Astrophysics: Extra

Question 1

Minimum angular resolution

Answer 1

Resolving power
The minimum angular separation which the instrument can resolve/distinguish
Measured in radians

Question 2

What does minimum angular resolution

Answer 2

How far away they are

How close they are

Question 3

Why do telescopes not focus stars to a perfect point

Answer 3

Focus as an airy disc/pattern

Seen as the diffraction pattern from a point of light as the light waves diffract through the aperture of a telescope

Question 4

Formula for minimum angular resolution assuming the limit is due to diffraction by the circular objective

Answer 4

Rayleigh criterion formula on formula sheet

Question 5

Explain the Rayleigh criterion formula

Answer 5

Theta is the minimum angle that can be resolved in radians
Wavelength of light in metres
D is the diameter of the objective lens or primary mirror in metres

Question 6

What is collecting power

Answer 6

An important parameter
A measure of its ability to collect incident electromagnetic radiation
Directly proportional to the square of the diameter

Question 7

Black body

Answer 7

Perfect absorber and emitter of electromagnetic radiation
Absorbs all the EM radiation that falls on it
And a perfect emitter

Question 8

What is a spectrum

Answer 8

A graph of intensity against wavelength or frequency

Question 9

What is the area of an intensity wavelength graph

Answer 9

How much energy is emitted for the wavelengths

Question 10

What can be said about the total amount of radiation from a body when the temperature increases

Answer 10

Total amount of radiation increases

Question 11

Explain the graph for Wein’s displacement law

Answer 11

Wavelength max on y
Temperature of x
Inversely proportional

Hotter stars will produce more light at the blue/violet end of the spectrum and will appear bluer or blue-white
Cooler stars look red as they produce more of their light at longer wavelengths

Question 12

Assumptions made for Wein’s displacement law

Answer 12

No radiation is absorbed in space/by earths atmosphere

Assume to be a perfect black body

Question 13

Why might ground-based observations of stars lead to erroneous conclusions regarding the temperature

Answer 13

Space is better because less impediment to radiation so gives a lower maximum wavelength
Atmosphere not equally transported to all wavelengths

Question 14

Stefan’s law

Answer 14

Relates the total power output, P, of a star to its black body temperature, T, and its surface area, A

Tells you if two stars have the same black body temperature (are the same spectral class), the star with the brighter absolute magnitude has the larger diameter

Question 15

Ratio equation for power output against diameter for two black bodies

Answer 15

P1/P2 = T1^4d1^2/T2^4d2^2

Question 16

Order of spectral classes

Answer 16

O: Oh
B: Bugger
A: A
F: Fucking
G: Goat
K: Killed
M: Me

Hottest temperature is for O (blue) and coolest is for M (red)

Question 17

Balmer series

Answer 17

Excitation from n=2

Useful since visible light so can be detected

Question 18

Spectral class O prominence of Balmer lines

Answer 18

Weak

Collisions of atoms at high energy levels ionise hydrogen

Question 19

Spectral class B prominence of Balmer lines

Answer 19

Slightly stronger

Collisions of atoms at high energies ionise hydrogen

Question 20

Spectral class A prominence of Balmer lines

Answer 20

Strongest

Collisions cause excitation from n=2

Question 21

Spectral class F prominence of Balmer lines

Answer 21

Weak
Too cool, collisions not enough energy to cause excitation from n=2
From n=1 instead

Question 22

Spectral class G prominence of Balmer lines

Answer 22

Very weak

Collisions with not enough energy to separate H2 into H atoms and causes excitation from n=2

Question 23

Spectral class K prominence of Balmer lines

Answer 23

Very weak

Collisions with not enough energy to separate H2 into H atoms and causes excitation from n=2

Question 24

Spectral class M prominence of Balmer lines

Answer 24

Very weak

Collisions with not enough energy to separate H2 into H atoms and causes excitation from n=2

Question 25

Explain the Hertzsprung Russell diagram

Answer 25

Scatter graph With stars as the point on the graph y axis is absolute magnitude from +15 to -10 x axis can be spectral class or surface temperature

Question 26

For the four bubbles on the H-R diagram

Answer 26

Top horizontal = red super giants Next slanty bubble = red giants Funky squiggle bubble = main sequence Bottom = white dwarf

