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Anne’s Image of the Day: Globular Cluster Messier 19
read more at Anne’s Astronomy News http://annesastronomynews.com/
August 28, 2013
Messier 19, a globular cluster in Ophiuchus
Image Credit: Doug Williams, REU Program/NOAO/AURA/NSF
Messier 19 (also known as NGC 6273) is a globular cluster of some 140 light-years across, located about 28,700 light-years away from Earth in the constellation of Ophiuchus (the Serpent-bearer). It is receding form us at approximately 135 kilometers per second.
In contrast to most other globular clusters Messier 19 lies very close to the Galactic Center, at only about 6,500 light-years.
Messier 19 is a rich, fairly dense globular cluster, and unlike most globular clusters (which are quite round in appearance) it is distinctly elongated in the north-south direction, what makes it the most elliptical of the known globular clusters. This may be due to its proximity to the Galactic Center, and the tidal forces this generates.
However, the flattening may not accurately reflect the physical shape of the cluster because the emitted light is being strongly absorbed along the eastern edge. When viewed in the infrared, the cluster shows almost no flattening.
Messier 19 contains an estimated 1,100,000 times the mass of the Sun and it is around 11.9 billion years old. It is made up of stars that are all the same age, which are amongst the oldest stars in our Milky Way galaxy. The brightest stars have an average magnitude of 14.8, and the stellar population includes four Cepheids (1) and RV Tauri (2) variables, plus at least one RR Lyrae variable (3).
Because the stars in a globular cluster are all the same age and all at the same distance from Earth, they provide an excellent laboratory in which astronomers can study stellar evolution.
Although its total absolute magnitude is -9, the apparent magnitude to us is 6.8. This is bright enough for easy viewing through binoculars and small telescopes. The cluster is located 4.5° WSW of Theta Ophiuchi and is just visible as a fuzzy point of light using 50 mm (2.0 inch) binoculars. Using a telescope with a 25.4 cm (10.0 inch) aperture, the cluster shows an oval appearance with a 3′ × 4′ core and a 5′ × 7′ halo.
Notes:
1. Cepheids variable stars are very luminous variable stars with a strong direct relationship between their variable’s luminosity and pulsation period. That secures their status as important standard candles for establishing the Galactic and extragalactic distance scales.
2. RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima which are tied to radial pulsations of their surfaces. They exhibit brightness variations between magnitudes +9.8 and +13.3 with a formal period of 78.7 days.
3. RR Lyrae stars pulse in a manner similar to Cepheid variables, but in contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor stars. They are much more common than Cepheids, but also much less luminous. Their period is shorter, typically less than one day, sometimes ranging down to seven hours. The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good standard candles for relatively near objects, especially within the Milky Way.
This picture was created from six images taken in July 1997 at the KPNO 0.9-meter telescope during the summer Research Experiences for Undergraduates (REU) program operated at the Kitt Peak National Observatory and supported by the National Science Foundation.
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Source: http://annesastronomynews.com/annes-image-of-the-day-globular-cluster-messier-19/
