Module 7: The Closest Star: Our Sun

Solar Weather

Solar activity is like "weather" on the Sun. Sunspots Solar Flares Solar Prominences Solar Wind All these activities are related to magnetic fields. Solar weather can affect Earth's magnetic field and can be severe enough to damage satellites in orbit and disrupt power grids on the ground.

Maunder Minimum

Early records of sunspots indicate that the Sun went through a period of inactivity in the late 17th century. Very few sunspots were seen on the Sun from about 1645 to 1715.

Fusion

Small nuclei stick together to make a bigger one Sun, stars (B) Fusion brings lightweight nuclei together to make heavier nuclei and energy.

Sun Facts

That the Sun contains 99.8% of the mass of the solar system. That the Sun has an interior and an atmosphere, but no solid surface. That the Sun is a ball of hot, glowing gas, called plasma. That the Sun is made up of 70%hydrogen (H), 28% helium (He), and 2% heavier elements. That after 4.57 billion years, the Sun will continue to shine for 6 or more billion years.

Viewing the Corona

The Extreme-ultraviolet Imaging Telescope (EIT) on the SOHO spacecraft images the corona every few minutes. By looking through filters that pass only light given off by gas at a very high temperature, the spacecraft can make images of the corona even in the center of the Sun's disk.

How the Sun shines

Because the Sun has been around for nearly 4.6 billion years, it must be able to generate a lot of energy over a long time. The source of this energy is the nuclear fusion of hydrogen to helium that takes place in the central core of the Sun. This fusion process is often called hydrogen burning (though technically burning is a chemical process). The fusion of hydrogen to helium is the one property all stars that lie along the main sequence share in common. (A) The Sun in visible light. (B) Charcoal briquettes glowing in an outdoor grill is an example of chemical burning.

Plasma

Because the corona is so tenuous, it serves as a unique and valuable celestial laboratory in which we may study gaseous "plasmas" in a near-vacuum. Plasmas are gases consisting of positively and negatively charged particles and can be shaped by magnetic fields. We are trying to learn how to use magnetic fields on Earth to control plasmas, retaining charged particles within a small volume, in order to provide energy through nuclear fusion. Plasma is the 4th state of matter: an electrically neutral, highly ionized gas composed of ions, electrons, and neutral particles. It is a phase of matter distinct from solids, liquids, and normal gases. Plasma is the most common state found in stars.

Fission

Big nucleus splits into smaller pieces Nuclear power plants (A) Fission splits heavy nuclei to make energy

Solar Neutrino Problem

By 2001, observations at the Sudbury Neutrino Observatory indicated the right number of neutrinos are being produced in the Sun's core, but some have changed form from a version we can "see" to ones that our detectors have a hard time counting. The Sudbury (Canada) Neutrino Observatory (SNO) operated between 1999 and 2006. Shown is the outside view of the completed neutrino detector before the acrylic enclosure in which the detector will sit is filled with heavy water. The "dots" are 9,600 photomultiplier tubes that will observe the light produced by relativistic electrons in the water created by the neutrino interactions. As relativistic electrons travel through a medium, they lose energy producing a cone of blue light through the Cherenkov effect, and it is this light that is directly detected. The SNO produced the first clear evidence that neutrinos oscillate (i.e., that they can transmute into one of three forms or "flavors") as they travel from the Sun. This oscillation in turn implies that neutrinos have non-zero masses. The total flux of all neutrino flavors measured by the SNO agrees well with the theoretical prediction for solar neutrino production.

How does the Sun Shine?

Chemical burning (of wood or coal) would only power an object the Sun's size for between 7,000 and 10,000 years. The first plausible theory to explain how the Sun shines was gravitational contraction. Calculations in the 19th century showed that the Sun could shine via gravitational contraction for 25 million years, far short of geologic time on Earth. By the late 19th century, advances in geology and paleontology indicated that the Earth was at least hundreds of million years old, if not older. A newly-forming star—a protostar—heats up and begins to shine through gravitational contraction. Only later, when the protostar gets massive enough, dense enough, and hot enough at its core, will nuclear fusion begin.

Sun's Exterior Structures

Photosphere: sphere that makes light visible surface of Sun (+ sunspots) 5,800 K temperature composed of boiling gas Chromosphere: "color sphere" seen only during total solar eclipses temperature is 10,000 K primary source of UV radiation Corona: "crown" seen only during total solar eclipses extends a few million kilometers above the solarsurface temperature is > 1,000,000 K though density is low region emits most of the Sun's X-rays Solar prominences (circled) are part of the chromosphere and can be seen along with the corona during a total eclipse such as this one in 1999. (UCAR/NCAR/High Altitude Observatory)

Sun Funnel

(A) The Sun Funnel safely projects an image of the Sun onto a special fabric used for rear screen projection. (L. Black) (B) The Sun on March 7, 2012. Sunspot AR1429 is responsible for a large flare and subsequent coronal mass ejection (CME).

Super-Kamiokande

(A) The Sun as observed by Super-Kamiokande, which can measure the direction of the neutrinos. (Institute for Cosmic Ray Research, U. Tokyo) (B) This neutrino detector, built a kilometer underground in a Japanese zinc mine, began operations in 1996. The huge stainless-steel vessel, 40 meters tall and 40 meters wide, has been filled with 50,000 tons of highly purified water. Its walls are lined with 13,000 light sensors, called photo-multiplier tubes (PMTs), that pick up a flash of light generated by electrons recoiling from neutrino collisions in the water. (Courtesy of Yoji Totsuka, Institute for Cosmic Ray Research, U. Tokyo.)

Magnetic Field Lines

(A) The magnetic field of a bar magnet is revealed by iron filings on paper. A sheet of paper is laid on top of a bar magnet and iron filings are sprinkled on it. The needle shaped filings align with their long axis parallel to the magnetic field. They clump together in long strings, showing the direction of the magnetic field lines at each point. (B) Magnetic field of an ideal cylindrical magnet with its axis of symmetry inside the image plane. (C) Schematic representation of Earth's magnetic field lines.

