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The Classification
of Galaxies
By Daniel Underwood
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Contents
The History of Galaxy Discovery – Page 3
The Different Types of Galaxies – Page 4
Galaxies Connected – Page 7
Conclusion – Page 8
References – Page 9
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The History of Galaxy Discovery
It was only last century in the 1920s that the presence of certain
nebulae outside the Milky Way we realised, and it became universally
accepted by astronomers that there were other galaxies than our own in
the cosmos. However, it wasn’t immediately recognised that these
nebulae were actually galaxies like our own, it took time to realise that
they weren’t gaseous, but actually massive collections of stars.
These masses outside the Milky Way were becoming more and
more noticed by astronomers, and they had their own characteristics
which helped identify them. But it didn’t end there...
Even though galaxies these days are widely recognised, even by
the average man on the street, their characteristics are not always
consistent. It was pointed out by certain astronomers (namely Wolf,
Hubble) that the structure of galaxies could be furthermore sub divided.
That is to say galaxies come in all shapes and sizes, but fall into certain
categories. It was only natural that rational physicists decided it was
necessary to develop a classification scheme in order to identify types of
galaxy.
Classification of galaxies was first attempted by Wolf in 1908. In
1926 Edwin Hubble published his own scheme for classifying, which is
what is still used today, but includes additions and amendments by
Holmberg (1958), de Vaucouleurs (1956, 1959) and van den Bergh
(1960).
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The Different Types of Galaxies
When it was established that galaxies had distinguishable features
which even differentiated them from them from other galaxies, a
classification scheme was devised.
The classification scheme used today was devised by Edwin Hubble
and later extended to include further ideas proposed by de Vaucouleurs.
The scheme can help identify evolutionary patterns and other properties
of the galaxies.
The types of galaxies are:
- Elliptical
- Spiral/Barred Spiral
- Lenticular
- Peculiar
They are explained under the following subheadings.
Elliptical Galaxies
These galaxies have no general
structure, and are basically large
groups of matter compacted together
in an elliptical shape, with a drop in
brightness from the centre to the edge
of the galaxy, suggesting that there is
a larger density of stars in the centre,
or brighter ones.
These galaxies are denoted by
the letter ‘E’ followed by an integer n
(e.g. E3), where the integer is the product of the galaxy’s ellipticity:
,
where e is the ratio:
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,
where a and b are the major and minor axes of the galaxy
respectively.
The value of n ranges from 0 to 7, where 0 denotes a circularshaped elliptical galaxy and 7 a more ‘cigar-shaped’ galaxy. Any galaxy
with a value of n greater than 7 are classed as lenticular galaxies,
explained later.
Spiral/Barred Spiral Galaxies
Spiral galaxies can be easily recognised by their structure; arms of
matter spiralling out from the bulging centre of the galaxy. There are two
types: barred spirals (SB) and
non-barred (SA), although if
distinction between the two
happens to be difficult then
“SAB” is used for denotation. A
barred spiral is a spiral type
(pictured on the right) but with a
bar-like feature in the centre off
which the spirals emanate. Like
with elliptical galaxies, spiral
galaxies can be sub-divided by
adding a letter to the SB/SA
symbol (e.g. SBc). The letters
run from “a” to “d”, with a further sub type “m” at the end.
As you move through “a” to “d” the following occurs:
- The central bulge becomes smaller compared to the galactic disc;
- The spiral arms spread further and become less compact;
- Increased clumping of arms into stars and star forming regions
(clumping means the degree of condensation of the regions).
The “m” type classes galaxies where no nucleus is clear and the
galaxy is generally loosely spread. An irregular galaxy is denoted by “Im”
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(or “IBm”) which is very similar to Sam/SBm types. The classification of
these galaxies relies mostly on the bulge and the disc-condensation.
Lenticular Galaxies
Lenticular galaxies appear to be intermediate between elliptical and
spiral galaxies, where their structure shows an elliptical style that is more
elongated, where 10e is greater than 7, although
spiral galaxy features may be present, if very
slightly. These galaxies are denoted by “SA0” or
“SB0”.
Peculiar Galaxies
These galaxies have no features that lie
under the common classification scheme.
They are simply labelled “P”. However,
sometimes galaxies can be classified and still
have peculiarities, and so “p” is added to the
end of the class denotation.
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Galaxies Connected
The galaxy types aren’t completely connected in any way that we
currently know of, but they are commonly visualised together by the
famous ‘tuning fork’ diagram:
SA0 SAa SAb SAc SAd SAm Im
E0 E1 E2 E 3 E4 E5 E6 E7
SB0 SBa SBb SBc SBd SBm IBm
‘Early type’
‘Late type’
The reason the two sides are labelled ‘early’ and ‘late’ respectively
is because it was previously believed that the diagram was an
interpretation of an evolutionary line, but this view doesn’t hold anymore.
However there are a few apparent physical trends along the sequence
that are being recognised moreover.
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Conclusion
The development of the galaxy classification scheme may seem
trivial, but in the greater scheme of things it has the substance to help
astronomers and physicists in general to discover more about the
universe. Simply analysing deep field images of space in concordance
with this scheme can lead to conclusions about the evolution of the
universe with respect to galaxies, by recognising patterns and
differences as the universe ages, ultimately helping towards the ultimate
goal and understanding the universe’s origins and its fate.
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References
Zeilik & Gregory – Introductory Astronomy and Astrophysics,
Chapter 21
University Of London Observatory – 1B30 Practical Astronomy,
Classification of Galaxies report sheet
Images
http://galaxyzoo.org
http://sci.esa.int/
http://content.answers.com/
http://www.cosmiclight.com/
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