Contrasting Star Colors in Perseus

August 05, 2012

Algol_Rho_persei_EPOD (2)

: Greg Parker
Summary Author: Greg Parker
A few years back, I took an image of the well known double star Albireo in the constellation of Cygnus, which has a close pair of blue and orange stars. In all the images I've taken since, I've never noticed such a color contrast in a pair of nearby, reasonably bright stars (there are actually an enormous number of much fainter red-blue close neighbors to be found). So I took a look at a star map to see if I could find any other colorful doubles to image that would fit into the field of view of my Takahashi Sky 90 refractor telescope. Surprisingly there was only one suitable pair of stars to fit the criteria -- Algol and Rho Persei in the constellation of Perseus. This would make an impressive pair of stars in a deep-sky image, and both are visible with the unaided eye, so they're pretty bright as well.

In this image, Algol is the bright blue star and Rho Persei is the bright red star. Algol (beta-Persei, SAO 38592) also known as the "Demon Star" is an eclipsing binary star, so an orbiting star eclipses Algol causing its magnitude to change from a relatively bright 2.1 to a much dimmer 3.4, every 2 days 20 hours and 49 minutes. Although Algol is an eclipsing binary, it's in fact a three-star system.

25-Rho Persei (SAO 56138) is less commonly known as Gorgonea Tertia (the third of the four Gorgons). Rho Persei is also a variable star; it's a semiregular variable of the Mu Cephei type whose magnitude varies between 3.3 and 4.0 with periods of 50, 120 and 250 days.

  • Rho Persei is 325 light years away,
  • 5 times as massive as the Sun,
  • 150 times the Sun's radius,
  • 2,290 times the Sun's luminosity, and
  • has a surface temperature of 4,111 K.
  • Color: the characteristic red-orange color of an M-type star.

For comparison, our Sun has a surface temperature of 5,778 K, hotter than Rho Persei, so it appears more yellow than red, but much cooler than the blue-white Algol at 9,200 K.

Later this week look toward Perseus in the northeastern sky (in the Northern Hemisphere) around midnight to see meteors from the annual Perseids Meteor shower. Since the Moon will be new when the Perseids peak (August 11-12), if skies are clear, you may be able to see 4 or 5 shooting stars per minute.

Photo details: Takahashi Sky 90 telescope; M25C one-shot color camera (APS size chip).