Internal Structure of a Typical Main Sequence Star the

About 4.6 billion years ago the Sun was born out of gases which had perhaps 73% hydrogen, 25% helium, and tiny amounts of heavy elements. The radius of the Sun is now 694,000 km and its energy output is 3.90 x 1026 W. This "light bulb" has apparently kept pretty much the same luminosity and size through historical time and from fossil evidence, for most of the geological history of the Earth.

We cannot look at its deep interior, but the conditions there can be deduced from the fact that the Sun is neither expanding nor contracting. The high density and the temperature at its center are necessary to keep the Sun from collapsing. Its internal properties are described by the gas sphere model in Table 19.1. Inspection of the table reveals that the temperature and the density change very steeply from the center to the surface, while the hydrogen fraction is about the same for the outer two-third of the Sun's radius and smaller only in the innermost core of the Sun (the result of the "burning" of hydrogen).

The Sun does not have a solid surface. Rather its light comes from different depths of a layer called the photosphere which is about 300 km in thickness. The temperature usually quoted as 5,500°C is a kind of average over different depths of the photosphere.

The coolest part of the Sun is at the top of the photosphere, about 4,300°C. Outside the photosphere lies the chromosphere, a layer about 2,000 km thick. Here the gas is rare and the temperature rises to 100,000°C at the top of the chromosphere. Outside the chromosphere starts the corona where the temperature is millions of degrees. The gas forming the extensive corona is very rarefied. It emits little visible light that is best seen when the Moon covers up the photosphere during a solar eclipse (see Fig 19.10).

The Sun is losing 3.90 x 1026 W (J/s) into space. If this energy is not replaced the Sun will not be at equilibrium. Now we know that the energy of a main sequence star

Table 19.1 Current internal properties of the Sun

Distance from

Mass within

Temperature

Density

the center (106 km)

this distance

(106 K)

(g/cm3)

Hydrogen (%)

0.00

0

15.7

158

36

0.10

20

11.3

59

65

0.20

60

7.1

15.2

72

0.32

90

3.9

1.84

73

0.48

99

1.73

0.117

73

0.62

99.955

0.66

0.0063

73

0.694

100

0.0045

3 x 10-8

73

comes from nuclear reactions that fuse hydrogen nuclei into helium nuclei. In lighter MS stars the basic reaction is the proton-proton chain, also happening in the Sun, while in stars considerably heavier the chain of reactions is more complicated. Such various routes from hydrogen to helium were found by German-American physicist Hans Bethe (1906-2005) in his theoretical studies in the late 1930s; briefly the processes are called burning of hydrogen (here "burning" is an energy-generating nuclear process). Bethe was among those scientists with Jewish family roots who had to leave his native country. He received a Nobel prize in physics for his work on stellar nuclear synthesis in 1967.

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