Transcript The Origin of the Milky Way

H205
Cosmic Origins
APOD
Today: The Milky Way (Ch. 19)
Hand in EP3
More Opportunities…
• Dava Sobel, the author of the best-selling books
LONGITUDE and GALILEO'S DAUGHTER, will speak
at the auditorium at Ivy Tech Indianapolis on April 18,
at 3 PM.
• If you plan to attend, please RSVP to:
John J. Cooney, MA, MBA
Humanities Program Chair
Ivy Tech Community College-Central Indiana
50 W. Fall Creek Pkwy, North Dr., NMC 562
Indianapolis, IN 46208-5752
(317) 916-7930
[email protected]
www.ivytech.edu/indianapolis
April Foolery
• 1976: The British astronomer Patrick Moore announced
on BBC Radio 2 that at 9:47 AM a once-in-a-lifetime
astronomical event was going to occur that listeners
could experience in their very own homes. The planet
Pluto would pass behind Jupiter, temporarily causing a
gravitational alignment that would counteract and lessen
the Earth's own gravity. Moore told his listeners that if
they jumped in the air at the exact moment that this
planetary alignment occurred, they would experience a
strange floating sensation. When 9:47 AM arrived, BBC2
began to receive hundreds of phone calls from listeners
claiming to have felt the sensation. One woman even
reported that she and her eleven friends had risen from
their chairs and floated around the room.
Introducing: The Milky Way
Our
very
own
spiral
galaxy
Four
Galaxies
similar
to the
MW
Barred
spirals
(seen
face-on)
The Andromeda
Galaxy
Almost a twin of
the Milky Way…
Just bigger
Exploring the Milky Way
Virtual Reality All-Sky Milky Way Panorama
The Milky Way….
Halo
Halo
Bulge
Disk
Dwarf Spheroidal Companions
Dark Matter Corona
Where is the Center of
the Milky Way?
• The discovery of
Measuring
the
certain types of
variable stars allowed True Size and
Harlow Shapley to
Shape of the
determine the
Milky Way
distances to globular
star clusters
• Globular clusters
concentrate near the
center of the galaxy.
Key Ideas
The Milky Way is a spiral galaxy
The Galaxy is shaped like a disk
The Sun is located at the inner edge of a
spiral arm about 2/3 of the way out from the
center to the edge
The main components of the Galaxy are the
disk, the bulge, and the halo
We can measure the mass of the Galaxy from
the orbits of stars
Milky Way Factoids
• The Sun orbits the center of the Galaxy
– moving in a direction toward Cygnus
– speed of about 220 kilometers/second
– One orbit takes about 240 MILLION
years
• The mass of the Galaxy is about 2x1011
times the mass of the Sun
The Milky Way – Review Vital Stats
• Consists of 100 billion stars.
• Stars are distributed in a central bulge,
a huge disk, and a galactic halo
surrounding both.
• The diameter of the disk is 30kpc
(100,000 light years).
• The thickness of the disk is only 300pc
(1000 light years) on average.
• The total detectable mass is 200 billion
solar masses.
Galactic Inhabitants
• Stars (of all masses) – disk/bulge
• Star clusters – open clusters (near the disk),
globular clusters in the halo
• Clouds – giant molecular clouds, H I clouds, H II
regions (disk)
• Nebulae – reflection nebulae, emission nebulae,
nova/supernova remnants, planetary nebulae –
disk
• The Halo
– Is virtually gas free which implies little star
formation and chemical enrichment
– Consists mostly of old, metal-poor stars (for
example, in the globular clusters)
• The Disk
– Gas – ionized hydrogen (H II), atomic hydrogen (H I),
molecular hydrogen (densest and coldest), stellar
ejecta
– Dust
– Stars and failed stars (brown dwarfs)
– Stellar remnants – black holes, neutron stars, white
dwarfs
Structure
of the Disk
The Milky Way
Origin of the Milky Way I
A huge, millionparsec-sized blob of
gas begins to
contract under
gravity. The first
stars and star
clusters form.
Origin of the Milky Way II
The rotating cloud of
gas begins to contract
toward its equatorial
plane.
Origin of the Milky Way III
Stars and clusters are
left behind in the
“halo” as the gas cloud
flattens.
Origin of the Milky Way IV
Stars and clusters
formed in the
flattened cloud
remain in the newly
formed “disk”
Origin of the Milky Way V
The disk is now very
thin, and the bulge has
formed
Throughout the process
smaller galaxies are
cannibalized as the Milky
Way grows
Origin of the Milky Way
A huge, million-lightyear-sized blob of gas contracts
under gravity. The first stars and star clusters form.
