Download Section 28.2 - CPO Science

UNIT NINE: Matter and Motion
in the Universe
 Chapter 26 The Solar System
 Chapter 27 Stars
 Chapter 28 Exploring the Universe
Chapter Twenty-Eight:
Exploring the Universe
 28.1 Tools of Astronomers
 28.2 Galaxies
 28.3 Theories about the Universe
Section 28.2 Learning Goals
 Classify galaxies according to shape.
 Examine the features of the Milky Way
galaxy.
 Tell how astronomers determine the
distances between objects in space.
28.2 Galaxies
 A galaxy is a huge
group of stars, dust,
gas, and other objects
bound together by
gravitational forces.
 The sun, along with an
estimated 200 billion
other stars, belongs to
the Milky Way galaxy.
28.2 Types of galaxies

The Milky Way is a flattened, rotating system
that contains young to middle-aged stars,
along with gas and dust.

Astronomers identify it as a spiral galaxy.
28.2 Types of Galaxies
Astronomers classify galaxies according to
their shape.
1. Spiral galaxies consist of a central, dense
area surrounded by spiraling arms.
2. Barred spiral galaxies have a bar-shaped
structure in the center.
3. Elliptical galaxies look like the central
portion of a spiral galaxy without the arms.
4. Lenticular galaxies are lens-shaped.
Can you classify these galaxies?
28.2 The central black hole theory
 Recent studies have suggested that a black
hole, with a mass of more than a million Suns,
exists at the very center of our galaxy.
 The evidence for a huge black hole comes
from measurements of the orbital velocities of
stars and gas at the center.
 One of the strangest predictions of Einstein’s
theory of relativity is the existence of black
holes.
28.2 The central black hole theory
 The minimum speed an
unpowered projectile
must have to escape the
planet’s gravity is called
the escape velocity.
 A black hole is an object
with such strong gravity
that its escape velocity
equals or exceeds the
speed of light.
28.2 Distances between galaxies

The distances between stars are 10,000
times greater than the distances between
planets.

The distances between galaxies are a
million times greater than the distances
between stars.
28.2 Distances between galaxies

Figuring out the distance between galaxies
is one of the more difficult tasks in
astronomy.

A faint object in the night sky could be a dim
object that is relatively nearby or a bright
object that is far, far away.
28.2 Distances between galaxies

The most reliable method
for estimating the distance
to a galaxy is to find a star
whose luminosity is known.

If the luminosity is known,
the inverse square law can
be used to find the distance
from the observed
brightness.
28.2 Distances between galaxies
 The inverse square law shows how the
brightness of an object decreases as you
move away from it.
28.2 Distances between galaxies
 The inverse square
law is important to
astronomers because
if they know the
brightness and
luminosity of an
object, they can
determine its distance
by rearranging
equation variables.
28.2 Distances between galaxies
 Astronomers have found a way to infer values
for luminosity (L) using a source of light called
a standard candle.
 A standard candle is an object, such as a star,
whose luminosity is known.
 By determining the unknown star’s
temperature (using a spectrometer), they can
infer its luminosity by choosing a similar main
sequence star on the H-R diagram.
28.2 Distances between galaxies
 A second type of
standard candle is called
a Cepheid star.
 Cepheid stars “pulsate”
in regular periods
ranging from a few days
to a few weeks.
 By measuring the period
of a Cepheid star,
astronomers determine
its luminosity and then
calculate its distance.
Investigation 28B
Astronomy and Light
 Key Question:
How do we measure the distance to stars and
galaxies?