Module 5 Life of Stars Flashcards
In full, describe the events which take place during the formation of a star
- 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
FORMATION OF A STAR
-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
1 gravitational attraction
2 temperature rises
3 in core
4 electrostatic or Coulomb repulsion
5 stable main sequence star
What causes gas clouds of hydrogen to contract?
gravitational attraction
Why does the temperature rise initially during the contraction of a gas cloud?
loss of GPE and gain in KE of particles
When is a stable main sequence star formed?
when outward thermal and radiation pressure = inward gravitational pressure
When is a star considered to be main sequence?
when outward thermal and radiation pressure = inward gravitational pressure
hydrogen nuclei are being fused into helium nuclei
When is a star considered to no longer be a main sequence star?
once it runs out of hydrogen nuclei in its core
Explain what defines how long a star remains as a main sequence star?
The mass
Higher the mass the higher the gravitational forces
Core is hotter and hydrogen is used up faster
Outline the evolution of a small star such as the sun?
main sequence to red giant to white dwarf
What is the difference between a satellite and a comet?
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
In full, describe the events which take place during the formation of red giant from a main sequence star
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
During the formation of a red giant, what causes the outwards pressure in the core to drop?
hydrogen in the core runs out, rate of nuclear fusion decreases
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?
loss in GPE gain in KE as core contracts
What causes the outer layers of a star to expand during the formation of a red giant?
increased outward thermal pressure due to the energy released as helium is fused
Each fusion reaction of Helium releases more energy than Hydrogen.
Why does a red giant become red?
Surface of a star is much further away from the core so it cools and becomes red
emits photons of lower energy (larger wavelength)
Why can white dwarfs not fuse elements beyond carbon or oxygen?
does not have a large enough mass and therefore not a large enough core pressure or temperature to fuse further
What is the name given to the outer layers of the core which are released during the formation of a white dwarf?
planetary nebulae
Describe, in full, the process of the formation of a white dwarf from a red giant star
1.Core contracts due to gravitational forces greater than outwards thermal pressure
2.The outer layers are lost from the core forming planetary nebulae
- The remaining core is small and very hot glowing white
4.No further fusion reactions can take place
What are the properties of a white dwarf?
Very high density core
No further fusion reactions
High surface temp apx 30,000K
Small radius and surface area so dim
What is the maximum mass of for the formation of a white dwarf?
1.44 solar masses (Chandrasekhar limit)
What type of pressure replaces the outward thermal pressure to maintain the stability of a white dwarf?
electron degeneracy pressure
What is electron degeneracy pressure?
a quantum effect that prevents all the electrons in the core occupying the lowest energy levels
What is the Chandrasekhar limit
the maximum mass of a star which will form a white dwarf, approx 1.44 solar masses
Compare the density of the sun, a white dwarf and a red giant
White dwarf is denser than the sun
Sun is denser than a red giant
What is another name for the total power output of a star?
luminosity
Compare the power of the sun, a white dwarf, and a red giant
Red giant has a higher luminosity than the sun
Sun has a higher luminosity than the white dwarf
Compare the intensity of the sun, a white dwarf and a red giant
highest for white dwarf, lowest for red giant
What does intensity of a star depend on?
its temperature
Explain why massive stars spend less time as a main sequence star?
Higher mass means greater gravitational forces
Greater force compressing the core leading to a higher core temp
Hydrogen nuclei fuse at a higher rate
Describe the evolution of massive stars to produce a supergiant
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
Describe the process that takes place in a supergiant allowing for the formation of neutron stars
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
Describe the process that takes place in a supergiant allowing for the formation of black holes
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
At what masses do stars form neutron stars?
between 1.44 and 2.5 solar masses
At what masses do stars form black holes?
greater then 2.5 solar masses
Explain why the light cannot escape a black hole
Density is so high that it creates a gravitational field so strong that
Escape velocity > speed of light (3x10^8 ms^-1)
Briefly explain the properties of a neutron star?
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
What is a Hertzsprung-Russell diagram?
a ‘graph’ with luminosity plotted on the y axis and with temperature on the x axis increasing from right to left
What is the y axis of a H-R diagram usually plotted as?
using the ratio of Luminosity in W /Lsun
(no units for the ratio)
Why are logarithmic scales usually used in the axis of a H-R diagram?
large range of values in both axis
Explain where stars with the highest mass are plotted on a Hertzsprung-Russell diagram
Highest mass means high surface temp and high luminosity and are therefore plotted in the top left
Where are main sequence stars plotted on a H-R diagram?
form a band diagonally to the left down the centre
Where are red giants plotted?
high luminosity, low surface temp so towards the top right
Where are white dwarfs plotted?
low luminosity and high surface temp so towards the bottom left
What is a black body?
An object which absorbs all incident EM radiation and emits its own spectrum depending on the surface temperature
What is a blackbody spectrum?
the spectrum containing a range of wavelengths and intensities of em radiation emitted from an object
What is the peak of a black body spectrum called?
peak wavelength
(lambda max in equations)
What is the relationship between the surface temperature of a star and it’s peak wavelength?
peak emission wavelength is inversely proportional to absolute surface temperature
What is the relationship between intensity for all wavelengths and temperature (in black body radiation)?
intensity increases with temperature
How can Wien’s displacement law be used in calculations for astrophysics?
Lambda max x T = constant
Define luminosity in full
Is the power of a star
Is the total energy emitted per unit of time, measured in Watts
What two factors is luminosity dependent on?
L is proportional to the surface area of the star
L is proportional to the 4th power of the absolute temperature in kelvin
What does AU stand for?
astronomical unit
What is an astronomical unit?
the mean distance between the earth and the sun
What is the value for an AU?
1.50*10^11 m
What is one light year?
the distance travelled by light in a year
What is stellar parallax?
the apparent movement of a star due to movements of the observers position against a background of distant stars
What is the unit for measurement of degrees of a parallax angle?
arc-second
Definition of a parsec?
a star at a distance one parsec produces a parallax angle of 1 arc-second in a triangle base of 1 astronomical unit
Describe the relationship between the parallax angle p and distance of a star d
p=1/d
2p is the angle measured when the observer on Earth moves 6 months
What is the symbol for parallax angle?
p
What is the symbol for distance of a star?
d
Describe the measurements that must be taken to determine the distances of stars
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
