Transcript solar.gmu.edu

ASTR 113 – 003
Lecture 09 March 29, 2006
Spring 2006
Introduction To Modern Astronomy II
Review (Ch4-5): the Foundation
1.
2.
3.
4.
5.
6.
7.
Sun, Our star (Ch18)
Nature of Stars (Ch19)
Birth of Stars (Ch20)
After Main Sequence (Ch21)
Death of Stars (Ch22)
Neutron Stars (Ch23)
Black Holes (Ch24)
Star (Ch18-24)
Galaxy (Ch 25-27)
Cosmology (Ch28-29)
Extraterrestrial Life (Ch30)
1. Our Galaxy (Ch25)
2. Galaxies (Ch26)
3. Active Galaxies (Ch27)
1.
2.
Evolution of Universe (Ch28)
Early Universe (Ch29)
ASTR 113 – 003
Lecture 09 March 29, 2006
Our Galaxy
Chapter Twenty-Five
Spring 2006
Guiding Questions
1. What is our Galaxy? How do astronomers
know where we are located within it?
2. What is the shape and size of our Galaxy?
3. How do we know that our Galaxy has spiral
arms?
4. What is most of the Galaxy made of? Is it stars,
gas, dust, or something else?
5. What is the nature of the spiral arms?
6. What lies at the very center of our Galaxy?
Short Movies
Galaxy: The Milky Way
Galaxies
Note: the movies can only run in-class
Milky Way
• Milky way, the band of light, is caused by the shining of
numerous stars in our own disk-shaped galaxy
• Milky way galaxy has about 200 billion stars
• The dark streaks across the milky way is due to the
presence of Interstellar dust within the disk, which causes
interstellar extinction
• Interstellar extinction makes distant objects dim or
dissapper
• In visible light, we could only see the nearby star
The Sun’s location in the Galaxy?
• Counting the stars in all directions, with the largest
number in the direction of the center
• It was thought that the Sun was in the center of the
galaxy, according to Herschel’s map that showed roughly
equal number of stars in all direction
• However, this bias is caused by interstellar extinction
Herschel’s Map
(1785)
The Sun’s location in the Galaxy?
• Counting the number of globular clusters
• A globular cluster is a spherical distribution of roughly 1
million stars packed in a small size; it is bright
• Many globular clusters exist in the halo outside the
obscuring disk
The Sun’s location in the Galaxy?
• The distance of globular cluster is determined using the
pulsating variable stars, Cepheid variables and RR Lyrare
variables.
• There is a well known period-luminosity relation for
variables
The Sun’s location in the Galaxy?
• Globular clusters forms a huge spherical distribution
centered not on the Earth but rather about a point in the
direction of Sagittarius
• Our Sun lies within the galactic disk, some 8000 pc
(26,000 ly) from the center of the Galaxy
The shape of the Galaxy
• Observations at non-visible wavelengths reveal the shape
of the galaxy
• The longer the wavelength, the less the amount of
interstellar extinction, because the dust scattering is less
efficient in longer wavelength
• Infrared wavelength can see farther into the plan of the
Milky way
• Radio wave can see through the entire disk of the Milky
way
The shape of the Galaxy
• Far-infrared light (~ 50 μm) is directly from the black-body
emission of the interstellar dust (~ 50 Kelvin)
• Near-infrared light (~ 2 μm) is from emission of cool stars,
including red giants
The shape of the Galaxy
•
There are
three
geometric
components
of our Galaxy
1. A Disk
2. A central
bulge
3. A Halo
The shape of the Galaxy
1. Disk
• About 50 kpc (160,000 ly) in diameter and about
600 pc (2000 ly) thick
• with a high concentration of interstellar dust and gas
in the disk
• Contains metal-rich (Population 1) stars
• The Sun, at 8 kpc, orbits around the center of the
Galaxy at a speed of about 220 km/s
• It takes about 220 million years to complete one
orbit
The shape of the Galaxy
2. Central Bulge
– The galactic center is surrounded by a large
distribution of stars called the central bulge
– About 2 kpc (6500 ly) in diameter
– Contain a mixture of Population 1 and
Population II stars
– This bulge is not perfectly symmetrical, but
may have a bar or peanut shape
The shape of the Galaxy
3. Halo
– The disk of the Galaxy is
surrounded by the galactic
halo
– Halo is composed almost
exclusively of old, metal
poor (Population II) stars
– These old stars are in
globular clusters or
isolated
The shape of the galaxy NGC 4565
• If we could view our Galaxy edge-on from a great
distance, it would look like this galaxy (NGC 4565, 15
million pc (50 million ly away from us).
ASTR 113 – 003
Lecture 10 April 5, 2006
Spring 2006
Introduction To Modern Astronomy II
Review (Ch4-5): the Foundation
1.
2.
3.
4.
5.
6.
7.
