Starship Asterisk*

Cosmic Riches
ESA/HEIC Hubble Picture of the Week | 2012 Oct 01

This dazzling image shows the globular cluster Messier 69, or M 69 for short, as viewed through the NASA/ESA Hubble Space Telescope. Globular clusters are dense collections of old stars. In this picture, foreground stars look big and golden when set against the backdrop of the thousands of white, silvery stars that make up M 69.

Another aspect of M 69 lends itself to the bejewelled metaphor: As globular clusters go, M 69 is one of the most metal-rich on record. In astronomy, the term “metal” has a specialised meaning: it refers to any element heavier than the two most common elements in our Universe, hydrogen and helium. The nuclear fusion that powers stars created all of the metallic elements in nature, from the calcium in our bones to the carbon in diamonds. Successive generations of stars have built up the metallic abundances we see today.

Because the stars in globular clusters are ancient, their metallic abundances are much lower than more recently formed stars, such as the Sun. Studying the makeup of stars in globular clusters like M 69 has helped astronomers trace back the evolution of the cosmos.

M 69 is located 29 700 light-years away in the constellation Sagittarius (the Archer). The famed French comet hunter Charles Messier added M 69 to his catalogue in 1780. It is also known as NGC 6637.

The image is a combination of exposures taken in visible and near-infrared light by Hubble’s Advanced Camera for Surveys, and covers a field of view of approximately 3.4 by 3.4 arcminutes.

Credit: NASA/ESA/Hubble

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This is an interesting picture, where some foreground stars look really strikingly red. The reason for the unusual HEIC globular cluster palette is that this is a four-filter picture, imaged through blue, green, orange (or clear) and infrared filters. This makes it possible to bring out a wide range of colors in the finished picture.

Why has M69 been imaged through four filters, even though most clusters get visited by only two filters, typically clear (or orange) and infrared ones? I would say that the probable reason is that M69 is unusually colorless as globulars go, and one of the aims here was to find out exactly how colorless it really is.

Most globulars are extremely metal-poor. They therefore typically have a rich population of blue horizontal branch stars, blue stars which represent a late evolutionary stage in very metal-poor stars. While I, of course, find the blue stars very beautiful, the blue stars are important for astronomers because they are associated with white variable stars called RR Lyra variables. The RR Lyra variables are very important as distance indicators in globulars, and they, like the blue horizontal branch stars, can only exist in very metal-poor populations.

Since M69 is very metal-rich as globulars go, we should not expect to find many, or any, blue horizontal branch stars in it. Therefore we should not expect to find many, or any, RR Lyra variables either.

I googled "M69 globular RR Lyrae", but came up empty-handed. I checked M69 with my Sky Catalogue 2000.0 Volume 2, which says that M69 has an intrinsic B-V index of +0.82, which is red for a Milky Way globular. I checked Burnham's Celestial Handbook, which mentions M69 but says nothing about any RR Lyra variables in it.

So I would indeed guess that the fact that M69 was imaged through four filters, among them a blue one, was to find out exactly how non-blue and RR Lyra-free this globular cluster really is.

Ann

Hi Ann,

I read your post with interest. Your findings with M69 in Burnham's Celestial Handbook coincide with The Night Sky Observers Guide where Craig Crossen explains that RR Lyrae stars are poor to non-existant in younger globulars such as M69, M71 and 47 Tuc for example, so your findings appear to be correct.

---Daniel