced by nuclear fusion.
As for the "supernova remnant" left over from the collapse, its
fate depends on its mass. If the star was not too massive, the remnant
(as explained) is a neutron star. It that star originally rotated around
its axis, that rotation is enormously speeded up; the remnant of the
supernova of the year 1054 (its ejected cloud, the "Crab Nebula," is
shown on the left) is spinning at about 30 revolutions per second! Any
magnetic field of the original star is also enormously amplified, and
associated phenomena can make it beam radio waves. Pulsars, pulsed radio
sources with remarkably stable pulsation periods, are produced that way.
By the way, the Crab Nebula is still expanding; see here
<http://antwrp.gsfc.nasa.gov/apod/ap011227.html> for a comparison of
two images, taken 30 years apart. Another image of the nebula, highly
detailed, is given here <http://antwrp.gsfc.nasa.gov/apod/ap991122.html>
.
Added 20 October 1999: The new Chandra <http://chandra.harvard.edu/>=20
orbiting X-ray telescope has taken a high-resolution picture in X-rays
of the central region of the Crab nebula. Before this, astrophysicists
guessed the remnant star might be surrounded by orbiting debris, with
high-energy particles shooting out along its magnetic axis, the one
direction in which magnetic field lines do not confine them. The picture
on the right suggests something like that might indeed be happening. For
the image of a supernova remnant in Centaurus, as seen by Chandra, see
here <http://antwrp.gsfc.nasa.gov/apod/ap011026.html> .
Theory suggests that a star much more massive than the Sun will
collapse even further and become a black hole. What happens then can
only be guessed and calculated, not observed, for the star's gravity in
the collapsed state is so strong that no light and no information can
return from it to the outside world. One therefore expects such objects
to be completely black; they are called "black holes" because the
general theory of relativity suggests that the matter in such a star
keeps falling indefinitely, as the star contracts to a point. Thus in
theory such stars are like the proverbial bottomless pit, although no
observation could ever confirm it.
Although astronomers cannot see such objects, they have considerable
evidence that they exist, at least in a number of locations. For some
time now it was believed that very massive black hole existed at the
center of our galaxy, and if so, probably also at the centers of other
galaxies, helping hold them together. We now have some pretty definite
proof, and also a good estimate of what the mass of that monstrous
object may be. The story of that discovery is given in the following
section, "The black hole at the center of our galaxy
<http://www-spof.gsfc.nasa.gov/stargaze/Sblkhole.htm> ".
Another take on Edna St. Vincent Millay's rhyme:
My ice cream cone drips at both ends
I eat it in great haste
But ah, my foes, and oh, my friends
It has a lovely taste!
--0-1001994255-5997001861=:2
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
<A name=3D"q24B">
<TABLE width=3D"650" border=3D"0">
<TBODY>
<TR>
<TD vAlign=3D"top" align=3D"left" width=3D"200" bgColor=3D"#d0fbff"><FONT s=
ize=3D"-1"><IMG height=3D"10" src=3D"http://www-spof.gsfc.nasa.gov/stargaze=
/Sfigs/ball.gif" width=3D"8"> <A href=3D"http://www-spof.gsfc.=
nasa.gov/stargaze/Smap.htm"><FONT color=3D"#000000"><B>Index</B></FONT></A>=
<BR><BR><FONT color=3D"#990000"><B>The Sun</B></FONT> <BR><BR><IMG height=
=3D"10" src=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/ball.gif" width=
=3D"8"> <A href=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sun2view.htm">S-2=
.Solar Layers</A> <BR><BR><IMG height=3D"10" src=3D"http://www-spof.gsfc.na=
sa.gov/stargaze/Sfigs/ball.gif" width=3D"8"> <A href=3D"http://www-spof.gsf=
c.nasa.gov/stargaze/Sun3mag.htm">S-3.The Magnetic Sun</A> <BR><BR><IMG heig=