Sun, Our star (Ch18)
Nature of Stars (Ch19)
Birth of Stars (Ch20)
After Main Sequence (Ch21)
Death of Stars (Ch22)
Neutron Stars (Ch23)
Black Holes (Ch24)
Star (Ch18-24)
Galaxy (Ch 25-27)
Cosmology (Ch28-29)
Extraterrestrial Life (Ch30)
1. Our Galaxy (Ch25)
2. Galaxies (Ch26)
3. Active Galaxies (Ch27)
1.
2.
Evolution of Universe (Ch28)
Early Universe (Ch29)
The shape of the Galaxy
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There are
three
geometric
components
of our Galaxy
1. A Disk:
~ 50 kpc
2. A central
bulge:
~ 2 kpc
3 A Halo
The Structure of the Disk
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The disk is not uniform
The disk has several
spiral arms
The Sun is located just
outside the Orion arm
A spiral arm is a
concentration of stars,
dust and gas
Observation at radio wavelength at 21 cm
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The 21-cm radio emission is from neutral hydrogen
When the spin of the binding electron flips, it emits a tiny
amount of energy, the radio photon at wavelength 21-cm
Radio observations can see through the whole galaxy,
because interstellar dust does not scatter radio waves
The Map of Galaxy at 21-cm
21-cm observation maps the 2-D disk
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Along one line of sight, hydrogen gas at different
distances has different velocity with respect to the
observer
Doppler shift reveals the velocity, thus the distance
Spiral structure of the disk
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The 21-cm observations map the distribution of neutral
hydrogen in the disk, revealing the spiral structure
What are in the Spiral Arm?
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Observations indicate spiral arms are concentration of
1. Young, hot and luminous O and B stars (in blue color)
2. HII regions, or emission nebulae (in red color) surrounding O,B stars
3. Neutral hydrogen gas (HI) that gives rise to star formation
•
Spiral arms are star-forming region
The Cause of Spiral arms
• Spiral arms are not caused by winding rotation of
assemblages of stars
• If they were, spiral arms would “wind up” too much and
disappear in a few hundred million years
Spiral arms are caused by density waves
• spiral arms are created by density waves that sweep
around the Galaxy
• A density wave is a slow-moving region where stars,
gas, and dust are more densely packed than in the rest
of the galaxy
• A density wave is like a cosmic traffic jam
Spiral arms are caused by density waves
• Interstellar matter or
gas moves around
the galactic center
rapidly
• It slows down and is
compressed as it
goes through the
slow-moving spiral
arms
• The high density of
gas in the spiral arms
triggers star formation
Dark Matter and Rotation Curve
•Kepler’s third law: period square = radius cube
•Modified Kepler’s Law (in Newton’s form): the total mass
of matter can be inferred from the period and distance of
an orbiter
•Or it can be inferred from the distance and velocity of an
orbiter
•E.g., The sun is 8 kpc from the center of the Galaxy with
a rotation speed of 220 km/s, the inferred mass of Galaxy
within the Sun’s orbit is about 1012 M
•By studying the rotation speed with distance, mass
distribution in the galaxy can be inferred
Dark Matter and Rotation Curve
• The orbital speed of stars and gas is almost uniform
throughout the Galaxy
Dark Matter and Rotation Curve
• The orbital motion is not a solid body; solid body
rotation implies faster speed in larger distance
Dark Matter and Rotation Curve
• Rotation curve is the distribution of the orbital speed
of stars and gas with respect to the distance from
galactic center
Dark Matter and Rotation Curve
• The orbital motion is not the Kepler orbits like in the
solar system; Kepler orbit means slower velocity at
larger distance
Dark Matter and Rotation Curve
• If the mass of Galaxy is from the matter that emits
light, the orbital speed would decline at larger
distance
• But observations show a flat rotation curve at large
distance
• It implies that most of the mass in the Galaxy is in
the form of dark matter, emitting no light at all
• Stars, gas and dust account for only about 10% of
the Galaxy’s total mass
The Center of the Galaxy
• The innermost part of the Galaxy, or galactic nucleus,
is located in Sagittarius, and 8 kpc from the Sun
• The center is close to Sagittarius A, a strong radio
source
The Center of the Galaxy
• Sagittarius A is one of the strongest radio source in the sky
• On either side of Sagittarius A, show lobes of hot gas as
seen in X-ray
The Center of the Galaxy
•The motion of star S2 is 5000 km/s, or 2% of speed of light
•This speed infers a central mass of 3.7 × 106 M, indicating the
presence of a supermassive black hole at the center
Key Words
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central bulge (of a galaxy)
dark matter
density wave
disk (of a galaxy)
far-infrared
flocculent spiral galaxy
galactic nucleus
galaxy
globular cluster
grand-design spiral galaxy
HI
halo (of a galaxy)
high-velocity star
interstellar extinction
Local Bubble
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magnetic resonance imaging (MRI)
massive compact halo object
(MACHO)
microlensing
Milky Way Galaxy
near-infrared
rotation curve
RR Lyrae variable
Sagittarius A*
self-propagating star formation
spin (of a particle)
spin-flip transition
spiral arm
21-cm radio emission
weakly interacting massive particle
(WIMP)
winding dilemma