Our Interesting Sun
Our sun's color is white, although from the surface of the earth it appears orange/yellow because of atmospheric scattering. The spectacular picture to the left shows the sun's granular surface, a few surface flares, and an amazing edge prominence called the "Handle". The sun's surface temperature is 5,800 Kelvin or about 10,000 °F. But... the temperature of its core is 15,700,000 Kelvin! The sun's diameter is 865,000 miles, which is 109 times that of the earth. Its surface area is approximately 12,000 times that of the earth. The sun's mass is 333,000 times that of earth and is about 99.9% of the total mass of our whole solar system. 73% of the sun's mass is hydrogen while 25% is helium. The 2% rest consists of heavier elements: oxygen, carbon, iron and a smattering of other elements.
The sun is about 93 million miles on average from earth's orbit, which is known as one Astronomical Unit (1 AU). It takes light 8 minutes and 19 seconds to travel this distance. The sun revolves around the galactic center of the Milky Way at a radius of approximately 26,000 light years. It completes its orbit once every 235 million years. The sun's orbital velocity with respect to the Cosmic Microwave Background (CMB) is about 828,000 miles per hour.
Because the sun exists in a plasma state and behaves like a heavy fluid (not a solid) it rotates faster at its equator than at its poles. This is known as differential rotation (see Convection Zone below), and is caused by fluid rotational differences in the sun's interior due to steep temperature gradients from its core outwards. The sun's period of rotation is 25.6 days at the equator and 33.5 days at the poles. However, due to our constantly changing vantage point from the earth as it orbits the sun, the apparent rotation of the sun at its equator is about 28 days. Top
The Sun's Regions
The sun does not have a definite boundary as rocky planets do. In its outer parts the density of its gases drops exponentially with increasing distance from its center. Nevertheless, it has a well-defined interior structure. The Sun's radius is measured from its "Core" to the edge of the "Photosphere". This is the layer above which gases are too cool to radiate light, and is therefore the surface visible to the naked eye.
The sketch to the left illustrates the various sections of the sun's interior and some exterior features. These are discussed below:
- 1. Core.
- 2. Radiative Zone
- 3. Convective Zone
- 4. Photosphere
- 5. Chromosphere
- 6. Corona
- 7. Sunspots
- 8. Granules
- 9. Prominence
1.) The Core of the sun is considered to extend from the center to about 25% of the solar radius. It has a density of about 150 times the density of water. The Core is the only section of the sun that produces heat through fusion (the conversion of hydrogen into helium). The temperature is 15,700,000 K! The rest of the sun is heated by energy that is transferred outward from the Core. The energy produced by fusion in the Core must travel through successive layers to the Photosphere before it escapes into space as sunlight.
2.) The Radiative Zone, from 25% to 70% of the solar radius, the Radiative material is hot and dense enough that thermal radiation (not fusion) transfers the intense heat of the Core outward. Heat is transferred by photon radiation. Very hot ions of hydrogen and helium emit photons which are absorbed in only a few millimeters of solar plasma and then are re-emitted again in random directions. This random radiation process takes a very long time for photons to reach the sun's surface as sunlight. A reasonable guesstimate of the "photon travel time" is about 100,000 years. The plasma density drops a hundredfold from the bottom to the top of the Radiative Zone. Between the Radiative Zone and the Convective Zone is a very narrow transition layer called the Tachocline. The Radiative Zone rotates like a normal solid body. The Tachocline is a region between the uniform solid rotation of the Radiative Zone and the conventional fluid rotation of the Convective Zone. The Tachocline's plasma rotation rate changes very rapidly causing an extreme shear - a situation where successive horizontal layers slide past one another.
3.) The Convective Zone rotates around the sun as a normal fluid with "differential rotation". That is, the plasma poles rotate slowly (33.5 days) with the convection current speeds constantly increasing until they reach the plasma equator which rotates much faster (25.6 days). In the Convective layer, from its surface down 30% of the solar radius, the solar plasma is not dense or hot enough to transfer the heat energy of the interior outward through radiation. As a result, thermal convection occurs as thermal columns carry hot material to the Photosphere surface of the sun. Once the material cools off at the surface, it plunges downward to the base of the Convective Zone, to absorb more heat from the top of the Radiative Zone and then repeats the cycle over and over. These thermal columns in the Convective Zone form an imprint on the surface of the sun and are called Solar Granules.
