Astrophysics Flashcards
What is normal adjustment of a telescope
When the distance between the objective lens and the eyepiece lens is the sum of their focal lengths. Principle focus for these two lenses is in the same place
What is magnifying power
larger angle over smaller angle
What is chromatic aberration
Focal length of red light is greater than that of blue light so they are focussed at different points. This causes an image with coloured fringing
What is spherical aberration
The curvature of a lens or mirror causes rays of light at the edge to be focused in a different position to those near the centre, leading to image blurring and distortion
Where is spherical aberration more pronounced
Lenses with larger diameter
How to avoid spherical aberration
use parabolic dishes
What is an Achromatic doublet
A convex lens made of crown glass and a concave lens made of flint glass cemented together in order to bring all rays of light into focus in the same position
Disadvantages of Refracting telescopes(6)
-Glass must be pure and free from defects (hard to achieve for a large diameter lens)
-Large lenses can bend and distort under their own weight due to how heavy they are
-Chromatic and spherical aberration both affect lenses
- Incredibly heavy and therefore can be difficult to manoeuvre
-Large magnifications require very large diameter objective lenses.
-Lenses can only be supported from the edges which can be an issue when they are large and heavy
Advantages of reflecting telescopes(6)
-Mirrors that are few nanometres thick can be made which give great image quality
-Mirrors unaffected by chromatic aberration and spherical can be fixed(parabolic)
-Mirrors aren’t as heavy as lenses so easier to manoeuvre to follow objects
-Large composite primary mirrors can be made from lots of smaller mirror segments
-Large primary mirrors are easy to support from behind since you don’t need to be able to see through them
Are reflectors or refractors preferred in modern telescopes
Reflectors
How do radio telescopes work
They use radio waves to create images of astronomical objects.
Are radio telescopes ground based
yes
Why should radio telescopes be in isolated locations
To avoid interference from nearby radio sources
Why can radio telescopes be ground based
The atmosphere is transparent to a large range of radio wavelengths so it does not absorb them
What are the Similarities of radio telescopes and optical telescopes (4)
-Both function in same way. Intercept and focus incoming radiation to detect its intensity
-Both can be moved to focus on different sources of radiation or to track a moving source
-Parabolic dish of a radio telescope is extremely similar to the objective mirror of a reflecting optical telescope
-Both can be build on the ground
What are the differences between radio telescopes and optical telescopes(4)
-Radio telescopes have to be much larger in diameter than optical telescopes in order to achieve the same quality image (resolving power)
-Construction of radio is cheaper and simpler because wire mesh is used instead of a mirror
-Radio must move across an area to build up an image, unlike optical telescopes
-Radio experience a lot more man-made interference.
Which has a larger collecting power out of radio and optical telescopes and why
Radio because they have a much larger diameter
How do infrared telescopes work
Use infrared radiation to create images of astronomical objects. They consist of large concave mirrors which focus radiation onto a detector.
What precaution has to be taken to infrared telescopes because all objects emit infrared radiation as heat
They must be cooled using cryogenic fluids to almost absolute zero. They must also be well shielded to avoid thermal contamination from nearby objects.
What are infrared telescopes used for
To observe cooler regions in space
Are infrared telescopes ground based
No, they must be launched into space
Why aren’t infrared telescopes ground based
The atmosphere absorbs most infrared radiation
How do ultraviolet telescopes work
Use ultraviolet radiation to create images of astronomical objects. They uses the the cassegrain configuration to bring ultraviolet rays to a focus. The rays are detected by solid state devices which use the photoelectric effect to convert UV photons into electrons and then pass around a circuit
What are UV telescopes used to observe
The interstellar medium and star formation regions
Are UV telescopes ground based
No
Why are UV telescopes not ground based
Ozone layer blocks all ultraviolet rays that have a wavelength of less than 300nm
How do X-ray telescopes work
Use X-rays to create images of astronomical objects. They are made from a combination of parabolic and hyperbolic mirrors which are extremely small. rays enter the telescope, skim off the mirrors, and are brought into focus on CCDs which convert
light into electrical pulses.
What are x ray telescopes used for
To observe high energy events such as black holes
Are X ray telescopes ground based
no
Why are X ray telescopes not ground based
All x rays get absorbed by the earths atmosphere
How do Gamma telescopes work
They use gamma radiation to create images of astronomical objects. They use a detector made of layers of pixels. As the gamma photons pass through, they cause a signal in each pixel they come into contact with.
