Download Astronomy Chapter 16 – The Milky Way Galaxy A. Main Ideas 1

Astronomy
Chapter 16 – The Milky Way Galaxy
A. Main Ideas
1. Discovering the Milky Way
• Shape of the Milky Way
⇒ By the eighteenth century, scientists knew that the Milky Way was a flattened swarm of
stars. At the end of the 18th century, William Herschel, by counting the stars visible in
different directions, deduced that the disk was about five times wider than it is thick.
• Size of the Milky Way
⇒ The Milky Way is more than 20,000 parsecs (65,200 light-years) in diameter, and the
Solar System lies about two-thirds of the way out in the disk
2. Overview of the Milky Way
• Content and Structure
⇒ The Milky Way consists of three main parts, a disk about 30 kiloparsecs in diameter, a
surrounding halo, and a flattened bulge of stars surrounding the core.
⇒ In addition to the stars in the Milky Way are huge clouds of gas and dust that amount to
nearly 15% of the disk’s mass
• Mass of the Milky Way and the Number of Stars
⇒ The mass of the Milky Way is estimated to be 5.0 x1011 solar masses. The Milky Way
galaxy contains 100 billion stars
• Age of the Milky Way
⇒ The oldest stars in the Milky Way are around 15 billion years old, roughly the age of the
universe. In about 10 billion years, the galaxy will lack material for making new stars and
will start to dim
3. Stars of the Milky Way
• Stellar Censuses
⇒ The Milky Way contains many types of stars: giants and dwarfs, hot and cold, young and
old, stable and exploding. Most of the stars in the galaxy are like the Sun: small, dim, and
cool.
⇒ The average mass for Milky Way stars is about one solar mass, ranging extending from
0.08 solar mass to over 30 solar masses
• Two Stellar Populations: Population I and Population II
⇒ Population I stars are blue stars normally located in the spiral arms of galaxies.
Population II stars are red star generally found in the bulges and halos of galaxies
⇒ The two types differ in nearly every aspect: color, age, location, motion, and composition.
Population I stars are young, follow approximately circular orbits, about 3% of their mass
is elements heavier than hydrogen and helium. Population II stars are old, move in highly
elliptical orbits, and only a few hundredths percent of their mass is composed of heavy
elements
• Star Clusters
⇒ Some of the Milky Way’s stars are bound together gravitationally into groups called
clusters. Open clusters contain up to a few hundred members in a volume with a radius
of between 7 to 20 light-years. Globular clusters contain far more stars than open
clusters, from a few hundred thousands to several million stars; the radius of these
clusters range for 40 to 60 light-years
4. Gas and Dust in the Milky Way
The space between stars in not empty it contains interstellar matter composed of gas and dust.
• Distribution and Composition of Interstellar Matter
⇒ Interstellar matter is scattered throughout the Milky Way, but gravity has pulled most of it
into a thin layer in the disk
⇒ The gas in interstellar clouds has a composition similar to that of the Sun and other stars:
about 71 % hydrogen and 27% helium with trace amounts of heavier elements
• Interstellar Dust: Dimming and Reddening
⇒ Dimming and reddening result when light strikes a tiny particle such as a dust grain. The
light is reflected from it in random directions in a process called scattering. Scattering
dims a star’s light because radiation that would have reached us in the absence of dust is
sent off in random directions and is lost to our sight
⇒ Because red light scatters less than blue, more of the star’s red light reaches us than its
blue. As a result the star looks redder than it really is
• Interstellar Gas
⇒ Interstellar gas is the material from which stars form, and it is the repository (storage
place) of matter blown off dying stars
5. Motion of Stars and Gas in the Milky Way
Although all stars within the Milky Way move around its center, the paths followed by stars in the
disk and halo are very different. The stars in the disk orbit in the same direction and in nearly the
same plane, giving the disk its flat shape. Stars in the halo and bulge follow orbits that are steeply
inclined to the disk’s plane. Motion within the disk also creates the Milky Way’s spiral arms.
6. Measuring the Milky Way
• Diameter of the Milky Way
⇒ Astronomers have several ways to measure the Milky Way’s diameter, but all methods
depend on first knowing how far from the center the Sun is. Astronomers believe the
Milky Way has a diameter spanning 40 kiloparsecs, but much of the vast outer part
beyond the Sun’s orbit is extremely sparsely populated
• Mass of the Milky Way
⇒ Astronomers can deduce the Milky Way’s mass from its gravitational attraction on matter
in it and near it. The calculated mass greatly exceeds the mass of detectable stars and
gas, which leads astronomers to hypothesize that our galaxy is embedded in a huge,
massive halo of dark matter
7. The Galactic Center
What lies at the center of the Milky Way galaxy? Astronomers cannot directly view the galactic
core because of the interstellar dust between the core and us. However, examination of radiation
from the center of galaxy in the radio wave, X-ray, and infrared portion of the electromagnetic
spectrum give some indication of what lies in the center. At the center of the Milky Way is a dense
swarm of stars and gas, and within this group of stars lies a black hole with a mass of one million
Suns.
8. Evolution of the Milky Way
One of the major unsolved problems in astronomy is how galaxies form
• Birth of Population I and II Stars
⇒ The Milky Way probably began as a vast, slowly rotating gas cloud, perhaps a million
light-years in diameter and containing about 1011 solar masses. The cloud was composed
of almost pure hydrogen and helium because no stars yet existed to make the heavier
elements
⇒ As gravity began shrinking the cloud, clumps of gas within it grew more dense and
gradually turned into stars. These stars were extremely massive and evolved quickly and
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blew up as supernovas. As the cloud continued to collapse, new stars formed with a
small amount of heavy elements; these are the Population II stars we see today
⇒ The surviving gas formed a rotating disk rich in heavy elements and began to form
Population I stars
Evolution by Mergers
⇒ The collapse model of galaxy formation cannot explain all features of the Milky Way. As
galaxies move through space they can collide with other galaxies and merge to form a
single, larger galaxy. There is evidence that supports the idea that the Milky Way gained
some matter by this process
Population III
⇒ The two-stage collapse model also predicts that the low-mass stars born in that first
generation that have not burned out should have no heavy elements, but no such stars
have been observed. These hypothetical ancient pure hydrogen and helium stars are
called Population III stars.
The Future of the Milky Way
⇒ There are three collisions in the Milky Way’s future: the Large and Small Magellanic
Clouds, two satellite galaxies gravitationally bound to the Milky Way, will eventually
merge with the Milky Way, and the Andromeda galaxy will collide with the Milky Way in
about 5 billion years.
⇒ Ultimately all the interstellar gas will be used to make stars, these stars will burn their
nuclear fuel and die, and finally our galaxy will fade, slowly spinning in space, a dark disk
of stellar cinders
B. Vocabulary
Association
Globular cluster
Population I stars
Selection effect
Bulge
Halo
Population II stars
Self-propagating star formation
Dark matter
HII regions
Population III stars
Spiral arms
Dark nebula
Interstellar matter
Reddening
Star cluster
Density-wave model
Milky Way
Refection nebula
21-centimeter radiation
Disk
Nucleus
Rotation curve
Zone of avoidance
Emission nebula
Open cluster
Scattering
Discussion Questions
1. How do we know our galaxy is a flat disk?
2. How do we know interstellar matter exists?
3. What evidence makes astronomers believe the Milky Way has spiral arms?
4. How can we determine the Milky Way’s diameter?
5. How can we determine the Milky Way’s mass?
6. Describe one model for the origin of the Milky Way. How does this model explain the difference
between Population I and Population II stars?
7. What are Population III stars?