Transcript Space, time & Cosmos Lecture 4: Our Galaxy

Space, time & Cosmos
Lecture 4:
Our Sun, the Solar System & Earth
Prof. Ken Tsang
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The Sun
a huge sphere of mostly ionized gas.
It powers photosynthesis in green plants, and is ultimately the source of all food
and fossil fuel.
A handle-shaped cloud of plasma erupts from the Sun.
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The Sun
The Sun, a yellow dwarf, is the star at the center of the Solar System, which by
itself accounts for about 98.6% of the Solar System's mass. The mean distance
of the Sun from the Earth is approximately 149.60 million km (it takes light 8.3
minutes to travel this distance). This distance is known as an astronomical unit
(abbreviated AU), and sets the scale for measuring distances all across the
solar system.
The surface of the Sun consists of hydrogen (about 74% of its mass, or 92% of
its volume), helium (about 24% of mass, 7% of volume), and trace quantities of
other elements, including iron, nickel, oxygen, silicon, sulfur, magnesium,
carbon, neon, calcium, and chromium.
Energy from the Sun, in the form of sunlight, supports almost all life on Earth via
photosynthesis. The connection and interactions between the Sun and Earth
drive the seasons, ocean currents, weather, and climate.
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Life cycle of the Sun
The Sun was born about 4.6 billion years ago from the gravitational
collapse of a vast cloud of gas and dust. Material in the center of the cloud
was squeezed so tightly that it became hot enough to ignite nuclear fusion.
The Sun is about halfway through its evolution, during which nuclear fusion
reactions in its core fuse hydrogen into helium. Each second, more than 4
million metric tons of matter are converted into energy within the Sun's
core, producing neutrinos and solar radiation; at this rate, the Sun will have
so far converted around 100 Earth-masses of matter into energy. The Sun
will spend a total of approximately 10 billion years as a main sequence
star.
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Future of the Sun
The Sun will continue to burn its hydrogen for several billion years more. As
it depletes the supply of hydrogen, its core will shrink and temperatures will
climb high enough for it to burn helium instead. The Sun's surface will puff
up like a balloon, growing cooler, brighter, and redder, forming a red giant.
Eventually, as the Sun burns helium to form heavier elements, it will reach a
critical point where fusion cannot release enough energy to form new
elements, so fusion will end.
After that, the Sun will shed its outer layers, surrounding itself with a colorful
bubble of gas called a planetary nebula. As the nebula dissipates,
distributing carbon, oxygen, and other elements into the galaxy, only the
Sun's collapsed core will remain -- a dense ball no bigger than Earth,
containing about 60 percent of the Sun's original mass. This dead remnant
is called a white dwarf. Over many billions of years, the white-dwarf Sun will
cool and fade from sight, leaving behind a dark cosmic ember.
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Venus Transit Date: 06.08.2004
NASA's TRACE satellite captured this image of Venus crossing the face
of the Sun as seen from Earth orbit. The last event occurred in 1882. The
next Venus transit will be visible in 2012.
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Eclipsed Earth Date: 08.11.1999
Here is what the Earth looks like during a solar eclipse. The shadow of the Moon
can be seen darkening part of Earth. This shadow moves across the Earth at
nearly 2,000 kilometers per hour. Only observers near the center of the dark circle
see a total solar eclipse - others see a partial eclipse where only part of the Sun
appears blocked by the Moon.
This spectacular
picture of the Aug. 11,
1999 solar eclipse was
one of the last ever
taken from the Mir
space station. Mir was
decommissioned after
more than ten years of
use.
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The structure of the Sun
1. Core
2. Radiative zone
3. Convective zone
4. Photosphere
5. Chromosphere
6. Corona
7. Sunspot
8. Granules
9. Prominence
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1. Core. The Sun's nuclear "furnace," where fusion reactions initially combine
hydrogen atoms to produce helium, yielding energy in the process.
2. Radiative Zone. Energy moves through a surrounding envelope of gas toward
the Sun's surface.
3. Convection Zone. Big "bubbles" of hot gas transport energy to the surface.
Most of this energy is in the form of gamma-rays and X-rays. As the energy works its way to the surface
-- a process that takes centuries -- it is absorbed by other atoms, then re-radiated at other wavelengths.
When it reaches the surface, where it can escape into space, most of the energy is in the form of visible
light.
4. Photosphere. The Sun's visible surface.
Because of its high temperature, it glows
yellow. Chromosphere - a thin layer just above the photosphere,
roughly 2,000 kilometers deep. The name comes from the fact that it
has a reddish color. The photosphere is closer to the surface of the
sun and its temperature is around 4000 K to 6400 K but the
chromosphere is about 4500 K to as high as 20,000 K.
5. Sunspot. A magnetic "storm" on the Sun's
surface.
