File:Caldwell 89 (50328205511).jpg

This shining collection of stars captured in infrared light by Hubble’s Wide Field Camera 3 is part of Caldwell 89, also known as NGC 6087. This open cluster consists of approximately 40 stars, the brightest of which (near the center here) is the variable star S Normae.

Cepheid variable stars like S Normae change brightness at a regular and predictable rate. There is also a relationship between a Cepheid’s pulsation rate and its intrinsic brightness. A Cepheid’s pulsation rate tells us how bright it truly is, so how bright it appears to us tells us how far away it is. (The farther a Cepheid is, the dimmer it appears.) This makes Cepheids important “standard candles” for measuring distances in the universe. Hubble’s observations of Caldwell 89 were made to improve the calibration of S Normae as a standard candle.

Caldwell 89 has an apparent magnitude of 5.4 and is located 3,500 light-years away in the constellation Norma. S Normae fluctuates between apparent magnitudes of 6.12 to 6.77 over an approximately 10-day period. The cluster can be seen in binoculars, and some of its stars can be resolved with a small telescope. Caldwell 89 is best observed in the Southern Hemisphere’s winter (or in summer for Northern Hemisphere observers close to the equator). It is not far from Caldwell 95 (NGC 6025), another open star cluster that’s about the same size and brightness.

Credit: NASA, ESA, and A. Riess (Johns Hopkins University); Processing: Gladys Kober (NASA/Catholic University of America)