Is this the Brightest Star in the Milky Way?


Figure 1:
First Generation Palomar Sky Survey field
Trumpler 16 & Eta Carinae (lower left)
Trumpler 14 & HD93129AB (upper right)
See text below for details and source.

The image in figure 1 is a 15 x 15 arcminute field from the original Palomar Sky Survey, and I got it from the MAST Digitized Sky Survey, hosted by the Space Telescope Science Institute. The small cluster that contains HD 93129A is in the upper right, and is designated Trumpler 14. The nebulosity in the center and lower left is the Carina Nebula, NGC 3372. The conspicuously bright star in the lower right is the luminous blue variable & supermassive star Eta Carinae, another of the candidates for brightest star in the Galaxy. The cluster that surrounds Eta Carinae is Trumpler 16 (it is now common to see them combined as trumpler 14/16, as astronomers now believe that they are two halves of the same open cluster, separated visually by a dense dust lane in the foreground).


Table I: HD 93129A data as reported in two main references
star
name		 star
location		 log Teff Mv Mbol log L/Lsun Mass
(solar)		 log age
(Myr)		 D (kpc) spectral
class		 Reference
HD 93129AB Trumpler 14/16 4.705 -7.5 -12.1 6.74 >120 5.94 3.2 O3 If* Massey et al., 2001
HD 93129A Trumpler 14/16 4.716±0.009 6.4±0.1 120 O3 If* Taresch et al., 1997

Is HD 93129A the brightest star in the Milky Way?

The star HD 93129A is located at right ascension 10h43m57.40s and declination -59°32'51.0" (J2000), and is usually classified as spectral type O3 If*, though the ORFEUS EUV spectrometer log says O3 Iab. Both Massey et al. and Taresch et al. agree that it is a very massive star, weighing in about 120 solar masses. Massey et al. give it a higher bolometric brightness, about 5,500,000 solar luminosities. But theirs was a survey in the optical. Taresch et al. gave HD 93129A personal attention in their paper, folding in UV, far UV and optical data. They give it a mere 2,500,000 solar luminosities. Massey et al. report the absolute bolometric magnitude of -12.1, and the luminosity given by Taresch et al. translates into -11.3. If the latter is correct, HD 93129A falls out of the running for brightest honors, though it remains exceptionally bright. On the other hand, if the Massey et al. brightness is correct, then HD 93129A remains in the running for brightest honors, tied with Eta Carinae and a mere 0.1 magnitudes behind Cyg OB2-304.

HD 93129A might be the brightest, but the detailed study of Taresch et al. seems likely to be the more reliable, and put HD 93129A out of the running.

How far away is HD 93129A?

The distance that I show from Massey et al. is what I get from their reported true distance modulus of 12.5 for Trumpler 14/16, or about 3.2 kpc. This agrees, for instance, with the 10,000 lightyear (3.1 kpc) distance to Eta Carinae that shows up on the seds Eta Carinae page. The Hipparcos Space Astrometry Mission does not include either Eta Carinae (HD 93308) or HD 93129A in its catalog, but it does include HD 93250, which is in the Trumpler 14/16 cluster (Massey et al.). Hipparcos reports a very uncertain parallax of 0.26 milli arcseconds for HD 93250, which translates into 3.4 kpc. Based on these sources, it would seem that HD 93129A & Eta Carinae are about 3.1-3.4 kpc (10,000-11,000 ly) distant.

However, Humphreys & Davidson, 1997, give the distance as about 2.5 kpc (8100 ly) to the Car OB1 association, the broad stellar group that includes Trumpler 14/16, and Collinder 228. They cite several other observational projects, all of which derive distance between 2.1 and 2.5 kpc (6800-8100 ly), and finally recommend 2.3 kpc (7500 ly) as the recommended distance. Hillier et al. adopt that recommendation, and base their analysis on an assumed distance of 2.3 kpc. Furthermore, they report that it differs little from a 1993 derivation by Hillier & Allen of 2.2±0.2 kpc (7100±650 ly).

So there appears to exist a dichotomy of distances in the literature. Is the Trumpler 14/16 cluster 10,000 or 7,000 ly away? It makes a difference, as the distance is an important factor in determining the extinction & reddening of the source, and hence the true brightness of the source. The greater the distance & extinction, the more luminous the source must be, to look as bright as it looks to us. If the shorter distances are real, then perhaps Eta Carinae & HD 93129A are both intrinsically dimmer than we think.

I do not know how to work around the issue, until a better determination of the distance is made.

References

Tim Thompson's homepage
Tim's bright star page.