Module 5 Life of Stars

Question 1

In full, describe the events which take place during the formation of a star

Answer 1

• Large gas cloud of hydrogen and dust particles contract due to gravitational attraction
• Loss of gravitational potential energy and gain of KE so temp rises
• Eventually temperature in core becomes high enough for nuclear fusion to take place
• Hydrogen nuclei overcome electrostatic repulsion fuse to become helium nuclei
  Fusion releases energy and photons
• Outwards thermal and radiation pressure = inwards gravitational pressure
  A stable main sequence star is formed

Question 2

• Large gas cloud of hydrogen and dust particles contract due to ________1__________
• Loss of gravitational potential energy and gain of KE so _______2__________
• Eventually temperature ____3_____ becomes high enough for nuclear fusion to take place
• Hydrogen nuclei overcome _____4_______
  and fuse to become helium nuclei
  Fusion releases energy and photons
• Outwards thermal and radiation pressure = inwards gravitational pressure
  _____________5_____________ is formed

Answer 2

1 gravitational attraction

2 temperature rises

3 in core

4 electrostatic or Coulomb repulsion

5 stable main sequence star

Question 3

What causes gas clouds of hydrogen to contract?

Answer 3

gravitational attraction

Question 4

Why does the temperature rise initially during the contraction of a gas cloud?

Answer 4

loss of GPE and gain in KE of particles

Question 5

When is a stable main sequence star formed?

Answer 5

when outward thermal and radiation pressure = inward gravitational pressure

Question 6

When is a star considered to be main sequence?

Answer 6

when outward thermal and radiation pressure = inward gravitational pressure

hydrogen nuclei are being fused into helium nuclei

Question 7

When is a star considered to no longer be a main sequence star?

Answer 7

once it runs out of hydrogen nuclei in its core

Question 8

Explain what defines how long a star remains as a main sequence star?

Answer 8

The mass

Higher the mass the higher the gravitational forces

Core is hotter and hydrogen is used up faster

Question 9

Outline the evolution of a small star such as the sun?

Answer 9

main sequence to red giant to white dwarf

Question 10

What is the difference between a satellite and a comet?

Answer 10

A satellite is an object which orbits another

a comet is a body comprised of ice and dust particles which orbits the sun (sublimates when it gets close to the sun), they usually have an elliptical orbit

Question 11

In full, describe the events which take place during the formation of red giant from a main sequence star

Answer 11

Hydrogen in core is used up

Rate of nuclear fusion decreases - outward pressure drops so gravitational forces cause the core to contract and heats up

Shell of hydrogen around the core now fuses and helium nuclei in the core produce larger nuclei such as carbon and oxygen

Outward thermal pressure increases so outer layers of star expand a lot
Surface of star is much further away from core so it cools and becomes red

Question 12

During the formation of a red giant, what causes the outwards pressure in the core to drop?

Answer 12

hydrogen in the core runs out, rate of nuclear fusion decreases

Question 13

Describe the conversion of energies which take places in the core of a star during the formation of a red giant which causes the increases in temperature?

Answer 13

loss in GPE gain in KE as core contracts

Question 14

What causes the outer layers of a star to expand during the formation of a red giant?

Answer 14

increased outward thermal pressure due to the energy released as helium is fused
Each fusion reaction of Helium releases more energy than Hydrogen.

Question 15

Why does a red giant become red?

Answer 15

Surface of a star is much further away from the core so it cools and becomes red
emits photons of lower energy (larger wavelength)

Question 16

Why can white dwarfs not fuse elements beyond carbon or oxygen?

Answer 16

does not have a large enough mass and therefore not a large enough core pressure or temperature to fuse further

Question 17

What is the name given to the outer layers of the core which are released during the formation of a white dwarf?

Answer 17

planetary nebulae

Question 18

Describe, in full, the process of the formation of a white dwarf from a red giant star

Answer 18

1.Core contracts due to gravitational forces greater than outwards thermal pressure

2.The outer layers are lost from the core forming planetary nebulae

3. The remaining core is small and very hot glowing white

4.No further fusion reactions can take place

Question 19

What are the properties of a white dwarf?

Answer 19

Very high density core
No further fusion reactions
High surface temp apx 30,000K
Small radius and surface area so dim

Question 20

What is the maximum mass of for the formation of a white dwarf?

Answer 20

1.44 solar masses (Chandrasekhar limit)

Question 21

What type of pressure replaces the outward thermal pressure to maintain the stability of a white dwarf?

Answer 21

electron degeneracy pressure

Question 22

What is electron degeneracy pressure?

Answer 22

a quantum effect that prevents all the electrons in the core occupying the lowest energy levels

Question 23

What is the Chandrasekhar limit

Answer 23

the maximum mass of a star which will form a white dwarf, approx 1.44 solar masses

Question 24

Compare the density of the sun, a white dwarf and a red giant

Answer 24

White dwarf is denser than the sun

Sun is denser than a red giant

Question 25

What is another name for the total power output of a star?

Answer 25

luminosity

Question 26

Compare the power of the sun, a white dwarf, and a red giant

Answer 26

Red giant has a higher luminosity than the sun Sun has a higher luminosity than the white dwarf

Question 27

Compare the intensity of the sun, a white dwarf and a red giant

Answer 27

highest for white dwarf, lowest for red giant

Question 28

What does intensity of a star depend on?

