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Sun
The Sun has been burning
for about 5 billion (5,000,000,000) years. Our Galaxy is more than 10 billion
years old, and new stars are forming all the time, so our Sun is neither young
nor old, and is almost right in the middle of stellar ages. -- Dr. Eric
Christian, NASA
New
Views of the Sun
An image of the Sun taken in ultraviolet light reveals gas at 1.5 million
degrees Celsius shaped by magnetic fields. The Solar and Heliospheric
Observatory (SOHO), launched in late 1995, is a spacecraft that is increasing
our understanding of the Sun and Size:
30K
Surprises
from SOHO include tornadoes on the Sun
British scientists discovered the solar tornadoes in images and data from
SOHO's scanning spectrometer CDS. One of SOHO's main tasks is to trace the
sources of the wind from the Sun that pervades the Solar System. The newly
discovered tornadoes may cont Size:
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Cosmic
and Heliospheric Learning Center -- The Sun
The chromosphere and corona can only be seen during solar eclipses, or
with instruments that simulate a solar eclipse. In one of LASCO's images of
activity on the Sun, shown here, blobs of plasma (the solar wind stream) can be
seen to be emitted from the Size:
15K
Ask
the Space Scientist about : The Sun
Why is a magnetic field 'frozen into' the solar wind? Would the solar
wind knock a planet off its orbit if the planet didn't have a magnetic field? Do
all stars produce solar winds?
Size: 10K
NASA: Ask A Physicist
Sun
To learn a lot more about the Sun, check out the Cosmic
and Heliospheric Learning Center's page
on the Sun.
Is it possible to see sunspots with the naked eye?
Please DON'T EVER LOOK DIRECTLY AT THE SUN! It
will cause permanent eye damage.
There's a web
site at Ohio State University that will give you more information
about solar observing. Section Five on this site may answer some of your
questions about sunspots, if you have others. Of course you can always
check out our
page on sunspots.
One solar mass is just the mass of our Sun, 2 x 1030
kg. It's used for convenience, because saying that a star is 4 solar
masses (4 times the mass of our Sun) is easier to visualize than saying it
has a mass of 8 x 1030 kg.
Are there more different kinds of gases on the Sun
than on any of the planets?
There is more hydrogen and helium in the Sun than
there is on Earth because the Earth's gravity is not strong enough to keep
all the hydrogen and helium gas from escaping. The gas giant planets
(Jupiter, Saturn, Uranus, and Neptune) are big enough to hold on to all
that gas and are made up of the same stuff the Sun is. The biggest
difference between the gas of the Sun and the planets is that the Sun is
in a plasma
state, where the electrons are separated from the gas nuclei.
I am curious to know why the Sun shines. I understand
some solar rays are x-rays but isn't the Sun much too cool to produce
these?
The Sun gives off light and heat because it is
essentially a giant nuclear reactor that is fusing (burning) hydrogen into
helium inside. When hydrogen combines to form helium, it gives off energy.
Fusion is a very efficient way of converting mass to energy (light and
heat); only a very, very, very tiny amount of the Sun is used up. There's
a good Web page at the
University of Oregon.
Most of the visible light from the Sun comes just from the
fact that it is hot. The surface (what we see) is about 5800 Kelvin. The
center is over 15,000,000 Kelvin. A few of the thermal photons extend up
into the x-rays, but most of the x-rays and gamma rays come from nuclear
reactions that are taking place in the Sun (the fusion that powers the Sun
is a nuclear reaction, and there are lots of peripheral reactions going
on).
Incidentally, the Sun also releases particles continuously
in the solar
wind, and even more so during solar
activity.
I heard at one time the amount of matter that is
converted by the Sun into energy and released, but have been unable to
remember the quantity stated. It was given as the number of Earth masses
that are converted every month or year.
The Sun consumes about 600 million tons of
hydrogen per second. (That's 6 x 108 tons.) For comparison, the
mass of the Earth is about 1.35 x 1021 tons. This would mean
the Sun consumes the mass of the Earth in about 70,000 years.
I have read the following expression, related to the
Sun's photosphere: "Below the photosphere the solar gas is opaque.
This opacity is primarily due to a small concentration of negative
hydrogen ions in the region immediately below the photosphere." I
want to know what "negative hydrogen ions" are. I can not
imagine a negative hydrogen ion, and I do not understand how it can be
produced.
