Lives and deaths of sibling stars: NGC 3293 from ESO La Silla

The star cluster NGC 3293
Image: ESO. More on the formation and lives of sibling stars, as they age in slowly dissipating open clusters:

Most of the stars seen here are very young, and the cluster itself is less than 10 million years old. Just babies on cosmic scales if you consider that the Sun is 4.6 billion years old and still only middle-aged. An abundance of these bright, blue, youthful stars is common in open clusters like NGC 3293, and, for example, in the better known Kappa Crucis cluster (VLT image at bottom), otherwise known as the Jewel Box or NGC 4755. These open clusters each formed from a giant cloud of molecular gas and their stars are held together by their mutual gravitational attraction. But these forces are not enough to hold a cluster together against close encounters with other clusters and clouds of gas as the cluster’s own gas and dust dissipates. So, open clusters will only last a few hundred million years, unlike their big cousins, the globular clusters, which can survive for billions of years, and hold on to far more stars. Despite some evidence suggesting that there is still some ongoing star formation in NGC 3293, it is thought that most, if not all, of the nearly fifty stars in this cluster were born in one single event. But even though these stars are all the same age, they do not all have the dazzling appearance of a star in its infancy; some of them look positively elderly, giving astronomers the chance to explore how and why stars evolve at different speeds.

Bearing in mind that stars can be coeval and therefore have the same chemical composition has led another group of astronomers to detect remnant differences in chemical composition caused by the formation of a giant planet, adding weight to the core accretion hypothesis. In an update on the differential abundance analysis, evidence for the accretion of the known radial velocity planet 16 Cyg Bb is inferred by the difference in composition between its host star and 16 Cygni A, the other major component of this system:

Difference in chemical composition between the stars 16 Cyg A and 16 Cyg B, versus the condensation temperature of the elements in the proto-planetary nebula. If the stars had identical chemical compositions then the difference (A-B) would be zero. The star 16 Cyg A is richer in all elements relative to star 16 Cyg B. In other words, star 16 Cyg B, the host star of a giant planet, is deficient in all chemical elements, especially in the refractory elements (those with high condensation temperatures and that form dust grains more easily), suggesting evidence of a rocky core in the giant planet 16 Cyg Bb. Credits: M. Tucci Maia, J. Meléndez, I. Ramírez, Canada-France-Hawaii Telescope.

The fingerprints detected by the astronomers are twofold. First, they found that the star 16 Cygni A is enhanced in all chemical elements relative to 16 Cygni B. This means that 16 Cygni B, the star that hosts a giant planet, is metal deficient. As both stars were born from the same natal cloud, they should have exactly the same chemical composition. However, planets and stars form at about the same time, hence the metals that are missing in 16 Cygni B (relative to 16 Cygni A) were probably removed from its protoplanetary disk to form its giant planet, so that the remaining material that was falling into 16 Cygni B in the final phases of its formation was deficient in those metals.

“A Snapshot of the Jewel Box cluster with the ESO VLT” by ESO/Y. Beletsky – ESO. Licensed under Creative Commons Attribution 3.0 via Wikimedia Commons. Note this is taken with an 8 metre mirror, whereas the MPG/ESO Telescope used to image NGC 3293 above, with a much longer exposure, is only 2.2 m! Finally on a more whimsical note James Kaler also refers recently to the dissipation of open clusters in his discussion of the 17 Com system:

“About 400 million years old, the cluster as a whole is falling apart, its stars gradually escaping its weak gravitational grip”.

Update: You may well have heard something of NASA’s double whammy of discoveries yesterday, confirmation first of a subsurface liquid water ocean at Enceladus but then this. (Full story at, and now the proper skepticism has been rounded up by Perhaps the situation is best captured here.


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