Astrophysics Flashcards
Complete diagram to show action of converging lens on a parallel beam of light, not parallel to the principal axis. Label principal focus F and label focal length f
Complete diagram to show action of converging lens on a parallel beam of light, not parallel to the principal axis. Label principal focus F
How big an object or image looks to the eye depends on the?
angle to which it is subtended to at the eye
What can you say about the rays from objects with a very long distance
then rays from one point on the object are almost parallel. (Points are small and close to the eye)
Use constructional rays to find position and properties of image formed by a converging lens of an object at infinity
equation for angular magnification and equation for length of telescope
length = fo + fe
M = fo / fe
A refracting astronomical telescope has a magnifying power (same as angular magnification) of 40 and a length of 820 mm. What is the focal length of each lens?
fo / fe = 40
fo + fe = 820
solve simultaneously to get
fo = 800 mm
fe = 20 mm
draw a diagram to show a parabolic mirror fixing spherical aberration
One way of using a converging mirror uses an additional convex mirror and an eyepiece.
You need to show the paths of two rays in a Cassegrain up to the eyepiece
What is dispersion
when refractive index of light varies with wavelength
do refracting and reflecting telescopes suffer from chromatic aberration
refractors will unless corrected
reflecting telescopes will not due to their primary mirrors
Advantages and disadvantages of reflecting and refracting telescopes
Sketch the appearance of a diffraction pattern caused by a small circular hole in front of a laser and underneath it a sketch graph of the variation in intensity in one direction.
How do you increase the collecting power
increase diammeter of the object
as collecting power is proportional to (diammeter)^2
Similarities and differences between radio telescopes and optical telescopes
be ground-based because the atmosphere is transparent to a large range of radio wavelengths. Compared to an optical telescope, a radio telescope has a low angular resolution because of the dependence on wavelength in the Rayleigh criterion, θ ≈ λ /D
Unlike optical telescopes, radio telescopes can operate during the day as well as at night
Limitations of ground based telescopes
atmospheric absorption and distortion in the visible region of the electromagnetic spectrum are limiting factors in image
Ozone, oxygen, water vapour and carbon dioxide all contribute to the absorption of light, from the ultraviolet through visible to infrared. Dust within the atmosphere also absorbs and scatters light on its way to the telescope, and atmospheric turbulence (due to convection currents) reduces image quality. Such problems are avoided by building observatories in dry, pollution-free areas at high altitude, or, better, by putting telescopes in orbit around the Earth beyond the atmosphere
What are infrared telescopes used for
used to make observations of cool regions
What are UV telescopes used for
Ultraviolet (UV) telescopes are used to examine objects in the UV part of the electromagnetic spectrum
Explain what is meant by the Rayleigh criterion
Explain what is meant by the Rayleigh criterion
(Minimum angle is when) the central maximum of (the diffraction pattern of light from) one object coincides with the first minimum of (the diffraction pattern) of
the second object.
What you need to know for a CCD
- quantum efficiency of CCD is higher than that of an eye
- resolution of CCD higher
- The CCD is better for scientific purposes (e.g., astronomy, detailed imaging) due to its high quantum efficiency and resolution.
The eye excels in convenience for real-time use but is limited in its detection and storage capabilities.
Define quantum efficiency
Quantum efficiency is the ratio of the number of photons detected to the number of photons incident on the detector.
Define resolution
Resolution refers to the ability to distinguish between closely spaced objects or details.
A telescope uses wavelengths in the range 90 nm to 120 nm.
Explain why this telescope must be located in space.
Go on to discuss one advantage that this telescope has compared to a telescope with the
same aperture that uses visible light.
Wavelengths in the range 90 nm to 120 nm fall within the ultraviolet (UV) region
- Which is absorbed by (ozone in) the atmosphere so must be in space
- UV light gives better resolution, as it has shorter wavelength, eqaution linked is theta = wavelength / diammeter
one telescope has a spherical disc shape and another has a parabolic disc shape. State which telescope will produce a more detailed image
Parabolic as it has no spherical aberration
Two telescopes have different diammeters. Which one would be better at resolving images of two objects that are closer together
One with larger diammeter because its minimum angular resolution depends on 1/D
A spacecraft passes Pluto at a distance of 12 500 km. The telescope on board has an aperture of diameter 0.21 m and operates at a wavelength of 450 nm.
Discuss whether this telescope is suitable for studying a crater with a diameter of approximately 1 km on Pluto.
Theta = wavelength / d
= 2.14 x 10^-6 rad
smallest detail = 2.14 × 10^−6 × 12.5 × 10^6 = 27 m
The telescope can resolve features as small as 27 m, which is much smaller than the diameter of the crater, 1km. Telescope is suitable
How do you work out the collecting power when given diammeter
power is proportional to area
so calculate area to find power
problems of refractors vs reflecting
Can suffer spherical aberration and chromatic aberration.
