Stellar magnetic field

A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively increasing the pressure without a comparable gain in density. As a result, the magnetized region rises relative to the remainder of the plasma, until it reaches the star's photosphere. This creates starspots on the surface, and the related phenomenon of coronal loops.[1]

Mass eject
The magnetic field of the Sun is driving this massive ejection of plasma. NOAA image.
Holly Gilbert, NASA GSFC solar scientist, explains a model of magnetic fields on the sun.

Measurement

ZeemanEffect
The lower spectrum demonstrates the Zeeman effect after a magnetic field is applied to the source at top.

The magnetic field of a star can be measured by means of the Zeeman effect. Normally the atoms in a star's atmosphere will absorb certain frequencies of energy in the electromagnetic spectrum, producing characteristic dark absorption lines in the spectrum. When the atoms are within a magnetic field, however, these lines become split into multiple, closely spaced lines. The energy also becomes polarized with an orientation that depends on orientation of the magnetic field. Thus the strength and direction of the star's magnetic field can be determined by examination of the Zeeman effect lines.[2][3]

A stellar spectropolarimeter is used to measure the magnetic field of a star. This instrument consists of a spectrograph combined with a polarimeter. The first instrument to be dedicated to the study of stellar magnetic fields was NARVAL, which was mounted on the Bernard Lyot Telescope at the Pic du Midi de Bigorre in the French Pyrenees mountains.[4]

Various measurements—including magnetometer measurements over the last 150 years;[5] 14C in tree rings; and 10Be in ice cores[6]—have established substantial magnetic variability of the Sun on decadal, centennial and millennial time scales.[7]

Field generation

Stellar magnetic fields, according to solar dynamo theory, are caused within the convective zone of the star. The convective circulation of the conducting plasma functions like a dynamo. This activity destroys the star's primordial magnetic field, then generates a dipolar magnetic field. As the star undergoes differential rotation—rotating at different rates for various latitudes—the magnetism is wound into a toroidal field of "flux ropes" that become wrapped around the star. The fields can become highly concentrated, producing activity when they emerge on the surface.[8]

The magnetic field of a rotating body of conductive gas or liquid develops self-amplifying electric currents, and thus a self-generated magnetic field, due to a combination of differential rotation (different angular velocity of different parts of body), Coriolis forces and induction. The distribution of currents can be quite complicated, with numerous open and closed loops, and thus the magnetic field of these currents in their immediate vicinity is also quite twisted. At large distances, however, the magnetic fields of currents flowing in opposite directions cancel out and only a net dipole field survives, slowly diminishing with distance. Because the major currents flow in the direction of conductive mass motion (equatorial currents), the major component of the generated magnetic field is the dipole field of the equatorial current loop, thus producing magnetic poles near the geographic poles of a rotating body.

The magnetic fields of all celestial bodies are often aligned with the direction of rotation, with notable exceptions such as certain pulsars.

Periodic field reversal

Another feature of this dynamo model is that the currents are AC rather than DC. Their direction, and thus the direction of the magnetic field they generate, alternates more or less periodically, changing amplitude and reversing direction, although still more or less aligned with the axis of rotation.

The Sun's major component of magnetic field reverses direction every 11 years (so the period is about 22 years), resulting in a diminished magnitude of magnetic field near reversal time. During this dormancy, the sunspots activity is at maximum (because of the lack of magnetic braking on plasma) and, as a result, massive ejection of high energy plasma into the solar corona and interplanetary space takes place. Collisions of neighboring sunspots with oppositely directed magnetic fields result in the generation of strong electric fields near rapidly disappearing magnetic field regions. This electric field accelerates electrons and protons to high energies (kiloelectronvolts) which results in jets of extremely hot plasma leaving the Sun's surface and heating coronal plasma to high temperatures (millions of kelvin).

If the gas or liquid is very viscous (resulting in turbulent differential motion), the reversal of the magnetic field may not be very periodic. This is the case with the Earth's magnetic field, which is generated by turbulent currents in a viscous outer core.

