DEEP SKY
  Gavin James
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  gavin@gjmultimedia.co.uk
 
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Caroline's Rose

Caroline’s Rose (NGC 7789) is an open cluster in the constellation of Cassiopeia. It lies around 7600 light years from Earth. It was discovered by Caroline Herschel in 1783, during the period that she lived with her brother, William, at 19 New King Street, Bath. 1783 was the first year of her own astronomical explorations, using a telescope that William had made for her. It was the start of a career that was to prove highly productive. The siblings discovered over 2400 astronomical objects together during their twenty years of searching. Caroline Herschel was the first woman to be awarded a Gold Medal of the Royal Astronomical Society (1828) and to be named an Honorary Member of the Royal Astronomical Society (1835).

Recent estimates age the cluster at around 1.6 billion years old, making it one of the oldest known. Caroline's Rose shows its age. All the stars in the cluster were likely born at the same time, but the brighter and more massive ones have more rapidly exhausted the hydrogen fuel in their cores. These have evolved from main sequence stars like the Sun into the many red giant stars shown with an orange-yellow cast in this image. Using measurements of colour and brightness, astronomers can model the mass and hence the age of the cluster stars just starting to leave the main sequence and become red giants. According to most sources the mass of the cluster is around 6600 solar masses. Given the diameter of the cluster is about 35 light years, the volume is approximately 22,000 cubic light years. This makes the density about 0.3 solar masses per cubic light year.

The density of this cluster and its relatively old age beg the question: what is the difference between an open cluster and a globular cluster? Caroline's Rose has an appearance that is edging towards that of a globular cluster, but can an open cluster become a globular cluster? It is not likely that this is the case as globular clusters are thought to be the remnants of small galaxies consumed by the larger host galaxy that they orbit. They are densely packed, graviationally bound clusters. While this open cluster may look like the beginnings of a globular cluster, it is more likely that time will see its component stars drift away from each other on their own individual cosmic journeys.

Research Assistant: Charlie Wright

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Caroline's Rose
NGC 7789
Open Cluster
Cassiopeia
7,600 light years
6.7
16 arcminutes
35 light years
1783, Caroline Herschel
October
22h 37m 55s
+34º 30’ 43”
Skywatcher Esprit100 ED
7 nights in September & October 2018
LRGB = 18 x 600s each
12 hours

 

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