Anne’s Picture of the Day: Globular Cluster Messier 68

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February 5, 2012

Messier 68, a globular cluster in Hydra

Image Credit: ESA/Hubble & NASA

Messier 68 (also known as NGC 4590) is a globular cluster of some 106 light-years across and a mass of 223,000 solar masses, located about 33,600 light-years from Earth in the constellation of Hydra (The Female Water Snake), while it is approaching us at 112 – 116 kilometers per second.

More than 150 of these objects surround our Milky Way galaxy. Remarkably, Messier 68 is located almost opposite the center of our galaxy in the night sky. Most globular clusters are concentrated in the area around this center.

Mutual gravitational attraction among the cluster’s hundreds of thousands of stars keeps stellar members in check, allowing Messier 68 to hang together for many billions of years.

This spherical, star-filled region of space contains mainly very old stars. The age of the cluster is estimated at around 11.2 billion years, meaning that it began forming just 2.5 billion years after the Big Bang.

Astronomers can measure the ages of globular clusters by looking at the light of their constituent stars. The chemical elements leave signatures in this light, and the starlight reveals that stars of globular clusters typically contain fewer heavy elements, such as carbon, oxygen and iron, than stars like the Sun. Since successive generations of stars gradually create these elements through nuclear fusion, stars having fewer of them are relics of earlier epochs in the Universe. Indeed, the stars in globular clusters rank among the oldest on record, dating back more than 10 billion years.

Messier 68 has at least 42 known variable stars of which 27 RR Lyrae stars. RR Lyrae variables are periodic variable stars. They are pulsating horizontal branch stars (stars in a stage of stellar evolution that immediately follows the red giant phase), with a mass of around half the Sun’s. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.

RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but in contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor stars. They are much more common than Cepheids, but also much less luminous. Their period is shorter, typically less than one day, sometimes ranging down to seven hours.

The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good standard candles for relatively near objects, especially within the Milky Way. This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra.

For amateur observers in the northern hemisphere, this globular difficult to observe because of its southern declination. In more southern regions however, M68 is good observable.

The image was taken by Hubble’s Wide Field Camera of the Advanced Camera for Surveys and combines visible and infrared light.

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