Download The Sun Notes

ASTRONOMY
THE SUN
Our Star: The Sun
MURCHISON
1) By studying the Sun, we not only learn about the properties of a particular star, but also can study
processes that undoubtedly take place in more distant stars as well.
2) The quite Sun is a description of the solar phenomena that typically appear every day.
3) The active Sun is a description of solar phenomena that appear non-uniformly on the Sun and vary over
time.
a) The Sun is entering a period of maximum solar activity
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Basic Structure of the Sun
1) The Sun is thought of as a bright ball of gas that appears to travel across our sky every day. In reality we
see only one layer of the Sun, the photosphere, which is part of the Sun’s atmosphere.
a) photosphere simply means the sphere from which light comes (from the Greek photos – light)
b) The photosphere is about 110 Earth diameters across, so over 1 million Earths could fit inside
the Sun
2) About 94 percent of the atoms and nuclei in the outer parts are hydrogen , about 5.9 percent are
helium, and a mixture of all other elements makes up the remaining 1/10th of a percent
a) The overall composition of the Sun’s interior is not very different
3) The Sun is a typical star in the sense that stars much hotter and much cooler, and stars intrinsically much
brighter and much fainter, exist. Radiation from the photosphere peaks (and is strongest) in the middle
of the visible spectrum.
a) Remember that our eyes evolved over time to be sensitive to that region of the spectrum
because the greatest amount of solar radiation is emitted there
4) The Sun isn’t yellow, though it is thought of being that color
a) The Sun appears yellow (or orange or red when it is close to the horizon): The blue and green
light is selectively absorbed and scattered by Earth’s atmosphere
b) The scattered blue light produces the color of the daytime sky.
c) Also, on many photographs of the Sun, the filter used to cut down the solar brightness to a safe
levels favors the yellow
5) Beneath the photosphere is the solar interior.
a) All the solar energy is generated there at the solar core which is about 10 percent of the solar
diameter at this stage in the Sun’s life
b) The temperature at the core is about 15,000,000 K
6) The photosphere is the lowest level of the solar atmosphere
a) Though the Sun is gaseous throughout with no solid parts, the term atmosphere is still used for
the upper part of the solar material because it is relatively transparent
7) Just above the photosphere is a jagged, spiky layer about 10,000 K thick, only about 1.5 per cent of the
solar radius called the chromosphere (from the Greek chromos – color)
a) This layer glows colorfully pinkish when seen during an eclipse, when the photosphere is hidden
b) Above the chromosphere a ghostly white halo called the corona (from Latin – crown) extends
tens of millions of kilometers into space and in this form is called the solar wind. The Earth is
bathed in solar wind
MURCHISON
The Photosphere
1) The Sun is a normal star with a surface temperature of about 5800 K, neither the hottest nor the coolest
star.
2) When we study the solar surface in white light – all the visible radiation taken together – with typical
good resolution, a salt-and- pepper texture called granulation can be seen
a) The effect is similar to that seen in boiling liquids on Earth
3) On close examination, the photosphere as a whole oscillates – vibrating up and down slightly
a) Astronomers have realized that the Sun vibrates with many different periods, and that studying
these periods tells us about the solar interior.
b) This method works similarly to the way terrestrial geologists investigate the Earth’s interior by
measuring seismic waves on the Earth’s surface; the studies on the Sun are called by analogy,
“solar seismology
c) To study long periods of solar seismology, astronomers have observed the Sun from the south
pole where the Sun stays above the horizon for months on end.
i) In addition they use the Global Oscillation Network Group (GONG), a program centered at
the National Solar Observatory that has erected a network of telescopes around the world
to study solar oscillations.
ii) In addition, a NASA/European Space Agency spacecraft called SOHO (the Solar and
Heliospheric Observatory) is stationed in space at a location from which it continuously
views the Sun and has also assembled long runs of observations lasting many months.
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d) Studies of solar vibrations thus far have told us about the temperature and density at various
levels in the solar interior, and about how fast the interior rotates
4) The spectrum of the solar photosphere, like that of almost all stars, is a continuous spectrum crossed by
absorption lines
a) Hundreds of thousands of these absorption lines, which are also called Fraunhofer lines, have
been photographed and catalogued.
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The Chromosphere
1) During total solar eclipses, when first the photosphere and then the chromosphere are completely
hidden from view; a faint white halo around the Sun becomes visible.