Reflecting are lighter. Reflecting are shorter. Mirrors do not suffer
from chromatic aberration.
problems of reflecting telescope
Spider/secondary mirror block some of the light/reduce image
brightness/cause diffraction effects.
The Griffith telescope is used to observe two point objects which subtend an angle of 1.8 × 10^–6 rad at the unaided eye.
The typical human eye has a minimum angular resolution of approximately 3.2 × 10^–4 rad
Calculate the focal length of the eyepiece lens so that an observer can just resolve the two
objects when observing them through the Griffith telescope. focal length 5.03 m
M = 3.2 × 10^–4 / 1.8 × 10^–6
= 178
fe = fo / M
fe = 5.03 / 178
The asteroid Apophis has a diameter of 325 m
It has been calculated that, in 2029, its distance of closest approach to the Earth’s surface
will be 3.0 × 10^4 km
The Griffith telescope may be used to view Apophis using the eyepiece lens of 1.8 x 10^6 rad
Deduce whether this telescope is suitable to obtain a detailed view of Apophis.
Support your answer with a calculation.
What are the shapes of the primary and secondary mirrors in a Cassegrain telescope?
primary : concave
secondary : convex
The Triesnecker Crater on the Moon has a diameter of 23 km. Calculate the angle subtended by the image of this crater when viewed through a telescope of angular magnification 50 on the Earth.
distance from Earth to Moon = 3.8 × 10^5 km
s = r x theta
theta = 23 / 3.8 x 10^5 = 6.053 x 10^-5
M = theta 2 / theta 1 = to give θ2 = 50 × θ1 = 3(.026) × 10–3 (rad)
what is the ratio of resolving power of reflector / resolving power of refractor
Theta reflector / theta refractor = d refractor / d reflector
Give two reasons why the secondary mirror in the Cassegrain telescope affects the clarity of the image.
- mirror blocks light so less light hits objective mirror
- light diffracted passing secondary mirror affects image
The Moon is 380 000 km from the Earth and has a diameter of 3 500 km. Calculate the angle subtended by the image of the full Moon when viewed through the telescope. Telescope made from two cardboard tubes of slightly different diameter, two convex lenses of focal lengths 0.10 m and 0.50 m respectively
fo / fe = 5
5 x 3500 / 380 000 = 0.046 rad
The telescope suffers from chromatic aberration. Describe how this affects the appearance of an image.
edges on image will appear coloured
Huygens made an astronomical telescope with an angular magnification of approximately 100. Using this telescope, Huygens discovered Titan, a satellite of Saturn. At this angular magnification, the image of Titan subtends an angle 4.0 × 10–3 radians when it is approximately 1.3 × 109 km from the Earth. Calculate the diameter of Titan.
(4 x 10^-3) / 100 = 4 x 10^-5
a = d / r
d = 5.2 x 10^4 km
State and explain two optical advantages reflecting telescopes have compared with refracting telescopes.
no chromatic aberration - mirrors do not refract light (1) no spherical aberration - use of parabolic mirror (1) no distortion - mirror can be supported more strongly (1) better resolving power or greater brightness - mirrors can be larger (1) more light gets through (image brighter) - lens absorbs more light (1)
A telescope is made from two converging lenses of focal lengths 2.50 m and 0.020 m. The telescope is used to observe a planet which subtends an angle of 5.0 × 10–5 rad at the objective. Calculate the angle subtended at the eye by the final image.
M = 2.5 / 0.02 = 125
125 x 5.0 × 10–5 = β = 6.2(5) × 10–3 rad
Convex mirror between objective and F1
State what is meant by chromatic aberration and explain the effect it would have on the image in an uncorrected refracting telescope.
chromatic aberration -different wavelengths (1) refracted different amounts or different speeds in glass (1) image with coloured edges or different focus for different colours (1)
Explain why the Cassegrain telescope would be almost free of chromatic aberration.
no refraction (by mirrors), as telescopes use mirrors. some chromatic aberration in eyepiece lens
what is minimum angular resolution proportional to
1/D from theta = wavelength / diammeter
compare the quantum efficiency of a CCD with that of the eye.
For CCD QE = 80%
For eye QE = 1%
explain what is meant by spherical aberration when applied to a concave mirror.
- different focal points for rays at different distances from axis
- shortest focal length for paraxial rays
explain what is meant by chromatic aberration
- light of different wavelengths refracted to different foci
- a diagram showing refraction with blue focal length closest to lens
State two similarities between a radio telescope and an optical reflecting telescope.
- collect electromagnetic radiation
- curved dish to collect radiation
- reflected to a focus / image or reciever
The dish of a radio telescope has holes of diameter 20 mm spaced close together in its reflecting surface in order to reduce the weight of the dish. Explain why the performance of this telescope will be far more satisfactory when receiving signals of frequency 7.5 × 10^8 Hz than when receiving signals of frequency 1.5 × 10^10 Hz.
lambda = c / f
use c = 3 x 10^8
sub in both frequencies to get lambda 1 as 0.4 m and lambda 2 as 0.02 m
λ2 ≈ dimensions of holes and signal reduced
lower frequency gives λ > dimensions of holes, signal not affected
State, with reasons, two optical advantages which the reflecting telescope normally has over a refracting telescope.
