The Sun is a prodigous source of energy. As such, it
is of fair amount of interest to us.
Question: How energetic is the Sun?
Answer:
A not too useful statement is that the Sun radiates at a
rate of 3.9x1023 kilo-Watts (kW).
This is a high rate as it corresponds to
the brightness of 3.9x1024 100 W light bulbs!
A more striking statement
is that at $0.10 a
kW-hour, the Sun produces
$ = 3.9x1023 kW x 1 hour x $0.10
===> $ 3.9 x 1022
worth of energy in one hour!!
An aside--
A Watt is a measure of power, the rate at which energy is used or
produced. A Watt is 1 Joule (J) per second (s) and the Sun's
luminosity is 3.9x1026 J/s.
Question: How much is a Joule?
- If I drop a rock of mass 1 kilogram (1 kg = 2.2 pounds on Earth) from
a perch 5 meters (m) off the ground,
the rock will hit the
ground with speed of ~10 m/s. The
kinetic energy (energy of motion) of the rock when it hits the ground is
the 0.5 x mass x speed2. If I measure the mass in
kg and the speed in m/s, the energy of the falling rock will be
in Joules. The rock hits the ground kinetic energy of ~50 J.
- A slow moving mosquito (1 gram) has an energy of about 10-7 J
===> a pack of 10 million mosquitos has an energy of ~ 1 J.
Question: Are there more useful yardsticks for the
power of the Sun?
Answer:
Yes, in 1972 the US consumed 1.7 trillion kW-hours of energy =
6x1018 J (with a fairly
large increase over the last 30 years--note one BTU = 1,055 Joules).
By 2002, the U.S. consumed energy
at the rate
~ 3.4x109 kW. The Sun produces power at the rate
4x1023 kW; the Sun would not notice us stealing energy.
This, however, is not the complete picture.
Question: How much of this energy is available to us on the Earth?
Answer:
Since we are 93,000,000 miles from the Sun, we
cannot utilize all of the energy produced by the Sun.
Because of our great distance from the Sun, we
intercept only a small fraction of
the Sun's total energy output. At the orbit of the Earth, we
intercept only 0.46
billion-ths of the Solar output. The energy flow from the Sun falls off
with an inverse
square law dependence. At the top of our atmosphere, the flow
amounts to ~1,365 Watts per square meter (W/m2).
A flow of energy
per unit area is referred to as an energy flux
. The energy flux from the Sun at the Earth's orbit
is called the Solar Constant.
The amount of energy that actually reaches the
surface of the Earth is smaller. The
amount that doesn't penetrate the atmosphere and cloud layer and
which is thus prevented from reaching the ground is
variable--it ranges from 30 % to more than 50 %. The
globally averaged flux that reaches the surface of the Earth
is ~ 341 W/m2 of which 31 % is further reflected.
In the absence of an atmosphere, this energy input would heat
the surface of the Earth
to only 250 Kelvin (~ -23 Celsius or ~ -9 F which is well
below the freezing point of water at standard temperature and pressure).
However, the heated surface produces infrared
radiation which is absorbed in the lower atmosphere
by
Greenhouse gases. This absorption with its
subequent remission process traps some of the heat
raising the temperature in the lower atmosphere
of the Earth to roughly 288 K (~60 F, a nice
comfortable temperature for life as we know it).
Although 341 W/m2
does not sound like a large flux of energy,
if we could tap it efficiently we would easily
satisfy our energy needs. (Question:
If we had a Solar collector that was
100 % efficient, how large would it need to be to acquire enough energy to
satisfy the energy needs of the U.S.?)
Question: How does the Sun produce energy?
Answer:
This question was a mystery for many years given that the Earth (and
therefore the Sun) is very old and the Sun is very bright. The
combination of these two things means that the Sun must have a large
store of energy. The understanding of the energy source for the
Sun (and hence other stars) relied on the work of Einstein
in the early 1900's. The pioneering workers
(e.g., R. d'E. Atkinson in the 1920's)
realized that if the Sun generated energy through
nuclear fusion
(
fusing hydrogen
into helium) then it had ample energy to radiate at
its current rate for around
10 billion years, easily long enough to
explain lifetime of the Solar
System. We return to this important topic later.
Other sources of energy such as fossil fuel burning (chemical burning),
gravity, stored internal heat are just not efficient enough to have
powered the Sun from 4.6 billion years (the lifetime of the Solar System).
Solar-Terrestrial Connections
The Sun is a variable star. The Sun shows
shows variability on time scales ranging from
milliseconds to billions of years. As an example (because of its possible
effects on short-term global
climatic changes) we consider what is known as the
Solar Activity Cycle.
The principal observable manifestation of the Solar Activity Cycle is
the variable number of
sunpots visible on the surface of the Sun.
As discovered in 1843 by Heinrich
Schwabe, a German apothecary, the number of Sunspots varies roughly
periodically with a period of 11 years (the cycle based on other markers
suggests that the period is actually 22 years). There are other features
of the Sun that vary with the number of sunspots
e.g., prominences (quiescent and
active), faculae, and
coronal activity.
Although, it is clear that the appearance (activity) of the Sun varies
with strongly with the Solar Activity Cycle, the actual
change in the luminosity of the Sun is small, less than 1 %.
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Based on the historical record, it has been shown that the Sunspot Cycle
can stop, that is, there are times when there are no visible Sunspots.
There have been several such intervals in recent history. The most
prominent one is referred to as the
Maunder Minimum
which occured
in the 1600's. The interesting thing is that during this time Europe
experienced what is known as the Little Ice Age when the average
temperature was depressed by over a degree. There is a clear indication
that the varying activity of the Sun affects the overall climate of the
Earth. The annoying thing is that the
changes in the luminosity of the Sun
are small. The challenge is to figure out how small changes in the Sun
can lead to global climatic changes. |
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*
The IAU therefore resolves that planets and other
bodies in our Solar System be defined into three distinct categories
in the following way:
- A planet is a celestial body that (a) is in
orbit around the Sun, (b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape, and (c) has cleared the neighbourhood around its
orbit.
- A dwarf planet is a celestial body that
(a) is in orbit around the Sun, (b) has sufficient mass for
its self-gravity to overcome rigid body forces so that it assumes
a hydrostatic equilibrium (nearly round) shape, (c) has not
cleared the neighbourhood around its
orbit, and (d) is not a satellite.
- All other objects except satellites orbiting the Sun shall be
referred to collectively as Small Solar-System
Bodies