What is the Brightest Star in the Milky Way?

high resolution solar spectrum courtesy of Larry Webster and the UCLA 150-ft Solar Tower Telescope at Mt. Wilson.

Table I: The Brightest Stars
log Teff Mv Mbol log L/Lsun Mass
log age
D (kpc) spectral
Pistol Star (H) Quintuplet Cluster 4.33±0.01 -10.4 -13.3 7.20±0.20 >85 6.28 7.7 LBV Figer et al., 1998
Cyg OB2 #12 Cyg OB2 Association 4.270 -10.6 -12.2 6.78 92 6.43 1.7 B5 Ie Massey et al., 2001
HD 93129A Trumpler 14/16 4.705 -7.5 -12.1 6.74 >120 5.94 3.2 O3 If* Massey et al., 2001
Eta Carinae Carina Nebula
NGC 3372
4.477 -9.6 -12.1 6.74 >120 <6.48 2.3? LBV Hillier, et al., 2001
Davidson & Humphreys, 1997
QPM-241 Quintuplet Cluster -9.0 -11.9 6.66 7.7 WN9/Ofpe Figer et al., 1999
Pistol Star (L) Quintuplet Cluster 4.15±0.01 -8.9 -11.8 6.61±0.20 >85 6.28 7.7 LBV Figer et al., 1998
HDE 319718 Pismis 24 4.705 -7.3 -11.8 6.61 120 5.86 2.5 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

The Brightest Stars

In a personal quest to discover which star really was the most luminous in the Milky Way, I searched through the astronomical literature. The results of my search are summarized in table I, above. In many cases I had to derive some of the tabulated data from that presented in the primary reference, or perhaps consult other sources on a few points. The resulting table is the best I can do at constructing a list of all those stars that are viable candidates for the honor of most luminous star in the Milky Way. Every Milky War star that I know of, with an absolute bolometric magnitude of -11.8 or greater, is on this list. But there are several stars at least that luminous, in the LMC cluster R136, in the 30 Doradus region. I picked -11.8 arbitrarily. Any star with a dimmer absolute magnitude is not in the running for brightest star in the Milky Way, and there are quite a few of them. Even pushing the limit down to -11.0 would mean more typing than I feel like doing for now. Maybe later. Each star is linked to another page, with much more detailed info & images on that particular star. I didn't bother to write one for Eta Carinae, as there are already plenty of pages on that star. The others I wrote myself, when I discovered a dearth of information about them on the web. The page for Cyg OB2 #12 also includes a short discussion of the Cygnus OB2 association.

The Winner and Champion Star

Cyg OB2 #12, in the Cygnus OB2 Association, about 6,000,000 solar luminosities, is the brightest star known, both in visible light, and bolometrically. Its absolute visual magnitude of -10.6 is the brightest known. It's absolute bolometric brightness of -12.2 is also the brightest known, with one possible exception. The true brightness of the Pistol star is unknown, as there are two families of model that fit the star's observed photometry. If the high temperature model reflects reality, then the Pistol will dethrone Cyg OB2 #12. I don't think that's likely, as the mass implied for such a model is improbably high (over 300 solar masses). If the low temperature model reflects reality, then Cyg OB2 #12 is the winner, and brightest star known in the Milky Way.


Despite declaring Cyg OB2 #12 the winner, there is plenty of room for one of its close competitors to jump ahead. Most of the stars listed, some of which appear on no other lists but mine, have no experimental uncertainties attached to their brightness. That's because none were reported, and I couldn't derive one from what was presented. And, in the case of the Pistol star and HD93129A, there are two entries for each star, because the observers can't agree with each other. So, really, any of the stars listed here might turn out to be the real, brightest star known in the Milky Way. But I think that it is virtually certain that the brightest star that we know of so far, in the Milky Way, is on this list of stars.

New Champion?

Astronomers: Star may be biggest, brightest yet observed. This is a press release from the University of Florida (UF). According to the release, a team led by Steve Eikenberry, UF Professor of Astronomy, has identified the star LBV 1806-20 as perhaps the new champion, possibly as bright as 40,000,000 solar luminosities (the brightest Pistol Star model above is only about 25,000,000 solar luminosities). If so, LBV 1806-20 goes from obscurity to champ in one step. The release references a paper "recently submitted" to the Astrophysical Journal, but it is not on the pre-print servers as far as I can see. However, there is this: The Connection between W31, SGR 1806-20, & LBV 1806-20: Distance, Extinction, and Structure, by Corbel & Eikenberry, which locates the star and describes its environment. When the paper or pre-print comes available, I will add additional updates.

Bolometric magnitudes

Visual magnitudes refer to brightness in "visible light", much as your eye would see it. Our own sun emits most of its radiation as visible light, but stars that are significantly cooler or warmer do not. Red giants & supergiants emit most of their radiation in the invisibly long waves of the infrared (the eyeball light we see from Betelgeuse represents only about 13% of its total). Likewise, hot stars such as those listed here, emit most of their radiation in the invisibly short waves of ultraviolet light. You can see this difference illustrated in the difference between the absolute visual magnitude (Mv) and absolute bolometric magnitude (Mbol), as listed in the table I.

Absolute magnitudes are all scaled to the standard distance of 10 parsecs, as opposed to an apparent magnitude, which is whatever your eye actually sees. The word bolometric refers to brightness integrated (added up), over all wavelengths (or frequenceies), and derives from a Greek root word that means "beam of light". The bolometric magnitude is the brightness integrated over all wavelengths, while the visual magnitude is the brightness only over visible wavelengths. The difference is exaggerated by temperature. So, for instance, the absolute visual magnitude for HD 93129A is -7.5, but its absolute bolometric magnitude is -12.1, brighter by 4.6 magnitudes!


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