Milky Way I

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Transcript Milky Way I

A105
Stars and Galaxies
Today’s APOD
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This week’s units: 70, 71, 72, 73
News Quiz Today
Milky Way homework due Thursday
3 observing events
Announcements…
• Kirkwood Obs. open
–Weds night 7-9 PM
• Rooftop Thursday 8 PM
• Transit of Mercury Weds
–2:15 – 5:15
–Kirkwood & Sample Gate
Transit of
Mercury
• Or view it online at:
www.exploratorium.edu/transit/
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
certain types of
variable stars allowed
Harlow Shapley to
determine the
distances to globular
star clusters
• Globular clusters
concentrate near the
center of the galaxy.
Measuring the
True Size and
Shape of the
Milky Way
Globular Cluster Locations
The Sun
15
10
The Galactic
Center
Kiloparsecs
5
The distance to
the Galactic
Center is about 8
kiloparsecs
(8,000 parsecs),
or about 26,000
light years
0
-5
-10
-15
-20
-20
-15
-10
-5
0
Kiloparsecs
5
10
15
20
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
THE MILKY
WAY
Draw two sketches of the Milky Way
from different perspectives
Add as many details as you can
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.
Our Local Neighborhood
• The Sun is located on the outskirts.
• Most stars in our neighborhood are cool, low mass
stars
• There are no massive (O or B) stars nearby
• The interstellar gas in our neighborhood is still quite
hot  It is a graveyard of recent massive star deaths.
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
Stellar “Populations”
Stars formed at a particular “epoch” of the
history of the Milky Way share similar
properties such as composition and motion.
Structure of
the Disk
What Sustains the Spiral
Pattern?
• The stars in the galaxy revolve around the
galactic center due to gravity
• The galactic disk rotates differentially – stars
near the center move faster than those farther
away
• This produces a spiral structure but should
quickly wind up after a few rotations and
disappear
• What then preserves the arms?
One origin of density waves
The Density Wave Theory
• According to this theory, the spiral arms are not
material but are rather waves or ripples which
cause greater concentrations of stars and
interstellar dust and gas.
• Stars move in special, precessing elliptical orbits
which gives rise to the spiral pattern.
• The spiral pattern moves as a shock through the
galaxy, compressing gas into molecular clouds,
promoting star formation at the trailing edges.
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
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
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
 Read Units 70, 71, 72, 73
 Milky Way HW Due THURS.
 3 observing events available