Question 27

What does Stefan's law tell us

Answer 27

That hot stars should be relatively bright | Cools stars relatively dim

Question 28

What does Hertzsprung Russel diagram tell us about white dwarfs

Answer 28

Very dim but very hot | Although T is very high, a really low surface area means a lower lower and hence brightness

Question 29

Exoplanet/Extra-solar planet

Answer 29

Any planet not in our solar system Hard to detect don't emit light satellite of any star other than the sun

Question 30

Why are exoplanets hard to find

Answer 30

Orbiting stars much brighter than them Can't be seen directly because they mostly only reflect light and are too dim to be seen from Earth Subtend too small of an angle with their star for resolution in a telescope Only a few of the largest hottest and nearest exoplanets that are furthest away from their star can be seen directly using specially built telescopes

Question 31

Two methods to find exoplanets

Answer 31

Doppler shift/radial velocity method | Transit method

Question 32

Explain the Doppler shift for exoplanets

Answer 32

Measures how much lines from a star has been red and blue shifted from its mean value over a period of time Due to the star rotating about the centre of mass of the star planet system and wobbles towards us in part of the orbit giving a relative blue shift and away giving a relative red shift Changes are small since stars have a high mass relative to planet This method can be used to get the mass of the planet Stars movement and plane of orbit must be in line with the observers line of sight

Question 33

Quasar

Answer 33

Galaxy Particularly large concentration of gas, dust and matter near to the black hole at its centre is draw in and its gravitational potential energy decreases, kinetic and thermal increase So high energy radiation emit above and below accretion disc due to the black hole and the thermal energy

Question 34

Which galaxies are believed to have a black hole at the centre

Answer 34

All

Question 35

Why are Quasars hard to detect/condition to detect quasar

Answer 35

Have to be in line with the jets of high energy radiation (directly above or below) We observe very large radio wave signals because the gamma wavelengths have been red shifted So the quasar is moving away from us Because it is massive (one end of spectrum to the other) they must be the most distant observable things moving very very quickly

Question 36

Explain the transit method for

Answer 36

Changes in apparent magnitude as an exoplanet travels in front of a star are observed when some of the light is blocked from Earth's view Leading to a dip in the apparent magnitude observed So can find the radius But the chance of the planets path being perfectly aligned so its crosses the line of sight between the star and the Earth is very low

Question 37

What radiation is emit/detected and why from a quasar

Answer 37

We observe very large radio wave signals because the gamma wavelengths have been red shifted So the quasar is moving away from us Because it is massive (one end of spectrum to the other) they must be the most distant observable things moving very very quickly

Question 38

What has the greatest redshift

Answer 38

Quasars Make the graph curve upwards Was unexpected, so suggests the universes expansion is accelerating

Question 39

The universe is expanding...

Answer 39

At a constant rate

Question 40

What provides evidence that the universe is expanding at an increasing rate

Answer 40

Quasars

Question 41

Why are Quasars different

Answer 41

Have dense mater and dust and gas round their centre | GPE into thermal means they emit jets of high energy radiation

Question 42

What happens when the hydrogen stars to run out in the core of the sun

Answer 42

``` Hydrogen is used up Decreases radiation pressure Core contracts rapidly Big increase in temperature Enough so helium can start fusing Expansion of the outer layers which cool ```

Question 43

What elements may be produced by fusion reactions in the sun

Answer 43

Helium forms beryllium Increase in temperature for carbon to form Increase in pressure further for oxygen to form Gravitational collapse raises temperature

Question 44

What is a planetary nebula

Answer 44

Final fusion processes stops due to lack of fuel Core shrinks to raise temperature and outer layers blown off to lose about 50% of mass Remainder shrinks to a high density

Question 45

Lifecycle of a star

Answer 45

Stellar nebula Average size star = red giant = planetary nebula = white dwarf Massive star = red super giant = supernova = Neutron star OR if massive a black hole

Question 46

Explain the universe creation

Answer 46

From gluons pairs of antiquarks and quarks were formed which annihilated and gave off more gluons Somewhere along this time matter won over antimatter and 1 billion and 1 matter particles were made for 1 billion antiparticles Universe expanded so leads to a decrease in temperature Quarks form hadrons (p and n) Temperature cools to be cold enough for neutrons to decay into protons and form hydrogen Atoms form out of hadrons and electrons Pressure increases so stars and galaxies can form and emit radiation