Dating a Cluster – A New Trick

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Finding the ages of things in astronomy is hard. While it is undoubted that the properties of objects change as they age, the difficulty lies in that the initial parameters are often so varied that, for most cases, finding reliable ages is challenging. There’s some tricks to do it though. One of the best ones, taught conceptually in introductory astronomy courses, is to use the “main sequence turn-off” of a cluster. Of course, applying any of these methods is easier said than done, but a new method may help alleviate some of the challenges and allow for smaller errors.

The largest difficulty in the main sequence turn-off method lies in the inherent scatter caused by numerous sources that must be accounted for. Stars that lie along the same line of sight as the cluster being observed can add extraneous data points. Any interstellar reddening caused by gas may make stars appear more red than they should be. Close binary stars that cannot be spatially resolved appear brighter than they should be as an individual star. The amount of heavy elements in the star will also effect the fitting of the model. All of these factors and more contribute to an uncertainty in any calculation that requires an accurate Hertzsprung-Russell Diagram. Tricks to correct for some of these factors exist. Others cannot (yet) be accounted for.

Thanks to all these problems, fitting the data can often be challenging. Finding the point where the cluster “peels away” from the main sequence is difficult, so one of the tricks is to look for other points that should have significant numbers of stars to provide extra reference points for fitting. Examples of this include the horizontal branch and the red clump.

The new technique, developed by a large team of international astronomers, uses “a well defined knee located along the lower main sequence” which they refer to as the Main Sequence Knee (MSK). This “knee” appears in H-R diagrams of the clusters taken in the near-infrared and is largely independent of the age of the cluster. As such, it provides a stable reference point to improve corrections for the general main sequence turn-off method. Additionally, since this system uses infrared wavelengths, it is less prone to contamination between gas and dust.

To test this new method, the group selected a globular cluster (NGC 3201) as a test case. When their method was applied, they found that their derived age for the cluster was consistent with ages derived by other methods.

However, the new method is not without difficulties of its own. Since the knee is at the faint end of the main sequence, this requires that exposure times for target clusters be sufficiently long to bring out such faint stars. Fortunately, with new telescopes like the the James Webb Space Telescope, these faint stars should be in reach.

Telescopes Open Up the Jewel Box

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Nothing in my jewelry box compares to the Kappa Crucis Cluster, also known as NGC 4755 or simply the “Jewel Box.” This object is just bright enough to be seen with the unaided eye, but a combination of images taken by three exceptional telescopes, the Very Large Telescope, the 2.2-meter telescope at the La Silla observatory and the Hubble Space Telescope, has allowed the stunning Jewel Box star cluster to be seen in a whole new light. Above is the image from ESO’ Very Large Telescope, which zooms in for a close look at the cluster itself. This new image is one of the best ever taken of this cluster from the ground, taken with an exposure time of just 5 seconds.

A Hubble gem: the Jewel Box.  Credit: NASA/ESO
A Hubble gem: the Jewel Box. Credit: NASA/ESO

The Hubble Space Telescope can capture light of shorter wavelengths than ground-based telescopes can, and this new HST image of the core of the cluster represents the first comprehensive far ultraviolet to near-infrared image of an open galactic cluster. It was created from images taken through seven filters, allowing viewers to see details never seen before. It was taken near the end of the long life of the Wide Field Planetary Camera 2, Hubble’s workhorse camera up until the recent Servicing Mission, when it was removed and brought back to Earth, and replaced with an new and improved version. Several very bright, pale blue supergiant stars, a solitary ruby-red supergiant and a variety of other brilliantly colored stars are visible in the Hubble image, as well as many much fainter ones. The intriguing colors of many of the stars result from their differing intensities at different ultraviolet wavelengths.

Wide Field Image  of the Jewel Box.  Credit:  ESO
Wide Field Image of the Jewel Box. Credit: ESO

A new image taken with the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile shows the cluster and its rich surroundings in all their multicolored glory. The large field of view of the WFI shows a vast number of stars. Many are located behind the dusty clouds of the Milky Way and therefore appear red.

Composite image of the Jewel Box. Credit: ESO
Composite image of the Jewel Box. Credit: ESO

Star clusters are among the most fascinating objects in the sky. Open clusters such as NGC 4755 typically contain anything from a few to thousands of stars that are loosely bound together by gravity. 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.

Source: ESO