Is this the Brightest Star in the Milky Way?


Figure 1
HST/NICMOS image of the Pistol Star & Pistol Nebula

The Pistol Star

The Pistol star is located at right ascension 17h43m4.8s and declination -2848'57" (B1950), and appears to be a member of the Quintuplet cluster (AFGL 2004, see image below). It is about 25,000 light years from Earth, and at a projected distance of about 100 light years from the center of the Galaxy. The image presented as figure 1 above, was taken by the NICMOS infrared camera, on the Hubble Space Telescope, September 17, 1997. The field of view of the image is about 4.8 light years across. Table 1 presents the most reliable list of basic data on the Pistol star that I can make, based mostly on the intensive study of that star, carried out by Donald Figer. The following text will explain in more detail. bright star page.

Table I: The Pistol Star
star
name
star
location
log Teff Mv Mbol log L/Lsun Mass
(solar)
log age
(Myr)
D (kpc) spectral
class
Reference
Pistol star (H) Quintuplet cluster 4.330.01 -10.4 -13.3 7.200.20 >85 6.28 7.7 LBV Figer et al., 1998
Pistol star (L) Quintuplet cluster 4.150.01 -8.9 -11.8 6.610.20 >85 6.28 7.7 LBV Figer et al., 1998

Is this the brightest star in the Milky Way?

Although the Space Telescope Institute press release only claims that the Pistol star is "one of the intrinsically brightest stars in our galaxy", it also says that it shines 10,000,000 times brighter than the sun. If true, then the Pistol star definitely is not just one of the brightest, but indeed the brightest star that we know of, in or out of the Milky Way. But Figer's more detailed study doesn't quite back up the enthusiastic press release.

So how bright is the Pistol star, really? The biggest problem trying to find out, is that there is a lot of dust between us & the Pistol, and there are several estimates as to how strong the extinction of the Pistol starlight really is. Another problem is that supermassive stars commonly sport prodigious stellar winds, much stronger than our own wimpy solar wind. That wind acts as a coolant in stellar atmosphere models. We know that the Pistol star has a strong stellar wind, but cannot measure it directly. Other complications include the true ratio of helium to hydrogen in the atmosphere, which is a problem for massive, luminous stars, because they are hot enough to invoke ionized helium absorption.

Table II: The Pistol Star Model Parameters
star
Model
star
log(L*/Lsun)
log Teff Radius
(Solar)
Wind Mass Wind Speed
H 7.20 4.33 300 2.8 95
L 6.61 4.15 340 3.8 95

That brings us to table II, which summarizes the more pertinent (in my opinion) features of the two model families, a shortened version of table 5 in Figer et al., 1998. The column "wind mass" is the mass loss rate in the model stellar wind, in units of 10-5 solar masses per year. The column "wind speed" is the terminal wind speed, in km/sec. The spectral energy density was measured using the Keck I 10 meter telescope, and the models are based on that spectral data. The H model features a photospheric temperature about 21,400 Kelvin, 300 solar radii, whereas the L model is a mere 14,000 Kelvins and 340 solar radii. By comparison, one astronomical unit (the average Earth-sun distance) is 215 solar radii, and the photospheric effective temperature of the sun is 5777 Kelvins.

Both of these model families reproduce the observed spectral energy density (as measured with the NIRC instrument on the Keck I 10-meter telescope), and so both have a shot at being a good example of reality. If the H model is what the Pistol star really looks like, then it is the winner & champion, the brightest star in the galaxy, anywhere from 10,000,000 to 25,000,000 times brighter than the sun. On the other hand, if the L model is what the Pistol star actually looks like, then its luminosity falls into the range 2,500,000 to 6,300,000 suns. That would be bright, but not necessarily the brightest.

Although the H models fit, the L models have to get the nod for being more likely. As Figer shows, if the initial mass of the Pistol star was between 150 and 250 solar masses, then stellar evolution models will reproduce the L family. But even initial masses as high as 300 solar masses will not reproduce the H models. Stellar evolution models are notoriously bad at recovering the masses of high end, very massive, very luminous stars, and that may explain the failure to reproduce the H models.

