Module 5.5 - Astrophysics And Cosmology

Question 1

Explain gravitational collapse

Answer 1

The inward movement of material in a star due to the gravitational force caused by its own mass
Star formation is due to the gradual gravitational collapse of a cloud of dust and gas (nebula)
Occurs in a mature star when the internal gas and radiation pressure can no longer support the star’s own mass

Question 2

Define radiation pressure

Answer 2

Due to the momentum of photons released in fusion reactions and acts outwards (direction of energy flow)

Question 3

How does gas pressure act inside a star?

Answer 3

Acts in all directions at a point inside a gas (e.g. inside a star)

Question 4

What is a main sequence star?

Answer 4

A star in the main part of its life cycle, where it is funding hydrogen to form helium in its core
Shown as a curved band on a plot of luminosity against temperature (Hertzsprung-Russell diagram)

Question 5

What is a red giant?

Answer 5

A star in the later stages of its life that has nearly exhausted the hydrogen in its core and is now funding helium nuclei
Bigger than a normal star as its surface layers have cooled and expanded

Question 6

What is a white dwarf?

Answer 6

The end product of a low-mass star, where the outer layers have dispersed into space
Very dense, high surface temperature, low luminosity

Question 7

What is a planetary nebula?

Answer 7

An expanding, glowing shell of ionised hydrogen and helium ejected from a red giant at the end of its life

Question 8

Define electron degeneracy pressure

Answer 8

Pressure that stops the gravitational collapse of a low-mass star (below the Chandrasekhar limit of 1.4 solar masses)
Pressure that prevents a white dwarf from collapsing

Question 9

What is the Chandrasekhar limit?

Answer 9

Maximum possible mass for a stable white dwarf and is equal to 1.4 times the mass of our Sun
White dwarfs with masses above this will collapse further to become neutron stars or black holes

Question 10

What is a red super giant?

Answer 10

A star that has exhausted all the hydrogen in its core and has a mass much higher than the Sun

Question 11

What is a supernova?

Answer 11

A huge explosion produced when the core of a red super giant collapses

Question 12

What is a neutron star?

Answer 12

The remains of a core of a red super giant after it has undergone a supernova explosion
Extremely dense and composed mainly of neutrons

Question 13

What is a black hole?

Answer 13

The core of a massive star that has collapsed almost to a point
Very dense and very small, with a gravitational field so strong that light can’t escape (the escape velocity is greater than the speed of light)

Question 14

What is the Hertzsprung-Russell (HR) diagram?

Answer 14

A luminosity-temperature graph

Question 15

Define luminosity

Answer 15

The total energy emitted (by a star) per second

Question 16

Describe the formation of a star up until it becomes a main sequence star

Answer 16

Dust and gas come together (nebula) by gravitational collapse
As gravity pulls more matter together, more work is done on particles so they gain more kinetic energy and increase temperature (can be hot enough to glow) - Protostar
Temperature at the core of the protostar will reach millions of degrees Kelvin and increased KE increases chance of fusion occurring
Fusion of hydrogen nuclei to helium nuclei occurs
Very large amounts of energy released, momentum of photons released by fusion leads to outwards acting radiation pressure
When radiation pressure and gas pressure = gravitational collapse, the star is stable and is now a main sequence star
Will remain in this state for most of its life, converting hydrogen to helium through nuclear fusion

Question 17

Describe the effect of the fusion of hydrogen nuclei to helium nuclei

Answer 17

Four protons are converted into one helium-4 nucleus with the production of 2 gamma ray photons, 2 neutrinos and 2 positrons

Question 18

Describe the fate of low-mass stars

Answer 18

Occurs if the star is less than 1.4 times the mass of the Sun
Begins when most hydrogen nuclei have been fused into helium, so nuclear fusion stops
Radiation pressure will also stop and star will experience net inwards force due to gravitational collapse
Core will contract and its temperature will increase
Still a large amount of hydrogen surrounding core, and will get hotter as core contracts and releases thermal energy
Outer layers cool and expand to cover greater volume than original star, forming a red giant
Process in the core continues, and eventually core is hot enough for fusion of helium nuclei, producing heavier elements (e.g. carbon and oxygen)
Large amounts of energy released with these reactions, increasing radiation pressure
When helium fusion is finished, star is not hot enough for further fusion so process stops
Star becomes unstable and begins to collapse again
Outer layers of gas may be ejected back into space, forming planetary nebula
Rest of the star continues to collapse under own mass and heat up to a point where it can collapse no further, leaving a white dwarf