Observing the Sun using white light filters

(A) White light solar filters are made either of a special Mylar or coated glass. (L. Black) (B) The Sun in white light is mostly featureless, except for sunspots and granulation (which requires exceptional seeing and high magnification).

Photosphere in white light

(A) White light solar image taken during the Mercury transit of May 9, 2016. (L. Black) (B) Image of the Sun in white light taken by the Solar and Heliospheric Observatory (SOHO) on October 28, 2011. Note the numerous sunspots and sunspot groups. Some of the sunspots are larger than the Earth. (SOHO/ESA and NASA)

Charged Particles Spiral AlongMagnetic Field Lines

(A, B) Electrons will spiral along magnetic field lines. (C) These tracks were made by charged particles in a bubble chamber (a technology used in the 1970s). A magnetic field perpendicular to the image produces a force that curves the orbits of charged particles. (CERN) (D) TRACE image of the Sun's magnetic fields lines: charged particles (plasma) follow the loops and streamers.

Proton-Proton Chain-2

(C) In Step 3, two helium-3s (He3) collide to produce a normal helium-4 nucleus (He4) with the release of two protons (H1). These protons are then available to continue the fusion process. The mass of the helium-4 nucleus (often called an alpha particle) is slightly less than the mass of the original 4 protons by about 0.7%. (The mass of a neutron is slightly less than the mass of a proton.) The "missing" mass has been converted to energy via Einstein's famous equation: E = mc2. (NASA/NSSTC/Hathaway)

Coronal Mass Ejection

A coronal mass ejection (CME) is a massive burst of energetic charged particles (solar wind) and magnetic fields rising above the solar corona or being released into space. CMEs are often associated with other forms of solar activity, most notably solar flares, but a causal relationship has not been fully established. Most ejections originate from active regions on the Sun's surface, such as groupings of sunspots associated with frequent flares. Leaves the sun.

Sunspots Temperature

A large sunspot might have a temperature of about 4,000 K (about 3,700 °C or 6,700 °F). This is much lower than the 5,800 K (about 5,500 °C or 10,000 °F) temperature of the bright photosphere that surrounds the sunspots. However, if you could cut an average sunspot out of the Sun and place it in the night sky, it would be about as bright as a full moon.

Photon on Random Walk

A photon on a "random walk" takes 100,000 or more years to travel from the Sun's core to its surface. Energy gradually leaks out of radiative zone in the form of randomly bouncing photons. Photons cannot travel quickly and directly from the core through the radiative zone to the surface. The free electrons in the dense plasma of the core and radiative zone deflect and scatter the photons after they have moved only a very short distance (typically around 10−4 m or 0.1 mm).

Plage

A plage is a bright region in the chromosphere of the Sun, typically found in regions of the chromosphere near sunspots. They mark areas of nearly vertical emerging or reconnecting magnetic field lines.

Solar Prominences

A prominence is a large, bright, gaseous feature extending outward from the Sun's surface, often in a loop shape. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's corona. Unlike the corona's extremely hot plasma, prominences contain much cooler plasma, similar to that of the chromosphere. Prominence plasma is typically a hundred times cooler and denser than coronal plasma. A prominence forms over timescales of about a day, and prominences may persist in the corona for several weeks or months. A typical prominence extends over many thousands of kilometers; the largest on record was estimated at over 800,000 km (500,000 mi) long—roughly the radius of the Sun. This eruptive solar prominence was imaged over about a five-hour period on March 19, 2011, by the Solar Dynamics Observatory (SDO) in the extreme ultraviolet centered around 304 Å (30.4 nm). The prominence became unstable and erupted into space with a distinct twisting motion. Many solar astronomers consider this eruptive prominence seen June 28, 1945, to be the "granddaddy" of all time.

Coronal Loops Sizes

A schematic depicting the different scales of coronal loop that exist in the lower corona and transition region. Many scales have been observed, and it is believed there are many sub-resolution structures below the transition region threading through the chromosphere. Highly radiating coronal loops may share the same footprint location as open flux tubes (i.e., solar wind and coronal holes) and therefore may share similar physics leading to the conjecture that coronal loops may share similar heating mechanisms as the solar wind.

Coronal Magnetic Field Loops

A slow rotating tour of a data-based coronal loop model. The solar model is constructed from magnetogram data collected by SOHO/MDI. Coronal loops are visible at the higher temperatures of ultraviolet light, in this case, 1950 nm, the filter wavelength of SOHO/EIT. This animation shows convoluted magnetic field lines extending out all over the Sun.

Solar Flares

A sudden brightening observed over the Sun's surface or the solar limb and represents a large energy release of up to 6 x 10<25 joules (about a sixth of the total energy output of the Sun each second). They are often followed by a large coronal mass ejection (CME). The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day or two after the event. 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. The same energy releases may produce CMEs, although the relationship between CMEs and flares is still not well established. Twisting magnetic fields beneath the surface of the sun erupt into a large solar flare.

Solar Fountains

At the core of the solar magnetic field, immense jets of hot gas are forced to the surface through increases in pressure. Just like an earthly geyser, when the pressure releases, the gases fall back towards the Sun's surface. But what causes the pressure? Solar fountains are caused by rearrangements of the Sun's magnetic field, a continual process that results in looping cycles of increasing and decreasing pressure.

Coronal Holes

Coronal holes are regions where the corona is dark. These features were discovered when X-ray telescopes were first flown above the Earth's atmosphere to reveal the structure of the corona across the solar disc. Coronal holes are associated with "open" magnetic field lines and are often found at the Sun's poles. The high-speed solar wind (800 km/s) is known to originate in coronal holes.

Coronal Loops

Coronal loops are found around sunspots and in active regions. These structures are associated with the closed magnetic field lines that connect magnetic regions on the solar surface. Many coronal loops last for days or weeks but most change quite rapidly. Some loops, however, are associated with solar flares and are visible for much shorter periods. These loops contain denser material than their material than their surroundings. The three-dimensional structure and the dynamics of these loops is an area of active research.