ht=3D"10" src=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/ball.gif" wid=
th=3D"8"> <A href=3D"http://www-spof.gsfc.nasa.gov/stargaze/Simfproj.htm">S=
-3A. Interplanetary</A> <BR> <A href=3D"http://w=
ww-spof.gsfc.nasa.gov/stargaze/Simfproj.htm">Magnetic Fields</A> <BR><BR><I=
MG height=3D"10" src=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/ball.g=
if" width=3D"8"> <A href=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sun4spec=
.htm">S-4. Colors of Sunlight</A> <BR><BR><IMG height=3D"10" src=3D"http://=
www-spof.gsfc.nasa.gov/stargaze/Sfigs/ball.gif" width=3D"8"> <A href=3D"htt=
p://www-spof.gsfc.nasa.gov/stargaze/Scolors.htm">S-4A.Color Expts.</A> <BR>=
<BR><IMG height=3D"10" src=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/=
ball.gif" width=3D"8"><A href=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sun=
5wave.htm"> S-5.Waves & Photons</A> <BR><BR><FONT color=3D"#990000"><B>=
Optional: Quantum Physics</B></FONT> <BR><BR><IMG height=3D"10" src=3D"http=
://www-spof.gsfc.nasa.gov/stargaze/Sfigs/ball.gif" width=3D"8"> <A href=3D"=
http://www-spof.gsfc.nasa.gov/stargaze/Q1.htm">Q1.Quantum Physics</A> <BR><=
BR><IMG height=3D"10" src=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/b=
all.gif" width=3D"8"> <A href=3D"http://www-spof.gsfc.nasa.gov/stargaze/Q2.=
htm">Q2. Atoms </A> (and 6 more) <BR>-------------------------- <BR><=
BR><IMG height=3D"10" src=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/b=
all.gif" width=3D"8"> <A href=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sun=
6new.htm">S-6.The X-ray Sun </A><BR><BR><IMG height=3D"10" src=3D"http://ww=
w-spof.gsfc.nasa.gov/stargaze/Sfigs/ball.gif" width=3D"8"> <A href=3D"http:=
//www-spof.gsfc.nasa.gov/stargaze/Sun7enrg.htm"><FONT color=3D"#000000"><B>=
S-7.The Sun's Energy</B></FONT></A> <BR><BR><IMG height=3D"10" src=3D"http:=
//www-spof.gsfc.nasa.gov/stargaze/Sfigs/ball.gif" width=3D"8"> <A href=3D"h=
ttp://www-spof.gsfc.nasa.gov/stargaze/Sblkhole.htm">S-7A. The Black Hole at=
</A> <BR> <A href=3D"http://www-spof.gsfc.nasa.g=
ov/stargaze/Sblkhole.htm">our Galactic Center</A> <BR><BR><IMG height=3D"10=
" src=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/ball.gif" width=3D"8"=
> <A href=3D"http://www-spof.gsfc.nasa.gov/stargaze/Ls7adisc.htm">LS-7A. Di=
scovery</A> <BR> <A href=3D"http://www-spof.gsfc.n=
asa.gov/stargaze/Ls7adisc.htm">of Atoms and Nuclei </A><BR><BR><IMG height=
=3D"10" src=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/ball.gif" width=
=3D"8"> <A href=3D"http://www-spof.gsfc.nasa.gov/stargaze/Snuclear.htm">S-8=
.Nuclear Power</A> <BR><BR><IMG height=3D"10" src=3D"http://www-spof.gsfc.n=
asa.gov/stargaze/Sfigs/ball.gif" width=3D"8"> <A href=3D"http://www-spof.gs=
fc.nasa.gov/stargaze/Snucweap.htm">S-9.Nuclear Weapons</A></FONT></TD></TR>=
</TBODY></TABLE>
<H2><FONT color=3D"#bb0000"><I>Nuclear Physics</I></FONT></H2> =
Gradually the picture became clearer. Atoms were found to consist of heavy=
nuclei, consisting of electrically positive <B>protons</B> and uncharged <=
B>neutrons, </B>while around the nuclei swarmed lightweight <B>electrons,</=
B> with a negative electric charge. An electron has about 1/1840 the mass o=
f the proton, which is also the nucleus of hydrogen.=20
<P>
<UL> [<B>Please excuse </B>the recitation of dry facts, but to=
explain how all this was established would be asking too much from such a =
brief review. A few of the benchmarks in the discovery of atoms and nuclei =
are listed <A href=3D"http://www-spof.gsfc.nasa.gov/stargaze/Ls7adisc.htm">=
here</A>.] </UL>
<P> Electrons and nuclei were kept together by electric attrac=
tion (negative attracts positive). Furthermore, electrons were sometimes sh=
ared by neighboring atoms or transferred to them (by processes of quantum p=
hysics), and this link between atoms gave our world its many chemical compo=
