Galactic Center Stars

In this exercise, you will be presented with background information on the topic of interest, including lecture notes. After reviewing that material, you should download and print out the PDF of the problem. Finally with the problem in front of you, open up the image tool and use it to analyse the images in order to answer the questions on the PDF. Write on the printout of the problem and bring it to class.




Galactic Center Stars

Over the last decades, it has become a standard paradigm that massive black holes are located at the centers of most, if not all, galaxies. They are believed to be the central engines powering the enormous energy ouput and luminosity of active galactic nuclei and of quasars, the most distant and most luminous objects observed in our universe. Observations of other galactic nuclei have already provided strong evidence that these massive black holes exist. However, this evidence has always been indirect and did not exclude alternative explanations. By showing in a direct and straightforward way that such a massive black hole does indeed exist in the heart of our own galaxy, the Milky Way, we provide a firm and compelling fundament for this key hypothesis of modern astronomy.

Since the early 1990's two research groups have observed the central parsec of the galactic center in Near Infrared (K-Band). Observations by a research group at Max-Planck-Institut für extraterrestrische Physik (MPE) in Germany and by a research group at UCLA have measured the velocities and accelerations of stars in the Galactic Center over the course of the last decade have already provided strong evidence that Sgr A* (the radio point source at the center of our galaxy) is in fact a black hole of about 2.6 million solar masses.

Proper Motion Observations

Galactic Center Stars K-band

A research group at the Max-Planck-Institut für extraterrestrische Physik (MPE) in Germany uses a speckle imaging camera developed at MPE (called SHARP) at the European Southern Observatory's (ESO) NTT telescope in La Silla Chile. Speckle imaging is a special technique in order to compensate the atmospheric turbulence and to obtain sharp images at the theoretical diffraction limit of a telescope despite of the degrading influence of the Earth's atmosphere. Speckle imaging involves taking hundreds or thousands of short exposure images of an astronomical object and the later reconstruction of a sharp image with the aid of a computer.

Another way of compensating atmospheric turbulence is the technique of adaptive optics. It is more costly and elaborate than speckle imaging, but provides usually significantly better results. At the end of 2001/beginning of 2002 the near-infrared adpative optics system/infrared camera NAOS/CONICA was installed on one of ESO's four 8.2 meter VLT telescopes. The combination of these two new powerful instruments provided us with the deepest images that were taken of the Galactic Center with an 8 meter-class telescope up to date (see image above).

Movie of Stellar Motions in the Galactic Center

Movie of Stellar Motions in the Galactic Center

In an article in 17 Oct 2002 issue of Nature the MPE group reports on having observed 2/3 of a complete orbit of the star currently closest to the enigmatic radio source Sagittarius A*, which is thought to mark the location of our Milky Way's massive central black hole. As explained below, this orbit provides overwhelming evidence that Sgr A* is indeed a supermassive black hole of more than 2 million solar masses. See this star (called S2) swing around the black hole in this movie.

PDF of Assignment

Print out the following PDF file, print out and answer using the interactive tool below.

PDF of Assignment

Interactive Measurement Tool

Clicking on the link below will bring up a tool where you can estimate the mass of the black hole by measuring the orbits of one of the fast-moving stars near the black hole.

Open Interactive Measurement Tool