Solar Activity's Effect on Humans

Coronal mass ejections are large bubbles of charged particles ejected from the corona as a result of flares and other solar storms. These CMEs have strong magnetic fields and on reaching Earth can create a geomagnetic storm. Geomagnetic storms create auroras and can disrupt radio communications and electric power grids and damage electronic circuits in orbiting satellites. Solar activity surged on December 12, 2010, when the Sun erupted three times in quick succession, hurling a trio of bright coronal mass ejections (CMEs) into space.

Nuclear Fusion in the Sun

During fusion, the sequence of steps that occurs in the Sun is called the proton-proton chain. In the proton-proton chain, four protons fuse into one helium-4 nucleus. Gamma rays and neutrinos carry off the energy released in the reaction. The response of the core pressure to changes in the nuclear fusion rate is essentially a thermost at that keeps the Sun's central temperature steady. The proton-proton chain (Fusion Technology Institute, U. WI-Madison) The accompanying image of the Sun was taken in the extreme ultraviolet portion of the electromagnetic spectrum at 304 Å (30.4 nm). Because UV radiation is invisible to human eyes, NASA uses false color to designate particular wavelengths. Orange is used for images taken at 304 Å, green for 195 Å, and blue for 171 Å.

Edward Maunder

Edward Walter Maunder (1851-1928) was an English astronomer best remembered for his study of sunspots and the solar magnetic cycle that led to his identification of the period from 1645 to 1715 that is now known as the Maunder Minimum. In Europe and North America, this period was also known as the "Little Ice Age" as winters were particularly harsh in these years.

Thomas Harriot

Englishman Thomas Harriot (1560-1621), a contemporary of Galileo, also observed and drew sunspots. He was the first person to observe sunspots with a telescope on December 8, 1610. (Shakespeare wrote the plays Cymbeline and The Tempest during this time period.)

Temperature of the Corona

Even though the temperature of the corona is very high (1-2 million K), the actual amount of energy in the solar corona is not large. Coronal temperature is actually a measure of how fast individual particles (electrons, in particular) are moving. There are not very many coronal particles, even though each particle has a high speed. The corona has less than one-billionth (10−9) the density of the Earth's atmosphere. A) The Sun's corona is visible during a total solar eclipse. This is the eclipse of March 9, 1997. (B) On the microscopic scale, temperature can be defined as the average energy (and therefore speed) of the particles in a system.

Galileo's Drawings of Sunspots

Galileo published a description of sunspots in 1613 titled Letters on Sunspots.

Solar Granules

Granules are small (about 1000 km across) cellular features that cover the entire Sun except for those areas covered by sunspots. These features are the tops of convection cells where hot fluid rises up from the interior in the bright areas, spreads out across the surface, cools and then sinks inward along the dark lanes. Individual granules last for only about 20 minutes. The granulation pattern is continually evolving as old granules are pushed aside by newly emerging ones. The flow within the granules can reach supersonic speeds of more than 7 km/s (15,000 mph) and produce sonic "booms" and other noise that generates waves on the Sun's surface.

Vibrations on the Sun

Helioseismology is the study of the propagation of wave oscillations, particularly acoustic pressure waves, in the Sun. Unlike seismic waves on Earth, solar waves have practically no shear component (s-waves). Solar pressure waves are believed to be generated by the turbulence in the convection zone near the surface of the Sun. Certain frequencies are amplified by constructive interference. In other words, the turbulence "rings" the Sun like a bell. The acoustic waves are transmitted to the outer photosphere of the Sun. These oscillations are detectable on almost any time series of solar images but are best observed by measuring the Doppler shift of photospheric absorption lines.

Helmet Streamers

Helmet streamers are large cap-like coronal structures with long pointed peaks that usually overlie sunspots and active regions. We often find a prominence or filament lying at the base of these structures. Helmet streamers are formed by a network of magnetic loops that connect the sunspots in active regions and help suspend the prominence material above the solar surface. The closed magnetic field lines trap the electrically charged coronal gases to form these relatively dense structures. The pointed peaks are formed by the action of the solar wind blowing away from the Sun in the spaces between the streamers.

Temperature and Nuclear Fusion

High temperature enables nuclear fusion to happen in the core of the Sun. However, this fusion process is significantly aided by quantum tunneling. (slide 14) Extreme heat and pressure, which are found in the core of a star, are necessary for nuclei to overcome electromagnetic repulsion.

Hydrostatic Equalibrium

In a star, there is hydrostatic equalibrium between the outward thermal pressure from below and the weight of the gas above pressing inward. The uniform gravitational field compresses the star into the most compact shape possible. A rotating star in hydrostatic equalibrium is an oblate spheroid up to a certain (critical) angular velocity. (A) Compression due to gravity is balanced by the outward pressure of fusion. (B) The greater gravity compresses the gas—making it denser and hotter—the more the outward pressure increases

Proton-Proton Chain-1

In stars like the Sun, the nuclear burning takes place through a three-step process called the proton-proton or pp chain. he proton-proton chain. (A) In Step 1, two protons (H1) collide to produce deuterium (D2), a positron (e+), and a neutrino (ν). (B) In Step 2, a proton (H1) collides with the deuterium (D2) to produce a helium-3 nucleus (He3) and a gamma ray (γ). In the actual process, four protons are required. (NASA/NSSTC/Hathaway)

The butterfly diagram

In the second half of the 19th century, it was noted that as the cycle progresses, sunspots appear first at mid-latitudes, and then closer and closer to the equator until solar minimum is reached. This pattern is best visualized in the form of the so-called butterfly diagram, first constructed by the husband-wife team of Edward Walter and Annie Maunder in the early twentieth century

The Sun in X-Rays

In this X-ray image, the corona appears bright in places where magnetic fields trap hot gas. The X-rays we detect from the Sun actually come from the solar corona, not the solar surface.

