| Mass | 2 x 1030 kg (333,000 Earth mass) |
| Radius | 7 x 108 m (109 times Earth radius) |
| Surface Temperature | 5800 K |
| Central Temperature | 1.5 x 107 K |
| Central Pressure | 2.0 x 1011 atmospheres |
| Age | 4.6 x 109 years |
| Composition (by mass) | 75% hydrogen, 25% helium, 0.1% heavier elements |
1868--Helium (from Greek: Helios) was discovered first in the Sun by Pierre Jansen and Norman Lockyer.
1925--Cecilia Payne-Gaposchkin discovered that the Sun is
made mostly of hydrogen. (Payne was the first woman to earn a PhD in astronomy
in the USA).
Photosphere Light-emitting layer (5780K). Just above it the temperature drops to about 4000 K.Chromosphere Color-layer; layer of small, intense red flames (50,000 K; 2500 km thick).
Corona Rarified gas at 2 million K.
Sunspots Cooler regions (4000-4500 K) of intense magnetic fields. Galileo realized in 1613 that the spots are actually on the Sun's surface.
Granules Formed by convective motion--1000-2000 km cells rising at 2 to 3 km/s, lasting minutes.
Spicules Columns of very hot gas (10,000 km long). Basically, they are giant flames. (Earth compared to size of the spicules.)Prominences and Flares. Giant eruptions of gas in the chromosphere.
The Sun's surface activity varies in a cycle of about 11 years. At its most active the Sun is covered in sunspots and other features. At its least active the Sun can remain featureless for several months at a time. Here is a picture showing the Sun's surface at sunspot minimum and maximum.
Power Output
We receive about 1370 Watts/m2 of power from the Sun over all wavelengths. When integrated over a sphere whose radius is the Earth-Sun distance this implies that the Sun generates
3.826 x 1026 Watts of power.
Recall that one Watt is 1 Joule/second. A Joule is a unit of energy.
Example: A 100 watt light bulb converts electrical energy into light (and heat) at the rate of 100 Joules/second.
It is only in this century that we have finally understood how the Sun and other stars generate such huge amounts of power.
1871--Hermann von Helmholtz computed that the solar energy output is equal to the energy released by burning 1500 lb of coal per hour for every square foot of the solar surface!
1919--Sir Arthur Eddington showed that the conversion of hydrogen to helium could provide enough energy to explain the power of the Sun.
1930s--Hans Bethe and Carl von Weizsacker developed Eddington's ideas and showed how nuclear fusion, (fission and fusion) could produce the required amount of energy.
Nuclear Fusion
When 4 hydrogen nuclei are fused together into 1 helium nucleus (via a complicated chain of intermediate nuclear reactions), energy is released in the form of photons and sub-nuclear particles called neutrinos.
The probability for two protons to fuse, in the environment of the Sun's core, is extremely low: even at the rate of 100 million collisions per second, one would have to wait on average 14 billion years to see two protons fuse! Happily, there are a lot of protons so that enough of them fuse to maintain a steady supply of energy.
The energy arises from the fact that mass, in accordance with Einstein's famous formula, can be converted to energy (and vice versa).
But where does the mass come from? It turns out that the mass of 4 hydrogen nuclei is less than that of 1 helium nucleus, so after fusing 4 hydrogen nuclei there is a little bit of mass left over. This mass deficit appears as the energy of light and neutrinos.
The mass of 4 hydrogen atoms is
4 x 1.007825 u = 4.03130 u
where u = 1/12 the mass of an atom of carbon. The mass of 1 helium atom is
4.00268 u
The mass loss, 0.02682 u (which is 0.71% of the mass of the initial hydrogen atoms), appears as energy.
Einstein's Equation (1905)
In 1905, in one of three revolutionary scientific papers, Einstein showed that matter and energy are related by the famous formula
E = mc2
E is the energy
m is the mass
c is the speed of light
Using this equation we can compute how much mass must be converted into energy every second to power the Sun. In one second the sun releases
E = 3.8 x 1026 J
This corresponds to a mass of
m = E/c2, that is,
m = 3.8 x 1026 J/(3.0 x 108 m/s)2
m = 4.2 x 109 kg.
To sustain its energy output the Sun must convert about
4 million metric tons of hydrogen every second to energy!
It is rather difficult to comprehend the enormity of this level of destruction of matter. But let's try.
Consider the explosion of 1 megaton of TNT. This is the explosive power of a typical hydrogen bomb (H-bomb) in the US and Russian nuclear arsenals. Such an explosion is enough to destroy a town like Tallahassee. A 1 megaton explosion releases roughly 4 x 1015 Joules of energy. So to match the Sun's output we would have to explode
3.8 x 1026 J/s / 4.0 x 1015 J
that is, about
100 billion H-bombs every second!
It is very impressive that the Sun has maintained this colossal power output for at least a billion years, and will continue to do so for several more!
The Sun is a copious source of neutrinos as well as of photons. Neutrinos are neutral sub-nuclear particles with zero (or very small) mass that interact very weakly with matter. Consequently, neutrinos can easily travel through vast stretches of material without being stopped.
Therefore, whereas light takes about one million years to percolate from the core to the surface, neutrinos reach the Sun's surface in seconds.So unlike light, neutrinos provide, in principle, a way to probe the Sun's core.Right now about 60 billion neutrinos per second are passing through every square centimeter of your body. It is remarkable that these elusive particles were created within the Sun's core a mere 8.5 minutes ago!
There is a puzzle, called the Solar Neutrino Problem, which is: Why are fewer neutrinos reaching us, from the center of the Sun, than we predict from calculations of the nuclear reactions that occur in the core? It could be that the theories of the Sun's structure are wrong or that the experiments that have measured the neutrino flux are wrong. Or, perhaps, something strange is happening to the neutrinos as they journey earthwards.
Sound waves travel throughout
the Sun. Indeed, the Sun can be thought of as a giant ringing (spherical)
bell. Many thousands of different oscillations modes (tones, if you like)
have been measured. The study of these solar oscillations, called helioseismology,
has provided a great deal of detailed information about the internal
structure of the Sun.
