Dwarf confusion

[Ann]

I bought a magazine called Discover the Universe: All About Space the other day. There was, among other things, a nice article about red dwarf stars in that magazine. My favorite part of that article was a fine illustration of the colors and sizes of typical "dwarf" (hydrogen-fusing) stars. A typical M-type dwarf has a radius of 0.3 times the Sun, we were told. An orange K-type dwarf has a radius 0.8 that of the Sun, and stars like the Sun are the size of the Sun, we were told. "Yellow-white" (I beg to differ) F-type dwarfs have radii 1.3 that of the Sun. Blue-white B-type dwarfs like Regulus have radii 5 times that of the Sun, and blue O-type dwarfs, perhaps like Theta C Orionis, have radii 10 times that of the Sun.

Which leaves only the A-type hydrogen-fusing stars like Vega and Sirius. In the illustration, these stars were called... you guessed it. They were called "white dwarfs".

Imagine! White dwarf star Sirius has a white dwarf companion! And the white dwarf has a much larger radius than the white dwarf! Of course the white dwarf is much brighter than the white dwarf in ultraviolet light!

But the illustration was really nice and illustrative. I liked it!

Ann

[THX1138]

Hmmm only diffraction spikes on one and not from the smaller or more distant one also?
So much for my photographic know-how.

[Ann]

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