Strong Force

It is stronger than the weak force is only felt by quarks. It behaves like elastic because the farther apart you pull the two quarks, the stronger the strong force gets between them. It's observable in two areas, on the smaller scale (less than about 0.8 fm, the radius of the nucleon), it is the force (carried by gluons) that holds quarks together to form a proton, the neutron, and pion (collectively called hadrons). The theory of quantum chromodynamics (QCD), is the theory of quark-gluon interactions. Unlike all other forces-electromagnetic, weak, and gravitational-the strong force does not diminish in strength with increasing distance. After a limiting distance (about the size of a hadron) has been reached, it remains at a strength of about 10,000 newtons, no matter how much farther the distance between the quarks. (Slide 11) Here a proton is composed of three quarks—up, up, down—which are connected by gluons. Gluons are thought to interact with quarks and other gluons because all carry a type of charge called "color charge." Color charge is analogous to electromagnetic charge, but it comes in three types rather than two, and it results in a different type of force, with different rules of behavior.

Magnetic Butterfly Diagram

It is widely believed that the Sun's magnetic field is generated by a magnetic dynamo within the Sun. The fact that the Sun's magnetic field changes dramatically over the course of just a few years, and the fact that it changes in a cyclical manner indicates that the magnetic field continues to be generated within the Sun. The reversal of the polar magnetic fields near the time of cycle maximum as seen in the magnetic butterfly diagram

Nuclear Fusion makes the Sun shine

It was not until the 20th century that scientists realized that only nuclear forces in the form of nuclear fusion could produce the energy that makes the Sun shine. Gravitational equilibrium or hydrostatic equilibrium is the balance between gravity pulling inward and underlying pressure from nuclear fusion pushing outward. About 5.5 billion years from now, the Sun will exhaust its nuclear fuel, fusion will cease, and contraction will begin again as no outward force will oppose gravity. When nuclear fusion began in the Sun's core, the contraction due to gravity stopped. Fusion pressure outward was balanced by gravity pulling inward, which is known as hydrostatic equilibrium.

Solar Granulation

Japan's Solar Optical Telescope (SOT) aboard the HINODE spacecraft took this image of solar granulation in 2006. Each granule is as big as a terrestrial continent. The SOT can achieve an angular resolution of 0.2 arcseconds.

Sun's Interior Structure

Just inside the Sun is the convection zone, where energy generated in the core travels upward transported by columns of rising hot gas that replaces falling cooler gas. A third of the way to the core is the radiation zone, where energy is carried upward by photons of light, and the temperature rises to almost 10 million K. Energy produced in the dense core of the Sun can take 100,000 or more years to reach the surface. The internal structure of the Sun cannot be seen visually, so astronomers use techniques such as solar vibrations, neutrinos coming from nuclear reactions in the Sun's core, and supercomputer modeling of astrophysical theory to explore these regions.

Sunspot pairs

Loops of bright gas often connect sunspot pairs. Magnetic fields of sunspots suppress convection and prevent surrounding plasma from sliding sideways into sunspots, creating a cooler region that appears dark against the hotter and brighter surface of the Sun. (A) The magnetic loops that connect two sunspots are always positive on one end and negative on the other. Like a bar magnet, one end has the opposite polarity of the other end. (NASA) (B) Animated image of two sunspots connected by magnetic field lines. The hot, glowing plasma allows us to see the invisible magnetic field lines.

Sun's Luminosity

Luminosity is the total power output of the Sun Capturing and storing 1 second of the Sun's luminosity would power human energy needs for 500,000 years L(Sun circle symbol)= 3.846 x 10<26 Watts (3.846 x 10<26 j/s) By measuring the total energy produced by one square meter of the Sun, the total energy output of the Sun—its luminosity —can be estimated.

Magnetism and Solar Flares

Magnetic activity causes solar flares that send bursts of X-rays and charged particles into space. TRACE (Transition Region and Coronal Explorer) observed this M1 flare at 171 Å (most sensitive to emission from gas around 1 million degrees). The frame on the left shows the bright, flaring loops only 15 minutes after observations began. During the first hour and a half after the flare, nothing much spectacular happens: the post-flare loop system shows up, with brightening and dimming of cooling loops, gradually increasing with height. Then, high loops brighten, but the spacecraft goes through a radiation zone and stops observing. When it resumes observing about 20 minutes later, a tangled web of high loops shows up (middle image). Coronal rain streams out of them as cooling material falls back to the surface. The system seems to simplify dramatically, but it is unclear whether the field is reorganizing rapidly, or whether the simpler configuration of other field lines replaces that seen in the central image. Almost three hours after the main flare (with other, smaller flares still going off), a much simplified loop system continues to cool and drain (right image). The field of view in the images is 115,000 km by 155,000 km.

Sun's plasma trace magnetic field lines

Magnetic field lines between two active regions (sunspots) extended across about one-third of the Sun to make their connections (July 23-24, 2012). The magnetically powerful active regions were just rotating into view, giving a wonderful profile of their activity. The looping arcs above each active region show off the magnetic field lines. The image was taken in the extreme ultraviolet.

Magnetic field lines and electric currents

Moving electrons (current in a wire) also produce magnetic field lines, demonstrating that magnetism and electricity are manifestations of the same force: electromagnetism (A) Current, I, through a wire produces a magnetic field, B. The field is oriented according to the right-hand rule. (B) Right-hand rule states that a current flowing in the direction of the white arrow produces a magnetic field shown by the red arrows. Your fingers curl in the direction of the magnetic field, and your thumb points in the direction of the current, (C) Moving charges create magnetic fields, so when an electric current flows through a coil of wire (called a solenoid), an electromagnet is created.

