Photosphere

The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to it being a spherical surface that is perceived to emit light. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately 2/3[1], or equivalently, a depth from which 50% of light will escape without being scattered. In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.

Temperature

The surface of a star is defined to have a temperature given by the effective temperature in the Stefan–Boltzmann law. Stars, except neutron stars, have no solid or liquid surface.[2] Therefore, the photosphere is typically used to describe the Sun's or another star's visual surface .

Composition of the Sun

The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant.

Sun

Sun Atmosphere Temperature and Density SkyLab
Solar atmosphere: temperature and density [3]. See here for meanings of extra lines in the graph.

The Sun's photosphere has a temperature between 4,500 and 6,000 K (4,230 and 5,730 °C)[4] (with an effective temperature of 5,777 K (5,504 °C))[5] and a density of about 1×106 kg/m3[6]; increasing with depth into the sun.[3] Other stars may have hotter or cooler photospheres. The Sun's photosphere is around 100 kilometers thick, and is composed of convection cells called granules—cells of plasma each approximately 1000 kilometers in diameter with hot rising plasma in the center and cooler plasma falling in the narrow spaces between them, flowing at velocities of 7 kilometer per second. Each granule has a lifespan of only about twenty minutes, resulting in a continually shifting "boiling" pattern. Grouping the typical granules are super granules up to 30,000 kilometers in diameter with lifespans of up to 24 hours and flow speeds of about 500 meter per second, carrying magnetic field bundles to the edges of the cells. Other magnetically-related phenomena include sunspots and solar faculae dispersed between the granules[7]. These details are too fine to be seen when observing other stars from earth.

Other layers of the Sun

The Sun's visible atmosphere has other layers above the photosphere: the 2,000 kilometer-deep chromosphere (typically observed by filtered light, for example H-alpha) lies just between the photosphere and the much hotter but more tenuous corona. Other "surface features" on the photosphere are solar flares and sunspots.

References

  1. ^ Carroll, Bradley W. & Ostlie, Dale A. (1996). Modern Astrophysics. Addison-Wesley.
  2. ^ As of 2004, although white dwarfs are believed to crystallize from the middle out, none have fully solidified yet [1]; and only neutron stars are believed to have a solid, albeit unstable [2], crust [3]
  3. ^ a b John A. Eddy (1979). "SP-402 A New Sun: The Solar Results From Skylab". NASA.
  4. ^ The Sun – Introduction
  5. ^ World Book at NASA – Sun
  6. ^ Stanford Solar Center (2008). "The Sun's Vital Statistics".
  7. ^ "NASA/Marshall Solar Physics". NASA.

External links

43 Aurigae

43 Aurigae is a star located 382 light years away from the Sun in the northern constellation of Auriga. It is just bright enough to be barely visible to the naked eye with an apparent visual magnitude of 6.33. The star is moving closer to the Earth with a heliocentric radial velocity of −3.4 km/s.This is an aging giant star with a stellar classification of K2 III, having exhausted the hydrogen at its core and expanded off the main sequence. Roughly three billion years old, this star has 1.43 times the mass of the Sun and 11 times the Sun's radius. It is radiating 49 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,552 K.

62 Aquilae

62 Aquilae is a single star located about 427 light years away from the Sun in the equatorial constellation of Aquila. 62 Aquilae is its Flamsteed designation. It is visible to the naked eye as a dim, orange-hued star with an apparent visual magnitude of 5.67.This is an aging giant star with a stellar classification of K4 III, having exhausted the supply of hydrogen at its core and expanded to 23 times the girth of the Sun. It is 11.2 billion years old with 0.89 times the Sun's mass. The star is radiating 153 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,246 K.

62 Aurigae

62 Aurigae is a star located 559 light years away from the Sun in the northern constellation of Auriga. It is visible to the naked eye as a dim, orange-hued star with an apparent visual magnitude of 6.02. This object is moving further from the Earth with a heliocentric radial velocity of +25 km/s. It is an aging giant star with a stellar classification of K2 III, having exhausted the supply of hydrogen at its core then expanded to 22 times the Sun's radius. 62 Aurigae is radiating 167 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,389 K.

Atmosphere

An atmosphere (from Ancient Greek ἀτμός (atmos), meaning 'vapour', and σφαῖρα (sphaira), meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in place by the gravity of that body. An atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the atmosphere is low.

The atmosphere of Earth is composed of nitrogen (about 78%), oxygen (about 21%), argon (about 0.9%), carbon dioxide (0.04%) and other gases in trace amounts. Oxygen is used by most organisms for respiration; nitrogen is fixed by bacteria and lightning to produce ammonia used in the construction of nucleotides and amino acids; and carbon dioxide is used by plants, algae and cyanobacteria for photosynthesis. The atmosphere helps to protect living organisms from genetic damage by solar ultraviolet radiation, solar wind and cosmic rays. The current composition of the Earth's atmosphere is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms.

The term stellar atmosphere describes the outer region of a star and typically includes the portion above the opaque photosphere. Stars with sufficiently low temperatures may have outer atmospheres with compound molecules.

