Hi there,

Thanks for this kind of response. It was exactly what I was hoping for and very helpful.

On 18 August 2014 15:22, Tim <tim@little-possums.net> wrote:

On Mon, Aug 18, 2014 at 10:20:35AM +0100, Timothy Collinson wrote:
> I'm trying to describe the view on a world where this is given as
> stellar data:
> F7V M1IV
> So I know the main star looks yellow-white. Not sure about the other - is
> it even visible?

I would expect so! It's a subgiant, and likely more luminous than the
F7V by a factor of 10-20 or so.

Ah. My mistake, must have been looking at the wrong thing - thought it was a dwarf and almost something to ignore... glad I asked. Not, I suspect, that there'd be many who'd read something about the planet and bother to actually check up on this! Still, I feel I should try and fit it with the data where possible.

The F7 main sequence star is not very much hotter than the Sun; I
don't think anybody would notice the difference without measurement or
some sort of side-by-side comparison.

OK.

> The other clue that I have is that the UWP is C565436-5 so I'm
> assuming shirt sleeve conditions are possible for at least portions
> of the year on some of the planet.

For similar temperature to Earth, the primary star would be slightly
smaller in the sky, though a little more "dazzling" than our Sun from
Earth. Without knowing the orbit of the companion star,

(that's what I assume I can pretty much just decide)

it could be
anywhere from visually much larger and brighter (if they're a close
binary pair inside the planet's orbit), similar or smaller disk with
dimmer/yellower light (medium binary), to just a bright pointlike star
easily visible during the day (distant binary).

The middle case is possibly the most interesting, with substantial
light and some heat during the primary's "night" for much of each
year. If the companion was about 10 times further out but 20 times as
luminous, it would contribute about 5% of the light and 20% of the
heat. That 5% light might not seem much in numbers, but indoor
lighting is usually less than 1% as bright as daylight, so it would be
much brighter than most artificial lighting. Likewise the 20% heat
influx would be enough to noticeably reduce heat loss during the
(primary) night or add to warming during the day.

OK. You're right - that is the most interesting. I was inclined to go with distant binary just to make it 'easier', but I don't think it will affect the story I'm trying to tell, and could be interesting, so I might go with that. See how it goes.

One other thing to remember is that an F-class star puts out a fair
bit more energy into ultraviolet than our Sun does. UV-A in
particular would be more intense at the surface of an otherwise
Earth-like world, so beings not adapted to such flux would likely need
to avoid exposing unprotected skin and eyes for too long.

OK, I'm guessing that the inhabitants would be inured - but useful to know for the ummm, visitors.

I hope this is a start towards some sort of help.

Really excellent, thank you.