Download How do stars orbit in our galaxy?

Our Galaxy
The Milky Way Revealed
Our goals for learning
• What does our galaxy look like?
• How do stars orbit in our galaxy?
REVIEW:
Recall from our Introduction that stars are the building blocks
of huge conglomerations called galaxies. Our Sun is one star in
a huge assembly of stars called the Milky Way.
Living inside
the Milky Way
This hazy band of
light is our galaxy
seen from our
vantage point
inside it.
Galileo Galilei was
the first person to use
a telescope to resolve
the Milky Way into
numerous individual
stars.
All-sky picture taken with fish-eye lens.
All-Sky View
This projection shows the entire sky
A GALAXY is any gravitationally-bound conglomeration of billions of stars.
The galaxy containing our Sun is called the Milky Way and we can only see it
from the inside. It is a very large structure.
Edge-on, our galaxy looks like a flattened “disk” of stars
with a central concentration or “bulge” of stars. Note the
location of the Sun, very far from the Center. The disk is
1,000 light-years thick but 100x that is width – very thin!
Primary features: disk, bulge, halo, globular clusters
If we could view the Milky Way from above the disk, we would see
that it has Spiral Arms. Many other galaxies look the same, for
example the nearby spiral galaxy (M31) in Andromeda.
How do stars orbit in
our galaxy?
Stars in the disk all orbit in the same direction with a
little up-and-down motion (exaggerated here).
How do stars orbit in
our galaxy?
Disk plane
Center
Orbits of stars in
the Bulge and
Halo have random
orientations, even
crossing through
the disk plane.
Note: stars don’t collide – too small compared
to distance between them.
Thought Question
Why do orbits of Disk stars bob up and down?
A. They’re stuck to interstellar medium
B. Gravity of disk stars pulls toward disk
C. Halo stars knock them back into disk
Thought Question
Why do orbits of Disk stars bob up and down?
A. They’re stuck to interstellar medium
B. Gravity of disk stars pulls toward disk
C. Halo stars knock them back into disk
Disk of stars
The net gravitational effect of a large flat disk of mass is to
pull objects vertically toward the disk. Objects with a slight
motion out of the plane of the disk are pulled back.
Orbital Velocity Law
r´v
Mr =
G
2
• The orbital speed (v) and radius (r) of an object on a
circular orbit around the center of the galaxy tells us
the mass (Mr) within that orbit.
THE MASS OF
THE GALAXY
The Sun’s orbital
motion (radius
and velocity) tells
us the mass within
the Sun’s orbit:
~1.0 x 1011 MSun
What have we learned?
• What does our galaxy look
like?
• The Milky Way Galaxy
consists of a thin disk of
stars about 100,000 lightyears in diameter with a
central bulge and a spherical
region called the halo that
surrounds the entire disk.
The disk also contains the
gas and dust of the
interstellar medium,
while the halo contains very
little gas.
What have we learned?
• How do stars orbit in our
galaxy?
• Stars in the disk all orbit
the galactic center in
about the same plane and
in the same direction.
• Halo and bulge stars also
orbit the center of the
galaxy, but their orbits are
randomly inclined to the
disk of the galaxy.
Galactic Recycling
Our goals for learning
• How is gas recycled in our galaxy?
• Where do stars tend to form in our galaxy?
How is gas recycled in our galaxy?
Star-Gas-Star cycle: gas from old stars is recycled into new stars
Let’s follow the
sequence from here.
High-mass stars have strong stellar winds that blow
bubbles of hot gas …
Lower mass stars return gas to interstellar space
through stellar winds and planetary nebulae …
X-rays from
very hot gas
in Supernova
Remnants
reveal newlymade heavy
elements that
get thrown
back into the
interstellar
medium…
As the supernova
remnant cools it
begins to emit
visible light as it
expands.
New elements
made by the
supernova mix into
the interstellar
medium and we
detect them using
spectroscopy.
Multiple
supernovae
create huge hot
bubbles that can
blow out of the
galactic disk.
Gas clouds
cooling in the
halo can “rain”
back down on
disk.
Atomic hydrogen gas (H) forms as hot
gas cools, allowing electrons to join
with protons to make normal atoms.
Molecular clouds form next, after the
gas cools enough to allow atoms to
combine into molecules; mostly H2.
Molecular clouds in Orion
Composition:
• Mostly H2
• About 28% He
• About 1% CO
• Many other molecules
Gravity forms
stars out of
the gas in the
molecular
clouds, thus
completing
the star-gasstar cycle.
Radiation from
newly formed
stars is eroding
these starforming clouds.
Gas Cools
Summary of Galactic Recycling
• Stars make new elements by fusion
• Dying stars expel gas and new elements,
producing hot bubbles (~106 K)
• Hot gas cools, allowing atomic hydrogen clouds
to form (~100-10,000 K)
• Further cooling permits molecules to form
(mainly H2), making molecular clouds (~30 K)
• Gravity forms new stars (and planets) in
molecular clouds
Thought Question
Where will the gas be in 1 trillion years?
A. Blown out of galaxy
B. Still recycling just like now
C. Locked into white dwarfs and low-mass
stars
Thought Question
Where will the gas be in 1 trillion years?
