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Week 8
Galaxies
Reading:
Chapter 15, Sections 1, 3 (9 pages)
Galaxy/Not a Galaxy?
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Types of Galaxies
The Hubble Classification System:
• Spirals
• Barred Spirals
• Ellipticals
• Irregulars
Spiral Galaxies
Have nuclear bulges and spiral arms.
Spiral arms have clouds that are forming
new stars.
Three Sub-types:
Spiral galaxies
are classified
according to the
size of their
central bulge and
the tightness of
their arms.
Nuclear
Bulge
Spiral Arms
Sa
Big
Tightly
wound
Sb
Moderate
Sc
Small
Moderately
wound
Loosely
wound
Spiral Galaxies (Face-on)
Sa
Tight arms,
big bulge
Sb
Sc
Loose arms,
small bulge
Spiral Galaxies (Edge-on)
Sa
Tight arms,
big bulge
Sb
Sc
Loose arms,
small bulge
Face-on
Edge-on
Barred Spiral Galaxies
• Like normal spirals, but have central bars
• Bars are extended, linear bulges
• Half as common as “normal” spirals
SBa
SBb
SBc
Spiral Density Waves
Spirals arms like traffic jams – jam always
there, but different cars.
Stars and gas clouds rotate around galaxy
faster than the spiral density wave.
Clouds get compressed when passing through
wave; new star formation is triggered.
Spiral Density Waves
Elliptical Galaxies
Elliptical galaxies have no spiral arms and no
disk. They come in many sizes, from giant
ellipticals of trillions of stars, down to dwarf
ellipticals of less than a million stars.
Ellipticals also contain very little, if any, cool gas
and dust, and show no evidence of ongoing star
formation.
Elliptical Galaxies
Elliptical in shape
Mostly older stars (note yellowish color)
Seven Sub-types: E0, E1, E2, E3, E4, E5, E6, and E7
E0 – almost perfectly round
E3 – somewhat elongated
E7 – very elongated
Elliptical Galaxies
E0
E3
E6
Ellipticals are classified according to their
shape from E0 (almost spherical) to E7 (the
most elongated). Which class depends on
angle; E0 can look like E7 if seen “edge-on”.
Giant Elliptical Galaxies
~20x as
big as the
Milky Way
Dwarf Elliptical Galaxies
So few stars
that you can
see right
through them.
S0 (lenticular) and SB0 galaxies have a disk and
bulge, but no spiral arms and no interstellar gas:
Irregular Galaxies
Irregular galaxies have a wide variety of shapes. Both
these galaxies appear to be undergoing interactions with
other galaxies.
Irregular Galaxies
SMC
LMC = Large
Magellenic Cloud
Galaxy classification tutorial
www.galaxyzoo.org
Galaxy Zoo
How We Know that
Galaxies are Far Away
Cepheid variables
1912 – Henrietta Leavitt’s Cepheid PeriodLuminosity Relation
Cepheid Variables in Andromeda
1923 – Edwin
Hubble discovers
Cepheids in the
Andromeda
“Nebula”, M31.
It is 2.2 million
light-years beyond
the Milky Way
The Local Group
The Local Group is the cluster of
galaxies to which the Milky Way belongs.
• LG is relatively poor: ~40 galaxies
• Andromeda (M31): largest and most massive
• More than 1/3 are dwarf ellipticals
• 3 are spirals (MW, M31, M33)
• New dwarf ellipticals being discovered
The Local Group
Here is the
distribution of
galaxies within
about 1 Mpc of
the Milky Way.
A nearby galaxy cluster is
the Virgo cluster; it is much
larger than the Local Group,
containing about 3500
galaxies.
Very Distant Galaxies
Most galaxies are too far away to pick out their
Cepheid variables.
How do we measure distances to them?
Using atomic spectra, Doppler shifts and the
Hubble law.
Cosmic Fingerprints
Spectral lines are like fingerprints – they
identify the element that produces them.
We use these fingerprints to study the chemical
composition and distances of objects in space.
Doppler Shifts
A moving source of light appears to produce
different lines (fingerprints) than a stationary
source of light.
That is, a moving object’s fingerprints will
be shifted with respect to the fingerprints
from a stationary object.
This effect is called a Doppler Shift.
The Doppler Effect
Depends only on
the relative motion
of source and
observer.
If one is moving
toward a source of
radiation, the
wavelengths seem
shorter; if moving
away, they seem
longer.
Light moving towards you is blueshifted.
Light moving away from you is redshifted.
Very Distant Galaxies
Most galaxies are too
far away to pick out
their Cepheid variables.
Their distances are
instead found through
their Doppler shifts.
All galaxies seem to be
moving away from us,
with the redshift of their
motion correlated with
their distance:
Hubble’s Law
These plots show the relation between
distance and recessional velocity
The relationship (slope of the
line) is characterized by
Hubble’s constant H0:
The value of Hubble’s constant is currently
uncertain, with most estimates ranging
from 50 to 80 km/s/Mpc.
Note that the Universe is expanding. More
on this in the last week of class.