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Messier 4
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Messier 4 (also M4, NGC 6121 and GCL 41) is a magnitude +5.9 globular cluster located 7,200 light years away in the constellation Scorpius.
It is one of the easiest globular clusters to find, being located only 1.3 degrees west of the bright star Antares.

The cluster was discovered by Swiss astronomer Philippe Loys de Chéseaux using a a 2-feet focal length Gregorian reflector in Lausanne in 1746. 
The French astronomer Charles Messier catalogued the object on May 8, 1764, and was the first to realise it was not a nebula as previously thought, but a globular cluster, when Messier resolved its brightest stars, becoming the first globular cluster discovered. 

Right Ascension 16h 23m 35.22s, Declination -26° 31' 32.7"

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NGC 6121
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Star close to Earth that resists ageing discovered

Scientists have claimed that a member of a nearby cluster of stars in the Milky Way may hold the secret to eternal youth.
This glob, scientifically known as a globular cluster was observed by the European Southern Observatory (ESO)'s La Silla Observatory in Chile.

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Messier 4
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A Cluster with a Secret 

eso1235a.jpg

A new image from ESOs La Silla Observatory in Chile shows the spectacular globular star cluster Messier 4. This ball of tens of thousands of ancient stars is one of the closest and most studied of the globular clusters and recent work has revealed that one of its stars has strange and unexpected properties, apparently possessing the secret of eternal youth.
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Title: A second neutron star in M4?
Authors: J. Kaluzny, A. Rozanska, M. Rozyczka, W. Krzeminski, I. B. Thompson

We show that the optical counterpart of the X-ray source CX 1 in M4 is a 20th magnitude star, located in the colour-magnitude diagram on (or very close to) the main sequence of the cluster, and exhibiting sinusoidal variations of the flux. We find the X-ray flux to be also periodically variable, with X-ray and optical minima coinciding. Stability of the optical light curve, lack of UV-excess, and unrealistic mean density resulting from period-density relation for semidetached systems, speak against the original identification of CX 1 as a cataclysmic variable. We argue that the X-ray active component of this system is a neutron star (probably a millisecond pulsar).

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Title: The helium content of globular clusters: NGC6121 (M4)
Authors: S. Villanova, D. Geisler, G. Piotto, R. Gratton

He has been proposed as a key element to interpret the observed multiple MS, SGB, and RGB, as well as the complex horizontal branch (HB) morphology. Stars belonging to the bluer part of the HB, are thought to be more He rich (\Delta Y=0.03 or more) and more Na-rich/O-poor than those located in the redder part. This hypothesis was only partially confirmed in NGC 6752, where stars of the redder zero-age HB showed a He content of Y=0.25±0.01, fully compatible with the primordial He content of the Universe, and were all Na-poor/O-rich. Here we study hot blue HB (BHB) stars in the GC NGC 6121 (M4) to measure their He plus O/Na content. We observed 6 BHB stars using the UVES@VLT2 spectroscopic facility. In addition to He, O, Na, and Fe abundances were estimated. Stars turned out to be all Na-rich and O-poor and to have a homogeneous enhanced He content with a mean value of Y=0.29±0.01(random)±0.01(systematic). The high He content of blue HB stars in M4 is also confirmed by the fact that they are brighter than red HB stars (RHB). Theoretical models suggest the BHB stars are He-enhanced by \Delta Y=0.02-0.03 with respect to the RHB stars. The whole sample of stars has a metallicity of [Fe/H]=-1.06±0.02 (internal error). This is a rare direct measurement of the (primordial) He abundance for stars belonging to the Na-rich/O-poor population of GC stars in a temperature regime where the He content is not altered by sedimentation or extreme mixing as suggested for the hottest, late helium flash HB stars. Our results support theoretical predictions that the Na-rich/O-poor population is also more He-rich than the Na-poor/O-rich generation and that a leading contender for the 2^{nd} parameter is the He abundance.

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