Tuesday, January 21, 2014

A pair of colorful gems

While I normally observe galaxies and nebulae, sometimes I turn my telescope on another class of objects, double and multiple star systems. When the skies are too bright due to moonlight for my usual quarry, I look at them when the seeing is steady. Two such systems are especially noteworthy for their ease of separation in modest telescopes and their strong colors.
40 Eridani, or Omicron Eridani-2 is a very interesting triple star system only 16.5 light years away. The primary star is an orange dwarf of spectral type K1V, a hydrogen fusing star like the Sun but smaller, fainter and cooler. In orbit around 40 Eridani A is a white dwarf star that orbits once every 8,000 years with an average separation of 400 A.U. or nearly 40 billion miles. In orbit around the white dwarf is a red dwarf that circles it every 252 years with an average separation of 35 A.U., or roughly Pluto's distance from the Sun. The magnitudes of the stars are 4,4, 9.5, and 11 respectively. The spectral types of the B and C stars are DA4 and M4.5Ve, the masses of the three components are .84, .5 and .2 Solar masses, with the C star also being a flare star. The B component was evidently the most massive of the three, given the age of 5.6 billion years for the system. It apparently ran through it's life cycle and became a white dwarf while the other two are still Main Sequence stars. 40 Eridani B is the easiest white dwarf to see with amateur telescopes, it's separation of 82 arc-seconds from 40 Eridani-A ensures there is no interference from the primary star.
Sigma Persei is a binary star with a very strong color contrast between the two components. The brighter of the two is a K-type giant star some 13,000 times brighter than the Sun, while the dimmer blue star may in fat be an unrelated star along the same line of sight. The bright star is itself 1,300 light years from Earth. If it is indeed a companion to Eta Persei A, it must be orbiting at least 11,500 A.U away from it. That extreme distance makes it more likely the two stars are merely travelling through space with almost the same velocity and direction rather than a true binary star system. Eta Persei B is a Main sequence star with a spectral type of B9V, making it far less massive than the A star, which is believed to have 9 to 11 times the mass of the Sun. It's diameter is some 220 times that of the Sun, or nearly as big as the Earth's orbit around the Sun. Either it will explode as a core-collapse supernova, or end it's life as a very rare type of white dwarf that is made mostly of neon. Such stars are also perilously close to the point where they collapse then detonate as a supernova.
Double and multiple stars are not my favorite kinds of objects to observe, but they have the advantage of being observable from virtually anywhere and do not require large telescopes either. Some have dramatic colors that make them the jewels of the night sky while most galaxies and nebulae appear only in shades of gray. These two double stars are only two of thousands of double and multiple systems in range of small telescopes.