Last week I wrote a bit about the Star of Bethlehem, and how I think it is not much more than a fish story; something that started small and has grown in significance over the millennia. I also talked about it on Coast to Coast AM last night; they had a link on their site about a Notre Dame astronomer who thinks it might have been a conjunction of planets plus a supernova.
Ignoring for the moment my own objections to this story in general (the star in the story stays in the East, which is impossible, and why do people who believe in miracles try to back them up with science, negating their entire premise?), I still have doubts over the cause. We don't know when Jesus was born, making this a difficult proposition to start with. There were several planetary conjunctions in the time range in question (say, 8 to 2 BC), and a few comets as well.
The new thing in that article linked above is the idea that the Star may have been inspired in part by a supernova, an exploding star. The astronomer, Grant Mathews, points to a supernova remnant called Kesteven 75 as the possible culprit (note in the article that name is misspelled).
I looked up that remnant, and found a few papers. The age of the expanding gas cloud (the debris from the star's explosion) is difficult to determine, and ranges from a few hundred to several thousand years. That doesn't help much. However, when the core of the star collapsed, it formed a fantastically dense neutron star. This star has a strong magnetic field and spins rapidly, making it a pulsar: a star that appears to emit blips of light like a lighthouse does.
The ages of pulsars can be determined with more confidence (the rate of spin of a pulsar slows as its magnetic field sweep up material, like a parachute catching the air, and this can be used to backtrack when the star first formed). The age estimates determined for the pulsar in Kesteven 75 all converge to around 700 or so years, far too young to be the Star in the legend. While these are still estimates, and may be off by quite a bit, it's a stretch to say this explosion could have been the basis of the Star of Bethlehem.
So I still stand by my original premise: the very fact that people are scrounging around looking for the possible cause of the Star makes it very unlikely to have been a single, real, spectacular event. Had it been such, the root source would be easier to find (and would be in several cultures' legend and writings). This, coupled with the uncertain date of Jesus' birth, the vague nature of Biblical writing, and the very real tendency to exaggerate such events when writing them down to increase their perceived importance, makes me think that the Star is a nice story, but very unlikely to be true. Maybe there is some basis in fact, but it's almost certainly been puffed up over the centuries.