Astrophysics

Question 1

What ways can we gather information about stars and galaxies?

Answer 1

From observing electromagnetic radiation, matter, neutrinos and gravitational waves. Collecting matter is limited to nearby regions, neutrinos rarely interact and are hard to detect, and gravitational waves are also difficult to detect, as they are perturbations of the field.

Question 2

How does light propagate through Earth’s atmosphere?

Answer 2

The optical range of the earth’s atmosphere is mostly transparent, so a large fraction of radiation energy reaches the earths surface, however clouds, rain, and water vapour absorb light.

Turbulence in the atmosphere also blurs observed images across space.

Question 3

What is a blackbody?

Answer 3

Within stars and gaseous objects, local thermodynamic equilibrium is maintained, which can be approximated as a blackbody, absorbing all radiation incident on it.

Question 4

What is bolometric flux?

Answer 4

σT^4

Question 5

What is the Rydberg formula?

Answer 5

Gives the wavelength at which electron energy transitions occur:

1/λ = R(1/nl^2 - 1/nu^2)

Question 6

What is the diffraction limit of a telescope?

Answer 6

Where a star’s image has an angular diameter instead of a point, caused by the diffraction of the telescope mirror not being infinitely large.

Question 7

How has photon detection changed?

Answer 7

Photographic plates used to be used, but the quantum efficiency, the fraction of incident photons detected, was 1-2%, only just better than the human eye.

Photomultiplier tubes were used, but charge-coupled devices replaced them, which a quantum efficiency of 90%.

The angular diameter of many optical telescopes are around 0.5 arcmin, but some are 2 degrees.

Question 8

What is photometry?

Answer 8

Using a telescope to measure the flux of an astronomical object.

Question 9

What is the signal to noise ratio?

Answer 9

The ratio of total light detection to the background light detection. The total light incident on a telescope is not just from a star, so the background radiation must be subtracted to measure flux.

Question 10

What does a higher signal to noise ratio show and how is it calculated?

Answer 10

That the telescope is more sensitive to detecting fake stars out from the background light.

S/N = Nstar/(Ntot)^0.5

Nsky = nsky*πr^2

Question 11

What is an optical filter?

Answer 11

Ensures a fixed wavelength is recorded by an optical telescope.

Question 12

What is apparent magnitude?

Answer 12

A system from Hipparcos, where stars are divided into classes of brightness. A higher flux, gives a lower magnitude. It is logarithmic since the response of the human eye is close to logarithmic.

mx = -2.5log10(Fx/Fx,0)

where Fx is the observed flux, and Fxo is the reference flux for a filter band.

x is a wavelength band given by a filter.

Question 13

How is surface brightness defined for extended objects?

Answer 13

S = F/Ω

F: flux
Ω: angular area in sky given as arcsec^2

Question 14

What is the colour index?

Answer 14

Gives the slope of a broadband spectrum between two wavelength bands.

(B - V) = mb - mv
is the difference in apparent magnitude in the B and V bands.

Question 15

What is absolute magnitude?

Answer 15

mx - Mx = 5log10(D) - 5

Question 16

What does spectroscopy show?

Answer 16

Gives an idea of the broadband optical spectrum, which can tell us about an objects chemical composition, luminosity, and relative motion.

Question 17

What is radial velocity?

Answer 17

It is due to it’s relative motion toward or away from an observer, measured via the Doppler effect.

When an object is receeding, each wave peak is emitted further away, and takes a longer time to reach the observer. As λ = cT, a larger wavelength is recorded, therefore this is called redshift.

Question 18

What is the ISM composed of?

Answer 18

99% gas, 1% dust

Question 19

What are properties of a molecular cloud?

Answer 19

The densest and coldest regions of the ISM.

Fill 1-5% of the milky way, but make up for half of the ISM mass, but the number density is still 10^6 cm^-3

Made of molecular hydrogen formed when atomic hydrogen interacts with dust, which acts as a catalyst for HI to combine into H2.

Due to the high density, they are shielded from cosmic rays and high energy photons, therefore they remain very cold.

Question 20

How can stars be traced in molecular clouds and why is it difficult?

Answer 20

At low temperatures of ~10K, there are no excited emission lines, so the H2 is mostly invisible. The gas density is also too low to emit thermal radiation, but CO is excited so it traces the H2, estimating an abundance conversion of CO to H2.

Another method is to use absorption by the dust of near infrared light from stars located behind the cloud. The measured opacity can be used to estimate the column density of dust with a relationship to the abundance of H2.

Question 21

What type of stars are seen in GMC?

Answer 21

Young stellar clusters are seen, but older star clusters are not located within. This proves they have the conditions to form stars.

Question 22

What is the star formation rate?

Answer 22

M* = Nm/t*

m: ~0.5Msun
t ~ 2Myr

Question 23

What is the gas consumption timescale?

Answer 23

Mgas/M*

Question 24

What is efficiency for star formation?