A. Blown out of galaxy
B. Still recycling just like now
C. Locked into white dwarfs and low-mass
stars
1 trillion years = 1,000 billion years, much longer than the lifetime of the Sun
THE MILKY WAY AT DIFFERENT WAVELENGTHS
We observe star-gas-star cycle operating in Milky Way’s disk using
many different wavelengths from gamma rays to radio waves.
FOR EXAMPLE,
Radio (atomic hydrogen)
Visible
21-cm radio waves emitted by atomic hydrogen
show where gas has cooled and settled into disk
IR (dust)
Visible
Long-wavelength infrared emission shows where young
stars are heating dust grains inside molecular clouds
Visible
X-rays
X-rays are observed from very hot gas above and
below the Milky Way’s disk
Where do stars tend to
form in our galaxy? Ionization nebulae are
found around short-lived
high-mass stars, signifying
active star formation
Orion Nebula
Reflection nebulae scatter
the light from stars. The
radiation from less
massive young stars does
not ionize the cloud.
Why do reflection nebulae
look bluer than the nearby
stars?
For the same reason that
our sky is blue!
Blue light (shorter
wavelength) is “scattered”
more than red light.
Halo: No ionization nebulae, no blue stars
 no star formation
Disk: Ionization nebulae, blue stars  active star formation
Much of the star
formation in the
disk happens in the
Spiral Arms.
Ionization Nebulae
Blue Stars
Gas Clouds
Spiral Arms are best
explained as “density
waves” – a bunching up of
moving material as it
orbits the Galactic Center
Whirlpool Galaxy
Spiral arms are waves of star formation
1. Gas clouds get
squeezed as they
move into spiral
arms
2. Squeezing of clouds
triggers star
formation
3. Young stars flow
out of spiral arms
Density Wave: think of traffic! Cars on the freeway bunch up as they approach a slow
moving construction crew working along the hard shoulder. Bunching up is the density wave.
The wave keeps moving forward, and each car gets through … eventually.
What have we learned?
• How does our galaxy recycle gas into stars?
• Stars are born from the gravitational collapse of gas clumps in
molecular clouds. Near the ends of their lives, stars more massive
than our Sun create elements heavier than hydrogen and helium
and expel them into space via supernovae and stellar winds. The
supernovae and winds create hot bubbles in the interstellar
medium, but the gas within these bubbles gradually slows and
cools as they expand. Eventually, this gas cools enough to collect
into clouds of atomic hydrogen. Further cooling allows atoms of
hydrogen and other elements to collect into molecules, producing
molecular clouds.These molecular clouds then form stars,
completing the star–gas–star cycle.
What have we learned?
• Where do stars tend to form
in our galaxy?
• Active star-forming regions,
marked by the presence of
hot, massive stars and
ionization nebulae, are found
preferentially in spiral
arms. The spiral arms
represent regions where a
spiral density wave has
caused gas clouds to crash
into each other, thereby
compressing them and
making star formation more
likely.
The History of the Milky Way
Our goals for learning
• What clues to our galaxy’s history do halo
stars hold?
• How did our galaxy form?
Halo Stars:
Contain only 0.02-0.2% heavy elements (O, Fe, …)
only old stars, not much reprocessing by gas-star-gas cycle
Conclusion: Halo stars formed first then
stopped; disk stars formed later and kept
forming.
Disk Stars:
2% heavy elements,
stars of all ages, lots of reprocessing of material
How did our galaxy form?
Our galaxy probably formed from a really giant gas cloud
Halo stars formed first as gravity caused cloud to contract
Remaining gas settled into a spinning disk
Warning: This model
is oversimplified
Stars continuously form in the disk as galaxy grows older
Detailed studies: Halo stars formed in clumps that later merged to
form a single, larger protogalactic cloud. In other words, the early
Milky Way suffered collisions and mergers.
What have we learned?
• What clues to our galaxy’s history do halo
stars hold?
– Halo stars are all old, with a smaller
proportion of heavy elements than disk stars,
indicating that the halo formed first
• How did our galaxy form?
– Our galaxy formed from a huge cloud of gas,
with the halo stars forming first and the disk
stars forming later, after the gas settled into a
spinning disk
The Mysterious Galactic Center
Our Goals for Learning
• What lies in the center of our galaxy?
A BLACK HOLE!
The Galactic Center lies 28,000 light-years from the Sun, hidden by
layers of dusty spiral arms. Visible light cannot penetrate. But …
Infrared light from center
Radio emission from center
Zooming in …
Radio emission from center
Swirling gas near center
Strange radio sources in galactic center (Sgr A*).
Orbiting stars near center
Swirling gas near center
Infrared image obtained by
Adaptive Optics methods.
Location of Sgr A*
Stars at galactic center: we can now map out the motions
over many years, and use Kepler’s 3rd Law to get orbits.
Stars appear to
be orbiting
something very
massive but it is
invisible … a
black hole?
The orbits of
stars indicate a
mass of about
4 million MSun
We will see later that this is
not unusual; many galaxies
harbor very massive Black
Holes.
GALACTIC CENTER
X-ray image from
the Chandra X-ray
telescope in space
Further
Evidence:
X-ray flares from
the Galactic
Center suggest
that the tidal
forces of the
suspected black
hole occasionally
tear apart chunks
of matter that are
about to fall in.
What have we learned?
• What lies in the center of our galaxy?
– Orbits of stars near the center of our galaxy
indicate that it contains a black hole with
about 4 million times the mass of the Sun
– This is evidence for black holes much larger
than stellar-mass objects