Astrophysics and Cosmology

Question 1

What is one Astronomical Unit (AU)?

Answer 1

The mean distance between the sun and the earth

Question 2

What is nuclear fusion?

Answer 2

The process of two nuclei joining together and releasing energy from a change in binding energy

Question 3

What is a light year?

Answer 3

The distance that it takes one year for light to travel

Question 4

What is a parsec?

Answer 4

The distance at which the mean radius of the earth’s orbit subtends an angle of one second of arc.

Question 5

What is the distance to a nearby star given by in terms of parsecs?

Answer 5

d (distance in parsecs) = 1 / p (parallax in arcseconds)

Question 6

What is gravitational collapse?

Answer 6

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 gas and dust. Gravitational collapse occurs in a mature star when the internal gas and radiation pressure can no longer support the star’s own mass.

Question 7

What is radiation pressure?

Answer 7

Pressure acting outwards due to the momentum of photons released in fusion reactions.

Question 8

What is a main sequence star?

Answer 8

A star in the main part of its life cycle. Where it is fusing hydrogen to form helium in its core.

Question 9

What is a red giant?

Answer 9

A star in the later stages of its life, that has nearly exhausted all the hydrogen in its core and is now fusing with helium nuclei. It is bigger than a normal star because its surface layers have cooled and expanded.

Question 10

What is a white dwarf?

Answer 10

The end product of a low-mass star, when the outer layers have dispersed into space. A white dwarf is very dense, with a high surface temperature and luminosity.

Question 11

What is a planetary nebula?

Answer 11

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

Question 12

What is electron degeneracy pressure?

Answer 12

The pressure that stops the gravitational collapse of a low-mass star (below the Chandrasekhar limit of 1.4 solar masses), stopping a white dwarf from collapsing.

Question 13

What is a red super giant?

Answer 13

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

Question 14

What is a supernova?

Answer 14

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

Question 15

What is a stellar parallax?

Answer 15

The apparent shifting in position of a star viewed against a background of distant stars when viewed from different positions of the earth such as at different positions of the earth’s orbit around the sun.

Question 16

What is the life cycle of a low-mass star?

Answer 16

stellar nebula -> main sequence star -> red giant -> planetary nebula -> white dwarf

Question 17

What is the life cycle of a high-mass star?

Answer 17

stellar nebula -> massive star -> red super giant -> supernova -> black hole/neutron star

Question 18

What is a neutron star?

Answer 18

The remains of the core of a red super giant after it has undergone a supernova explosion. It is extremely dense and composed of mainly neutrons.

Question 19

What is a black hole?

Answer 19

The core of a massive star that has collapsed almost to a point. Black holes are very dense and small with a gravitational field so strong that light can’t escape.

Question 20

What is the luminosity of a star?

Answer 20

The total energy the star emits per second.

Question 21

What does the Hertzsprung-Russel diagram show?

Answer 21

This shows the positions of stars on a scatter graph based on their luminosities and their temperatures and temperatures.

Question 22

What is a continuous spectrum?

Answer 22

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

Question 23

What are energy levels?

Answer 23

They are the specific energies that electrons can have when occupying certain orbits. Electrons can only occupy these discrete energy levels and cannot exist at other energy values between them.

Question 24

What is an emission line spectrum of an element?

Answer 24

The spectrum of frequencies of EM radiation emitted due to electron transitions from a higher energy level to a lower one within an atom of that element. Since there are many possible electron transitions for each atoms, there are many different radiated wavelengths. A line spectrum consists of a series of bright lines against a dark background.

Question 25

What is an absorption line spectrum?

Answer 25

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 gas. The dark lines represent the wavelengths absorbed,

Question 26

What is Wein’s displacement law?

Answer 26

λmax ∝ 1/T

Question 27

What is the Doppler effect/Doppler shift?

Answer 27

The change in wavelength of waves due to the relative motion of the wave source and its observer.

Question 28

What is red shift?

Answer 28

The apparent increase in wavelength of EM radiation due to a source moving away from the observer.

Question 29

What does Hubble’s law state?

Answer 29

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

Question 30

What do spectral lines show in relation to red shift?

Answer 30

Spectral lines examined from stars travelling away from us have a pattern of lines shifted towards the red end of the spectrum.

Question 31

Why can’t the value of Hubble’s constant be determined to a high degree of accuracy?

Answer 31

It is difficult to measure accurately the distance of galaxies that are far away.

Question 32

What is cosmic microwave background radiation?

Answer 32

Radiation originating from the big bang, when the universe has cooled to 3000 K. As the universe has expanded this radiation is now just a faint microwave glow with peak wavelength corresponding to 2.7 K.

Question 33

What does the big bang theory state?

Answer 33

That the universe was created from a single “point” where all of the universe’s current mass was situated. At the time of its creation, the universe was much smaller, hotter and denser than it is now. Time and space were both created at the instance of the big bang.

Question 34

What does the cosmological principle state?

Answer 34

On a large scale, the universe is isotropic (same in all directions) and homogeneous (uniform density in large volumes)

Question 35

What is dark matter?

Answer 35

matter which cannot be seen and that does not emit or absorb EM radiation. It is not detected directly but is directed based on its gravitational effects relating to either the rotation of galaxies or by gravitational lensing of starlight.

Question 36

What is dark energy?

Answer 36

A type of energy that permeates the whole universe and opposes the attractive force of gravitation because of negative pressure. It is not directed directly but it is known to exist as the universe is accelerating as it expands.

Question 37

What are the four main contenders for dark matter?

Answer 37

WIMPs (Weakly Interacting Massive Particles), MACHOs (Massive Astrophysical Compact Halo Objects), Axions, Neutrinos

Question 38

What was the universe like at 10-43 seconds?

Answer 38

Infinitely dense, small and hot.

Question 39

What was the universe like from 10-43 to 10-4 seconds?

Answer 39

The period of inflation, matter and antimatter were formed, quarks, leptons and photons formed.

Question 40

What was the universe like at 10-4 seconds?

Answer 40

The universe has cooled enough for quarks to form protons and neutrons but no atoms. Lost of photons due to matter-antimatter annihilation.

Question 41

What was the universe like at 100 seconds?

Answer 41

The universe has cooled to the temperature of the core of a star. Helium and lithium nuclei formed, matter in the universe in plasma form.

Question 42

What was the universe like at 250 000 years?

Answer 42

hydrogen and helium formed, photons travel freely through space which form the CMBR we see today.

Question 43

What was the universe like at 1 million years?

Answer 43

tiny density fluctuations cause galaxies to begin to be formed.

Question 44

What was the universe like at 1 billion years?

Answer 44

Heavy elements are formed by the gravitational collapse of stars.