2) This corona is the outermost part of the solar atmosphere and technically extends throughout the Solar
System.
3) At the lowest levels, the corona’s temperature is about 2,000,000 K.
4) Even though the temperature of the corona is high, the actual amount of energy in the solar corona is
not large.
a) The temperature quoted is actually a measure of how fast individual particles (electrons) are
moving.
b) There aren’t very many coronal particles, even though each particle has a high speed.
c) The corona has less than one-billionth the density of the Earth’s atmosphere and would be
considered a very good vacuum in a laboratory on Earth
d) For this reason the corona serves as a unique and valuable celestial laboratory in which gaseous
plasmas are studied in a near-vacuum
i) Plasmas are gases consisting of positively and negatively charged particles and can be
shaped by magnetic fields
ii) Scientists on Earth are trying to learn how to use magnetic fields to control plasmas in order
to provide energy through nuclear fusion
5) Photographs of the corona show that it is very irregular in form.
6) Beautiful long streamers extend away from the Sun in the equatorial regions.
7) The shape of the corona varies continuously and is thus different at each successive eclipse.
8) The structure of the corona is maintained by the magnetic field of the Sun
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b) They represent sets of spectral lines from most of the chemical elements.
i) Iron has many lines in the spectrum; the hydrogen lines are strong but few in number
c) From the spectral lines we can figure out the percentages of the elements
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a) The TRACE (Transition Region and Coronal Explorer) spacecraft makes extremely high-resolution
observations of the solar corona by observing in the ultraviolet. It shows clearly how the corona
is made up of loops of gas, which are held in their shapes by the Sun’s magnetic field.
9) The corona is normally too faint to be seen except at an eclipse of the Sun because it is fainter than the
everyday blue sky.
a) But at certain locations on mountain peaks on the surface of the Earth, the sky is especially clear
and dust free and the innermost part of the corona can be seen with special telescopes.
10) Among the major conclusions of the research is that the corona is much more dynamic than we had
thought.
a) For example, many blobs of matter were seen to be ejected from the corona into interplanetary
space, one per day or so.
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b) These coronal mass ejections (CME) sometimes even impact the Earth, causing surges in power
lines and zapping – even occasionally destroying – satellites that transmit television or
telephone calls.
c) SOHO and other spacecraft far above the Earth in the direction of the Sun give us an early
warning when solar particles pass them.
d) SOHO web-page: http://www.nasa.gov/mission_pages/soho/index.html
11) The visible region of the coronal spectrum, when observed at eclipses, shows continuous radiation,
absorption lines, and emission lines.
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On the sun, coronal mass ejections occur when solar magnetic field lines snake around each other, forming the
letter "S". Usually, they go past each other. But if they connect, it's like a short circuit. The mid-section breaks
loose and drives out a coronal mass ejection. (http://www.nasa.gov/mission_pages/solar-b/solar_005_prt.htm)
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e) The density of gas in those areas is lower than the density in adjacent areas
f) There is usually a coronal hole at one or both of the solar poles – less often there are additional
coronal holes at the lower solar latitudes
i) The regions of the coronal holes seem very different from other parts of the Sun.
ii) The solar wind flows to Earth mainly out of coronal holes, so it is important to study the
coronal holes to study our environment in space.
g) The most detailed x-ray images support the more recent ultraviolet high-resolution images in
showing that most, if not all - the radiation appears in the forms of loops of gas joining separate
points on the solar surface.
i) We must understand the physics of coronal loops in order to understand how the corona is
heated.
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a) The emission lines do not correspond to any normal spectral lines known in laboratories on
Earth or other stars, and from many years their identification was one of the major problems in
solar astronomy.
12) The gas in the corona is so hot that it emits mainly x-rays, photons of high energy.
a) The photosphere on the other hand, is too cool to emit x-rays.
b) As a result, when photographs are taken of the Sun in the x-ray region of the spectrum the show
the corona and its structure. These photographs are taken from satellites since x-rays cannot
pass through Earth’s atmosphere.
c) The x-ray images reveal very dark areas at the Sun’s north pole and extending downward and
across the center of the solar disk.
d) These dark locations are coronal holes – regions of the corona that are particularly cool and
quiet.
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Sunspots and other Solar Activity
1) Many solar phenomena vary with an 11 year cycle, which is called the solar activity cycle.