- collects more light, mirrors can be larger than lens
- better resolution, larger diammeter
- no chromatic aberration
- no spherical aberration
Many astronomical observations rely on a Charge Coupled Device (CCD) to obtain an image. Describe the structure and operation of the CCD and discuss the advantages of using a CCD for astronomical observations.
- The CCD is a silicon chip
- The chip is divided into picture elements
- Each picture element is associated with a potential well in the silicon
- Incident photons are focused on the CCD
- The photons cause the release of electrons within the semiconductor
- The number of electrons liberated is proportional to the intensity of the light.
- Electrons are trapped in the potential wells
- An electron pattern is built up which is identical to the image formed on the CCD
- When exposure is complete the charge is processed to form an image.
Advantages: High quantum efficiency > 70%
Light integration – using long exposure times to capture faint images. Device can be directly linked to computer for capture and analysis.
define luminosity
the total power radiated by a star at all wavelengths
define intensity
the power per unit area at the observer
equation = power / area
Why is it important that we are able to measure the brightness of stars?
If we know this and how far away the stars are, we can work out how big / hot the star is. Then we can know what type of star it is
Q2.1 How many times brighter is a fourth magnitude star than a fifth magnitude star?
Q2.2 How many times brighter is a fourth magnitude star than a sixth magnitude star?
Q2.3. How many times brighter is a second magnitude star than a ninth magnitude star?
How many seconds of an arc are there in one degree?
3600
What is 3.1416° expressed using minutes and seconds?
Find the value of tan(1”)
tan (1 degree divided by 3600) = 4.85 x 10^-6
How far in pc. is the nearest star to the sun?
1.3 pc
- don’t fill in the blanks, Q3.3 is find value of tan(1’’)
Two stars, i and q, in the constellation of Auriga appear equally bright, but i is known to be at a distance of 330 light-years from us, and q is 108 light-years away. Which star is the most luminous and by how many times?
What two basic assumption have been made in this section?
Assumptions :
- there is no absorption of light in the space looked at
- the radiation is isotropic (same in any direction)
Define absolute magnitude
the absolute magnitude of a star is the apparent magnitude it would have at a distance of 10pc from an observer
apparent magnitude of betelgeuse is 0.45 and absolute magnitude is -5.47. How far away in light years is betelgeuse. State an assumption you have made
There has been no light absorption in space
Part c means which is further away
What is the spectrum
a graph of intensity against wavelength or frequency
What happens when the current is gradually increased in a filament bulb - how is this explained by the graphs.
The temperature of the bulb increases as part of the spectrum shifts to the left
The sun is yellowish, Vega is bluish - what does that tell us?
Vega has a greater surface temperature
so its peak wavelength is further to the left, towards the blue side
The peak wavelength of our sun is 5.02 x 10^-7 m. What is its surface temperature?
b) what two assumptions have been made in this calculation
Wiens displacement law is
Max wavelength x temperature = 2.9 x 10^-3mK
so for this calculation
5.02 x 10^-7 x temp = 2.9 x 10^-3
temp = 5800 Kelvin
b) assumed star as black body
assumed no absorption of radiation in space
Sketch a graph of lambda max against T.
Why might ground-based observations of stars lead to erroneous conclusions regarding the temperature
Atmosphere not equally transparent to all wavelengths
What does Stefans Law state
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.
Consider two stars of black body temperatures T1 and T2. If the ratio of their power output (P1/P2) is known, show how Stefan’s law can be used to calculate the ratio of their diameters, d1/d2
For this spectral class
- state the intrinsic colour
- temperature range (kelvin)
- prominent absorption lines
- spectral class O
Blue
25000 - 50000
He^+, He, H
For this spectral class, B
- state the intrinsic colour
- temperature range (kelvin)
- prominent absorption lines
Blue
11,000 - 25,000
He, H
For this spectral class, A
- state the intrinsic colour
- temperature range (kelvin)
- prominent absorption lines
Blue - white
7,500 - 11,000
H (strongest) ionised metals
For this spectral class, F
- state the intrinsic colour
- temperature range (kelvin)
- prominent absorption lines
White
6,000 - 7,500
ionised metals
For this spectral class, G
- state the intrinsic colour
- temperature range (kelvin)
- prominent absorption lines
Yellow - white
5000 - 6000
ionised & neutral metals
For this spectral class, K
- state the intrinsic colour
- temperature range (kelvin)
- prominent absorption lines
- orange
3,500 - 5,000
neutral metals
For this spectral class, M
- state the intrinsic colour
- temperature range (kelvin)
- prominent absorption lines
- red
- less than 3,500
- neutral atoms, TiO
Draw the hertzsprung-Russel (H-R) diagram
What happens when hydrogen starts to run out in the core, describe formation of the red giant
hydrogen used up leads to
- loss of radiation pressure
- core contracts rapidly
- large temperature rise
- helium fumes
- expansion of outer layers, which cool
What is a planetary nebula
- final fusion process halts owing to lack of fuel
- core shrinks raising temperature
- outer layers blown off, about 50% of stars masses
a)
W - protostar / gas cloud
X - main sequence star
Y - Red giant
Z - White dwarf
Theta Carinae is a star with a radius five times that of the Sun. It has a surface temperature of 31
000 K. An astronomer suggests that an Earth-sized planet orbits Theta Carinae. Explain one difficulty with using the transit method to detect this planet
- Transit method measures how much light is blocked by a planet
- Planet is small and star is very big so little light will be blocked
Theta Carinae is a star with a radius five times that of the Sun. It has a surface temperature of 31
000 K. The astronomer suggests that the Earth-sized planet receives a similar amount of power
from Theta Carinae as the Earth does from the Sun.
