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Our Place in the
Cosmos
Lecture 14
Galaxies
Nebulae
• The planets and stars we have studied so far
have all been part of our own galaxy, the Milky
Way, which we will come back to in a later
lecture
• Deep images of the sky, such as the Hubble
Space Telescope Ultra-Deep Field show many
faint and diffuse smudges of light
• Diffuse patches of light in the night sky were
originally referred to as nebulae due to their
nebulous appearance
Hubble
Space
Telescope
Ultra-Deep
Field
Messier Catalogue
• In 1784 the French astronomer Charles Messier
published a catalogue of 110 nebulae
• [Primary motivation was as an aid to comet
hunters - nebulae and comets are similar in
appearance, but nebulae do not move]
• Within another 20 years, largely thanks to
Caroline and William Herschel, the number of
known nebulae had reached 2500
• Three types of nebulae became apparent diffuse and amorphous, elliptical and spiral
Nature of the Nebulae
• Many astronomers pre-1920 thought that the
Universe consisted entirely of the Milky Way
Galaxy
• It was suggested that the spiral nebulae might
be planetary systems in formation
• Immanuel Kant (1724-1804) proposed that the
nebulae were island universes separate from
our own Galaxy, a belief shared by William
Herschel, but which could not be proved with
existing telescope technology
Nature of the Nebulae
• We now know that Kant was correct
• Our Milky Way Galaxy is just one of Kant’s island
universes, which are now referred to as galaxies
• The word Universe now refers to the full expanse of
space and its contents
• While most diffuse nebulae are nearby clouds of gas and
dust within the Milky Way, the elliptical and spiral
nebulae are galaxies located well beyond the limits of
the Milky Way
• The Universe contains uncounted billions of galaxies,
each of which in turn contains millions to hundreds of
billions of stars
The Great Debate
• The key to understanding the nature of nebulae lay in
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determining their distances and hence sizes
Early astronomers did not know of the existence of dust
which limits our view, and under-estimated the size of
our Galaxy as about 6,000 light years across
In the mid 1910s Harlow Shapley determined that the
Galaxy is over 300,000 light years in size
Due to this large size, Shapley believed our Galaxy to
contain the observed nebulae
In 1920 the Great Debate took place between Shapley
and Heber Curtis, who argued for the island universe
hypothesis
Edwin Hubble
• The debate itself was inconclusive, but it did
focus attention on the nebulae
• The debate was resolved in 1924 by Edwin
Hubble who used the Mt Wilson 100-inch
telescope to observe Cepheid variable stars in
the Andromeda Nebula
• Using the period-luminosity relation for these
stars, and comparing with their observed
(apparent) brightness, Hubble was able to
estimate their distances to be far greater even
than Shapley’s measurement of the size of the
Milky Way
Edwin Hubble
• Hubble’s estimate of the
distance to Andromeda (M31)
showed it to be of comparable
size to the Milky Way
• Almost overnight our world view
of the Universe changed - the
Milky Way became but a speck
adrift in a Universe full of
galaxies
Galaxy Classification
Galaxies are like a
handful of coins thrown
into the air - some are
seen face-on, some edgeon and most somewhere
in between
The intrinsic 3d shape is
seen in projection on the
sky
HST image of a
group of
galaxies
Galaxies come
in a wide range
of shapes and
sizes
Hubble Tuning Fork
Ellipticals: E0  E5 more elongated
Spirals: Sa  Sd more open, knottier arms, smaller bulge
Lenticulars: S0 - lenticular shape but no spiral arms
Irregulars: Irr - none of the above
Elliptical Galaxies
• Galaxies are not solid objects but collections of
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stars, gas and dust orbiting under the galaxy’s
gravitational field
In elliptical galaxies stars are moving in all
directions on different shaped orbits
Random motions  spherical galaxy
Faster motions in any direction elongate the
galaxy in that direction
Note that an E0 (apparently round) elliptical
galaxy is not necessarily spherical - it could be a
rugby-ball shape seen end-on
Spiral Galaxies
• Spiral galaxies possess a flattened rotating disk
• Most stars in the disk follow nearly circular orbits in the
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same direction about the centre of the galaxy (much like
the planets in our Solar System)
The disk also contains the eponymous spiral arms
Spiral galaxies have a central bulge that is quite similar
to a small elliptical galaxy
Some bulges are bar shaped
Barred spirals are classified in parallel with unbarred
spirals
Sa or SBa  tightly-wound arms, prominent bulge
Sd or SBd  loosely-wound arms, small bulge
