Transcript ppt document - FacStaff Home Page for CBU

Part 4: The Stars
In Part 3 we looked at the Solar System, that
is, we looked at all the parts except for the
central element: the sun.
In Part 4 we are going to look at stars. Since
the sun is the closest star, we will now look
at the sun, and then we will look at the other
stars.
The Sun
This image was
taken by SOHO's
EIT (ExtremeUltraviolet Imaging
Telescope) and is
courtesy of the EIT
Consortium.
The Sun: basic facts
The sun is 93 million miles (150 million km)
away from the earth, and makes an angle of
0.5o at the earth. This means that the sun
has a diameter of about 800,000 miles
(1,300,000 million km) – which is about
1/100 of the earth-sun distance (AU), and
is about 100 times bigger (in diameter)
than the earth.
The Sun: its structure
The sun does seem to have a definite
edge, or surface. We call this
Corona visible during a solar
“surface” the photosphere.
eclipse – Marshal Space Flight
Center.
During solar eclipses, we see that the sun
does seem to have an “atmosphere” above the
“surface” (photosphere) of the sun. Although this
atmosphere is very dim relative to the surface of
the sun, it is bright compared to the blackness of
space. This visible atmosphere of the sun is
called the corona. It can extend as far as another
radius of the sun.
For an image, see:
http://sunearthday.nasa.gov/2006/images/gal_045.jpg
The Sun: its structure
When we look more closely at the photosphere
(“surface”), we see relatively small spikes on the
photosphere that measure about 400 miles wide
and 4,000 miles high (small compared to the diameter
of the sun of 800,000 miles). This is called the
chromosphere and is considered part of the sun’s
atmosphere along with the corona.
See the web site: http://www.nasa.gov/mission_pages/solar-b/solar_019.html
Surface Temperature of the Sun
As we saw in Part 2, when we look at the
visible spectra of the sun, we see that it’s
intensity peaks at about 500 nm (green
light). From the equation:
 = b/T (where b = 2.9 x 10-3m*K)
we get: T = b/ = (2.9 x 10-3m*K) / 500 x 10-9m
 6,000 K  10,000 oF .
Power output of the sun
From the relation:
P = AT4 where  = 5.67 x 10-8 m2 *K4
and the size of the sun (radius = 700,000 km =
7 x 108 m; A = area = 4r2 = 6 x 1018m2),
we get: P = (1) * (5.67 x 10-8 m2*K4) *
(6x1018m2) * (6000K)4  4 x 1026 Watts.
Intensity of sunlight at the earth’s
orbit
At the earth’s distance from the sun
(93 million miles = 1.5 x 1011m),
the intensity of sunlight we receive is about
I = P/A = (4 x 1026Watts)/(4**[1.5x1011m]2)
 1600 W/m2.
However, due to reflection off the atmosphere and
due to day/night and slanting angle of sunlight
during the day, the average intensity striking the
earth is only about 250 W/m2.
Sunspots
Images from NASA Marshal Space Flight Center
http://solarscience.msfc.nasa.gov/surface.shtml
Sunspots
When Galileo viewed the sun with the aid of a
telescope, he noticed there were spots on the sun’s
surface. Like the corona, sunspots are dark when
compared to the very bright normal surface of the
sun (at 6,000 K or 10,000 oF), but actually they are
bright compared to the dark of space – the “dark”
areas are still at 4,500 K).
For images, see:
http://www.nso.edu/current_images.html
http://sohowww.nascom.nasa.gov/sunspots/
Sunspots – cont
While the sunspots themselves are
Solar Flair
Marshal Space Flight Center
relatively cool, the areas around
sunspots are very active with the
result that when there are more sunspots there is
generally a little more power being put out by the
sun. Solar flares often are seen in these areas
around sunspots. Solar flares last about 20
minutes on average, and extend far out into the
corona. They emit significant amounts of energy
in all the various types of light from radio to xrays. See:
http://solarscience.msfc.nasa.gov/flares.shtml
The Sun’s Rotation
Sunspots with their associated flares rotate
with the sun’s photosphere. Unlike solid
objects, the sun is a gaseous object, and the
rotation of the sun is different at different
places. The equator of the sun rotates
(counterclockwise as viewed from above the North pole
like almost all of the rest of the solar system) with a
period of about 26 days at the equator and
up to about 30 days at higher latitudes.
Sunspot cycle
The number and location of sunspots varies over
time in more or less a periodic fashion. Sunspots
start appearing around 30o latitude, reach a
maximum in number appearing at lower latitude,
then start decreasing in number as they start
forming near the sun’s equator. The next cycle of
sunspot activity starts before the old cycle is
finished. This cycle lasts about 11 years on
average, with the maximum number of sunspots
being between 100 and 200 per year, and the
minimum number being between 0 and 15 a year.