Baily's beads

The Baily's beads effect, or diamond ring effect, is a feature of total and annular solar eclipses. As the Moon covers the Sun during a solar eclipse, the rugged topography of the lunar limb allows beads of sunlight to shine through in some places while not in others. The effect is named after Francis Baily, who explained the phenomenon in 1836. The diamond ring effect is seen when only one bead is left, appearing as a shining "diamond" set in a bright ring around the lunar silhouette.Lunar topography has considerable relief because of the presence of mountains, craters, valleys, and other topographical features. The irregularities of the lunar limb profile (the "edge" of the Moon, as seen from a distance) are known accurately from observations of grazing occultations of stars. Astronomers thus have a fairly good idea which mountains and valleys will cause the beads to appear in advance of the eclipse. While Baily's beads are seen briefly for a few seconds at the center of the eclipse path, their duration is maximized near the edges of the path of the umbra, lasting 1–2 minutes.

After the diamond ring effect has diminished, the subsequent Baily's beads effect and totality phase are safe to view without the solar filters used during the partial phases. By then, less than 0.001% of the Sun's photosphere is visible.

Observers in the path of totality of a solar eclipse see first a gradual covering of the Sun by the lunar silhouette for over an hour, followed by the diamond ring effect (visible without filters) as the last bit of photosphere disappears. As the burst of light from the ring fades, Bailey's beads appear as the last bits of the bright photosphere shine through valleys aligned at the edge of the Moon. As the Baily's beads disappear behind the advancing lunar edge (the beads also reappear at the end of totality), a thin reddish edge called the chromosphere (the Greek chrōma meaning "color") appears. Though the reddish hydrogen radiation is most visible to the unaided eye, the chromosphere also emits thousands of additional spectral lines.

CI chondrite

CI chondrites, sometimes C1 chondrites, are a group of rare stony meteorites belonging to the carbonaceous chondrites. Samples have been discovered in France, Canada, India, and Tanzania. Compared to all the meteorites found so far, their chemical composition most closely resembles the elemental distribution in the sun's photosphere.

Chromosphere

The chromosphere (literally, "sphere of color") is the second of the three main layers in the Sun's atmosphere and is roughly 3,000 to 5,000 kilometers deep. The chromosphere's rosy red color is only apparent during eclipses. The chromosphere sits just above the photosphere and below the solar transition region. The layer of the chromosphere atop the photosphere is homogeneous. A forest of hairy-appearing spicules rise from the homogeneous layer, some of which extend 10,000 km into the corona above.

The density of the chromosphere is only 10−4 times that of the photosphere, the layer beneath, and 10−8 times that of the atmosphere of Earth at sea level. This makes the chromosphere normally invisible and it can be seen only during a total eclipse, where its reddish color is revealed. The color hues are anywhere between pink and red.

Without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere beneath.

The density of the chromosphere decreases with distance from the center of the Sun. This decreases logarithmically from 1017 particles per cubic centimeter, or approximately 2×10−4 kg/m3 to under 1.6×10−11 kg/m3 at the outer boundary. The temperature decreases from the inner boundary at about 6,000 K to a minimum of approximately 3,800 K, before increasing to upwards of 35,000 K at the outer boundary with the transition layer of the corona.

Chromospheres have been observed also in stars other than the Sun. The Sun's chromosphere has been hard to examine and decipher, although observations continue with the help of the electromagnetic spectrum.

Corona

A corona (meaning 'crown' in Latin derived from Ancient Greek 'κορώνη' (korōnè, “garland, wreath”)) is an aura of plasma that surrounds the Sun and other stars. The Sun's corona extends millions of kilometres into outer space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph.

Spectroscopy measurements indicate strong ionization in the corona and a plasma temperature in excess of 1000000 kelvin, much hotter than the surface of the Sun.

Light from the corona comes from three primary sources, from the same volume of space. The K-corona (K for kontinuierlich, "continuous" in German) is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines spreads them so greatly as to completely obscure them, giving the spectral appearance of a continuum with no absorption lines. The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the zodiacal light. The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition.

Coronal loop

Coronal loops are huge loops of magnetic field beginning and ending on the Sun's visible surface (photosphere) projecting into the solar atmosphere (corona). Hot glowing ionized gas (plasma) trapped in the loops makes them visible. Coronal loops range widely in size up to several thousand kilometers long. They are transient features of the solar surface, forming and dissipating over periods of days to months. They form the basic structure of the lower corona and transition region of the Sun. These highly structured loops are a direct consequence of the twisted solar magnetic flux within the solar body. Coronal loops are associated with sunspots; the two "footpoints" where the loop passes through the sun's surface are often sunspots. This is because sunspots occur at regions of high magnetic field. The high magnetic field where the loop passes through the surface forms a barrier to convection currents, which bring hot plasma from the interior to the sun's surface, so the plasma in these high field regions is cooler than the rest of the sun's surface, appearing as a dark spot when viewed against the rest of the photosphere. The population of coronal loops varies with the 11 year solar cycle, which also influences the number of sunspots.

Granule (solar physics)

Granules on the photosphere of the Sun are caused by convection currents (thermal columns, Bénard cells) of plasma within the Sun's convective zone. The grainy appearance of the solar photosphere is produced by the tops of these convective cells and is called granulation.

