Solar System Essay, Research Paper
The Sun is an ordinary G2 star, one of more than 100 billion stars in our galaxy.
diameter: 1,390,000 km.
mass: 1.989e30 kg
temperature: 5800 K (surface)
15,600,000 K (core)
The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total
mass of the Solar System (Jupiter contains most of the rest).
The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it
Sol.
The Sun is, at present, about 75% hydrogen and 25% helium by mass (92.1% hydrogen and
7.8% helium by number of atoms); everything else (”metals”) amounts to only 0.1%. This changes
slowly over time as the Sun converts hydrogen to helium in its core.
The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once
every 25.4 days; near the poles it’s as much as 36 days. This odd behavior is due to the fact that the
Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential
rotation extends considerably down into the interior of the Sun but core of the Sun rotates as a solid
body.
Conditions at the Sun’s core are extreme. The temperature is 15.6 million Kelvin and the pressure
is 250 billion atmospheres. The core’s gases are compressed to a density 150 times that of water.
The Sun’s energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by
nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about
695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma
rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at
lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light.
For the last 20% of the way to the surface the energy is carried more by convection than by
radiation.
The surface of the Sun, called the photosphere, is at a temperature of
about 5800 K. Sunspots are “cool” regions, only 3800 K (they look dark
only by comparison with the surrounding regions). Sunspots can be very
large, as much as 50,000 km in diameter. Sunspots are caused by
complicated and not very well understood interactions with the Sun’s
magnetic field.
A small region known as the chromosphere lies above the photosphere.
The highly rarified region above the chromosphere, called the corona,
extends millions of kilometers into space but is visible only during eclipses
(left). Temperatures in the corona are over 1,000,000 K.
The Sun’s magnetic field is very strong (by terrestrial standards) and
very complicated. Its magnetosphere (also known as the heliosphere
extends well beyond Pluto.
In additio
particles (mostly electrons and protons) known as the solar wind which
propagates throughout the solar system at about 450 km/sec. The solar wind and
the much higher energy particles ejected by solar flares can have dramatic effects
on the Earth ranging from power line surges to radio interference to the beautiful
aurora borealis.
Recent data from the spacecraft Ulysses show that the solar wind emanating from the polar
regions flows at nearly double the rate, 750 kilometers per second, that it does at lower latitudes.
The composition of the solar wind also appears to differ in the polar regions. And the Sun’s magnetic
field seems to be surprisingly uniform.
Further study of the solar wind will be done by the recently launched Wind, ACE and SOHO
spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about
1.6 million km from Earth.
The solar wind has large effects on the tails of comets and even has measurable effects on the
trajectories of spacecraft.
Spectacular loops and prominences are often visible on the Sun’s limb (left).
The Sun’s output is not entirely constant. Nor is the amount of sunspot activity.
There was a period of very low sunspot activity in the latter half of the 17th century called the
Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes
known as the Little Ice Age. Since the formation of the solar system the Sun’s output has increased
by about 40%.
The Sun is about 4.5 billion years old. Since its birth it has used up about half of
the hydrogen in its core. It will continue to radiate “peacefully” for another 5 billion
years or so (although its luminosity will approximately double in that time). But
eventually it will run out of hydrogen fuel. It will then be forced into radical changes
which, though commonplace by stellar standards, will result in the total destruction
of the Earth (and probably the creation of a planetary nebula).
The Sun’s satellites
There are nine planets and a large number of smaller objects orbiting the Sun. (Exactly which
bodies should be classified as planets and which as “smaller objects” has been the source of some
controversy, but in the end it is really only a matter of definition.)
Distance Radius Mass
Planet (000 km) (km) (kg) Discoverer Date
——— ——— —— ——- ———- —–
Mercury 57,910 2439 3.30e23
Venus 108,200 6052 4.87e24
Earth 149,600 6378 5.98e24
Mars 227,940 3397 6.42e23
Jupiter 778,330 71492 1.90e27
Saturn 1,426,940 60268 5.69e26
Uranus 2,870,990 25559 8.69e25 Herschel 1781
Neptune 4,497,070 24764 1.02e26 Galle 1846
Pluto 5,913,520 1160 1.31e22 Tombaugh 1930