Answer 28

its temperature

Question 29

Explain why massive stars spend less time as a main sequence star?

Answer 29

Higher mass means greater gravitational forces Greater force compressing the core leading to a higher core temp Hydrogen nuclei fuse at a higher rate

Question 30

Describe the evolution of massive stars to produce a supergiant

Answer 30

Once hydrogen in the core runs out, outward thermal pressure falls Core contracts and heats up Helium fusion begins in the core plus a shell of surrounding hydrogen Increasing outward pressure Outer layers of the star expand producing a super giant This process continues through many elements producing layers

Question 31

Describe the process that takes place in a supergiant allowing for the formation of neutron stars

Answer 31

As more iron is formed in nuclear fusion, fewer atoms are available for nuclear fusion Outward thermal pressure decreases Gravitational attraction causes the collapse of the star/core forming a supernova Core collapses causing protons and electrons to fuse forming a mass of entirely neutrons

Question 32

Describe the process that takes place in a supergiant allowing for the formation of black holes

Answer 32

As more iron is formed in nuclear fusion, less atoms are available for nuclear fusion Outward thermal pressure decreases Gravitational attraction causes the collapse of the star/core forming a supernova Core collapses to form a black hole where the density causes a gravitational field so strong that escape velocity is > c

Question 33

At what masses do stars form neutron stars?

Answer 33

between 1.44 and 2.5 solar masses

Question 34

At what masses do stars form black holes?

Answer 34

greater then 2.5 solar masses

Question 35

Explain why the light cannot escape a black hole

Answer 35

Density is so high that it creates a gravitational field so strong that Escape velocity > speed of light (3x10^8 ms^-1)

Question 36

Briefly explain the properties of a neutron star?

Answer 36

Core composed of entirely neutrons (As gravitational forces have made the protons and electrons combine) Very small radius and an extremely high density Rotate rapidly and emit radio waves at poles known as pulsars

Question 37

What is a Hertzsprung-Russell diagram?

Answer 37

a 'graph' with luminosity plotted on the y axis and with temperature on the x axis increasing from right to left

Question 38

What is the y axis of a H-R diagram usually plotted as?

Answer 38

using the ratio of Luminosity in W /Lsun (no units for the ratio)

Question 39

Why are logarithmic scales usually used in the axis of a H-R diagram?

Answer 39

large range of values in both axis

Question 40

Explain where stars with the highest mass are plotted on a Hertzsprung-Russell diagram

Answer 40

Highest mass means high surface temp and high luminosity and are therefore plotted in the top left

Question 41

Where are main sequence stars plotted on a H-R diagram?

Answer 41

form a band diagonally to the left down the centre

Question 42

Where are red giants plotted?

Answer 42

high luminosity, low surface temp so towards the top right

Question 43

Where are white dwarfs plotted?

Answer 43

low luminosity and high surface temp so towards the bottom left

Question 44

What is a black body?

Answer 44

An object which absorbs all incident EM radiation and emits its own spectrum depending on the surface temperature

Question 45

What is a blackbody spectrum?

Answer 45

the spectrum containing a range of wavelengths and intensities of em radiation emitted from an object

Question 46

What is the peak of a black body spectrum called?

Answer 46

peak wavelength (lambda max in equations)

Question 47

What is the relationship between the surface temperature of a star and it's peak wavelength?

Answer 47

peak emission wavelength is inversely proportional to absolute surface temperature

Question 48

What is the relationship between intensity for all wavelengths and temperature (in black body radiation)?

Answer 48

intensity increases with temperature

Question 49

How can Wien's displacement law be used in calculations for astrophysics?

Answer 49

Lambda max x T = constant

Question 50

Define luminosity in full

Answer 50

Is the power of a star Is the total energy emitted per unit of time, measured in Watts

Question 51

What two factors is luminosity dependent on?

Answer 51

L is proportional to the surface area of the star L is proportional to the 4th power of the absolute temperature in kelvin

Question 52

What does AU stand for?

Answer 52

astronomical unit

Question 53

What is an astronomical unit?

Answer 53

the mean distance between the earth and the sun

Question 54

What is the value for an AU?

Answer 54

1.50*10^11 m

Question 55

What is one light year?

Answer 55

the distance travelled by light in a year

Question 56

What is stellar parallax?

Answer 56

the apparent movement of a star due to movements of the observers position against a background of distant stars

Question 57

What is the unit for measurement of degrees of a parallax angle?

Answer 57

arc-second

Question 58

Definition of a parsec?

Answer 58

a star at a distance one parsec produces a parallax angle of 1 arc-second in a triangle base of 1 astronomical unit

Question 59

Describe the relationship between the parallax angle p and distance of a star d

Answer 59

p=1/d 2p is the angle measured when the observer on Earth moves 6 months

Question 60

What is the symbol for parallax angle?

Answer 60

p

Question 61

What is the symbol for distance of a star?

Answer 61

d

Question 62

Describe the measurements that must be taken to determine the distances of stars

Answer 62

Telescope is used to measure the angle by which a star moves in six month intervals in arc seconds This angle is 2p Then the formula of d=1/p to find the distance in parsecs