Evi Hummel and Wolfgang Beisker
The phenomenon of flare stars was discovered first by Hertzsprung in 1924. He found that a faint star increased its intensity by about 2 magnitudes. In 1949 transient outbursts were found on UV Cet, YZ CMi and WX UMa. UV Cet is some kind of prototype for all flare stars in the solar neighbourhood. Flare stars are stars which produce rapid optical bursts. Because all these stars have a relatively low luminosity, only stars in the solar neighbourhood can be observed. All known flare stars have spectral class M and are dwarf stars. Most of them have emission-line spectra with a presence of strong hydrogen emission lines in the optical wavelength range.
If UV Cet is compared with the Sun, its diameter is 1/6th, its mass is 1/8th, the effective temperature is 3000 K (sun 5800 K). The coronae of the dwarf stars are hotter and denser. Due to the high temperatures of the corona, the gas cannot be gravitationally bound to the star. To a lesser degree this is true for the sun, too. In case of the sun the magnetic field is assumed to confine the gas.
Stellar flares are qualitativly similar to solar flares, but differ largely in its magnitude. Intensity changes can be observed throughout all wavelength, but with different amplitudes and time responses.
The Galilee Astrophysical Observatory maintains a special WWW page with the address http://kineret.yarden.ac.il/bloss/stel.htm.
From there you can get more information about flare stars and its observation. To give a short introduction about these stars, in the following paragraph we copy the information from the Galilee Astrophysical Observatory's home page :
More than 90 percent of the stellar population of our Galaxy are flare stars. The nearest star to the sun, Proxima Centauri, is a flare star. Flare stars are small and cold stars, in other words, red dwarfs. Each of these stars is smaller than the Sun by approximately 2-5 times. The temperature of such a star is around 2000-4000 degrees, while the temperature of the solar surface is about 6000 degrees. The interesting phenomena is that in spite of the fact that such a star is so small and weak, every few hours their magnitude jumps during several seconds between 10 to 600 times more than "normal". It has to be stressed that the explosion we are speaking of doesn't only not destroy the star, but it even doesn't influence it. Our current knowledge about stellar stars is that the energy we see during the explosions accumulates prior to the explosion as a magnetic field energy over a period of time. Once the magnetic field is no longer able to contain the accumulated energy there is an abrupt release of a vast amount of energy and we see the flare. In spite of the fact that this process we see in flare stars as well as on the sun, the details of the process are still being extensively researched all over the world.
In order to register the outbursts of these stars, fast photometry is necessary. For stellar occultation measurements, the European Section of the International Occultation Timing Association has developed a CCD system capable to record images as fast as necessary for the flare star registration. It is described in the following.
High speed CCD photometry can very well be used to measure fast intensity variations in different variable stars. Flare stars are possible objects for photometry with sampling frequency in the 1 to 10 second time range. In contrast to standard CCD detector systems, which provide relatively large areas with many pixels, we have developed a slow scan CCD camera based on a commercially available chip, the TC245 manufactured by Texas Instruments, which can record small images at a rate starting from 18 images per second. The chip is mounted in a closed thermoelectrically cooled environment (temperature ~-30C) and can be attached to any small or large telescope. It is directly connected with a standard PC equipped with a 12 Bit AD converter card. An operating program can display the images in real time and record the images with the exact time (obtained with a DCF-77 time receiver) on the computer's hard disk. The camera has been developed by IOTA/ES and was used before to record lunar as well as planetary occultations of different stars.
We propose to compare the possibilities of visual observations with the results from such a CCD system. For the observations we plan to attach the camera to the brandnew 10 inch Schaer refractor of the Public Observatory of Munich (Volkssternwarte München) or to a portable instrument.
Because we are located in the Munich area with its relatively poor weather conditions, we have to find a way to let you know, when the weather conditions are favorable here for observations.Each of the participants, who want to contribute to this project, should send his/her email address to
All subscribed participants will receive via email the exact coordinates, magnitude and further informations about the flare star (frequency of flares, magnitude change, color change and more literature references) we select for observation.
We think to be ready for observations between the 24th of November and the 6th of December 1996. In case of good weather conditions we will inform all subscribed participants via email that we are going to take CCD image sequences of the selected star. The mail will inform about the exact observing time with the CCD camera system. During this time period the participants should try to visually observe the selected star and record the UT time, estimated intensity as well as duration of possible flares. All data should be send as soon as possible to the email addresses given above with the subject "Flare Project Results". Don't forget to send informtions about your telescope size, focal length, magnification, observing site, weather conditions and visibility conditions (scintillation etc.).
We will compare all results with our camera data and publish the complete evaluation in the Astronomy On-Line websites.
More information about the Public Observatory of Munich and the International Occultation Timing Association (IOTA) can be obtained from our webservers :
Last update: Nov 17, 1996