"Space fantasy" is correct. I've long ago stopped expecting so-called science fiction films to actually make technical and scientific sense, but it's doubly disappointing that a film that has so much good going for it gets so much wrong. Or perhaps the scriptwriters simply had no scientific education. The absurdities are many, including the following. (Spoiler warning!)

The whole premise of the sun's suddenly going a bit dark is very iffy, but just to be nice, let's allow the film that. It's somewhat more iffy to posit that there's going to be any effect on whatsoever on the sun by dropping an object minuscule compared to it, even if the object is "the size of Manhattan", but let's allow the film that, too. But if only this were it.

One would think that humankind would send its best and its brightest, since its fate depends on the success of the ship's mission. Instead, the eight crewmembers have fistfights; one turns out to be a spineless coward; the psychologist is nuts; and, early in the film, the captain commits suicide by sun, for no apparent reason.

The ship is a long structure, like the spaceship sent to Jupiter in 2001, but fronted by a giant curved mirror. A smooth mirror surface would make the most sense, since there's no conceivable reason you wouldn't want the mirror to reflect the sun's rays away, but for some unexplained reason, the mirror surface consists of panels. These open to expose some unexplained equipment underneath, some of which mysteriously break because the navigator made a one-degree error in steering the ship. (As if a computerised guidance system wouldn't be perfectly accurate, especially decades in our future.) Why would you want to create a situation where the only way to reach such equipment is through the shield, making safe access impossible (unless you ignore geometry), and thus creating crevices that, no matter how tiny, would leak some of the intense radiation through?

Two crew members go on a maintenance mission to fix the broken equipment mentioned above. To enable them to open one of the panels of the mirror surface, the ship is rotated so that section of the mirror shield is in shade. However, in real life, where geometry works, that would expose most of the ship to the sun.

In a couple of scenes, crewmembers don spacesuits to float from one ship to another. In space, they're weightless; but once they enter a ship, they somehow experience an apparently full-one-gee gravity field. No, the living quarters do not rotate to create a centrifugal force. (However, there is an antenna that constantly rotates around the ship's axis, for some completely unknown reason.)

The ship's interior has some "cooling fins", perhaps leading heat leaked through the mirror shield, to some extremely cool liquid. However, in a closed system like a ship, heat transferred in such a way still remains in the system. It has to be led away from the system, so all you achieve with such a setup is to add an unnecessary step to the heat transfer. The only thing that would actually do the job is radiating the extra heat out to space.

On the way to the sun, one of the crew members notices that Mercury is transiting the sun; i.e., the planet moves across the disc of the sun as seen from the spacecraft. This seems to be a surprise to everyone, although we have long been able to calculate the orbits of the planets for thousands of years in advance, if not millions.

On the way to the sun, the ship makes a little detour to Mercury. However, as seen during the transit, Mercury's orbit was perpendicular to the ship's direction of travel, so it's a complete myster how the ship is able to match velocities with this fast-moving planet.

And, as a previous poster remarked, why should the whole mission have any crew at all? Adding humans, and life support sufficient for at least a one-way trip, makes the design immensely more difficult. What conveivable use is there for a human in a bomb-dropping mission?

And so on, and so forth. For anyone who knows some basic facts about physics or astronomy, Sunshine is a groanfest. And as Zacharek writes, the plot descends into tedium, anyway. 2001 this ain't, to mention one of the many films it copies elements from. I thought Boyle's Trainspotting was brilliant, but thoughtful science fiction is obviously not his forte.