Stellar Motion and Black Holes

Written by: RC Davison • Edited by: RC Davison
Updated May 19, 2011 • Related Guides: Milky Way | Night Sky

Do stars really move? We look at the night sky and the stars appear fixed in the heavens. But they do move and you can see them move if you know where and how to look. But the most exciting thing about this is that the driving force behind this motion is a black hole at the center of our galaxy!

Stellar Motion

If you watch the night sky for a lifetime you will notice the planets wander among the stars, along with the occasional comet now and then and the Moon going through its monthly cycle; you might even be lucky to see a nova or supernova, but you will never see the stars move. They are moving, rest assured, but even with the high velocities they are traveling at, the tremendous distance that separates us makes the distance (their proper motion) they would move in a lifetime imperceptible—especially to the naked eye. But, there is a place where moving stars move fast enough so that you would notice their relative motion in a few years and that is at the galactic center of the Milky Way.

Below, in the infrared image from the Two Micron All Sky Survey, a more encompassing view of the Milky Way Galaxy can be seen, emphasizing the galactic core.

How Do We See the Motion of the Stars?

The galactic center contains a high concentration of stars and a massive black hole. It also is shrouded in dust that obscures our visual observation of the individual stars. We know there is a black hole there by the intense X-ray and radio emissions that are observable in the constellation Sagittarius, which frames our view of the center of the Milky Way galaxy. The object that is specifically associated with the black hole is Sagittarius A*. To pierce the veil of dust that hides the stars at the center we need to look beyond the visual into other parts of the electromagnetic spectrum—the infrared region.

Looking at the longer wavelengths in the infrared region of the spectrum gives us a view of the heat signature of the stars, which passes through the dust while the visual wavelengths are absorbed by the dust. Looking in the right part of the spectrum is not all you need. You will need an optical system that can resolve, or separately image the individual stars at the center and that takes some special technology, namely adaptive optics and speckle imaging.

Both of these tools are used to compensate for the turbulence we have in our atmosphere due to temperature, pressure and moisture changes. The adaptive optics system adjusts the surface of the mirror of the telescope many times a second after measuring the atmospheric distortion and calculating the best mirror shape to compensate for it. The speckle imaging technique takes many short exposure images, on the order of hundreds to thousands of pictures and then uses a computer and a mathematical algorithm to combine the images to provide a much sharper image.

Utilizing these two advances in technology a team of astronomers lead by Dr. Reinhard Genzel at the Max Planck Institute was able to image the stars at the galactic center over a period of six years, from 1994 to 2000. They produced an amazing movie that shows the stars moving in different orbits about a central mass. The gallery below contains three images taken over this time span that clearly show the motion of the stars. Look closely at the stars in the center of the images and you will see that the pattern changes from frame to frame.

Images Showing Stellar Motion at the Galactic Center

Movies of the Stars in Motion

To actually see these stars at the galactic center in motion please view this movie, which covers a period from 1992 to 2002 and shows very clearly the stars moving about a central point marked by the yellow cross—the suspected black hole. (Even more detail can be seen in this movie showing a zoomed-in view of the activity.)

After analysis of the images they were able to produce this simulation the orbits of these stars about an unseen mass. In the figure below you can see a detailed image of the orbit of the star - S2, which moves around Sagittarius A*.

Our Universe is teeming with activity—from the occasional supernova to the more mundane comet streaking through our Solar System to the mysterious dance of stars about a massive black hole at the center of the Milky Way galaxy. We live in a very dynamic cosmos only limited by what we can perceive with our senses, technology and imagination.

For more information on this fascinating place in our galaxy visit the Galactic Center Research site at the Max Planck Institute and be sure to view the slide show that is available and gives more information about this topic.

12/11/2008 Update: The ESO, the European Organisation for Astronomical Research in the Southern Hemisphere, has just released a wonderful video summarizing 16 years of observations of moving stars at the galactic center. There are some nice 3D graphics to illustrate the orbits of the stars about the massive black hole that hides at the core of the Milky Way.

