The matter in our Galaxy emits different kinds of
radiation.
The ground state in hydrogen is actually
two energy levels and if it is left alone
long enough…
Called the “spin flip” transition
l = 21 cm
And there’s lots of Hydrogen out there!!!
Radio observations help map the
galactic disk
• Looking for 21-cm
wavelengths of light …
– emitted by interstellar
hydrogen
– as we look along the
disk of the Milky Way
(from inside), we see
21-cm photons Doppler
shifted varying amounts
– this allows the
interstellar hydrogen to
be mapped
A Map of the Milky Way Based on
21-cm wavelength light mapping
Zone of
Avoidance
Spiral Galaxy M83 observed in both visible
light and radio wavelengths.
Don’t galaxies look like they spin?
d
d
We investigate the
“Rotation Curve”
of our Galaxy by
plotting the
velocity as a
function of radial
distance d from
the center.
Orbital Velocities in the Disk
rotation curve – a plot of rotational
speed
(orbital) speed .vs. distance from the center
radius
Do galaxies rotate like a Merry-Go-Round ?
(solid body rotation)
Differential Rotation
of the Galaxy
The Sun orbits at 230 km/s
or about 500,000 mph
Do they rotate like planets
in our solar system ?
(Keplerian falloff)
The Galaxy’s Rotation Curve
Most of the matter in the Galaxy
has not yet been identified
• According to Kepler’s
Third Law, the farther a
star is from the center,
the slower it should orbit
• Observations show that
speed actually increases
with distance from the
center
• This could be due to gravity from extra mass we
cannot see - called DARK MATTER.
Orbital Velocities in the Disk
Stars in the Galactic disk should orbit according to Kepler’s Laws
Here is what we observe:
• The flat rotation curve of our
Galaxy implies that:
• its mass in not concentrated in
the center
• its mass extends far out into the
halo
• But we do not “see” this mass
• we do not detect light from most
of this mass in the halo
• so we refer to it as dark matter
Zone of
Avoidance
Halo
The Very Large Array (VLA) in New Mexico
Center of the Galaxy
Radio
Although dark in visual light,
there are bright radio, IR, and
X-ray sources at the center of
the Galaxy, known as Sgr A*.
X-ray
Center of the Galaxy
in Sagittarius
Infrared
Visual
Center of the Galaxy
• We measure the orbits of fast-moving stars near the Galactic
center.
• these measurements must be made in the infrared
• in particular, this star passed within 1 light-day of Sgr A*
• using Kepler’s Law, we infer a mass of 2.6 million M for Sgr A*
• What can be so small, yet be so massive?
X-ray Flare from Sgr A*
• The rapid flare rise/drop time
(< 10 min) implied that the
emission region is only 20
times the size of the event
horizon of the 2.6 million M
black hole.
• Observations are consistent
with the existence of a
supermassive black hole at the
center of our Galaxy.
• Energy from flare probably
came from a comet-sized lump
of matter…torn apart before
falling beneath the event
horizon!
Chandra image of Sgr A*
The Star–Gas–Star Cycle
Halo vs. Disk Stars
• Stars in the disk are relatively young.
• fraction of heavy elements same as or greater than the Sun
• plenty of high- and low-mass stars, blue and red
• Stars in the halo are old.
• fraction of heavy elements much less than the Sun
• mostly low-mass, red stars
• Stars in the halo must have formed early in the Milky
Way Galaxy’s history.
• they formed at a time when few heavy elements existed
• there is no ISM in the halo
• star formation stopped long ago in the halo when all the gas
flattened into the disk
Stellar Orbits in the Galaxy
• Stars in the disk all orbit the
Galactic center:
• in the same direction
• in the same plane (like planets do)
• they “bobble” up and down
• this is due to gravitational pull
from the disk
• this gives the disk its thickness
• Stars in the bulge and halo all
orbit the Galactic center:
• in different directions
• at various inclinations to the disk
• they have higher velocities
• they are not slowed by disk as
they plunge through it
• nearby example: Barnard’s Star
Mass of the Galaxy
• We can use Kepler’s Third Law to estimate the mass
• Sun’s distance from center: 28,000 l.y. = 1.75 x 109 AU
• Sun’s orbital period: 230 million years (2.3 x 108 yr)
• P2 = 42/GM a3  mass within Sun’s orbit is 1011 M
• Total mass of MW Galaxy : 1012 M
• Total number of stars in MW Galaxy  2 x 1011
Galaxies
Galaxies seem to take one of four
different appearances
I. SPIRALS
SPIRALS
SPIRALS
The tightness of a spiral
galaxy’s arms is correlated to
the size of its nuclear bulge
Type Sa
Type Sb
Type Sc
Variety of Spiral Arms
Flocculent spirals
(fleecy)
Grand-design spirals
(highly organized)
We easily see these spiral arms
because they contain numerous
bright O and B stars which illuminate
dust in the arms.
However, stars
in total seem
to be evenly
distributed
throughout the
disk.
Spiral Structure
• Our Galactic disk does not appear solid.
• it has spiral arms, much like we see in other
galaxies like M51
• These arms are not fixed strings of stars
which revolve like the fins of a fan.
• They are caused by compression waves
which propagate around the disk.
• such waves increase the density of matter at
their crests
• we call them density waves
• they revolve at a different speed than
individual star orbit the Galactic center
M 51
• Note how the spiral arms appear bluer
compared to the bulge or the gaps
between the arms.
Compression Wave
Spiral Arms
• The compression caused by density waves triggers star formation.
• molecular clouds are concentrated in arms…plenty of source matter for stars
• short-lived O & B stars delineate the arms and make them blue & bright
• long-lived low-mass stars pass through several spiral arms in their orbits
around the disk
Galaxies seem to take one of four
different appearances
II. BARRED SPIRALS
BARRED SPIRALS
BARRED SPIRALS
Bars of stars run through the
nuclear bulges of barred spiral
galaxies
Type SBa
Type SBb
Type SBc
Galaxies seem to take one of four
different appearances
Type E0
Type E3
Type E6
III. ELLIPTICALS
ELLIPTICALS
Elliptical galaxies display a
variety of sizes and masses
• Giant elliptical
galaxies can be 20
times larger than the
Milky Way
• Dwarf elliptical
galaxies are
extremely common and
can contain as few as a
million stars
Galaxies seem to take one of four
different appearances
IV. IRREGULAR
Galaxies seem to take one of four
different appearances
• Spirals
• Barred Spirals
• Ellipticals
• Irregulars
This classification scheme is known as the
Hubble Tuning Fork Scheme