IS Rapid irregular variable stars

- Rapid irregular variables having no apparent connection with diffuse nebulae and showing light changes of about 0".'5-l".'0 for several hours or days. There is no strict boundary between rapid irregular and Orion variables. If a rapid irregular star is observed in the region of a diffuse nebula, it is considered an Orion variable and is designated by the symbol INS. To attribute a variable to the type IS, it is necessary to take much care to be certain that its light changes are really not periodic. Quite a number of the stars assigned to this type in the GCVS 3rd Ed., turned out to be eclipsing binary systems, RR Lyrae variables, and even extra-galactic BL Lac objects. ISA (subtype) - Rapid irregular variables of the early spectral types B-A or Ae. INB (subtype) - Rapid irregular variables of the intermediate and late spectral types F-M and Fe-Me. GCVS

In general, another group of stars displaying duplicitous characteristics probably assembled by the Universe with the intent to challenge your observational skills. Within the GCVS, you will find approximately 230 rapid irregular variables including early (ISA), and intermediate through late spectral types (ISB) (see Figure 3.3).

Most of these stars are considered faint but are within the range of medium-to-large telescopes. A small number are within the reach of binoculars or small telescopes. As with the group as a whole, little information will be found on individual stars but rather than viewing this as an obstacle, you should


Faint stars Q Small amplitude ^ Short periods <3> Visual, CCD/PEP

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Figure 3.3. Light curve of the INSA-type varioble star, RR Ta Data provided by VSNET Used with permission.

consider this an excellent opportunity to conduct some original research, perhaps resulting in ill-defined stars being better defined and properly placed within the classification scheme of the GCVS.

For example, CV Dra was discovered to be a short-period variable in 1960 but no further details were given other than that it was variable. The star was further investigated in 1961 but no confirmation of periodicity was evident so the star was classified as a rapid irregular variable. It is listed as one of the brighter rapid irregulars within the GCVS 4th Ed. However, in 1988, after further study, the star was determined to probably be an eclipsing binary of W UMa type.


Bright stars large amplitudes ^^ long periods <3> Visual

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