- distinguish between the different classifications of galaxies
- discuss the evolutionary theories of galaxies
- predict the evolution of the Milky Way galaxy
When it was discovered in 1923 that there existed
galaxies other than the Milky Way (see Unit 8, Lesson 1),
astronomers turned their telescopes in earnest to the fuzzy “island
universes” in order to determine their structure and distance from our
galaxy. What they discovered was not only the spiral galactic structure
of the nearby Andromeda galaxy, but other structures in other galaxies.
It turns out there are quite a few!
was Edwin Hubble (of recent Hubble Space Telescope fame) who made the
initial measurement of the distance to the Andromeda Galaxy. It was
also Hubble, three years later, who proposed that galaxies be separated
into three main categories, based on their appearance. This method of
classification is now known as the Hubble Sequence
have a central bulge and a flat disk of stars forming arms. They are
also spinning, creating a spiral, pinwheel pattern seen in the arms.
The Andromeda galaxy is an example of a spiral, as is the Milky Way.
Spiral galaxies are further broken down into barred spirals
or normal spirals
– the former have slightly elongated bulges, while the latter have more spherical bulges.
have very little structure compared to the bulge and arms of spiral
galaxies. They are oval in shape, and the stars move in random patterns
within them. They are classified further by their ellipticity – how
flattened the oval is. An elliptical galaxy classified as E0 is nearly
spherical, E7 would be quite flattened, and with E3 would be somewhere
The final classification of galaxy is the catch-all for any galaxy which doesn’t fit into the first two categories: Irregular galaxies
. Irregulars have no set shape or organizational structure, and have a huge range in size.
Like stars, galaxies are sometimes found clustered together. In this particular cluster, a large
Did You Know ?
elliptical galaxy is found near the centre of the image, surrounded by dozens of other galaxies.
The only objects in this image which are not galaxies are foreground stars (with cross-hair-like
spikes). How many spirals, ellipticals and irregulars can you find?
Milky Way galaxy, and its closest neighbour, the Andromeda galaxy, are
on a collision course with each other! This collision between these two
spiral galaxies will likely occur in about 3 billion years, and the
resulting galaxy is expected to be an elliptical.
Hubble originally laid out his classification in the form of a tuning-fork diagram
He initially thought that his diagram represented a sort of galactic
evolution i.e., that all galaxies started off as spherical ellipticals,
slowly flattened into an E7, eventually becoming a spiral galaxy and
finally dissipating into an irregular shape. He believed that in
locating different classifications of galaxies in the night sky, one
was seeing galaxies in various stages of evolution. This theory turned
out being incorrect, but was an important first step in separating
galaxies into different classifications.
van den Burgh, a Canadian astronomer, made a significant modification
to Hubble’s classification scheme. Dr. van den Burgh introduced a
method of further classifying spiral galaxies based on the definition
of the spiral arms observed. His research included further study of
galaxy morphology, and he showed that Hubble’s classification applies
only to young galaxies. He has served as President of the Canadian
Astronomical Society, and has won numerous medals and awards. For 6
years he served as Vice-President of the International Astronomical
Millions of galaxies arescattered throughout the
universe. Instead of all living side-by-side in a universal
subdivision, most galaxies are found in densely-packed clusters,
themselves each with thousands of galaxies. Famous examples include the
Coma cluster and Hercules cluster. Within these clusters are found
galaxies of each of the three types, all in motion based on the
gravitational interactions with their galactic neighbours. It stands to
reason that so much mass (recall that each galaxy contains hundreds of
billions of stars) is bound to tug and pull on all of the surrounding
mass. The result is that often galaxies will “collide” – run into each
other and disrupt the existing structure (spiral or otherwise).
galaxies do collide, interesting gravitational effects occur. Because
there is a large amount of space between stars within a galaxy, the
chance of a star colliding with another – even while the galaxies are
colliding – is quite small. Instead, the stars pull and tug at each
other gravitationally, destroying the structure of the galaxies. What
is left at the end of the collision – which could take hundreds of
millions of years in total – is a large collection of stars,
gravitationally bound, but exhibiting no structure; essentially, it is
an elliptical. In fact it is now thought, contrary to Hubble’s initial
theory of galaxy evolution, that elliptical galaxies may be formed by
the interactions of spiral galaxies.
evolution is perhaps one of the most interesting areas of theoretical
astronomy today. Since collisions happen on such large time scales, it
is nearly impossible to observe the ongoing effects of two galaxies
A pair of spiral galaxies commonly referred to as “The Mice” are currently in the process
of colliding. It is thought that the two galaxies have already passed through each other
– which is
why we can see some disruption in structure – but that they will continue to
with each other until they form a single galaxy.
Professor John Dubinski, of the Department of Astronomy and
Astrophysics at the University
of Toronto, and the
Canadian Institute for Theoretical Astrophysics (CITA), is one of the world’s
leading researchers on galactic formation and dynamics. He was an amateur
astronomer in his teens before completing degrees in Physics and Astronomy from
the University of Waterloo and the University of Toronto.
He has pioneered work using parallel supercomputers, which has recently allowed
for simulations of models using billions of particles, similar to the most
complex galaxies. His work on the collision and evolution of galaxies is
referenced by astronomers around the world.