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

Questions1. 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 LuminositySurface

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