Topic 7Astronomy - Session 3

Life Cycle of Stars

10X2 Physics Mr Ram Week beginning 4th May 2020 (week 5). (if you have any questions contact: r.ram@bishopchalloner.bham.sch.uk)

Aims of session 3

• Describe the evolution of stars of similar mass to the Sun (including nebula, main sequence star, red giant, white dwarf).

• Describe the forces acting on a star in terms of thermal expansion and gravity.

• Explain how the balance of thermal expansion and gravity affects the life cycle of stars.

• Describe the evolution of stars with a mass larger than the Sun.

Quick Re-cap (answer in your book)

1. What does g stand for?

2. What are the units for g?

3. Name two factors that affect the weight of an object.

4. Name two factors that affect the surface gravity of a planet.

5. How does mass affect g?

6. What happens if a satellite speeds up in its orbit?

7. Why is the velocity of a satellite always changing?

Quick recap - Check your answers

1. What does g stand for?

Gravitational field strength

2. What are the units for g?

N/kg (also, m/s2)

3. Name two factors that affect the weight of an object.

Mass and gravitational field strength

4. Name two factors that affect the surface gravity of a planet.

Mass and radius of the planet

5. How does mass affect g?

The larger the mass of planet the larger the value of g

6. What happens if a satellite speeds up in its orbit?

Moves to a higher orbit

7. Why is the velocity of a satellite always changing?

Velocity is a vector, so a change in direction will cause a change in velocity

Task 1:- Use the information in the video clip to complete the flow chart (copy out).

Title – Life Cycle of Starshttps://www.youtube.com/watch?v=PM9CQDlQI0A

ProtostarFormed from…

R_____ S_____ G_______

S_________

B_______ H______

R____ G_______

W______ D______N_______

S____

_______ _______ Star

Stars b

igger th

an th

e S

unS

mal

l st

ars,

lik

e

our

Sun

a huge cloud of dust and hydrogen called a nebula

main sequence

ed iant ianted uper

hite warf

upernova

eutrontar

lackole

Check your answers

It is important that you learn the names and the order of these stages.

Task 2 You will need learn what is happening during each stage. I have summarised the key pieces of information over the next few slides, you will need to make notes on this. (you may find it

useful to construct a flowchart similar to the one on the previous slide)

Stars form in clouds of dust and gas called stellar nebulae. Gravitational forces cause the large masses of gas to be pulled together.

Stellar nebula Protostar

As the clouds contract, gravitational energy is converted to thermal energy, heating up the centre of the cloud. At the centre of the cloud, a protostar is formed.

Main Sequence Star

• Once the core reaches a high enough temperature NUCLEAR FUSION begins. Hydrogen nuclei are fused together to make helium releasing ENERY.

• The radiation pressure produced by nuclear fusion is balanced by the gravity during the main sequence phase.

• This is the longest, most stable period of a star’s life. (our own sun is around 4.6 billion years old)

Task 2 Continued

Task 2 Continued

Red Giant

• A stars expands as it grows old.

• As the core runs out of hydrogen and then helium, the core contracts and the outer layers expand, cool, and become less bright; forming a red giant.

• During this time the star begins to convert helium into carbon, by nuclear fusion.

• Our Sun will run out of fuel in ~5 billion years when it will expand, forming a red giant engulfing Mercury and Venus.

Small stars, like our sun.

Task 2 Continued

• At the end of the red giant phase nuclear fusion stops and the corefurther collapses.

• As the core collapses, the outer layer is expelled, forming a planetary nebula.

• What remains is a very hot dense core called a white dwarf.

• Eventually as the white dwarf cools further, it becomes a black dwarf.

• Black dwarfs have not yet formed, as the Universe has not been in existence long enough.

Small stars, like our sun.

White Dwarf

A planetary nebula occurs at the end of a red giant’s life.

Stars much bigger than our sun. Task 2 Continued

• Initially its same as the formation of red giants. • But during the red supergiant phase the carbon atoms

continue to be pulled together by gravity, increasing the temperature.

• Nuclear fusion continues forming oxygen, nitrogen, and eventually iron.

• Once the core contains mainly iron, nuclear fusion stops, because iron is a very stable element.

Stars much bigger than our sun. Task 2 Continued

The Crab nebula was formed in 1054 AD when a supergiant star exploded in a supernova.

Supernova• Since nuclear fusion has

stopped there is no outward radiation pressure.

• Gravity causes the star to collapse rapidly into a super dense core.

• This rapid collapse results in a cataclysmic explosion known as a supernova.

• Energy released causes medium mass elements to fuse, forming heavy elements (up to Uranium).

Stars much bigger than our sun. Task 2 Continued

• If the mass of the core of a supernova is less than 2.5 solar masses, it becomes a neutron star.

• Made almost entirely from neutrons compressed together. They are very dense, very strong gravity.

• A star with a mass greater than 20 times the mass of our Sun may produce a black hole at the end of its life.

• Black holes are objects so dense that not even light can escape their gravity and since nothing can travel faster than light, nothing can escape

Task 3 Complete online test using the link below:-

https://www.bbc.co.uk/bitesize/guides/zwv8xfr/test

Task 4 Answer the exam questions in sentences in your book.

Task 5 check your answers and make any corrections.

Task 6 Assignment 1 – Online work on SENECA – Due in 11th May 2020

Follow instructions on how to log on as shown.Complete Assignment 1 by the 11th May 2020.