unds.=20
<P> But <B>something else</B> was needed to hold nuclei togeth=
er, since all protons carried positive charges and <B>repelled</B> each oth=
er. Electric forces are definitely <B>not</B> the glue that holds nuclei to=
gether, they act in the wrong direction! Besides, binding neutrons to nucle=
i clearly requires a non-electrical attraction. <A name=3D"q22B">
<P> All that suggested a different kind of force, a <B>nuclear=
force, </B>was holding nuclei together. That force had to be <B>stronger</=
B> than the electric repulsion at short distances, but <B>weaker</B> far aw=
ay, or else different nuclei might have tended to clump together, too. In o=
ther words, it had to be a <B>short-range force, </B>like the force between=
two small magnets--very hard to separate when stuck together, but once pul=
led a short distance apart, the force between them drops almost to zero (pl=
ease do not take this analogy too literally!). <A name=3D"q106A">
<P> Actually <B>two</B> kinds of force are active in the nucle=
us, known simply as the "strong force" and the "weak force," or more common=
ly the "strong interaction" and the "weak interaction" (because their main =
effect is in converting and creating particles). The <B>weak interaction</B=
> also affects electrons and other particles, but in the nucleus its main r=
ole is to maintain a balance between protons and neutrons, which except for=
their electric charge are very similar particles (diferent kinds of "nucle=
ons"). Nuclear structure (in light nuclei, at least) favors nuclei containi=
ng <B>equal</B> numbers of protons and neutrons, and although moderate ineq=
ualities can also exist (in "isotopes"), when they get too big, the weak in=
teraction can convert nucleons of one kind to the other, emitting an electr=
on (or a positron, its positive counterpart) in the process. That is known =
as <B>beta radioactivity</B> and will not be discussed any further.=20
<P> The <B>strong nuclear force</B> (the only nuclear force co=
nsidered from here on) can bind protons and neutrons into bigger nuclei. Be=
ing positively charged, all these nuclei repel each other, and therefore, e=
xcept in the presence of extreme temperatures and pressures--such as exist =
in the core of the Sun--two different nuclei are not likely to combine into=
one. Their electric repulsion does not allow them to get close enough for =
the nuclear force to take over.=20
<P><A name=3D"h68">
<H2><FONT color=3D"#bb0000"><I>The Binding Energy of Nuclei</I></FONT></H2>=
Nature contains nuclei of many different sizes. In hydrogen t=
hey contain just one proton, in heavy hydrogen ("deuterium") a proton and a=
neutron; in helium, two protons and two neutrons, and in carbon, nitrogen =
and oxygen--6, 7 and 8 of each particle, respectively. The <B>weight</B> of=
all these nuclei has been measured, and an interesting fact was noted: a h=
elium nucleus weighed less than the sum of the weights of its components. T=
he same held even more for carbon, nitrogen and oxygen--the carbon nucleus,=
for instance, was found to be slightly lighter than three helium nuclei.=20
<P> The reason for this "mass defect" has to do with Einstein'=
s famous formula E=3Dmc<SUP>2</SUP>, expressing the equivalence of energy a=
nd mass. By this formula, adding energy also increases mass (both weight an=
d inertia), removing energy, decreases it.=20
<P> If a combination of particles contains <B>extra energy</B>=
--for instance, in a molecule of the explosive TNT--weighing it will reveal=
some <B>extra</B> mass (compared to its end products--an unmeasurably smal=
l difference, for TNT). If on the other time we need <B>invest</B> energy t=
o separate it into its components, the weight will be <B>less</B> than that=
of the components.=20
<P> The latter is the case with nuclei such as helium: to <B>b=