References and Credits

Galactic Center Research: http://www.mpe.mpg.de/ir/GC/index.php?lang=en

Sagittarius: http://www.spitzer.caltech.edu/images/1540-ssc2006-02a-A-Cauldron-of-Stars-at-the-Galaxy-s-Center

Two Micron All Sky Survey: Atlas Image obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. http://www.ipac.caltech.edu/2mass/gallery/showcase/allsky_stars/index.html

Images of stellar motion: Courtesy of the Galactic Center Research site at the Max Planck Institute - http://www.mpe.mpg.de/ir/GC/res_mass.php?lang=en

ESO stars at the galactic center: http://www.eso.org/public/news/eso0846/

Orbit of Star S2: Courtesy of the Galactic Center Research site at the Max Planck Institute - http://www.mpe.mpg.de/ir/GC/res_s2orbit.php?lang=en

RC Davison Apr 3, 2010 3:36 PM

Black Holes

Sam,
It's a pleasure to help someone learn about, and understand the wonderful Universe that we live in.
Remember that the black holes that are in the center of the galaxy are part of the galaxy, gravitationally bound together with the stars at the core. Even though they are extremely massive, the black holes are a small part of the overall collection of matter that makes up the galaxy and is bound together by gravity.

Sam Apr 3, 2010 2:07 PM

Sir,
Thanks for your patience. It seems that all galaxies are accelerating; this implies that the black holes suspected to be at the centre of the galaxies must also be accelerating. Isn't it strange that objects, as powerful as black holes, are also accelerating away along with their neighbouring galaxies? One would have expected black holes to remain stationary in the Universe while various galaxies passed them by.

RC Davison Apr 3, 2010 11:21 AM

Black Holes and Acceleration

Hi Sam,
To address your questions:
1) The time it takes an object to orbit about a common center in a gravitationally bound system depends on how far it is from this center. Mercury orbits the Sun in 88 days, the Earth in 365 days, and Jupiter takes 4331 days to orbit the sun. The stars that are shown in the videos linked in the article are relatively close to the black hole controlling their motion. The video covers a period of about 10 years. Remember that this article is discussing this special case of these stars moving about the black hole at the center of our galaxy.
2) One must be careful with all the theories, ideas and facts being presented today. I believe that you are referring to the motion of galaxies, not stars, when you are referring to "stellar bodies accelerating away from each other". Aside from the galaxies in clusters, and gravitationally bound groups, like the one that attracts the Milky Way to Andromeda and other galaxies in our local group, galaxies are accelerating away from each other as the Universe expands. The farther away the galaxies are the faster they are moving.
3) I'm not sure what you mean about the black holes being propelled at immense speeds. There are high speed jets of gas that are ejected by black holes as they draw in matter from the surrounding space.
4) If the motion of the Earth is constant, then it is easy to subtract that motion from observations of the motion of other objects to determine their true motion. This link (http://cseligman.com/text/stars/propermotion.htm) may be useful in understanding how the motion of celestial objects is determined. We can use very distant objects to determine the relative motion of the Earth and use that to compensate the measurements of the motion of stars we are studying.

RC Davison

Sam Apr 3, 2010 3:07 AM

Black Holes and Acceleration.

Sir,
1) The orbit of any star around a central point must be taking millions of years to complete; how can astronomers with records of around 200 years claim that a star is orbiting any point?
2) The theory that stars orbit a central point is at odds with the observation that all stellar bodies are accelerating away from each other; one of these facts must be false.
3) If black holes are as powerful as claimed, then, what is the force which is propelling the black holes at immense speeds ?
4) Since all observations of the stars are made from earth which is claimed to be in constant motion, can it be proved that the motion of the stars is independent of our motion? How?

RC Davison Apr 2, 2010 7:27 PM

Movement may be an illusion

Sam,
You raise some interesting points about one's reference to make measurements of stellar motion, or any motion for that matter. It's all relative! Einstein's early thought experiments were all about relative motion in different reference frames and from those thought experiments the Special and General Theories of Relativity were born.
In the case presented in the article, the stars are moving about the unseen black hole. One can easily observe the stars moving relative to each other, as well as orbiting around a common point. This motion is independent of our motion, as these stars are relatively close together and bound in the gravitational well of the black hole.
Astronomers know how fast the Earth is moving about the Sun, how fast the Sun is moving around the galaxy and the relative speed of the Milky Way compared to other galaxies, so with this information they can compensate their measurements of the position and speed of other celestial objects.

RC Davison

Sam Apr 2, 2010 3:45 PM

Movement may be an illusion.

Movement may be actual or an illusion. A star may appear to be receding away from us but the truth may be that it is stationary and it is we who are hurtling away from it. Movement is always relative to a stationary point. Is there any stationary point in the universe which we can use as a reference point to determine movement of ourselves? How can we comment about other stars when the reality of its movement can only be known if we are present in its system?