Download Astronomy Chapter 17 – Galaxies A. Main Ideas 1. Discovering

Astronomy
Chapter 17 – Galaxies
A. Main Ideas
1. Discovering Galaxies
• Early Observations of Galaxies
⇒ All galaxies are extremely distant from Earth, the nearest is more than 150,000 light-year
away. Few can be seen with the naked eye. In the Northern Hemisphere the galaxy M31
appears as a pale smudge in the constellation Andromeda.
⇒ In the 18th and 19 centuries astronomers began catalogue the objects they observed in
the night sky, many of these objects were galaxies. Two prominent catalogues were the
Messier and the New General Catalog
• Types of Galaxies
⇒ Galaxies can be divided into three main types on the basis of their shape: spirals,
elliptical, and irregular. Spiral galaxies have two or more arms winding out from the
center. Elliptical galaxies have a smooth and featureless appearance and a generally
elliptical shape. Irregular galaxies generally have stars and gas scattered in random
patches
• Differences in the Stellar and Gas Content of Galaxies
⇒ Astronomers have discovered that spiral, elliptical, and irregular galaxies differ not only in
their shape bur also in the types of stars they contain. Spiral galaxies contain a mix of
young and old stars, but elliptical galaxies contain mostly old stars. This difference is
understandable because spiral galaxies contain more star making gas and dust than
elliptical do.
• The Evolution of Galaxies: Collisions and Mergers
⇒ Galaxies interact with each other on a regular basis. Two galaxies may collide and the
gravitational interaction will change the shape of both galaxies. When galaxies collide
very few stars in either galaxy actually collide with each other because the stars are so
far apart in each galaxy. Galaxies may merge with each other with the result being the
formation of a single, larger galaxy.
2. Measuring Properties of Galaxies
• Galaxy Distances
⇒ Galaxies are separated from each other by enormous distances. Astronomers cannot use
parallax to measure such large distances because the angle by which the galaxy’s
position changes as we move around the Sun is too small to be measured. Astronomers
use the method of standard candles to measure the distance to distant galaxies. Using
the inverse-square law and the known luminosity of Cepheid variables scientists are able
to reasonable measurements of the distances involved.
• Redshift and the Hubble Law
⇒ In the 1920s, astronomers discovered that galaxies are moving away from each other.
This discovery was made by studying the spectrum of the galaxies. If a galaxy is moving
either toward or away from us its spectral lines will be Doppler-shifted. Motions away
from us lengthens the wavelength of the lines and produces the so-called redshift.
⇒ With few exceptions, nearly are the galaxies are moving away from the Milky Way galaxy,
and the velocity of galaxy is larger for galaxies that are farther away. Edwin Hubble
determined the relationship between a galaxy’s velocity and its distance from the Milky
Way. The relationship can be summarized by the following equation: V = HD, where V is
velocity, D is distance, and H is the Hubble constant.
• Measuring the Diameter of a Galaxy
⇒ Astronomers measure the diameter of a galaxy using the same method for measuring the
diameter of the Moon or the Sun.
•
Measuring the Mass of a Galaxy
⇒ We can find the mass of a galaxy by using the modified form of Keplar’s third law. The
modified third law relates a body’s mass to the distance and period of an object orbiting
around it. Using this method the calculated mass of almost all galaxies is larger than the
mass of stars and interstellar matter detectable by optical, infrared, and radio telescopes.
3. Dark Matter
The amount of dark matter to account for the difference between the observed and calculated
mass is very large; the amount of dark matter surrounding a galaxy 10 times greater than the
visible mass.
4. Active Galaxies
Active galaxies are galaxies whose centers emit abnormally large amounts of energy from a tiny
region in their core.
• Radio Galaxies
⇒ Radio galaxies emit large amounts of energy in the radio part of the EM spectrum. They
are usually elliptical galaxies, and their radio emission differs in two important ways from
that of ordinary galaxies. The energy is emitted from the core of the galaxy and from
regions outside of the galaxy, and the emission is million of times stronger than normal
galaxies emit
• Seyfert Galaxies
⇒ A Seyfert galaxy is a spiral galaxy whose nucleus is abnormally luminous. The core
luminosity of a Seyfert galaxy is immense, amounting to the entire radiation output of the
Milky Way, but coming from a region less than one light-year across
• Quasars
⇒ Quasars are extremely luminous, extremely distant, active galaxies. Some are powerful
radio sources, others are similar to both radio and Seyfert galaxies in that they eject hot
gas from their centers.
• A Unified Model of Active Galaxies
⇒ Astronomers hypothesize that the cores of active galaxies contain something unusual. No
ordinary single star can be so luminous. No ordinary group of stars could be packed into
so small a region. It is possible that the cores of active galaxies contain truly huge black
holes about the size of the Earth’s orbit around the Sun and having the mass of a
hundred million or more Suns.
5. Probing Intergalactic Space
• Gravitational Lenses
⇒ A gravitational lens forms an image because light bends as it passes through the curved
space around a massive object such as a galaxy. The galaxy’s gravitational force bends
the space around it so that light rays that would otherwise travel off on other directions
and never reach the Earth are bent so that they do
6. Galaxy Clusters
• The Local Group
⇒ The Milky Way belongs to a very small galaxy cluster called the Local Group. The Local
Group contains about 30 members. The Local Group has three large spiral galaxies,
small satellite galaxies, and small, faint, dwarf galaxies.
•
•
The Relationship of Cluster Size and Galaxy Type
⇒ Rich galaxy clusters contain hundreds to thousands of member galaxies, most of which
are elliptical. Rich clusters also contain large amounts of extremely hot gas that emits Xrays, and they contain a large amount of dark matter.
⇒ Poor galaxy clusters contain relative few member galaxies. The Local Group is a typical
poor cluster.
⇒ Based on observations, galaxy clusters form after the individual galaxies form and are
drawn together by their mutual gravitational attraction
Superclusters
⇒ The great mass of galaxy clusters creates gravitational interactions between them and
holds them together into larger structures called superclusters. A supercluster contains a
half dozen to several dozen galaxy clusters spread throughout a region of space tens to
hundreds of millions of light-years across
⇒ Superclusters have irregular shapes and are often part of yet larger groups. Two known
clusters of superclusters are the “Great Wall” and the “Great Attractor”. The Great Wall is
a structure that is 330 million light-years away from us and stretches across a region
about 500 x 200 x 16 million light-years. The Great Attractor is about 130 million lightyears away from us and is about 260 million light-years in diameter. The Great Attractor
is pulling the Local Group toward its center.
B. Vocabulary
Active galaxy
Galaxy
Irregular galaxy
Radio galaxy
Spiral galaxy
Barred spiral
Galaxy cluster
Jets
Recession velocity
supercluster
Dark matter
Gravitational lens
Local Group
Red shift
Elliptical galaxy
Hubble constant
Megaparsecs
Rich cluster
Supermassive
Black Hole
Galactic cannibalism
Hubble law
Quasars
Seyfert galaxy
C. Discussion Questions
1. What are the three types of galaxies?
2. Which of the basic galaxy types has the smallest proportion of Population I stars?
3. How do astronomers measure the mass of a galaxy?
4. What process generates the radio emission in radio galaxies?
5. How might a large black hole form in a galaxy’s core?