Stellar magnetic field
Encyclopedia
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
generated by the motion of conductive plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
inside a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
. This motion is created through convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
, which is a form of energy transport involving the physical movement of material. A localized magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
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
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
. This creates starspot
Starspot
Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...
s on the surface, and the related phenomenon of coronal loop
Coronal loop
Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle; it...
s.
Measurement
Zeeman effect
The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...
. Normally the atoms in a star's atmosphere will absorb certain frequencies of energy in the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
, 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.
A stellar spectropolarimeter is used to measure the magnetic field of a star. This instrument consists of a spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
combined with a polarimeter
Polarimeter
A polarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance....
. 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
Pic du Midi de Bigorre
The Pic du Midi de Bigorre or simply Pic du Midi is a mountain in the French Pyrenees famous for its astronomical observatory, the Observatoire du Pic du Midi de Bigorre , part of the Observatoire Midi-Pyrénées .-Pic du Midi Observatory:Construction of the observatory began in 1878 under the...
in the French Pyrenees
Pyrenees
The Pyrenees is a range of mountains in southwest Europe that forms a natural border between France and Spain...
mountains.
Various measurements—including magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
measurements over the last 150 years; 14C in tree rings; and 10Be in ice cores—have established substantial magnetic variability of the Sun on decadal, centennial and millennial time scales.
Field generation
Stellar magnetic fields, according to solar dynamoSolar dynamo
The solar dynamo is the physical process that generates the Sun's magnetic field. The Sun is permeated by an overall dipole magnetic field, as are many other celestial bodies such as the Earth. The dipole field is produced by a circular electric current flowing deep within the star, following...
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.
The magnetic field of a rotating body of conductive gas or liquid develops self-amplifying electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
s, 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 multitwisted. 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 pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
s. Another feature of this dynamo model
Dynamo theory
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
'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 time, the sunspot
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....
s activity is maximized (because of the lack of magnetic braking
Magnetic braking
Magnetic braking is a theory explaining the loss of solar angular momentum due to material getting captured by the solar magnetic field and thrown out at great distance from the surface of the Sun.-The problem:...
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
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
).
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
StarspotStarspot
Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...
s are regions of intense magnetic activity on the surface of a star. (On the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
they are termed sunspot
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....
s.) These form a visible component of magnetic flux tube
Flux tube
A flux tube is a generally tube-like region of space containing a magnetic field, such that the field at the side surfaces is parallel to those surfaces...
s that are formed within a star's convection zone
Convection zone
The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation...
. 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. Coronal loop
Coronal loop
Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle; it...
s often form above starspots, forming from magnetic field lines that stretch out into the corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
. These in turn serve to heat the corona to temperatures over a million kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s.
The magnetic fields linked to starspots and coronal loops are linked to flare activity, and the associated coronal mass ejection
Coronal mass ejection
A coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....
. The plasma is heated to tens of millions of kelvins, and the particles are accelerated away from the star's surface at extreme velocities.
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
Maunder Minimum
The Maunder Minimum is the name used for the period roughly spanning 1645 to 1715 when sunspots became exceedingly rare, as noted by solar observers of the time....
. Measurements of the time variation in stellar activity can be useful for determining the differential rotation rates of a star.
Magnetosphere
A star with a magnetic field will generate a magnetosphereMagnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
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
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
, which then move along these field lines. As the star rotates, the magnetosphere rotates with it, dragging along the charged particles.
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
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
from the star to the surrounding space, causing a slowing of the stellar rotation
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....
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.
Magnetic stars
T Tauri star
T Tauri stars are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.-Characteristics:...
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
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
to the surrounding protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...
. This allows the star to brake its rotation rate as it collapses.
Small, M-class stars (with 0.1–0.6 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es) that exhibit rapid, irregular variability are known as flare star
Flare star
A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to magnetic reconnection in the atmospheres of the stars. The brightness increase is...
s. 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.
Planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
e are created when a red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
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.
After some massive stars have ceased thermonuclear fusion, a portion of their mass collapses into a compact body of neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s called a neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
. 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
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
, which emits a narrow beam of energy that can periodically point toward an observer.
Compact and fast-rotating astronomical objects (white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
s, neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
s and black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
s) 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 pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
s).
An extreme form of a magnetized neutron star is the magnetar
Magnetar
A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious high-energy electromagnetic radiation, particularly X-rays and gamma rays...
. These are formed as the result of a core-collapse supernova. The existence of such stars was confirmed in 1998 with the measurement of the star SGR 1806-20
SGR 1806-20
|- style="vertical-align: top;"| Distance | 50,000 light-years SGR 1806-20 is a magnetar, a particular type of neutron star. It has been identified as a soft gamma repeater. SGR 1806-20 is located about 14.5 kiloparsecs from Earth on the far side of our Milky Way galaxy in the constellation of...
. 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 burst
Gamma ray burst
Gamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...
s.
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.
See also
- Alpha2 Canum Venaticorum variableAlpha2 Canum Venaticorum variableAn Alpha2 Canum Venaticorum variable is a type of variable star. These stars are chemically peculiar main sequence stars of spectral class B8p to A7p. They have strong magnetic fields and strong silicon, strontium, or chromium spectral lines...
- Dynamo theoryDynamo theoryIn geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
- Earth's magnetic fieldEarth's magnetic fieldEarth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
- Fossil stellar magnetic fieldFossil stellar magnetic fieldFossil stellar magnetic fields or fossil fields are proposed as possible interstellar magnetic fields that became locked into certain stars....
- Intermediate polarIntermediate polarAn Intermediate Polar is a type of cataclysmic variable binary star system. In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf star in the form of an accretion disk. In intermediate polar systems, the accretion disk is disrupted...
- Magnetic fieldMagnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
- Peculiar starPeculiar starIn astrophysics, peculiar stars have distinctly unusual metal abundances, at least in their surface layers.Chemically peculiar stars are common among hot main sequence stars...
- Polar (cataclysmic variable)Polar (cataclysmic variable)A Polar is a type of cataclysmic variable binary star system with a very strong magnetic field.In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf star in the form of an accretion disk. In polar systems, the magnetic field of the...
- SX Arietis variableSX Arietis variableSX Arietis variables are a class of variable stars. They are high-temperature analogues of Alpha2 Canum Venaticorum variables and exhibit strong magnetic fields and intense He I and Si III spectral lines. They have brightness fluctuations of approximately 0.1 magnitudes with periods of...