As the Stars in the Night Sky

Wow.

That’s all I can say about one of the latest images from the European Southern Observatory in Chile. This is the “Jewel Box” cluster, one of the loveliest open clusters in the sky.  It’s not one of the brightest things to see — you can just barely make it out with the naked eye. But, if you look at it through binoculars or  a smal telescope, you can start to see the jewel-like stars that give this cluster its name.  There are some amazing color contrasts between the brightest stars in the cluster — ranging from pale blue to golden orange stars.

Open clusters like this one can have anywhere from a few stars to thousands of them. They travel together through space, held together by their mutual gravitational attraction. They form together and stay together for a long time as they move through space.  Because the stars all formed together from the same cloud of gas and dust their ages and chemical makeup are similar, which makes them ideal laboratories for studying how stars evolve.

Okay, this is gorgeous to look at in a wide field of view, but what if you looked at the heart of the Jewel Box?  You’d use a  telescope such as the FORS1 instrument on the ESO Very Large Telescope (VLT) at ESO’s Paranal Observatory in Chile, and you’d get a very sharp closeup view of the heart of the cluster. You can start to distinguish stars from each other and their colors are really quite strikingly different, glittering like diamonds on a fancy brooch.

You start to notice how the brightnesses of the different stars contrast with each other. That huge variety in brightness is because the brighter stars are 15 to 20 times the mass of the Sun, while the dimmest stars are less than half the mass of the Sun. More massive stars shine much more brilliantly. They also age faster and make the transition to giant stars much more quickly than their faint, less-massive siblings.  This is another reason why astronomers like to study stars in clusters — their masses, ages, and sizes give them a range of stellar evolution to study.

Okay, so what if you wanted to really zero in on the stars in the Jewel Box?  You’d aim the Hubble Space Telescope at the cluster and use the  multi-wavelength capability of the telescope to give you optical, infrared, and ultraviolet views of those stars. And the view would be just as exhilirating as the images from Chile.

This new Hubble image of the core of the Jewel Box cluster is the first comprehensive far ultraviolet to near-infrared image of an open galactic cluster.  HST imaged it using seven filters, which permit details of the stars at different wavelengths to shine through.

The image was taken near the end of the long life of the Wide Field Planetary Camera 2 — Hubble’s workhorse camera. You can see several very bright, pale blue supergiant stars, a solitary ruby-red supergiant and a variety of other brilliantly colored stars in HST’s view. There are also many very faint stars, showing just how populous this cluster and its environment are. The intriguing colors of many of the stars result from their differing intensities at different ultraviolet wavelengths, which tell astronomers a great deal about the temperatures and chemical compositions of those stars and their gaseous atmospheres. So, as you can see, there’s value in the wide-field view and the zoom-in — and each view tells astronomers a great deal about this starry jewel box. Enjoy!

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