Astrophysics Flashcards
What is quantum efficiency?
The proportion of the incident photons that are detected. For a CCD it’s typically 80% or more. For the eye it is around 1%.
What are the visible spectrum characteristics of the spectral class O?
Blue stars, temperature 25,000-50,000 K. Strongest spectral lines are helium ion and helium atom absorptions, since these need a really high temperature. They also have weak hydrogen Balmer lines
What does the huge red shift from the emission lines of quasars tell us?
They’re a huge distance away (billions of light years)
Which part of the light spectrum is detectable by a CCD?
Infrared, visible and UV light
Type 1a supernovae
- happen the same way for stars of the same mass
- all have the same absolute magnitude curve so it can be used as a standard candle.
What is the equation for the Rayleigh criterion?
θ = λ/D
What happens when a light source moves away from us?
The wavelengths of light become longer and the frequencies become lower. This light shifts towards the red end of the spectrum and undergoes red shift
For a converging lens to form a real image, where most of the object be?
Further away from the lens than the focal length
How do radio astronomers overcome the problem of the long wavelengths of radio waves and how this affects the revolving power of radio telescopes?
By linking lots of telescopes together. The data from these telescopes can be combined to form a single image, this is the equivalent to one huge dish the size of the separation of the telescopes. This allows resolutions thousands of times better than optical telescopes to be achieved
What is a standard candle?
A class of astrophysical objects, such as supernovae or variable stars, which have known luminosity due to some characteristic quality possessed by the entire class of objects
What is the Balmer series (ONLY IN VISIBLE PART?)
A series of spectral lines in the visible and UV spectrum of atomic hydrogen. From n = 2 to n = 6
What are the problems with the Doppler shift method of detecting exoplanets
The movement of the star needs to be aligned correctly with the observer’s line of sight so that the shift from the star is detectable
How are neutron stars formed?
If the star is massive enough, electron degeneracy pressure can’t stop the core contracting, happens when the core is 1.4x the mass of the sun. The electrons get squashed onto nuclei and combine with protons to form neutrons and neutrinos. Core collapses to form neutron star. Outer layers hit the surface of the neutron star, rebounding and creating huge shock waves in the form of a supernova.
Why do infrared telescopes need to be cooled to very low temperatures? What is used to cool them?
1) Because they produce their own infrared radiation do you to their temperature
2) liquid helium or refrigeration units
Properties of neutron stars
- very dense, about 4 x 10^17 kg m^-3
- very small, about 20 km across
- can rotate very fast, up to 600 times a second
- some emit radio waves
What happens after low mass stars become red giants?
The form white dwarfs. This is because the carbon-oxygen core isn’t hot enough for any further fusion so the star continues to contract under its own weight. Once Earth-size, electron degeneracy pressure stops it from collapsing further. Helium shell becomes more unstable as the core contracts and the star pulsates, ejecting its outer layers into space, leaving behind a hot dense solid core
Why is glass a problem in UV telescopes?
Glass absorbs UV light
1 parsec is equal to about
3.26 light years
What electromagnetic wavelengths does the atmosphere block out?
All except visible, radio, and a few wavelengths infrared and UV
Why are reflecting telescopes better than refracting telescopes?
Large mirror is of good quality are much cheaper to build than a large lenses. They can also be supported from underneath so they don’t distort as much as lenses. They also don’t suffer from chromatic aberration
Why do the mirrors in UV telescopes have to be made more precisely than in optical reflecting telescopes, but the mirrors in infrared telescopes can be less perfectly shaped than the ones in optical telescopes?
The longer the wavelength of the radiation, the less it’s affected by imperfections in the mirror
What is a quasar?
A very powerful galactic nucleus containing a huge active black hole
Describe the Hipparchus scale for apparent magnitude
- it is a scale to show apparent magnitude, the brighter and object looks, the more negative it’s apparent magnitude on the Hipparchus scale
- logarithmic scale so a magnitude 1 star has an intensity 100 times greater than a magnitude 6 star.
- this means a difference of one magnitude corresponds to a difference in intensity of 100^0.2 times. So a magnitude 1 star is about 2.51 times brighter than a magnitude 2 star
How can you calculate the brightness or intensity ratio between two stars using the Hipparchus scale?
I2/I1 = 2.51^(m1 - m2), where I is intensity and m is apparent magnitude
In X-ray telescopes, how can X-rays be detected?
Using a modified Geiger counter or a fine wire mesh. New X-ray telescopes use highly sensitive X-ray CCD cameras
What is the collecting power of a telescope?
The energy collected per second. A big dish or mirror collects more energy from object in a given time, this gives a more intense image so the telescope can observe fainter objects.
Power is proportional to the Diameter²
What are the problems with reflecting telescopes?
If the shape of the mirror isn’t quite the parabolic, then parallel rays reflecting off different parts of the mirror do not all converge on to the same point. This is known as spherical aberration
How does the spatial resolution of the eye compare to the spatial resolution of CCDs?
The spatial resolution of the eye is around 100 μm, see CDs can have a spatial resolution of around 10 μm. So see CCDs are better for capturing fine detail
fo + fe =
Telescope length
What things are similar about radio telescopes and optical telescopes?
- it has A parabolic dish that works in exactly the same way as the object in the mirror open up to come reflecting telescope
- an antenna is used as a detector at the focal point instead of an eye or camera in an optical telescope, but there is no equivalent to the eye lens
- most radiotelescopes are manoeuverable than allowing the sort of waves to be tracked, in the same way as optical telescopes
Do stars behave as black bodies?
Yes and their black body radiation produces a continuous spectrum
For a converging lens to form a virtual image, where most the object to be?
Close up to the lens and the focal length
What does the brightness of a star in the night sky depend on?
It’s luminosity and its distance from us
What are the visible spectrum characteristics of the spectral class B?
Blue stars, temperature 11,000-25,000 K. Spectra show strong helium atom and hydrogen absorptions
What two factors limit the resolving power of a telescope?
- the Rayleigh criterion
- the quality of the detector
How can exoplanets be detected?
- using Doppler shift
- the transit method
What does 1 Mpc equal in metres?
3.08 x 10^22 m
Do you like to Mrs can just be distinguished if the centre of the… from one source is…
…Airy disc… is at least as far away as the minimum of the other source.
What is spatial resolution?
How far apart different parts of the object being viewed need to be in order for them to be distinguishable
What happens when a light source moves towards us?
The wavelengths become shorter and the frequencies become higher, the light undergoes blue shift
What is the equation for the power output of a star?
P = σAT^4, where A is surface area, T is temperature in kelvin, σ is Stefan’s constant = 5.67 x 10^-8 Wm^-2K^-4
An object is at infinity, what assumption can you make about the light rays entering a telescope?
They are parallel
Explain the transit method for detecting exoplanets
It measures the change in apparent magnitude as an exoplanet travels in front of a star. As the exoplanet crosses in front of the star, some of the light from the star is blocked from Earth’s view. This causes a dip in the light curve observed on earth. From this, the radius of the exoplanet can be found
Why is an Airy disc formed?
When a beam of light passes through a circular aperture, a diffraction pattern is formed, then central circle of this pattern is called the Airy disc
What are the emission lines detected from Quasars?
The Balmer series of Hydrogen but red shifted enormously
The intensity of a star is
The power of radiation per square metre. If the energy has been emitted from a point or sphere (star) it obeys the inverse square law. I = P/(4πd²) where d is distance in m
What are PULSARS?
Rotating neutron stars and that emit radio waves which can be observed from earth as radio pulses
What are CCDs?
Silicon chips about the size of a postage stamp, divided up into a grid of millions of identical pixels
What does the intensity of the Balmer lines in a spectrum of hydrogen depend on?
The the temperature of the star
What is an exoplanet/extrasolar planet?
A planet that is not in our solar system
What are the visible spectrum characteristics of the spectral class G?
Yellow-white stars, 5000-6000 K. Both metal ion and a metal atom absorptions
What problem is there with the transit method of detecting exoplanets?
The chances of the planet’s path being perfectly lined up so it crosses the line of sight between the star and the Earth is incredibly low. Therefore you can only confirm exoplanets that are observed, not rule of the locations of any
The Hot Big Bang Theory
The universe started off very hot and very dense (perhaps as an infinitely hot, infinitely dense singularity) and has been expanding ever since
What happens when photons hit the silicon in a pixel on a CCD chip?
They cause electrons to be released via the photoelectric effect. These electrons alter the charge on each pixel, this charge can be measured and used to create a digital signal.
How are exoplanets detected using Doppler shift?
- an exoplanet causes tiny variations in the orbit of the star its orbiting
- this wobble cause tiny red and blue shifts in the star’s emissions which can be detected from Earth
- from this the minimum mass of the exoplanet can also b calculated
What is cosmological red shift?
When light waves are stretched due to the expansion of the universe
How to get accurate measurements using a telescope
- place observatories at high altitudes
- send satellites above the atmosphere
What are the similarities between IR and UV telescopes and optical reflecting telescopes?
- they both have the same parabolic mirror set up to focus the radiation onto a detector
- in both cases CCDs or special photographic paper are used as the radiation detectors, just as an optical telescopes.
How do radiotelescopes compare to optical reflectors?
- instead of a polished mirror, a wire mesh can be used since the long wavelength of radio waves don’t notice the gaps. This makes the construction easier and cheaper than optical reflectors
- the shape of the dish has to be have a precision of about λ/20 to avoid spherical aberration so the dish doesn’t have to be anywhere near as perfect as a mirror
- however, unlike an optical telescope, a radio telescope has to scan across the radio source to build up an image
What is the lens equation?
1/f = 1/u + 1/v. u is the distance between the object and lens axis, v is the distance between the image and lens axis. If the image is real v is +ve, if the images virtual v is -ve
What is the absolute magnitude, M, of a star or galaxy?
What it’s apparent magnitude would be if it were 10 parsecs away from earth
How can the magnification of a telescope be calculated?
M = θi/θo or M = fo/fe
What is the value for Ho (Hubble’s constant)?
Between 65 and 80 km s^-1 Mpc^-1
When do black holes form?
When the core of the star remaining after a supernova is more than 3x the Sun’s mass, the neutrons can’t withstand the gravitational forces. The core collapses to an infinitely dense point, the singularity
A star is exactly one parsec away from earth if the angle of parallax, θ, is:
1 arcsecond = (1/3600)°
1 light year is equivalent to
about 9.46 x10^15 m
What is the Doppler shift
The change in wavelength and frequency of EM radiation coming from an object moving towards or away from us
What are the visible spectrum characteristics of the spectral class M?
Red stars, < 3500 K. Molecular band absorptions from compounds like titanium oxide are present in the spectra of these stars, since they’re cool enough for molecules to form
What are light curves for supernovae?
A plot of absolute magnitude M, against time since the supernova began
What evidence is there for the Hot Big Bang model?
- cosmic microwave background radiation
- the large abundances of hydrogen and helium in the universe (about 74% H and 24% He), universe must have been hot enough for H fusion to occur, studying the ratio of helium to hydrogen allows us to work out a time frame for this fusion
What are the visible spectrum characteristics of the spectral class K?
Orange stars, 3500-5000 K. At this temperature most spectral lines are from neutral metal atoms
What is the equation that links M (absolute magnitude) and m (apparent magnitude)
m - M = 5 log(d/10) where d is the distance in parsecs
How are X-ray telescopes designed?
With a series of nested mirrors so that the direction of the X-rays can be gradually altered, until they can be brought to focus on a detector. This type of telescope is known as a grazing telescope
What allows electrons to be in a state higher than n=1?
The temperature of stars as the collisions between atoms give the electrons extra energy
How can the spectra of binary stars be used to calculate the period of their orbit?
By looking at the Doppler shift from of spectra from the two stars and seeing how long it takes for the lines to go from zero separation to maximum separation. This is the time for half a period. Double this for the period
What is the SI unit for Ho?
s^-1
Why do main sequence stars become red giants?
When the hydrogen in the core runs out, outward pressure stops. The helium core contracts and heats up under the weight of the star. The outer layers expand to form a red giant. Eventually core of star is hot enough to fuse helium into carbon and oxygen, this releases huge amounts of energy and the outer layers are pushed outwards. When all the helium is used up, the C and O core contracts and shell helium burning occurs
What are the problems with refracting telescopes?
1) Glass refracts different colours of light by different amounts. This is called chromatic aberration and blurs the image
2) Any bubbles and impurities in the glass absorb some of the light, which means very faint objects aren’t seen
3) Large lenses are very heavy and can only be supported from the edges, so their shape can become distorted
4) For a larger magnification, The objective lens needs to have a very long focal length. Therefore reflecting telescopes have to be very long, leading to large and expect expensive buildings needed to house them.
In a reflecting telescope, the objective lens… the eyepiece lens…
- … converges the rays from the object to form a real image.
- … acts as a magnifying glass on this real image to form a magnified virtual image at infinity
The cosmological principle
On a large scale the universe is homogeneous (every part is the same as every other part) and isotropic (everything looks the same in every direction) – so it doesn’t have a centre
Why are X-ray telescopes designed in the way they are?