Surface activity

Starspots are regions of intense magnetic activity on the surface of a star. (On the Sun they are termed sunspots.) These form a visible component of magnetic flux tubes that are formed within a star's convection zone. Due to the differential rotation of the star, the tube becomes curled up and stretched, inhibiting convection and producing zones of lower than normal temperature.[9] Coronal loops often form above starspots, forming from magnetic field lines that stretch out into the corona. These in turn serve to heat the corona to temperatures over a million kelvins.[10]

The magnetic fields linked to starspots and coronal loops are linked to flare activity, and the associated coronal mass ejection. The plasma is heated to tens of millions of kelvins, and the particles are accelerated away from the star's surface at extreme velocities.[11]

Surface activity appears to be related to the age and rotation rate of main-sequence stars. Young stars with a rapid rate of rotation exhibit strong activity. By contrast middle-aged, Sun-like stars with a slow rate of rotation show low levels of activity that varies in cycles. Some older stars display almost no activity, which may mean they have entered a lull that is comparable to the Sun's Maunder minimum. Measurements of the time variation in stellar activity can be useful for determining the differential rotation rates of a star.[12]

Ssn yearly

Magnetosphere

A star with a magnetic field will generate a magnetosphere that extends outward into the surrounding space. Field lines from this field originate at one magnetic pole on the star then end at the other pole, forming a closed loop. The magnetosphere contains charged particles that are trapped from the stellar wind, which then move along these field lines. As the star rotates, the magnetosphere rotates with it, dragging along the charged particles.[13]

As stars emit matter with a stellar wind from the photosphere, the magnetosphere creates a torque on the ejected matter. This results in a transfer of angular momentum from the star to the surrounding space, causing a slowing of the stellar rotation rate. Rapidly rotating stars have a higher mass loss rate, resulting in a faster loss of momentum. As the rotation rate slows, so too does the angular deceleration. By this means, a star will gradually approach, but never quite reach, the state of zero rotation.[14]

Magnetic stars

Suaur
Surface magnetic field of SU Aur (a young star of T Tauri type), reconstructed by means of Zeeman-Doppler imaging

A T Tauri star is a type of pre–main sequence star that is being heated through gravitational contraction and has not yet begun to burn hydrogen at its core. They are variable stars that are magnetically active. The magnetic field of these stars is thought to interact with its strong stellar wind, transferring angular momentum to the surrounding protoplanetary disk. This allows the star to brake its rotation rate as it collapses.[15]

Small, M-class stars (with 0.1–0.6 solar masses) that exhibit rapid, irregular variability are known as flare stars. These fluctuations are hypothesized to be caused by flares, although the activity is much stronger relative to the size of the star. The flares on this class of stars can extend up to 20% of the circumference, and radiate much of their energy in the blue and ultraviolet portion of the spectrum.[16]

Straddling the boundary between stars that undergo nuclear fusion in their cores and non-hydrogen fusing brown dwarfs are the ultracool dwarfs. These objects can emit radio waves due to their strong magnetic fields. Approximately 5-10% of these objects have had their magnetic fields measured.[17] The coolest of these, 2MASS J10475385+2124234 with a temperature of 800-900 K, retains a magnetic field stronger than 1.7 kG, making it some 3000 times stronger than the Earth's magnetic field.[18] Radio observations also suggest that their magnetic fields periodically change their orientation, similar to the Sun during the solar cycle.[19]

Planetary nebulae are created when a red giant star ejects its outer envelope, forming an expanding shell of gas. However it remains a mystery why these shells are not always spherically symmetrical. 80% of planetary nebulae do not have a spherical shape; instead forming bipolar or elliptical nebulae. One hypothesis for the formation of a non-spherical shape is the effect of the star's magnetic field. Instead of expanding evenly in all directions, the ejected plasma tends to leave by way of the magnetic poles. Observations of the central stars in at least four planetary nebulae have confirmed that they do indeed possess powerful magnetic fields.[20]

After some massive stars have ceased thermonuclear fusion, a portion of their mass collapses into a compact body of neutrons called a neutron star. These bodies retain a significant magnetic field from the original star, but the collapse in size causes the strength of this field to increase dramatically. The rapid rotation of these collapsed neutron stars results in a pulsar, which emits a narrow beam of energy that can periodically point toward an observer.

Compact and fast-rotating astronomical objects (white dwarfs, neutron stars and black holes) have extremely strong magnetic fields. The magnetic field of a newly born fast-spinning neutron star is so strong (up to 108 teslas) that it electromagnetically radiates enough energy to quickly (in a matter of few million years) damp down the star rotation by 100 to 1000 times. Matter falling on a neutron star also has to follow the magnetic field lines, resulting in two hot spots on the surface where it can reach and collide with the star's surface. These spots are literally a few feet (about a metre) across but tremendously bright. Their periodic eclipsing during star rotation is hypothesized to be the source of pulsating radiation (see pulsars).