The average power output of the Sun is 3.8 × 10^26 W.
Determine the orbital radius of the Earth-sized planet orbiting Theta Carinae.
Colour of Schaefer = orange
Colour of Caph = white
- Use Wiens law to calculate temperature
For Caph = T = 2.9 x 10^-3 / 410
x 10^-9 = 7250 Kelvin
For Schedar = T = 2.9 x 10^-3 / 660 x 10^-9 = 4400 Kelvin
Schedar has longer wavelength hence it has a more redder colour
The temperatures match the spectral class colour
As Caph is in spectral class F & Schedar is in spectral class K
Tsih has a mass over 15 times the mass of the Sun.
Tsih may eventually collapse to form a black hole.
Calculate the radius of the event horizon for a black hole with a mass 15 times that of the
Sun.
Use of equation
R = 2GM / c^2
= 4.4 x 10^4m
D
- make sure you remember the point at which it peaks as you will have to remember this graph.
Type 1a supernovae can be used as standard candles.
Explain what is meant by a standard candle.
Object whose absolute magnitude is known and whose apparent
magnitude can be measured.
Measurements of type 1a supernovae in 1999 led to a controversy concerning the behaviour of the Universe.
Describe this controversy and how the measurements led to it.
- Measurements of supernovae do not agree with predictions (from
Hubble’s Law) - So Universe must be expanding at increasing rate/accelerating
- (Controversial as) no known energy source for expansion or
reference to dark energy
b) Explain, with reference to the SI units involved, how the curve you have drawn can be used to determine the black-body temperature of the star.
for a) just draw a single peak
b) Use peak wavelength and sub into Wiens law equations. Temperature must be measured in Kelvin, wavelength in metres
Define a binary star system
two stars orbiting a common centre of mass
Define a black body radiator
A perfect emitter and absorber of all possible wavelengths of radiation.
The Hipparcos scale is another name for what diagram? Give a picture of what it looks like
Hertzsprung-Russel diagram
State why a b class star will have more prominent Hydrogen balmer absorption lines than other spectral classes apart from O
They are hot enough to electrons/hydrogen in the n=2 state
Deduce which star, Gamma A or HD 66141, has the larger diameter.
Both are in same spectral class but Gamma A has a higher absolute magnitude
- same spectral class so similiar temperature
- absolute magnitude of gamma A is higher so it has a higher power output and is brighter.
- Use of stefans law equation, Gamma A has a larger area and hence a larger diammeter
Astronomers recently used the radial velocity method to discover an exoplanet orbiting
HD 66141.
Describe the main features of the radial velocity method in the detection of planets.
The radial velocity method detects exoplanets by measuring periodic shifts in a star’s spectral lines caused by the Doppler effect as the star moves towards and away from Earth due to the gravitational pull of an orbiting planet, as they are orbiting around the same centre of mass.
- as the star moves towards and away from Earth due to the gravitational pull of an orbiting planet.
What is the doppler effect
As the star moves toward Earth, its light experiences a blueshift (shorter wavelengths).
As the star moves away, its light undergoes a redshift (longer wavelengths).
These shifts are detectable through precise spectroscopic measurements.
Discuss why the evolution of a supergiant star in the local part of our galaxy could be dangerous for life on Earth.
- Star will undergo a supernova collapse
- star will form a neutron star / black hole
- which produces a gamma ray burst
- cause consequences for life such as damaged DNA or kills cells
e) A star much more massive than the Sun may become a supernova and then a black hole.