Well, I'm not an expert on stellar interiors, but
it is possible for a proton to attract two electrons. Although a
proton-electron pair is electrically neutral, close in it has an electric
field, due to the fact that the electron and proton are not at the same
location. What I think your quote is referring to, however, is what is
also called the "Free-Free" absorption of light. A free electron
can scatter light, but can't absorb the photon, due to conservation of
energy and momentum. In the presence of something else (such as a hydrogen
atom), the electron can absorb the photon. Since the electron has to be
close (within the electric field) of the hydrogen atom, you could consider
this to be a negative hydrogen atom, but the electron is usually unbound
(on a hyperbolic orbit). But the free-free absorption is an important
component to stellar opacity.
On a PBS science show I heard a statement made about
the length of time it took for light to go from the center of the Sun to
its outer edge. The time was in thousands of years, which does not sound
plausible, but someone else on the show confirmed the statement. Can you
provide any information about this?
Yes, it does take light thousands of years to get
out of the Sun. The important thing to realize is that the Sun (especially
at the center) is quite opaque, that is, light travels through it only
slightly better than light travels through a rock. What happens is that
light only travels a short distance before it is absorbed. It is then
re-emitted, but in a random direction. It eventually random
"walks" it's way out of the Sun, but that takes a long time.
If the Sun is as old as it is and it still has 99.9%
of its energy left, then what is the potential relative life span of the
Sun remaining? What will be left at 15,000 A.D.?
Our Sun is about halfway through the "main
sequence" part of its life. During this part, the Sun
"burns" hydrogen into helium (fusion), which is what generates
the heat and light. The Sun has been doing this for about 5 billion years,
so in 13,000 years (15,000 A.D.) there will be no real difference from the
energy left now. In about 5 billion more years, the useable hydrogen (not
all the hydrogen) will have been converted to helium, and the Sun will
start burning helium, and become a red giant. After that the Sun will
recollapse down to a white dwarf and last for billions of years more.
How does the Sun last so long? Could the Sun ever
crash into a planet, and that planet crash into another planet, until
there are no more planets left? How big could the Sun get?
I'll answer these questions together. The Sun
lasts so long and gives off so much light because it is a giant nuclear
reactor that is fusing (burning) hydrogen gas into helium gas. (see Why
the Sun Shines) Eventually (more than 5 billion years from now) the
Sun will use up most of its hydrogen and will get larger and cooler. At
this time, Mercury and Venus, and possibly the Earth, will be swallowed up
by the Sun. But the Sun will never get large enough (hundreds of millions
of miles in radius) to "crash" into the outer planets.
The Sun is shrinking at five feet per year.
Considering the temperature of the Sun, how would the average temperature
of the Earth be affected by increasing the size of the Sun by 5 million
feet per million years? If we went back say, one million, ten million, or
a hundred million years?
It is incorrect to say that the Sun is shrinking
and it has been since the "creation" of the Universe. The Sun is
not shrinking at a consistent rate. The data that were used to derive that
were both wrong and misinterpreted. See http://www.skepticfriends.org/hoax1.htm.
I am a university undergraduate trying to understand
the nature of the extremely high temperatures in the corona. I think it is
due to b-fields interacting with plasma. Am I right or on the wrong path
altogether?
Why the corona is so hot, when the region below it
is several orders of magnitude cooler, is one of the open questions in
solar physics. Magnetic fields and turbulence in the plasma are certainly
involved, but the exact mechanism is not understood. One suggestion is
that large numbers of "microflares" are the cause. NASA is
developing a mission that should study this problem (and others) called Solar
Probe.
How long has the sun been burning? Compared to other
stars, is our Sun young or old?
The Sun has been burning for about 5 billion
(5,000,000,000) years. Our Galaxy is more than 10 billion years old, and
new stars are forming all the time, so our Sun is neither young nor old,
and is almost right in the middle of stellar ages.
The Sun's output is 3.8 x 1033
ergs/second, or about 5 x 1023 horsepower. How much is that? It
is enough energy to melt a bridge of ice 2 miles wide, 1 mile thick, and
extending the entire way from the Earth to the Sun, in one second.
