Starspot

Starspots are stellar phenomena, so-named by analogy with sunspots. Spots actually at the size of sunspots would be very hard to detect on other stars because they are too small to cause detectable fluctuations in brightness. The commonly observed starspots are in general much larger than those on the Sun: up to about 30% of the stellar surface may be covered, corresponding to starspots 100 times larger than those on the Sun.

Detection and measurements

To detect and measure the extent of starspots one uses several types of methods.

  • For rapidly rotating stars – Doppler imaging and Zeeman-Doppler imaging.[1]

    With the Zeeman-Doppler imaging technique the direction of the magnetic field on stars can be determined since spectral lines are split according to the Zeeman effect, revealing the direction and magnitude of the field.

  • For slowly rotating stars – Line Depth Ratio (LDR).

    Here one measures two different spectral lines, one sensitive to temperature and one which is not. Since starspots have a lower temperature than their surroundings the temperature-sensitive line changes its depth. From the difference between these two lines the temperature and size of the spot can be calculated, with a temperature accuracy of 10K.

  • For eclipsing binary stars – Eclipse mapping produces images and maps of spots on both stars.[2]
  • For stars with transiting extrasolar planets – Light curve variations.[3]

Temperature

Observed starspots have a temperature which is in general 500–2000 kelvins cooler than the stellar photosphere. This temperature difference could give rise to a brightness variation up to 0.6 magnitudes between the spot and the surrounding surface. There also seems to be a relation between the spot temperature and the temperature for the stellar photosphere, indicating that starspots behave similarly for different types of stars (observed in G–K dwarfs).

Lifetimes

The lifetime for a starspot depends on its size.

  • For small spots the lifetime is proportional to their size, similar to spots on the Sun.[4]
  • For large spots the sizes depend on the differential rotation of the star, but there are some indications that large spots which give rise to light variations can survive for many years even in stars with differential rotation.[4]

Activity cycles

The distribution of starspots across the stellar surface varies analogous to the solar case, but differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the Maunder minima for the Sun.

Flip-flop cycles

Another activity cycle is the so-called flip-flop cycle, which implies that the activity on either hemisphere shifts from one side to the other. The same phenomena can be seen on the Sun, with periods of 3.8 and 3.65 years for the northern and southern hemispheres. Flip-flop phenomena are observed for both binary RS CVn stars and single stars although the extent of the cycles are different between binary and singular stars.

Notes

  1. ^ Cameron 2008
  2. ^ Cameron 2008. Eclipse movies show spots on two imaged binaries
  3. ^ Sanchis-Ojeda, Roberto; Winn, Joshua N.; Marcy, Geoffrey W.; et al. (2013). "Kepler-63b: A Giant Planet in a Polar Orbit Around a Young Sun-like Star". The Astrophysical Journal. 775 (1): 54. arXiv:1307.8128v2. Bibcode:2013ApJ...775...54S. doi:10.1088/0004-637X/775/1/54. ISSN 0004-637X.
  4. ^ a b Berdyugina 5.3 Lifetimes

References

Further reading

12 Ophiuchi

12 Ophiuchi is a variable star in the constellation Ophiuchus. No companions have yet been detected in orbit around this star, and it remains uncertain whether or not it possesses a dust ring.

This star is categorized as a BY Draconis variable, with variable star designation V2133. The variability is attributed to large-scale magnetic activity on the chromosphere (in the form of starspots) combined with a rotational period that moved the active regions into (and out of) the line of sight. This results in low amplitude variability of 12 Ophiuchi's luminosity. The star also appears to display rapid variation in luminosity, possibly due to changes in the starspots. Measurements of the long-term variability show two overlapping cycles of starspot activity (compared to the Sun's single, 11-year cycle.) The periods of these two cycles are 4.0 and 17.4 years.

This star is among the top 100 target stars for NASA's planned Terrestrial Planet Finder mission [1]. However, the mission is now postponed indefinitely.

Its abundance of heavy elements (elements heavier than helium) is nearly identical to that of the Sun. The surface gravity is equal to , which is somewhat higher than the Sun's. The space velocity is 30 km/s relative to the solar system. The high rotation period and active chromosphere are indicative of a relatively young star.

58 Eridani

58 Eridani is a main-sequence star in the constellation Eridanus. It is considered a solar analogue, which means it has similar physical properties to the Sun. The star has a relatively high proper motion across the sky, and it is located about 43 light years distant. It is a probable member of the IC 2391 moving group of stars that share a common motion through space.This is a BY Draconis variable with the designation IX Eridani, which ranges in magnitude from 5.47 down to 5.51 with a period of 11.3 days. The X-ray emissions from this star's corona indicate an age of less than a billion (109) years, compared to 4.6 billion for the Sun, so it is still relatively young for a star of its mass. Starspot activity has also been detected, which varies from year to year.

A circumstellar disc of dust particles has been detected

in orbit around 58 Eridani.