Why don’t gamma telescopes use mirrors
Gamma rays have so much energy that they would just pass straight through
Describe short lived GRBs (gamma ray bursts)
They last anywhere between 0.01 and 1 second. They are thought to be associated with merging neutron stars (forming a black hole), or a neutron star falling into a black hole
Describe Long lived GRBs (gamma ray bursts)
They last between 10 and 1000 seconds. They are associated with a Type II supernova (death of massive star)
What is collecting power
A measure of the ability of a lens or mirror to collect incident EM radiation
What is collecting power directly proportional to
Objective diameter squared
How does a greater collecting power affect the images
The images are brighter
What is resolving power
the ability of a telescope to produce separate images of close-together objects
What is required for an image to be resolved
the angle between the straight lines from the earth to each object must be at least the minimum angular resolution
what is angular resolution measured in
Radians
What does the Rayleigh criterion state
Two objects will not be resolved if any part of the central maximum of either of the images falls within the first minimum diffraction ring of the other
What is the target like shape that light diffracts into in a telescope
airy disc
What is a CCD (charged coupled device
Array of light sensitive pixels, which become charged when they exposed to light by the photoelectric effect
What is quantum efficiency
the percentage of incident photons which cause an electron to be released
what is spectral range
detectable range of wavelengths of light
what is pixel resolution
Total number of pixels used to for the image on a screen
what is spatial resolution
the minimum distance two objects must be apart in order to be distinguishable
what is the quantum efficiency of a CCD
around 80%
what is convenience (with regard to ccds)
how easy the images are to form and use
what is the spectral range of CCDs
Infrared, UV, visible
what is the quantum efficiency of the human eye
between 4 and 5 percent
what is the pixel resolution of a CCD
varies, but is around 50 mega pixels
what is the spectral range of the human eye
Only visible light
what is the spatial resolution of a CCD
10 micrometers
what is the pixel resolution of the human eye
around 500 megapixels
what is the spatial resolution of the human eye
100 micrometers
what is the convenience of a CCD
Needs to be set up but images produced are digital
what is the convenience of the human eye
simpler to use as there is no need for extra equipment
define luminosity
rate of light energy released of a star ( power output of a star)
define intensity
the power received from a star (luminosity) per unit area
What does the intensity of a star follow
the inverse square law
what is apparent magnitude
how bright the object appears in the sky
what does the apparent magnitude of a star depend on
the stars luminosity and distance from earth
what is the hipparcos scale used for
to classify astronomical objects by their apparent magnitudes
what is the apparent magnitude of the brightest stars on the hipparcos scale
1
what is the apparent magnitude of the dimmest stars on the hipparcos scale
6
in the hipparcos scale if the magnitude changes by 1 what does the intensity change by
2.51
define absolute magnitude
what somethings apparent magnitude would be if it were placed 10 parsecs away from earth
define parallax
The apparent change of position of a nearer star in comparison to distant stars as a result of the orbit of the earth around the sun
what is the astronomical unit
the average distance between the centre of the earth and the centre of the sun
what is a parsec
the distance at which 1 au subtends an angle of 1 arcsecond
what is a black body radiator
perfect emitter and absorber of all possible wavelengths of radiation
what are hydrogen balmer lines
absorption lines which are caused by the excitation of hydrogen atoms from the n = 2 state to higher energy levels.
which spectral classes are balmer lines found
O B and A
why are hydrogen balmer lines not present when the temperature is too high
the majority of atoms will be excited to higher levels than n=2 or electrons might even become ionised
why are hydrogen balmer lines not present when the temperature is too low
the hydrogen atoms are unlikely to become excited
Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in O
-Blue
-25000-50000k
-He+ He H
- weak
Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in B
-Blue
-11000-25000k
-He H
-Slightly stronger than O
Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in A
-Blue/white
-7500-11000k
-H Ionised metals
-strongest
Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in F
-White
-6000-7500 k
-ionised metals
-weak
Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in G
-Yellow/ white
-5000-6000k
-Ionised and neutral metals
-none
Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in K
-Orange
-3500-5000k
-Neutral metals
-None
Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in M
-Red
- <3500k
-Neutral atoms, Titanium oxide
-None
Y axis of HR diagram
Absolute magnitude
Y axis of HR diagram moving up in 5s
+15, +10, +5, 0, -5, -10, -15
X axis of HR diagram
Temperature /K and spectral class
What happens to main sequence star when it uses up all the hydrogen in its core
It moves up and to the right on the HR diagram and becomes a red giant
How is a red giant different from a main sequence star
It is brighter and cooler
What happens when a red giant uses up all the helium in its core
It will eject its outer layers and will move down and to the lift on the HR diagram to become a white dwarf
How is a white dwarf different to a main sequence star
it is hotter and dimmer
what is one solar mass
the mass of the sun
