Download formation of stars

FORMATION OF
STARS
SES4U
OBJECTIVES
1.
2.
3.
Name, describe, and give examples of several
kinds of nebulae and explain the relationship
between nebulae and stars.
Describe the formation of red giants and
dwarfs.
Describe the formation of novas, supernovas,
neutron stars, and black holes.
Topic 10 Origin of Stars
Background: In parts of space between stars exist
huge clouds of very low density dust (1%) and
gas (99% - mostly hydrogen). The grains of the
strange dust are very tiny with diameters of
about 1/10 000 cm [silicon carbide, graphite,
diamond, and minor amounts of nitrogen and
other elements.]
It is believed that this gas comes from the explosion
of stars that have become novas or supernovas
[Topic 13] that have scattered this material over
vast areas.
TOPIC 10 Origin of a Star
Background: An average cloud is about 25 lightyears in diameter and may contain enough
material to form many stars. A force from
outside the cloud [shockwave from a supernova]
causes the cloud to begin to condense into stars
by triggering the force of gravity that exists
between the gas atoms and dust grains. Huge
areas become denser throughout the cloud and
temperatures increase as the areas contract.
TOPIC 10 Origin of a Star
(a)
(b)
The great nebulae are clouds of gas and dust
in space of which most are invisible. A
diffuse nebulae is made visible from the light
of a nearby bright star. [Great Nebula in the
constellation Orion.]
Other nebulae, called dark nebula show up as
a dark patch against the more distant stars.
[The Horsehead Nebula in Orion]
TOPIC 10 Origin of a Star
(c) According to theory, stars continually form
wherever dense clouds of gas and dust exist.
(d) As the cloud contacts under the influence of
gravity, the temperature increases and, if the
cloud is large enough, parts of it will start to
glow. These large glowing cloud sections are
called protostars.
TOPIC 10 Origin of a Star
(e) As contraction continues, the protostars
become hotter and brighter. Eventually the
centre is so hot that a fusion reaction begins
and the protostar has become a star.
(f) The star stops contracting when the release of
energy from the fusion of hydrogen in the
centre counterbalances the force of gravity. At
this point the star has reached a stable state.
TOPIC 11: Formation of Red Giants
(a)
(b)
In a stable state a star’s diameter and radiation
remain constant for billions of years. When so
many of the core’s light nuclei are used up that the
energy of fusion no longer balances the force of
gravity the star loses its stability.
When the star loses its stability the centre of the star
contracts again. The core gets so hot that it causes
the star’s outer layers to expand increasing the star’s
surface area. The star again radiates more light and
appears brighter. The fusion reaction starts occuring
in the outer layers. The star expands further and
becomes a red giant or supergiant.
TOPIC 12: Formation of White
Dwarfs
(a) When most of the fuel for the fusion reaction
is used up the temperature and pressure of the
core can no longer support the weight of its
outer layers. The giant collapses, the nuclei of
the atoms are squeezed tightly together and the
star becomes a white dwarf and is probably no
larger than Earth.
TOPIC 12: Formation of White
Dwarfs
(b) A nova [new star] forms from a white dwarf that
has flared up brilliantly, brightening a hundred a
hundred to a million times. A nova may be the result
of the bombardment by a companion star. Novas
fade to their former luminosity in a few years at
most.
(c) The sun is thought to be 5 billion years old still in
its stable stage. It is expected to remain stable for
another 5 billion years before it swells to a red giant
and eventually collapses to a white dwarf.
TOPIC 13: Supernovas
(a)
(b)
Stars with at least 7 times the sun’s mass become red giants
in a relatively short few million years.
When fusion has stopped, it leaves a central iron core. As
the star starts to cool, the core collapses. With the collapse,
the pressures and temperatures within the core rise
dramatically and the iron nuclei become fused into heavier
elements. In a rush toward further collapse, the star
explodes so violently that half its mass is blown away as a
great cloud. The star flares up into an intensely bright
object called a supernova. [see page 387, figure 21.10]
TOPIC 13: Supernovas
(d) The Crab Nebula is the constellation of
Taurus the Bull is a great expanding cloud of
gas formed from a supernova observed by
Chinese astronomers in a.d. 1054. this brilliant
star faded after a year.
TOPIC 14: Neutron Stars and Black
Holes
13(b) A supervova removes only about one-half of the
exploding stars mass. The mass of what remains after
the explosion is what astronomers call a neutron star.
14(a) Astronomers think that in the core of a supernova,
the forces are so great that every atom’s electrons are
crushed into the nucleus combining with protons to
form neutrons. All of the core’s nuclei merge into a
single, dense mass of neutrons. A typical neutron star
is about 10 km in diameter and trillions of times
more dense than the sun.
TOPIC 14: Neutron Stars and Black
Holes
(b) In very massive stars, the nuclear forces
between neutrons become overwhelmed by the
gravitational forces and the star collapses into a
very small volume. These objects have
gravitational forces so powerful that even light
cannot escape. These invisible objects are called
black holes.