The Central Stellar Cluster

A small population of faint blue main-sequence B-stars with a few tens of solar masses, called the S-cluster after their identifying labels, surrounds the Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*) in the inner arcsecond of the Galactic Center. The ideal band for the observation of these stars is the near-infrared around 2.2 micrometers, where the extinction is less than 3 magnitudes. Fitting Keplerian orbits to the motion of these stars is the most powerful evidence for the existence of a black hole and allows the estimation of its mass and our distance to the Galactic center, namely 4.15 million solar masses and 8.2 kiloparsecs, respectively. The S-cluster was studied by Eckart & Genzel (1996) for the first time. Currently, the proper motion (on the sky plane) of 95 S-stars is known, and among them, 37 have complete orbital fits.

The observation of the S-stars provides the chance to study the physics near a supermassive black hole and conduct dynamical tests of General relativity. Relativistic corrections to the Keplerian orbits can show an advance in the periapse after each orbit. S2, a bright member of the cluster with a small periapse distance and a short orbital period, is the ideal source for measuring the periapse shift. We designed a new method to investigate the relativistic orbits of the stars in the gravitational field near the black hole. With this technique, and using images and spectroscopic data acquired with the NACO and SINFONI instruments on the ESO Very Large Telescope, we were able to show for the first time, the relativistic effects in the orbit of S2. The estimated variation in the argument of periapse of the S2 orbit was 14 arcminutes, which agrees well with the theoretical expectations. Later observations made with the high resolution interferometer GRAVITY allowed us, together with our large group of collaborators, to narrow down this value to 12 arcminutes and further measure the relativistics effects on the S2 velocity at the periapse passage.

Central arcseconds of the Milky Way observed in 2010 with NACO VLT. The K-band deconvolved image shows the S-star cluster members and all known orbits coloured in the same way as their corresponding stars.

Credits: Eckart group, Ph1-UniKoeln.

Selected publications:

• Gravity Collaboration, 2018: Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole
  http://adsabs.harvard.edu/abs/2018A%26A...615L..15G
• Parsa et al. 2017: Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center
  http://adsabs.harvard.edu/abs/2017ApJ...845...22P