You might even teach a little basic science and engineering, like how airlocks work, and why their doors have to be made to only open inwards…

However, not all binaries are close binaries. In the Alpha Centauri system, the planets around A and B probably had too variable a history to have nurtured life. However, at Proxima (providing its orbit is roughly circular and doesn’t come too near its primaries) a life bearing planet might evolve.

What we don’t have to worry about is “pinball systems”. Multiple stars form out of their birth nebulae in a morphological hierarchy. Two stars orbit a common center of gravity. Add a third star and it orbits far enough away that it sees the other two as one gravitational attractor. Add another and you get either two stars orbiting the central pair (like two satellites orbiting a planet) or two pairs orbiting each other (One binary star in orbit around another binary star).

Although higher numbers are physically possible, like three stars orbiting equidistantly from each other, they are higly unstable and easily perturbed. The stellar multiples are always pairs, two centers of gravity orbiting each other, (although each center of gravity may consist of multiple CGs as well.

This is the famous double-double, epsilon lyrae, two double stars orbiting each other. You will note that the separation between pairs is much greater than the separation of each pair. This is what I mean by “hierarchical”.

I believe the most complex multiple system known contains 11 members, but others may have been found even greater by now.

The separations can vary enormously, in some cases, the pair is in actual contact, and exchanging material at the Lagrangian point between them. (Contact binaries)

In cases like this, the two stars actually affect each other’s evolution, and the system itself evolves. For example a star can enter its red giant phase and swell up to fill its Roche lobe, at which point it starts transfering matter to its partner. Remember, the size of a star is not related directly to its mass. The smaller star may be much more massive. And stars will change size as well as temperature(color) as they evolve. And many stars vary in size and brightness over very short time periods, between hours and months, oscillating due to complex internal instabilities.

There are many strange creatures in this zoo.

Even a system like Alpha Centauri could offer a rather dynamic version of a Habitable Zone, could it not? It might not be of much practical use to a planet, but mapping such a zone could be an interesting challenge.

Also, look at figures 4 and 5 in this article.

https://www.gl.ciw.edu/static/users/bmilitzer/papers/34_seager_kuchner_hier-majumder_militzer_2007.pdf

or here, for density as a function of radius:

The formula for gravitational acceleration g at the surface of a planet is

g = GM/R**2 (the test mass is irrelevant because it is assumed to be much, much lighter than the planet)

where

g is the gravitational acceleration in m/s**2
G is the constant of proportionality that makes it all come out in the right units
M is the mass of the planet in kg
R is the radius of the planet in meters

Let’s normalize the data by redefining the above variables to 1

1.5 is the gravitational acceleration in earth g’s
5 is the planet mass in earth masses
r is the unknown radius we are trying to determine in earth radii
(Normalizing will require a new value of G, so let’s call it Gprime, although we don’t need to know it)

The equation now becomes

1.5 = Gprime5/r**2 and reordering terms,

r**2 = (5Gprime)/1.5 since M and g are now both normalized to 1.0

therefore

r**2 = (3.33333…)Gprime

r = SQRT (3.333…) Gprime drops out as all the units cancel out

r = 1.8257 earth radii

This stands up to the eyeball check: if the planet massed 4 times as much it would have to have twice the radius to give the same surface gravity, due to inverse square law, so my result seems to be at least in the ballpark.

I know the formula is Mm/r2 but what little incentive I had for ciphering has left me. I’m guessing, probably twice the radius of Earth would not be too bad. I would like to see it scaled.

Obviously there is a point where the planet density would get pretty mushy, but the consistency of the planet would also be a factor. (a thin crusted planet)

Our nearest neighbor, Alpha Centauri, about 4 light years away, is a triple star system. Alpha Centauri A and B are a close binary pair. They are both sunlike stars. The third member, C, circles the center of gravity of A and B, and is currently on the side of its orbit that makes it the closest star to earth, only 4.2 light years. C is a faint red dwarf. It is often referred to by its nickname, Proxima.

If the above comes across as plain text, then someone’s gonna hafta show this old dog a new WordPress trick.

It’s about half as bright as the Sun, so its habitable zone is much smaller, but the spectral type is similar, it appears to be a fairly stable G8 subdwarf, not identical to the Sun, but fairly similar.

The planet is 5 times as massive as earth, but depending on its radius, it need not have 5 times our surface gravity. It might be a fairly nice place to visit.

The article does not mention if the orbit is highly eccentric or near
circular. The latter would be better for life, but the former would be easier to detect using the wobble method.

The really good news is statistical, it suggests reasonable size planets in the habitable zones of main sequence subdwarfs are not a fluke, and are probably quite common.