Fusion creates Neutrinos

Neutrinos (ν) created during nuclear fusion fly directly through the Sun from its core. Observations of these solar neutrinos can tell us about the nuclear reactions that are occurring in the Sun's core. Begun in the 1960s, the Homestake experiment (built by R. Davis, Jr.) led to the solar neutrino problem where fewer neutrinos are observed coming from the Sun than are predicted by theory. The discrepancy has since been resolved by a new understanding of neutrino physics, requiring a modification of the Standard Model of particle physics—specifically, neutrino oscillation. Because neutrinos have a small mass, they can change from the type that had been expected to be produced in the Sun's interior into two types that would not be caught by the detectors in use at the time.

The Sun's Color

Note that the Sun's color is white, not yellow, though we often think of it as yellow. The Sun only appears yellow, or even orange or red, when it is close to the horizon: The blue and green light is selectively absorbed and scattered by Earth's atmosphere. (Short-wavelength blue light scattered by air molecules produces the daytime blue sky.) Also, on many photographs, the filter used to reduce the solar brightness to a safe level adds a yellowish tint to the image. (A) The Sun appears white to our eyes. (B) A mosaic of extreme ultraviolet images. These false color images show the Sun's atmospheres at a range of different temperatures. Clockwise from top left: 1 million K (171 Å), 1.5 million K (195 Å), 60,000-80,000 K (304 Å), and 2.5 million K (286 Å).

Twin Solar Eruptions

On January, 28, 2011, two major eruptions occurred on the Sun. Separated by more than 1 million km, the two blasts occurred almost simultaneously on opposite corners of the solar disk. On the lower left, a magnetic filament became unstable and erupted, hurling a portion of itself into space. On the upper right, departing sunspot 1149 produced an M1-class solar flare and a bright coronal mass ejection. New research shows that eruptions on the Sun can "go global" with widely separated blasts unfolding in concert as they trigger and feed off of one another.

The sound of a Type II Solar Radio Burst

On March 26, 2008, the Sun unleashed an M2-class solar flare which in turn created a large coronal mass ejection (CME), propagating away from the solar disk. The CME was not directed toward Earth. A radio astronomer in New Mexico, Thomas Ashcraft, recorded the sound coming from his 21 MHz radio telescope during the event. He heard a strange "heaving sound" as the shock wave on the leading edge of the CME generated radio waves. CME generated radio waves that were converted to sound.

Magnetic Flux Transport Dynamo and Sunspots

On the image in the previous slide, the red inner sphere represents the Sun's radiative core and the blue mesh represents the solar surface. In between is the convection zone where the solar dynamo resides and sunspots originate. (a) Shearing (twisting) of the poloidal field (direction of magnetic field is toward the poles) by the Sun's differential rotation occurs near the bottom of the convection zone. Because the Sun is not a rigid body, it rotates faster at the equator than at the pole. (b) Toroidal field (magnetic field lines are parallel to latitude) are produced due to this shearing (twisting) by the differential rotation. (c) When the toroidal field is strong enough, buoyant loops of magnetism rise to the surface, twisting as they rise due to rotational influence. Sunspots (two black dots) are formed from these loops. (d,e,f) Additional magnetic flux emerges (d,e) and spreads (f) in latitude and longitude from decaying spots. (g) Meridional flow (north-south circulation indicated by yellow arrows) carries surface magnetic flux towards the poles, causing the polar fields to reverse in polarity (sign). (h) Some of this flux is then transported downward to the bottom and towards the equator. These poloidal fields have the opposite to sign those at the beginning of the sequence, in frame (a). (i) This reversed poloidal flux is then sheared again near the bottom by the differential rotation to produce a new toroidal field opposite in sign to that shown in (b). The solar cycle begins again.

Magnetic Storm

Plasma released by the Sun directly affects Earth and the rest of the solar system. Solar wind shapes the Earth's magnetosphere, and a magnetic storm can result. These frequently occurring storms can disrupt communications and navigational equipment, damage satellites, and even cause blackouts. A schematic of a solar storm as it approaches Earth. (Distances are not to scale.) The white lines represent the solar wind; the purple line is the bow shock line; and the blue lines surrounding the Earth represent its protective magnetosphere. The magnetic cloud of plasma can extend to 48 million km (30 million miles) wide by the time it reaches Earth.

Polar Plumes

Polar plumes are long thin streamers that project outward from the Sun's north and south poles. We often find bright areas at the footpoints of these features that are associated with small magnetic regions on the solar surface. These structures are associated with the "open" magnetic field lines at the Sun's poles. The plumes are formed by the action of the solar wind in much the same way as the peaks on the helmet streamers. Polar plumes emanate from the Sun's poles.

Quantum Tunneling

Refers to quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the Sun. Quantum tunneling through a barrier. The energy of the tunneled particle is the same but the amplitude is decreased. Explained by using the Heisenberg uncertainty principle and the wave-particle duality of matter. Purely quantum mechanical concepts are central to the phenomenon, so quantum tunneling is one of the novel implications of quantum mechanics. (slide 16)

Sunspot Growth

SDO took a closer look as a barely emerging sunspot group (left) grew into a large sunspot group (right) over less than three days (January 6-8, 2012). Sunspots are almost always changing, but the speed of growth here was quite striking. Though sunspots are often the source for solar storms, this particular group did not produce any major storms. For a size comparison, the spot on the left of the group is easily larger than Earth

The Sun's Radius

Scientists have measured the diameter of the Sun with unprecedented accuracy by using a spacecraft to time the transits of the planet Mercury across the face of the Sun in 2003 and 2006. They measured the Sun's radius as 696,342 km (432,687 mi) with an uncertainty of only 65 km (40 mi) or 0.009%. This accuracy was achieved by using the solar telescope on NASA's SOHO (Solar and Heliospheric Observatory) satellite, thereby bypassing the blurring caused by Earth's atmosphere that occurs when observations are made from the ground. The measurements were made with the MDI (Michelson Doppler Imager) onboard SOHO. Mercury's path across the solar disk as seen from SOHO on November 8, 2006. (NASA)

Facule

Solar faculae are bright spots that form in the canyons between solar granules, short-lived convection cells several thousand kilometers across that constantly form and dissipate over timescales of several minutes. Faculae are produced by concentrations of magnetic field lines. Strong concentrations of faculae appear in solar activity, with or without sunspots. The faculae and the sunspots contribute noticeably to variations in the "solar constant." The chromospheric counterpart of a facular region is called a plage. Faculae are bright areas that are usually most easily seen near the limb, or edge, of the solar disk. These are also magnetic areas but the magnetic field is concentrated in much smaller bundles than in sunspots. While the sunspots tend to make the Sun look darker, the faculae make it look brighter. During a sunspot cycle the faculae actually win out over the sunspots and make the Sun appear slightly (about 0.1%) brighter at sunspot maximum than at sunspot minimum.