4.) The Photosphere - the visible surface of the sun, is the layer below which the sun becomes opaque. Above the Photosphere, sunlight is free to propagate into space and its energy escapes the sun entirely. The visible light we see is produced as electrons react with hydrogen atoms to produce hydrogen ions. The Photosphere is hundreds of kilometers thick. Because the upper part of the Photosphere is cooler than the lower part, an image of the sun appears brighter in the center than on the edge. Shown in the diagram at the left is how the energy from the sun is distributed by wavelength and photon energy. A nice 46% of the radiation is in the visible range. Another 49% is in the infrared range which we feel as heat. The remaining 5% is in the ultraviolet range which tans the skin. During early studies of the photosphere, some absorption lines were found in the solar spectrum that did not correspond to any chemicals then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were a new element which he dubbed "helium", after the Greek Sun God Helios. It was 25 years later that helium was isolated on Earth.
5.) The Chromosphere is a layer of hot gases about 2,500 kilometers thick. The Chromosphere cannot normally be seen because it is washed out by the over-whelming brightness of the Photosphere. However, the remarkable picture of the Chromosphere on the left was taken by Luc Viatour of France during the 1999 total eclipse of the sun at just the right moment. During eclipses of the sun the Chromosphere can be seen by the naked eye. The temperature in the Chromosphere "increases" gradually, ranging from 4,000 K at its bottom to 20,000 K at the top.
6.) The Corona is the outer atmosphere of the Sun which is extremely large. In the lower part of the Corona is a thin "Transition Layer" (about 120 miles thick) in which the temperatures rise from 20,000 K at the bottom of the Transition Layer to temperatures of 1,000,000 K and above! How this happens is a solar mystery. The average temperature of the Corona is 1,000,000 to 2,000,000 K. However, in the hottest regions it is 8,000,000 to an unbelievable 20,000,000 K! The Corona continuously extends into outer space forming the Solar Wind.
7.) Sunspots are a temporary phenomena on the surface of the Photosphere that appear as dark spots compared to the surrounding regions. They are caused by intense magnetic activity, which inhibits convection, forming areas of lower surface temperatures. If a Sunspot were isolated from its surrounding Photosphere, it would be brighter than
an electric arc. Sunspots expand and contract as they move across the surface of the sun. They can be as large as 50,000 miles in diameter making the larger ones visible from Earth. To the left are three sunspots in the suns's northern hemisphere as seen on July 7, 2011 (none in the southern hemisphere). For much more information on sunspot activity see the Sunspots Page.
8.) Solar Granules are very hot thermal columns formed in the Convective Zone which rise to the surface of the Photosphere, cool down, and then plunge back down to the base of the Convective Zone, receive more heat from the Radiative Zone, then cycle up and down again. The grainy appearance of the Photosphere is produced by the tops of these Convective cells. A typical Granule has a diameter of about 600 miles and lasts only 8 to 20 minutes before dissipating. Just below the Photosphere is a layer of "Super Granules", up to 20,000 miles in
diameter whose life span is up to 24 hours.
9.) Solar Prominences rise up through the Chromosphere from the Photosphere, sometimes reaching altitudes of 100,000 miles. These gigantic plumes of gases, often in a loop shape, are the most spectacular of the solar phenomena. The Prominence at the left was recorded in April, 2010. While the Corona consists of extremely hot ionized gases which do not emit much visible light, Prominences contain much cooler plasma which emit quite a bit of light. The mass within a Prominence is typically on the order of 100 billion tons. A Prominence forms in about a day and if stable can persist in the Corona for several months. Some Prominences break apart and morph into Coronal Mass Ejections (CMEs). Scientists are currently researching how Prominences and CMEs are formed and ejected. It is believed that they are caused by intense magnetic activity beneath the surface of the Photosphere. See the NASA CME Video. Top
The Sun's Magnetic Properties
The sun is a very active "magnetic star". Its internal regions are 100% plasma. Plasma is a gas whose temperature has risen to such a high level that it becomes sensitive to magnetism. The sun's rotating magnetic fields affect the gases of the Solar Wind creating the Magnetic Current Sheet, which is a humongous continuous magnetic wave of ion particles in the Heliosphere. The spiral wavy shape, known as the Parker Spiral illustrated on the left, can be compared to a rotating lawn sprinkler, except that the waves keep growing until they encounter the Termination Shock. Extending throughout the Heliosphere, the Magnetic Current Sheet is considered the largest structure in our solar system. The sun's strong changing magnetic field varies from year to year and amazingly reverses itself on average every 10.7 years. The "differential rotation" of the Convective Zone (explained above in the Convective Zone section) causes the magnetic field lines to become twisted over time which then causes magnetic field loops to erupt on the sun's surface triggering the formation of dramatic Solar Prominences
and Coronal Mass Ejections (CMEs).
