Astrophysics And Cosmology

Question 1

What is the universe

Answer 1

The universe is everything that exists - this includes plenty you can see, like stars and galaxies, and plenty that you can’t see, like microwave radiation, dark energy and dark matter.

Question 2

What are galaxies

Answer 2

Galaxies, like our Milky Way galaxy, are clusters of stars and planets that are held together by gravity.

Question 3

What is inside our galaxy (the Milky Way)

Answer 3

Inside the Milky Way is our Solar System, which consists of the Sun and all of the objects that orbit it. This includes the planets and their planetary satellites (including moons, artificial satellites and anything else that’s orbiting them), comets, and asteroids.

Question 4

Name all the planets in our galaxy in order (closest distance from the sun first).

Answer 4

The planets (in order) are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The planets (and the asteroid belt) all have nearly circular orbits. Pluto is a dwarf planet beyond Neptune.

Question 5

What is a planet?

Answer 5

A planet is an object in orbit around a star with three important characteristics:
• it has a mass large enough for its own gravity to give it a round shape (unlike the irregular shape of asteroids)
• it has no fusion reactions (unlike a star)
• it has cleared its orbit of most other objects (asteroids, etc.).

Question 6

What is a planetary satellite (with example)

Answer 6

A planetary satellite is a body in orbit around a planet. This includes moons and man-made satellites.

Question 7

What is a comet

Answer 7

They are small irregular bodies made up of ice, dust, and small pieces of rock. (Dirty snow balls)

Question 8

Describe the orbit of comets

Answer 8

The orbits of the comets we see are highly elliptical.
They orbit the Sun about 1000 times further away than Pluto does (in the “Oort cloud”). Occasionally one gets dislodged and heads towards the Sun. It follows a new elliptical orbit, which can take millions of years to complete. Some comets (from closer in than the Oort cloud) follow a smaller orbit and they return to swing round the Sun more regularly.

Question 9

What is astronomical units

Answer 9

One astronomical unit (AU) is defined as the mean distance between the Earth and the Sun.

Question 10

What is the size of one AU

Answer 10

now know that 1 AU is equal to about 150 million km.

Question 11

What is a light year

Answer 11

All electromagnetic waves travel at the speed of light, c, in a vacuum
(c = 3.00 x 10% ms-*). The distance that electromagnetic waves travel through a
vacuum in one year is called a light-year (ly). 1 ly is equivalent to about 9.5 × 10^13m.

Question 12

How does observation of something light years away from you relate to the age of that object

Answer 12

If you see the light from a star that is, say, 10 light-years away then you’re actually seeing it as it was 10 years ago. The further away the object is, the further back in time you are actually seeing it. So when we look at the stars we’re looking back in time, and we can only see as far back as the beginning of the universe. This means we can work out the size of the observable universe

Question 13

What is one arc second in degrees

Answer 13

There are 60 arcminutes in 1°
There are 60 arcseconds in each arcminute.
Therefore, 1 arcsecond = (1/3600)°

Question 14

What can a parsec be defined as ?

Answer 14

The parsec is defined as the distance at which a radius of one AU subtends an angle of one arcsecond.

Question 15

Since the radius of the Earth is a constant and theta (the angle subtended by the radius) is small what expression can be made about a parsec

Answer 15

Using trig: d = r/ tan(θ)

Since tan(θ) = θ (small angle approximation)

d= (constant) / θ
Where d is the distance in parsecs

Since the distance between the Earth and Sun is 1 AU and θ is measured in arc seconds

d = 1/p

Question 16

What is a parsec in meters - explain how you got your answer

Answer 16

1 pc = 1AU / tan (1/3600) = 1.5x10^11 / tan(1/3600) = 3.1x10^16

Question 17

What is the Stellar parallax?

Answer 17

Stellar parallax is a technique used to determine the distance to stars that are relatively close to the Earth, at distances less than 100 pc.

Question 18

What is parallax (in relation to stars)?

Answer 18

Parallax is the apparent shift in the position of a relatively close star against the backdrop of much more distant stars as the Earth orbits the Sun.

View Kerboddle pg 384 or screen shot

Question 19

In the equation relating to parallax : d= 1/p
What are d and p measured in ?

Answer 19

d is in parsec and p is the angle of parallax in arc seconds

Question 20

How is a protostar formed?

Answer 20

Stars are born in a cloud of interstellar dust and gas, most of which was left when previous stars blew themselves apart in supernovae. The denser clumps of the cloud contract (very slowly) under the force of gravity.
When these clumps get dense enough, the cloud fragments into regions called protostars, that continue to contract and heat up.

Key word “contract”

Question 21

How does a protostar eventually become main sequence

Answer 21

Eventually the temperature at the centre of a protostar reaches a few million degrees, and hydrogen nuclei start to fuse together to form helium.
As the star’s temperature increases and its volume decreases (remember, its contracting), the gas pressure increases.
There is also radiation pressure in the star - a pressure exerted by electromagnetic radiation on any surface it hits. It’s usually too tiny to notice, but becomes significant in stars because of the enormous amount of electromagnetic radiation released by fusion.
The combination of gas pressure and radiation pressure counteract the force of gravity, preventing the star from contracting further.
The star has now reached the MAIN SEQUENCE and will stay there, relatively unchanged, while it fuses hydrogen into helium.

Question 22

What happens to a main sequence start when it begins to run out of fuel

Answer 22

When the hydrogen in the core runs out, nuclear fusion stops, and with it the outward pressure stops.
The core contracts and heats up under the weight of the star. The outer layers expand and cool, and the star becomes a RED GIANT.
The material surrounding the core still has plenty of hydrogen. The heat from the contracting core raises the temperature of this material enough for the hydrogen to fuse. This is called shell hydrogen burning.
(Very low-mass stars stop at this point. They use up their fuel and slowly fade away…)
The core continues to contract until, eventually, it gets hot enough and dense enough for helium to fuse into carbon and oxygen. This is called core helium burning. This releases a huge amount of energy, which pushes the outer layers of the star outwards.
When the helium runs out, the carbon-oxygen core contracts again and heats a shell around it so that helium can fuse in this region - shell helium burning.

Question 23

What happens for the majority of a main sequence stars existence

Answer 23

Stars spend most of their lives as main sequence stars. The pressure (radiation and gas) produced from hydrogen fusion in their core balances the gravitational force trying to compress them. This stage is called core hydrogen burning.

Question 24

What happens to low mass stars like the sun when the core cools

Answer 24

In low-mass stars, the core isn’t hot enough for any further fusion and so it continues to contract under its own weight. Once the core has shrunk to about Earth-size, electrons exert enough pressure (electron degeneracy pressure) to stop it collapsing any more

For stars below the Chandrasekhar limit, the helium shell becomes increasingly unstable as the core contracts.
The star pulsates and ejects its outer layers into space as a planetary nebula, leaving behind the dense core.
The star is now a very hot, dense solid called a WHITE DWARF, which will simply cool down and fade away.

Question 25

What is the Chandrasekhar limit?

Answer 25

For stars with a core mass over about 1.4 solar masses, the electron degeneracy pressure isn't enough to counteract the gravitational force and the star collapses (going on to become a supernova ). The maximum mass for which the electron degeneracy pressure can counteract the gravitational force is called the Chandrasekhar limit.

Question 26

Names each stage of the star cycle

Answer 26

Clouds of dust and gas Protostar Main sequence Red giant White dwarf

Question 27

Which spend longer in the main sequence stage, high or low mass stars?

Answer 27

Stars with a large mass have a lot of fuel, but they use it up more quickly and don't spend so long as main sequence stars.

Question 28

What happens when the core a massive star runs out of fuel

Answer 28

If the star's core is larger than the Chandrasekhar limit, electron degeneracy pressure can't stop the core contracting. This happens when the mass of the core is more than 1.4 times the mass of the Sun. The core of the star continues to contract, and as it does, the outer layers fall in and rebound off the core, setting up huge shockwaves. These shockwaves cause the star to explode cataclysmically in a SUPERNOVA, leaving behind a NEUTRON STAR or (if the star was massive enough) a BLACK HOLE. The light from a supernova can briefly outshine an entire galaxy.

Question 29

What’s happens when a massive star runs out out of hydrogen

Answer 29

When they are red giants the 'core burning to shell burning' process can continue beyond the fusion of helium, building up layers in an onion-like structure to become SUPER RED GIANTS (or red super giants). This fusion of helium causes the star to expand For really massive stars, fusion can go all the way up to iron. Nuclear fusion beyond iron isn't energetically favorable, so once an iron core is formed then very quickly it's goodbye star. Note that is basically the same explanation as for a smaller star, only with more elements undergoing fusion

Question 30

How do Neutron stars form

Answer 30

As the core of a massive star contracts, electrons get captured by protons, combining with protons to form neutrons and neutrinos (hence the name neutron star'). If a white dwarf's core is 1.4 to 3 times the mass of the Sun then this is as far as the star can contract. The core suddenly collapses to a neutron star, causing a supernova.

Question 31

Neutron stars describe the properties of neutron starts

Answer 31

Neutron stars are incredibly dense (about 4 x 10'7 kgm^3). They're also very small, typically about 20 km across, and they can rotate very fast (up to 600 times a second). either: extremely / very dense or: has a very strong gravitational field or: supported by neutron degeneracy pressure or: fusion is no longer taking place 1.4 x solar mass or: core has mass above Chandrasekhar limit / (and below limit for black hole / 3 solar masses)

Question 32

How do Black holes form?

Answer 32

If the core of a star is more than 3 times the Sun's mass, the neutrons can't withstand the gravitational forces and the star continues to collapse. For something of this size, there are no known mechanisms left to stop the core collapsing to an infinitely dense point called a singularity.

Question 33

What are the characteristics of a black hole

Answer 33

Extremely/infinitely dense not even light cab escape Up to a certain distance away the gravitational pull is so strong that nothing, not even light, can escape its grasp — it's called a black hole. The boundary of this region is called the event horizon.

Question 34

What are on the axis of the Hertzsprung-Russel Diagram

Answer 34

Y axis - luminosity (in solar luminosity units) X axis - decreasing temperature in Kelvin (non linear scale)

Question 35

Draw the hertzspeung- Russel diagram

Answer 35

.

Question 36

Why don’t you see stars in transitional periods on the hertzbirg Russell diagram

Answer 36

The reason you can see these areas is because stars exist in these stable stages of their life cycle for long periods of time. You don't see groups of stars in any transitional period on the H-R diagram because they are unstable and the transitions happen quickly Luminosity (in solar luminosity units) 10° Super Red Giants -Betelgeuse 10* Red Giants 10° 1 Main Sequence Vega 0.01 White Dwarfs Sun (compared with the life of the star).

Question 37

When making flash card on timeline of the universe compare CGP and kerboodle (very different)

Question 38

How can you use a diffraction grating to find the wavelength of light

Answer 38

Cgp pg 131

Question 39

What does a spectrum being continuous mean

Answer 39

A continuous spectrum contains all wavelengths of light

Question 40

Describe the spectrum of white light…

Answer 40

1) The spectrum of white light is continuous. 2) If you split white light up with a diffraction grating, the different wavelengths within the white light are diffracted by different amounts. 3) Each order in the pattern becomes a spectrum, with red on the outside and violet on the inside. The zero order maximum stays white because all the wavelengths just pass straight through.

Question 41

Describe the spectra of hot objects

Answer 41

Hot things emit a continuous spectrum in the visible and infrared regions. If an object is hot enough, the spectrum can reach into shorter wavelengths, like ultraviolet.

Question 42

How are electrons arranged in an atom

Answer 42

Discrete energy levels

Question 43

What happens when electrons move down an energy level?

Answer 43

Electrons can move down an energy level by emitting a photon. Since these transitions are between definite energy levels, the energy of each photon emitted can only take a certain allowed value.

Question 44

How do you find the carried by each photon

Answer 44

The energy carried by each photon is equal to the difference in energies between the two levels.

Question 45

How do you find the wavelength of a photon produced from its energy

Answer 45

E=hc/(lmabha

Question 46

How is an emission spectra produced?

Answer 46

If you heat a gas to a high temperature, many of its electrons move to higher energy levels. 2) As they fall back to the ground state, these electrons emit energy as photons. 3) If you split the light from a hot gas with a diffraction grating, you get a line spectrum. Each line on the spectrum corresponds to a particular wavelength of light emitted by the source. Since only certain photon energies are allowed, you only see the corresponding wavelengths. You can calculate the wavelength of each line in a line emission spectrum using the formula “dsin0=n λ if you measure the angle from the zero order line and you know which order maxima the spectrum is. 5) Different atoms have different electron energy levels and so different sets of emission spectra. This means you can identify a gas from its emission spectrum.

Question 47

Explain how you get an absorption spectrum

Answer 47

You get a line absorption spectrum when light with a continuous spectrum of energy (white light) passes through a cool gas: • At low temperatures, most of the electrons in the gas atoms will be in their ground states. • Photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels. • These wavelengths are missing from the continuous spectrum when it comes out on the other side. • You see a continuous spectrum with black lines in it corresponding to the absorbed wavelengths.

Question 48

How do emission and absorption spectra compare

Answer 48

See pt 2 CGP pg 132

Question 49

What can be assumed about the radiation the sun emits

Answer 49

It emits a continuous spectrum

Question 50

How can you find out what a star is made up of

Answer 50

You get absorption lines in the spectra of light from stars. Stars can be assumed to emit radiation in a continuous spectrum. This radiation has to pass through a large amount of gas at the surface of the star (the star's 'atmosphere') before travelling to Earth. This gas absorbs particular wavelengths of light depending on the elements it consists of. Comparing the absorption spectra of stars to sets of emission spectral lines from the lab therefore allows you identify elements within a star. 5) The most common element in most stars is hydrogen, so the spectral lines for hydrogen are usually the clearest. This makes these lines the easiest to identify and measure.

Question 51

What is the luminosity of a star

Answer 51

The luminosity of a star is the total energy it emits per second (it power output) Luminosity is measured in watts

Question 52

What is Stephan’s law

Answer 52

L=4 πr^2 σT^4 Where L is the luminosity of the star (in W), r is its radius (in m), T is its surface temperature (in K) and (a lower case 'sigma') is the Stefan constant.

Question 53

What is emitted by objects because of their temperature

Answer 53

EM radiation (mostly infra red)

Question 54

How does and wavelength of the EM radiation and relative intensity vary with temperature

Answer 54

The most common wavelength becomes shorter as the surface temperature of the star increases. This is called the **peak wavelength**, λ max Have to get used this this as a term “peak wavelength” See graph pg 133

Question 55

What is wien’s displacement law?

Answer 55

λ(max) ∝ 1/ T

Question 56

What is the cosmological principle

Answer 56

COSMOLOGICAL PRINCIPLE: on a large scale the universe is homogeneous (every part is the same as every other part), the density of the universe is (approximately) uniform. isotropic (it is the same in every direction) can be true even if the universe is not the same density throughout and the laws of physics are universal (the same everywhere).

Question 57

Why does the frequency of wavelength change as something producing a sound moves towards you

Answer 57

The frequency and the wavelength change because the waves bunch together in front of the source and stretch out behind it. The amount of stretching or bunching together depends on the velocity of the source. - this is know as the Doppler effect Important to mention both frequency and wavelength

Question 58

What happens to lights wave length as light moves away from us or towards us

Answer 58

This happens with light too - when a light source moves away from us, the wavelengths become longer and the frequencies become lower. This shifts the light towards the red end of the spectrum and is called red shift. When a light source moves towards us, the opposite happens and the light undergoes blue shift.

Question 59

How can the amount of red shift or blue shift be determined

Answer 59

Δ λ/λ = Δf/f = v/c Δ λ is the difference between the observed and emitted wavelengths, λ is the emitted wavelength Δ f is the difference between the observed and emitted frequencies, f is the emitted frequency, v is the velocity of the source in the observer's direction and c is the speed of light.

Question 60

What does red shift show

Answer 60

the spectra from galaxies (apart from a few very close ones) all show red shift - so they're all moving away from us.