An extreme form of a magnetized neutron star is the magnetar. These are formed as the result of a core-collapse supernova.[21] The existence of such stars was confirmed in 1998 with the measurement of the star SGR 1806-20. The magnetic field of this star has increased the surface temperature to 18 million K and it releases enormous amounts of energy in gamma ray bursts.[22]

Jets of relativistic plasma are often observed along the direction of the magnetic poles of active black holes in the centers of very young galaxies.

Star-Planet Interaction Controversy

In 2008, a team of astronomers first described how as the exoplanet orbiting HD 189733 A reaches a certain place in its orbit, it causes increased stellar flaring. In 2010, a different team found that every time they observe the exoplanet at a certain position in its orbit, they also detected X-ray flares. Theoretical research since 2000 suggested that an exoplanet very near to the star that it orbits may cause increased flaring due to the interaction of their magnetic fields, or because of tidal forces. In 2019, astronomers combined data from Arecibo Observatory, MOST, and the Automated Photoelectric Telescope, in addition to historical observations of the star at radio, optical, ultraviolet, and X-ray wavelengths to examine these claims. Their analysis found that the previous claims were exaggerated and the host star failed to display many of the brightness and spectral characteristics associated with stellar flaring and solar active regions, including sunspots. They also found that the claims did not stand up to statistical analysis, given that many stellar flares are seen regardless of the position of the exoplanet, therefore debunking the earlier claims. The magnetic fields of the host star and exoplanet do not interact, and this system is no longer believed to have a "star-planet interaction."[23]

See also

References

  1. ^ Brainerd, Jerome James (July 6, 2005). "X-rays from Stellar Coronas". The Astrophysics Spectator. Retrieved 2007-06-21.
  2. ^ Wade, Gregg A. (July 8–13, 2004). "Stellar Magnetic Fields: The view from the ground and from space". The A-star Puzzle: Proceedings IAU Symposium No. 224. Cambridge, England: Cambridge University Press. pp. 235–243. doi:10.1017/S1743921304004612.
  3. ^ Basri, Gibor (2006). "Big Fields on Small Stars". Science. 311 (5761): 618–619. doi:10.1126/science.1122815. PMID 16456068.
  4. ^ Staff (February 22, 2007). "NARVAL: First Observatory Dedicated To Stellar Magnetism". Science Daily. Retrieved 2007-06-21.
  5. ^ Lockwood, M.; Stamper, R.; Wild, M. N. (1999). "A Doubling of the Sun's Coronal Magnetic Field during the Last 100 Years". Nature. 399 (6735): 437–439. Bibcode:1999Natur.399..437L. doi:10.1038/20867.
  6. ^ Beer, Jürg (2000). "Long-term indirect indices of solar variability". Space Science Reviews. 94 (1/2): 53–66. Bibcode:2000SSRv...94...53B. doi:10.1023/A:1026778013901.
  7. ^ Kirkby, Jasper (2007). "Cosmic Rays and Climate". Surveys in Geophysics. 28 (5–6): 333–375. arXiv:0804.1938. Bibcode:2007SGeo...28..333K. doi:10.1007/s10712-008-9030-6.
  8. ^ Piddington, J. H. (1983). "On the origin and structure of stellar magnetic fields". Astrophysics and Space Science. 90 (1): 217–230. Bibcode:1983Ap&SS..90..217P. doi:10.1007/BF00651562.
  9. ^ Sherwood, Jonathan (December 3, 2002). "Dark Edge of Sunspots Reveal Magnetic Melee". University of Rochester. Retrieved 2007-06-21.
  10. ^ Hudson, H. S.; Kosugi, T. (1999). "How the Sun's Corona Gets Hot". Science. 285 (5429): 849. Bibcode:1999Sci...285..849H. doi:10.1126/science.285.5429.849.
  11. ^ Hathaway, David H. (January 18, 2007). "Solar Flares". NASA. Retrieved 2007-06-21.
  12. ^ Berdyugina, Svetlana V. (2005). "Starspots: A Key to the Stellar Dynamo". Living Reviews. Retrieved 2007-06-21.
  13. ^ Harpaz, Amos (1994). Stellar evolution. Ak Peters Series. A. K. Peters, Ltd. p. 230. ISBN 978-1-56881-012-6.
  14. ^ Nariai, Kyoji (1969). "Mass Loss from Coronae and Its Effect upon Stellar Rotation". Astrophysics and Space Science. 3 (1): 150–159. Bibcode:1969Ap&SS...3..150N. doi:10.1007/BF00649601. hdl:2060/19680026259.
  15. ^ Küker, M.; Henning, T.; Rüdiger, G. (2003). "Magnetic Star-Disk Coupling in Classical T Tauri Systems". The Astrophysical Journal. 589 (1): 397–409. Bibcode:2003ApJ...589..397K. doi:10.1086/374408.
  16. ^ Templeton, Matthew (Autumn 2003). "Variable Star Of The Season: UV Ceti". AAVSO. Archived from the original on 2007-02-14. Retrieved 2007-06-21.
  17. ^ Route, M.; Wolszczan, A. (20 October 2016). "The Second Arecibo Search for 5 GHz Radio Flares from Ultracool Dwarfs". The Astrophysical Journal. 830 (2): 85. arXiv:1608.02480. Bibcode:2016ApJ...830...85R. doi:10.3847/0004-637X/830/2/85.
  18. ^ Route, M.; Wolszczan, A. (10 March 2012). "The Arecibo Detection of the Coolest Radio-flaring Brown Dwarf". The Astrophysical Journal Letters. 747 (2): L22. arXiv:1202.1287. Bibcode:2012ApJ...747L..22R. doi:10.1088/2041-8205/747/2/L22.
  19. ^ Route, M. (20 October 2016). "The Discovery of Solar-like Activity Cycles Beyond the End of the Main Sequence?". The Astrophysical Journal Letters. 830 (2): L27. arXiv:1609.07761. Bibcode:2016ApJ...830L..27R. doi:10.3847/2041-8205/830/2/L27.
  20. ^ Jordan, S.; Werner, K.; O'Toole, S. (January 6, 2005). "First Detection Of Magnetic Fields In Central Stars Of Four Planetary Nebulae". Space Daily. Retrieved 2007-06-23.
  21. ^ Duncan, Robert C. (2003). "'Magnetars', Soft Gamma Repeaters, and Very Strong Magnetic Fields". University of Texas at Austin. Archived from the original on 2013-05-17. Retrieved 2007-06-21.
  22. ^ Isbell, D.; Tyson, T. (May 20, 1998). "Strongest Stellar Magnetic Field yet Observed Confirms Existence of Magnetars". NASA/Goddard Space Flight Center. Retrieved 2006-05-24.
  23. ^ Route, Matthew (February 10, 2019). "The Rise of ROME. I. A Multiwavelength Analysis of the Star-Planet Interaction in the HD 189733 System". The Astrophysical Journal. 872 (1): 79. arXiv:1901.02048. Bibcode:2019ApJ...872...79R. doi:10.3847/1538-4357/aafc25.

