S0-102

Scientists at the University of California recently announced the discovery of a star with the shortest known period orbiting around the black hole at the center of the Milky Way (Sagittarius A*) at only 11.5 years. The new star has been designated “S0-102”.

A diagram accompanying the announcement shows the orbit of both S0-102 and S0-2, the star with the previous shortest known period (as well as the orbit of various other stars buzzing around the black hole.)

To provide a sense of scale, I added a small sub-diagram showing the relative size of the orbits of Sedna and Pluto. Sedna has the largest known aphelion of any body orbiting the sun other than some long-period comets; still, this shows that the neighborhood of Sagittarius A* is comparable in scale to that of the Sun, though of course with far greater gravitational intensity.

Some basic trigonometry indicates that the .2 arc seconds shown on the diagram represents about 8.8 light days. Which is amazingly compact given the usual distances associated with stars. (Or course, that is over 140 billion miles, so it is only relatively compact.) Computing the relative sizes of Senda’s and Pluto’s orbits are equally straightforward, coming out to .1277 arc seconds and .0103 arc seconds respectively.

To get a sense of the enormity of the Milky Way’s black hole, consider that Sedna orbits the Sun in about 11,700 years. Sagittarius A* pulls S0-102 through its orbit in only 11.5 years. S0-102 reaches over 1% the speed of light at perihelion.