5.5 Astrophysics and Cosmology

Question 1

What are Nebulae?

Answer 1

Nebulae are gigantic clouds of dust and gas (mainly consisting of hydrogen)

Question 2

How are Nebulae formed?

Answer 2

Nebulae are formed over millions of years, as the tiny gravitational attraction between particles of dust and gas pull the particles towards each other, eventually forming the large clouds of dust.

Question 3

Describe how a star is born

Answer 3

1. As particles of dust and gas exert forces of gravitational attraction on each other, they form gigantic clouds together called a nebula.

• As the dust and gas get closer together, this gravitational collapse accelerates.
• Due to tiny variations in the nebula, denser regions begin to form
• These regions pull in more dust and gas, gaining mass and getting denser, and also getting hotter as gravitational energy is eventually transferred to thermal energy
• In one part of the cloud, a protostar forms - this is not yet a star but a very hot, very dense sphere of dust and gas
• For a protostar to become a star, nuclear fusion of hydrogen nuclei into helium nuclei needs to start in its core
• Many protostars never reach this stage, as high pressures abs temperatures in the core are needed in order to overcome the electrostatic repulsion between hydrogen nuclei - in order to fuse them together to form helium nuclei
• In some cases, more and more mass is added to the protostar; it grows so large and the core becomes so hot that the kinetic energy of the hydrogen nuclei is large enough to overcome the electrostatic repulsion.
• The hydrogen nuclei are forced together to form helium nuclei as nuclear fusion begins.

A star is born

Question 4

What happens once a star is formed?

Answer 4

• Once a star is formed it maintains equilibrium with almost a constant size
• Gravitational forces act to compress a star
• But the radiation pressure from the photons emitted during fusion and the gas pressure from the nuclei in the core push outwards
• The force from the radiation and gas pressure balances the force from the gravitational attraction and maintains equilibrium

Stars in this stable phase are describes as being on their main sequence

Question 5

What are the factors that affect how long a star is stable and in its main sequence?

Answer 5

The size and mass of its core

• The cores of massive supergiant stars are much hotter than those of smaller stars, releasing more power and converting the available hydrogen into helium in a much shorter time.

Question 6

Compare the lifespan of massive stars to smaller stars

Answer 6

Really massive stars are only stable for a few million years, whereas smaller stars like our Sun are stable for teens of billions of years