reak them up</B> into protons and neutrons, we would have to <B>invest</B> =
energy. On the other hand, if a process existed going in the opposite direc=
tion, by which hydrogen atoms could be <B>combined</B> to form helium, a lo=
t of energy would be <B>released</B>-- namely, E=3Dmc<SUP>2</SUP> per nucle=
us, where m is the difference beween the mass of the helium nucleus and the=
mass of four protons (plus 2 electrons, absorbed to create the neutrons of=
helium).=20
<P></P></A>
<P>
<CENTER><IMG height=3D"160" src=3D"http://www-spof.gsfc.nasa.gov/stargaze/S=
figs/bindenrg.gif" width=3D"450"> </CENTER>
<P></P>
<CENTER>
<TABLE width=3D"550" border=3D"0">
<TBODY>
<TR>
<TD>
<P> As we go on to elements heavier than oxygen, the energy wh=
ich can be gained by assembling them from lighter elements decreases, up to=
<B>iron. </B>For nuclei heavier than iron, one actually gains energy by <B=
>breaking them up</B> into 2 fragments. That, of course, is how energy is e=
xtracted by breaking up uranium nuclei in nuclear power reactors.=20
<P> The reason the trend reverses after iron is the growing <B=
>positive charge</B> of the nuclei. The electric force may be weaker than t=
he nuclear force, but its <B>range</B> is greater: in an iron nucleus, each=
proton repels 25 other protons, while (one may argue) the nuclear force on=
ly binds close neighbors.=20
<P><A name=3D"q106A1">As nuclei grow bigger still, this disruptive effect b=
ecomes steadily more significant. By the time uranium is reached (92 proton=
s), nuclei can no longer accomodate their large positive charge, but emit t=
heir excess protons in the process of <B>alpha radioactivity</B>--the emiss=
ion of helium nuclei, each containing two protons and two neutrons. (Helium=
nuclei are an especially stable combination.) Still heavier nuclei are not=
found naturally on Earth<A name=3D"p1">.=20
<P>
<UL> [<B>Why are helium nuclei especially stable?</B> Some tim=
e ago I received e-mail from a teacher asking for suggestions of ways of de=
monstrating fusion to her students, "e.g. using M&Ms." One can do so (a=
lthough tennis balls are better, being visible even from the last row of th=
e classroom!) by showing how 4 balls (or candy pieces) can be stacked in a =
pyramid, each touching the other 3. Considering that the nuclear forces has=
a short range, this "close packing" gives the tightest binding. Maybe that=
is why helium (4 nucleons) is such a stable combination, as shown by its p=
eak on the curve of binding energy above. No larger number can be stacked w=
ith every single ball touching all the others!] </UL>
<P>
<H2><FONT color=3D"#bb0000"><I>The Sun's Energy Source</I></FONT></H2> =
; It is believed that the Sun is about 5 billion years old, formed w=
hen gravity pulled together a vast cloud of gas and dust, from which the Ea=
rth and other planets also arose. The gravitational pull released energy an=
d heated the early Sun, much in the way Helmholtz had proposed. <A name=3D"=
q23B">
<P> Heat is the motion of atoms and molecules: the higher the =
temperature, the greater is their velocity and the more violent are their c=
ollisions. When the temperature at the center of the newly-formed Sun becam=
e great enough for collisions between nuclei to overcome their electric rep=
ulsion, nuclei began to stick together and protons were combined into heliu=
m, with some protons changing in the process to neutrons (plus positrons, p=
ositive electrons, which combine with electrons and are destroyed). This re=
leased nuclear energy and kept up the high temperature of the Sun's core, a=
nd the heat also kept the gas pressure high, keeping the Sun puffed up and =
stopping gravity from pulling it together any more.=20
<P> That, in greatly simplified terms, is the <B>"nuclear fusi=
on"</B> process which still takes place inside the Sun. Different nuclear r=
