A binary star system is where two stars are close enough together to be in orbit around each other, instead of individually orbiting the centre of the galaxy. Instead they orbit the centre of the galaxy together, much the same way as the Earth and the moon orbit the sun together. Also sort of like on Tatooine from Star Wars.
|Artist's impression of the Kepler 16 system.|
Credit: NASA/JPL-Caltech/R. Hurt
As far as we know, there are three bodies in the Kepler 16 system: the two stars which are designated Kepler 16A and B (and together Kepler 16(AB). As per usual convention, the more massive star will be A while the smaller will be B. The usual convention, for non-binary systems, is that the star is designated Whatever-a and planets in order of discovery then mass (so mass when several are discovered simultaneously) are designated Whatever-b, Whatever-c etc. The lower-case a for the stars is a bit redundant here, though. Masses and distances for the system are as follows:
- Star A:
- Mass: 0.69 solar masses
- Size: 0.65 solar radii
- Temperature: 4450 Kelvin (about 4180º C or 7600º F)
- Star B:
- Mass: 0.20 solar masses
- Size: 0.23 solar radii
- Stars AB together:
- Orbital period: 41 days
- Orbital separation: 0.224 AU (for comparison, Mercury's orbit is 0.387 AU from the sun)
- Planet Kepler 16(AB)b:
- Mass: 0.33 Jovian masses
- Size: 0.75 Jovian radii
- Orbital period: 229 days
- Orbital radius: 0.70 AU (Venus is 0.72 AU from the sun)
For a bit of fun, let's work out how big and bright each star would be from the planet, relative to the sun. I should note that this is a gas giant planet and so any possible light would be more likely to exist on one of its moons, not the planet. The suns would appear the same size from a moon, though.
First, how big would the suns appear? (See this post for details on the calculation.) Remember for comparison that the sun (and moon) have an angular diameter of about 0.5º. So, sizes:
- Star A as seen from planet on average: 0.49º, so about the same as the sun
- Range: 0.43º to 0.59º
- Star B as seen from planet on average: 0.18º, which is about a third of the diameter of the sun
- Range: 0.15º to 0.21º
- Star A: 410 Joules per square metre per second, which is a bit less than a third of the light energy the Earth gets from the sun. It is also going to be redder light, thanks to the cooler temperature of the star.
- Star B: assuming its temperature is about 2500 K which is within the plausible range for M type stars (star A is K type, if you're curious), I get about 5 Joules. That's 3-4% of the light from the sun hitting Earth. On the other hand, it's still a lot brighter than the full moon in Earth's sky (and significantly redder). About 1400 times brighter, in fact. And about as bright as 260 100 Watt light bulbs at a distance of 10 m. Except it would probably feel dimmer thanks to the redness.
And if you want to read more, here is a nice New Scientist article about it. And, below, a video from APOD: