Chapter 20 - Cosmology

Question 1

Planets

Answer 1

Planets are objects with mass sufficient for their own gravity to force them to take a spherical shape, where no nuclear fusion occurs, and the object has cleared its orbit of other objects.

Question 2

Dwarf planets

Answer 2

Planets where the orbit has not been cleared of other objects.

Question 3

Planetary Satellites

Answer 3

Bodies that orbit a planet.

Question 4

Asteroids and Comets

Answer 4

Asteroids - objects which are too small and uneven i shape to be planets, with a near circular orbit around the sun.
Comets - small, irregularly sized balls of rock, dust and ice. They orbit the sun in eccentric elliptical orbits.

Question 5

Solar Systems

Answer 5

The systems containing stars and and orbiting objects like planets.

Question 6

Galaxies

Answer 6

A collection of stars, dust, and gas. Each galaxy contains around 100 billion stars.

Question 7

Formation of a Star

Answer 7

· Stars form in galaxy nebulae.
· Gravitational attraction causes clouds of dust and gas to form
· The cloud becomes denser, causing the gravitational collapse to accelerate.
· GPE is converted to Thermal Energy.
· Resultant sphere is very hot and dense, this is a protostar.
· To finally become a star the temperature and pressure must be high enough to overcome the electrostatic forces of repulsion and allow nuclear fusion to occur.

Question 8

What is solar mass?

Answer 8

Solar mass is the mass of our sun’s core.
Mₒ = 1.99 x 10³⁰ kg

Question 9

Evolution of Low Mass Star

Answer 9

· Low mass stars has a core mass between 0.5Mₒ and 10Mₒ
· React slower so stay in main sequence for longer.
· Once hydrogen supply is low, the gravitational forces inwards the radiation and gas pressure outward. Causing the star to collapse inward. Evolving into a red giant.
· Red giant core is too cool for helium to fuse, but the pressure in the outer shell is great enough for fusion to occur.
· As the helium nuclei run low the red giant evolves into a white dwarf. Outer shells drift off into space as a planetary nebula.

Question 10

White Dwarfs

Answer 10

3000K
No fusion
Emit photons to cool
Electron degeneracy pressure prevents the core from collapsing
White dwarf is stable if it is below the Chandrasekhar limit, this is if the core is below 1.44Mₒ.

Question 11

Evolution of High Mass Star

Answer 11

· Stars Mass exceeds 10Mₒ
· As hydrogen supply decreases, the star fuses helium nuclei as it has enough temperature and pressure. Forming a red super giant.
· Red supergiants have layers of elements, with the heaviest in the centre, iron core.
· No fusion above Iron.
· Iron core causes star to become unstable, causing a supernova to take place and the core collapse.
· If remaining mass of core is greater than 1.44Mₒ a neutron star is formed.
· If remaining mass of core is greater than 3Mₒ a black hole is formed.

Question 12

Energy levels of electrons

Answer 12

Electrons are bound to fixed energy levels.
When electrons move up energy levels from their ground state they absorb energy.
When electrons drop down energy levels they release energy.

Question 13

Types of Spectras

Answer 13

Emission Spectra - Black Background with coloured lines
Absorption Spectra - Coloured Background with black lines.
Continuous Spectra - Full coloured spectrum.

Question 14

Wavelength of Light from emission and absorption

Answer 14

ΔE = hc/λ

Question 15

What is a diffraction grating and its equation

Answer 15

Components with regularly spaced slits that can diffract light.
dsinϴ = nλ

Question 16

Wein’s Law

Answer 16

The black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature of the object.
λmax ⍺ 1/T
λmax = Wein’s Constant / T
Weins constant = 2.9 x 10⁻³

Question 17

Stefan’s Law

Answer 17

For a black body, the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature.
L ⍺ 4πr²T⁴
L = 4πr²T⁴σ
σ = Stefans Constant = 5.67 x 10⁻⁸

Question 18

What is an Astronomical Unit?

