# Stars/Flares

Flare stars are a type intrinsic eruptive variable star. That brightly glowing dot right at the middle of the image is the dust and gas swirling around Sgr A*, from Sagittarius A$^*$.
The sequence of frames on the left shows the decrease in intensity that occurred with time as compared with the stars SO-2 and SO-17.
"The strange brightening was observed on May 13, [2019], two hours condensed down to a few seconds."
"The black hole is always variable, but this was the brightest we've seen in the infrared so far. It was probably even brighter before we started observing that night!"
"But - have a look at the timelapse again. See that bright dot at around 11 o'clock from the black hole? That's S0-2, a star on a long, looping, 16-year elliptical orbit around Sgr A*. Last year, it made its closest approach, coming within 17 light-hours of the black hole."
"One of the possibilities is that the star S0-2, when it passed close to the black hole last year, changed the way gas flows into the black hole, and so more gas is falling on it, leading it to become more variable."

## Flares

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. The brightness increase is across the spectrum, from X rays to radio waves. The first known flare stars (V1396 Cygni and AT Microscopii) were discovered in 1924. Most flare stars are dim red dwarfs. The more massive RS Canum Venaticorum variables (RS CVn) are also known to flare.
Nine stars similar to the Sun have also been seen to undergo flare events.
Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth.
"All parameters seem to have broad or unimodal distributions, suggesting that flares and CMEs form a continuum with the same underlying physics."

## AD Leonis

AD Leonis is listed as a flare star in Gurzadyan's (1980) list of flare stars.
It is a flare star that undergoes random increases in luminosity.
AD Leonis is one of the most active flare stars known, and the emissions from the flares have been detected across the electromagnetic spectrum as high as the X-ray band. The net magnetic flux at the surface is about 3 kG. Besides star spots, about 73% of the surface is covered by magnetically active regions. Examination of the corona in X-ray shows compact loop structures that span up to 30% of the size of the star. The average temperature of the corona is around 6.39 MK.

## AT Microscopii

AT Microscopii is listed as a flare star in Gurzadyan's (1980) list of flare stars.
AT Microscopii is a binary star system located at a distance of 35 ly (11 pc) from the Sun in the constellation of Microscopium. Both members are flare stars. This pair lies physically near the red dwarf star AU Microscopii, which may mean they form a wide triple star system.
Both members of this system have active coronae, show luminosity variations of the BY Draconis type, and are X-ray emitters. The average flare rate for the pair is 2.8 per hour. Their X-ray spectrum is consistent with a plasma density of around 3 × 10$^{10}$ cm$^{−3}$ and a magnetic field strength of at least 100 G in the flare regions.

## AZ Cancri

At right is a close-up of the SDSS DR6 image of AZ Cancri in real (visual) color. According to SIMBAD, AZ Cancri (AZ Cnc) is a spectral type M6.0V flare star, that is also an X-ray source detected by the ROSAT satellite. AZ Cancri (AZ Cnc) is a M-type flare star in the constellation Cancer.
The star is in NGC 2632 designated Haro, Chavira, and Gonzalez (HCG) 4. NGC 2632 is an open cluster, also called Messier 44, and the Praesepe Cluster.
The visual star is spectral type M6e, specifically M6.5Ve.

## Beta Boötis

Beta Boötis is a flare star.
In 1993, ROSAT observed an X-ray flare on Beta Boötis, which released an estimated 1.7 × 10$^{32}$ erg, making this the first such observation for a low-activity star of this type.
The flare may be explained by an as yet unobserved M-type red dwarf companion star.

## EQ Pegasi

EQ Pegasi is listed as a flare star in Gurzadyan's (1980) list of flare stars.
EQ Pegasi is a system of two red dwarf stars of spectral types M3.5V and M4.5V, located in constellation Pegasus at 20 light-years from Earth.

## Proxima Centauri

Although it has a very low average luminosity, Proxima Centauri is a flare star that undergoes random dramatic increases in brightness because of magnetic activity.
Proxima Centauri was announced as a flare star in 1951.
Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known.
In 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, and the X-ray emissions of smaller, solar-like flares were observed by the Japanese Advanced Satellite for Cosmology and Astrophysics (ASCA) satellite in 1995. Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and the Chandra X-ray Observatory.

## UV Ceti

UV Ceti is listed as a flare star in Gurzadyan's (1980) list of flare stars.

## Wolf 359

Wolf 359 (CN Leonis, Gliese 406, Gliese-Jahreiss 406) is listed as a flare star in Gurzadyan's (1980) list of flare stars.
In 1969, a brief flare in the luminosity of Wolf 359 was observed, linking it to the class of variable stars known as flare stars.
At an estimated 9% of the Sun's mass, Wolf 359 is just above the lowest limit at which a star can perform hydrogen fusion through the proton–proton chain reaction: 8% of the Sun's mass. (Substellar objects below this limit are known as brown dwarfs.) The radius of Wolf 359 is an estimated 16% of the Sun's radius, or about 110,000 km. For comparison, the equatorial radius of the planet Jupiter is 71,492 km, which is 65% as large as Wolf 359's.
The spectrum of its corona showed emission lines of Fe XIII, which is heavily ionized iron that has been stripped of twelve of its electrons. The strength of this line can vary over a time period of several hours, which may be evidence of microflare heating.
Observations with the Hubble Space Telescope detected 32 flare events within a two-hour period, with energies of 10$^{27}$ ergs (10$^{20}$ joules) and higher. The mean magnetic field at the surface of Wolf 359 has a strength of about 2.2 kG (0.22 teslas), but this varies significantly on time scales as short as six hours. By comparison, the magnetic field of the Sun averages 1 gauss (100 µT), although it can rise as high as 3 kG (0.3 T) in active sunspot regions. During flare activity, Wolf 359 has been observed emitting X-rays and gamma rays.

## Wolf 630

Wolf 630 is listed as a flare star in Gurzadyan's (1980) list of flare stars.

## YY Geminorum

Castor is 51 light-years away from Earth, determined from its large annual parallax. The two brightest stars are both A-type main-sequence stars, more massive and brighter than the Sun.
The properties of their red dwarf companions are difficult to determine, but are both thought to have less than half the mass of the Sun.
The two red dwarfs of Castor C are almost identical, with masses around a half of solar mass and luminosities less than 10% of the Sun.
A third star is 73" distant from the main components.
It was discovered to vary in brightness with a regular period, and was given the variable star designation YY Geminorum. It is an eclipsing binary with additional variations due to areas of different brightness on the surface of one or both stars, as well as irregular flares.
YY Geminorum is listed as a flare star in Gurzadyan's (1980) list of flare stars.
All the red dwarfs in the Castor system have emissions lines in their spectra, and all are flare stars.

## YZ Canis Minoris

YZ Canis Minoris is listed as a flare star in Gurzadyan's (1980) list of flare stars.
It is a flare star, so called for its solar flares being more powerful than those of the Sun, and is roughly three times the size of Jupiter.