30 HER (30 Herculis). Some particular kinds of stars fascinate, those that are highly colored, those of great luminosity, those of low luminosity, those like the Sun, those just weird. Here is an example of the first of these, a classic red giant. Known mostly by Flamsteed number 30 in Hercules, it is also strangely listed in the literature as "g Herculis," the "g" from the obscure extension of Bayer's Greek letter system, wherein he appended lower case, then upper case Roman letters. Very few stars are so know, the best example being the cluster "h Persei" (one of pair known as the "Double Cluster," the other "Chi Persei"). A lovely sight in binoculars or telescope, fifth magnitude (5.0) 30 Herculis is a quite reddish class M (M6) giant that lies 360 light years away. "Giant" is an encompassing term that incorporates a variety of different kinds stars: those that have dead helium cores and are brightening, those that are more or less stable helium burners (like Arcturus and Aldebaran), and those brightening with dead carbon cores (the carbon the result of helium fusion) like Mira. "30" is among the latter, and as such is a "semi-regular" variable star that goes from roughly magnitude 4.7 to 6.0 and back over a rather regular 93 day period (superimposed on a much longer 833 or so day period). The semi- regulars are not all that irregular, and are really more or less
Thirty Her is seen to vary by up to a magnitude or so over the 30 years of observation, with a principal period of 93 days. The scale on the bottom is the "Julian Date" of 2440000 plus the number that appears, where the Julian Date is the number of days since January 1, 4713 BC of the Julian Calendar and is commonly used for variable phenomena in astronomy. JD 2446500 corresponds to March 11, 1986. The left-hand scale expresses the difference between the apparent visual magnitude of 30 Herculis and a nearby comparison star. (From an article in the Publications of the Astronomical Society of the Pacific by J. R. Percy, J. B. Wilson, and G. W. Henry.)
low-amplitude Mira-type stars. Though distant, 30 Her is so large its angular diameter is easily measured, showing it to have a radius of 228 times that of the Sun, or 1.06 Astronomical Units, slightly larger than the size of Earth's orbit. Stars this huge lose mass at a fast rate, and this one is no exception, its wind blowing at a loss rate of 2.6 ten-millionths of a solar mass per year, some 10 million times that from the solar wind. The wind from 30 Her has produced a large circumstellar shell around the star that is rich in metallic oxides. For all the action, however, there is little evidence that the by-products of nuclear fusion have made it to the stellar surface (as it does in many other similar giants). The luminosity is not well known. The measured temperature of 3000 Kelvin and the radius give 3800 Suns. The luminosity from the visual brightness and an estimate of the amount of infrared radiation, however, give 8700 solar. A temperature of 3200 Kelvin that is more appropriate to the spectral class gives 2900. Whatever it is, it is surely high. Even so, the mass is not all that great: the star probably started life with not too much more than one or two solar masses, showing what in several billion years will happen to our own Sun. Like the classic highly evolved star Mira, 30 Her is preparing to slough off its outer envelope to bare its nuclear burning heart, which in turn will become a dim, shrunken white dwarf.
Written by Jim Kaler. Return to STARS.