Measuring the Movement of Globular Star Clusters Visualization
[Left] — This is a Hubble Space Telescope image of a portion of the globular star cluster NGC 5466. It is among 150 compact islands of stars that orbit our Milky Way galaxy like bees buzzing around a hive. The orbital speed of the cluster can be used to estimate the total mass of our galaxy. The more massive the Milky Way, the faster the cluster is moving under the pull of gravity.
[Right] — To clock the cluster's velocity, Hubble images taken ten years apart were compared. Hubble's view is so sharp it can be used to measure the motion of the cluster's stars by their offset positions between observing epochs. A grid in the background helps to illustrate the stellar motion in the foreground cluster (located 52,000 light-years away). Notice that background galaxies (top right of center, bottom left of center) do not appear to move because they are so much farther away, many millions of light-years. The precise measurements, when combined with other data, show that the Milky Way weighs in at about 1.5 trillion solar masses. Only a few percent of this comes from its population of approximately 200 billion stars. Most of the rest is locked away in an invisible mass called dark matter.
Publication: March 7, 2019