2) The most obvious are sunspots which appear relatively dark when seen in white light.
3) Sunspots appear dark because they are giving off less radiation per unit area than the photosphere that
surrounds them.
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a) Thus they are relatively cool (about 2000 K cooler than the photosphere) since cooler gas
radiates less than hotter gas.
b) Sunspots are not “dark” - If you could remove a sunspot from the Sun’s surface and place it in
space, it would appear bright against the dark sky; a large one would give off as much light as
the full moon seen from Earth
4) A sunspot includes a very dark central region called the umbra (from the Latin for shadow) – (plural:
umbrae). The umbra is surrounded by a penumbra (plural: penumbrae) which is not as dark
MURCHISON
5) To understand sunspots, we much understand magnetic fields.
a) When iron filings are put near a magnet, the filings show a pattern. The magnet is said to have a
north pole and a south pole and the magnetic field linking them is characterized by what we call
magnetic-field lines
b) The structure seen in the solar corona, including the streamers, shows matter being held by the
Sun’s magnetic field.
6) The strength of the Sun’s magnetic field is revealed in the spectra.
a) George Ellery Hale showed in 1908 that the sunspots are regions of very high magnetic-field
strength of the Sun, thousands of more powerful than the Earth’s magnetic field.
b) Sunspots usually occur in pairs, and often these pairs are part of larger groups. In each pair, one
sunspot will be typical of a north magnetic pole and the other will be typical of a south magnetic
pole.
7) Magnetic fields are able to restrain charged material.
a) The strongest magnetic fields on the Sun occur in sunspots. The magnetic fields in sunspots keep
energy from being carried upward toward the surface.
b) As a result, sunspots are cool and dark.
8) Sunspots were discovered in 1610 independently by Galileo and others.
a) In about 1850, it was realized that the number of sunspots varies with an 11-year cycle – the
sunspot cycle.
b) Every 11-year cycle, the north magnetic pole and south magnetic pole on the Sun reverse; what
had been a north magnetic pole is then a south magnetic pole and vice versa.
c) Therefore it is 22 years before the Sun returns to its original state making the real period of the
solar-activity cycle 22 years
9) Careful studies of the solar-activity cycle are now increasing our understanding of how the Sun affects
the Earth.
a) For many years scientists were skeptical of the idea that solar activity could have a direct effect
on the Earth’s weather, scientists currently seem to be accepting more and more the possibility
of such a relationship.
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b) An extreme test of the interaction may be provided by the interesting probability that there
were essentially no sunspots on the Sun from 1645 to 1715.
i) This period called the Maunder minimum was largely forgotten until recently – an
important conclusion is that the solar-activity cycle may be much less regular than we had
thought
ii) Much of the evidence for the Maunder minimum is indirect and has been challenged, as
has the specific link of the Maunder minimum with the colder climate during that period.
c) Precise measurements made from spacecraft have shown that the total amount of energy
flowing out of the Sun varies slightly, by up to 0.002 (0.2 per cent).
i) On a short time scale, the dips in energy seem to correspond to the existence of large
sunspots – astronomers are attempting to figure out what happens to the blocked energy
ii) On a longer time scale, the effect goes the other way. Spacecraft observations have shown
that as sunspot minimum is reached, the Sun becomes overall slightly fainter.
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2) These solar storms begin over a few seconds and can last for up to four hours.
3) Temperatures in the flare can reach 5 million Kelvins, even hotter than the quiet corona.
4) Ultraviolet and x-rays that are given off reach Earth at the speed of light in 8 minutes and can disrupt
radio communications, because they ionize the upper part of Earth’s atmosphere.
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Flares
1) Violent activity sometimes occurs in the regions around sunspots. Tremendous eruptions called solar
flares can eject particles and emit radiation from all parts of the spectrum into space.
ASTRONOMY
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5) Flare particles that are ejected reach the Earth within a few hours or days and can cause the aurorae
and even surges in power lines that lead to blackouts of electricity.
6) Because of these solar-terrestrial relationships, high priority is placed on understanding solar activity
and being able to predict it
7) No specific theory for explaining the eruption of solar flares is generally accepted – but it is agreed that a
tremendous amount of energy is stored in the solar magnetic fields in sunspot regions. Something
unknown triggers the release of energy
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3) They mark the locations of zero magnetic field lines that separate regions of the magnetic field pointing
in opposite directions.
