The selection below discusses the different stages in the life of a star. This topic is crucial for understanding the next lesson. A printable copy of the link is provided at the bottom. Unlike the previous handout, this includes the follow-up questions for the article (Activity # 3: Stellar Formation and Evolution).
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THE LIFE
CYCLE OF A STAR
(taken from: http://www.telescope.org/pparc/res8.html)
Outlined
below are the many steps involved in a stars evolution, from its formation in a
nebula, to its death as a white dwarf or neutron star.
NEBULA
A
nebula is a cloud of gas (hydrogen) and dust in space. Nebulae are the
birthplaces of stars. There are different types of nebula. An Emission Nebula
e.g. such as Orion nebula, glows brightly because the gas in it is energized by
the stars that have already formed within it. In a Reflection Nebula, starlight
reflects on the grains of dust in a nebula. The nebula surrounding the Pleiades
Cluster is typical of a reflection nebula. Dark Nebula also exist. These are
dense clouds of molecular hydrogen which partially or completely absorb the
light from stars behind them e.g. the Horsehead Nebula in Orion.
Planetary
Nebula are the outer layers of a star that are lost when the star changes from
a red giant to a white dwarf.
STAR
A
star is a luminous globe of gas producing its own heat and light by nuclear
reactions (nuclear fusion). They are born from nebulae and consist mostly of
hydrogen and helium gas. Surface temperatures range from 2000⁰C to above 30,000⁰C, and
the corresponding colors from red to blue-white. The brightest stars (high-mass
stars) have masses 100 times that of the Sun and emit as much light as millions
of Suns. They live for less than a million years before transitioning to a
Supergiant and exploding as Supernovae. The faintest stars are the red dwarfs,
less than one-thousandth the brightness of the Sun.
The
smallest mass possible for a star is about 8% that of the Sun (80 times the
mass of the planet Jupiter), otherwise nuclear reactions do not take place.
Objects with less than critical mass shine only dimly and are termed brown
dwarfs or a large planet. Towards the end of its life, a star like the Sun
swells up into a red giant, before losing its outer layers as a Planetary
Nebula and finally shrinking to become a white dwarf.
RED GIANT
This
is a large bright star with a cool surface. It is formed during the later
stages of the evolution of an intermediate-mass star like the Sun, as it runs
out of hydrogen fuel at its center. Red giants have diameters between 10 and
100 times that of the Sun. They are very bright because they are so large,
although their surface temperature is lower than that of the Sun, about
2000-3000⁰C.
RED DWARF
These
are very cool, faint and small stars, approximately one tenth the mass and
diameter of the Sun. They burn very slowly and have estimated lifetimes of 100
billion years. Proxima Centauri and Barnard's Star are red dwarfs.
WHITE DWARF
This
is very small, hot star, the last stage in the life cycle of a star like the
Sun. White dwarfs have a mass similar to that of the Sun, but only 1% of the
Sun's diameter; approximately the diameter of the Earth. The surface
temperature of a white dwarf is 8000⁰C or
more, but being smaller than the Sun their overall luminosity's are 1% of the
Sun or less.
White
dwarfs are the shrunken remains of normal stars, whose nuclear energy supplies
have been used up. White dwarf consist of degenerate matter with a very high
density due to gravitational effects, i.e. one spoonful has a mass of several tons.
White dwarfs cool and fade over several billion years.
SUPERNOVA
This
is the explosive death of a star, and often results in the star obtaining the
brightness of 100 million suns for a short time. There are two general types of
Supernova:-
Type
I: These occur in binary star systems in
which gas from one star falls on to a white dwarf, causing it to explode.
Type
II: These occur in stars ten times or
more as massive as the Sun, which suffer runaway internal nuclear
reactions at the ends of their lives, leading to an explosion. They leave
behind neutron stars and black holes. Supernovae are thought to be main source
of elements heavier than hydrogen and helium.
NEUTRON STARS
These
stars are composed mainly of neutrons and are produced when a supernova
explodes, forcing the protons and electrons to combine to produce a neutron
star. Neutron stars are very dense. Typical stars having a mass of three times
the Sun but a diameter of only 20 km. If its mass is any greater, its gravity
will be so strong that it will shrink further to become a black hole. Pulsars
are believed to be neutron stars that are spinning very rapidly.
BLACK HOLE
Black
holes are believed to form from massive stars at the end of their lifetimes.
The gravitational pull in a black hole is so great that nothing can escape from
it, not even light. The density of matter in a black hole cannot be measured.
Black holes distort the space around them, and can often suck neighboring
matter into them including stars.
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