13. Astrophysics Flashcards
What are the two types of optical telescopes?
- Refracting telescope
- Reflecting telescope
What are converging lenses also called?
Convex lenses
What are diverging lenses also called?
Concave lenses
What is the principle focus?
Where rays meet on the principle axis
What are axial rays?
Rays that are parallel to the principle axis
What happens to the focal length if parallel rays enter the lens at different angles?
The focal length doesn’t change
Where do parallel rays converge when they enter the lens at different angles?
On the focal plane
What are non-axial rays?
Rays that are not parallel to the principle axis
For a single lens to make an image appear diminished and inverted where should the object be placed? Where will the image be positioned?
- More than 2f away from the lens
- Between f and 2f
For a single lens to make an image appear the same size and inverted, where should the object be placed? Where will the image be positioned?
- At 2f
- At 2f
For a single lens to make an image appear magnified and inverted, where should the object be placed? Where will the image be positioned?
- Between f and 2f
- Beyond 2f
For a single lens to make an object appear as an upright, virtual object, where should the object be placed? Where will the image be positioned?
- Closer than f
- Same side as object
What is the key difference between real and virtual images?
Real images are inverted and virtual images are not
What type of images does a flat mirror produce?
Virtual images
What types of images can a lens produce?
Both real and virtual images
What is the total length of a refracting telescope?
The sum of the two focal lengths of the lenses
How can a clear image be achieved by a refracting telescope?
The focal lengths must meet at the same point
How do refracting telescopes work? What do they consist of? What do they form?
They use two convex lenses to form a magnified image
How do reflecting telescopes work? What do they consist of?
- They use a parabolic mirror to focus incoming light into a point
- A secondary mirror is placed before the focal point to reflect the rays back
- The rays cross over then pass into a lens
- The rays emerge parallel
What is a reflecting telescope also known as?
Cassegrain telescope
What is a CCD?
Charge-coupled device
What are CCDs used for?
Used to take digital images
What are the physical characteristics of a CCD?
- Consist of a series of very small silicon pixels
- Beneath each pixel is a potential well which can trap electrons
- Above each one is a filter to only allow certain colour photons through
How does a CCD work?
- The filter only allows certain wavelengths of photon to hit the pixels
- The photons cause electrons in the pixels to be released into the potential wells
- The charge is then collected from each potential well
- 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
How does the number of electrons liberated relate to the intensity of the light?
The number of electrons liberated is proportional to the intensity of the light incident (number of photons)
What is quantum efficiency?
The ratio of number of photons that are actually detected to the number of photons incident on the CCD
What is the quantum efficiency of a CCD?
80% or more
What is the quantum efficiency of photographic film?
4%
What is the quantum efficiency of the human eye?
1%
What happens if film is exposed to too much light?
It becomes saturated and the image gets ruined
What are the advantages of CCDs?
- They do not get saturated
- They can detect a wider spectrum of light that the human eye (infrared, visible and UV)
- They can capture more fine detail
- They can have long exposures to capture very faint images
What is the minimum resolvable distance (spatial resolution) of a CCD?
10 micrometers
What is the minimum resolvable distance (spatial resolution) of the human eye?
100 micrometers
What is chromatic aberration?
When different wavelengths of light refract through glass by different amounts, causing the different colours to focus in different places after passing through a lens
What is the effect of chromatic aberration of the image produced?
The edges of the image will appear coloured
Which type of telescope does chromatic aberration have a greater effect on?
Refracting telescope because the larger lens in the refracting telescope is affected more than the smaller lens in the reflecting telescope
What are the four main problems of refracting telescopes?
- Chromatic aberration
- Impurities
- Lens distortion
- Length of telescope
What effect do impurities have on refracting telescopes?
Any bubbles or impurities in the glass absorb and scatter light so very faint objects can’t be seen
What effect does lens distortion have on refracting telescopes?
Large lenses are heavy and can only be supported at the edge so that shape can become distorted
What is the disadvantage of the length of refracting telescopes?
For large magnification, long focal lengths are needed so telescopes are very long, requiring big expensive buildings
What are the four main advantages of reflecting telescopes?
- Cost
- Support structure
- Collecting power
- Better resolving power
What is the advantage of the cost of reflecting telescopes?
Large mirrors of good quality are much cheaper than large lenses
What is the advantage of the support structure of reflecting telescopes?
Lenses can only be supported around the edge to prevent blocking light, but as no light passes through the mirror, they can be supported from the back, making them less likely to distort
What is the advantage of the collecting power of reflecting telescopes?
- The larger the telescope, the more light you collect and the dimmer the objects you can see
- Reflecting telescopes are easier to make larger
What are the two main disadvantages of reflecting telescopes?
- Spherical aberration
- Secondary mirror
What is spherical aberration?
If the shape of the primary mirror isn’t a perfect parabola, the outer rays will focus too close and the inner rays will focus too far away
What is the effect of spherical aberration on the image produced?
It causes images to be blurring
Which type of telescope is affected by spherical aberration?
Both reflecting and refracting telescopes suffer from spherical aberration
How does the secondary mirror reduce the clarity of the image produced by a reflecting telescope?
The secondary mirror can block and diffract some of the incoming light
What happens when light diffracts through a circular opening?
- The light diffracts
- Creating a circular diffraction pattern
- Consisting of bright rings (maxima) and dark rings (minima)
What is the central maxima of the circular diffraction pattern produced by the diffraction of light through the opening of a telescope called?
An airy disc
What is the Rayleigh criterion?
Two objects are just resolved when the centre of the airy disc in the diffraction pattern of one object coincides with the first minimum of the diffraction pattern of the other object
What is resolving power?
The smallest angle between two stars where they can be seen as two distinct stars (resolved)
How can resolving power be increased?
- Increase the size (diameter) of the dish
- Look at smaller wavelengths
What is 1 arcsecond?
1/3600 of a degree
What do the majority of telescopes use to focus electromagnetic radiation onto a point?
A parabolic dish
What do visible, UV and infrared telescopes place at the focal point of radiation on the parabolic dish?
A CCD
What do radio telescopes place at the focal point of radiation on the parabolic dish?
A combination of amplifiers are used to boost weak signals and a tuner is used to focus on specific frequencies
Why do X-ray telescopes require different structure?
X-rays are absorbed by the dish
What is the structure of X-ray telescopes?
They use a series of ‘grazing’ mirrors to focus the X-rays, making X-ray telescopes very long
What is used as a detector in X-ray telescopes?
A Geiger counter, a CCD or charged metal mesh
What is the maximum size imperfections in a telescope can be?
1/20th of the wavelength
What counts as imperfections in a telescope?
Bumps or holes
Which telescopes have to be the most perfect? What does this mean?
- UV telescopes
- This makes them the most expensive
Which telescopes can be the least perfect? What does this mean?
- Radio telescopes
- This makes them cheaper to build and much larger
What are radio telescopes often made of?
A mesh
Which telescopes have a much higher resolving power?
UV and X-ray telescopes
Which telescopes have the worst resolving power?
Radio telescopes
Why is it easier to improve the resolving power in a radio telescope than a UV telescope?
- Increasing the diameter of a telescope increases the resolving power
- Large radio telescopes are cheaper to make
- Large UV and X-ray telescopes are very expensive to make
How is collecting power related to the diameter of a telescope?
Collecting power is proportional to area which is proportional to diameter squared
Why can a larger telescopes allow you to see the dimmest stars?
Larger telescopes can collect more photons
Which telescopes have the best collecting power?
Radio telescopes
Which telescopes have the worst collecting power?
UV and X-ray telescopes
What affect does the atmosphere have on telescopes?
The atmosphere blocks some wavelengths more than others
Why do some telescopes need to be placed in space? Which ones?
- The atmosphere blocks some wavelengths more than others
- It blocks most infrared, ultraviolet and X-rays
Where can visible and radio telescopes be placed?
Visible and radio waves can pass through the atmosphere so these telescopes can be placed on the ground
Where might visible telescopes be more effective? Why?
Some visible light is blocked by the atmosphere so these telescopes are more effective in space
What is the issue with infrared telescopes?
- As infrared radiation is heat, infrared telescopes have to cool themselves
- This supply of coolant only lasts a few years
What is the limitation of the images produced by a telescope?
The images produced by a telescope are only as good as the detector (how many pixels are on the CCD)
What is an astronomical unit?
The radius of the Earth’s orbit around the Sun
What is a parsec?
One parsec is the distance at which 1 Au subtends an angle of 1 arc second
What is a light year?
The distance light travels in 1 year
What affects a star’s brightness?
- Its power output
- It’s distance from us
What is meant by power output (luminosity)?
The total amount of energy emitted (in the form of EM radiation)
In which direction does a star emit radiation?
In all directions (spherical shape)
What is apparent magnitude?
The brightness of a star as seen from Earth
What is absolute magnitude?
How bright a star appears from 10 parsecs away
What is a standard candle?
Any object whose absolute magnitude is known
What are type 1a supernovae?
Unique supernovae which, when they explode and rapidly increase in brightness, all have the same peak in absolute magnitude
What is the peak absolute magnitude of type 1a supernovae?
-19.3
What is a blackbody?
An object which absorbs and emits all wavelengths of light/all types of electromagnetic radiation
What does a blackbody radiation curve show?
The amount of radiation being produced by each wavelength
How does the blackbody curve of a hot star compare to that if a cool star?
