The Milky Way II
AST 112
Interstellar Medium
• The space between stars is not empty!
– Filled with the Interstellar Medium (ISM)
• Star formation is not random
– Gas-rich regions: MORE
– Gas-poor regions: LESS
• The ISM doesn’t just sit there!
Star-Gas-Star Cycle
• Matter in the Milky Way goes in cycles
– Matter is taken out of the ISM when stars are born
– Matter is altered and returned to the ISM
throughout the star’s life and when it dies
– This is called the Star-Gas-Star Cycle
Composition of the Milky Way
• Heavy elements seem to have mixed
thoroughly into the Milky Way
• Supernovae eject heavy elements at greater
than the Milky Way’s escape velocity
– They’re somewhat localized
• So how do we get good mixing?
Star-Gas-Star Cycle
Star-Gas-Star Cycle
• Stars return material to
ISM by:
– Stellar wind
– Blowing up
Star-Gas-Star Cycle
• Material from a
supernova collides with
ISM material
– Forms hot shock waves
– Gradually slows, cools
Bubbles and Superbubbles
• Supernova expands, ISM
“piles up” at the edge
• Can grow to 100 LY before
slowing, merging with
ISM
• “Superbubbles” happen
when cluster stars die at
the same time
– Can grow to thousands of
LY
Galactic Fountain
• What happens when
the bubble becomes
larger than the disk?
– Blasts out of the Milky
Way’s disk
– Gas cools, rains back
down
– “Galactic fountain”
mixes heavier elements
into entire galaxy
Galactic Fountain
Galactic Fountains
Credit: ESA
Hydrogen Clouds
• Milky Way is full of warm (neutral) hydrogen
gas
– Occasionally find cold molecular hydrogen clouds
– Often find hot, ionized hydrogen clouds
• How do we map hydrogen in the Milky Way?
Warm Hydrogen Clouds: 21 cm Line
• Consider a hydrogen atom.
– The proton is spinning and so is the electron.
– Parallel spins have a higher energy than antiparallel spins
– Parallel spins can suddenly switch to anti-parallel
spins and emit radio waves
• Wavelength of these waves is 21 cm
Warm Hydrogen Clouds
Distribution of atomic H gas is shown by
mapping the 21 cm hydrogen line.
Atomic Hydrogen Clouds
• The disk is full of hydrogen gas. The bulge
doesn’t have much.
• We see hydrogen clouds at high latitudes that
are falling down onto the disk
– This supports the galactic fountain idea!
• Hydrogen stays in clouds for a long time, cools
Aside: Magellanic Stream
• The Magellanic Stream is a trail of hydrogen
gas that starts at the Magellanic clouds and
ends at the Milky Way
Star-Gas-Star Cycle
• Hydrogen cools and forms
H2 (molecular clouds)
• Heavy, dense; settle to
middle of Milky Way
• Can see this!
Molecular Hydrogen Clouds
• But as we know…
• Molecular clouds continue to collapse
– They heat
– They fragment
– They form stars!
Star Formation in the Milky Way
• Star forming regions are indicated by:
• Open clusters of young blue stars
– Hot massive stars don’t last long
– Don’t have time to stray far from their “brothers and
sisters”
• Ionized hydrogen that is glowing from collapse and
newborn stars
– Glows red
– Called HII (“H two”) regions
Star Formation in the Milky Way
HII Region (Rosette Nebula)
NGC 265 (Open Cluster)
Star-Gas-Star Cycle
• So the enriched gas has turned into stars.
• They will explode, supplying enriched material
and gas to the ISM.
• This material expands, cools, and collapses back
into stars…
• … and on and on and on.
Star-Gas-Star Cycle
Star-Gas-Star Cycle
• It can’t go on forever.
– Brown dwarfs don’t explode
– White dwarfs, neutron stars and black holes aren’t
turning into gas anytime soon (i.e. EVER)
– The Milky Way is therefore running out of the gas
that stars are born from
Star formation in the Milky Way will decrease
over the next 50 billion years, at which point it
will cease entirely.
At the center of our galaxy lurks…
… a teapot!
The Center of the Milky Way
• The bulge looks like it’s at the center
– So, just more bulge stars?
• Use radio, infrared and see!
The Center of the Milky Way
• IR: stars and gas within
1000 LY of center
The Center of the Milky Way
• Radio: magnetic fields
and turbulence
The Center of the Milky Way
• More radio: gas is
swirling around a
strong, unusual radio
source
• Source is called
Sagittarius A*
The Center of the Milky Way
• IR: stars shown within 1
LY of Sagittarius A*
(arrows)
• Several hundred stars in
this area
The Center of the Milky Way
• We can again use Kepler’s 3rd Law to measure
the mass of the object that these stars are
orbiting
• Assume a mass of 100 suns and a distance
from the Milky Way’s center of 0.017 LY
(about 1000 AU)
• What is the orbital period?
The orbital period of the star in question
(called S2) has been precisely measured to be
15.8 years!
Comparison of S2 and Pluto Orbits
• So this star is 30x
farther from the center
of its orbit than Pluto…
• And completes its orbit
about 16x faster!
Center of the Milky Way
• So there are several
hundred stars in the
center of the Milky Way
• Stars orbit Sagittarius A*:
– NOT consistent with a
central mass of a few
hundred stars!
– ARE consistent with a
central mass of a few
MILLION stars – but they
aren’t there!
Center of the Milky Way
A supermassive black hole (SMBH) is thought
to live at the center of the Milky Way. Its mass
is calculated at 3-4 million solar masses.
Center of the Milky Way
The circled dot is an
x-ray flare.
Any ideas why these
might occur here?