Free Astronomy Magazine July-August 2020

29 GALAXIES tions, narrows at each passage of the peri- center. Sagittarius has a diameter of about 10,000 light-years, a mass many times smaller than that of the Milky Way, is essen- tially composed of old and low-metal stars, and is almost gas-free. Its center is occu- pied by the globular cluster M54, the only easily observable Sagittarius component. Astronomers estimate that the first closed orbit of Sagittarius around the Milky Way began approximately 1 billion years ago and that, in the repeated and ever-closer pericenter passages, it has lost at least half of its mass to the Milky Way’s advantage. T he globular cluster M54 is located in the cen- ter of the Sagit- tarius galaxy. It is its most striking component. [ESA/ Hubble & NASA]