The pistol Nebula

The Pistol nebula, featured in figure 1, is almost certainly a creation of the Pistol star and its powerful stellar wind. The relative velocity of the star (130 km/sec) along our line of sight, matches that of the nebula. The nebula holds about 15 solar masses of material, but is much too massive to have been created by the current stellar wind, from either the H or L models. It may well be that the Pistol star, like other luminous blue variables (Eta Carinae, for instance) experiences episodic mass loss, so that the mass in the nebula comes from past episodes.

The Arches & Quintuplet Clusters


Figure 2: The Arches & Qunituplet Clusters

The Pistol Star is seen in this image of the Quintuplet cluster, the brightest star, just below the center of the image. The Quintuplet cluster is a massive, young cluster, about 25,000 light years away, and only about 100 light years from the region of the center of the galaxy. There is a lot more dust closer to the center of the galaxy, which is not evident on these images, since they are infrared images, and can see through dust that would block visible light. The Quintuplet cluster appears to be between 3,000,000 and 5,000,000 years old, and probably totals about 10,000 solar masses. The Arches cluster may be as massive as 40,000 solar masses, and has a half-mass radius of 0.35 pc. Both clusters emit copious UV radiation, and are responsible for ionizing a large volume of surrounding space. It is also evident that there are other stars in the Quintuplet cluster that rival the Pistol star for total luminosity, including more Wolf-Rayet class stars than are found in any other cluster in the Milky Way. The Arches cluster is brighter and more massive, but also much smaller. (Figer et al., 1999; Zwart et al., 2002).

The Other Quintuplet Star

Table II: The other QPM star
star
name
star
location
log Teff Mv Mbol log L/Lsun Mass
(solar)
log age
(Myr)
D (kpc) spectral
class
Reference
QPM-241 Quintuplet Cluster -9.0 -11.9 6.66 7.7 WN9/Ofpe Figer et al., 1999

The Pistol star is not the only bright star in the Qunituplet cluster. The study of Figer, McLean & Morris, 1999, tabulates data for 33 stars they observed in the Quintuplet cluster (their table 3). One other of those stars meet the minimum criterion I set for my own list, an absolute bolometric magnitude (or equivalent given luminosity) of -11.8, and that star are listed here. The "QPM" nomenclature comes from Figer, McLean & Morris. Its location is RA 17h43m4.7s, DEC -2848'30.2". Its spectral type WN9 indicates a nitrogen Wolf-Rayet star, the only Wolf-Rayet star on the candidates list for brightest. This is not surprising, since Wolf-Rayet stars tend to be so thoroughly reddened by their own circumstellar shells, as to be nearly invisible.

Figure 3:
The Heart
of the Quint

In figure 3 above, the Pistol star is conspicuously bright at the bottom center. Above the Pistol, a line of bright white stars points up and to the right, towards a small, tightly clustered bunch of white stars, upper right center. QPM-241 is one of those stars, but its image is blended with the others in the small cluster. Despite its small appearance, that small cluster has about 15 stars in it.

I know nothing else about the star. It is not discussed individually. In the data table, Figer, McLean & Morris cite Najarro et al., 1997 as a reference for the tabulated data on this star. Unfortunately, they don't use the same nomenclature, and don't mention which of the objects in Najarro et al. is supposed to be their QPM-241. A perusal of the data in Najarro et al. reveals no star with the same luminosity, so there's just no way for now, to know any more about it than this. Maybe the brightest star that we know of in the Milky Way languishes in obscurity?


References

Figer, D.F., et al.
The Pistol Star
Astrophysical Journal 506(1): 384-404, Part 1, October 10, 1998

Figer, D.F., McLean, I.S. & Morris, M.
Massive stars in the quintuplet cluster
Astrophysical Journal 514(1): 202-220, Part 1, March 20, 1999

Zwart, S.F.P., Makino, J., McMillan, S.L.W. & Hut, P.
The lives and deaths of star clusters near the Galactic center
Astrophysical Journal 565(1): 265-279, Part 1, January 20, 2002

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