Stars are the most basic entity within the universe, for they are present to varying degrees in galaxies, nebulae, open
and closed clusters as well as supernova remnants. They have graced the evening sky ever since man first started becoming
aware of his existence and have been intricately intertwined in culture, religion, superstition and many facets of human
behaviour.
Our galaxy is believed to contain 300 billion (3x108) stars
whereas over 70 sextillion (7x1022) stars are believed to exist within the universe. The greatest accumulation
of stars is in the form of galaxies where hundreds of billions
of stars form these island universes. Less populous but still large accumulations of stars are observed in
globular clusters where up to one million stars are
strongly held together gravitationally (approximately 150 such clusters exist in our galaxy).
Open clusters also involve a collection of stars (usually in
the hundreds) which are weakly held gravitationally and are more frequent than globular clusters (at least 1500 open
clusters exist in our galaxy).
Note: Star HE 1523-0901 lying in Libra is amongst a sample of bright metal-poor stars selected
from the Hamburg-ESO Survey and whose age
using various chronometers has been estimated to be 13.2 billion years old, thus making it one of the oldest known stars
to date and which was created a mere 500 million years following the Big Bang. Using techniques similar to carbon-14 dating,
the star's thorium and uranium abundance (amongst others) has helped establish the star's age as well as set a lower bound
for the creation of the universe. This "galactic fossil" lies two degrees east of Zubeneschamali (â-Lib, mag 2.57) and
reaches its greatest altitude at the southern meridian during summer and around midnight. For further details, see the
article by Frebel et al (2007)
here as well as the online article from Astronomy
Magazine here.
Note: With a declination of -9.2°, HE 1523-0901 is a difficult imaging target for mid-northern
latitudes and this task is further complicated by the geosynchronous satellite belt.
Please click on the image below to display in reverse colour (900x1200)