eactions may predominate at different stages of the Sun's existence, includ=
ing the proton-proton reaction and the carbon-nitrogen cycle which involves=
heavier nuclei, but whose final product is still the combination of proton=
s to form helium. <!-- A more detailed qualitative account, by astrophysici=
sts of the University of California at Berkeley, can be reached <a href=3D"=
http://spectrum.lbl.gov/education/elements/stellar/stellar_b.html">here</a>=
.=20
-->
<P> A branch of physics, the study of "controlled nuclear fusi=
on," has tried since the 1950s to derive useful power from "nuclear fusion"=
reactions which combine small nuclei into bigger ones--power to heat boile=
rs, whose steam could turn turbines and produce electricity. Unfortunately,=
no earthly laboratory can match one feature of the solar powerhouse--the g=
reat mass of the Sun, whose weight keeps the hot plasma compressed and conf=
ines the "nuclear furnace" to the Sun's core. Instead, physicists use stron=
g magnetic fields to confine the plasma, and for fuel they use heavy forms =
of hydrogen, which "burn" more easily. Still, magnetic traps can be rather =
unstable, and any plasma hot enough and dense enough to undergo nuclear fus=
ion tends to slip out of them after a short time. Even with ingenious trick=
s, the confinement in most cases lasts only a small fraction of a second.=20
<P> The Sun today still consists mostly of hydrogen. The fuel =
supply which has seen it through its first 5 billion years should be good f=
or about as long in the future<A name=3D"q28B">.=20
<P>
<H2><FONT color=3D"#bb0000"><I>The Evolution of Stars</I></FONT></H2> =
Apart from the 5 planets, every star we see at night is a sun: some=
are bigger than ours, some smaller, some are at an earlier stage of their =
developments, some at a later one, and some have evolved altogether differe=
ntly, for a variety of reasons. The telescope allows astronomers to observe=
and compare stars of different size, at different stages of evolution. The=
ir smooth spectra tell about their temperatures, their spectral lines revea=
l some of their composition, and based on these a general theory of "stella=
r evolution" has been formulated, which also applies to our own Sun, a typi=
cal "main sequence" star.=20
<P> All such stars burn hydrogen to produce helium, where "bur=
n" refers to nuclear processes, not to the (completely inadequate) chemical=
process of fire. Big stars burn rapidly and brightly, like the candle in E=
dna St. Vincent Millay's poem=20
<P></P></A></TD></TR></TBODY></TABLE></CENTER>
<CENTER>
<TABLE width=3D"300" border=3D"0">
<TBODY>
<TR>
<TD>My candle burns at both ends;<BR>It will not last the night;<BR>But ah,=
my foes, and oh, my friends<BR>It gives a lovely light!<BR></TD></TR></TBO=
DY></TABLE></CENTER>
<CENTER>
<TABLE width=3D"500" border=3D"0">
<TBODY>
<TR>
<TD>(Another take on this rhyme, <A href=3D"http://www-spof.gsfc.nasa.gov/s=
targaze/Sun7enrg.htm#icecream">below</A>) <BR> S=
mall stars last longer and many are dim; but whatever a star's size, ultima=
tely it runs out of hydrogen. It can still release energy by "burning" heav=
ier nuclei and combining them into bigger ones, up to iron: theory suggests=
this does happen, but it provides much less energy and does not greatly ex=
tend the star's lifetime. When all the fuel is gone, gravity again becomes =
the dominant source of energy, and the star again begins collapsing inwards=
.=20
<P> The Earth keeps its size because its gravity is not strong=
enough to crush the minerals of which it consists. Not so with a star mass=
ive enough to sustain nuclear burning. A small star may crush all its atoms=
together, creating a "white dwarf"--e.g. of half the mass of the Sun, but =
only as big as the Earth. Some energy release continues (hence "white") but=
ultimately, the star probably becomes a dark cinder.=20