Question 61

What is recessional velocity

Answer 61

how fast the galaxy is moving away from us

Question 62

How is the recessional velocity of a galaxy related to distance from us

Answer 62

Plotting recessional velocity against distance shows that they're proportional — i.e. the speed that galaxies move away from us depends on how far away they are. Therefore the universe is expanding

Question 63

What is Hubble law

Answer 63

v=H ₀d Where v is recessional velocity in Kms^-1 d is distance is Mpc H ₀ is Hubble constant in kms^-1Mpc^-1

Question 64

What are the Si unit of Hubble constant (actual value given in exams) And how do you covert to it (important see kerboodle image)

Answer 64

The Si unit is s^-1 To get it in SI units you need v in ms^-1 and d in m

Question 65

What is the Big Bang theory

Answer 65

The universe started off very hot and very dense (perhaps as an infinitely hot, infinitely dense singularity) and has been expanding ever since.

Question 66

What was there before the Big Bang

Answer 66

According to the Big Bang theory, before the Big Bang, there was no space or time — space-time began with the Big Bang, (when time = 0 and the radius of the universe = 0) and has been expanding ever since.

Question 67

How has cosmic microwave background radiation occurred and what is it evidence for

Answer 67

The Big Bang model predicts that loads of gamma radiation was produced in the very early universe. This radiation should still be observed today (it hasn't had anywhere else to go). 2) Because the universe has expanded, the wavelengths of this cosmic background radiation have been stretched and are now in the microwave region.

Question 68

How was CMBR discovered

Answer 68

4) In the late 1980s a satellite called the Cosmic Background Explorer (COBE) was sent up to have a detailed look at the radiation. It found a continuous spectrum corresponding to a temperature of about 2.7 K.

Question 69

What can be said about the nature of CMBR

Answer 69

The radiation is largely the same everywhere (homogeneous) and in all directions (isotropic), in line with the Cosmological principle.

Question 70

What is an expression for the relative age of universe? What assumption has been made about to come to this conclusion?

Answer 70

t= H ₀ ^-1 This relies on the assumption that the universe has been expanding at the same rate for its whole life,

Question 71

How old is the universe

Answer 71

Assuming H0 is 70kms^-1Mpc^-1 age is 14 billion years

Question 72

Go over kerboddle story of universe table in images

Answer 72

.l

Question 73

Why did the initially created fundamental particles not immediately form large particles like proton and neutrons

Answer 73

The universe is a sea of quarks, antiquarks, leptons and photons. The quarks aren't bound up in particles like protons and neutrons, because there's too much energy around.

Question 74

Why is the universe not collapsing in on itself but instead it’s expansion is accelerating? What is the evidence for this.

Answer 74

1) Everything in the universe is attracted to everything else by gravity. This means the expansion of the universe should be slowing down. 2) Historically, astronomers debated whether this would slow the expansion of the universe enough to cause it to contract back in on itself (in a so called 'Big Crunch'), or if the universe would go on expanding forever 3) In the late 1990s, astronomers discovered something entirely unexpected. Rather than slowing down, the expansion of the universe appears to be accelerating. Astronomers are trying to explain this acceleration using dark energy — a type of energy that fills the whole of space. They observed a particular type of supernova, a type ll supernova, which produces a characteristic kind of light. On studying this light it was found to be less intense than predicted. The only possible conclusion was that the expansion of the Universe was accelerating.

Question 75

What is the current expected composition of the universe

Answer 75

Based on current observations, dark energy makes up about 70% of the universe. As dark matter makes up another 25%, this means that only about 5% of the universe is made up of ordinary matter.

Question 76

What is the evidence for dark matter.

Answer 76

In the late 1970s, astronomers studying the Doppler shift in light from galaxies found that the velocity of the stars in the galaxies did not behave as predicted. It was expected that their velocity would decrease as the distance from the centre of the galaxy increases. The observations can be explained if the mass of the galaxy is not concentrated in the centre. However, most of the matter we can see is in the centre. The current thinking is that there must be another type of matter which we cannot see. This dark matter is spread throughout the galaxy, explaining the observations.

Question 77

What are speculations as to what dark matter is

Answer 77

There are exotic speculations as to what it could be: black holes, gravitinos, weakly interacting massive particles (wimps)

Question 78

How does redshift work on emissions line spectra

Answer 78

When redshifted, emissions lines all undergo the same fractional wavelength increase. Longer wavelength have a large absolute increase

Question 79

What temp does CMBR correspond to

Answer 79

2.7K