Question 19

Describe the fate of high-mass stars

Answer 19

Core contracts under gravity and outer layers cool and expand to many times the star’s original size to form a red super giant
As core collapses and heats up, more nuclear fusion occurs, with fusion of heavier elements possible at higher temperatures and pressures
In each stable fusion phase, electron degeneracy and radiation pressure prevent gravitational collapse
Fusion continues until iron core builds up, which then collapses
Core will undergo gravitational collapse, and loss of gravitational potential energy produces intense heating
During final seconds of collapse the gravitational pressure forces protons and electrons to combine to form neutrons
Triggers explosive blowing out of outer shell, releasing energy (supernova)
Core can remain intact as a neutron star
If core mass is greater than 3 or 4 solar masses then the pressure on the core is so large that the neutron star would collapse to the point where the density becomes infinite, so the gravitational field is so strong that nothing, including light, can escape

Question 20

What is a continuous spectrum?

Answer 20

A spectrum that appears to contain all wavelengths over a comparatively wide range

Question 21

Define energy levels

Answer 21

Quantised energy that electrons can have when occupying specific orbits

Question 22

What is the emission line spectrum of an element?

Answer 22

The spectrum of frequencies of EM radiation emitted due to electron transitions from a higher energy level to a lower one

Question 23

What is an absorption line spectrum?

Answer 23

The pattern of dark lines in a continuous spectrum from a light source and is caused by light passing through an absorbing medium such as a gas
Dark lines represent absorbed wavelengths

Question 24

What is a transmission diffraction grating?

Answer 24

A glass surface with a large number of very fine parallel grooves/slits
Used to produce optical spectra by diffraction of transmitted light

Question 25

Which energy level is the lowest in an atom?

Answer 25

The level closest to the nucleus

Question 26

What is the energy required to excite an electron the same as?

Answer 26

The energy difference between the two levels

Question 27

What happens when an excited electron falls from a higher energy level to a lower one?

Answer 27

Electromagnetic radiation of a fixed energy and wavelength is emitted

Question 28

State Wien’s displacement law

Answer 28

Maximum wavelength is proportional to 1/temperature

Maximum wavelength x temperature = constant = 2.89 x 10^-3 m K

Question 29

What is Wien’s law used for?

Answer 29

Estimate the peak surface temperature of a star from the wavelength at which the star’s brightness is maximum

Question 30

Define luminosity

Answer 30

The total energy emitted by a star per second

Question 31

Difficulties in using Stefan’s and Wien’s laws accurately

Answer 31

Earth’s atmosphere only allows certain wavelengths of EM radiation through
Dust and light pollution will affect the quality of data collected
Detectors are often wavelength-sensitive, so some detectors do not respond to some wavelengths, giving an inaccurate result for max. wavelength of a star which radiates mainly in that region

Question 32

How to collect more accurate results for Stefan’s and Wien’s laws

Answer 32

Place telescopes at high altitudes
Place telescopes on spacecraft orbiting the Earth
(Escape Earth’s atmosphere, light pollution and dust)

Question 33

Define the astronomical unit (AU)

Answer 33

The mean distance from the centre of the Earth to the centre of the Sun

Question 34

Define the parsec

Answer 34

A unit of distance that gives a parallax angle of 1 second of arc (1/3600 of a degree), using the radius of the Earth’s orbit (1 AU) as the baseline of a right-angled triangle

Question 35

Define stellar parallax

Answer 35

The apparent shifting in position of a star viewed against a background of distant stars when viewed from different positions of the Earth’s orbit around the Sun

Question 36

Define a light year

Answer 36

The distance travelled by light in one year

Question 37

Define the Doppler effect/shift

Answer 37

The change in wavelength caused by the relative motion between the wave source and an observer

Question 38

Define red shift

Answer 38

The apparent increase in wavelength of EM radiation caused when the source is moving away, relative to the observer

Question 39

State Hubble’s law

Answer 39

The recessional velocity of a galaxy is directly proportional to its distance from the Earth

Question 40

Describe the evolution of the universe from the Big Bang to present

Answer 40

Universe started as infinitely hot and infinitely dense singularity
Big Bang occurred, caused sudden expansion and cooling
First few seconds of universe’s existence was as a quark ‘soup’
Cools enough for protons/hydrogen to form
Universe behaves like core of a star, helium begins to form
Continues cooling to current temperature of 2.7K
Due to cooling, energy is lost so the original high energy radiation has been stretched/expanded with red shift to microwave radiation (CMBR)

Question 40

Cosmological principle

Answer 40

Universe is isotropic (same in all directions)
Universe is homogeneous (of uniform density)
Universe is subject everywhere to same physical laws and models applied on Earth (laws of physics are universal)