External links

102 Herculis

102 Herculis is a single star in the northern constellation of Hercules. It is visible to the naked eye as a faint, blue-white hued star with an apparent visual magnitude of 4.37. Based upon parallax measurements, it is located around 920 light years away from the Sun. The star is moving closer to the Earth with a heliocentric radial velocity of −15 km/s.The stellar classification of this object matches a massive, early B-type star with a luminosity class of IV or V, corresponding to a subgiant or main sequence star, respectively. It is 20 million years old with nearly ten times the mass of the Sun and is spinning with a projected rotational velocity of 41 km/s. The strength of the stellar magnetic field has been measured at (209.5±135.4)×10−4 T. The star is radiating 3,632 times the Sun's luminosity from its photosphere at an effective temperature of 22,420 K.

Fossil stellar magnetic field

Fossil stellar magnetic fields or fossil fields are proposed as possible interstellar magnetic fields that became locked into certain stars.

Gamma Virginis

Gamma Virginis (γ Virginis, abbreviated Gamma Vir, γ Vir), officially named Porrima , is a binary star system in the constellation of Virgo. It consists of two almost identical main sequence stars at a distance of about 38 light years.

Glossary of astronomy

This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.

Herbig–Haro object

Herbig–Haro (HH) objects are bright patches of nebulosity associated with newborn stars. They are formed when narrow jets of partially ionized gas ejected by said stars collide with nearby clouds of gas and dust at speeds of several hundred kilometers per second. Herbig–Haro objects are ubiquitous in star-forming regions, and several are often seen around a single star, aligned with its rotational axis. Most of them lie within about one parsec (3.26 light-years) of the source, although some have been observed several parsecs away. HH objects are transient phenomena that last around a few tens of thousands of years. They can change visibly over quite short timescales of a few years as they move rapidly away from their parent star into the gas clouds of interstellar space (the interstellar medium or ISM). Hubble Space Telescope observations have revealed the complex evolution of HH objects over the period of a few years, as parts of the nebula fade while others brighten as they collide with the clumpy material of the interstellar medium.

