Chapter 20: Galaxies So far we have talked about small things
like stars, nebulae and star clusters. Now its time to get
big!
Slide 2
What are the Spiral Nebulae? The Shapley Curtis Debate April
1920 Harlow ShapleyThe Milky Way is huge so the spiral nebulae must
be located within it Heber CurtisThe spiral nebulae are island
universes like our Milky Way and thus are not part of it No
definitive conclusion was reached because no one had any data on
the distance to the spiral nebulae or their size
Slide 3
Edwin Hubble: late 1924 Hubble looked for Cepheid variable
stars in the closest of the spiral nebulae, the Andromeda nebulae.
He found it is over 2 million lightyears away and so must be at
least 200,000 light- years in diameter. It cannot be part of the
Milky Way. The Hubble Space Telescope still measures Cepheid
variables in distant galaxies
Slide 4
The Hubble Galaxy Classification Scheme After observing lots of
other galaxies, Hubble came up with a classification scheme based
on shape Elliptical Galaxies: spherical to elliptical in shape
Spiral Galaxies: Normal Spirals and Barred Spirals Irregular
Galaxies: No distinct shape
Slide 5
The Hubble Tuning Fork diagram The diagram shows the Hubble
classification scheme. When he first devised it, Hubble thought it
may be related to galaxy evolution. We now know it does not show
galactic evolution!
Slide 6
S0 Spiral Galaxies have dust lanes but no arms
Slide 7
Sa Spirals have tightly wound arms and a large nucleus
Slide 8
Sb Spirals have looser arms and a smaller nucleus
Slide 9
Sc Spirals have very loose arms and a small nucleus
Slide 10
Barred Spirals are like normal spirals except with a bar across
their center
Slide 11
Barred Spirals are classified SBa, SBb, and SBc SBaSBbSBc
Slide 12
Elliptical Galaxies contain no gas or dust, theyre just big
balls of stars
Slide 13
Ellipticals are classified according to how stretched out they
areE0 E7 E0 E3 E6
Slide 14
Irr Galaxies are irregular galaxies The Large and Small
Magellenic Clouds
Slide 15
What are the Spiral Arms? If the spiral arms are due to the
rotation of stars and nebulae around the center of the galaxy, they
should wind up.
Slide 16
Observed spiral arms dont wind up We can see lots of spiral
galaxies and none of them show evidence of wind-up
Slide 17
The Spiral Density Wave Model
Slide 18
Stars exist in all areas of disk but brightest ones are in
arms
Slide 19
How fast do the stars and nebulae move around spiral
galaxy?
Slide 20
Possible Velocity distributions
Slide 21
Expected Rotation Curve Predicted velocities are based on the
amount of matter we can see: stars, gas & dust
Slide 22
Observed velocity distributions in galaxies The stars in spiral
galaxies orbit too fast far from the center
Slide 23
The velocity curves imply Dark Matter must exist Since the
velocity doesnt fall off at larger distance as predicted, there
must be a lot of mass out there that we cant see. Since we cant see
it at any wavelength, we call it Dark Matter. It seems that dark
matter makes up about 90% of the mass of galaxies.
Slide 24
The dark matter surrounds galaxies While the luminous matter of
the disk may go out 50,000 to 75,000 lightyears from the center,
the spherical halo of dark matter may go out several hundred
thousand lightyears from the center.
Slide 25
What is Dark Matter? MACHOs: Massive Compact Halo Objects : old
lone white dwarfs, old lone neutron stars (non-pulsar),
non-accreting, lone black holes, brown dwarfs and free floating
planets WIMPs: Weakly Interacting Massive Particles: the neutrino
is a weakly interacting particle but it isnt massive. It must be
some other types of exotic elementary particles Most theories tell
us only a small fraction of the dark matter can be MACHOs. Most of
it is non-neutrino WIMPs.
Slide 26
Seeing MACHOs by gravitational lensing A number of searches
have been conducted and they find about as many as theory predicts.
Only enough to account for a few percent of the dark matter
Slide 27
Looking for WIMPs
Slide 28
Quasars and Active Galaxies Centaurus A Galaxy X-ray over
Visible
Slide 29
Quasars: QUAsi StellAr Radio Source Discovered in the 1950s and
60s, these objects are star-like in visible light but also bright
radio sources.
Slide 30
In visible light, quasars look like stars Diffraction spikes
come from the arms that hold the secondary mirror. They only appear
around bright point sources of light.
Slide 31
Quasars have LARGE redshifts A large redshift means they are
moving away at high speed. Edwin Hubble had discovered the universe
is expanding so this meant they were very far away
Slide 32
The HST has shown that quasars are associated with distant
galaxies
Slide 33
The Missing Link to Quasars: Seyfert Galaxies There are no
quasars nearby but there are active galaxies
Slide 34
Some large elliptical galaxies have enormous jets
Slide 35
When Jets Collide with intergalactic gas they produce radio
lobes
Slide 36
Blazars (BL Lacertae objects) are extremely luminous galaxy
cores with no spectral features NGC 3185
Slide 37
Blazars can change brightness in less than a day
Slide 38
How quickly something changes brightness gives clues to its
size If a blazar can change its brightness in less than a day, it
must be less than a lightday across.
Slide 39
The energy source of blazars, quasars and Seyfert galaxies is a
supermassive black hole
Slide 40
Difference between quasar and blazar is the view angle
Slide 41
Galaxy collisions can feed the monster black hole at the center
of a galaxy
Slide 42
Galactic Collisions also lead to Starburst Galaxies
