Expert: Philip Stahl - 6/25/2007

Question
In a  science-fiction book I've just read, by Poul Anderson("Ensign Flandry"), there was a mention of a wandering  planet from outside the relevant solar system colliding (off the elliptic plane) into an F5 star. The F5 star is set to go supernova, normally,  in c. 1 billion years, as it's at near the end of its evolution, but the impact of the planet's collision with the star is supposed to release 3 days worth of that sun's solar-output within a few mere seconds, thus forcing a supernova, 1 billion years before it should.

Is this a scientifically plausible event? I assume that the book was using the F5 classification as from a modern-day astronomical textbook, but, in case it was just an arbitrary SF measurement, I (think) that the star in question may possibly have been a blue giant. And the planet is described as being not that much larger than the Earth's Moon.  

Answer
Hello,

You don't provide the date (year) the book was written, but I'd warrant it was probably before the mechanism for supernovae was well understood. Definitely, a spectral type F star is a most implausible candidate for a supernova - whether Type I or Type II.

Type II supernovae, to refresh memory, occur in type O and B stars much more massive than the Sun. (E.g. at least five times the Sun's mass). An F star would never attain such a mass so we can rule out a Type II immediately.

Type I supernovae are more complex to be sure, and *can* allow a star with a mass as low as the Sun's (which might be approached by some F-type stars) to detonate violently as a supernova.

However, the scenario here is usually NOT for main sequence stars but for white dwarfs at or near the Chandrasekhar limit (1.4 solar masses). They also need a companion, like a red giant (class M) star.

What happens is that the fantastically high gravity of the white dwarf pulls mass off from the red giant. As the mass flows to the dwarf it eventually exceeds the 1.4 solar mass limit and collapses - since its gas -radiation pressure can no longer support its gravity.

Then you get the Type I supernova.

Anderson's scenario still can't work like this. Here the F-star will simply not have the adequate gravity to pull enough mass off the wayward planet to exceed the Chandrasekhar limit. Indeed, it isn't likely the planet would have ample mass for the mechanism to work in the first place. Even Jupiter, big as it is, has nowhere near 1 solar mass. Indeed, at about 1.9 x 10^-27 kg it is nearly a thousand times less massive than the Sun.

Thus, even if the whole planet somehow agglomerated onto the F-star, the maximum resulting mass would be barely 1.001 solar masses, or still below the 1.4 limit.

The planet being not much larger than Earth's Moon doesn't help matters.

It also seems implausible to me that Anderson would have used an F5 spectral type for a blue giant star. Even then the spectral sequence was known and a writer of the caliber of Anderson would never have succumbed to such an egregious error and wouldn't have used some "amorphous" classification system that was so close the actual one.

In the end, treat the story as fiction - maybe based on science - but fiction nonetheless.