Beyond the stars
Both spiral types are further divided into sub-types a, b and c, which
indicate the tightness of their spiral arms. Type Sa and SBa galaxies are
tightly wound while Sc and SBc have very open arms. The spiral arms of
galaxies are regions where material is being compressed and stars are
being born. The arms often look bluish in colour due to the hot, young blue
stars which are forming there. The core region, by contrast, often looks
yellow due to the large population of older, redder stars that can be found all
around there.
The Hubble Tuning Fork Diagram isn’t perfect and we now know of
several types of galaxy which the tuning fork is unable to classify properly.
The De Vaucouleurs system is designed to address this shortcoming
by adding another fork to the diagram for weakly barred spirals. The De
Vaucouleurs system is also able to classify galaxies surrounded by a ring of
stars, compact galaxies, dwarf galaxies, peculiar galaxies and galaxies with
active galactic nuclei.Just as stars are found gravitationally bound into open clusters, galaxies too
are found in gravitationally bound groups.
A telescope on a low power will show many stars in the region that
appear slightly fuzzy. These are individual galaxies and here you’ll find a
large collection of all types including spiral, lenticular and elliptical. There
may be as many as 2000 members in the Virgo cluster alone.
Galaxy Classification
The shape of galaxies is often represented by what’s known as the Hubble
Tuning Fork Diagram, after the famous astronomer Edwin Hubble. The
diagram depicts the progression of different observed galaxy shapes,
from elliptical through to spiral. Spiral galaxies are arranged in two parallel
branches, one for normal spirals and one for barred spirals. The diagram is
normally arranged so that the shape of the galaxy types gets more extreme
towards the right side of the diagram. The ellipticals are arranged in order of
eccentricity, or how elliptical they appear. They are identified by the letter E
followed by an index number.
The shapes range from E0, which is spherical, through to E7, which
is highly elliptical. Beyond this point the flatness of the galaxy cannot be
sustained. The most common type of elliptical galaxy is E3. The point
where the "handle" meets the "forks" is where a special class of galaxy
known as a "lenticular" sits. This is a flattened galaxy type similar in profile
to a spiral except that it has no spiral structure in the disc of stars that
surrounds the core. Lenticulars are given the designation S0. From here the
branches of spirals, identified by the letter S, and barred spirals, identified by
SB, begin.
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