Help in determining the features of an unusual, fictional star system

Question

(Hope this is the right Stack-exchange site for this question)
I'm working on a sci-fi RPG campaign, set on a very atypical location. Since this is a work of fiction, there's enough room for speculation and artistic license. But I'd still like to start from a more astronomically sound perspective.

Background

Consider the following system:

1. A single moon, with properties roughly comparable to Earth's so that it can support human life (1G gravity, Nitrogen-Oxygen-Carbon atmosphere, soil and liquid water etc.). This moon is tidally locked with it's planet.

2. A single gas-giant planet, with a size roughly comparable to Saturn. Orbiting it's star so that it completes a cycle about every 12 earth years.

3. A Star, larger than Sol and emitting enough energy to keep the moon in a comfortably warm temperature.

The features above may be altered to ensure possibility of human survival on the moon, and if possible, to keep the moon tidally locked with a large planet.

Now, the complications (inspired by a short story by Larry Niven):

1. The whole system (star, planet and moon) are traveling at a relativistic speed across the galaxy.
2. (Based on the theory that some parts of the universe might be made from anti-matter) The whole system is made from anti-matter, but is currently traveling through a normal-matter area of space. Note: the "human life" on the moon are also based on anti-matter...

Finally, the story will begin with a small community of primitive "anti-matter humans", living on the moon in a place from which the planet fully visible, and at the zenith.

The Questions

I'd appreciate ideas, insights and comments on the following:

1. From the moon, how will the sky look like?
2. What will the equivalent of a "day and night cycle" (sun-up to sun-up) look like?
3. Will the winds on the moon have any "non-earthlike" patterns?
4. Considering that at relativistic speeds, the odds of the bodies in the system hitting hydrogen particles in the vacuum of interstellar space are significant (is that even correct?), will that create any special aura or energy streams from the sun or planet?

Thank you for your help.

Answer 1

First it will be important to consider the term 'relativistic speed'. If by that you mean something like 0.1c, it will only change the colour of the stars as you mentioned in the bounty description. However, if it means something with higher Lorentz Gammas (like 0.9c or 0.99c), all other relativistic effects come into play. There's relativistic beaming and aberration combined, which means that all the light that you receive from systems with respect to which you're moving at relativistic speeds will only be received in small beams towards the direction you're moving in and those will be strongly blueshifted and intensified. Watch this animated video for an illustrated view of what I mean - https://www.youtube.com/watch?v=JQnHTKZBTI4. (It's not a high quality video, but it is very effective in demonstrating what I just wrote.)

So, in effect, you will possibly see only three major sources of light in the sky:

1. The parent planet
2. The parent star
3. The bright patch of the sky in the direction of motion, highly blueshifted, with brightness dependent on your speed

Their relative brightnesses will depend on:

1. distance to parent star
2. distance to parent planet
3. albedo of the parent planet
4. speed of motion through the ism

For a day and night cycle:

• The parent planet will obviously be at the zenith the whole time (or fixed location in the sky if you're at a different location).
• The parent star will appear to move around the sky with the same period as the revolution period of the moon - say a few days at the most (will be fairly small as compared to the 12 year revolution period of the planet, so you can ignore 'synodic' effects to the first order for a single day/night cycle).
• The sky will also move with roughly the same period as the sun, ignoring synodic effects.

Winds will be very complexly affected by the three sources (since heating due to radiation affects winds). There will also be tidal effects due to the parent planet which also play a role in the winds (similar to how tides work on the earth). So, you will have to take into account heating from the star, heating from the planet (which is constant due to tidal locking), tidal effects of the planet (again constant) and heating from the sky. Now the radiation from the sky can also act in a weird way, since blueshifting can give rise to strong emissions in UV, X-ray and Gamma ray photons, plus relativistic cosmic rays (everything you encounter will be in the form of cosmic rays due to the relativistic speed of your motion). These can ionize the atmosphere, heat it, cause there to be particle showers and aurorae, which can all affect wind patterns in complex ways, which I cannot exactly say much about, given my limited knowledge in atmospheric sciences.

As I mentioned above, yes, all interactions will be relativistic and will be similar to cosmic ray bombardment onto the moon you live on. Things will get nasty and you'll be having geomagnetic storms all the time. Satellite communications will probably never work. Expect these particles to arrive in the previously mentioned bright region of the sky due to aberration still applying to this case. You will see similar aurora like regions on the parent planet (http://www.spacetelescope.org/static/archives/postcards/screen/hst_postcard_0008.jpg), and the star will probably not be affected that much.

Oh and by the way, due to the antimatter nature of your system and the matter nature of the ism, the atmosphere would thin over time, and later these antimatter cosmic rays will start bombarding the moon. So if you stay in the ism for long enough, living on the moon will be difficult. Also, I am assuming that life evolved on your moon in an antimatter ISM and you have entered the matter ism only recently, as otherwise, the moon won't be stable for long enough for life (intelligent life) to evolve.

I will try to see if I can think of some other effects, but for now, I guess this will do?!

Answer 2

1) From the moon, how will the sky look like?

If the moon is tidally locked in a circular orbit then the giant gas planet will always be in the same spot in the sky. It wouldn't move. It would always be there.

But if the orbit is eccentric, as is that of Mercury, then it would turn and expose all its sides to its mother planet, even though it is tidally locked.

The mother gas giant planet would change its phases and its atmospheric storms. Its hugeness and nearness would take up a really large fraction of the sky for anyone on the side of that moon which permanently faces the planet. You couldn't have a better GPS...

2) What will the equivalent of a "day and night cycle" (sun-up to sun-up) look like?

Day, as in dawn to dusk, for a moon would be its orbital period. For most large moons in the Solar system, their orbital periods are a few days to a week or so. Not extremely different from our Earth day rhythm, but likely a few times longer.

3) Will the winds on the moon have any "non-earthlike" patterns?

Titan of Saturn is the only moon which has atmosphere, and thus winds. And its winds seem to be very calm. And the winds on Mars too move grains of sand quite slowly most of the time. It's more of a geological phenomenon than a weather phenomenon, if I've understood it correctly. But weather patterns in gas giant planets can be enormous. As in the atmospheres of stars. And then there's the Venusian atmosphere. There are four atmospheres of rocky/icy worlds in the Solar system, and they are all very different. Very different. So your fiction may be believable almost anyway you make it in that respect.

Here's a SETI talk about tidally locked exoplanetary insolation which I think might interest you: http://www.seti.org/weeky-lecture/patterns-sunlight-extra-solar-planets 13 minutes into it maybe it gets more interesting.

And I think that the solar wind from the star would keep all the interstellar medium anti-anti-matter from interacting with its planets. ISM is about one atom per cubic centimeter. If its matter or antimatter I think it wouldn't change much as our Sun plows through it.