Download 25.2 Stellar Evolution

25.2 Stellar Evolution
By studying stars of different ages,
astronomers have been able to piece
together the evolution of a star.
Star Birth
The birthplaces of stars are dark, cool
interstellar clouds, such as the one in Figure
8.
These nebulae are made up of dust and
gases.
In the Milky Way, nebulae consist of 92
percent hydrogen, 7 percent helium, and less
than 1 percent of the remaining heavier
elements.
Protostar Stage
A protostar is a developing star not yet hot
enough to engage in nuclear fusion.
When the core of a protostar has reached
about 10 million K, pressure within is so great
that nuclear fusion of hydrogen begins, and a
star is born.
A stable main-sequence star is balanced
between two forces: gravity, which is trying to
squeeze it into a smaller sphere, and gas
pressure, which is trying to expand it.
Main-Sequence Stage
Typically, hydrogen fusion continues for
a few billion years.
An average star spends 90 percent of
its life as a hydrogen-burning mainsequence star.
With the exception of the least-massive
red stars, a star can delay its death by
fusing heavier elements and becoming
a giant.
Red-Giant Stage
The red-giant stage occurs because the zone
of hydrogen fusion continually moves
outward, leaving behind a helium core.
As the core contracts, it grow hotter.
Energy it radiated outward, increasing
hydrogen fusion in the star’s outer shell.
This energy in turn heats and expands the
star’s outer layer.
The result is a giant body hundreds to
thousands of times its main-sequence size,
as shown in Figure 10.
Burnout and Death
All stars, regardless of their size,
eventually run out of fuel, and collapse
due to gravity.
Death of Low-Mass Stars
Low-mass stars remain as stable mainsequence stars until they consume their
hydrogen fuel and collapse into a white
dwarf.
Death of Medium-Mass Stars
During their giant phase, sunlike stars
fuse hydrogen and and helium fuel at a
fast rate.
Once this fuel is exhausted, these stars
also collapse into white dwarfs.
They cast off their outer layer, creating
a round cloud of gas called planetary
nebulae.
Death of Massive Stars
Stars with masses three times that of
the sun end their lives in a brilliant
explosion called a supernova.
During a supernova, a star becomes
millions of times brighter than its
prenova stage.
H-R Diagrams and
Stellar Evolution
Hertzsprung-Russell diagrams have
been helpful in formulating and testing
models of stellar evolution.
Stellar Remnants
Eventually, all stars consume their
nuclear fuel and collapse into one of
three documented stages:
White Dwarf
Neutron Star
Black Hole
White Dwarfs
White Dwarfs are the remains of lowmass and medium-mass stars.
The last stage of a white dwarf must be
a small, cold body called a black dwarf.
Neutron Stars
Neutron stars are thought to be the
remnants of supernova events.
Supernovae
During a supernova, the outer layer of
the star is ejected, while the core
collapses into a very hot neutron star
about 20 kilometers in diameter.
The pulsar found in the Crab Nebula is
undoubtedly the remains of the
supernova of 1054.
Black Holes
During a supernova event, remnants of
stars three times more massive than the
sun apparently collapse into objects
even smaller and denser then neutron
stars.
Their gravity is so strong that not even
light can escape their surface.
25.3 The Universe
Galaxies are groups of stars, dust, and
gases held together by gravity.
The Milky Way Galaxy
The Milky Way is a large spiral galaxy
whose disk is about 100,000 light-years
wide and about 10,000 light-years thick
at the nucleus, as shown in Figure 17A.
Our solar system orbits the galactic
nucleus about every 200 million years.
Types of Galaxies
Spiral Galaxies
Elliptical Galaxies
Irregular Galaxies
Galaxy Clusters
Spiral Galaxies
Usually disk-shaped, with a somewhat
greater concentration of stars near their
centers
Appearance of a pinwheel
The Milky Way may be a barred spiral
galaxy
Elliptical Galaxies
About 60 percent of galaxies are
classified as elliptical galaxies
Does not have spiral arms
Irregular Galaxies
Only 10 percent
Large and Small Magellanic Clouds
They are our nearest galactic neighbors
In addition to shape and size, one of the
major differences among different types
of galaxies is the age of their stars.
Galaxy Clusters
Like stars, galaxies are grouped in
clusters.
Some clusters may contain thousands
of galaxies.
Our own cluster, called the Local Group,
contains at least 28 galaxies.
The Expanding Universe
When a source is moving away, its light
appears redder than it actually is,
because its waves appear lengthened.
Objects approaching have their light
waves shifted toward the blue or shorter
wavelengths.
Therefore, the Doppler effect reveals
whether a star or other body in space is
moving away from Earth or toward
Earth.
Red Shifts
Discoveries made by Edwin Hubble
Red shift occurs because Earth and the
source are moving away from each
other.
Galaxies that exhibit the greatest red
shifts are the most distant.
Hubble’s Law
Most galaxies – except for a few nearby
– are moving away from us.
Hubble’s Law states that galaxies are
retreating from us at a speed that is
proportional to their distance.
The red shifts of distant galaxies
indicate that the universe is expanding.
Figure 22
The Big Bang
According to the big bang theory, the
universe began as a violent explosion
from which the universe continues to
expand, evolve, and cool.
The big bang theory states that at one
time, the entire universe was confined
to a dense, hot, supermassive ball.
Then, about 13.7 billion years ago, a
violent explosion occurred, hurling this
material in all directions.