Evolution of Stars

Question 1

Nebulae

Answer 1

Stars are formed from the gravitational collapse of gas and dust mainly in the spiral arms of galaxies. These clouds can be 15kpc across and can have enough raw materials to form several thousand stars.

Question 2

What is the Orion Nebula?

Answer 2

A stellar nursery, as it is an emission nebula.

Question 3

How big can nebulae be?

Answer 3

15kpc across

Question 4

Protostars

Answer 4

The clouds of gas and dust begin to collapse and form smaller collapsing ‘knots’.

Question 5

Energy

Answer 5

Gravitational energy is converted into kinetic energy, which is converted into thermal energy. The star’s central temperature begins to rise and can reach as high as 15 million K.

Question 6

Nuclear fusion

Answer 6

Due to these high temperatures, nuclear fusion of hydrogen into helium can begin to take place. This releases energy at all stages.

Question 7

Stabilisation

Answer 7

The outwards radiation pressure from nuclear fusion balances the inwards pressure from gravity. This makes the star stay at a stable size and halts further collapse.

Question 8

Main sequence

Answer 8

The star can stay at this stable size for millions of years depending on its mass. If the star is large, such as Spica, it stays at this stage for 100 million years. If the star is small, such as Barnard’s star, the star can spend a million million years as a Main Sequence star.

Question 9

How far is the sun through its main sequence?

Answer 9

The sun is approximately halfway through its main sequence of 10,000 million years.

Question 10

Star collapses

Answer 10

The star then runs out of hydrogen fuel. With no more radiation pressure, the star collapses under its own gravity.

Question 11

Further hydrogen fusion

Answer 11

This heats up the shell around the star’s core (which is already rich in helium), meaning that more hydrogen fusion can take place.

Question 12

Supergiant / Red Giant stage

Answer 12

The star expands and cools to become a red giant or supergiant.

Question 13

Star collapses again

Answer 13

The hydrogen fuel eventually depletes again, causing further collapse.

Question 14

Helium fusion

Answer 14

because the star collapses, temperatures rise to as high as 100 million K. This allows helium fusion into carbon to take place.

Question 15

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Answer 15

The helium fuel depletes again and the red giant loses its outer layers in an expanding gas layer. This forms a planetary nebula.

Question 16

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Answer 16

The core is left behind after this. It cools to become a white dwarf, then a brown and black dwarf. A white dwarrf has the size of the earth but contains approximately the mass of the sun.

Question 17

(>8 sol) Fusion continues

Answer 17

Fusion continues with further collapse as the star fuses elements down to iron.

Question 18

(>8 sol) Supernova

Answer 18

The core collapses once more. Temperature and density increases. Then, a final burst of fusion takes place as iron combines to make helium and electrons and protons combine to make neutrons. This causes an explosion as temperatures reach 100 billion K. 10% of the mass is converted into energy, causing collapse at speeds up to 70,000km/s and the outer layers to be blown away at speeds of up to 5,000km/s. This leaves behind a supernova remnant.

Question 19

What nebulae are associated with the birth of stars?

Answer 19

Emission and absorption.

Question 20

What are also associated with the birth of stars?

Answer 20

Open clusters.

Question 21

How do emission nebulae glow?

Answer 21

Stellar radiation excites the hydrogen gas, which makes it appear red in colour.

Question 22

Give an example of an emission nebulae.

Answer 22

The Eagle Nebula

Question 23

Why do absorption nebulae appear as they do?

Answer 23

They block out the light from the stars behind them.

Question 24

Give an example of an absorption nebula.

Answer 24

Dark Horse Nebula

Question 25

Why do reflection nebula appear as they do?

Answer 25

Dust scatter the light from stars. They appear blue in colour.

Question 26

Give an example of a reflection nebula.

Answer 26

Witch Head Nebula

Question 27

What are planetary nebula?

Answer 27

The outer layer of gas from a dying star of less than 8 solar masses. They are dust and gas ejected into space.

Question 28

Give an example of a planetary nebula.

Answer 28

Ring Nebula

Question 29

What are open clusters? Relate to the birth of stars.

Answer 29

Open clusters are groups of around 1000 stars.
They are bright, implying that they are young.
Close to each other in space, thus they are formed in the same nebula from the same material.
Found around the galactic plane.

Question 30

Describe the nature of neutron stars.

Answer 30

Have the mass of the sun enclosed in a sphere of 20km diameter.
Due to mass they have extremely high forces of gravity.
This gravity causes them to rotate on their own axis.
They emit radio waves in regular pulses from their poles, so are detected as pulsars.

Question 31

What do neutron stars emit?

Answer 31

Radio waves

Question 32

Describe the nature of black holes.

Answer 32

Extremely dense supernova remnant with extremely strong gravity.
Gravity so strong that no electromagnetic radiation can escape.
Emit X-rays from event horizon.
Impossible to view directly.
May account for much of the hidden mass in the universe.

Question 33

Give evidence for the existence of black holes.

Answer 33

• Spiralling charged particles from accreting matter heat up due to friction and emit X-rays, which can be detected.
• Gravitational lensing: observed multiple or distorted images of galaxies due to light being bent by gravity.
• Galaxy’s speed of rotation is faster than expected, there must be hidden mass in black holes near its centre.

Question 34

What do black holes emit?

Answer 34

X-rays

Question 35

Give evidence for the existence of neutron stars.

Answer 35

They are pulsars that regularly emit radio waves from their polar regions.