What sun? It exploded. Yes. However, a nova or supernova always leaves a remnant. Mirkheim is the remnant, the blasted core, of a companion planet too large to have been entirely vaporized or disintegrated by the stellar explosion. Similarly, the supernova itself left a remnant, imperceptibly shrinking towards black hole status. Thus, two unusual heavenly bodies still orbit in close proximity. Coya Conyon says that they had originated as a double stellar system, not as a star with a planetary system.
The stellar remnant is six Solar masses compressed into one Terrestrial volume. Therefore, it "...has stupendous gravitational power..." (David Falkayn: Star Trader, p. 676) which warps space so that "...the laws of nature take on an eerie aspect." (ibid.) Anderson does not elaborate although readers of popular science and science fiction have learned that spatial distortions affect time and matter in counter-intuitive ways described by Einstein, not by Newton or by common sense. (Common sense generalizes our experience on a macroscopic scale but at sub-relativistic velocities.)
Secondly, the fantastically rapid spin of a dwarf star also "...generates relativistic forces, describable only if you have determined the precise quantities involved." (ibid.) Thirdly (I think that this is a separate point), pulsations usually found only inside atomic nuclei stretch across interplanetary distances. Fourthly, as with any star, there is radiation although this is comparatively easy to guard against, dim visible light but lurid gamma and X-rays.
The Ythrian spaceship, Dewfall, chartered by Nicholas van Rijn, finds the sun - so small as to be undetectable except within a few parsecs but van Rijn had known where to look. Maximum photoamplification reveals a white spark but makes other stars painfully brilliant. Expensive equipment installed by van Rijn detects the light-bending due to a sub-stellar object between forty and fifty astronomical units from the star. To establish its position, they need a long baseline which would take weeks at sublight speeds. However, Dewfall's engines are well-tuned enough to use hyperdrive even this deep in a gravity well.
As Dewfall approaches Mirkheim, three Supermetals warships move to intercept and block escape. The Ythrian ship's approach has been observed. Relative speeds and distances are such that one ship can engage until the others arrive. They intend to detain crew and passengers but will fire if Dewfall attempts to escape. Having come prepared, van Rijn takes charge and accelerates on negagrav back towards the sun. No enemy missiles can accelerate fast enough "...to get past defensive fire..." (p. 675) Dewfall goes hyper, though not at full speed, and reverts to normal state when nearly at the sun but still continues toward, then around, it. Prior computations ordered by van Rijn and readings taken by Coya as they approached the sun are in the data banks. Now Coya inserts current data that enable van Rijn and Hirharouk to steer. Commodore Nadi of Supermetals can neither pursue nor predict where his enemy will emerge.
The Ythrian beats his wings while the man kneels in prayer. They must cope with a meteoroid swarm and a plasma cloud but:
"They won past reef and riptide..." (p. 677)
I have found it rewarding to summarize other battles in space. See here and here.
