Space, time & CosmosLecture 4:

Our Sun, the Solar System & Earth

Prof. Ken Tsang

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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.

The Sun a huge sphere of mostly ionized gas.

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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.

The Sun

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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.

Life cycle of the Sun

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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.

Future of the Sun

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Venus Transit Date: 06.08.2004NASA'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: 08.11.1999Here 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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1. Core2. Radiative zone3. Convective zone4. Photosphere5. Chromosphere6. Corona7. Sunspot8. Granules9. Prominence

The structure of the Sun

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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.

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.

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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.

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.

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.

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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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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.

Sunspots, solar flares and the sunspot cycle

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Three large sunspot groups shine brightly in this March 26 X-ray image from the orbiting SOHO solar observatory.

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Sunspot LoopsEven 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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The solar wind 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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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.

AU: Astronomical Unit

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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 long-period 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.

How Big is 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 mid-spring. 

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Stellar NurseriesThe 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

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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 OrbitThe 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, tide-generating 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.

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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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The magnetosphere 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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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 Systemhttp://www.youtube.com/watch?v=B1AXbpYndGcWelcome to the Universe: Nebula & Galaxies: A Cosmic Journeyhttp://www.youtube.com/watch?v=X5zVlEywGZg&NR=1

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