Energizing the

Art Paintings

IMPRESSION OF THE RISING SUN This portrayal of sunrise by the French artist Claude Monet (1840-1926) gave the Impressionist Movement its name. (Courtesy of the Musée Marmottan, Paris.)


tte Sun is extremely massive, containing 99.9 percent of the total mass of the Solar System, tte gravitational pull of the Sun's large mass controls the movements of everything in its vicinity, ttat is the reason the planets belong to the Solar System; their motion is dominated by the Sun.

Our home star is also very big, with a radius of about 109 times that of the Earth. And such a large, hot object must shine brightly.

tte Sun's luminosity is diluted by the square of the distance as it spreads out into the increasing volume of space. By measuring the amount of solar radiation intercepted by the Earth, at a mean distance of about 150 million kilometers from the Sun, we can extrapolate back to infer the total energy emitted by the Sun every second. It radiates a power of 380 million billion billion watts, or 3.8 x 1026 watts, which is known as the absolute luminosity of the Sun and designated by the symbol Le (Table 2.1).

So the Sun is relentlessly losing its energy, radiating it away at an enormous rate. In just one second, the Sun emits more energy than humans have used since the beginning of civilization. Its' fire is too brilliant to be perpetually sustained; after all, nothing can stay hot forever, and all things wear out with time.

Why does the Sun stay hot, and how long has it been shining? A normal fire of the Sun's intensity would soon burn out. ttat is, no ordinary fire can maintain the Sun's steady supply of heat for long periods of time. If the Sun were composed entirely of coal, with enough oxygen to sustain combustion, it would be burned away and totally consumed in a few thousand years.

In the mid-nineteenth century, the German physicist and physiologist Hermann von Helmholtz (1821-1894) proposed that the origin of the Sun's radiated energy is the gravitational contraction of its large mass. If the Sun were gradually shrinking, the infalling matter would heat the solar gases to incandescence, just as the air inside a tire pump warms when it is compressed; in more scientific terms, the Sun's gravitational energy would be slowly converted into the kinetic energy of motion and heat the Sun up. Helmholtz also made the first precise formulation of the principle of conservation of energy, in which the total energy of a system and its surroundings remain constant even if it may be changed from one form of energy to another.

tte Irish physicist William ttomson (1824-1907), later Lord Kelvin, then showed that the Sun could have illuminated the Earth at its present rate for about 100 million years by slowly contracting. In his article entitled "On the Age of the Sun's Heat", published in 1862, ttomson wrote:

It seems, therefore, on the whole most probable that the Sun has not illuminated the Earth for 100 million years, and almost certain that he has not done so for 500 million years. As for the future, we may say, with equal certainty, that inhabitants of the Earth cannot continue to enjoy the light and heat essential to their life, for many million years longer, unless sources now unknown to us are prepared in the great storehouse of creation.5

ttomson incidentally developed the scale of temperature that starts from absolute zero - the temperature at which atoms and molecules cease to move and have no kinetic energy; it is now known as the Kelvin temperature scale and is widely used

TABLE 2.1 The Sun's Physical Properties"

Mean distance, AU 1.495 978 7 X 10" m, and about 150 million km

(from radarmeasurements ofthe distance to Venus andKepler's thirdlaw)

499 seconds

6.955 X 10s meters (109 Earth radii)

1.412 X 1027 m3 (1.3 million Earths) 1.989 X 1030 kg (332,946 Earth masses)

Light travel time from Sun to Earth Radius, R

(from distance and angular extent) Volume

Mass, M

(from distance and Earth's orbitalperiod using Kepler's third law)

Escape velocity at photosphere 6.178 X 105 m s_1

Mean density 1,409 kg m~3

Solar constant,/^ 1.366 J s"1 m~2 = 1,366 W m~2

Luminosity, L 3.854 X 1026 J s"1 = 3.854 X 1026 W

(from solar constant, distance and Stefan-Boltzmann law)

Principal chemical constituents

(from analysis ofFraunhofer lines)

(By Number Of Atoms)

(By Mass Fraction)

Hydrogen Helium All others

92.1 percent 7.8 percent 0.1 percent

X = 70.68 percent Y = 27.43 percent Z = 1.89 percent

(from age ofoldestmeteorites)

4.6 billion years

Density (center)

151,300 kg m~3

Pressure (center)

2.334 X 10"


Pressure (photosphere)

0.0001 bar

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