The ultraviolet photo on the left from NASA's Solar Dynamic Laboratory (SDO) shows the sun's whole northern hemisphere exploding as examples of huge Solar Flares and a Coronal Mass Ejections. Different colors in the image represent different gas temperatures. Here is a quote from NASA,: "On August 1, 2010, almost the entire earth facing side of the sun erupted in a tumult of activity. This image shows the large solar flare (white area on the left), a solar tsunami (wave-like structure upper right) multiple filaments of magnetism lifting off the stellar surface, large scale shaking of the solar Corona, radio bursts, a Coronal Mass Ejection, and more."
For three months NASA scientists have been working hard to under-
stand what really happened on August 1st, 2010. The events on the sun's surface were not isolated events, they were all magnetically collated into one massive instantaneous explosion. NASA announced "Explosions on the sun are not localized or isolated events. Instead, solar activity is interconnected by magnetism over breath-taking distances. Solar flares, Tsunamis, Coronal Mass Ejections - they can go off all at once, hundreds of thousands of miles apart, in a dizzyingly complex concert of mayhem".
The magnetic forces were traced by the SDO spacecraft. Previously these types of events were thought to be isolated from one another. While scientists know a lot about what happened that day, they are still piecing together the causes. It may be that a new theory has to evolve to explain massive explosions like this one. Not all explosions are massive, so there may be more than one type of activity going on.
The CME from August 1st, 2010 headed directly towards the earth and three days later the earth's magnetic field reverberated from the CME's impact which sparked auroras as far south as Wisconsin and Iowa. However, this particular intense solar storm was an exception. Months had gone by without a single sunspot. The sun was coming out of a exceptionally long, low period of activity in August, 2010, the longest low in more than a century. Sun activity cycles are measured by the number of sunspots recorded in a given year. Sunspots are dark regions caused by strong magnetic fields that only appear dark because the local magnetic field is so strong that it blocks the upward flow of heat from the sun's interior. For more information on sunspot activity see the Sunspots Page. Top
Solar Flares And CMEs
A solar flare is a very large explosion on the surface of the sun with its plasma suddenly roaring to millions of degrees. Flares occur in the active regions around Sunspots. The flare shown on the left was an X-class flare (most powerful) which occurred on August 9, 2011. The image is an extreme ultraviolet picture taken by NASA's SDO satellite. While this flare produced a Coronal Mass Ejection (CME), the CME was not aimed towards the earth and no local effects were observed. The energy emitted from a flare is about one sixth of the sun's total energy output each second. Strong flares eject streams of electrons, ions, and atoms (called Solar Storms) into outer space.
Flares are formed when intense magnetic fields from below the sun's surface link up with magnetic fields in the outer Corona in a process called "Magnetic Reconnection" (see the Reconnection section). Flares are powered by the sudden release of magnetic energy stored in the sun's Corona. The same release may also produce a CME, but not always. And, sometimes CMEs form without flares. The connection between flares and CMEs is not well understood.
The solar flare at the left from June 7th, 2011 was a medium sized flare, an M2 on the x-ray scale, but it produced a spectacular CME. Its size was in the top 5% of all CMEs recorded in the "Space Age". The plasma contained blobs that were as big as planets, many larger than the earth. NASA reported "They rose and fell ballistically, moving under the influence of the sun's gravity like balls tossed in the air, exploding like bombs when they hit the surface."
Flares produce radiation across the electromagnetic spectrum, although with different intensities. Most of their energy goes to frequencies outside of our visual range so the majority of flares are not visible to the naked eye and must be observed with special instruments. While not very intense at white light, they can be very bright at particular frequencies.
Solar flares are classified as A, B, C, M or X according to the peak flux of X-rays at a specified frequency range. Each class has a peak flux ten times greater than the preceding one. Within a class there is a linear scale from 1 to 9, so an X2 flare is twice as powerful as an X1 flare. Solar flares strongly influence space weather in the vicinity of the earth. They can produce streams of highly energetic alpha particles in the Solar Wind.