4) When filaments happen to be on the Sun’s edge, the project into space, often in beautiful shapes –
these are called prominences
a) Prominences can be seen with the eye at solar eclipses and glow pinkish at that time because of
their emission of hydrogen and a few other spectral lines.
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Filaments and Prominences
1) Studies of the solar atmosphere through filters that pass only hydrogen radiation also reveal other types
of solar activity.
2) Dark filaments are seen threading their way for up to 100,000 km across the Sun in the vicinity of
sunspots.
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b) Prominences appear to be composed of matter in a condition of temperature and density
similar to matter in the quiet chromospheres, somewhat hotter than the photosphere.
c) Sometimes prominences can hover above the Sun, supported by magnetic fields, for weeks or
months – other prominences change rapidly.
Eclipses
1) Because the Moon’s orbit around the Earth and the Earth’s orbit around the Sun are not precisely in the
same plane, the Moon usually passes slightly above or below the Earth’s shadow at full moon, and the
Earth usually passes slightly above or below the Moon’s shadow at new moon.
2) Up to seven times a year, full moons or new moons occur when the Moon is at the part of its orbit that
crosses the Earth’s orbital plane. At those times we have a lunar eclipse or a solar eclipse – thus up to
seven eclipses (mostly partial) can occur in a given year
3) At a total lunar eclipse, the Moon lies in the Earth’s full shadow and sunlight is entirely cut off from it
(neglecting atmospheric effects)
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Lunar Eclipse
1) The partial phase, when the Earth’s shadow gradually covers the Moon, usually last over an hour, similar
to the duration of a total solar eclipse
2) But then the total phase of a lunar eclipse, when the Moon is entirely within the Earth’s shadow, can
also last for over an hour, in dramatic contrast with the few minutes of a total solar eclipse.
3) During the total lunar eclipse, the sunlight is not entirely shut off from the Moon – a small amount is
refracted around the edge of the Earth by our atmosphere.
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a) So for anyone on the entire hemisphere of Earth for which the Moon is up, the eclipse is visible.
b) In a total solar eclipse, on the other hand, the alignment of the Moon between the Sun and the
Earth must be precise, and only those people in a narrow band on the surface of the Earth see
the eclipse
c) Therefore it is much rarer for a typical person on Earth to see a total solar eclipse – when the
Moon covers the whole surface of the Sun – than a total lunar eclipse
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MURCHISON
4) Most of the blue light is scattered out during the sunlight’s passage through our atmosphere – this
explains how blue skies are made for people part-way around the globe from places at which the Sun is
high in the sky. The remaining light is reddish, as is the light from the Sun we see at sunset, and this is
the light that falls on the Moon.
a) Therefore, the eclipsed Moon appears reddish
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Solar Eclipses
1) The outer layers of the Sun, known as the corona, are fainter that in the blue sky.
2) A total solar eclipse allows astronomers to study the corona
3) Solar eclipses arise because of a very precise circumstance
a) The Moon is 400 time smaller in diameter than the solar photosphere (the disk of Sun we see
every day) – it is also about 400 times closer to Earth
b) Because of this coincidence, the Sun and the Moon cover almost exactly the same angle in the
sky – about ½o.
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4) Occasionally the Moon passes close enough to the Earth – Sun line that the Moon’s shadow falls upon
the surface of the Earth.
5) The central part of the lunar shadow barely reaches the Earth’s surface. This lunar shadow sweeps
across the Earth’s surface in a band up to 300 km wide. Only observers stationed within this narrow
band can see the total eclipse
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6) From anywhere outside the band of totality, one sees only a partial eclipse.
a) Sometimes the Moon, Sun, and the Earth are not precisely aligned and the darkest part of the
shadow – called the umbra – never hits the Earth.
b) We are then in the intermediate part of the shadow, which is called the penumbra.
c) Only a partial eclipse is visible on Earth under these circumstances
7) TO SEE A PARTIAL ECLIPSE OR THE PARTIAL PHASE OF A TOTAL ECLIPSE, YOU SHOULD NOT LOOK AT
THE SUN EXCEPT THROUGH A SPECIAL FILTER.
a) Alternatively, you can project the image of the Sun with a telescope or a pinhole camera onto a
surface)
b) You need the filter to protect your eyes because the photosphere is visible throughout the
partial phases before and after totality.
c) Its direct image on your retina for an extended time could cause burning and blindness
d) You still need the special filter to watch the final minute of the partial phases. As the partial
phase ends, the bright light of the solar photosphere passing through the valleys on the edge of
the Moon glistens like a series of bright beads, which are called Bailey’s beads.
