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Stars & Life Cycle of Stars
Birth of Stars Steps
1st a huge cloud of nebula
 2nd as the density of the gas grows so
does gravitational pull (more particles
are collected)
 3rd the forming star is called a Protostar.
 4th Density increases as gravity
continues to crunch the matter together

Birth of Stars
5th When the central temperature reaches
about 10 million Kelvins hydrogen begins
to fuse to make helium (This marks the
change from a protostar to a star)
 6th The pressure from the fusion (outward
force) and gravity (inward force) reach
equilibrium, A stable star is formed

Stars
The material that makes up a star depends
upon how old the universe was when the star
formed.
 Older stars were mostly hydrogen and helium
 But through the birth and death of stars in the
past, younger stars contain elements heavier
than hydrogen and helium
 Heavier elements were made inside of a star,
basically we are made from those reused and
left over elements

Hydrogen nuclei
collide to form
helium-3
Gamma ray
Two helium-3
nuclei collide.
Large Amount of
Energy
Released!
Helium-4 and
hydrogen nuclei
form.
Green particles are protons
Purple particles are neutrons
Stars & Life Cycle of Stars
The mass of a star determines the path
of its evolution.
Nebula

A group of bright
young stars can
be seen in the
hollowed-out
center of the
Rosette Nebula.
Supernova

The above two photographs are
of the same part of the sky. The
photo on the left was taken in
1987 during the supernova
explosion of SN 1987A, while the
right hand photo was taken
beforehand. Supernovae are one
of the most energetic explosions
in nature, making them like a 1028
megaton bomb (i.e., a few
octillion nuclear warheads).
Supernova

The Crab Nebula is
the remnant of a
supernova explosion
that was observed on
Earth in A.D. 1054.
The supernova was
so bright that people
could see it in the
daytime.
Possible end results for stars
Neutron Stars
 A neutron star is about 20 km in diameter and has the mass of about 1.4
times that of our Sun. This means that a neutron star is so dense that on
Earth, one teaspoonful would weigh a billion tons! Because of its small
size and high density, a neutron star possesses a surface gravitational
field about 2 x 1011 times that of Earth. Neutron stars can also have
magnetic fields a million times stronger than the strongest magnetic fields
produced on Earth.
 Neutron stars are one of the possible ends for a star. They result from
massive stars which have mass greater than 4 to 8 times that of our Sun.
After these stars have finished burning their nuclear fuel, they undergo a
supernova explosion. This explosion blows off the outer layers of a star
into a beautiful supernova remnant.
 The central region of the star collapses under gravity. It collapses so much
that protons (+) and electrons (-) combine to form neutrons (No
charge). Hence the name "neutron star".
Neutron Star
The Hubble Space
Telescope succeded in
taking an image of a
neutron star located less
than 400 light-years away
from Earth.
This star was previously
detected by its Xray
radiation, indicating a
surface temperature
around 700,000°.
Its diameter is less than 28
km.
28 kilometers = 17.4 miles
Roughly Cville to Lebanon

Black Holes
Black holes are the end result of a massive
star’s death. The force of gravity is so strong
that the escape speed is greater than the speed
of light. Hence the name black hole.
 Once you cross the event horizon you will
continue to be stretched and squeezed until you
reach the singularity.
 They are detected by X-rays given off by matter
entering the black hole and their gravitational
effects on neighboring stars.

Properties of Stars
Betelgeuse,
Procyon, and Sirius
are three of the
brightest stars in
the sky. Betelgeuse
is a much cooler
star than the others.
Properties of Stars
Brightness
Astronomers have discovered that the brightness
of stars can vary by a factor of more than a billion.
Stars that look bright may actually be farther away
than stars that appear dim.
Properties of Stars
These streetlights all
have about the same
absolute brightness, but
the closer lights appear
brighter.
Properties of Stars
The sun appears very bright to us because it
is much closer than other stars.
The brightness of a star as it appears from
Earth is called its apparent brightness.
The apparent brightness of a star decreases
as its distance from you increases.
Properties of Stars
Absolute brightness is how bright a star
really is.
A star’s absolute brightness is a
characteristic of the star and does not
depend on how far it is from Earth.
You can calculate a star’s absolute
brightness if you know its distance from Earth
and its apparent brightness.
Properties of Stars
Size and Mass
Once astronomers know a star’s temperature and
absolute brightness, they can estimate its
diameter and then calculate its volume.
The masses of many stars can be determined by
observing the gravitational interaction of stars that
occur in pairs.
For most stars, there is a relationship between
mass and absolute brightness.
The Hertzsprung-Russell Diagram
H-R diagrams are used to estimate the sizes
of stars and their distances, and to infer how
stars change over time.
The Hertzsprung-Russell Diagram
Stars can be classified by locating them on a
graph showing two easily determined
characteristics.
Such a graph is called a Hertzsprung-Russell
diagram, or H-R diagram.
An H-R diagram is a graph of the surface
temperature, or color, and absolute
brightness of a sample of stars.
The Hertzsprung-Russell Diagram
The vertical axis of the H-R diagram shows
absolute brightness, with the brightest stars
at the top and the faintest at the bottom.
The absolute brightnesses of stars vary even
more than temperature, ranging from about
one ten-thousandth to a million times that of
the sun.
The Hertzsprung-Russell Diagram
A star’s
placement on
an H-R diagram
indicates its
absolute
brightness and
surface
temperature (or
color).

Polaris was used
for navigation
because it is
located above
the 23.5O tilted
rotational axis of
the earth.
The Hertzsprung-Russell Diagram
Main-Sequence Stars
Stars occur only in certain places on the H-R diagram.
Most stars are found along a diagonal band running from
the bright hot stars on the upper left to the dim cool stars
on the lower right. Astronomers call this diagonal band
on the H-R diagram the main sequence.
About 90% of all stars are found on the main
sequence. The sun lies near the middle of this band.
The Hertzsprung-Russell Diagram
Giants and Dwarfs
In general, two factors determine a star’s absolute
brightness: its size and its surface temperature.
An H-R diagram shows a star’s absolute
brightness and surface temperature.
• If you compare two stars at the same temperature,
the brighter one must be larger.
• Hotter stars are brighter than cooler stars of the same
size.
The Hertzsprung-Russell Diagram
The very bright stars at the upper right of the
H-R diagram are called supergiants.
Supergiants are much brighter than mainsequence stars of the same temperature, so
they must be very large compared with mainsequence stars.
The Hertzsprung-Russell Diagram
Supergiants range in size from 100 to 1000
times the diameter of the sun.
Just below the supergiants on the H-R
diagram are the giants—large, bright stars
that are smaller and fainter than supergiants.
The Hertzsprung-Russell Diagram
Supergiants range in size from 100 to 1000
times the diameter of the sun.
Just below the supergiants on the H-R
diagram are the giants—large, bright stars
that are smaller and fainter than supergiants.
The Hertzsprung-Russell Diagram
Below the main sequence in the lower part of
the H-R diagram are white dwarfs.
• A white dwarf is the small, dense remains of a
low- or medium-mass star.
• White dwarfs are hot but dimmer than mainsequence stars of the same temperature.
The Hertzsprung-Russell Diagram
The diameter of a red giant is
typically 10–100 times that of the
sun and more than 1000 times that
of a white dwarf.
Galaxies
Astronomers classify galaxies into four main
types: spiral, barred-spiral, elliptical, and
irregular.
Galaxies
A galaxy is a huge group of individual stars,
star systems, star clusters, dust, and gas
bound together by gravity.
• There are billions of galaxies in the universe.
• The largest galaxies consist of more than a
trillion stars. Galaxies vary widely in size and
shape.
Galaxies
Spiral and Barred-Spiral Galaxies
Spiral galaxies have a bulge of stars at the
center, with arms extending outward like a
pinwheel.
• These spiral arms contain gas, dust, and many bright
young stars.
• The Milky Way is a spiral galaxy.
Galaxies
Some spiral galaxies have a bar through the
center with the arms extending outward from
the bar on either side. These are called
barred-spiral galaxies.
Galaxies
Elliptical Galaxies
Elliptical galaxies are spherical or oval, with no
trace of spiral arms.
• Elliptical galaxies come in a wide range of sizes.
• Elliptical galaxies have very little gas or dust between
stars. They contain only old stars.
Galaxies
Irregular Galaxies
A small fraction of all galaxies are known as irregular
galaxies.
Irregular galaxies have a disorganized appearance.
They have many young stars and large amounts of
gas and dust.
Irregular galaxies come in many shapes, are typically
smaller than other types of galaxies, and are often
located near larger galaxies.
Galaxies
A. A spiral galaxy in the constellation Coma
Berenices
B. A barred-spiral galaxy in the Fornax
cluster
Galaxies
C. Elliptical galaxy M87
D. An irregular galaxy with many areas of
star formation
Galaxies
The Milky Way Galaxy
The Milky Way galaxy has an estimated 200 to
400 billion stars and a diameter of more than
100,000 light years.
Every individual star that you can see with the
unaided eye is in our galaxy.
The solar system lies in the Milky Way’s disk
within a spiral arm, about two thirds of the way
from the center.
Galaxies
In a side view, the Milky Way appears as a flat
disk with a central bulge. An overhead view of
the Milky Way shows its spiral shape.
Location of
solar system
Central bulge
Nucleus
Overhead View of Our Galaxy
Disk of spiral arms
containing mainly
young stars
Central bulge
containing mainly
older stars
Halo containing
Nucleus
oldest stars About 100,000 light-years
Side View of Our Galaxy
Galaxies
The Milky Way’s flattened disk shape is
caused by its rotation.
The sun takes about 220 million years to
complete one orbit around the galaxy’s center.
Recent evidence suggests that there is a
massive black hole at our galaxy’s center.
Stars are forming in the galaxy's spiral arms.
Are we made of star remains?

The elements themselves (carbon, nitrogen,
oxygen, etc.) were synthesized, cooked up as it
were, in the nuclear furnaces that are the deep
interior of stars (The elements were fused together
to make heavier elements). These elements are
then released at the end of a star's lifetime when it
explodes (supernova), and subsequently used to
make a new generation of stars -- and into the
planets that form around the stars, and the life
forms that originate on the planets.