Answer 18

The average distance from the Earth to the sun. 1AU = 1.5 x 10^11 m

Question 19

What is a Light Year?

Answer 19

The distance light travels in one year. 1 ly = 9.46 x 10^15 m

Question 20

What are arcminutes and arcseconds?

Answer 20

In terms of angles, 1 degree contain 60 arcminutes and 3600 arcseconds.

Question 21

What is a Parsec?

Answer 21

A parsec is defined as the distance at which a radius of 1 AU subtends an angle of 1 arcsecond. 1 pc = 3.1 x 10^16 m
parsec = 1AU / tan(θ)

Question 22

What is Stellar Parallax?

Answer 22

Stellar Parallax can be used to measure the distance to nearby stars. Parallax is the apparent shift in position of an object against a backdrop of distance objects.
Accurate up to 100 pc, angles becomes too small to measure.
Equation:
d = 1/p
Where d is the distance and p is the parallax angle. in parsecs and arcseconds.

Question 23

What is the Cosmological Principle?

Answer 23

Defines the formation of the universe on a large scale.
The universe is isotropic and homogenous, and the laws of physics are universal.
Isotropic means that the universe is the same in all directions to every observer , having no center nor edges.
Homogenous means that matter is uniformly distributed, for a large volume of the universe the density is the same.

Question 24

What is the Doppler Effect?

Answer 24

The Doppler Effect is the apparent shift in wavelength when the source of the wave is moving relative to the observer.
If the source is moving towards the observer the wavelength decreases.
If the source is moving away from the observer the wavelength increases.
Actual wavelength stays the same only appears to change.

Question 25

How can the Doppler Effect be used?

Answer 25

In star light, the Doppler Effect shifts the position of spectral lines. By comparing this to an unshifted hydrogen emission spectrum we can determine the relative speed of a star.
Change in Wavelength / Wavelength in lab = Relative Speed / Speed of Light

Question 26

What is Hubble’s Law

Answer 26

Hubble’s law states that the recessional velocity of a galaxy is proportional to its distance from earth.
Meaning that the further away a star is from Earth, the faster it is moving.
V = H0 x d
Hubble’s Constant (H0) = 67.8 kms^-1 Mpc^-1

Question 27

What does Hubble’s Law tell us?

Answer 27

Hubble’s Law is evidence that the universe is expanding. Almost all light from distant galaxies is red shifted, showing that they are moving away from Earth.
This suggests that the fabric of space and time is expanding, and any point in the universe is moving away from any other point.

Question 28

How can Hubble’s Law be used to estimate the age of the universe?

Answer 28

Hubble’s Law can be used to estimate the age of the universe. If initially all points in the universe were together, then the distance of a galaxy from Earth and its speed are related to the time taken for the galaxy to reach this distance away from Earth.
V = H0 x D and t = d / V
t = 1 / H0, with H0 in seconds

Question 29

What is the Big Bang Theory?

Answer 29

The Big Bang Theory describes how the early universe was a singularity, which suddenly exploded and expanded outward. And is still now expanding.

Question 30

What evidence is there for the Big Bang Theory?

Answer 30

Hubble’s Law shows the universe is expanding, through red shift.
Microwave background radiation, scientist found a constant radiation interference. There was originally high energy gamma photons, but as the universe expanded, the wavelength of these photons was stretched into the microwave region. Only a theory as we can’t recreate the conditions of the Big Bang.

Question 31

What is Dark Energy?

Answer 31

Dark energy is a hypothetical form which fills all of space and accelerates expansion. Should make up around 68% of the total energy in the universe.

Question 32

What is Dark Matter?

Answer 32

Observable mass is concentrated in the center but experiments showed mass is not concentrated in the center of the galaxy, so there must be Dark Matter we cant see. Around 27% of all mass is dark mass.

Question 33

What are the two theories for the future of the universe?

Answer 33

In the future the universe could either be open, closed or flat.
Open - it would continue expanding for all of time.
Closed - stop expanding and collapses in on itself.
Flat - stop expanding and remain at a fixed size.