6. Prominence. An eruption of hot gas that
can extend thousands of miles into space.
7. Corona. The Sun's outer atmosphere,
which is heated by the magnetic field to
millions of degrees.
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The Sun with some sunspots is visible. The two small spots in the
middle have about the same diameter as our planet Earth.
It has a surface temperature of approximately 5,500 °C giving it a white color that often,
because of atmospheric scattering, appears yellow when seen from the surface of the Earth.
This is a subtractive
effect, as the preferential
scattering of shorter
wavelength light removes
enough violet and blue
light, leaving a range of
frequencies that is
perceived by the human
eye as yellow.
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The two STEREO spacecraft were launched together in Oct. 2006 from Cape Canaveral. In the
following months they were placed in two separate orbits about the Sun - one (the Ahead spacecraft)
moving ahead of Earth's orbit, the other (Behind) moving behind Earth's orbit. Both spacecraft are
separating from each other and Earth. The spacecraft now have four degrees of separation, enough to
provide true 3D images of the Sun and solar storms for the very first time.
The images shown are produced by the STEREO Extreme Ultraviolet Imaging
Telescopes (EUVI). These show the Sun's super-hot atmosphere in ultraviolet
wavelengths of light invisible to the human eyes and unobtainable from the
Earth's surface. This hot, ionized material is shaped by the sun's magnetic fields
so that observing the Sun's atmosphere in ultraviolet light allows us to study its
magnetic field.
The Sun's atmosphere, the corona, is shaped by the Sun's complex and
dynamic magnetic field. The magnetic field is also the source of solar activity.
Complex magnetic fields rearrange and reconnect to form simpler magnetic
structures and in the process release energy in the forms of flares and coronal
mass ejections.
At the time these images were taken in late March 2007 the two STEREO spacecraft were about 10
million km apart. This is far enough to give each spacecraft a distinct point of view of the structures in
the Sun's lower atmosphere and makes 3D images of the Sun possible for the first time.
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Cross-section of a solar-type star
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During a total
solar eclipse, the
solar corona can
be seen with the
naked eye.
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Coronal Loops
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Extending above the photosphere or visible surface of
the Sun , the faint, tenuous solar corona is measured
to be hundreds of times hotter than the photosphere
itself.
What makes the solar corona so hot? Astronomers have long sought the source of
the corona's heat in magnetic fields which loft monstrous loops of solar plasma
above the photosphere. Still, new and dramatically detailed observations of coronal
loops from the orbiting TRACE satellite are now pointing more closely to the
unidentified energy source. Recorded in extreme ultraviolet light, this and other
TRACE images indicate that most of the heating occurs low in the
corona, near the bases of the loops as they emerge from and
return to the solar surface. The new results confound the conventional theory
which relies on heating the loops uniformly. This tantalizing TRACE image shows
clusters of the majestic, hot coronal loops which span 30 or more times the diameter
of planet Earth.
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Taken by Hinode's Solar Optical Telescope on January 12, 2007, this image
of the Sun reveals the filamentary nature of the plasma connecting regions
of different magnetic polarity.
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Sunspots, solar flares and the sunspot cycle
The motions of the hot gas below the Sun's surface create a powerful
magnetic field. The field encircles the Sun with lines of magnetic
force. These lines become entangled, forming relatively cool, dark
magnetic storms on the Sun's surface known as sunspots.
Occasionally, the entangled lines "snap," triggering enormous
explosions of energy known as solar flares. Magnetic effects also
pull out big streamers of hot gas from the Sun's surface, and they heat
the Sun's thin outer atmosphere to more than one million degrees.
The number of sunspots and flares peaks every 11 years, when the
Sun's magnetic field flips over. It takes two "flips" to complete a full
cycle.
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Three large
sunspot groups
shine brightly in
this March 26 Xray image from
the orbiting
SOHO solar
observatory.
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Sunspot Loops
Even a relatively quiet day on the Sun is busy. This ultraviolet image shows
bright, glowing arcs of gas flowing around the sunspots.
A sunspot viewed
close-up in
ultraviolet light,
taken by the
TRACE
spacecraft.
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Sunspots are relatively cool magnetic storms
in the Sun's atmosphere. Sunspots have been rare
over the last few years as the Sun passed through
the quiet phase of its 11-year magnetic cycle. A new
cycle has just started, and will build to a peak around
2013. At the cycle's peak, sunspots like these will be
a common sight. The high levels of magnetic activity
may affect communications with orbiting satellites,
create problems with electric grids, and cause other
technological mischief.
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Solar prominence is a large bright feature extending outward from the Sun's
surface, often in a loop configuration. Prominences are anchored to the Sun's
surface in the photosphere, and extend outwards into the Sun's corona. While the
corona consists of extremely hot ionized gases, known as plasma, which do not
emit much visible light, prominences contain much cooler plasma, similar in
composition to that of the chromosphere. A prominence forms over timescales of
about a day, and stable prominences may persist in the corona for several
months. Some prominences break apart and give rise to coronal mass ejections.