Spicules

Spicules are cylindrical jets of gas about 500 km across and 7,000 km tall in the Sun's chromosphere. Spicules move rapidly upward from the photosphere at 20 km/s (44,700 mph). Spicules have lifetimes between 5 and 15 minutes, and there may be 300,000 or more of them on the Sun's surface at any given moment. Current research indicates the cause of spicules is sound-like waves that flow over the Sun's surface but leak into the Sun's atmosphere. They are found in the chromosphere and are usually associated with regions of high magnetic flux. Starting out as tall tubes of rapidly rising gas, spicules eventually fade as the gas peaks and then fall back down to the Sun.

Nuclear Fusion

Splitting a big nucleus into two smaller nuclei is nuclear fission. Combining small nuclei is nuclear fusion. In fusion, nuclei have to be pushed together to overcome the electromagnetic repulsion between two positively charged nuclei (1/r2) so that the short-range strong force(1/r) can take over. (A) At very short distances the attractive nuclear force is stronger than the repulsive electrostatic force. The problem is getting the nuclei close enough to fuse. (B) The strong nuclear force cuts off very sharply at a range of about 1 femtometer (10−15 m).

Sunspot Cycle II

Sunspot activity practically ceased between 1645 and 1715, a period known as the Maunder minimum. Though the precise mechanism of the Sun's magnetic fields and the sunspot cycle are not well understood, the current thinking is that convection brings weak magnetic fields from the solar interior, amplifying them as they rise. The Sun's rotation then stretches and shapes these fields. We think solar activity is linked to climate changes on Earth, but the mechanism by which this works is unknown. The cylindrical loops connect one magnetic polarity to the other.

Sun Spots

Sunspots are dark regions that appear on the "surface" of the Sun (also known as the photosphere). Sunspots are cooler than other parts of the Sun's surface. Sunspots are regions with strong magnetic fields. The Sun's complex magnetic field creates this cool region by inhibiting hot material from entering the spot.

Sunspots Size

Sunspots are very big structures. Though they look small compared to the Sun, the Sun has a diameter of 1.4 million km (870,000 mi). Many sunspots, like the one shown in the image, are as large as Earth. Most spots range in size from about 1,500 km (932 miles) to around 50,000 km (31,068 miles) in diameter. Once in a while, huge sunspots the size of Jupiter show up on the Sun's surface.

Sunspots and Magnetism

Sunspots, the most striking surface feature of the Sun, are magnetic "knots" that bubble up from the Sun's interior, eventually popping through the surface. Sunspots are "cooler" than the surrounding area because strong magnetic fields prevent hot plasma from entering the sunspot region. Sunspots are composed of two main parts: a dark central area called the umbra and a lighter outer area called the penumbra.

Super Granules

Supergranules are much larger versions of granules (about 35,000 km or 21,700 mi across) but are best seen in measurements of the "Doppler shift" where light from material moving toward us is shifted to the blue while light from material moving away from us is shifted to the red. These features also cover the entire Sun and are continually evolving. Individual supergranules last for a day or two and have flow speeds of about 0.5 km/s (1,000 mph). The fluid flows observed in supergranules carry magnetic field bundles to the edges of the cells where they produce the chromospheric network.

Properties of the Sun

The Sun is denoted by the symbol of a circle with a dot in it. Radius(Sun symbol)= 696,342 km (109 times REarth) Mass (Sun symbol)= 1.989 x 10<30 kg (333,000 times MEarth) Sun's mass is 70% H, 28% He, 2% heavier elements Luminosity L (Sun symbol)= 3.846 x 10<26 Watts (3.846 x 10<26 j/s) Rotation Rate= 25 days (equator) to 30 days (poles) Surface Temperature= 5,800 K (ave); 4,000 K sunspots Core Temperature= 15.6 Million K (1.56 x 10<7 K) Core Density= 150 g/cm<3 = 8X gold density Core mass= 35% H, 63% He, 2% heavier elements Solar age= 4.57 x 10<9 y

Differential Rotation of the Sun

The Sun rotates more slowly than the Earth, taking 25 days at the equator and 35 days at the poles to make one rotation. The Sun rotates faster at the equator because it is not solid (rigid) like the Earth. This phenomena is known as the differential rotation. (A) Differential rotation of the Sun showing rotation rates at different latitudes. (B) Sunspots moving across the face of the Sun. Galileo was the first to link the movement of sunspots to the Sun's rotation.

Solar Wind, Corona Holes, and Magnetic Fields

The Sun's atmosphere is threaded with magnetic fields (yellow lines). Areas with closed magnetic fields give rise to slow, dense solar wind (short, dashed, red arrows), while areas with open magnetic fields—so-called "coronal holes"—yield fast, less dense solar wind streams (longer, solid, red arrows). In addition to the permanent coronal holes at the Sun's poles, coronal holes can sometimes occur closer to the Sun's equator, as shown here just right of center. This image was taken on September 18, 2003, from the SOHO Extreme ultraviolet Imaging Telescope (EIT).

Observing the Sun in Hα

The Sun's chromosphere can be observed by the use of special filters that only allow a very narrow wavelength of light to pass. These narrow bandwidth filters center on the hydrogen-alpha (Hα) spectral line—656.3 nm, which is in the far red portion of the visible spectrum.