<P> This may be the fate of our Sun, too. In the final transit=
ion strange changes occur--the star becomes a "red giant," diffuse and enor=
mously large, and later much of the material is blown to space where it for=
ms a "planetary" nebula, but there is no explosion. See "The Complexity of =
Stellar Death" by Yervant Terzian, "Science" vol. 256 p. 425-6, 15 October =
1999. <A name=3D"q4B">
<H2><FONT color=3D"#bb0000"><I>Supernovas</I></FONT></H2><IMG src=3D"http:/=
/www-spof.gsfc.nasa.gov/stargaze/Sfigs/SN1987A.gif" align=3D"right"> =
Stars several times the size of our Sun have enough gravity to crus=
h together not just atoms but even nuclei, compressing all their matter to =
a sphere perhaps 15 kilometers across. After their collapse they become "ne=
utron stars" consisting only of neutrons (the protons all switching form), =
giant nuclei as dense as the ones in atoms. A huge amount of energy is libe=
rated in that final collapse which is quite rapid, blowing off the top laye=
rs of the collapsing star and also producing elements heavier than iron.=20
<P>
<UL> [You may wonder why that collapse is so rapid, considerin=
g that Helmholtz and Kelvin--cited at the start of this section--found the =
gravitational energy of the Sun was sufficient to keep it hot for tens of m=
illions of years. The answer was given by George Gamov and the Brazilian ph=
ysicist Mario Schenberg in 1941: enormous energy is indeed generated, but t=
he extreme temperature produces nuclear processes which generate <B>neutrin=
os</B> and these remove energy very, very quickly. The neutrino is an almos=
t massless uncharged particle with very weak interactions, able to move uni=
mpeded through thick layers of material--even the entire solid Earth, even =
the Sun. Neutrinos escaping through the outer layers of the supernova carry=
away its energy, in what Gamov facetiously named <B>"<!--<A HREF=3D"http:/=
/www15.vianetworks.com.br/casadasrosas/mario/1urca.htm">-->the Urca process=
,</A>"</B> comparing it to the rapid way in which a gambler's money disappe=
ared at the roulette tables of the Casino da Urca in Rio de Janeiro. <A nam=
e=3D"SN1987"><FONT color=3D"#990000">The process was dramatically confirmed=
</FONT> by the <B>supernova of 1987</B></A> (picture above shows it some ye=
ars later; a larger picture, with added links, <A href=3D"http://antwrp.gsf=
c.nasa.gov/apod/ap020331.html"><B>here</B></A>) whose observation coincided=
with a burst of 11 neutrinos, detected by the sensitive Kamiokande undergr=
ound observatory in Japan, and by 8 registered independently on a detector =
in Ohio]</UL><A name=3D"q23A"> That catastrophic event is know=
n as a <B>supernova</B> explosion (technically, a "type 2 supernova"). Tych=
o Brahe was fortunate to have seen one that occured in our galaxy, outshini=
ng Venus and visible even in the daytime. The Chinese observed one in the y=
ear 1054, in the Crab constellation of the zodiac, and still another occure=
d in Kepler's lifetime. Since then, however, none seemed to have occured cl=
ose to Earth. The most notable event of this type was observed (quite exten=
sively) in 1987 in the Large Magellanic Cloud, a small galaxy neighboring o=
urs (see image above; the inner cloud is the one produced in the explosion,=
the rings seem older). For more about supernovas, see <A href=3D"http://ww=
w.kingsu.ab.ca/~brian/astro/a200l21a.htm">here</A>=20
<P> The material blown off by a supernova explosion ultimately=
scatters throughout space, and some of it is incorporated in clouds of dus=
t and gas which later form new suns and planets. All elements on Earth heav=
ier than helium (except, possibly, a small amount of lithium) must have arr=
ived that way: products of nuclear burning in some pre-solar star, released=
or created in the explosion accompanying its final collapse. <B>Our bodies=