Discuss whether supernovae and black holes can be placed on the HR diagram in.
e) - Absolute magnitude of supernova (is about –20), beyond scale of HR diagram
- Supernova is shortlived / varies so cannot be assigned a position
- Temperature of a supernova (peak) is too high (greater than 50 000 K)
- Black hole – escape velocity greater than c
- no light emitted /absolute magnitude too dim to fit on scale
- Temperature of a black hole would be too low (less than 2500 K
Equation for age of universe (in seconds)
T = 1 / hubbles constant
Define the parsec
Distance at which 1AU subtends an angle of 1/3600th degree
OR
distance to an object subtending 1 sec of arc to Earth’s orbit
Which star has the brightest absolute magnitude
Rasalgethi
- don’t focus on which is the largest number but which is the most negative (highest negative)
- get help on it just in case
A) which of the three stars appear the brightest
B) deduce which of the 3 stars is the largest
A) lowest value of apparent magnitude indicates brightest star, Vega has lowest so it brightest
B) - similar temperature, bigger absolute magnitude, due to stefans law equation, to have greater p power, must have greatest area
State the difference between apparent magnitude and absolute magnitude.
apparent: brightness of star as seen (from Earth) absolute: the apparent magnitude at a distance of 10 pc
Sirius has an intensity of 1.18 × 10–7 Wm^–2 at the Earth. The distance between Sirius and the Earth is 8.13 × 10^13 km.
Calculate the luminosity of Sirius.
Give an appropriate unit for your answer.
Intensity = power / area
re arrange to get power = 9.8 x 10^27
units = watts
State which star is closer to earth
Sirius
- Sirius is 10pc away as apparent magnitude = absolute magnitude
- Data shows Regel would appear brighter if at 10pc but is seem to be dimmer
- it appears brighter even though it has a lower luminosity / lower surface temperature
The star Betelgeuse has a mean apparent magnitude of +0.5 and an absolute magnitude of –5.7. Use the definitions you have given above to deduce (without calculation) that the distance between Betelgeuse and Earth is greater than 10 pc.
apparent magnitude = +0.5 indicates dim light absolute magnitude = –5.7 indicates brightness, as star becomes nearer it appears brighter so distance > 10pc
The star Proxima is much closer to Earth than 10 pc and its mean apparent magnitude is +11.0. Giving a reason, suggest a value for its absolute magnitude.
as it moves away from earth at 10pc, it would be less bright
hence absolute magnitude should be anything greater than 11
P
- lowest peak wavelength
- wavelengthmax x temperature = constant, so the lower the wavelenght, the higher the temperature
- has highest peak intensity, Use stefans law equation to see that higher intensity / power, higher temp
More detailed analysis of the hottest star’s spectrum revealed the presence of Hydrogen Balmer absorption lines.
For which two spectral classes are these lines the prominent feature?
B) Describe how these absorption lines are produced in the spectrum of a star.
A and B
b) light from the star passes through the atmosphere of the star (1) which contains hydrogen with electrons in n = 2 state (1) electrons in this state absorb certain energies and (hence) frequencies of light (1) the light is re-emitted in all directions, so that the intensity of these frequencies is reduced in any given direction, resulting in absorption lines (1
0.5, 1, 1.5, 2
Notice the units not in its usual nanometres.
Assuming power output of Arcturus is 100 times greater than that of the sun, show that’s its surface area must be approximately 200 times greater
A) is likely to be orange in colour
B) appears brightest from earth
A) sheddir: class K towards red end
B) Sheddir, had lowest value of apparent magnitude
Define light year
distance travelled by light in one year
The two stars of 95 Herculis are separated by an angle of 1.8 × 10^–3 degrees. Calculate the minimum diameter of an aperture which would just allow these stars to be resolved wavelength of the light = 5.0 × 10^–7 m
use of rayleigh criterion equation
theta = wavelength / diammeter
- remember to convert degrees to radians
Hamal has a lower apparent magnitude than Arietis, which appears brighter
Hamal
State what is meant by a supernova.
An object that produces a rapid increase in brightness
State the defining properties of a neutron star.
- extremely dense
- made up of neutrons
Some scientists are concerned about the consequences for the Earth of a supernova occurring in a nearby part of the galaxy.
Explain the cause of this concern.
Collapsing star can produce gamma ray bursts with energy similar to total output of Sun
Highly collimated – if in direction of Earth, could cause mass extinction event
Explain what is meant by the event horizon of a black hole.
boundary where the escape velocity = c
Why will stars of large mass undergo a supernova event
large gravitational field in a big stars core to cause further contraction at High speeds
What are the two possibilities for the remainder of the star after a supernova event? What determines which of these will be formed.
neutron star
black hole is big enough mass
what is the defining feature of a black hole
its escape velocity is greater than the speed of light
state an advantage of a type1a supernova as standard candles
that they are very bright. This means they can be used to measure the distance to the furthest galaxies
what is the event horizon
The boundary at which the escape velocity is equal to the speed of light
What is the doppler effect, state all key points
- The Doppler effect is the compression or spreading out of waves that are emitted or reflected by a moving source
- The Doppler effect causes the line spectra of distant objects to be shifted either towards the blue end of the visible spectrum when they move towards the Earth (blue-shift) or towards the red
end of the spectrum when they move away from the Earth (red-shift).
What is evidence for an expanding universe
Red-shift is used as evidence for the expanding universe, as distant objects are red-shifted. The more distant the object, the greater its red-shift.
What can the doppler effect be used for
The Doppler effect can be used to identify binary star systems, where two stars are orbiting a common centre of mass.
Also state what eclipsing binaries are
when the plane of the orbit of the stars is in the line of sight from Earth to the system, meaning that the stars cross in front of each other as they orbit
What is Hubble’s law
a galaxy’s recessional velocity is directly proportional to its
distance from the Earth. It essentially states that the universe is expanding from a common starting point.
What is the big bang and what is the evidence for it
The Big Bang theory suggests that the universe began with a huge explosion from this point. When the Big Bang happened, it is thought that there was high-energy radiation everywhere, and as the universe
expanded and cooled, this radiation would have lost energy and been red-shifted. The remains of this radiation is what we call Cosmological Microwave Background Radiation (CMBR), which is
microwave radiation that has been detected from all directions in space. This provides evidence for the Big Bang.
characteristics of a quasar and what is a quasar
a supermassive black hole surrounded by a disc of
matter which causes jets of radiation to be emitted from the poles
● Extremely large optical red-shifts
● Very powerful light output
● Their size being not much bigger than a star
What are exoplanets
Exoplanets are planets that are not within our solar system; they orbit other stars. They can be difficult to detect directly as they tend to be obscured by the light of their host
stars
Two methods to detect exoplanets
radial velocity method and transit method
What is the radial velocity method
The star and planet orbit a common centre of mass, which causes the star to ‘wobble’ slightly. This causes a Doppler shift in the light received from the star (this method is sometimes referred to as
the ‘wobble effect’ for this reason). The effect is most noticeable with high-mass planets since they have a greater gravitational pull on the star. The line spectrum of the star is blue-shifted when it moves towards the Earth, then red-shifted when it moves away. This
shows that there is something else near the star that is exerting a gravitational force on it the exoplanet. The time period (T) of the planet’s orbit is equal to the time period of the Doppler shift.
What is the transit method
- Apparent magnitude of star is measured over a long period of time ✔
- When planet passes in front of star (as seen from Earth), some of the light from star is absorbed and therefore the amount of light reaching Earth reduced ✔
- This produces a light curve showing constant value with a dip periodically as the planet passes in front of the star ✔
Describe the links between galaxies, black holes and quasars.
Quasars are produced by supermassive black holes
These black holes are at the centre of active galaxies
A typical quasar is believed to be approximately the size of the solar system, with a power output similar to that of a thousand galaxies.
Estimate, with reference to the inverse-square law, how much further the most distant visible quasar is likely to be compared to the most distant visible galaxy.
According to NASA nearly 2000 exoplanets had been discovered by 2016, and the search continues. One aim of this search is to find an Earth-like planet orbiting a Sun-like star.
Discuss the difficulties associated with the detection of an Earth-like planet orbiting a Sun-like star.
In your answer you should compare the methods that are used in the search and suggest which may be the most successful.
Transit – dips in brightness as planet crosses in front of star from
our point of view.
Alignment must be correct for planets to eclipse, so many possible
candidates not observed. Earth-like planet could be observed
provided not too far away.
Radial velocity (Doppler) – periodic shift in spectra of star due to
star’s movement around common centre of mass with planet.
Earth-like planet mass much less than mass of Sun-like star so
effect slight. Earth-like planet could be detected with highly
sensitive spectrometers.
Direct observation – very unlikely as Earth-like planet to small and
too near star and too cool to be detected against the brightness of
the Sun-like star. Unlikely to be detected.
Explain what is meant by cosmological microwave background radiation and how its existence supports the Big Bang theory
It is the radiation coming from all parts of the Universe ✔
When the Universe cooled sufficiently for matter and radiation to ‘decouple’, with the combination of protons and electrons to form neutral atoms ✔
This radiation has been red-shifted into the microwave region as the Universe has expanded
Explain how the relative abundance of hydrogen and helium supports the Big Bang theory.
The Big Bang theory suggests that a very brief period of fusion occurred when the Universe was very young, resulting in the production of helium from fusing hydrogen. ✔
Fusion stopped as the Universe then expanded and cooled ✔
Resulting in a relative abundance of hydrogen and helium in the ratio of 3:1/ cooled too rapidly for the creation of larger nuclei,
Quasars are the most distant measurable objects.
Discuss one problem associated with the determination of the distance from the Earth to a quasar.
Distant quasars are very faint
associated galaxy would be very faint ✔
Due to dark energy/accelerating universe, ✔
use of Hubble’s Law/inverse square law not reliable over large distances.
Suggest why the quasar stopped emitting radiation.
Quasars are formed around black holes
Black holes no longer have matter falling into it
IC2497 has a red shift of 0.0516
Determine the distance from the Earth to IC2497.
Give an appropriate unit for your answer.
