I note that pictures of the Milky Way "interacting" with Earthly sceneries often become APODs. Maybe a little too often for my tastes.
But I like today's APOD. As usual, I'm fascinated by the individual stars. Fortunately, the stars are bright and colorful and are seen just in front of the Milky Way, which makes them easy to identify.
There are two prominent orange stars in the picture, one at upper left, one at upper center-right. The one at upper left is noticeably more orange than the one at upper center-right. That star, the paler one, is Avior, Epsilon Carina. It is a fascinatingly bright star, likely more than 5,000 times as bright as the Sun. Orange stars as bright as that tend to be very
orange, but the B-V index of Avior is only +1.19. That's not a lot for a bright orange giant. Compare the B-V index of Avior with the B-V index of, say, Arcturus: +1.23. Okay, not such a big difference. But what about the difference in brightness? The luminosity of Arcturus is only about 110 times the Sun, or 110 L☉
. (Okay, Wikipedia says 170 L☉
, big deal.)
But Avior is some 5,000 times as bright as the Sun, or at least 20 times brighter than Arcturus!
Both Avior and Arcturus are classified as "giants". Arcturus is classified as a K2III star (okay, K2IIIp), while Avior is classified as a K3III+B2V star. The classifications are similar, and the presence of the blue B-star in Avior explains the overall "orange paleness" of this binary star. But how can we explain the difference in brightness between Avior and Arcturus? Avior is certainly at least 20 times brighter than Arcturus, and in my experience, when I have used my software Guide to check the Hipparcos parallaxes and the intrinsic luminosities that can be derived from them for various stars, stars of spectral class B2V are almost never optically brighter than 1,000 L☉
, and indeed they are hardly ever even that bright. Since Avior is classified as a binary star with components of spectral classes K3III and B2V, we are still left with an evolved star of spectral class K3 that is classified as a "giant", not a "bright giant", and still its optical luminosity must be at least
Take it from me: You can't look at a star's spectral classification and guess how bright it will be.
So what about the other orange star, the one at upper left? That's Lambda Velorum, and its B-V index is indeed redder than the B-V index of Avior, +1.66 versus +1.19. Lambda Velorum is also classified as either a "bright giant" or a "lesser supergiant", K4Ib-II. The "K4" means that the star is cooler and in all probability larger than the evolved component of Avior, whose classification is K3III.
But guess what, Lambda Velorum is less bright than Avior in optical light: some 3,000 L☉
versus at least 4,000 L☉
In my opinion, there is nothing wrong with either the classification or the optical brightness of Lambda Velorum. Being such a cool star, most of its total energy must be emitted in the infrared part of the spectrum. Jim Kaler
calculated the bolometric (total) luminosity of Lambda Velorum (also known as Suhail) at 11,000 L☉
. (Actually though, Jim Kaler pointed out, probably very correctly, that Lambda Velorum should be called a bright giant rather than a supergiant.)
It's interesting to read what Jim Kaler wrote about Avior:
If there is a stellar category of "bright stars getting no respect," Avior probably holds the record.
Yes, because no one seems to know much about Avior, and no one seems to care much! But the star is remarkably bright!
It's not like me to talk so much about two orange stars, is it? Ah, but there is one strikingly blue star in the picture, too. And guess what star that is? It is Gamma Velorum, the star I talked about in the recent APOD featuring the Vela supernova remnant.
In my comment there, I said that Gamma Velorum is not sitting in a reflection nebula, even if the APOD in question suggested that it does.
But now take a look at the appearance of Gamma Velorum in this APOD (at top). Surely there is a large blue halo surrounding Gamma Velorum! The star is "burning blue"
And just because I talked about the spectral classes, B-V indexes and optical luminosities of Avior and Lambda Velorum, let's look at these things for Gamma Velorum, too. Spectral class: WC8+O9I. (One hot but shrunken Wolf Rayet component and one supergiant of spectral class O9.) B-V index: -0.145. (Not as blue as I'd like, but I would guess that the star is self-reddened due to the dust that is likely produced when the winds of the two components collide.) Optical luminosity: Almost 20,000 L☉
. Bolometric (total) luminosity
: Perhaps 280,000 L☉
The Gum Nbeula. Photo: P. Horálek/ESO (cropped by Szczureq)
Oh, and by the way, that faint reddish nebula seen below and to the left of Gamma Velorum is the Gum Nebula. You can see it better in the picture at left. It is thought to be a supernova remnant.
And the bright white star at right in the picture is of course Canopus, the second brightest star in the sky after Sirius.