Re: Will the light from betelgeuse be hot?

Betelgeuse is a large red super-giant star, and is in the right mass range that we expect it to end its life in a core-collapse supernova explosion sometime "soon". If and when this happens (and in truth, it could still be in tens of thousands of years time!), we'd expect the explosion to reach a peak power output of around a billion to 10 billion times the power of the Sun. That's a lot of power, but of course Betelgeuse is a long way away from us -- about 600 light years -- which means that in our sky it would still be much fainter than the Sun, by a factor of about 100 thousand or so.

At this point you may be wondering how that qualifies as an additional Sun! I guess the answer (apart from a bit of journalistic licence) is that this is still brighter than the full Moon for a few weeks, so it would certainly be easily visible in the daytime, and would make nights a lot brighter (and cast shadows etc.).

As you can imagine, though, such an event wouldn't provide you with much in the way of warmth. However, it is possible that a supernova could at some point occur much closer to the Earth, so that it genuinely would rival the Sun in brightness. It so happens that during its bright phase, supernovae have spectra which are rather similar to the Sun -- most of the light comes out in visible and infrared wavelengths, meaning the colour would appear similar to the Sun (yellowish-white), and the heat we'd receive would also be rather comparable.

Of course, all that extra heat would not be very healthy for life on Earth, and even worse would be the huge flux of energetic cosmic rays which would then proceed to bombard the planet, altering the chemistry of the atmosphere, and very likely leading to mass extinctions (indeed it has been suggested that some of the mass extinctions in the fossil record could be due to nearby supernova explosions). On the positive side, though, we know such events are extremely rare, and we also know that the nearest big massive stars like Betelgeuse are much too far away at the moment to be of any concern!