GORGONEA TERTIA (Rho Persei). The most famed star of Perseus, the Hero, is not the brightest, Mirfak (Alpha Persei), but Algol (Beta Per), the "Demon Star," an eclipser that represents the head of the Medusa, one of the three Gorgons of classical mythology. Three fainter stars lie to the south, making a rough box a couple degrees across, the quartet called the Gorgonea (never minding that there are but three Gorgons), of which Algol is "Gorgonea Prima." In order from the west, the other three (still mixing Greek and Latin) are fifth magnitude (4.69) Gorgonea Secunda (the "Second Gorgon," Pi Persei), third magnitude (3.39) Gorgonea Tertia (Third Gorgon, Rho Persei), and fifth magnitude (4.61) Gorgonea Quarta (Fourth Gorgon, Omega Persei). Other than the mythological connection, the four have little to do with one another. While the fainter three actually lie at close to the same distance (respectively 325, 317, and 305 light years), they are moving in very different directions, and do not come close to forming any kind of assembly. Much brighter Algol is only a third as far away. They are also very different kinds of stars. Algol is an eclipsing double made of a class B8 blue-white dwarf and a cool K giant. Pi (Gorgon number 2) is a rather ordinary white class A2 dwarf, while Omega (number 4) is a common K0 yellow-orange giant. That leaves number 3, our "GT," Rho Per, a far more unusual cool (3460 Kelvin) red class M (M4) bright giant. From its 317 light year distance, it radiates 3470 times more energy than the Sun, the bulk of it in the infrared. Temperature and luminosity combine to give a radius 164 times that of the Sun, while direct measure of angular diameter combined with distance give 157 solar radii, the closeness of the two figures showing that all the measured parameters are close to the mark. Even more interesting is the evolutionary status. Far more advanced along its ageing pathway, Rho Per is a three solar mass 440 million year old "second ascent" giant, one that is brightening for the second time. In its first ascent as a giant, Rho had a dead helium core. Then it went through the act of fusing its helium into carbon, like Omega Per is doing. Rho is now becoming brighter and larger than ever with a dead carbon and oxygen core. Consistent with this configuration, it is also a "semi-regular" (SRb) variable star with a period of around 50 days (and a possible longer variation of 100 days) that changes between magnitudes 3.3 and 4.0, enough to be sensed with the naked eye. "Semi-regular variable" is now considered something of a misnomer, as such stars are really low-amplitude versions of the magnificent Mira stars that vary so much that they can disappear from naked eye view entirely. Closing in on its last stage of life, Rho Per is losing mass, and before long will expose the ancient nuclear-burning core at its center, which will become a white dwarf. The sight from Rho Per would be magnificent, as its Gorgon sisters Pi and Omega average but a dozen light years away, and would shine in Rho's sky at magnitude -5, about the brightness of our Venus at maximum. From Pi or Omega, the sight would be even grander, Rho shining over another magnitude brighter. Going their own way, however, the stars will soon separate, their current closeness becoming but a fleeting memory in Galactic time.
Written by Jim Kaler. Return to STARS.