12
The Milky
Way Galaxy
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CHAPTER OUTLINE
1.
2.
3.
4.
Discovery of the Galaxy
Spiral arms and star formation
Origin and history of the Galaxy
The nucleus of the Galaxy
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ASTRO3 | CH12
2
The Great Star System
LO 1
• The Herschels concluded that:
• The star system has a disk shape and consists of
some noticeable holes lacking stars around its
edges
• The Sun and Earth are near the center of the
star system
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ASTRO3 | CH12
3
Finding the Size of the Milky Way
LO 1
• Cepheid variable stars: Variable stars with
pulsation periods of 1 to 60 days
• Period of variation is related to their luminosity
• Helped in measuring the size of the Galaxy
• Instability strip: Region of the H–R diagram in
which stars are unstable to pulsation
- Star evolving through this strip becomes a
variable star
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Finding the Size of the Milky Way (continued)
LO 1
• Period–luminosity relation
• Relation between period of pulsation and
intrinsic brightness among Cepheid variable
stars
• Massive stars are larger and pulsate slower
• Lower-mass stars are less luminous and pulsate
faster as they are smaller
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LO 1
Star Clusters and the Center of the Galaxy
• Astronomer Harlow Shapley noticed that:
• Different kinds of star clusters have different
distributions in the sky
• Globular clusters were more common toward
the constellations Sagittarius and Scorpius than
the open clusters
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LO 1
Measuring the Distances to Globular Clusters
• Globular clusters are far away to have
measurable parallaxes, but they contain
variable stars
• Average distances of a few nearest Cepheids can
be found with the help of proper motion
- Proper motion: Measures the rate at which a
star moves across the sky in arc seconds per
year
• Astronomers suspect that some of the faint
patches of light visible through telescopes are
other galaxies
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Globular Clusters
LO 1
Shapley plotted the direction and distance to the globular clusters
and found that they form a great swarm that is not centered on the
Sun
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LO 1
Components of the Galaxy
• Disk component
• Material confined to the
plane of the Galaxy
• Spherical component
• Includes all matter in the
Galaxy
• Scattered in a spherical
distribution around the
center (the halo and
central bulge)
The face-on and edge-on view
of the Milky Way Galaxy
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9
Mass of the Galaxy
LO 1
• Determined by noticing the orbital motions
of the stars within the Galaxy
• Every star in the Galaxy follows an orbit around
the center of mass of the Galaxy
• Stars follow parallel circular orbits in the disk of
the Galaxy
• Astronomers believe that the disk rotates
- Rotation allows an estimate of the mass of
the Galaxy
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10
Rotation of the Galaxy and the Orbits of the
Stars within the Galaxy
LO 1
• Astronomers find the orbits of stars by
studying their movement
• Velocity of the star perpendicular to the
radial motion can be found by measuring:
• The distance to the star
• The proper motion of the star
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LO 1
Mass of the Galaxy (continued 1)
• Orbital motion of the Sun helps find the
mass of the Galaxy inside the Sun’s orbit
• Sun completes a single orbit in about 225 million
years
• Milky Way Galaxy must have a mass of
more than 100 billion solar masses
• Based on the assumption that the Sun and the
center of mass of the Galaxy are two objects
orbiting each other
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Mass of the Galaxy (continued 2)
LO 1
• Not concentrated at its center
• If most of the mass were inside the orbit of the
Sun, then orbital velocities should decline at
greater distances
- Observations conclude that velocities do not
decline and actually increase at a greater
distance
▸ Hence, larger and larger orbits enclose more and
more mass
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13
Spiral Arms
LO 2
• Contain swarms of hot, blue stars; clouds of
dust and gas; and young star clusters
• Involve star formation
• Can be studied through locating the stars
• Stars are detected by finding their distances with
the help of spectroscopic parallax
• Spiral tracer: Object used to map the spiral
arms
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14
Spiral Tracers
LO 2
• Youth of spiral tracers provides information
