Chris Peterson wrote:
I think Ann believes a star representative of Christmas should be blue.
owlice wrote:I didn't understand why Ann wanted to discuss someone else's image of a completely different target on this thread. I was hoping for enlightenment. This thread should be about its topic -- today's APOD -- rather than someone else's topic. I remain confused.
Today's image is not only interesting, but very beautiful. A stellar portrait, indeed!
You got me, Chris!
Personally, I think blue is about the least "Christmasy" color there is!
Most people would agree, I'm sure.
Today's APOD is boldly colored, which gives the image a wonderfully festive quality that is just so appropriate for the season. I love it!
In reality, most of the stars in the image apart from Aldebaran are rather pale in color. The blue stars in it are bluer than the Sun but yellower than Sirius and Vega. The fact that the Hyades is about 800 million years old means that all the O- and B-class stars that once belonged to this cluster have burnt out and disappeared long ago. And O- and B-class stars are the ones that are usually thought of as blue.
There are in fact two intrinsically blue class B stars in today's APOD, however. Neither of them is a member of the Hyades, because they are simply too hot and blue and therefore too young for that. One is a small blue star about midway between the brightish blue star in the upper left corner and the orange star designated ε, or Epsilon Tauri. That small blue star is a modest star of spectral class B9, HU Tau or HD 29365. This star is blue not only because it belongs to spectral class B (if just barely), but also because it has a (small) negative color index, -0.019 ± 0.007. This makes it bluer than Sirius and Vega.
The other blue star is the brightish star in the upper left corner. It's a star of spectral class B3, so it's both brighter and bluer than its small B9 "cousin". The B-V index of the brighter star is -0.112 ± 0.003. I love the designation of this star: it's called Tau Tau!
As for Tau Canis Majoris, however, it is so much brighter and bluer than either HU Tau or Tau Tau. While Tau Tau is about as bright as 250 stars like the Sun, Tau Canis Majoris may shine 13,000 times as bright in visual light
as the Sun, and then it pumps out incredible amounts of invisible ultraviolet light, too.
But as I said about today's APOD, it makes the Hyades look festive indeed. The cluster does resemble a Christmas tree decorated with colorful baubles and a brilliant orange "top star" that for some reason has fallen from the top!