[WARNING: Spoilers for The Force Awakens]
From memory, it looked like it was much closer to the star than Mercury’s orbit. That planet would have been fried by solar radiation the moment it open its shields.
The star seemed pretty small in comparison to Starkiller, which itself was quite small in relation to a planet. That causes problems, as smaller stars = greater density = more surface gravity = more energy needed to escape that surface gravity. It’s not clear if Starkiller consumes the entire star or just most of it, as the Kylo Ren/Rey duel was bathed in moonlight yet that base has no moon. At any rate, it probably takes more energy to siphon off that much gas than it does to run Starkiller’s big gun. The surface temperature of the star may be a mere few thousand degrees (or about 6000 if you want it to have the same colour as the Sun), but as you peel back layers you expose gasses that are over a million degrees in temperature. Add that to the energy containment woes.
Next problem: Starkiller contains most of the mass of a star, packed even more tightly than before. Even if all that mass were converted to energy, the resulting gravitational field would be off the charts. Star Wars’ universe does have magic gravity modifiers, which would solve that problem (as well as providing Earth-like gravity on a planet much smaller than Earth)… at least until the explosions disabled them. At which point the base should have gone full-on supernova, but instead it turned into a Starkiller-sized star.
Plus, who planted the trees? While you can Earth-ize the gravity here, any pre-existent life would have been adapted to a much weaker gravitational field. The native trees would have collapsed under their own weight. Similar arguments apply to the terrain, which looks suspiciously Earth-like and lacks the soaring peaks you’d expect from a low-gravity place
And how close were all those planets that Starkiller obliterated? No system with four planets that close would last a hundred years before three-body interactions chopped down their number or tore them apart. Meanwhile, the heroes are gawking from the surface of a planet close enough to see all this chaos. It must be in the same system, yet it wasn’t targeted even though Starkiller can destroy multiple planets.
That beam, too; as I recall, it was a single column until it hit a certain spot, at which point portions of it made ninety degree turns and fanned out towards their targets. Imagine the energy needed to make a car complete a right angle turn within half a second, and extrapolate upwards towards a car with a significant fraction of a star’s mass.
I know, it’s nitpicky to point out these things in a movie that greatly bends physics without care. The problem is, most of them are easily fixable. Don’t park so close to a sun; don’t run Starkiller directly off a sun’s mass, just use a Dyson swarm or massive solar collector instead; don’t go the poetic route, just blow the damn planet completely; don’t have one beam but multiple ones, and never show all the planets together. Compare these changes with something like a hyper-crowded asteroid belt; they’re just as much a cheat, yet there’s no easy dodge that doesn’t kill the story as well.
Lazy writing pulls you out of the story, and I lost my suspension of disbelief when I started thinking about some of these things in the theatre. Put some effort into it, please. And I’m not the only one to spot issues like these.
As a bonus, here’s a scene rewrite which makes a lot more sense than the original.
SCENE: VARIOUS COCKPITS ABOVE THE RESISTANCE PLANET OF D’QAR.
POE: An encrypted channel, Han? You’re scaring me.
HAN: Oh, I haven’t even started yet. Forget what I told the General, our actual plan is to come out of hyperspace UNDER Starkiller’s shields.
PILOT #1: Impossible!
PILOT #2: You’ll kill us all!
POE: … Riiight. Han, how much time have you spent in the bar lately?
HAN: It’ll get us past the shields AND nice and low, under their sensors. Then my team and the other landing party will hand out some nice housewarming gifts.
POE: But if we get it wrong…
HAN: … with a dozen ships, at least one of us won’t.
POE: Can you even come out of hyperspace that close to a planet? Why didn’t the Rebels use that against the Empire before?
HAN: What can I say, I’m a genius.
POE: You’d have to be, to keep that bit of garbage running.
PILOT #1: So us fighters blast the regulators while the landing parties work?
HAN: Nah, that’ll attract their attention. The fighters will hang tight and quietly guard the ships, until the fireworks start.
HAN: I have a Space Chess set.
POE: Will the Wookie be playing?
POE: This plan is crazy and suicidal. MY FAVORITE KIND!!!
PILOT #1: It’s better than the alternative, I guess.
PILOT #2: Are the First Order hiring?
HAN: I hear they just had a new position open up.
CUT TO FINN, LISTENING TO ALL THIS. HE’S A BUNDLE OF ANXIETY AND DEEPLY CONFLICTED.