THX1138 wrote:Hmmm only diffraction spikes on one and not from the smaller or more distant one also?
So much for my photographic know-how.
The picture with diffraction spikes shows the ultraviolet emission from Sirius A, a "normal" star about twice as heavy and about 22 times as bright as the Sun, and from its hot shrunken degenerate companion, Sirius B. Sirius B was once a larger and hotter star than Sirius A, the star that we now call Sirius. But this larger and more massive star used up its internal fuel faster. Sirius B never exploded, but it "cast off" its outer envelope and bared its extremely hot, shrunken interior. When that happened, the small hot core lit up the cast-off envelope and made it glow splendidly as a
planetary nebula. But planetary nebulae only last for a few thousand years or so, until they disperse.
There is no sign of the cast-off outer atmosphere of Sirius B. But the hot core is still there. It is extremely faint in visual light, because the core is so small, but it is still very hot. Because it is so hot, it is brighter in ultraviolet light than Sirius A (the star we think of as Sirius). The picture with the diffraction spikes showed these two stars, which are very close together in space, and the apparently brighter star is the hot faint companion.
The diffraction spikes presumably reflect internal properties of the camera or telescope.
Ann