First observed in the late 19th century by Sherburne Wesley Burnham, Herbig–Haro objects were not recognized as being a distinct type of emission nebula until the 1940s. The first astronomers to study them in detail were George Herbig and Guillermo Haro, after whom they have been named. Herbig and Haro were working independently on studies of star formation when they first analysed the objects, and recognized that they were a by-product of the star formation process. Although HH objects are a visible wavelength phenomena, many remain undetectable at these wavelengths due to dust and gas envelope and can only be seen at infrared wavelengths. Such objects, when observed in near infrared, are called Molecular Hydrogen emission-line Objects (MHOs).

Index of physics articles (F)

The index of physics articles is split into multiple pages due to its size.

To navigate by individual letter use the table of contents below.

Index of physics articles (S)

The index of physics articles is split into multiple pages due to its size.

To navigate by individual letter use the table of contents below.

Interstellar medium

In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.

The interstellar medium is composed of multiple phases, distinguished by whether matter is ionic, atomic, or molecular, and the temperature and density of the matter. The interstellar medium is composed primarily of hydrogen followed by helium with trace amounts of carbon, oxygen, and nitrogen comparatively to hydrogen. The thermal pressures of these phases are in rough equilibrium with one another. Magnetic fields and turbulent motions also provide pressure in the ISM, and are typically more important dynamically than the thermal pressure is.

In all phases, the interstellar medium is extremely tenuous by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 106 molecules per cm3 (1 million molecules per cm3). In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10−4 ions per cm3. Compare this with a number density of roughly 1019 molecules per cm3 for air at sea level, and 1010 molecules per cm3 (10 billion molecules per cm3) for a laboratory high-vacuum chamber. By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, by number 91% of atoms are hydrogen and 8.9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, known as "metals" in astronomical parlance. By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements. The hydrogen and helium are primarily a result of primordial nucleosynthesis, while the heavier elements in the ISM are mostly a result of enrichment in the process of stellar evolution.

The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, which ultimately contributes to molecular clouds and replenishes the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.

Voyager 1 reached the ISM on August 25, 2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the mission's end in 2025. Its twin, Voyager 2 entered the ISM in November 2018.

List of stellar properties

Pages Related to Stellar properties, Pages using the word stellar in a physics context.

Stellar aberration

Stellar aberration (derivation from Lorentz transformation)

Stellar age estimation

Stellar archaeology

Stellar astronomy

Stellar atmosphere

Stellar birthline

Stellar black hole

Stellar cartography

Stellar chemistry

Stellar chonography

Stellar classification

Stellar cluster

Stellar collision

Stellar core

Stellar coronae

Stellar density

Stellar disk

Stellar distance

Stellar drift

Stellar dynamics

Stellar engine

Stellar engineering

Stellar envelope see stellar atmosphere

Stellar evolution

Stellar flare

Stellar flux

Stellar fog

Stellar halo

Stellar interferometer

Stellar isochrone

Stellar kinematics

Stellar limb-darkening

Stellar luminosity

Stellar magnetic field

Stellar magnitude

Stellar mass

Stellar mass black hole

Stellar mass loss

Stellar molecule

Stellar navigation

Stellar near-collision

Stellar neighborhood

Stellar nucleosynthesis

Stellar nursery

Stellar occultation

Stellar parallax

Stellar physics

Stellar planetary

Stellar population

Stellar precession

Stellar pulsations

Stellar quake

Stellar radius

Stellar remnant

Stellar rotation

Stellar scintillation

Stellar seismology

Stellar spectra

Stellar spheroid

Stellar spin-down

Stellar structure

Stellar surface fusion

Stellar system

Stellar triangulation

Stellar uplift

Stellar variation

Stellar vault

Stellar wind

Stellar wind (disambiguation)

Stellar wobble

Stellar X-ray astronomy

Stellar-wind bubble

Other

Catalog of Stellar Identifications

Fossil stellar magnetic field

General Catalogue of Stellar Radial Velocities

General Catalogue of Trigonometric Stellar Parallaxes

Interstellar cloud

Inter-stellar clouds

Interstellar medium

List of stellar angular diameters

List of stellar streams

Low-dimensional chaos in stellar pulsations

Mark III Stellar Interferometer

Michelson stellar interferometer

NEMO (Stellar Dynamics Toolbox)

Non-stellar astronomical object

Quasi-stellar object

Substellar object

Sub-stellar object

Sydney University Stellar Interferometer

TD1 Catalog of Stellar Ultraviolet Fluxes

Timeline of stellar astronomy

Utah state stellar cluster

Young stellar object

Magnetic braking

Magnetic braking is a theory explaining the loss of stellar angular momentum due to material getting captured by the stellar magnetic field and thrown out at great distance from the surface of the star. It plays an important role in the evolution of binary star systems.

