Pollux

NASA Observatorium




See a discussion of
the "main sequence"
as part of stellar
evolution and death.

The Star

Pollux is a orange-red giant star of spectral and luminosity type K0 IIIb. It has about 1.7 +/- 0.4 times Sol's mass (Allende-Prieto and Lambert, 2000; and Drake and Smith, 1990), 8.8 +/- 0.1 times its diameter (Hatzes et al, 2006), and 32 times its visual luminosity. Since 1990, it has been measured to be around 85 to 155 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Hatzes et al, 2006; and Drake and Smith, 1990). According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 2990, there are "pronounced emissions" of magnesium-II.

Given that Pollux has a higher mass and is more evolved than Sol, the star is likely to be much younger than Sol's 4.6 billion years. Its relatively high mass estimates, however, suggest that it has not yet suffered substantial mass loss. Thus, Pollux may have evolved to the first ascent of the red-giant branch or to the post-helium-flash clump-giant stage (Drake and Smith, 1990). Pollux is a New Suspected Variable star designated as NSV 3712, and it may have as many as six optical, stellar companions that are not gravitationally bound to the star. Useful catalogue numbers and designations for the star include: Bet Gem, 78 Gem, HR 2990*, Gl 286, Hip 37826, HD 62509, BD+28 1463, SAO 79666, FK5 295, LHS 1945, LTT 12065, LFT 548, and ADS 6335 A.

NASA Observatorium


See a discussion of
"core helium burning"
as part of stellar
evolution and death.

As a star that has evolved out of the "main sequence," Pollux has fully shifted from the fusion of hydrogen to helium at its core to the fusion of helium to carbon and oxygen, with trace activity of other nuclear processes. This helium-burning, orange-red giant stage is relatively brief, lasting tens to hundreds of million years (e.g., lasting around 700 million years for a star of one Solar mass like the Sun).

Eventually, the star will lose much of its current mass, from an intensified stellar wind that eventually puffs out its outer gas envelopes of hydrogen and helium (and lesser amounts of higher elements such as carbon and oxygen) into interstellar space as a planetary nebula. The result will be a planet-sized, white dwarf core that gradually cools and fades in brightness from the shutdown of thermonuclear fusion. (Nearby white dwarfs include solitary Van Maanen's Star and the dim companions of Sirius, Procyon, and 40 (Omicron2) Eridani.)

H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA -- larger image
(White dwarfs are remnant stellar cores that have cast off their outer gas
layers, like planetary nebula NGC 2440.)

Planet "b"

On June 16, 2006, a team of astronomers (including Artie P. Hatzes, William D. Cochran, E. Endl, E.W. Guenther, S.H. Saar, G.A.H. Walker, S. Yang, M. Hartmann, M. Esposito, and D.B. Paulson) confirmed the presence of a Jupiter-class planet "b" around Pollux, that was first detected in 1993 (Hatzes et al, 2006; and Hatzes and Cochran, 1993). Assuming that Pollux has 1.7 Solar-masses, planet b has a minimum mass of 2.3 +/- 0.45 Jupiter-masses. It moves around Pollux at an average distance of 1.64 +/- 0.27 AUs in a nearly circular orbit (e= 0.02 +/- 0.03) that takes 1.6 years (589.64 +/- 0.81 days) to complete. The presence of the planet has been confirmed by continued monitoring (Quirrenbach et al, 2011). (See an animation of the planet's orbit around Pollux, with a table of basic orbital and physical characteristics.)

NASA

Cassini-Huygens Mission
to Saturn and Titan


Larger image.


Planetary candidate "b"
is a Jupiter-class planet
(shown here with Europa).

Currently, the orbit of an Earth-like planet (with liquid water) around Pollux may be centered around 5.7 AU -- just outside the orbital distance of Jupiter in the Solar System. Astronomers would find it very difficult to detect an Earth-type planet in the water zone of this star using present methods.