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 310, 308, and 388 light years with uncertainties of 5.7. and 6;
second Hipparcos reductin), 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 308
light year distance, it radiates 3275 times more energy than the Sun, the bulk of it in the infrared.
Temperature and luminosity combine to give a radius 160 times that
of the Sun, while direct measure of angular diameter combined with
distance give 152 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 1/16/04. Return to STARS.