An overview of active galaxies
An overview of active galaxies

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

An overview of active galaxies

4.2 Activity 3: Stars orbiting our local supermassive black hole

Activity 3

Read through the following notes, and then watch the animation below.

Keywords: speckle imaging, enclosed mass

Stellar proper motions in the central 0.1 pc of the Galaxy

The animation shows the proper motions of stars near to the centre of our Galaxy over the course of about ten years between 1992 and 2002. The field of view seen here in this infra red image is about 3 arc seconds along each side, which corresponds to a linear scale of about 0.1 pc at the distance of the galactic centre.

The individual frames which comprise this movie are each the result of adding together several thousand images with individual exposure times of around 0.3s to 0.5s. With conventional, longer exposure times, the motion of pockets of air in the Earth's atmosphere will cause a variable refraction of the light rays from each point in the field of view, and so cause the images of point sources (stars) to become slightly blurred. In contrast, this so called speckle imaging technique can remove much of the effect of atmospheric distortions by using such short exposures that the motion of pockets of air is negligible within the exposure time. By carefully aligning all the thousands of short exposure time speckle images, the resulting point spread function, which characterises the extent of the image of a point source, is only 0.15 arc seconds wide in these frames instead of the more typical 1 arc second in the best conventional ground-based imaging.

The fastest moving stars seen here are those closest to the yellow cross which marks the position of Sgr A*, the presumed supermassive black hole at the centre of the Galaxy. Some of these stars have speeds in excess of 1500 km s−1. Taking account of all the stellar motions seen, the implication is that the enclosed mass within < 0.015 pc of Sgr A* is 2.45 × 106 M – and this probably takes the form of a single massive blackhole.

The journal paper which first reported these results is ‘Stellar proper motions in the central 0.1pc of the Galaxy’ by A. Eckart and R. Genzel which appeared in the Monthly Notices of the Royal Astronomical Society, volume 284, pages 576–598, in 1997. A copy of this paper [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]   is availableif you wish to read more details of their research.

The following short animation was made by Thomas Ott and Rainer Schoedel, copyright © Max-Plank-Institut für extraterrestriche Physik. Please note that some of the stellar motions indicated are still subject to analysis and should be regarded as of an illustrative nature.

View their research web pages at

Please note that this animation will loop, please press pause when you are ready to stop the animation.

Star motion clip1.1MB Video file (3GP)

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus