Chapter 14
The Universe
The Night Sky
• A light year is the distance that light travels in
one year, about 6 x 1012 mi. This is 8 times the
distance around the earth traveled in 1 second.
• Celestial coordinates are similar to earth
coordinates. The celestial sphere is the space
that extends from the earth’s sphere.
• There is a celestial equator. It is adjacent to the
earth’s equator.
• The earth’s north pole touches the celestial
sphere at the north celestial pole and the
earth’s south pole at the south celestial pole.
Stars
• There are billions of stars in the universe.
• The sun is only one of many. It is a star of
average brightness.
• All stars are massive, dense balls of gases
with a surface heated to incandescence by
energy released by fusion reactions deep
within.
Origin of Stars
• Stars are born from swirling clouds of hydrogen
gas in the deep space between other stars.
• Interstellar (between stars) clouds are called
nebulae. They consist of random, swirling atoms
of gases that have little gravitational attraction
for one another because they have little mass.
• Complex motions of stars can produce a shock
wave that causes particles to move closer
together into a growing cluster and as the mass
increases the gravitational attraction of the
particles for each other increases.
• Eventually you end up with a protostar, an
accumulation of gases that will form a star.
A star is born
Nebulae - Swirling clouds of
hydrogen gas away from
other stars, eventually forms
protostar.
A protostar
Origin of Stars
• The protostar becomes more dense. As the
atoms accelerate towards the center of the star
they gain in kinetic energy which causes the
temperature to increase.
• After about 10 million years of contracting and
heating the temperature and density conditions
at the center of the protostar are sufficient to
start nuclear fusion reactions.
• After about 17 million years of this a star is born
and it will exist for the next 10 billion years.
Sections of a star
Sections of a Star-The Core
• The core is a dense, very hot region where
nuclear fusion reactions release gamma
rays and X-ray radiation.
• The density of the core is about twelve
times that of solid lead.
• Because of plasma conditions, however,
the core remains in a gaseous state even
at this density.
Sections of a Star- Radiation Zone
• The radiation zone is less dense than the core, it
has a density like water’s.
• Energy in the form of gamma and X-rays from
the core is absorbed and reemitted by collisions
with atoms in this zone.
• The radiation slowly diffuses outward because of
the countless collisions over a distance
comparable to the distance between the earth
and the moon. It could take millions of years
before this radiation finally escapes the radiation
zone.
Sections of a Star-Convection Zone
• The convection zone begins about 7/10 of
the way to the surface, where the density
of the gases is about 1% of the density of
water.
• Gases at the bottom of this zone are
heated by radiation from the radiation
zone below, expand from the heating and
rise to the surface by convection.
• The temperature is maintained at about
5500oC
The Sun
• At an average the sun converts about
1.4 x 1017 kg of matter to energy every
year as H nuclei are fused to produce
helium.
• The sun was born about 5 billion years
ago and has sufficient hydrogen in the
core to continue shining or another 4-5
billion years.
Other Stars
• Other stars have masses that are much greater or much
less than the mass of the sun so they have different life
spans.
• More massive stars generate higher temperatures in the
core because they have a greater gravitational
contraction from their greater masses.
• Higher temperatures mean increased kinetic energy,
more collisions between hydrogen nuclei and therefore
more fusion reactions.
• A more massive star uses up its hydrogen more rapidly
than a less massive star and vice versa.
• The life spans of stars range from a few million years for
large, massive stars, to ten billion years for an average
star, to trillions of years for small, less massive stars.
A star
4 Hydrogens combine to form Helium
Fusion fuel for all stars.
(10,000,000K required).
Larger stars burn up fuel
faster.
Sun’s lifetime - 10 billion
years (we’re half through).
Brightness of Stars
• The brightness of stars is related to:
1. The amount of light produced by the stars.
2. The size of each star.
3. The distance to a particular star.
• The apparent magnitude scale indicates the relative
brightness of stars. It goes from 1-6 with 1 being the
brightest. The stars are said to be first through sixth
magnitude stars.
• The sun has an absolute brightness of 4.8, which is the
brightness of a faint star.
• The absolute brightness is an expression of luminosity,
the total amount of energy radiated into space each
second from the surface of a star.
Luminosity of Stars
• If the sun is assigned a luminosity of 1,
then other stars range from a low of 10-6
sun units to a high of 105 sun units.
• The sun is somewhere in the middle of the
range of star luminosity.
Star Temperature
• The temperature of the stars is reflected in
their color.
• The hotter stars are bluish white, the in
between ones are yellowish, like the sun,
and the cooler ones are reddish.
• This is the same situation that we have
with any incandescent object.
The distribution of radiant energy emitted is different for stars
with different surface temperatures. Note that the peak radiation of a
cooler star is more toward the red part of the spectrum, and the peak
Radiation of a hotter star is more toward the blue part of the spectrum.
The mid mass stars eventually become planetary nebulae after a violent explosion.
They eventually add to the dust and gases between stars.
Both mid and low mass stars eventually becomes cold clumps of carbon in space.
The higher mass stars eventually collapse and rebound to a catastrophic explosion
called a supernova.
The blown-off outer layers of mid mass stars form ringstar structures
called nebulae.
Supernova
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• http://www.spaceimag
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Galaxies
• The basic unit of the universe is the galaxy.
• The sun is but one of about one hundred million
stars that are held together by gravitational
attraction in the Milky Way galaxy.
