Ring

This is the final book in Baxter's Zelee Sequence but was recommended to me as a worthwhile stand alone book.

As the sun approaches heat death Earth's scientists work out that it's happening much too soon and something must be happening to it. The Zelee series deals with alien wars etc and space travelers also work out that other stars are also degenerating too quickly.

By sending a probe into the sun they find the problem and realise that it's non-repairable and it looks like the whole galaxy is threatened by the same thing.

As the book approaches the end they work out that the aliens with whom they were at war for so long are really the solution to a galaxy wide problem.