238 Mpc
The Earth’s atmosphere absorbs electromagnetic radiation of certain wavelengths. What is responsible for the absorption
b) What effect can this absorption have on the measured temperature of a star? Explain your answer.
Ozone
b) lower temperature, larger wavelength
The atmosphere has little effect on radio waves between 30 MHz and 300 GHz. This radio window was first exploited in 1946 when a short pulse of radio waves of wavelength 2.7 m was transmitted from the Earth and reflected back by the Moon.
Show that the frequency of the transmitted waves falls within the radio window.
b) The experimenters had to take into account the relative movement of the Earth and Moon when tuning the receiver. The maximum difference between the frequency of the detected and transmitted waves was 300 Hz.
What is the name of this effect?
speed = frequency / wavelength
3 x 10^8 / 2.7 = frequency in range
b) Doppler effect
State what is meant by a black hole
an object with an escape velocity greater than the speed of light
State what is meant by the Hubble constant.
Gives the ratio of the recessional velocity of galaxies to distance from Earth
State one assumption that must be made to justify when estimating the age of the universe
Universe is expanding at constant / steady rate
State which property of the first identified quasar led to its discovery.
The quasar is a bright radio source.
3C48 is a quasar in the constellation Triangulum. It is believed to have a power output 4 × 10^11 times greater than that of the Sun. At the Earth, the Sun’s intensity is 1.4 × 10^17 times greater than that of the quasar.
Calculate, using the inverse square law, the distance from Earth to this quasar in AU.
Measurements of the red shift of the quasar suggest the expansion of the Universe has accelerated since the detected light left the quasar.
State the cause of this acceleration
Dark energy
Explain why it is important that there is more than one method of detection and not just transit method.
Dip in light curve can be caused by other effects ✔
Except for planets very close to star, periods likely to be very long and may take many years of observation using transit method alone ✔
The black body temperature of each star is approximately 9200 K.
Explain why a Hydrogen Balmer line was chosen for the analysis of wavelength variation.
The temperature (9200K) indicates that the star is in spectral class A. ✓
Hydrogen Balmer lines are strongest in A class stars and therefore would be more easily measured. ✓
complete the sentence
the lower the apparent magnitude, the … the star
brighter
Which of the following regions of the Hertzsprung-Russell diagram does 40 Eridani B belong to between dwarf star, giant star, main sequence star
its temperature is 16,000 & low radius
dwarf as it has a High temperature but is relatively small
Explain how an orbiting planet causes a Doppler shift in the spectrum of a star.
Planet and star orbit around common centre of mass that means the star vmoves towards/away from Earth as planet orbits ✓
Causes shift in wavelength of light received from star ✓
Explain how an orbiting planet causes a change in the apparent magnitude of a star.
When planet passes in front of star (as seen from Earth), some of the light from star is absorbed and therefore the amount of light reaching Earth reduced ✓
Apparent magnitude is a measure of the amount of light reaching Earth from the star ✓
The Andromeda galaxy is believed to be approaching the Milky Way at a speed of 105 km s–1
Calculate the wavelength of the radio waves produced by atomic hydrogen which would be detected from a source approaching the observer at a speed of 105 km s–1 wavelength of atomic hydrogen measured in a laboratory = 0.21121 m.
Don’t draw and label it but just say where you think they each may go
On a H-R diagram, mark with an X the current position of the Sun and draw a line to represent the evolution of the Sun, from its formation to its eventual state as a white dwarf.
Matar is a star in the same spectral class as the Sun.
State two properties common to Matar and the Sun.
temperature and colour
two stars have same radius and same apparent magnitude but one is F spectral class and the other is K spectral class. State one similarity and one difference
similarity : both would appear same brightness
difference : one with K spectral class would appear orange
Explain what is meant by red shift.
increase in wavelength (of em radiation) due to relative
recessive velocity between observer and source
explain why type 1a supernovae can be
used as standard candles to determine distances.
- all type 1a supernovae have the same peak in absolute magnitude
- apparent magnitude can be measured from equation m=-M = 5logd/10
Show how Hubble’s Law can be used to estimate the age of the Universe. State the assumption made.
Velocity/distance = Hubbles constant (H)
Time = distance/velocity
therefore Time = 1/H
The Charge Coupled Device (CCD) is an important part of the detection system of many modern telescopes due to its high quantum efficiency.
Explain what is meant by quantum efficiency and compare the quantum efficiency of a CCD with that of the eye
quantum efficiency = number of photons detected at detector / total arriving at detector
For CCD QE> 80%
For eye QE = 1%
Many astronomical observations rely on a Charge Coupled Device (CCD) to obtain an image. Describe the structure and operation of the CCD and discuss the advantages of using a CCD for astronomical observations.
The quality of your written communication will be assessed in this question.
The CCD is a silicon chip
* The chip is divided into picture elements
* Each picture element is associated with a potential well in the silicon
* Incident photons are focused on the CCD
* The photons cause the release of electrons within the semiconductor
* The number of electrons liberated is proportional to the intensity of the light.
