A twinkling field of stars clustering in the Hubble image
A sea of stars twinkles in this Hubble Space Telescope image. This view shows a massive star cluster called the Globular Cluster, located in the galaxy Messier 55.
A globular cluster is a group of stars made up of tens of thousands or even millions of stars held together by gravity. Because of this, these clusters tend to form spherical shapes as gravity holds the clusters together.
Difficult to observe from Earth, this cluster is faint and lacks a contiguous bright center. It is also in a region of the sky low above the horizon when viewed from the Northern Hemisphere, where interference from Earth’s atmosphere is strongest. But Hubble can observe individual stars in the cluster thanks to its position in space, where it sits above the water vapor in the atmosphere and therefore experiences less distortion.
This dazzling cluster of stars is just a small part of the galaxy Messier 55. Located 20,000 light-years away, the cluster is about 100 light-years across and makes up only a fraction of the entire galaxy. To give you a sense of the size of the entire galaxy, NASA shared this graphic showing the area covered by the Hubble image compared to an image of the entire galaxy taken as part of the Digital Sky Survey.
You can also see the wider view of the Messier 55 galaxy provided by the European Southern Observatory. This broader image was part of a project that took data from various astronomical surveys of the sky between 1983 and 2006 and created a digitized version. A more detailed picture of the galaxy has since been taken with the ground-based VISTA infrared telescope, showing the distribution of the stars.
Because the VISTA image was captured in the infrared and displayed in visible light, the stars are all a similar shade of yellow. But because Hubble looks at the optical wavelength, it can see different colors of starlight. The different colors correspond to the temperature of this star.