Starship Asterisk*

The most famous of all color-contrast binary stars is undoubtedly Albireo, also known as Beta Cygni. However, the binary is "paler" than you might think. The primary is a star of spectral class K3II, and this particular spectral class ought to ensure that the star would be rather red, with a B-V index of at least +1.5. Instead, however, the B-V index of the primary is only +1.08, about the same color as Pollux! That's really quite pale for a K-type bright giant. The reason for the diluted color of the K-type giant is that it has a hot companion, which orbits so close to the red giant star that it can only be detected spectroscopically. But this hot companion is of spectral type B0.5V and undoubtedly very blue, and its light dilutes the color of the primary of Albireo to a pale yellow hue.

As for the visible blue companion, it, too, is rather pale as blue stars go. It has a B-V color index of -0.095. That's blue, but not strikingly so. To me, therefore, Albireo is a double star consisting of a pale yellow primary and a pale blue secondary, and to me this means that Albireo isn't so great!
No, my own favorite color contrast pair - or rather, triple - is 30 and 31 Cygni. 31 Cygni is a star of spectral class K2II. This star, too, appears to have a hot unresolved spectroscopic companion, but even so, its B-V index is redder than the B-V index of the primary of Albireo: +1.270 for 31 Cygni versus +1.08 for Albireo.

However, 31 Cygni has a visible companion too, and this star is bluer than the blue component of Albireo: -0.133 versus -0.095 for Albireo.The color contrast is simply more impressive here than for Albireo!

Fascinatingly, 30 Cygni, a star of spectral class A5, is located close to the 31 Cygni pair. It can be seen in the same telescope field. The B-V index of 30 Cygni is +0.100, which means that it is not just bluer than the red component of 31 Cygni, but it is also visibly "paler" or "less blue" than the blue component of 31 Cygni! How fascinating!
Here is a color-enhanced picture of 30 and 31 Cygni. 31 Cygni is the yellow star, and 30 Cygni is the (too) blue star. The blue component of 31 Cygni can barely be seen in teh lower part of the yellow halo of 31 Cygni.

Ann

Ann
The binary stars where will be in the H R Diagram?
They have itself, different L , M and color-temperature

saturno2 wrote:Ann
The binary stars where will be in the H R Diagram?
They have itself, different L , M and color-temperature

I'll pretend to be Ann for a moment. Hope she doesn't mind!

If the each of the stars in a binary pair can be measured separately, they'll each have a spot of their own on an H-R Diagram. If not, then the blob they make together will have just one spot on an H-R Diagram, which will represent the net of their values. (I suspect a well-prepared H-R Diagram would just exclude such data.)

Location on an H-R Diagram has no relation to location in space, so each of the stars in a binary pair can be anywhere on the diagram. For example, there's a form of binary that results in Type 1a supernovas. Such a pair can be a white dwarf and perhaps a red giant. They'd be at nearly oppposite corners of an H-R Diagram -- if they could be separately measured.

Rob

Rob
Thank you for your explanation