AD Phoenicis

AD Phoenicis is a variable star in the constellation of Phoenix. An eclipsing binary, its apparent magnitude has a maximum of 10.27, dimming to 10.80 during primary and secondary eclipses, which are approximately equal. From parallax measurements by the Gaia spacecraft, the system is located at a distance of 655 light-years (201 parsecs) from Earth.AD Phoenicis is a contact binary of W Ursae Majoris type, composed of two stars so close that their surfaces touch each other. They are separated by 2.46 solar radii and orbit each other with a period of 0.3799 days. The primary star has a mass of 1.00 solar mass and a radius of 1.17 solar radii, while the secondary has 0.38 solar masses and 0.76 solar radii. Their surface temperatures are very similar, 6,155 and 5,835 K, which is the reason for the eclipses being equal-depth.In visible light, the primary star contributes 71.2% of the system's luminosity, while the secondary contributes the rest (28.8%). Previous analyses of the system suggested that the secondary star was eclipsed during the primary minimum and hence was hotter than the primary. The bolometric luminosity of the two stars combined is 2.298 L☉. The eclipse's light curve shows an asymmetric feature that is best explained by a large starspot in the surface of the primary, about 700 K cooler than the rest of the photosphere. Asymmetry in the light curve may also be caused by starspots on one or both components, which would result in slightly results for the physical properties of the two stars.Variations in the orbital period of the system have been detected, which were modelled as a continuous decrease in the period plus a cyclic oscillation. The period decrease of about 1.5×10−7 days per year is likely caused by mass transfer from the secondary to the primary star, while the oscillation can be explained by a third star in the system or by a magnetic activity cycle. In the third star hypothesis, its orbit would have a period of 56.2 ± 0.9 years and an eccentricity of 0.36 ± 0.01. A minimum mass of 0.257 solar masses is calculated, which corresponds to a red dwarf of spectral type M4–M5, consistent with the lack of photometric and spectroscopic evidence for this star.

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

CN star

A CN star is a star with strong cyanogen bands in its spectrum. Cyanogen is a simple molecule of one carbon atom and one nitrogen atom, with absorption bands around 388.9 and 421.6 nm. This group of stars was first noticed by Nancy G. Roman who called them 4150 stars.

Calypso Now

Calypso Now was part of the cassette culture movement. Calypso Now was founded in 1983 by the Swiss musician and concert promoter Hotcha (alias Rudi Tüscher), located in Biel/Bienne. Up to 1989, about 200 cassette releases were produced, mainly from Switzerland, Germany, UK, USA and Canada. The label was reactivated in autumn 2010, with reissues and new cassette releases.Calypso Now followed a licensing policy and often copied the cassettes on demand from master tapes of foreign labels, thus keeping the investment low. There were also bigger productions, mainly compilations of Swiss indie bands, targeted at record shops, with printed covers in the LP-sized 12" format Examples are This Is Guitar Town with bands from Geneva, Chart Attack, or a series of cassingles. Calypso Now also compiled tapes with Swiss underground artists for release by another label: Der Politische Katholizismus (1985) was released simultaneously in Japan, Germany and the USA; Sonique Suisse was initiated and released in 1988 by Carl Howard (at - Audiofile Tapes) in the USA, and the Starspot Compilation was made in 1989 for Bi-Joopiter in the UK.

Gerald Kron

Gerald Kron (April 6, 1913 – April 9, 2012) was an American astronomer who was one of the pioneers of high-precision photometry with photoelectric instrumentation. He discovered the first starspot and made the first photometric observation of a stellar flare.

A graduate of the University of Wisconsin-Madison, where he earned a Master of Science degree in mechanical engineering in 1934, Kron became interested in astronomy, which he studied under Joel Stebbins. Stebbins arranged for Kron to enter the University of California at Berkeley, where he received his doctorate in astronomy in 1938.

During World War II, Kron served with the Radiation Laboratory at the Massachusetts Institute of Technology, and participated in the development of microwave radar. He later became the head of the Special Devices Group at the Naval Ordnance Test Station (NOTS) at Inyokern, California, where he conducted studies on solid fuel rockets, and developed radio transponders for the Manhattan Project's Project Camel.

After the war Kron returned to the Lick Observatory, where he was one of the designers of the C. Donald Shane telescope. Using photometric techniques that he had pioneered before the war, he studied the stars, especially eclipsing binaries. In 1965, Kron became director of the United States Naval Observatory in Flagstaff, Arizona, and he was a regular visitor to the Australian National University's Mount Stromlo Observatory.

HD 66141

HD 66141, also known as HR 3145 and 50 G. Canis Minoris, is star in the constellation Canis Minor. It is an orange K-type giant, approximately 254 light years from Earth. Its apparent magnitude is +4.39.

When first catalogued it was in the Puppis constellation and was designated "13 Puppis", but it subsequently migrated to Canis Minor. Bode gave it the Bayer designation of Lambda Canis Minoris.Over 2003 to 2012 a starspot was periodically dimming its light.

Helium-weak star

Helium-weak stars are chemically peculiar stars which have a weak helium lines for their spectral type. Their helium lines place them in a later (ie. cooler) spectral type then their hydrogen lines.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

Photometric-standard star

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.A current set of photometric-standard stars for UBVRI photometry was published by Arlo U. Landolt in 1992 in the Astronomical Journal.

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, 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.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Starfield (astronomy)

A starfield refers to a set of stars visible in an arbitrarily-sized field of view, usually in the context of some region of interest within the celestial sphere. For example: the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

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.

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.

TY Pyxidis

TY Pyxidis is an eclipsing binary star in the constellation Pyxis. The apparent magnitude ranges from 6.85 to 7.5 over 3.2 days.The two components are both of spectral type G5IV, have a mass of 1.2 solar masses and revolve around each other every 3.2 days. Each star is around 2.2 times the diameter of the Sun.The system is classified as either a RS Canum Venaticorum variable or a BY Draconis variable, stars that vary on account of prominent starspot activity, and lies 184 ± 5 light years away. The system emits X-rays, and analysing the emission curve over time led Pres and colleagues to conclude that there was a loop of material arcing between the two stars.

XX Trianguli

XX Trianguli is a variable star in the constellation Triangulum. It is classified as a RS Canum Venaticorum variable orange giant with spectral type K0III. XX Trianguli is notable for having a huge starspot larger than the diameter of the Sun, discovered using Doppler imaging.

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

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