Magnetic field

A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. Magnetic fields are observed in a wide range of size scales, from subatomic particles to galaxies. The effects of magnetic fields are commonly seen in permanent magnets, which pull on magnetic materials (such as iron) and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges (electric currents) such as those used in electromagnets. Magnetic fields exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field vary with location. As such, it is an example of a vector field.

The term "magnetic field" is used for two distinct but closely related fields denoted by the symbols B and H. In the International System of Units, H, magnetic field strength, is measured in the SI base units of ampere per meter. B, magnetic flux density, is measured in tesla (in SI base units: kilogram per second2 per ampere), which is equivalent to newton per meter per ampere. H and B differ in how they account for magnetization. In a vacuum, B and H are the same aside from units; but in a magnetized material, B/ and H differ by the magnetization M of the material at that point in the material.

Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. Magnetic fields and electric fields are interrelated, and are both components of the electromagnetic force, one of the four fundamental forces of nature.

Magnetic fields are widely used throughout modern technology, particularly in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric motors and generators. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall effect. The Earth produces its own magnetic field, which shields the Earth's ozone layer from the solar wind and is important in navigation using a compass.

Murugesapillai Maheswaran

Murugesapillai Maheswaran (born October 12, 1939) is a mathematician, astrophysicist and educator. He was born in Colombo, Sri Lanka, and emigrated to the United States of America in 1985. Maheswaran has lived in Wausau, Wisconsin since 1986.

Outline of astronomy

The following outline is provided as an overview of and topical guide to astronomy:

Astronomy – studies the universe beyond Earth, including its formation and development, and the evolution, physics, chemistry, meteorology, and motion of celestial objects (such as galaxies, planets, etc.) and phenomena that originate outside the atmosphere of Earth (such as the cosmic background radiation).

Stellar rotation

Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface.

The rotation of a star produces an equatorial bulge due to centrifugal force. As stars are not solid bodies, they can also undergo differential rotation. Thus the equator of the star can rotate at a different angular velocity than the higher latitudes. These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field.The magnetic field of a star interacts with the stellar wind. As the wind moves away from the star its rate of angular velocity slows. The magnetic field of the star interacts with the wind, which applies a drag to the stellar rotation. As a result, angular momentum is transferred from the star to the wind, and over time this gradually slows the star's rate of rotation.

Superflare

Superflares are very strong explosions observed on stars with energies up to ten thousand times that of typical solar flares. The stars in this class satisfy conditions which should make them solar analogues, and would be expected to be stable over very long time scales.

The original nine candidates were detected by a variety of methods. No systematic study was possible until the launch of the Kepler satellite, which monitored a very large number of solar-type stars with very high accuracy for an extended period. This showed that a small proportion of stars had violent outbursts, up to 10,000 times as powerful as the strongest flares known on the Sun. In many cases there were multiple events on the same star. Younger stars were more likely to flare than old ones, but strong events were seen on stars as old as the Sun.

The flares were initially explained by postulating giant planets in very close orbits, such that the magnetic fields of the star and planet were linked. The orbit of the planet would warp the field lines until the instability released magnetic field energy as a flare. However, no such planet has showed up as a Kepler transit and this theory has been abandoned.

All superflare stars show quasi-periodic brightness variations interpreted as very large starspots carried round by rotation. Spectroscopic studies found spectral lines that were clear indicators of chromospheric activity associated with strong and extensive magnetic fields. This suggests that superflares only differ in scale from solar flares.

Attempts have been made to detect past solar superflares from nitrate concentrations in polar ice, from historical observations of auroras, and from those radioactive isotopes that can be produced by solar energetic particles. Although two promising events have been found in the carbon-14 records in tree rings, it is not possible to associate them definitely with a superflare event.

Solar superflares would have drastic effects, especially if they occurred as multiple events. Since they can occur on stars of the same age, mass and composition as the Sun this cannot be ruled out. However, solar-type superflare stars are very rare and are magnetically much more active than the Sun; if solar superflares do occur, it may be in well-defined episodes that occupy a small fraction of its time.

Formation
Evolution
Spectral
classification
Remnants
Hypothetical
Nucleosynthesis
Structure
Properties
Star systems
Earth-centric
observations
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