Keep an Eye on Hypergiant Rho Cassiopeiae

Mercury, January/February 2004 Table of Contents

Image courtesy of A Lobel.

by Alex Lobel

An eruption in 1946 of mighty Rho Cassiopeiae put the astronomy community on guard, and recent, exciting changes in the star may portend something big and explosive for it in the near future.

She goes without a proper name, but recently the 17th brightest star of the northern constellation Cassiopeiae is drawing the attention of amateur and professional astronomers worldwide. In the spring of 2000 Rho Cassiopeiae, or r Cas, brightened up to magnitude 4.0, then dimmed to an astonishing 5.3 over the next half year, while changing its usual yellowish-white color to the red-orange glare of Betelgeuse (a Orionis). Such a rapid, extraordinary change was also observed for the star in 1946, bringing it to the attention of astronomers everywhere.

Various types of variable stars are known to change their visual brightness in a rather predictable way—stars such as Mira (o Ceti) and Algol (b Persei). And the R Coronae Borealis stars can suddenly dim by several magnitudes; they are much fainter and less intrinsically luminous than r Cas, however.

Indeed, shining at about half a million times the Sun’s luminosity, Queen r Cas is known to be one of the most luminous, cool stars of our Galaxy. Tucked away in the Orion spiral arm of the Galaxy, at an approximate distance of ten thousand light-years, it is possibly the most distant star with a surface temperature comparable to that of our Sun that can easily be observed with the unaided eye.

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