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Image above: Left: The moon's shadow falls on Earth, as seen from the International Space Station 230 miles
above. Photo Credit: NASA
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e) At that time, the eclipse becomes safe to watch unfiltered, seems so bright that it looks like a
diamond on a ring – the diamond-ring effect
f) The phenomena of the darkening of the sky around you as totality approaches is dramatic
g) For a few seconds, the chromosphere is visible as a pinkish band around the leading edge of the
Moon. Then as totality begins, the corona comes into view.
h) Streamers of gas are seen near the Sun’s equator and finer plumes near its poles. The total
phase may last a few seconds, or it may last as long as about 7 minutes.
i) At the end of the eclipse, the diamond ring appears on the other side of the Sun, followed by
Bailey’s beads and then the more mundane partial phases.
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The Sun and the Theory of Relativity
1) In 1916, Einstein came up with his “general theory of relativity” which explains gravity and also deals
with accelerations.
2) In this theory, we think objects have gravity, but in truth they are simply moving freely in space that is
curved.
3) In fact, Einstein showed that time is a dimension, almost by not quite like the three spatial dimensions –
thus Einstein worked in a four-dimensional space-time.
a) Picture a ball rolling on a golf green. If the green were warped, the ball will seem to curve.
b) If the surface were flattened out, though, or if we could view it from a perspective in which the
surface were flat, we would see that the ball is rolling in a straight line (the shortest path
between two points).
c) This analogy shows the effect of a two-dimensional space (a surface) curved into an extra
dimension.
d) Einstein’s general theory of relativity treats mathematically what happens as a consequence of
the curvature of space-time
4) In Einstein’s mathematical theory, the presence of a mass curves the space, just as you bed’s surface
ceases to be flat when you put your weight on it.
a) Light traveling on Einstein’s curved space tends to fall into the dents, just as a ball rolling on your
bed would fall into its dents.
b) Einstein predicted that light traveling near the Sun would be slightly bent because the Sun’s
gravity must warp space
5) The Sun has been very important for testing some of the predictions of Einstein’s general theory of
relativity. The theory could be checked by three observational tests that depends on the presence of a
large mass like the Sun for experimental verification
6) First Einstein’s theory showed that the closest point to the Sun of Mercury’s elliptical orbit would move
more slightly around the sky over centuries
a) Such movement had already been noted
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The Scientific Value of Eclipses
1) Eclipse observations are a relatively inexpensive way, compared to space research, of observing the
outer layers of the Sun
2) Artificial eclipses made by spacecraft hide not only the photosphere, but also the inner corona
3) For some kinds of observations, space techniques have not yet matched ground-based eclipse
capabilities.
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b) Calculations with Einstein’s theory accounted precisely for the amount of movement.
7) The second test arose from a major new prediction of Einstein’s theory – Light from a star would act as
though it were bent toward the Sun by a very small amount.
a) We on Earth, looking back past the Sun, would see the star from which the light was emitted as
though it were shifted slightly away from the Sun
b) Only a star whose radiation grazed the edge (limb) of the Sun would seem to undergo the full
deflection – the effect diminishes as one considers stars farther away from the solar limb
c) To see the effect, one has to look near the Sun at a time when the stars are visible, and this
could be done only at a total solar eclipse.
i) The effect was verified at the total solar eclipse of 1919. Scientists hailed this as
confirmation of Einstein’s theory, and from the moment of the announcement, Einstein was
recognized by scientists and the general public alike as the world’s greatest scientist.
8) Similar observations have been made at subsequent eclipses, though they are very difficult to make.
a) The effect is constant through the spectrum and the test can now be performed more
accurately by observing how the Sun bends radiation from radio sources, especially quasars.
9) The results agree with Einstein’s theory to within 1 percent, enough to make the competing theories
very unlikely.
a) The effect is well tested throughout the Universe.
10) A third traditional test of general relativity was to verify the prediction of Einstein’s theory that gravity
would cause the spectrum to be redshifted.
a) The effect is very slight for the Sun, but has been detected
b) It has best been verified for extremely dense stars known as white dwarfs in which mass is very
tightly packed together