A solar flare is a violent explosion in a star's (like the Sun's) atmosphere
releasing as much energy as 6 × 1025 Joules. Solar flares affect all layers of the
solar atmosphere (photosphere, corona, and chromosphere), heating plasma to
tens of million Kelvin and accelerating electrons, protons and heavier ions to
near the speed of light. They produce radiation across the electromagnetic
spectrum at all wavelengths, from radio waves to gamma rays. Most flares
occur in active regions around sunspots, where intense magnetic fields
penetrate the photosphere to link the corona to the solar interior. Flares are
powered by the sudden (timescales of minutes to tens of minutes) release of
magnetic energy stored in the corona.
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A powerful explosion of particles and energy known as a solar flare
erupts from the Sun in this false-color image. The flare forms a bright red
loop at lower left. Such flares may trigger vibrations in the surface that
ripple all the way around the Sun.
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A streamer of hot gas that is
hundreds of thousands of miles
long erupts from the surface of the
Sun in this image from the SOHO
spacecraft. When the Sun reaches
the peak of its next magnetic
cycle, around 2011 or 2012, such
eruptions will be much more
common.
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This picture depicts the last three solar cycles as measured in solar
irradiance, sunspot numbers, solar flare activity, and 10.7 cm radio flux.
Solar irradiance, i.e the direct solar power at the top of the Earth's
atmosphere, is depicted as both a daily measurement and a moving annual
average. All other data are depicted as the annual average value.
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solar wind
The
is a stream of charged particles—a plasma—
ejected from the upper atmosphere of the sun. It consists mostly of electrons
and protons with energies of about 1 keV. These particles are able to escape
the sun's gravity, in part because of the high temperature of the corona, but
also because of high kinetic energy that particles gain through a process that
is not well-understood.
The solar wind creates the Heliosphere, a vast bubble in the interstellar
medium surrounding the solar system. Other phenomena include
geomagnetic storms that can knock out power grids on Earth, the aurorae
such as the Northern Lights, and the plasma tails of comets that always point
away from the sun.
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Comets have highly elliptical orbits. Note the two distinct tails: one is the plasma
tail, another is the dust tail. The
plasma tails of comets are
always point away from the sun.
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Heliosphere
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Heliosphere
This is an artist's concept illustrating the structures the solar wind forms around
our Sun. As we fly out from the Sun beyond the orbits of the planets, we come to
the termination shock (semi-transparent purple sphere). The termination shock
is where the solar wind, a thin stream of electrically charged gas blown
constantly from the Sun, is slowed abruptly by pressure from gas between the
stars. Beyond this region is the solar system's final frontier - the heliosheath. The
heliosheath is a vast region where the solar wind is turbulent and hot. The
interstellar wind collides with the heliosheath and forms a structure called the bow
shock (red and orange areas), forcing the heliosheath into a long, teardrop
shaped structure.
It is believed that Voyager 1 (launched September 5, 1977) crossed the termination
shock and entered the heliosheath in the middle of December 2004, at a distance
of 94 AU, and Voyager 2 (launched on August 20, 1977) crossed the termination
shock on August 30, 2007 at 84 AU. This image shows the positions of the
Voyager spacecraft in relation to these structures.
The heliopause is the theoretical boundary where the Sun's solar wind is
stopped by the interstellar medium; where the solar wind's strength is no longer
great enough to push back the stellar winds of the surrounding stars.
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The Heliosphere is a bubble in space produced by the solar wind.
Virtually all the material in the heliosphere emanates from the Sun itself (though
neutral atoms from interstellar space can penetrate this bubble).
The solar wind streams off the Sun in all directions at speeds of several hundred
kilometers per second. At some distance from the Sun, well beyond the orbit of
Pluto, this supersonic wind must slow down to meet the gases in the interstellar
medium. It must first pass through a shock, the termination shock, to become
subsonic. It then slows down and gets turned in the direction of the ambient flow
of the interstellar medium to form a comet-like tail behind the Sun. This subsonic
flow region is called the heliosheath. The outer surface of the heliosheath,
where the heliosphere meets the interstellar medium, is called the heliopause.
The solar wind consists of particles, ionized atoms from the solar corona, and
fields (magnetic fields in particular). As the Sun rotates once in about 27 days,
the magnetic field transported by the solar wind gets wrapped into a spiral.
Variations in the Sun's magnetic field are carried outward by the solar wind and
can produce magnetic storms in the Earth's own magnetosphere.
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AU: Astronomical Unit
Distances in space are so vast that astronomers use a
much larger measurement, called the astronomical
unit. This is the average distance from the Earth to the
Sun, or approximately 150 million kilometers.