Gas and Dust
• Spiral galaxies contain
large amounts of
molecular gas and
dust within their disks
• This can sometimes
be seen as a dust lane
in an edge-on spiral
galaxy
• Gas in ellipticals is
heated by supernovae
and is unable to cool
Lenticular Galaxies
• Lenticular galaxies lie between spirals and
ellipticals
• They have a lenticular (lense-shaped) stellar
disk but no spiral arms
• The zero in their designation S0 denotes the
lack of spiral arms
• Modern observations show that most elliptical
galaxies contain small rotating disks
Star Formation
• Stars form from dense clouds of molecular gas,
found in spiral and irregular galaxies but not
ellipticals
• Spiral and irregular galaxies are thus still
forming stars today, whereas ellipticals contain
mostly old stars
• This difference is seen in colour:
• Spiral disks are relatively blue due to light from hot,
massive, young stars
• Elliptical galaxies are relatively red due to the
dominant population of older, lower-mass stars
Luminosity and Size
• Galaxy luminosities range from around 1 million
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to 1000 billion Solar luminosities (106-1012 L)
Sizes range from about 3,000 to hundreds of
thousands of light years
No strong dependence of luminosity and size on
Hubble type
Mass is not so important in determining the
appearance of a galaxy as it is for a star
A large, distant galaxy looks similar to a nearby,
smaller galaxy
Which galaxy is 4 times larger and 10 times more
luminous than the other?
Large distant galaxy
Small closer galaxy
Spiral Arms
• What are the spiral arms?
• The overall distribution of stars is not strongly
concentrated in spiral arms, but the young,
massive and luminous stars are concentrated
here
• This is apparent when comparing the image of a
spiral galaxy seen in ultraviolet (UV) light
compared with red light
• The arms are much more prominent in UV light,
and also in hydrogen gas seen with a radio
telescope
Three Views of a Spiral
Galaxy
UV light is dominated by massive, hot young stars which are
concentrated in the arms
Hydrogen gas is also concentrated in the arms
Older stars seen in red light are less concentrated in arms
Spiral Structure
• Spiral structure is almost inevitable in a disk
which is rotating differentially
• Material close to the centre takes less time to
complete a revolution about the galaxy than
material further out [cf uniform rotation of a CD]
• If we started with a single linear arm, it would
soon be wound up into a spiral pattern as those
parts of the arm closer to the centre rotate faster
than outer parts
Spiral Structure
• Any “kick”, such as a gravitational interaction
with another galaxy, or an internal burst of star
formation, will lead to spiral structure
• Note that the spiral from a single “kick” will
eventually be wound up and disappear
• A repetitive kick, such as that from a rotating
central bar, is able to sustain two-armed spiral
structure
• These disturbances are known as spiral density
waves - they propagate as regions of increased
density
Spiral Density Waves
• The density wave does not necessarily rotate at
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the same rate or even in the same direction as
the material in the disk
The density wave thus passes through the
material in the disk, much like ocean waves
pass through water
As a density wave passes, gas is compressed,
leading to enhanced rates of star formation
The most massive stars die out by the time the
wave has passed and so are only seen in the
arms
Lower mass stars live on and are seen
throughout the disk
Dark Matter in Galaxies
• The total mass of a galaxy may be estimated by
studying the effects of its gravity on motion
• We can apply Kepler’s laws to the rotation of a
spiral disk to estimate the mass contained within
a given radius
• One can also predict how the mass should be
distributed by measuring the light distribution
• The predictions say that rotation speed should
decrease with radius, whereas the observed
rotation curves are roughly constant to large
distances
Dark Matter Halos
• The “flatness” of the observed rotation curves
implies that most of the mass (up to 95%) of a
galaxy is contained in a “halo” surrounding the
luminous material
• Since this matter is not luminous it is known as
dark matter
• The luminous part of a galaxy is akin to the tip of
an iceberg seen above the water - most of the
mass is hidden within the dark matter halo
Summary
• Galaxies were shown in 1924 by Edwin Hubble
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to be separate from our own Milky Way
They are classified into elliptical, lenticular, spiral
and irregular
Spiral galaxies are dominated by a rotating disk
with often-spectacular spiral arms
Stars in elliptical galaxies are on more random
orbits
The masses of all galaxies are dominated by
dark matter