Hope it brings a smile to Ann!
The picture does make me smile, but it also makes me mystified.
The reason for my smile is, of course, the vivid and colorful beauty of the picture. I love the brilliant red Ha nebulosity and the blue OIII nebulosity on the right.
I love the many colorful stars, but the star colors are precisely the reason for my mystification.
Consider the red star at upper right. It is an M0 star whose color index is more than +2.0: check! Its orange-red color is exactly what you would expect from such a star!
Consider the brilliantly blue star near the center. It is classified as a B8V star, and it has a tiny parallax, 1.86 ± 0.75 milliarcseconds. Assuming you can trust the parallax - which is far from certain since the measured parallax is well within the margin of error - the distance to this star is about 1700 light-years. I must admit that if you combine the apparent magnitude of the star with the tiny parallax of 1.86 ± 0.75 milliarcseconds, you get a luminosity for the star which seems very reasonable for a B8V star, about 85 times that of the Sun. So the distance may be correct.
This blue star is also really blue in color. Not only is it intrinsically blue since it is of spectral class B, but its "Hipparcos B-V index" is in fact negative, -0.012. It is remarkable that a star so far away, sitting in the dust lane of the Milky Way, should be so extremely little reddened. Anyway, I love the blue star!
Altair. Photo: Chris Picking.
Okay, but now lets proceed to WR 134 itself, or HD 191765, as I prefer to call it. Its color in today's APOD is a sort of pinkish purple. But its B-V index is relatively low and relatively close to zero. Its "Hipparcos B-V index" is 0.219, and its "Johnson B-V index" is as low as 0.03. That is definitely as blue as an A-type star. Consider Altair, for example, the alpha star of the constellation Aquila: This A7IV star has a luminosity of 11 stars like the Sun and a "Johnson B-V index" of +0.22. Its Hipparcos B-V index is +0.23. So WR 134 is bluer
than Altair - any way you look at it, it is bluer - but it looks redder. Hmmm.
Could the pinkish-purple color of WR 134 in today's APOD be due to Ha emission? But you'd think that a Wolf-Rayet star would have lost most of its outer hydrogen, so that the Ha emission would not come from the star itself. And my impression is that Wolf-Rayet stars tend to look bluish. Consider bipolar nebula NGC 6164 surrounding WR star HD 148937
. The apparent color of HD 148937 is redder (Johnson +0.32, Hipparcos + 0.30) than the apparent color of WR 134. Still WR 134 looks purple in today's APOD, while HD 148937 looks blue in most pictures.
I'd love to hear a comment from Don Goldman himself and hear him comment on the purple color of WR 134 in his picture.
But like I said, I agree with starsurfer that today's APOD is very beautiful!