The biggest solar storm ever recorded was the Carrington Flare of 1859, named after Richard Carrington, a prominent English astronomer who observed
it. It was the first solar flare ever recorded. Skies erupted in red, green, and purple auroras so brilliant that newspapers could be read in the dark as easily as in daylight. Stunning auroras pulsated as far south as Cuba, El Salvador, and Hawaii. Telegraph systems worldwide went haywire. Spark discharges shocked telegraph operators and set the telegraph paper on fire. The Carrington Flare was the largest flare in the past 500 years as measured by radiation particles locked in the polar ice. A similar solar flare that generates a "massive CME" could knock out current day electrical grid power for months.
Shown at the left (in green) is a super large solar flare from October 28, 2003. The Solar and Heliospheric Observatory (SOHO) spacecraft captured the image of the flare as it erupted from the sun. This was the most powerful flare ever measured with modern technology.
Another solar flare on January 20, 2005 released the highest concentration of protons ever measured and took only 15 minutes after initial observation to reach earth. This calculates out to be one-third the speed of light. Energetic protons can pass through the human body causing biochemical damage which presents a hazard to astronauts. This 2005 "radiation" storm would have given astronauts only 15 minutes to seek shelter. Most radiation storms take two hours from the time of visual detection to reach earth's orbit.
CMEs are clouds of plasma and particles that travel much slower than radiation storms (about 300 miles a second vs. 30,000). We do have satellites far out in space that monitor the sun day and night. NASA is working on a program to detect CME storms and give utilities an early warning of up to 30 minutes. See Protecting The Grid on the Smart Grid Page.
In 2011 NASA announced that about 1 in 7 flares experience an aftershock. About ninety minutes after the flare initially dies down, it springs to life again producing an extra surge of extreme ultraviolet radiation. The energy in the second phase can exceed the energy of the primary phase by as much as a factor of four.
The second phase is thought to result from some of the sunspot’s magnetic loops re-forming. The extra energy from the late phase can have a big effect on earth. Extreme ultraviolet wavelengths are particularly good at heating and ionizing earth’s upper atmosphere. When our planet’s atmosphere is heated by extreme UV radiation it puffs up, accelerating the decay of low-orbiting satellites. Furthermore, the ionizing action of extreme UV can bend radio signals and disrupt the normal operation of GPS satellites.
Living with a "restless" star can be difficult at times. Top
When you see the word "nano", one naturally expects the object to be something small. And sure enough, solar nanoflares are a "billion" times less energetic than ordinary solar flares. But, compared to an explosion here on earth, each nanoflare has the energy equivalent of 10,000 atomic bombs. The sun can go months without producing an ordinary solar flare. Nanoflares, on the other hand, are crackling on the sun non-stop and many go off at the same time.
X-rays stream off the sun in the dramatic image to the left taken by NASA's Nuclear Spectroscopic Telescope Array, NuSTAR, overlaid on an ultraviolet image taken by NASA's Solar Dynamics Observatory (SDO). This remarkable image was the first x-ray picture taken by NuSTAR of the sun. (NuSTAR's mission is to take x-ray images of objects in distant outer space, like quasars for example. Pointing NusSTAR at our relatively close sun was using it out of character.)
NuSTAR has just the right combination of sensitivity and resolution to study the x-ray flickers of nanoflares. This image, taken in December, 2014, removed any doubt. NuSTAR turned toward the sun and, working together with NASA's SDO, captured one of the most beautiful images in the history of solar astronomy.
The NuSTAR data, seen in green and blue, reveal solar high-energy emissions (green shows energies between 2 and 3 kilo-electron volts, and blue shows energies between 3 and 5 kilo-electron volts). These high-energy x-rays come from gas heated above 3 million degrees. The red channel represents ultraviolet light captured by SDO and indicates the presence of lower-temperature material at 1 million degrees.
For more than a half-century, scientists have trying to figure out what causes the sun's corona to be so hot. It is one of the most vexing problems in astrophysics. One theory is that nanoflares might be involved. They appear to be active throughout the 11 year solar cycle, which would explain why the corona remains hot during the Solar Minimum. And while each individual nanoflare falls very short of the energy required to heat the sun's corona, collectively they might have no trouble doing to job. Stay tuned. Top
Spicules And Alfvénic Waves
Spicules (spik'-cules) are the dark brown images in the picture to the left. They are dynamic jet spouts about 300 miles in diameter shooting up into the Chromosphere from the Photosphere (surface of the sun). An individual Spicule typically reaches up to 30,000 miles above the Photosphere At any one time there are about 60,000 to 70,000 active Spicules on the sun's surface. They are found in regions of strong magnetic fields. Spicules live for about 5 to 10 minutes. NASA compares the Spicules to "seaweed" in the ocean swaying back and forth in the ocean's waves. Only in the sun's Corona, magnetic field ripples called "Alfvénic Waves", cause the the Spicules to sway.
