So far we have confined ourselves to our own Galaxy. As recently as
the 1930s most of the cloud-like objects in the heavens were referred
to as nebulae (Latin for clouds). Some nebulae are indeed clouds
of gas in which fresh stars are being born. Others though are quite
different – they are huge island cities of stars called galaxies, which
exist outside of our own Milky Way.BEYOND THE STARS
Astronomer Book
refer to them as "standard candles" in space. We now know the number of
galaxies in our range is very great, each containing huge numbers of stars,
like our Milky Way. An average galaxy probably contains several hundred
thousand million stars. Astonishing as this figure is, current estimates put
the number of galaxies that may exist at from several hundred thousand
million, perhaps even up to a million million (a trillion).
Galaxies come in a variety of shapes and sizes and many of these can
be seen in the night sky with quite modest equipment. Our own Milky Way
Galaxy is thought to be what is known as a barred spiral – that’s a spiral
galaxy with a star-dense core that appears elongated along an axis.
One of the best galaxy hunting grounds in the entire night sky is in a region
of sky known as The Realm of Galaxies. This lies within a large arc of stars in
Virgo, the Virgin, often referred to as the "Bowl of Virgo". The region bounded
by the five stars that make up the bowl – Epsilon, Delta, Gamma, Eta and
Beta Virginis – and the star Beta Leonis, or Denebola, marks the direction to
two large clusters of galaxies known as the Virgo Cluster and Coma Cluster.M31, the giant galaxy in Andromeda, looks like a mass of stars, and was
previously known as a "starry nebula". It was thought to lie within our
Milky Way Galaxy, but is in fact an external galaxy in its own right, 2.5 million
light years away. It is the most remote object that can be seen clearly with
the naked eye. It was not until 1930 that Edwin Hubble, using the most
powerful telescope in the world, realized that this object could not be inside
our own Galaxy.
He did this by identifying so-called Cepheid variable stars in various
"starry nebulae", including Andromeda. Cepheid variables have periods of
several days or several weeks, and we always know how a Cepheid is going
to behave. In 1930 it was discovered that the real luminosity of a Cepheid
depends upon its period: the longer the period, the more powerful the star.
As soon as we can indentify a Cepheid and measure its period we know how
luminous it really is and this gives us a good estimate of its distance, since
there is a straightforward relationship between distance and luminosity.
Cepheids are very powerful so can be identified out to great distances. We[1] The Markarian Chain of galaxies in
the Virgo Cluster.
[2] Hubble’s tuning fork classification
for galaxies: the main characteristic
shapes are Sa, Sb, Sc along the upper
parallel branch, and on the parallel
branch below Sba, SBb and SBc
[3] The Sombrero Galaxy (Messier 104)
in Virgo.1