Core

The Sun's core is the central region where nuclear reactions consume hydrogen to form helium. These reactions, which release the energy that ultimately leaves the surface as visible light, are highly sensitive to temperature and density. The individual hydrogen nuclei (H+) must collide with enough energy to give a reasonable probability of overcoming the repulsive electrical force between these two positively charged particles. The temperature at the very center of the Sun is about 15,600,000 K, and the density is about 150 g/cm³ (about 8-10 times the density of gold or lead). Both the temperature and the density decrease as one moves outward from the center of the Sun. The nuclear burning is almost completely shut off beyond the outer edge of the core (about 25% of the distance to the surface or 175,000 km from the center). At that point the temperature is only half its central value, and the density drops to about 20 g/cm³. 1.56 x 10<7 K

Solar Wind and Coronal Holes

The Sun's magnetic field structures its atmosphere. The solar wind has charged particles flowing away from the Sun through coronal holes, where magnetic field lines extend away from the Sun. A) Coronal holes are the source for strong solar wind storms. (NASA/SOHO) (B) The solar wind is an outflow of hot plasma from the Sun. The Sun rotates quickly so the solar winds come out in arcs. Shown here is a qualitative reconstruction of the shape of the heliospheric current sheet between 1 and 5 AU. The constant solar rotation rate and the essentially constant radial solar wind outflow produce the characteristic Parker spiral signature.

Aurorae

The aurora borealis or northern lights (and aurora australis or southern lights) are the result of electrons colliding with the upper reaches of Earth's atmosphere. It was named by Pierre Gassendi in 1621 after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas.

Chromosphere in Calcium

The chromosphere is also visible in the light emitted by ionized calcium, Ca II, in the violet part of the solar spectrum at a wavelength of 393.4 nanometers (the Calcium K-line). This emission is seen in other solar-type stars where it provides important information about the chromospheres and activity cycles in those stars.

Chromosphere

The chromosphere is an irregular layer above the photosphere where the temperature rises from 6,000 K to about 20,000 K. At these higher temperatures hydrogen emits light that gives off a reddish color (H-alpha emission). This colorful emission can be seen in prominences that project above the limb of the sun during total solar eclipses. This is what gives the chromosphere its name ("color sphere").

The Convection Zone

The convection zone is the outer-most layer of the solar interior. It extends from a depth of about 200,000 km right up to the visible surface. At the base of the convection zone, the temperature is about 2,000,000 K, "cool" enough for the heavier ions such as carbon (C), nitrogen (N), oxygen (O), calcium (Ca), and iron (Fe) to hold onto some of their electrons. This makes the material more opaque so that it is harder for radiation to get through. This opacity traps heat that ultimately makes the fluid unstable, and it starts to "boil" or convect. Convection cells in a gravity field. In the Sun, hot gas rises while cool gas falls. The effects of this activity can be seen in solar granulation.

Sun's Surface Temperature

The effective temperature, or black-body temperature, of the Sun is 5,777 K. The Sun is spectral class G, which corresponds to a surface temperature around 6,000 K and the color yellow. The effective temperature, or black- body temperature, of the Sun (5777 K) is the temperature a black body of the same size must have to yield the same total emissive power.

Heliosphere and Heliopause

The heliopause is the boundary between the heliosphere and the interstellar medium outside the solar system. As the solar wind approaches the heliopause, it slows suddenly, forming a shock wave called the termination shock of the solar wind. The heliosphere and heliopause in relation to the Voyager spacecraft. On September 12, 2013, NASA announced that Voyager 1 had crossed the heliopause and entered interstellar space on August 25, 2012, making it the first manmade object to do so. As of 2013, the probe was moving with a relative velocity to the Sun of about 17 km/s.

Interface Layer (Tachocline)

The interface layer lies between the radiative zone and the convective zone. The fluid motions found in the convection zone disappear slowly from the top of this layer to its bottom where the conditions match those of the calm radiative zone. The tachocline is a transitional region between the radiative zone and the convection zone.

Diameter of the Photosphere

The photosphere is about 109 Earth diameters across, so over a million—1093 ≈ 1003 = (102)3or 106—Earths could fit inside the Sun. The Earth-Moon distance is about 30 Earth diameters. Thus, the Sun's diameter is about 3.6 times the distance from Earth to the Moon. The size of the Sun can be determined by knowing its distance and measuring its angular diameter. (Historically, the scale of the solar system was first found accurately by measuring transits of Venus—when Venus appears to cross the face of the Sun and is silhouetted against the Sun. Such events take place very rarely.) (A) 109 Earths would fit edge to edge across the diameter of the Sun. (B) Structures on the Sun are enormous when compared to the diameter of the Earth.

At the Sun's Surface

The photosphere is marked throughout by the bubbling pattern of granulation, which is produced by the underlying convection. Bright blobs are where hot gas is reaching the surface. (A) Size of granulation compared to Earth and its continents. (B) Solar granulation magnified.

Magnetic Flux

The product of the average magnetic field times the perpendicular area that it penetrates. (114)

Radiative Zone

The radiative zone extends outward from the outer edge of the core to the interface layer or tachocline at the base of the convection zone (from 25% of the distance to the surface to 70% of that distance. The radiative zone is characterized by the method of energy transport—radiation. The energy generated in the core is carried by light (photons) that bounces from particle to particle through the radiative zone. The density of solar material drops steadily as one leaves the core and moves through the radiative zone

Solar cycle

The solar cycle is a complex process that begins with magnetic fields generated deep within the Sun's convective zone. These fields are twisted (sheared) by the Sun's differential rotation. The fields break through the solar surface as loops of magnetism that form sunspots.