stages of stellar evolution for star 1.4<solar mass<3
Nebula-Protostar-Main sequence-Red Giant-Supernova-neutron star
Stages of stellar evolution for star <1.4 solar masses
Nebula-protostar-Main sequence- Red giant- white dwarf- black dwarf
Stages of stellar evolution for star >3 solar masses
Nebula-protostar-Main sequence-Red supergiant-Supernova-Black hole
Describe events of protostar
-clouds of gas and dust (nebula)have fragments that clump together under gravity to form denser centre(protostar)
-surrounded by circumstellar disc
-When protostar becomes hot enough it fuses elements producing a strong stellar wind
Describe events of a Main sequence star
-Inward force of gravity and outward force due to fusion are in equilibrium
-Hydrogen nuclei fused into helium
-greater the mass of the star, the shorter its main sequence period as it uses fuel more quickly
Describe the events of a red giant
-Once hydrogen runs out the temperature of the core increases and begins fusing helium nuclei to heavier elements
-outer layers of the star expand and cool
Describe the events of a white dwarf
-When a red giant uses up all of its fuel the fusion stops and the core contracts as gravity is now greater than the outward force
-Outer layers are thrown off
-Core becomes very dense
-eventually cools to a black dwarf
Describe the events of a red supergiant
-When a high mass star runs out of hydrogen nuclei the same process for red giant occurs on a larger scale
-The collapse of red supergiants in a supernova causes gamma ray bursts
-They can fuse elements up to iron
describe the events of a supernova
-When all fuel runs out, fusion stops and the core collapses inwards
-outer layers fall inwards, rebound and sent into space in a shockwave
-As shockwave passes through surrounding material, elements heavier than iron are fused and flung out into space
-supernova has rapidly increasing absolute magnitude
Describe the events of a neutron star
-Gravity is so strong that it forces protons and electrons together to form neutrons
-incredibly dense
-pulsars are spinning neutron stars
describe the events of a black hole
-Core of giant star collapses, the neutrons are unable to withstand gravity forcing them together
-light cant escape
-event horizon is the point at which the escape velocity becomes greater than the speed of light
-Schwarzchild radius is the radius of the event horizon
what is a binary system
system where two stars orbit a common mass
Describe a type 1 supernova
when a star accumulates matter rom its companion star in a binary system and explodes after reaching a critical mass
Describe a type 2 supernova
The death of a high mass star after it runs out of fuel
3 ways that supermassive black holes can form
-collapse of massive gas clouds
-normal black hole that has accumulated huge amounts of matter
-several normal black holes merging together
why is type 1a supernova dimmer than hubble expected
expansion of the universe is accelerating and is older than hubbles law estimates
what is dark energy
Something that has overall repulsive effect throughout the whole universe causing the expansion of the universe to accelerate
What is the doppler effect
The compression or spreading out of waves that are emitted or reflected by a moving source
how does distance effect red shift
greater distance means greater red shift
what colour shift is it if something is moving towards the earth
blue shift
what colour shift is it if something is moving away from earth
red shift
what is the units of d in the apparent magnitude and absolute magnitude equation
parsec
how is doppler effect used to identify binary star systems
-As the stars eclipse each other, they are travelling perpendicular to the line of sight from the
observer, so there is no Doppler shift in their emitted radiation.
-However, when one star is travelling away from the observer, the other is travelling towards the observer. This causes each spectral line to be split into two, where one is blue-shifted and the other is red-shifted
How do we know that quasars are very far away
They have very large red shift values
what is the big bang theory
Universe began from one point (a singularity that was infinitely small and hot). The universe began with a huge explosion from this point
How does background radiation provide evidence for the big bang
-Background radiation is the radiation coming from all parts of the universe
-The spectrum has a peak in the microwave region corresponding to a temperature of 2.7k
-It can be interpreted as the radiation left over from the big bang
How do the relative abundancies of helium and hydrogen provide evidence for the big bang
-When universe was very young fusion occurred resulting in the production of helium from fusing hydrogen
-Fusion stopped as the universe then expanded and cooled
-resulting in relative abundance of hydrogen and helium in the ration of 3 to 1
what is a quasar
supermassive black hole surrounded by a disc of matter
3 characteristics of quasars
-very large red shift
-very powerful light output
-size not much bigger than a star
what is the radial velocity method of detecting an exoplanet
-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 ( blue shifted when it moved towards earth and red shifted when it moves away
-The time period of the planets orbit is equal to the time period of the doppler shift
what is the transit method of detecting an exoplanet
-When a planet crosses in front of a star (transits) the intensity dips slightly
-if the intensity of the star dips regularly then this is a sign that an exoplanet is orbiting it
-size and orbital period of planet can be determined from the amount that the intensity falls and the duration of the dip
-it only works if the line of sight to the star is in the plane of the planets orbit