The solar constant is the amount of energy from
the Sun at the distance of the Earth (outside the atmosphere). It is 1367
Watts per meter squared. It is not really constant; it varies by less than
a percent due to solar activity.
Does anyone keep tract of the Sun's radiant, or heat
energy output as a function of time? Most global warming models I have
looked into use the Sun's energy output as a constant or a sine wave
cycle. One scientist informed me that the Sun's energy output has
increased over the past 100 years, but I have not seen any data to support
that contention. I would like to see a plot of the Sun's heat energy
output over the past 100 years.
"...the sun's output changes so slowly and solar
variability is so slight (less than 0.00425% of the total energy per
year on time scales of days), that continuous monitoring by
state-of-the-art instrumentation is necessary to detect changes with
climate significance. Scientists theorize that as much as 25% of the
20th century anticipated global warming of the Earth may be due to
changes in the sun's energy output. Systematic changes in irradiance as
little as 0.25% per century can cause the complete range of climate
variations that have occurred in the past, ranging from ice ages to
global tropical conditions. For example, scientists believe the
"Little Ice Age" that occurred in Europe in the late 17th
century could have been related to the minimum in sunspot activity (and
a correlated minimum in total solar irradiance) that occurred during the
same period."
This page includes a graph of total solar irradiance from
1980 to 1996, measured by the ACRIMSATs. The changes in energy shown there
are caused by the solar sunspot
cycle. There's another graph of total solar irradiance at the
Athena web site.
How long does it take heat created on the Sun's
surface to reach Earth? Is it the same as the speed of light?
Heat is transmitted through conduction,
convection, and radiation. The heat that reaches us from the Sun is
infrared radiation, which travels at the speed of light. So, it takes
about 8 minutes for it to reach Earth from the Sun.
You might be interested to know that some of your
questions about sunspots -- how are they formed and what causes them --
are the same ones that scientists are now trying to answer.
I'm still confused as to the difference between solar
flares and solar prominences. I've seen photos of these hugh solar loops,
which are sometimes identified as prominences and sometimes as flares.
Prominences are big loops of hot gas (plasma)
trapped by magnetic field lines. Flares are sudden increases in brightness
(not necessarily in the visible part of the spectrum) in a region. Here's
another web site in our lab that discusses this.
How many different solar cycles are there (for
example, sunspots)? When is the next maximum of the solar cycle? With the
advent of global (warming?), are there other cycles that we do not know
about?
The Sun is a variable star, and its major cycle is
the 11-year/22-year sunspot cycle. I say 11-year/22-year because, although
there is a maximum in solar activity and sunspot numbers every 11 years,
it is caused by a 22-year cycle of magnetic field flipping in the Sun.
There are just two maximum activity time periods every 22 year cycle. The
next solar maximum is approaching now, and should peak about 2002.
There are other changes in the solar activity that either
are irregular, or humans haven't been monitoring the Sun long enough to
see the cycles (we have complete coverage of sunspot numbers since the mid
1700's, and this is a pretty good measure of solar activity). For example,
there was a decrease in solar output in the early 1800's (known as the
"Maunder minimum"), which could be seen in the sunspot number
and caused a "mini" Ice Age in Europe.
I just read an article stating that scientists have
just "had their first glimpse of the Sun's hidden half". We
orbit around the Sun, and it takes us 365 days to do so, so how is it that
we have never seen the other side of the Sun?
Scientists on the SOHO spacecraft have
learned how to see some of what is happening on the far side of the Sun,
even though the spacecraft is on the same side as the Earth is. The Sun
rotates every 26 - 28 days (since the Sun isn't solid, different parts
rotate at different speeds), so we would see the hidden half in 13-14
days, but this new data gives us a way to see active areas (sunspots)
before they rotate around to the side we can see. The article is here.
The planets rotate and circle the Sun. Does the Sun
rotate and circle something larger?
Yes, the Sun is only one of many stars in our
galaxy, the Milky Way. It is located in one of the spiral arms about
30,000 light years from the center. It moves at a speed of 200 - 300
km/sec in its orbit around the Galactic nucleus, and takes roughly 200
million of our years to make one orbit of the Galaxy, or one
"Galactic year".