• The Milky way is but one of billions of galaxies
that are associated with other galaxies in clusters,
and these clusters are associated with one
another in superclusters.
The Milky Way galaxy
The Milky Way Galaxy
• Contains 3 parts:
1. Galactic Nucleus- Spherical concentration of
stars.
2. Galactic Disk-Rotating disk which contains the
bright, blue stars along with much dust and gas.
3. Galactic Halo-Spherical area with 150 globular
clusters located outside of the galactic disk.
• The sun is located in one of the arms of the
galactic disk, about 25-30 thousand light years
from the center.
• The galactic disk rotates and the sun completes
one full rotation every 200 million years.
The Structure of the Milky Way Galaxy.
The Milky Way (cont.)
• The diameter of the galactic disk is about 100,000 light-years.
• In spite of the one hundred billion stars in the Milky Way, it is mostly full of
emptiness.
• If the stars were the size of tennis balls the distance between them would
be the size across the state of Texas.
• The space between the stars is not actually empty, since it contains a thin
concentration of gas, dust, and molecules of chemical compounds.
• The gas outnumbers the dust 1012 : 1.
• The gas is mostly hydrogen, and the dust is is mostly solid iron, carbon,
and silicon compounds.
• Over forty different chemical molecules have been discovered in the space
between the stars, including many organic molecules (contain C, H, and/or
O and N).
• The gas plays an important part in the formation of new stars and the dust
plays an important part in the formation of new planets.
Other Galaxies
• The nearest galaxy is a dwarf spherical galaxy
80,000 light years from the solar system.
• It is a dwarf galaxy because its diameter is about
1,000 light years as opposed to the Milky Way
with about 100,000 light years in diameter.
• It is in the process of being pulled apart by the
gravitational pull of the Milky Way, which is now
known to have 11 satellite galaxies.
• The nearest galactic neighbor similar to the
Milky Way in size and shape is Andromeda, with
about 100 billion stars, gas, and dust turning in a
giant spiral pinwheel.
The Big Bang Theory
• This is the current model of how galaxies form and of
the creation of the universe.
• According to this theory the universe had an
explosive beginning.
• According to this theory all matter in the universe was
located together in a dense state from which it began
to expand, even today.
• This theory indicates that the universe was all
together at one point in time and the masses that
move faster are now further from the center.
Big Bang Theory
• A “big bang” of energy created a plasma of protons and neutrons.
This led to the formation of H nuclei.
• The H nuclei were pulled together by gravity into masses that would
become the stars. The H nuclei fused into He nuclei, releasing
enough energy that the star began to shine.
• The fusion process continued for billions of years, releasing energy
as heavier and heavier nuclei were formed. Eventually the star
materials were fused into nuclei around iron, the element with the
lowest amount of energy per nucleon and the star used up its
energy source.
• Larger, more massive dying stars explode into supernovas. Such an
explosion releases a flood of neutrons, which bombard medium
weight nuclei and build them up to more massive nuclei, all the way
from iron up to uranium. These elements are then spread into
space as dust which became the materials of which planets were
made.
• The force of gravitational attraction provided the initial energy for the
whole process.
As the universe expands the expansion carries galaxies away from each other
at speeds that are proportional to their distances from each other. It doesn’t
matter within which galaxy an astronomer resides, the other galaxies all appear
to be moving away.
Hubble Space Telescope
• Images never before seen due to absence
of atmosphere.
• An astronomical reflecting telescope with a
mirror 94.5 inches (2.4 meters) in
diameter; placed in orbit above the Earths
atmosphere in April 1990.
Age of the Universe
• It is believed that the big bang occurred
some 13.7 billion years ago expanding as
an intense and brilliant explosion from a
primeval fireball with a temperature of
some 1012 K.
• The separation of the galaxies is actually
accelerating, pushed by a mysterious
force known as “dark energy”.
Review Exercises Chapter 14
• Applying the Concepts p. 372-373:
# 9, 10, 12, 16
• New Book:
p. 406-409 # 2, 4, 5, 6, 10, 13, 16, 17, 28,
29, 30, 31, 38, 39, 41, 48, 49, 50.
Review Chapter 14
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Light year, celestial equator, north
and south poles.
Nebulae and protostars, what are
they?
3 sections of a star: core, radiation
zone and convection zone. The
densest is the core, the least
dense is the convection zone.
The larger the mass of the star the
lower the lifespan of the star. They
use fuel faster.
Fusion reactions occur in stars-4
Hydrogen atoms are converted to
Helium. This is how the energy is
produced.
Brightness of stars: light,
magnitude and distance.
Luminosity-brightness is a
measure of this. Scale is 1-6 with
1 the brightest. The sun has a
brightness of 4.8.
The sun is of average luminosity,
temperature, size, and mass.
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The color of the stars is related to
their temperature, like other
luminous objects. Form bluish
white for the hottest to yellowish
and then reddish for the coolest.
Supernovas-Explosions of stars
that leads to their destruction.
Galaxies-Clusters of stars. The
Milky Way is our galaxy.
Parts of the Milky Way: Nucleus,
disk and halo. The Sun is in one of
the disks.
The Andromeda galaxy-similar to
the Milky Way.
There is dust and gas in space.
The gas gives rise to stars, the
dust to planets.
The Big Bang Theory-The faster
moving galaxies are now further
away. Everything originated at the
same point.
The Hubble Space telescope-Is
out in space, there is no
interference from the atmosphere.