An ultraviolet image below by the Solar Dynamics Observatory (SDO) spacecraft shows Alfvénic Waves. Alfvénic Waves, named after Hannes Alfvén, are magnetically induced waves in electrically conducting fluids. The different colors represent various gas temperatures.
Conducting fluid examples are salt water, electrolytes, liquid metals, and of course plasmas. Alfvén discovered this phenomenon in 1942 and won the Nobel Prize In Physics for it in 1970. Alfvén suggested that large plasmas could carry huge electric currents capable of generating "galactic magnetic fields" (i.e. the sun's Magnetic Current Sheet. Afvénic Waves travel up and down a magnetic field line much the way a wave travels up and down a plucked guitar string.
NASA's SDO is now able to measure how much energy is carried by the Alfvénic Waves spewing out from the jets of Spicules. The research shows that the waves carry 100 times more energy than previously thought. While the Alfvénic Waves carry enough energy to drive the intense heating of the Corona to 200 times hotter than the sun's surface and the solar winds up to 1.5 million miles per hour, how much of this energy is actually transferred is unknown.
The Alfvénic Waves as we know them today could account for the energy of the Corona and Solar Wind, but there is not enough energy to account for the huge mass of plasma materials ejected during CMEs. Scott McIntosh at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, says "We still don't perfectly understand the process going on, but we're getting better and better observations. The next step is for people to improve the theories and models to really capture the essence of the physics that's happening. Now that the real power of the waves has been revealed in the Corona, the next step in unraveling the mystery of its extreme heat is to study how the waves transfer their energy to the plasma." Top
How Long Will The Sun Last?
The sun is estimated by two different methodologies to be 4.6 billion years old. The sun will last about 10 billion years as a "main sequence" star. Main sequence means that by nuclear fusion it converts hydrogen into helium plus some neutrinos and radiation. The sun does not have enough mass to explode like a "super-nova". In about 5 billion years the sun will enter a "red giant" phase. As the hydrogen in the core is consumed, the core will begin to contract and heat up. "Helium fusion" will then begin and the sun's outer layers will expand significantly. Following the red giant phase the sun will eject its outer layers forming a "planetary nebula", which is a large glowing shell of ionized gases. After all the outer layers have been ejected, that which remains will be the extremely hot "white core". The white core will slowly cool and fade away as a "white dwarf" over many billions of years. This is the typical end of a medium sized star. Top
The Big Questions
The sun's outer atmosphere, the Corona, is extremely hot (millions of degrees K) at its outer edges while the Photosphere (the visible surface of the sun) has a temperature of only 5,800 K. The processes that super heat the Corona, maintain it at extreme temperatures, and accelerate the Solar Wind are a still somewhat of a mystery. Usually temperatures decrease as you move away from a heat source. This is true in the sun's Core right up to the surface. Then over a relatively small distance, the temperatures suddenly rise to extreme highs - 20 million degrees K! While recent data show that Spicules expel enough energy to do the heating (see Spicules section above) how they transfer their heat to the Corona and how much heat is transferred is still unknown. Another candidate are nanoflares, but they too need to be proven as the source of the heat.
Nature Of Solar Flares And CMEs
Areas on the sun near Sunspots often flare up heating the plasma to millions of degrees in just seconds and blasting billions of tons of plasma material into space. The precise causes of Solar Flares and Coronal Mass Ejections (CMEs) are solar mysteries. We do know that Spicules do not have enough energy to expel CMEs. We think we understand the basic magnetic reconnection mechanisms. However, many pieces of the puzzle are missing. We can not predict when and where a flare will occur, i.e. the trigger mechanisms, or how big it will be.
Origin Of Sunspot Cycles
Over an approximate 11 year period, the monthly daily average number of Sunspots observed on the sun's surface increases from nearly zero to over 100 and then decreases to near zero again as the next cycle starts. The nature and under pinnings of the Sunspot Cycle constitute some of the great mysteries of solar astronomy. While we know many details about Sunspot Cycles and also about some of the processes that play a role in producing them, we are still unable to produce a scientific model that allows us to reliably predict future Sunspot occurrences using basic physics principles. For more information on sunspot activity see the Sunspot Page.
For a good source of information about the sun, see the NASA Sun-Earth Web Site.