Roger V. wrote:This picture is particularly beautiful.
One of the interesting things about the Heart Nebula is the bright star to the upper right of the center of this image. The one I am referring to is the upper one of the two brightest stars in the image. It is HD 15558, and it is a double star, a spectroscopic binary with an orbital period of 442 days, and masses estimated at 152 and 46 solar masses. (It looks like a visual double star in this image, but that is not the companion star I speak of. As a spectroscopic binary, the two stars are much too close together to resolve in any telescope.) Wikipedia has an informative article about this star, and it's interesting that so much has been learned about it.
At 152 solar masses, the primary is considered to be approximately the 8th most massive star known, and the most massive one visible to to those of us who cannot see the Large Megallanic Cloud in our northern skies. It's of spectral type O4.5, while its 46-solar mass companion is of type O7.
The pair has a visual magnitude of 8, easily seen in binoculars.
-- Roger V.
Thanks for calling my attention to this star, Roger V.! I have never taken much of an interest in IC 1805 (the nebula) or Melotte 15 (the stars), because of, obviously, the colors. The reddened stars, the flat red hue of the nebula, or the Hubble palette version of it - plus the scattered, almost sparse appearance of the cluster...
But the suggested mass of HD 15558 is just amazing!!
According to this list
, HD 15558
A is indeed the 8th most massive star known anywhere! Not only that, but the seven stars ahead of it on the list are all in the large Magellanic Cloud, in or near the titanic cluster of supermassive stars, R136! And the next Milky Way star on the list is in 12th place! So according to this list, HD 15558 A would be the most massive star
that we know of in the entire Milky Way!!!!
Sorry for all the exclamation marks, but I'm just stunned.
I'm just a tad doubtful, too. All the seven stars that are listed as more massive than HD 15558 A are all more than 3 million times more luminous than the Sun. HD 15558 A, by contrast, is "only" ~660,000 L☉
. That's a huge difference in luminosity. Admittedly six of the seven stars ahead of HD 15558 A are Wolf-Rayet stars, which are particularly luminous even for their mass, and HD 15558 A is an O-type giant. But there is another O-type star ahead of HD 15558 A on the list, Melnick 42
, whose luminosity is 3,600,000 solar. Of course, Melnick 42 is an "extreme" O-type star of spectral class O2If, with a surface temperature of 47,300 K and a radius of 21.1 solar, whereas HD 15558 A is "only" 39,500 K and 16.4 R☉
. So all in all, I can't help thinking that HD 15558 A appears somewhat faint, cool and small for the tremendous mass it is believed to contain.
The primary may itself be a double star, suggested by the improbably large mass found from the binary orbit when compared to the other stellar parameters.
That's what I thought, too.
We may compare HD 15558 A with NGC 3603-B
, the only other Milky Way star among the top twelve. NGC 3603-B is a Wolf-Rayet star of spectral class WN6h. At 2,900,000 L☉
it is more than four times more luminous than HD 15558 A, and at 42,000 K and 33.8 R☉
it is both hotter and (very much) larger. It gives every appearance of being a much more powerful star than HD 15558 A.
So I'm doubtful that HD 15558 A is as massive as >152 ± 51 M☉
. But clearly it is a fantastic star in any case, and I must really thank you, Roger V., for calling my attention to it!