* Electrons are trapped in the potential wells
* An electron pattern is built up which is identical to the image formed on the CCD
* When exposure is complete the charge is processed to form an image.
The last refracting telescope that could be called ‘the largest optical telescope in the world’ was one with an objective lens of diameter 0.90 m. It was superseded in 1889 by a reflecting telescope with an objective mirror of diameter 1.52 m. Calculate the ratio resolving power of reflector / resolving power of refractor
use of rayhelgi critertion equation
theta (refractor) / diameter (refractor) = theta (reflector) / diameter (reflector)
0.9 / 1.52 = 0.592
How to prevent spherical aberration in reflecting telescopes
Use a parabolic mirror
Give two reasons why the secondary mirror in the Cassegrain telescope affects the clarity of the image.
mirror blocks light so less light hits objective mirror (1)
light diffracted passing secondary mirror affects image (1)
Explain why the resolving power of a telescope is better in the visible region than in the infrared region.
visible wavelengths shorter than infrared
smaller resolving angle, better resolving power
What, in the atmosphere, is responsible for absorbing infrared radiation?
water vapour or carbon dioxide
The spectrum of light from a star can be used to determine its temperature. Explain why this absorption can lead to errors in the value.
- longer wavelengths absorbed
- shifts peak to shorter wavelengths
- stars appear hotter at shorter wavelengths due to Wiens law equation
Distance from Vespa to the sun is 2.57 AU. Calculate distance from Vespa to the earth
2.57 + 1 = 3.57 AU
(This works because 1AU is the distance between the sun and the earth)
3.57 x 1.5 x 10^11 = 5.36 × 10^11m
Show that when Vesta is at a distance of 1.73 × 10^11d m from Earth, the angle subtended by Vesta to an observer on Earth is about 3 × 10^–6 radian. diameter of vesta= 5.4 x 10^5m
5.4 x 10^5 / 1.73 x 10^11 = 3.12 x 10^-6 rad
Explain what is meant by a parsec. Draw a labelled diagram in support of your answer.
1 pc is the distance at which 1AU subtends an angle of 1 arc second
The Hipparcos satellite used the parallax method to measure the distance to more than 100 000 stars with a precision of 0.002 arc seconds. Calculate, in metres, the maximum distance measurable by Hipparcos
distance (in parsecs) = 1/(parallax angle) = 1/0.0002 = 500pc
= 2 x 10^19 m
ci) two components are 178 pc apart
Distance apart too great
ii) more distant star will not appear to move as much as nearest star
what is meant by the Rayleigh criterion
Two sources will be resolved if the central maximum of the diffraction pattern of one coincides with the first minimum of the other.
The supermassive black hole at the centre of the Milky Way galaxy has a mass equal to 4.1 million solar masses.
Calculate the Schwarzschild radius, Rs, for this black hole.
R = 2GM / c^2
mass of milky way = 4.1 x 10^6 M (were M is the mass of the sun)
(2 x 6.67 x 10^-11 x 4.1 x 10^6 x 1.99 x 10^30) = 1.2 × 10^10m
Explain why X-ray telescopes need to be in orbit.
x-rays are absorbed by the Earth’s atmosphere
Chandra makes use of a charge coupled device (CCD) to detect the X-ray photons.
Describe the processes involved in the detection of photons by a CCD
CCD consists of silicon chip
incident photons cause electrons to be released
electrons are trapped in potential wells in the CCD
Draw a ray diagram to show how spherical aberration is caused by
a concave spherical mirror.
State the defining properties of a neutron star.
- extremely dense
- made up of neutrons
State what is meant by a supernova.
An object that produces a rapid increase in brightness
Draw a ray diagram for an astronomical refracting telescope in normal adjustment.
Your diagram should show the paths of three non-axial rays passing through both lenses. Label the principal foci of the two lenses.
State two advantages of using an astronomical telescope that has a large diameter objective lens when making observations.
- larger diameter allows fainter objects to be viewed, as collecting power proportional to d^2
- better resolution
Draw a labelled diagram to show how a converging lens causes chromatic aberration.
Draw a ray diagram to show how a converging lens can be used to form a diminished image of a real object. Label the object, image and principal foci of the lens on your diagram.
A student experimented with a converging lens whose focal length was known to be approximately 50 cm. She placed an object and screen a fixed distance of 200 cm apart. With the lens 128 cm from the object, she observed a sharp image on the screen.
Calculate the focal length of the lens.
The telescope suffers from chromatic aberration. Describe how this affects the appearance of the image.
edges of the image will appear coloured
What is an equation you have to be familiar with in AQA Physicd, linked to sectors
S = R x theta
S (distance)
R (radius)
Draw a labelled diagram to define parsec
Outline circumstances during which a gamma ray burst will be emitted by Rigel.
Formed when supergiant collapses and neutron star is formed
State what is meant by normal adjustment when applied to an astronomical refracting telescope.
Final image at infinity