The rotating cloud of gas contracts toward its equatorial
plane.
The disk becomes very thin, and a “bulge” forms in the
center
The Milky Way
Is Still Growing
• Nearby dwarf galaxy
discovered in 1994 in
the direction of
Sagittarius
• Discovered by radial
velocity
• Distance about 88,000
light years
• Merging with the Milky
Way
Sagittarius
Tidal Stream
• Orbits the Milky Way
• Orbital period about a billion years
• “Tidal stream” of stars from Sagittarius circles the
Milky Way
• Sagittarius may contain significant dark matter
Yet Another New Galaxy!
• Canis Major Dwarf
• Nearest galaxy to the Milky Way (yet
discovered…)
• 25,000 light years from the Sun
• 44,000 light years from the center of the
Milky Way
• Discovered with IR light (hidden behind
dust in the MW’s disk)
Tidal Streams from CMa Wrap
around the Milky Way
Galaxies evolve
and change
Rotation and spiral structure
Galaxy interactions
Galactic recycling
Galactic Recycling – the
Star-Gas-Star Cycle
• The Galaxy
recycles
gas from
old stars
into new
star
systems
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
Multiple
supernovae
create huge
hot bubbles
that can blow
out of disk
Gas clouds
cooling in the
halo can rain
back down on
disk
Atomic hydrogen gas forms
as hot gas cools, allowing
electrons to join with protons
Molecular clouds form next,
after gas cools enough to allow
to atoms to combine into
molecules
Gravity
forms stars
out of the
gas in
molecular
clouds,
completing
the stargas-star
cycle
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, making molecular clouds (~30 K)
• Gravity forms new stars (and planets) in
molecular clouds
Measuring the Mass of the MW
• The force between two bodies is proportional to 1/r2
GM 1m2
Fg 
r2
• The acceleration of a body in circular orbit is given by:
a = v2/r
where v is the body’s orbital speed
• Set F = m2v2/r equal to F = GM1m2/r2 and solve for M1
GM1m2
v2
F 
 m2 a  m2
2
r
r
M1 =
2
(v r)/G
This expression relates the orbital velocity
and distance to the total mass within the
orbital radius for a star orbiting in the
Galactic gravitational potential.
Measuring the Mass of the MW
• The Galactic rotation curve measures
the orbital speed of stars around the
center of mass of the Milky Way
The “Missing Mass”
• Stars in the outer reaches of the
Galaxy orbit faster – as if the mass of
the Galaxy continues to increase. Even
when there is no visible matter in the
outer Galaxy…
• Dark Matter!
The Galactic Center!
At visual wavelengths,
this region is totally hidden
from us by gas and dust
that dim the light by a factor
of 10 billion!
The Galactic Center in the Near
Infrared
We can see through
the gas and dust, to
observe many of the
stars near the
Galactic center. But
the Galactic center
itself remains
undetected in the
infrared.
The Galactic Center Further in
the Infrared
Here we see not
only stars, but the
warm gas that
glows in the
infrared.
Galactic Center at Radio
Wavelengths – It’s a MESS!
•Sgr A is bright!
•Supernova remnants
•Arcs and threads
The Galaxy hosts a
super-massive black
hole at its center!
“A supermassive black hole at the
center of our galaxy is adequate
to explain the observations that
have been seen.”
Orbit of star S2 (followed for
ten years) around the central
mass is consistent with a 2.63.3 million solar mass object
within 10 light days of Sgr A*
Galactic Center Research at MPE
The Galactic Center in X-rays
This false-color image of the
central region of our Milky
Way Galaxy was made with
the Chandra X-ray telescope.
The bright, point-like source
at the center of the image
was produced by a huge X-ray
flare in the vicinity of the
supermassive black hole.
Galactic Center Detected in Infrared Light!
• Seen with ESO Very
Large Telescope (8-m)
and Keck 10-m
Telescope
• Flares in infrared light
• Within 10
Schwarzschild radii of
the black hole
• Cause still unknown
Key Ideas – The Galactic Center
• Powerful radio source
• Stars very densely packed
• Surrounded by ring of molecular gasempty in the center
• Central object is small – less than 4 AU
• Stars near center moving rapidly
• Black Hole! – 2 million times the mass of
the Sun
Black Holes
in the
Centers of
MOST
Galaxies
Left: Image of galaxy NGC4261, 45 million light years
from Earth. The orange part is radio signals represented
in false color.
Right: Hubble's space telescope image of the same
galaxy. It is suspected that there is a black hole at the
center of this image.
For Next Week
The Origin and Evolution of Galaxies
Chapter 20 – Galaxies
Chapter 21 – Galaxy Evolution