Longish short story or short novella. I originally read it in an anthology compiled by Isaac Asimov. (I don't think the actual story was an Asimov)
Story about intelligent beings native to stellar photospheres.
Starts as an in-flight conversation between two beings, scientists decompressing after a research trip. They have slightly different structures, because they're from different star systems. They are, however, comfortable in a mutually compatible environment. (Plasma contained in a collapsed-neutron hull)
(turns out that solar flares are starship launches, and sunspots are large structures that are related to their cities)
The beings start to speculate about hypothetical "exotic" phases of matter, and what behavior matter might display if it was insanely cold. (by this, they mean too cold to emit spectral lines)
One being doubts that anyone could ever find enough of such an exotic material to learn about it. (after all, you can't see the stuff!) Second being responds by telling a "sea story" about something he had witnessed. A research vessel near his home star gets inexplicably dragged off course by a "phantom" gravity well. Plotting the effect, it appears to be a fairly significant concentration of mass, but no matter is seen. The vessel cannot break free, and eventually seems to impact on the invisible mass. The hull is breached. The crew is lost. As the plasma bleeds out of the hull, the observers note emission spectra of iron, silicon, aluminum & other "heavy" elements....
Cut to earth where we see this crash from a human perspective. Humans get lucky that the impact is in an out of the way area. A man in the area recognizes that this is a constructed object, but the plasma leak melts the rock under the ship, and the hull material is so dense that it sinks long before the heat can dissipate.
The anthology seemed to be from the 70s or 80s, but the dated science makes me think the story itself may date back to the 40s
NOT any of the stories in the following anthologies: Before the Golden Age, The Winds of Change and Other Stories, The Early Asimov, Nine Tomorrows