are made of star stuff</B>--carbon, oxygen, nitrogen and the rest have all=
been produced by nuclear fusion.=20
<P><A name=3D"q20"><IMG src=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sfigs=
/crab_neb.gif" align=3D"left"> As for the "supernova remnant"=
left over from the collapse, its fate depends on its mass. If the star was=
not too massive, the remnant (as explained) is a neutron star. It that sta=
r originally rotated around its axis, that rotation is enormously speeded u=
p; the remnant of the supernova of the year 1054 (its ejected cloud, the "C=
rab Nebula," is shown on the left) is spinning at about 30 revolutions per =
second! Any magnetic field of the original star is also enormously amplifie=
d, and associated phenomena can make it beam radio waves. <B>Pulsars, </B>p=
ulsed radio sources with remarkably stable pulsation periods, are produced =
that way. By the way, the Crab Nebula is still expanding; <A href=3D"http:/=
/antwrp.gsfc.nasa.gov/apod/ap011227.html"><B>see here</B></A> for a compari=
son of two images, taken 30 years apart. Another image of the nebula, highl=
y detailed, is given <A href=3D"http://antwrp.gsfc.nasa.gov/apod/ap991122.h=
tml"><B>here</B></A>.=20
<P><A name=3D"chandra"><IMG height=3D"150" src=3D"http://www-spof.gsfc.nasa=
.gov/stargaze/Sfigs/Scrabxry.jpg" width=3D"150" align=3D"right">=20
<UL><FONT color=3D"#990000"><B>Added 20 October 1999:</B></FONT> The new <A=
href=3D"http://chandra.harvard.edu/">Chandra</A> orbiting X-ray telescope =
has taken a high-resolution picture in X-rays of the central region of the =
Crab nebula. Before this, astrophysicists guessed the remnant star might be=
surrounded by orbiting debris, with high-energy particles shooting out alo=
ng its magnetic axis, the one direction in which magnetic field lines do no=
t confine them. The picture on the right suggests something like that might=
indeed be happening. For the image of a supernova remnant in Centaurus, as=
seen by Chandra, <A href=3D"http://antwrp.gsfc.nasa.gov/apod/ap011026.html=
"><B>see here</B></A>.</UL>
<P> Theory suggests that a star <B>much more</B> massive than =
the Sun will collapse even further and become a <B>black hole. </B>What hap=
pens then can only be guessed and calculated, not observed, for the star's =
gravity in the collapsed state is so strong that no light and no informatio=
n can return from it to the outside world. One therefore expects such objec=
ts to be completely black; they are called "black holes" because the genera=
l theory of relativity suggests that the matter in such a star keeps fallin=
g indefinitely, as the star contracts to a point. Thus in theory such stars=
are like the proverbial bottomless pit, although no observation could ever=
confirm it.=20
<P> Although astronomers cannot see such objects, they have co=
nsiderable evidence that they exist, at least in a number of locations. For=
some time now it was believed that very massive black hole existed at the =
center of our galaxy, and if so, probably also at the centers of other gala=
xies, helping hold them together. <B>We now have some pretty definite proof=
</B>, and also a good estimate of what the mass of that monstrous object ma=
y be. The story of that discovery is given in the following section, "<A hr=
ef=3D"http://www-spof.gsfc.nasa.gov/stargaze/Sblkhole.htm"><B>The black hol=
e at the center of our galaxy</B></A>".=20
<P><A name=3D"icecream">
<HR SIZE=3D"2">
<P>Another take on Edna St. Vincent Millay's rhyme:=20
<UL>
<UL>My ice cream cone drips at both ends <BR>I eat it in great haste<BR>But=
ah, my foes, and oh, my friends<BR>It has a lovely taste!<BR></UL></UL></A=
></TD></TR></TBODY></TABLE></CENTER>
--0-1001994255-5997001861=:2--
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Sun Nov 30, 2008 11:05 am
"Joseph Zaidan" <jnadiaz@...>
jnadiaz
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