Solar Interior

The solar interior is separated into four regions by the different processes that occur there. Energy is generated in the core, the inner-most 25%. This energy diffuses outward by radiation, mostly gamma-rays and X-rays) through the radiative zone and by convective fluid flows (boiling motion) through the convection zone, the outermost 30%. The thin interface layer—the "tachocline"—between the radiative zone and the convection zone is where the Sun's magnetic field is thought to be generated. 4 Regions: Core Radiative Zone Interface Layer Convective Zone

Solar storm of 1859

The solar storm of 1859, also known as the Solar Superstorm, or the Carrington Event, was the most powerful solar storm in recorded history. From August 28, 1859, until September 2, 1859, numerous sunspots and solar flares were observed on the Sun. On September 1, the British astronomer Richard Carrington observed the largest flare, which caused a massive coronal mass ejection (CME). On September 1-2, 1859, the largest recorded geomagnetic storm occurred, causing the failure of telegraph systems all over Europe and North America. Auroras were seen around the world, most notably over the Caribbean. Today, such a storm could cause a trillion dollars or more in damage worldwide.

Solar Thermostat

The solar thermostat "works" because the Sun (as a main sequence hydrogen-burning star) is in a state of hydrostatic equilibrium. If the core were to cool, its fusion rate would decrease; the core would then contract, heat up (as a result of gravitational potential energy being converted into kinetic energy), and the fusion rate would be restored to its normal level. Conversely, if the core's temperature were to rise, the fusion rate would increase, the core would expand as fusion pressure pushed outward. An expanded core (with more surface area) would radiate away energy that in turn would cool and subsequently contract the core back to its equilibrium state.

Nuclear Fusion in the Sun-2

The solar thermostat depends on (1) gravitational equilibrium (pressure balances gravity) and (2) the flow of energy through the Sun must remain balanced. The Sun has gradually brightened by about 30% over the past 4.6 billion years. Radiative diffusion is the slow, outward migration of photons from the solar interior. Radiative diffusion occurs in the radiative zone, which extends from the core to about 70% of the Sun's radius. From there, energy moves upward via the convection zone. The three inner layers of the Sun where energy is made and then transported to the surface.

Solar Wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies between 1.5 and 10 keV. These particles can escape the Sun's gravity because of their high kinetic energy and the high temperature of the corona. The solar wind streams off of the Sun in all directions at an average speed of about 400 km/s (≈ 1 million mph). The solar wind creates the heliosphere, a bubble in the interstellar medium that surrounds the solar system out to 100 AU. The solar wind flows from an active region on the Sun, close to an equatorial coronal hole. Looking to the left of center, an outflow of plasma was measured to be moving at 10 km/s. Material outflow in regions like this one is thought to be a source of the low-speed solar wind. High-speed solar wind has speeds of 600-800 km/s.

Solar Wind and Comets

The solar wind is related to other solar phenomena such as geomagnetic storms that can knock out power grids on Earth, the aurorae, and the plasma tails of comets that always point away from the Sun. The first indication that the Sun might be emitting a "wind" came from comet tails, observed to point always away from the Sun. Kepler in the early 1600s guessed that those tails were driven by the pressure of sunlight, and his guess still holds true for the many comet tails which consist of dust. Comets, however, also have ion tails, shining in their own spectral lines, not just in scattered sunlight. Comet Hale-Bopp (shown above), a prominent comet that was at its brightest in March-April 1997, clearly exhibited such twin tails. While the dust tail was much brighter, the plasma tail had a different color, tending towards the blue. The Sun's solar wind pushes the ions in the plasma tail. Ion tails do not necessarily point straight away from the Sun because the flow velocity of the solar wind particles is not too many times larger than the velocity of the comet itself.

Sunspot Cycle I

The sunspot cycle averages 11 years from solar maximum to solar minimum. A total cycle is 22 years. Frequency of prominences and flares follows the sunspot cycle with these events being most common during solar maximum. Interval between solar maxima can vary between 7 and 15 years. The entire magnetic field of the Sun flip-flops about every 11 years. Sunspot Cycles: Past and Future. In red, solar physicist David Hathaway's predictions for the next two solar cycles and, in pink, Mausumi Dikpati's prediction for cycle 24. The current cycle (24) has so far produced fewer sunspots than Hathaway's prediction.

Transition Region

The transition region is a thin and very irregular layer of the Sun's atmosphere that separates the hot corona from the much cooler chromosphere. Heat flows down from the corona into the chromosphere and in the process produces this thin region where the temperature changes rapidly from 1,000,000 K (1,800,000 °F) down to about 20,000 K (40,000 °F). Hydrogen is ionized (stripped of its electron) at these temperatures and is therefore difficult to see. Instead of hydrogen, the light emitted by the transition region is dominated by such ions as C IV, O IV, and Si IV (carbon, oxygen, and silicon each with three electrons stripped off). These ions emit light in the ultraviolet region of the solar spectrum that is only accessible from space.

Probing the Tachocline

This graph shows the internal rotation in the Sun, with differential rotation in the outer convective region and almost uniform rotation in the central radiative region. The transition between these regions is called the tachocline. Results from helioseismology indicate that the tachocline is located at a radius of at most 0.70 times the solar radius measured from the core. Rotation rate is plotted as a function of fractional solar radius at selected latitudes (0°, 15°, 30°, 45°, 60°, 75°).

Interior of the Sun

We learn about the Sun's interior by Making mathematical models Observing solar vibrations Observing solar neutrinos We also observe vibrations of the Sun—solar quakes—that are caused by sound waves (waves of pressure). This type of study is called helioseismology. Data on solar vibrations agree very well with mathematical models of the solar interior. Still another way to study the Sun's interior is to observe the neutrinos(lightweight subatomic particles that interact weakly with matter) coming from the fusion reactions in the Sun's core.

Photospheric Limb Darkening

When viewed in visible light, the Sun appears to have a sharp outline, even though it has no solid surface. In visible light the center of the Sun appears brighter than the edge, or limb. This effect is known as limb darkening. Looking at the middle of the Sun allows us to see deeper into the Sun's interior than does looking at the Sun's edge. Because higher temperature means greater luminosity, the middle of the Sun appears brighter than its limb. We look through less material at the edges, and so it appears darker. An image of the Sun in visible light showing the limb darkening effect as a drop in intensity toward the edge or "limb" of the solar disk.