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Starts With A Bang

Mostly Mute Monday: How young stars appear in old clusters

These stars are too hot and blue for their age, and the leading theory just got a strike against it.

“Youth is the gift of nature, but age is a work of art.” –Stanislaw Jerzy Lec

If you want to know how old a star cluster is, all you have to do is measure the color and brightness of its members.

Image credit: ESO/M.-R. Cioni/VISTA Magellanic Cloud survey.

The hottest, brightest and bluest stars burn through their fuel the fastest, and so as they disappear, you know your cluster is aging.

Image credit: ESO / Wide Field Imager attached to the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. Via

In astronomy, we call this color-brightness relationship the Hertzsprung-Russell diagram, and the “turn-off point” tells you the cluster’s age.

Image credit: Christopher Tout, Nature 478, 331–332 (2011), via

But surprisingly, bluer, brighter, hotter stars often appear in these older clusters anyway!

Image credit: Francesco Ferraro (Bologna Observatory), ESA, NASA.

Even in cases where there’s no evidence of a recent star-forming event, these blue straggler stars can frequently be found.

Image credit: NASA, ESA, F. Ferraro (University of Bologna).

For a long time, we thought the occasional merger of two low-mass stars gave rise to a heavier, blue star. But there’s an alternative.

Image credit: NASA, ESA, W. Clarkson (Indiana University and UCLA), and K. Sahu (STScl), with Blue Stragglers circled.

In binary star systems, a denser, heavier star can siphon mass off of its companion, stripping it into a white dwarf.

Image credit: NASA, ESA, and A. Feild (STScI).

Recently, scientist Natalie Gosnell studied the old open cluster NGC 188. Out of 21 blue stragglers, 7 have white dwarf companions.

Image credit: K. Garmany, F. Haase NOAO/AURA, from NOAO’s website at

Combined with an earlier study, this causes a complete rethink of blue stragglers.

Image credit: Hubble / ESA / NASA, via the Hubble Legacy Archive at, from HST and Wikimedia Commons’ user Fabian RRRR. Color correction by E. Siegel.

Don’t be deceived: these young-looking stars are older than they appear!

Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images, video and no more than 200 words.

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