This fun little video came across my computer screen not long ago:

It comes from the European Southern Observatory and contains 20 years of observations of the galactic center of the Milky Way.  Over this period, about 20 stars have been observed to travel in small orbits, moving quite quickly at some points in their orbits.

Near the center of the screen, one star makes a complete orbit over the duration of the clip and it is very clear when the star is farthest and closest from the focus of its elliptical orbit. Even though the focus is a black hole and not another star, let’s call the furthest distance apastron, and the closest, periastron.

The star moves quite quickly during periastron and just like comets in our own Solar System, which spend most of their time far from the Sun where they travel more slowly, this star spends most of its timer far from the black hole.

Notice that nothing is visible at the focus of this orbital ellipse.  Yet, there must be a great deal of mass there, to pull a star around in an orbit with a period of only 20 years.

Students can use these data to calculate the mass of the black hole at the center of the Milky Way Galaxy.  We show them how in Learning Astronomy by Doing Astronomy; Activity Number 29.  This video makes a great supplement to that activity!