I've noticed for the last few years that the Sun seems
to be further north than it used to be for the corresponding time of year
in previous years. I live near Buffalo, New York. Might there be some
explanation for my observations?
There is some wobble of the Earth's axis of
rotation (called precession), but it is not noticible over just a few
years. The Sun appears pretty much in the same place at the same time of
the year.
The "Science Goals" link on the ACE
home page includes several goals about determining the difference in
composition between the Sun's corona and photosphere, but doesn't mention
the chromosphere. Why?
Since ACE measures only particles, and the
particles that come from the chromosphere have to "pass through"
the corona before they get out, there is essentially no way to separately
get the composition of the chromosphere. Photospheric particles are given
off in solar flares, so we can look at them.
Where can I get information on solar wind and the
solar corona?
At a college level, your best bet is the college
library. At a lower level, middle or high school, start with the heliosphere
in our web site (click on the Sun and Solar Wind to learn more). And these
are a few other sites that might be of help to you at the same level:
I am studying solar wind and have a few questions --
How does solar wind vary with time? How does the intensity of solar wind
vary? How does the solar wind affect the Earth?
The density, temperature, and velocity of the
solar wind all vary with time in a pretty complicated way. The magnetic
field associated with the solar wind also varies in amplitude and
direction. The ACE
spacecraft, currently at the Earth-Sun libration point (L1) a million
miles "upstream" of Earth in the solar wind, measures all of
these quantities. You can check ACE
Browse Data and ACE
Real-Time Solar Wind sites for plots of the solar wind parameters
(look at MAG and SWEPAM data).
The normal solar wind doesn't have much effect on the
Earth (it's deflected by the Earth's magnetic field), but bursts of plasma
and magnetic field, called coronal
mass ejections (CMEs), that travel with the solar wind from active
regions on the Sun, can cause "geomagnetic storms", which is
what NOAA is trying to predict with ACE. See the NOAA (ACE Real-Time Solar
Wind) page above for more about these.
What is the amount of erosion caused by the solar
winds, and how much is recovered in the form of meteorites?
If you are asking about erosion of the Earth's
surface, the solar wind doesn't really make it through the atmosphere or
the Earth's magnetic field. Even on the moon, which has no atmosphere or
magnetic field, the solar wind doesn't knock atoms off the surface fast
enough to escape the moon's gravity, so there isn't any lunar erosion
either. The Earth does lose some of the gas in its atmosphere just by
random diffusion away from the Earth, but it is not as much as the
approximately one ton per hour that the Earth gains from micrometeorites.
How far is it from the Sun to the edge of the
heliosphere (on average)?
It's not precisely known, since we haven't gotten
a spacecraft out that far yet. The heliopause, which is the boundary
between the gas from the Sun and the gas of interstellar space, is
probably between 150 and 300 AU out (150 to 300 times the distance from
the Earth to the Sun) in the direction that the Sun is traveling. There is
a tail in the other direction (just like the Earth's magnetosphere)
that extends furthur.
I had a discussion with a friend about how the Earth
revolves around the Sun. He stated that the Sun does not revolve around
anything. I disagreed. If our moon revolves around us, and we revolve
around the Sun, I don't see why it would stop there. Does our Sun or our
Galaxy revolve around something? Are all objects in space in some kind of
motion?
The Sun (and all the stars in the Milky Way
galaxy) revolves about the center of the Galaxy. It takes about 200
million years to go around once. The Milky Way is also moving relative to
the local group of galaxies. Gravity works even across extremely large
distances. Pretty much everything in the Universe is moving, due to
gravity and the initial velocity obtained in the Big Bang.
If there were a highway from the Earth to the Sun, how
long would it take to get to the Sun, driving at 65 miles per hour?
If the highway is straight, and you drive non-stop
24 hours a day with no meal or bathroom breaks, it should take 163 years
and 120 days to get to the Sun from the Earth:
93,000,000 miles/65 mph = 1,430,769 hours
1,430,769/24 hours in a day = 59,615 days
59,615/365 days in a year = 163 years and 120 days
If you figure out how to do this with current gas prices,
please let us know!
Page last updated: 8/29/00
Web curator: Beth Jacob, Raytheon ITSS
beth@gamma.gsfc.nasa.gov
Responsible NASA Official: Tycho von Rosenvinge NASA website
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