about spiral arms
• Suggests active star formation
• Radio maps show that:
• Spiral arms are rather irregular and interrupted
by branches
• Material needed to make stars, hydrogen gas, is
abundant in spiral arms
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15
Star Formation in Spiral Arms
LO 2
Spiral density
wave theory
• Proposes that spiral arms are waves
of compressions that move around
the Galaxy triggering star formation
Self-sustaining
star formation
• Process by which the birth of stars
compresses the surrounding gas
clouds and triggers the formation of
more stars
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16
LO 3
Age of the Galaxy
• Age of the disk of the Galaxy - At least 9
billion years
• Finding the age of clusters is difficult
because:
• Clusters differ slightly in chemical composition
• Astronomers must know the distance to a
cluster
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LO 3
Stellar Populations
Population I stars
• Stars with significant amounts of atoms heavier than
helium
• Relatively young stars mostly found in the galactic disk
Population II stars
• Stars poor in atoms heavier than helium
• Relatively old stars mostly found in the halo, globular
clusters, or the central bulge
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Population I Stars and Population II Stars
LO 3
Population I stars
Population II stars
• Known as disk population
stars
• Have circular orbits in the
plane of the Galaxy
• Example - Sun
• Known as halo population
stars
• Have randomly tipped
orbits with a wide range of
shapes
• Example - Lower-mass
main- sequence stars or
giants
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19
LO 3
The ElementBuilding Cycle
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The Element-Building Cycle (continued)
LO 3
• Gas from which the Galaxy originally
condensed must have contained about 90
percent hydrogen atoms and 10 percent
helium atoms
• First stars to form from this gas were metal poor
- Evolved and died
• Succeeding generations of stars that formed
from gas clouds were more enriched
• By the time the Sun formed, the elementbuilding process had added about 2 percent
metals
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21
LO 3
Top-Down Galaxy
Formation
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22
Nucleus of the Galaxy
LO 4
• According to Harlow Shapley’s study of
globular clusters, Sagittarius was the center
of the Galaxy
• High-resolution radio maps revealed
Sagittarius A* lying at the expected location
of the galactic core
• Sagittarius A*: Powerful radio source located at
the core of the Milky Way Galaxy
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23
LO 4
Points to Remember about Sagittarius A*
• Observations at radio wavelengths reveal
complex structures near Sgr A*
• Caused by magnetic fields and rapid star
formation
• Tremendous numbers of stars plus radiation
from Sgr A* heat the dust, producing strong
infrared emission
• Sgr A* - Supermassive black hole into which
gas is flowing
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24
Observations of the Nucleus
LO 4
• Milky Way Galaxy’s nucleus is home to a
supermassive black hole
• Sgr A* is not as bright in X-rays as it should
be if it had a hot accretion disk with matter
constantly flowing into the black hole
• Observations of X-ray and infrared flares
lasting only a few hours
• Suggested that mountain-size blobs of matter
may occasionally fall into the black hole and be
heated and ripped apart by tidal forces
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KEY TERMS
•
•
•
•
•
•
•
•
Kiloparsec (kpc)
Cepheid variable stars
Instability strip
Period–luminosity
relation
Proper motion
Calibrate
Disk component
Spiral arms
•
•
•
•
•
•
•
Spherical component
Halo
Central bulge
Rotation curve
Dark matter
Spiral tracer
Spiral density wave
theory
• Self-sustaining star
formation
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26
KEY TERMS
•
•
•
•
Population I star
Population II star
Metal
Monolithic collapse
(topdown) hypothesis
• Sagittarius A*
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27
SUMMARY
• Astronomers discovered the Milky Way
Galaxy in the early 20th century
• Galaxy’s history was reconstructed from the
fossil the galaxy left as it evolved
• Milky Way contains spiral arms that:
• Wind outward through the disk
• Have swarms of hot, blue stars; clouds of dust
and gas; and young star clusters
• Mysterious region of the Galaxy is its center,
the nucleus
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Copyright ©2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly
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