The rising part of the granules is located in the center where the plasma is hotter. The outer edge of the granules is darker due to the cooler descending plasma. (The terms darker and cooler are strictly by comparison to the brighter, hotter plasma. Since luminosity increases with the fourth power of temperature, even a small loss of heat produces a large luminosity contrast; this "cooler", "darker" plasma is still far hotter and vastly brighter than a thermite reaction.) In addition to the visible appearance, which would be explained by convective motion, Doppler shift measurements of the light from individual granules provides evidence for the convective nature of the granules.

A typical granule has a diameter on the order of 1,500 kilometers and lasts 8 to 20 minutes before dissipating. At any one time, the Sun's surface is covered by about 4 million granules. Below the photosphere is a layer of "supergranules" up to 30,000 kilometers in diameter with lifespans of up to 24 hours.

John Ernest Williamson

John Ernest Williamson (8 December 1881 – 15 July 1966) invented the "photosphere" from which he filmed and photographed undersea. He is credited as being the first person to take an underwater photograph from a submarine.

Shell star

A shell star is a star having a spectrum that shows extremely broad absorption lines, plus some very narrow absorption lines. They typically also show some emission lines, usually from the Balmer series but occasionally of other lines. The broad absorption lines are due to rapid rotation of the photosphere, the emission lines from an equatorial disk, and the narrow absorption lines are produced when the disc is seen nearly edge-on.

Shell stars have spectral types O7.5 to F5, with rotation velocities of 200–300 km/s, not far from the point when the rotational acceleration would disrupt the star.

Solar radius

Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3:

695,700 kilometres (432,300 miles) is approximately 10 times the average radius of Jupiter, about 109 times the radius of the Earth, and 1/215th of an astronomical unit, the distance of the Earth from the Sun. It varies slightly from pole to equator due to its rotation, which induces an oblateness in the order of 10 parts per million.

Spicule (solar physics)

In solar physics, a spicule is a dynamic jet of about 500 km diameter in the chromosphere of the Sun. It moves upwards at about 20 km/s from the photosphere. They were discovered in 1877 by Father Angelo Secchi of the Observatory of Roman Collegium in Rome.

Stellar atmosphere

The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.

Sun

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers (864,000 miles), or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System.

Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.

The Sun is roughly middle-aged; it has not changed dramatically for more than four billion years, and will remain fairly stable for more than another five billion years. It currently fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand to eventually become a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, and no longer produce energy by fusion, but still glow and give off heat from its previous fusion.

The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominant calendar in use today.

Sunspot

Sunspots are temporary phenomena on the Sun's photosphere that appear as spots darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic field flux that inhibit convection. Sunspots usually appear in pairs of opposite magnetic polarity. Their number varies according to the approximately 11-year solar cycle.

Individual sunspots or groups of sunspots may last anywhere from a few days to a few months, but eventually decay. Sunspots expand and contract as they move across the surface of the Sun, with diameters ranging from 16 km (10 mi) to 160,000 km (100,000 mi). Larger sunspots can be visible from Earth without the aid of a telescope. They may travel at relative speeds, or proper motions, of a few hundred meters per second when they first emerge.

Indicating intense magnetic activity, sunspots accompany secondary phenomena such as coronal loops, prominences, and reconnection events. Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars other than the Sun are commonly called starspots, and both light and dark spots have been measured.

Supergranulation

Supergranulation is a particular pattern of convection cells on the Sun's surface called supergranules. It was discovered in the 1950s by A.B.Hart using Doppler velocity measurements showing horizontal flows on the photosphere (flow speed about 300 to 500 m/s, a tenth of that in the smaller granules). Later work (1960s) by Leighton, Noyes and Simon established a typical size of about 30000 km for supergranules with a lifetime of about 24 hours.

TRACE

Transition Region and Coronal Explorer (TRACE) was a NASA heliophysics and solar observatory designed to investigate the connections between fine-scale magnetic fields and the associated plasma structures on the Sun by providing high resolution images and observation of the solar photosphere, the transition region, and the corona. A main focus of the TRACE instrument is the fine structure of coronal loops low in the solar atmosphere. TRACE is the fourth spacecraft in the Small Explorer program, launched on April 2, 1998, and obtained its last science image in 2010.The satellite was built by NASA's Goddard Space Flight Center. Its telescope was constructed by a consortium led by Lockheed Martin's Advanced Technology Center. The optics were designed and built to a state-of-the-art surface finish by the Smithsonian Astrophysical Observatory (SAO). The telescope has a 30 cm (12 in) aperture and 1024×1024 CCD detector giving an 8.5 arc minute field of view. The telescope is designed to take correlated images in a range of wavelengths from visible light through the Lyman alpha line to far ultraviolet. The different wavelength passbands correspond to plasma emission temperatures from 4,000 to 4,000,000 K. The optics use a special multilayer technique to focus the difficult-to-reflect EUV light; the technique was first used for solar imaging in the late 1980s and 1990s, notably by the MSSTA and NIXT sounding rocket payloads.

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