OK, here are some interesting tidbits, if we can play pretend.
First of all, it’s interesting to note that a huge star like a red giant (such as Betelgeuse, 4au in radius, roughly the size of Mars’ orbit) is not so much a different type of star as a star in a very different phase of stellar evolution. Our sun, as mentioned, has a fusion reacive core, in which superhot condensed hydrogen atoms fuse to form deuterium, and eventually helium. However, over the course of many millenia, the fuel for this nuclear fusion is gradually expended. The result is that the reactions occur less and less, and, as mass is now also less, the graviational pull of the star is weakened. The surface (“corona”) expands vastly further from the core in the weaker gravitational field. Also, with decreased nuclear reactions in the core, the star simultaneously cools at it expands, thus the change from a yellow sol star to a red giant. This is just one phase of solar evolution, by the way, very simplified. Blue Giants are in fact closer to just being different types of stars.
Now, your planet. ASSUMING we could have a terrestrial planet the size you are talking about, made of non-reactive nickel, iron, and carbon, like our own planet, here is what you might expect:
Everything comes back to gravity. A planet that large would have an immense pull, allowing it to maintain a VERY diverse atmosphere. See, Earth’s atmosphere developed, it is believed, over the course of the Earth’s cooling. Geological processes associated with molten components reacting in Earth’s early molten stage, and her subsequesnt cooling, yielded gaseous biproducts, creating the atmosphere sustained by the Earth’s gravity. The most notable were CO2, hydrogen and helium. Single-celled photosynthetic life forms then furnished the atmosphere with the O2 we enjoy today. Lighter elements, such as H and He, eventually escaped the Earth’s gravity, explaining their relative scarcity today (H also probably combusted often). The moon, in turn, (as well as Mercury and to an extent, Mars) is thought to have long since lost whatever atmosphere it might have had, due to its weak gravity. A planet that size would have an immense field and thus maintain a very diverse and thus unstable, reactive, and overall hostile atmosphere at an unimaginable pressure. It is therefore basically impossible that a planet this large would be able to support any for of life that we on earth are familiar with. Would some other form of life evolve to meet these conditions? Hey, nothing’s impossible.
With this in mind, I won’t (and frankly can’t) go into lunar phases, tides, etc. but hypothesize that stars and probably galaxies could orbit this planet.
All this is conjecture, by the way, based on my Conecptual Astronomy class Sophomore year. IANA astronomer.
flup