Because X-rays don’t reflect of surfaces in the same way as most other EM radiation. They are usually either absorbed by material or pass straight through it
A real image is formed when
Light rays from an object or made to pass through another point in space. The light rays are actually there and the image can be captured on a screen
One light year is
the distance that electromagnetic waves travel through a vacuum in one year
A black body
A body that absorbs all wavelengths of electromagnetic radiation and can emit all wavelengths of electromagnetic radiation. To a reasonably good approximation, stars behave as black bodies
Why is 1/Ho only an estimate for the age of the universe?
- no one knows the exact value of Ho
- the rate of expansion of the universe hasn’t been constant, it is accelerating and the rate of acceleration is increasing. The current explanation for this is dark energy
What are the visible spectrum characteristics of the spectral class F?
White stars, 6000-7000 K. These spectra have strong metal ion absorptions
What are the two defining features of Type I light curves?
- a sharp initial peak
- then a gradually decreasing curve
How can you estimate the age of the universe?
v = Hod, v = d/t, t = 1/Ho
Why are observatories placed at high altitudes away from cities in low humidity climates?
To reduce the negative effects of dust and man-made light pollution
Virtual image is formed when
Light rays from an object appear to have come from another point in space. The light rays aren’t where the image appears to be, so the image can’t be captured on a screen
How does a reflecting telescope form an image?
A parabolic concave mirror (the primary mirror) converges parallel rays from object, forming a real image. An eye lens magnifies the image to form a magnified virtual image at infinity
What are the spectral classes for stars?
O (hottest) B A F G K M
What is apparent magnitude based on?
How bright things appear from earth
What happens to high mass stars after they become red giants?
They can fuse elements all the way up to iron, building up layers in an onion-like structure to become red supergiants. Nuclear fusion beyond ion isn’t energetically favourable so the star contracts and explodes into a supernova. Very massive stars emit bursts of high energy gamma rays during this process. A neutron star is left behind, or a black hole if the star was massive enough
Power is also known as
Luminosity
What is the absolute magnitude of a star based on?
The luminosity of the star
Why is the resolving power of one radio telescope worse then the unaided eye?
Wavelengths of radio waves are about 1 million times longer than wavelengths of light. Due to the Rayleigh criterion, A longer wavelength means a larger minimum angular resolution, therefore the resolving power is weaker.
What are the benefits of a CCD?
- high quantum efficiency
- can detect Infrared visible and UV light
- low spatial resolution so better for capturing fine detail
- can produce digital images which can be stored copied and shared globally.
How does the signal generated on a CCD chip allow a digital image of an object to be created?
The signal describes where the light hits and its brightness/intensity as the charge on each pixel will vary depending on how many photons hit it. This allows an image to be created
What is the minimum angular resolution of a telescope
The smallest angular separation at which it can distinguish two points. The smaller the minimum angular resolution, the better the resolving power
One astronomical unit
The mean distance between the Earth and the Sun
What are the visible spectrum characteristics of the spectral class A?
Blue-white stars, 7000-11,000 K. Visible spectrum governed by the strongest hydrogen Balmer lines, also some metal ion absorptions
What is the equation for the Schwarzschild radius?
Rs = 2GM/c²
What is the luminosity of a star?
The total energy emitted per second
What are the properties of cosmic microwave background radiation?
- perfect black body spectrum corresponding to temperature of 2.73 K
- isotropic and homogeneous
- very tiny fluctuations of temperature
- shows a Doppler shift indicating Earth is rushing towards the Great Attractor (an unknown mass)
What happens in the centre of a star as it becomes a red giant?
core hydrogen burning → shell hydrogen burning → core helium burning → shell helium burning
For a specific temperature there is a peak on a black body curve, what equation can you use to calculate this?
λmax x T = constant = 2.9 x10^-3 mK (metre kelvin)
Where T is the temperature in kelvin
Why do emission and absorption spectra occur?
Electrons in an atom only exist at discrete energy levels
How do stars begin?
As clouds of dust and gas left from previous supernovae. Denser clumps contract under the force of gravity. Eventually protostars are formed, contraction continues and the protostar heats up. When it’s hot enough hydrogen fuse to form helium. This generates lots of energy and creates enough pressure to stop gravitational collapse. Star has now reached the main sequence
What is the Schwarzschild radius?
- The distance from the singularity in a black hole where not even light can escape. The boundary of this region is called the event horizon
- It is the distance at which the escape velocity is the speed of light
Describe an H-R diagram
Absolute magnitude on the y axis decreasing from 15 to -10 as you go up. Temperature on the x axis decreasing from 50,000 K to 2500 K as you go across, scale is non-linear. White dwarfs in the bottom left corner, main sequence diagonal from top left to bottom right, red supergiants in the top right corner. The sun is just to the right of the middle of the main sequence
