Dim, brief light. Almost nothing more.
The vast majority of the effects you describe would not occur.
There'd be little heat because the heat is a result of the radiation interacting with the atmosphere around it. Ditto for the mushroom cloud and the shock wave. (You need some medium for a shock wave to travel in). There'd be some visible light from the collisions of the radiation with the nuclear ash ("ash" as in the product of the fission, in the case of Uranium the most common are Barium & Krypton nucleii), as the ash absorbs and then re-emits, occasionally at visible frequencies, photons. Not very much, however, compared to the first-generation fission products, such as the photons produced from fission which are gamma rays, invisible to the naked eye. Keep in mind at the end of the day you don't really need that much material actually undergoing fission: U-235 will explode if it's in a ball not a whole lot larger than your head. At the end of the day, that's not very much material to provide much cross-section for the gamma rays to scatter off of, and the gamma rays are going to outpace all the other matter in the explosion: They move at the speed of light, and nothing else does.
Still, there'd be a ton of gamma rays, beta particles (naked electrons), and neutrons emitted, enough to cause plenty of death and destruction to anything nearby. It just wouldn't be immediately visible - Think the inside of a microwave oven, only more so.
Keep in mind everything I just described occurs with the detonation of a nuke on earth, only nobody notices, because in part it's attenuated by the atmosphere, and in part because the attenuation produces the mushroom cloud, the heat, and the shock wave which destroys anything affected by the radiation, only a fraction of a second later. The first-generation radiation itself pretty much attentuates to a vanishingly low amount after about a mile-and-a-half from the atmosphere.
The air force did some test with high-altitude nuclear explosions, but they weren't really in space. They were in the kind of space that John Glenn orbited, which still has a fair amount (on the order of 1%) of atmosphere. What I'm talking about is SPACE space. Vacuum.
Oh, it would NOT look like the sun, by the way. NONE of the light we see from the sun is a first-generation product of fusion. Sure, the fusion reaction produces photons (also gamma rays, actually) but they take an average of 100,000 YEARS to go from the core, where fusion actually takes place, all the way to the surface.
The light we see from the sun is just garden-variety black body radiation. A ginormous incandescent light bulb, only hotter. Light bulbs burn yellow because they're cooler than the surface of the sun, but the sun actually appears roughly the same color because it's depleted of violet, indigo, and to a lesser extent, blue light because of the absorption and scattering from the atmosphere.
Is VISIBLE light emitted from nuclear reactions? No, not usually. The photons emitted from uranium and plutonium fission have some wiggle room in their energies, but not enough for many of them to end up in the visible spectrum.