Multi-Star systems and light

[Spartan159]

I want to get an idea of how much light a given planet receives from stars. In the particular case I am working on I have a planet orbiting a class F9V star in orbit 4, with an M1V in orbit 14. I have postulated that the companion star is currently ~ 90 degrees trailing the planet relative to the primary. *think* that would mean that the second star would "rise" about noon on the planet and set around midnight From primary down to secondary down, how much light is available? I am guessing that I need to do some calculation involving luminosity and range of the companion?

Just for the record I'm talking about Pysadi

 

[aramis]

Solar radiation influx is about 410°K, reduced for albedo.

So 410° K * Luminosity / Distance².

With multiple stars, average the infall (I recommend 4 points for simplicity - 0°, +90°, ±180°, -90° relative)

Then multiply each's component by the albedo, then sum the result. That's a VERY rough estimate of temp.

It's good enough for first principles. (Actual temp on surface will vary by atmosphere, rotational speed, and distribution of albedo affecting components, as well as specific heat of the surface elements.

As long as the solar influx is enough for anywhere in 263° K to 303° K, the surface can be considered "habitable" by humans, and 263° to 383° K for life... (Yes, that's 120° range - but allows for periodicity and polar effects.

 

[Spartan159]

Thanks, Aramis, but how much light to see by would that be? Would the (I'm guessing) orangeish red color cause problems for human vision?

 

[aramis]

Thanks, Aramis, but how much light to see by would that be? Would the (I'm guessing) orangeish red color cause problems for human vision?

You won't see much difference for any star in the FGK range; all of them merely have peak output. Think of the difference between a Fluorescent and an incandescent bulb.

Luminosities will be about 1.25 sol @ 1 AU and 0.04 @ 1 AU.

If the M1V star orbits at .1 AU, and the planet at 1.1...
= 1.25/1.1² + Avg (0.04/1.104² , 0.04/1.104² , 0.04/1.0² , and 0.04/1.2²)
= 1.25/1.1² + Avg (0.033 , 0.033 , 0.04 , and 0.028)
= 1.03+ 0.033
= 1.066

Just a bit brighter.
Peak will be about 1.07, minimum will be about 1.06

If the M1 V is at 4.1 AU, instead, 0.04 luminosity and a divisor of 9 at closest approach, we get 0.0044 peak, and a divisor of 27.04 at opposition, for 0.0017...

 

[G. Kashkanun Anderson]

K-Type stars start out at near Solar-spectrum output, but move pretty far to the infrared by the time they get to the late end of their subtype. A typical K0V star might be close to 90% visual spectrum output, whereas a bottom end K7.5V is hovering around 30%, with most of the invisible output being infrared radiation. M-Type stars move even more dramatically in this direction.

As Aramis said, an M1V star like Pysadi's secondary will have a total output of about 4% of Sol's, but it will only look about 1% as bright, because three-quarters of this output is infrared. Note that infrared radiation would still have an effect on a planet's albedo if the star were close enough (higher, in some cases), but in this case the secondary is far too distant to affect Pysadi in this way.

Orbit 14 is 1200+ AU from the F9V primary. My rough estimate is that the M1V secondary will have an apparent magnitude of -11.4, regardless of its orbital relationship with Pysadi (a 3AU difference either way is not significant at the distances we are talking about here). That's about as bright as a half Moon here on Terra. As it's too far away to make out any angular diameter, however, the secondary will appear as a brilliant, singular point of glittering reddish light -- perhaps a little washed out in the daylight of the much brighter primary, but still quite visible.

If it flares (and stars like that typically do), it might briefly reach full Moon levels of brightness. Knowing the Pysadians, they probably attach some religious significance to that.

 

[Spartan159]

Excellent, just what I was looking for. Many thanks!