What sort of stars are at the Galactic Center?

25" X 25" H & K image of the Central Parsec Cluster. Credit: Gemini

High resolution near-infrared imaging has led to the discovery of three stellar superclusters at the Galactic Center. Since near-infrared wavelengths cut through the dense dust between Earth and the Galactic Center, we are able to see these superclusters. They include the Central Parsec, Quintuplet, and Arches clusters.

HST image of the Arches and Quintuplet clusters. Credit: NICMOS

The Arches cluster is a densely packed star cluster, which is surprising due to it's closeness to the supermassive black hole at the center of our Galaxy. Since black holes have a strong gravitational field, they affect the area around them with tidal forces. The Arches cluster's proximity should cause the cluster to be torn apart. The fact that it is still around means that it is probably a young cluster. Members of the Galactic Center Group at UCLA discovered circumstellar disks in the Arches Cluster (the presence of a circumstellar disk means that a star is really young, and it is in the process of forming), which supports the hypothesis that this cluster is very young (Stolte et al. 2010). The Quintuplet Cluster is named after the five red stars in the image to the left. These particular stars do not have any observable emission or absorption lines in their stellar spectra, which has kept their identity a mystery.

K-band mosaicked image of the Galactic Center taken at Keck Observatory.
This image also shows young stars at the Galactic Center.

In addition to the young stars found in the Arches cluster and the peculiar stars found in the Quintuplet cluster, there is a large number of late-type (not very massive and fairly cold, spectral type K and M) stars at the Galactic Center. These were discovered by looking for CO absorption bands in their stellar spectra. These are therefore cold stars, and they look red in the top Gemini image (red colors mean that the object is cool, while blue and white objects are hot). The brightest star in the Gemini image is known as IRS 7, and 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 viarable stars. The M and K sueprgiants were formed 3 - 7 million years ago, while the AGB stars are about 100 million years old. There is a lack of late-type stars in the central 10 arcseconds of the Galaxy, close to the supermassive black hole. Astronomers are trying to determine why this is the case, and where the young, early-type stars that exist there came from--if they were formed in the Galactic Center or if they were formed somewhere else and moved to the center of our Galaxy.

The young, massive, 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 in the mini-spiral. These stars have Helium (He I) and Hydrogen (H I) emission lines in their spectra, which are come from the fast winds flowing away from the stars. The most noticeable cluster of these stars are in the IRS 16 cluster, which are located close to Sgr A*. These stars are also 3 - 7 million years old.

The Galactic Center tends to host, on average, more massive stars than other clusters found in the local neighborhood. This is probably due to the extreme conditions found at the Galactic Center created by the supermassive black hole, the large magnetic field, and the UV radiation from early-type stars.