# Stellar magnetic field

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A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a main sequence (hydrogen-burning) 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]

## MeasurementEdit

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-space 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 decadel, centennial and millennial time scales.[7]

## Field generationEdit

Stellar magnetic fields are believed to be 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]

## Surface activityEdit

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]

## Magnetic starsEdit

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.[13]

Small, M-class stars (with 0.1–0.6 solar masses) that exhibit rapid, irregular variability are known as flare stars. These fluctuations are believed 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.[14]

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.[15]

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.

An extreme form of a magnetized neutron star is the magnetar. These are formed as the result of a core-collapse supernova.[16] 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.[17]

## ReferencesEdit

1. Brainerd, Jerome James (July 6, 2005). "X-rays from Stellar Coronas". The Astrophysics Spectator. Retrieved on 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: 235-243, Cambridge, England: Cambridge University Press. Retrieved on 2007-06-21.
3. Basri, Gibor (2006). "Big Fields on Small Stars". Science 311 (5761): 618–619. doi:10.1126/science.1122815. PMID 16456068. Retrieved on 4 February 2007.
4. Staff (February 22, 2007). "NARVAL: First Observatory Dedicated To Stellar Magnetism", Science Daily. Retrieved on 21 June 2007.
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. doi:10.1038/20867. Retrieved on 21 August 2008.
6. Beer, Jürg (2000). "Long-term indirect indices of solar variability". Space Science Reviews 94 (1/2): 53–66. Retrieved on 21 August 2008.
7. Kirkby, Jasper (2007). "Cosmic Rays and Climate". Surveys in Geophysics 28: 333–375. doi:10.1007/s10712-008-9030-6. Retrieved on 21 August 2008.
8. Piddington, J. H. (1983). "On the origin and structure of stellar magnetic fields". Astrophysics and Space Science 90 (1): 217–230. doi:10.1007/BF00651562. Retrieved on 21 June 2007.
9. Sherwood, Jonathan (December 3, 2002). "Dark Edge of Sunspots Reveal Magnetic Melee", University of Rochester. Retrieved on 21 June 2007.
10. Hudson, H. S.; Kosugi, T. (1999). "How the Sun's Corona Gets Hot". Science 285 (5429): 849. doi:10.1126/science.285.5429.849. Retrieved on 21 June 2007.
11. Hathaway, David H. (January 18, 2007). "Solar Flares". NASA. Retrieved on 2007-06-21.
12. Berdyugina, Svetlana V. (2005). "Starspots: A Key to the Stellar Dynamo". Living Reviews. Retrieved on 2007-06-21.
13. Küker, M.; Henning, T.; Rüdiger, G. (2003). "Magnetic Star-Disk Coupling in Classical T Tauri Systems". The Astrophysical Journal 589: 397–409. doi:10.1086/374408.
14. Templeton, Matthew (Autumn 2003). "Variable Star Of The Season: UV Ceti". AAVSO. Archived from the original on 2004-03-05. Retrieved on 2007-06-21.
15. Jordan, S.; Werner, K.; O'Toole, S. (January 06, 2005). "First Detection Of Magnetic Fields In Central Stars Of Four Planetary Nebulae", Space Daily. Retrieved on 23 June 2007.
16. Duncan, Robert C. (2003). "'Magnetars', Soft Gamma Repeaters, and Very Strong Magnetic Fields". University of Texas at Austin. Retrieved on 2007-06-21.
17. Isbell, D.; Tyson, T. (May 20, 1998). "Strongest Stellar Magnetic Field yet Observed Confirms Existence of Magnetars", NASA/Goddard Space Flight Center. Retrieved on 24 May 2006.