Mercury is only 0.39 astronomical units from the Sun, while
Jupiter orbits at a distance of 5.5 astronomical units. And Pluto
is way out there at 39.2 astronomical units.
The Kuiper Belt, where we find a Pluto, Eris, Makemake and
Haumea, extends from 30 astronomical units all the way out to
50 AU, or 7.5 billion kilometers.
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How Big is the Solar System?
In the furthest reaches of the Solar System is the Oort Cloud; a
theorized cloud of icy objects that could orbit the Sun to a
distance of 100,000 astronomical units, or 1.87 light-years
away. Although we can’t see the Oort Cloud directly, the longperiod comets that drop into the inner Solar System from time
to time are thought to originate from this region.
The Sun’s gravity dominates local space out to a distance of
about 2 light-years, or almost half the distance from the Sun to
the nearest star: Proxima Centauri. Believe it or not, any object
within this region would probably be orbiting the Sun, and be
thought to be a part of the Solar System.
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The solar system, in logarithmic scale, showing the outer extent of the
heliosphere, the Oort cloud and Alpha Centauri [半人馬座α星]
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The Sun and solar system move through a part of the galaxy referred to as the local
interstellar medium. It is built up from material released from the stars of our galaxy
through stellar winds, novae, and supernovae.
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People in the
Northern
Hemisphere
notice Sirius
(2.6 parsecs
8.6 ly) in the
southeast –
south – or
southwest on
evenings from
winter to midspring.
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Stellar Nurseries
The Eagle Nebula
Orion Nebula
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During a star’s main-sequence lifetime, the star
expands somewhat and undergoes a modest increase
in luminosity
The Death of
a Low-Mass
Star (Sun)
Such stars never
become hot enough
for fusion past
carbon to take place.
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The small star
Sirius B is a
white-dwarf
companion
of the much
larger and
brighter Sirius A:
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Earth
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Earth's Orbit
The elliptical orbits of the moon around the Earth and the Earth around the Sun
have a substantial effect on the the Earth's tides.
Once a month, at perigee, when the moon is closest to the Earth, tidegenerating forces are higher than usual, producing above average ranges in the
tides. About two weeks later, at apogee, when the moon is farthest from the
Earth, the lunar tide-raising force is smaller, and the tidal ranges are less than
average.
When the Earth is closest to
the Sun (perihelion), around
January 2, tidal ranges are
enhanced. At aphelion, when
the Earth is furthest from the
Sun, around July 2, tidal
ranges are reduced.
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Earth's magnetic field is approximately a magnetic
dipole, with one pole near the north pole and the other near the geographic
south pole.
An imaginary line joining the
magnetic poles would be inclined
by approximately 11.3° from the
planet's axis of rotation. The cause
of the field can be explained by
dynamo theory.
The Earth's magnetic field
effectively extends several tens of
thousands of kilometres into
space, and becomes the
magnetosphere.
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Earth is surrounded by a magnetosphere, which
was discovered in 1958 by Explorer 1 during the research
performed for the International Geophysical Year.
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magnetosphere
The
of Earth is a region in space whose
shape is determined by the extent of Earth's internal magnetic field, the solar
wind plasma, and the interplanetary magnetic field (IMF).
In the magnetosphere, a mix of free ions and electrons from both the solar
wind and the Earth's ionosphere is confined by magnetic and electric forces
that are much stronger than gravity and collisions.
On the side facing the Sun, the distance to its boundary (which varies with
solar wind intensity) is about 70,000 km (10-12 Earth radii or RE, where 1
RE=6371 km; all distances here are from the Earth's center). The boundary
of the magnetosphere ("magnetopause") is roughly bullet shaped, about 15
RE abreast of Earth and on the night side (in the "magnetotail" or "geotail")
approaching a cylinder with a radius 20-25 RE. The tail region stretches well
past 200 RE, and the way it ends is not well-known.
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Auroras Underfoot
If you think auroras look spectacular from Earth,
check out the view astronauts aboard the Space Shuttle and International
Space Station get when the Earth's magnetosphere is struck by a Coronal Mass
Ejection (CME) from our Sun.
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The Aurora Borealis, or Northern Lights, shines above Bear Lake, Eielson
Air Force Base, Alaska
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Red and green Aurora in Fairbanks, Alaska.
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Aurora australis
(September 11, 2005)
as captured by NASA's
IMAGE satellite, digitally
overlaid onto the The
Blue Marble composite
image.
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Home work for Lecture 4:
Birth of the Solar System
http://www.youtube.com/watch?v=B1AXbpYndGc
Welcome to the Universe: Nebula & Galaxies: A Cosmic Journey
http://www.youtube.com/watch?v=X5zVlEywGZg&NR=1
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