Star Dynamics

Using high resolution near-infrared imaging three stellar ``superclusters'' have been discovered at the Galactic Center: the Central Parsec, Quintuplet and Arches clusters.

25"X25" HK image of the Central Parsec cluster taken with the Gemini telescope

HST/NICMOS image of the Arches and Quintuplet Clusters

Above is an HST image of the Arches and Quintuplet clusters. The Arches is a surprisingly densely packed cluster considering its proximity to a supermassive black hole which should cause the cluster to be torn apart. This suggests it is a fairly young cluster. The Quintuplet Cluster is named after the five red stars in this image. These particular stars do not have observable emission or absorption lines in their stellar spectra which has kept their identity a mystery.

The late-type (K and M spectral class) stars at the Galactic Center are identified through the CO absorption bands seen in their stellar spectra. They are cool stars which have red colors as seen in the Gemini image. The brightest star in this image (on the lower right corner) is IRS 7. It is an M supergiant. Most of the late type stars are giants, supergiants or more evolved giant stars called AGB, OH-IR and Mira variable stars. The M and K supergiants were formed 3-7 million years ago while the AGB stars are 100 million years old. There is a noticeable absence of late-type stars in the central 10 arcseconds.

The early-type (O, A and B spectral class) stars are the brightest stars in the Galactic Center and contribute most of the ultraviolet radiation that ionizes the surrounding gas seen along the mini-spiral. These stars are characterized by the Helium (HeI) and Hydrogen (HI) emission lines seen in their spectra. These lines are created within the high winds flowing away from the stars. The most noticeable cluster of HeI/HI stars in the Galactic Center are all in the IRS 16 cluster (also in the lower right corner of the Gemini image). These stars formed along with the super giants and are also 3-7 million years old.

There are different classes of stars observed at the Galactic Center. One defining distinction among these stars is their tendency to be, on average, more massive than stars found in the local neighborhood. This is probably due to the extreme conditions found at the Galactic Center created by the supermassive black hole and large magnetic field.

One particular star at the Galactic Center, the Pistol Star, gained popularity with the media in the fall of 1997 as being the most massive star ever found, weighing in at 200-250 times the mass of the Sun! Such a massive star is a rare find, and has raised suspicions that the Pistol star, upon more detailed scrutiny, might prove to be a binary system. If it is indeed a single star, then it presents a challenge to current theories of stellar evolution - when massive stars, such as the Pistol Star, reach old age after only a few million years, high velocity stellar winds eject the outermost layers of the star's atmosphere. Thus, according to current theory, the single Pistol Star should not have evolved to such a massive size. The stellar winds associated with the Pistol Star as well as the winds of about 15 other hot, massive stars at the Galactic Center have recently been detected at radio wavelengths with the VLA.

HST/NICMOS image of the Pistol Nebula and Star

The Pistol star gets its name from its close proximity to the Pistol Nebula, an unusually shaped cloud of hot gas thought to have been, at one time, an outer atmospheric layer of the Pistol Star. The figure above depicts the filamentary nebula of ionized hydrogen gas that surrounds the supergiant Pistol Star. The hydrogen is believe to be ionized by the high energy ultra-violet radiation from the Pistol Star.