The sun was a gas giant at one stage in its formation, about #4.7# billion years ago. "Gas giant" is the term we use for large planets the bulk of which are gaseous. Once a gas giant accumulates enough mass from the surrounding nebula (say about #13# times the mass of Jupiter), then the internal pressure and density is enough to allow some nuclear fusion to start. Initially such a proto-star becomes a brown dwarf, fusing deuterium (#""^2H#) and lithium (#""^7Li#), but not yet massive enough to fuse hydrogen (#""^1H#).
Between about #80# times the mass of Jupiter and half the mass of our sun, the star is a red dwarf - fusing hydrogen (#""^1H#), but still somewhat cooler than our sun.
The reason we see the sun as a disc and not a point is simply that it is much closer to us than the other stars.
Only a few stars (including Betelgeuse) are large enough and near enough that we can resolve more than one pixel with our best telescopes.
As for landing on the sun, the surface temperature is about #5778#K, so it would be a little impractical, and that's nothing compared with the sun's corona which can vary between #1# and #10# million degrees K.
