Monday, February 6, 2012

What did Hans Bethe find out about the Sun?

Sun. Image NOOA Space Weather Prediction Center

The processes involved began to be understood early in the 20th century, when it was first realized that the energy released from nuclear reactions accounted for the longevity of the Sun as a source of heat and light. The prime energy producer in the sun is the fusion of hydrogen to helium, which occurs at a minimum temperature of 3 million kelvin.

Now if you really want to know what the celebrated scientist Hans Bethe said about the nuclear reactions in Sun and other stars this is not the place! For I am a humanist with no formal training in Nuclear Physics, Astronomy, higher Mathematics and other such cool stuff.

You would find people who really understand what Hans Bethe said and learn from them.

I did the same.

Here is a short summary of what I found out about the workings of Sun:

From 1935-1938, he studied nuclear reactions and reaction cross sections, carbon-oxygen-nitrogen cycle, leading to his important contribution to stellar nucleosynthesis. This research was later useful to Bethe in more quantitatively developing Niels Bohr's theory of the compound nucleus.

Carbon-oxygen-nitrogen cycle (CNO)

CNO cycle

The CNO cycle (for carbon–nitrogen–oxygen) is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain. Unlike the proton–proton chain reaction, the CNO cycle is a catalytic cycle.

Theoretical models show that the CNO cycle is the dominant source of energy in stars more massive than about 1.3 times the mass of the Sun. The proton–proton chain is more important in stars the mass of the Sun or less.

This difference stems from temperature dependency differences between the two reactions; pp-chain reactions start occurring at temperatures around 4×106 K, making it the dominant energy source in smaller stars.

A self-maintaining CNO chain starts occurring at approximately 15×106 K, but its energy output rises much more rapidly with increasing temperatures. At approximately 17×106 K, the CNO cycle starts becoming the dominant source of energy. The Sun has a core temperature of around 15.7×106 K.

He nuclei being produced in the Sun are born in the CNO cycle. The CNO-I process was independently proposed by Carl von Weizsäcker and Hans Bethe in 1938 and 1939, respectively.

Proton-proton chain or Fusion in the Sun

Proton-proton chain

The proton–proton chain reaction is one of several fusion reactions by which stars convert hydrogen to helium, the primary alternative being the CNO cycle. The proton–proton chain dominates in stars the size of the Sun or smaller.

In general, proton–proton fusion can occur only if the temperature (i.e. kinetic energy) of the protons is high enough to overcome their mutual electrostatic or Coulomb repulsion.

In the Sun, deuterium-producing events are so rare (diprotons being the much more common result of nuclear reactions within the star) that a complete conversion of the star's hydrogen would take more than 1010 (ten billion) years at the prevailing conditions of its core.

The fact that the Sun is still shining is due to the slow nature of this reaction; if it went more quickly, the Sun would have exhausted its hydrogen long ago.

Energy release
Thank you very much. This is all very nice information about the inner workings of Sun. But what is in it for us?

Light, my dear Watson, light. And plenty of energy!

Comparing the mass of the final helium-4 atom with the masses of the four protons reveals that 0.007 or 0.7% of the mass of the original protons has been lost. This mass has been converted into energy, in the form of gamma rays and neutrinos released during each of the individual reactions. The total energy yield of one whole chain is 26.73 MeV.

Only energy released as gamma rays will interact with electrons and protons and heat the interior of the Sun. This heating supports the Sun and prevents it from collapsing under its own weight.

Neutrinos do not interact significantly with matter and do not help support the Sun against gravitational collapse. The neutrinos in the ppI, ppII and ppIII chains carry away 2.0%, 4.0% and 28.3% of the energy in those reactions respectively.

What is 26.73 MeV?
meV stands for milli-electron Volt.
MeV stands for mega-electron Volt.

In physics, the electron volt (symbol eV) is a unit of energy equal to approximately 1.602×10−19 joule (symbol J).

By definition, it is the amount of energy gained by the charge of a single electron moved across an electric potential difference of one volt.

Thus it is 1 volt (1 joule per coulomb, 1 J/C) multiplied by the electron charge (1 e, or 1.602176565(35)×10−19 C).

Therefore, one electron volt is equal to 1.602176565(35)×10−19 J.

Historically, the electron volt was devised as a standard unit of measure through its usefulness in electrostatic particle accelerator sciences because a particle with charge q has an energy E=qV after passing through the potential V; if q is quoted in integer units of the elementary charge and the terminal bias in volts, one gets an energy in eV.

How much energy is one MeV?

I am not the only one wondering...

Take a look at the smart and compact answers given to this question in Physics Forums

Sun is shining about 8 light minutes away and we can feel the energy on our skins.

If it is shining very hot remember the adivse to wear sunscreen, dear! (The long-term benefits of using sunscreen has been proven by scientists. Trust Baz Luhman on this.)

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