Answer 24

Tff/Tgas

Question 25

What is the first phase of collapse of the molecular cloud?

Answer 25

Isothermal free-fall collapse. The number density of particles is still very low so almost all radiation produced can escape, aka the opacity is very low.

The radiation is caused by increasing density, and more collisions occur between particles. Trace amounts of CO and H2O release photons, which escapes and does not provide supportive pressure and gas can fall freely.

Question 26

What is the free fall timescale?

Answer 26

The time it takes for a particle at distance R to fall to radius r under only the influence of gravity.

Question 27

What happens to a molecular cloud in collapse?

Answer 27

Perturbations in the clouds density, complicating the assumed uniform density.

Density increases, so smaller regions reach Jeans density and collapse independently.

The temperature then starts to increase and increases thermal pressure, known as the adiabatic phase of collapse.

Question 28

What is the limit of smallest jeans mass?

Answer 28

When Lff = Lrad

Lff = 1/2 Egr/tff

Question 29

What is a protostar?

Answer 29

The collapse of a cloud produces a core of ~2000K and a large density in hydrostatic equilbrium, which is a protostar.

At this temperature, gravitational contraction occurs, and the temperature rises and further collapse occurs.

Mass is accreted from the surrounding molecular cloud, forming a disk.

Question 30

What are four supporting observations for dark matter?

Answer 30

The rotation curves of spiral galaxies show a discrepancy at large radial distances between the measured rotational speed and the expected value, showing if Newton’s laws are correct there must be more mass than visible.

Using the measured velocities along the line of sight, the total mass of a galaxy cluster can be estimated. Using this method, a much larger mass is produced than what we know as the average mass of a large galaxy, showing that galaxies do not constitute all of the mass.

Gravitational lensing: Due to the curvature of spacetime bending light from distant galaxies, a large mass such as a galaxy cluster will bend spacetime around it. The ISM acts as a lens bending the light, so the inferred mass is larger than expected from visible material.

The measured cosmic microwave background and models of the early formation of galaxies suggests dark matter, due to the shape of the angular power spectrum of the CMB.

Question 31

What is a protoplanetary disk?

Answer 31

Results from the collapse of a molecular cloud fragment into a protostar. The rotational motion increases as the radius of collapsing material decreases.

The central temperature increases due to collisions between material in the centre where grav pull is stronger.

Question 32

How do solar systems evolve?

Answer 32

Models suggest that planets are formed in the protoplanetary disk, and the system evolves from there.

Question 33

Describe the inner 4AU of the solar system.

Answer 33

High temperature so volatiles can’t condense, accretion of metals and silicates form mercury, venus, earth and mars. This is due to the greater gravitational attraction of large protoplanets absorbing smaller ones, concentrating disk material into the planets.

Question 34

Describe the middle of the solar system.

Answer 34

Between Mars and Jupiter is the frost like, ~150K, where volatiles condense into solid ice grains, and accretion of rock and ice core undergoes runaway accretion of gas, forming the gas giants. These are mostly liquid above the core. Uranus and Neptune are smaller gas giants.

Question 35

What is orbital migration?

Answer 35

Where a planet is formed at a larger distance beyond the frost line, and it then migrates inward. This can cause planets similar to Jupiter to be located much closer to their host star.

It could also be due to the transfer of angular momentum to the gas and dust, causing inward motion.

Question 36

What is an exoplanet and how can they be detected?

Answer 36

A planet orbiting around a star other than the sun.

• Direct imaging, however there is contrast between stellar light reflected by the planet and the star, but a coronagraph can remove this light from the star making it easily to see fainter objects.

Question 37

What is the probability of observing a transit?

Answer 37

P = sin(θm) = R/a

Question 38

How does observed flux relate to the sizes of planets?

Answer 38

|ftran - f|/f = Ap/A* = Rp^2/R^2

Question 39

How is the impact parameter related to the transit angle?

Answer 39

sin(θ) = cos(i) = bR/a

Question 40

How does the doppler method help with detecting exoplanets?

Answer 40

for a star and planet system that share a centre of mass, when the star moves away the light will be redshifted, and blueshifted when moving towards. The stars radial velocity can be measured.

Question 41

How have exoplanet discoveries changed?

Answer 41

The first exoplanet was discovered using the radial velocity method, however in about 2008 the transit method began and it became vastly more popular in 2014.

Question 42

What is direct imaging more useful for?

Answer 42

Detecting massive young planets at large orbital distances, because they are further from a star and they can be more easily resolved. Larger mass means more thermal radiation emission.

Question 43

What is the transit method more useful for?

Answer 43

Planets with closer orbits, and a shorter period of orbits which increases the number of transits observed per observation time.

Question 44

What planets are easier to observe via fractional change in flux?

Answer 44

Higher mass and larger radius because the fractional change in flux during transit is higher.

Question 45

What is the radial method more useful for detecting?

Answer 45

Larger radial velocities are detected when there is a larger planets mass to orbital period ratio. The orbital distance must also be small.

Question 46

What is the equation for reaction energy?

Answer 46

Q = Ertoti - Ertot f = Ktotf - Ktoti

Question 47

What are the conditions for fusion in a star’s core

Answer 47

• High temperature in very dense region
• Quantum tunnelling allows temperature to be lower than predicted necessary, and some nuclei have higher thermal energies than the mean value.

Question 48

What is the solar neutrino problem?

Answer 48

The pp chain gives a prediction for the no of neutrinos travelling from the suns core to earth, but only a 3rd of the expected flux of solar neutrinos was measured on earth due to the fusion models. A theoretical solution is that neutrinos can change flavour, and Homestake had only measured one flavour.

Question 49

Describe the lifecycle of a main sequence star?

Answer 49

The core contracts and temperatures rise, as thermal energy increases when gravitational energy is released. The hydrogen undergoes fusion into helium, defining the end of the main sequence life, moving onto a Giant.

The surface temp drops due to an increased stellar radius.

Question 50

What is a pulsating star?

Answer 50

Caused by instability in the outer envelope, they alternate from high temp and lower pressure, to lower temp and high pressure as they expand and contract.

Question 51

What is a degenerate core?

Answer 51

When fusion stops in the core and it contracts until it is only supported by the degeneracy pressure of electrons. At higher density electrons repel each other by increasing their momentum.

Question 52

What are the late phases for stars similar to the sun versus massive stars?

Answer 52

For the sun, the core contracts and helium begins converting to carbon, resulting in a carbon and oxygen core. The stars envelope is distended, and colder outer layers are ejected to form a planetary nebula. the nebula can become a white dward if it is illuminated by the ionising radiation from the exposed inner star.

For amssive stars, fusion proceeds to more massive elements, so the degenerate core grows until it reaches the Chandrasekhar mass limit. The core contracts and photodisintegration and electron capture occurs. A neutron star is then created if the cores mass is small enough, if not it will become a black hole.

Question 53

What is a white dwarf?

Answer 53

Steller remnant of any star of mass lower than 8Msolar, and will not undergo nuclear fusion, cooling over time.

Question 54

What is a neutron star?

Answer 54

Remnant of stars between 8 and 25 Solar mass. They are supported against collapse by degeneracy pressure, but free neutrons repel eachother rather than electrons.

Question 55

What is a pulsar?

Answer 55

When a stellar core collapses to a neutron star the spin and magnetic field become higher, and so due to conservation of angular momentum, the neutron star has a spin period in milliseconds.

Question 56

What is Ltot of a pulsar equal to?

Answer 56

-dE/dt = -d/dt(1/2 Iω^2) = -Iω dω/dt

Question 57

What is a supernova remnant?

Answer 57

When a massive star collapses, the infalling material reaches very high speeds, and when it meets the compressed neutrons is bounces outwards creating a shock front with a very high kinetic energy.

Question 58

What is Eddington luminosity?

Answer 58

When Frad = Fgr

Question 59

What is a CV system?

Answer 59

A binary system of a white dwarf and a star, with a close enough orbit so that gravity attracts the stars outer layers towards the white dwarf, forming an accretion disk.

The material can reach high temperatures where nuclear fusion occurs and a bright outburst is produced.

Question 60

What is a type Ia supernova?

Answer 60

When nuclear fusion begins in a CV system, the carbon and oxygen in the white dwarf fuses into heavier elements increasing the internal temperature. The star explodes as a type Ia supernova, producing a shock wave.

Type Ia supernovae have similar luminosities to eachother due to white dwarfs having similar masses as a uniform fusion process. The distance to Ia supernovae can be estimated from the observed flux.

Question 61

What is Hubble’s law?

Answer 61

Hubble collected distance estimates of galaxies and applied the Doppler effect method to estimate the radial velocities. more distant galaxies had higher recessional velocities from earth.

Question 62

What is the winding problem?

Answer 62

Brighter spiral arms imply a higher density of gas and stars, but if this was due to a constant orbital velocity with respect to radius, differential rotation would occur. The outer disk would have a slower angular speed, causing a trailing spiral, but over time the arms would wind tighter. This is not observed in spiral galaxies.

Question 63

What indicates a black hole lies in the centre of the galaxy?

Answer 63

High mass, small region, not very luminous.

Question 64

Describe the three types of galaxy:

Answer 64

Mergers: The observed time between galaxy collisions is 5000Gyr, but this should be lower. This is because gravitational effects need to be included, and density increases centrally, so it is more like 9Gyr. The gas and stars settle into a merged galaxy.

Elliptical: Most likely mergers, they have a smooth ellipsoidal shape and visible dust lanes suggesting the galaxy was 2 merged. The stars have randomly oriented velocities, and a much more massive supermassive black hole.

Active galaxies: Much higher luminosity in their nucleus than expected, and they are active galactic nuclei. Luminosity is expected to originate from an accretion disk and efficiencies near the eddington limit.