ASTROPHYSICS

Question 1

Red giant

Answer 1

A giant star with a relatively low surface temperature. That fuses helium nuclei in its core to form carbon

Question 2

Nebula

Answer 2

A cloud of gas or dust in interstellar space

Question 3

White dwarf

Answer 3

High surface temperature and low luminosity very small radius so small surface area remnant of a low mass star

Question 4

Black dwarf

Answer 4

The theoretical end point of the cycle of a low - mass star

Question 5

Red supergiant

Answer 5

Very hot, massive, luminous star with a short life span. iron is the heaviest element produced in the core as it has the highest binding energy per nucleon and cannot be fused together

Question 6

Supernova

Answer 6

The explosion that occurs when a massive star’s core no longer undergoes fusion and the core collapses under gravity and the outer layers explode in a supernova

Question 7

Neutron star

Answer 7

A dense ball of neutrons that remains at the core of a star after a supernova explosion. A star’s core becomes a neutron star if its between 1.4 -2.5 solar masses.

Question 8

Pulsar

Answer 8

A rotating neutron star that emits energy in the form of radio waves at regular intervals

Question 9

Black hole

Answer 9

A remnant of a massive star formed when its matter collapses to a point (singularity). A star’s core only becomes a black hole if its is more than 2.5 solar masses

Question 10

Binary star

Answer 10

A system of two stars orbiting around their common centre of gravity

Question 11

Black body

Answer 11

An object that is a perfect emitter and absorber of radiation, a black body absorbs all wavelengths that fall upon it and radiates all wavelength, the spectrum of wavelengths emitted only depends on its temperature.

Question 12

Black body radiation

Answer 12

The radiation emitted from an object due to its temperature.

Question 13

Hertzsprung Russell diagram

Answer 13

Logarithmic scaled graph of luminosity of stars against their surface temperature, on the temperature scale at each interval temperature halves and high temperatures are on the left

Question 14

Stefan Boltzmann law

Answer 14

L= 4pi sigma r^2 T^4

where:
L= Luminosity
r= radius of star
T= temperature of star

Question 15

Boltzmann constant,K

Answer 15

Relates the average energy of a molecule to its absolute temperature

Question 16

Wien’s law

Answer 16

lambda peak x T= 2.898 x 10^-3 mk

Question 17

Emission spectra

Answer 17

Characteristic colours (frequencies) of light given out by an element when electrons are excited.

Question 18

Main sequence

Answer 18

Main group of stars on a Hertzsprung: Russell diagram, these are starts that fuse hydrogen nuclei to helium in their core and remain at a constant diameter.

Question 19

Light year

Answer 19

Distance travelled by light in one year

Question 20

Astronomical unit

Answer 20

The radius of the orbit of the Earth around the sun, 1Au = 1.5 X 10^11m

Question 21

Trigonometric parallax

Answer 21

The apparent displacement of an object when viewed along two different lines of sight, relative to a fixed background of stars

Question 22

Stellar parallax

Answer 22

The apparent shifting of stars against a fixed background when observed from different positions in the Earth’s orbit around the sun

Question 23

Parsec

Answer 23

Distance that gives a parallax angle of 1 arc second, 3.26 light years

Question 24

Standard candles

Answer 24

Astronomical objects of known luminosity

Question 25

Luminosity

Answer 25

Rate at which a source radiates energy

Question 26

Big bang theory

Answer 26

The universe evolved from a single hot explosion

Question 27

Red shift

Answer 27

EDIT

Question 28

Closed universe

Answer 28

The universe will expand until a point where the universe’s density is so high that the high gravitational forces cause the universe to contract ‘Big Crunch.’ This happens when the density of the universe is greater than the critical density.

Question 29

Big Crunch

Answer 29

A possible fate for the universe where it collapses back on itself to a singularity

Question 30

flat universe

Answer 30

The universe will continue to expand until an infinite amount of time . The Universe has a density equal to the critical density

Question 31

Critical density

Answer 31

The density of the universe that just allows it to stop the expansion of space after infinite time

Question 32

Dark matter

Answer 32

Matter in the universe that can’t be detected by emission or absorption of radiation

Question 33

Hubble’s law

Answer 33

The speed of a receding galaxy is proportional to its distance from the observer

Question 34

Hubble’s constant

Answer 34

Ratio of the speed v of a receding galaxy to its distance d from an observer

Question 35

Using parallax to determine distance:

Answer 35

1. Record the angular displacement of a nearby star relative to a fixed background of distant stars
2. Over a period of 6 months
3. We know the distance from the earth to the sun is 1AU
4. Use trigonometry to determine the distance to the star

Question 36

Using standard candles to determine distance

Answer 36

1. Identify a standard candle in the star cluster/galaxy and know its luminosity.
2. Measure the intensity of the star on earth (I).
3. Rearrange I = L/ 4πd2 to d = √𝐿4𝜋𝐼

Question 37

Explain how Hubble’s law predicts an expanding universe

Answer 37

1. Hubble’s law states that the velocity of a galaxy is directly proportional to the distance of the galaxy
2. So the most distant galaxies are receding the fastest
3. This means that at the start of the universe, all matter must have been concentrated in a single point and has been expanding ever since the big bang
4. So all of space has been expanding since the beginning of time

Question 38

Using Hubble’s law and redshift determine how to calculate distance:

Answer 38

1. Measure the wavelength of spectral lines from distant galaxies and compare it to laboratory values of the spectral lines λ
2. Calculate apparent shift in wavelength Δλ of spectral lines of distant galaxies and determine redshift z = Δλ /λ
3. Use the equation z = v/c to determine the recessional velocity v of the galaxy (c is the speed of light)
4. Rearrange Hubble’s law d = v/H0 to calculate the distance to the galaxy

Question 39

Explain what happens to a star like our sun at the end of its main sequence

Answer 39

1. When hydrogen fusion stops in core (main sequence ends), Temperature cools down gravity is greater than the outwards pressure due to nuclear fusion .
2. Core contracts
3. Temperature and density increases enough for fusion of helium nuclei in core to start
4. Helium fusion stops the core collapses again under gravity
5. For stars the same size as our sun: the outer layers of the sun are ejected into space
6. The temperature does not rise enough for further fusion to begin
7. The core becomes a white dwarf

Question 40

The Doppler effect:

Answer 40

An increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other.

Question 41

Calculating the age of the universe

Answer 41

1. Using speed= distance / time ( V=d/t)
2. v= H0d
3. Equating them so : d/t = H0d
4. Cancelling d
5. 1/t = H0
6. t= 1/H0

Question 42

Explain why the exact age of the universe is uncertain

Answer 42

1. Hubble’s constant is determined from measurements of the distance to distant galaxies and redshift.
2. Measurements of distance have high uncertainties.
3. So Hubble’s constant is uncertain.
4. So age of universe = 1/H0 is uncertain.
5. Supernovae observations indicated dark energy causing universe to accelerate in its expansion.
6. Assumption in derivation of t=1/H0 is that universe has expanded at a constant rate (if it has been accelerating due to dark energy this is not true)

Question 43

Explain why the ultimate fate of the universe is uncertain

Answer 43

1. We cannot observe dark matter directly – we are uncertain about how much there is.
2. So calculating the average density of the universe has uncertainty.
3. We do not know the nature of dark energy and how it interacts with gravity.
4. So we cannot predict how gravity/dark energy will affect the ultimate fate.