See: http://solarscience.msfc.nasa.gov/SunspotCycle.shtml
Sunspot activity
Sunspots seem to be associated with magnetic
fields. Sunspots usually appear in pairs,
with one spot associated with a North
magnetic pole, and the other with a South
magnetic pole.
Actually the sun’s overall magnetic field flips
each cycle, and so actually the full sunspot
cycle is actually composed of two 11 year
cycles (on average) for an overall period of
about 22 years.
Sunspot Activity
Because solar output and solar flares are associated
with sunspot activity, the sunspot cycle is
important. Solar activity and solar flares in
particular affect power transmission and
communications, and may even influence the
earth’s weather.
It appears that there were few if any sunspots for
about a 70 year period from 1645 – 1715 (sunspots
were first observed in 1610 by Galileo and others), and
this corresponds to the “Little Ice Age” in Europe
and to an extended drought in the southwestern
US. There are other possibilities for the “Little Ice
Age”, but this is one of them.
The Solar Wind
The solar wind is a mixture of ions and
electrons that are streaming out from
the sun. This wind decreases in
density as it moves away from the sun,
but it extends at least all the way to
Neptune (as measured by our satellites).
See the web page at:
http://solarscience.msfc.nasa.gov/
Solar Energy – what is the fuel?
We’ve looked at the surface and the
atmosphere, but what is the source of the
sun’s energy (its fuel), and how does it
“burn” the fuel?
Source of the sun’s energy
a) Gravity is an energy source. As things fall
down, the gravitational energy can be
converted to other forms like kinetic energy,
heat, and light. We use gravitational energy
when we build a dam and store water above
the dam. We then convert the gravitational
energy into electrical energy using turbines.
Source of the sun’s energy
a) Gravity – Using the sun’s gravity as a
source and knowing the present energy
output of the sun, we figure the sun could
only last a few million years using gravity
as its source.
Source of the sun’s energy
b) “burning” = chemical energy. We burn
fossil fuels like coal and oil to generate heat
that can be converted into other forms of
energy like kinetic energy (of cars and
trucks) and steam which can drive a turbine
to generate electrical energy.
Source of the sun’s energy
b) Chemical energy – If all of the sun were
carbon and oxygen, the sun would only last
a few thousand years burning this fuel into
carbon dioxide. Note: we do NOT see lots
of carbon, oxygen, and carbon dioxide in
the sun. It is mainly hydrogen with a little
helium and very small amounts of other
atoms.
Source of the sun’s energy
c) Nuclear energy – either fission or fusion.
c-1) nuclear fission (atomic bombs and
nuclear reactors use this) converts very
heavy atoms like uranium and plutonium
into mid-size atoms and releases a
tremendous amount of energy.
Source of the sun’s energy
c-1) Nuclear fission – we do NOT see any
significant amounts of uranium or
plutonium in the sun and we do NOT see
any significant amounts of the mid-size
atoms that should be the “ashes”.
Source of the sun’s energy
c-2) Nuclear fusion. This energy comes
from combining light elements into slightly
heavier elements (like hydogen into helium,
or helium into carbon). This is the source of
the extra energy in a hydrogen bomb.
We do NOT have any fusion reactors, but we
are working on them. The main problem is
that it takes a fairly high density of atoms
raised up to about a million degrees to get
this process going.
Source of the sun’s energy
c-2) Nuclear fusion – This energy source
could power the sun for 100 billion years if
all of the hydrogen were converted into
helium. We DO see lots of hydrogen and
some helium in the sun. The sun’s gravity
would keep the density high, and the sun’s
gravity would also provide the initial energy
to ignite the process.
We think this is the fuel that the sun is
using right now.
The sun’s interior
Our present view of the sun is that nuclear fusion
(converting hydrogen into helium) is occurring at
the sun’s core where it is extremely hot (on the
order of 15 million Kelvin or 28 million oF) and
the gases are very dense – due to gravity.
The energy initially leaves the core in the form of
radiation, so the layer that surrounds the core is
called the radiative zone.
The sun’s interior
As the radiation moves out from the core
through the radiative zone, it essentially
spreads out and is converted into heat – the
kinetic energy of the atoms. The layer
where this occurs, above the radiative zone,
is called the convective zone. The heat is
then “convected” onto the surface of the
sun.
The sun’s interior
At the surface, the photosphere, the heat then
radiates the energy into space.
Magnetic fields associated with the hot atoms
(actually ions since many electrons have
been stripped off due to the heat – this is
called a plasma) then cause the surface of
the sun to erupt in the flares we talked about
earlier.