Transcript Slide 1

The formation of stars
Learning Objective:
• How do stars form?
The formation of stars
• All of you will describe the basic steps
in star formation
• Most of you will explain how mass
relates to star structure
• Some of you will calculate how many
stars may be formed in a given
interstellar cloud
How stars form: the basic process
1. A cold cloud of gas and dust starts to contract,
pulled together by gravity. It breaks up into several
smaller clouds and each continues to contract.
2. Within a contracting cloud, each particle attracts
every other particle, so that the cloud collapses
towards its centre. It forms a rotating swirling disc.
How stars form: the basic process
3. As the gas particles are attracted towards
the centre, they move faster, which means
the gas gets hotter.
4. Eventually, the temperature of this material
is hot enough for fusion reactions to occur
and a star is born.
5. Material further out in the disc clumps
together to form planets
Formation
• Stars form in clusters
• Planets form at the same time
• In these early stages, as the star
forms, it is known as a protostar
• This stage in the Sun’s life is thought to
have lasted between 100,000 to 1 million
years
Getting Warmer
Here are two ways to thinking
about when a protostar when it
gets hot enough for fusion to
start
The Gas Idea
• The star starts from a cloud of gas
• When a gas is compressed its
temperature rises
• In this case, the force doing the
compressing is gravity
The Particle Idea
• Every particle in the cloud attracts every other
particle
• As they ‘fall’ inwards, they move faster (gravitational
potential energy is being converted to kinetic energy)
• The particles collide with each other, sharing their
energy
• The fastest particles are at the centre of the cloud
(they have fallen the furthest), and fast-moving
particles mean a high temperature
Seek and Find
• Astronomers’ ideas about star
formations come from observations and
computer model which test their ideas
• Computer models can help explain why
spherical material collapses to form a
flattened disc
• This explains why orbiting planets lie in
a plane, such as in the solar system
Seek and Find
• Some models
predict that, as
protostar forms, it
spins faster and
faster
• Eventually, it blows
out giant jets of hot
gas, at right angles
to the planetary
disk
Questions
1. From what materials does a protostar form?
2. If astronomers see a protostar glowing, does this
indicate that nuclear fusion is taking place?
3. Imagine a sky-rocket exploding in the night sky. A
small, hot explosion results in material being thrown
outwards.
a Describe the energy changes that are going on.
b Now imagine the same scene, but in reverse. How
is this similar to the formation of a protostar? How
does it differ?
4. Suggest reasons why a cold cloud of gas is more likely
to contract than a hot one.
Modelling the Sun
• The Sun’s surface is about 5800K:
far too ‘cold’ for nuclear fusion to
take place.
• You cannot tell exactly what it is
inside the Sun, but there are clues
that help physicist make intelligent
guesses
Modelling the Sun
• Nuclear fusion, the source of the Sun’s
energy, requires temperatures of
millions of degrees
• Energy leaves the Sun from its surface
layer, the photosphere, whose
temperature is about 5800K
Modelling the Sun
The photosphere
has a granular
appearance, which
is continually
changing.
Something is going
on under the
surface
Modelling the Sun
• A star like the Sun can burn steadily for
billions of years, so it must radiate
energy at the same rate that it
generates it from fusion reactions
Physicist can use these ideas to develop
models of the inside of a star.
Layer upon layer
• The core is the hottest part, with a temperature
around 14 million K. This is where fusion occurs
• Radiation (photons) travel outwards through the
radiative zone
• Close to the surface, the temperature falls to just 1
million K. Matter can flow, and convection currents
are set up, carrying heat to the photosphere. This
causes the convective cells
• EM radiation is emitted by the photosphere
Other main-sequence stars
• Other MS stars are modelled in the
same way as the Sun.
• All MS stars are fusing H in their cores
to make He
• A MS star has a steady luminosity and
temperature whilst is fusing H in its
core- millions, or even billions, of years.
Other main-sequence stars
• Differences in MS stars are due to their
different masses
• The more massive the star, the hotter its
core and the more rapidly it turns H into He.
• The most massive MS stars are also the
hottest and most luminous
• The lifetime of a MS star depends on its mass
and the rate at which it turns H to He
Mass of Star
Luminosity
Surface
Temperature (K)
0.5 x Sun
0.03 x Sun
3800
1 x Sun
1 x Sun
5800
3 x Sun
60 x Sun
11,000
15 x Sun
17,000 x Sun
28,000
Questions
5. Why does hydrogen fusion take place only in the core
of a main-sequence star?
6. On a sketch copy of an H–R diagram, label the ends of
the main sequence to show the most massive and the
least massive stars.
Questions
7. The helium made by a main-sequence star stays in its
core. Look at the cross-section diagrams below and
put forward a reason for this.
8. The greater the mass of a star, the shorter the time
it spends on the main sequence. Suggest an
explanation for this.
Glossary
• Convection zone (of a star): the layer of a star
above its radiative zone, where energy is transferred
by convective currents in the plasma
• Photosphere: the visible surface of a star, which
emits electromagnetic radiation
• Protostar: the early stages in the formation of a new
star, before the onset of nuclear fusion in the core
• Radiation: the flow of information and energy from a
source. Light and infrared are examples. Radiation
spreads out from its source, and may be absorbed,
reflected, or transmitted by objects in its path