- A hotter star produces more radiation than a cool star (area under line)
- They ‘peak’ at a shorter wavelength, meaning they appear bluer
- Cooler stars ‘peak’ at longer wavelengths, meaning they appear redder
What does Wein’s displacement law link?
The peak wavelength to the surface temperature of the star
What does it mean by peak wavelength?
The wavelength being emitted at which maximum intensity occurs
Why are non-optical telescopes important?
- Extremely hot objects like black hole event horizons can peak in the gamma part of the spectrum
- This can mean that in the visible part of the spectrum they appear less bright if most of the radiation being produced is invisible
What two main things can affect the power output of a star?
- The temperature of the star
- The surface area of the star
What does Stefan’s law link?
Stefan’s law links the power output of a star to its surface area and temperature
What are the spectral classes stars are grouped into based on?
- Their surface temperature (which affects their colour)
- Their absorption spectra (which is determined by what they are made of)
What are the 7 spectral classes?
O, B, A, F, G, K, M
How can you determine what a star is made of?
The star’s absorption spectra
Why do stars have an absorption spectrum?
- As stars are black bodies, they produce all types of radiation
- Including a continuous spectrum in the visible section
- But stars have an ‘atmosphere’ of cooler gases around them
- The gasses in this absorb some of the photons produced by the star
What affects the absorption spectra of stars?
Temperature
How is an absorption spectrum affected when a star is hotter?
- The hotter the star, the more energy the electrons have
- Meaning they sit in higher and higher energy levels
- Therefore, there are less photons that can be absorbed meaning there will be less absorption lines in a hotter star than a cooler star
What are hydrogen balmer lines?
Spectral lines corresponding to the visible wavelengths absorbed when electrons move from n=2 to higher energy levels
Which class of stars have the highest intensity of Balmer lines?
- A class
- A class stars are at a specific temperature where the majority of electrons are in n=2
Which stars have very intense helium lines?
Hotter stars
Which stars form heavy elements like metals?
Cooler stars
What can the coolest stars form?
Molecules
What are the three characteristics for each spectral class?
- Colour
- Temperature
- Absorption lines
What are the three characters of O class stars?
- Colour: Blue
- Temperature: 50,000 - 25,000
- Absorption lines: Strong He and He+ lines, weak H lines
What are the three characteristics of B class stars?
- Colour: Blue
- Temperature: 25,000 - 11,000
- Absorption lines: Strong He and H lines
What are the three characteristics of A class stars?
- Colour: Blue-White
- Temperature: 11,000 - 7500
- Absorption lines: Strongest H lines, some metal ions
What are the three characteristics of F class stars?
- Colour: White
- Temperature: 7500 - 6000
- Absorption lines: Strong metal ion lines
What are the three characteristics of G class stars?
- Colour: Yellow-white
- Temperature: 6000 - 5000
- Absorption lines: Metal ion and neutral metal lines
What are the three characteristics of K class stars?
- Colour: Orange
- Temperature: 5000 - 3500
- Absorption lines: Strong neutral metal lines
What are the three properties of M class stars?
- Colour: Red
- Temperature: <3500
- Absorption lines: Strong neutral atoms and molecular compounds lines like titanium oxide (TiO)
What is a Hertzsprung-Russell diagram?
A graph of absolute magnitude against temperature
What are the three distinct areas on an H-R diagram?
- Main sequence
- Red giants
- White dwarfs
What is the spectral class, approximate temperature and absolute magnitude of our Sun?
- Spectral class: G class
- Temperature: 5700K
- Absolute magnitude: +5
How is the main sequence show on an H-R diagram?
The long, diagonal band
What are main sequence stars?
Main sequence stars are in their long-lived stable phase where they are fusing hydrogen into helium
Where are red giants found on an H-R diagram?
In the top right hand corner
What are red giants?
Stars that have moved off the main sequence, and fusion reactions other than hydrogen to helium are occurring
What are the main characteristics of red giants?
- High luminosity (large negative absolute magnitude)
- High power output
- Relatively low surface temperature
- Huge surface area (due to Stefan’s law)
Where are white dwarfs found on an H-R diagram?
In the bottom left hand corner
What are white dwarfs?
Stars at the need on their lives, where all of their fusion reactions have stopped and there are slowly cooling to black dwarfs
What are the main characteristics of white dwarfs?
- Low luminosity (large positive absolute magnitude)
- Low power output
- Very high surface temperature
- Tiny surface area (due to Stefan’s law)
When is a star ‘alive’?
When fusion is taking place
What determines whether fusion takes place in a star?
- Is there enough fuel for fusion?
- Is it hot enough anywhere in the star for the fuel to fuse?
What is a stellar nebula?
A cloud of gas and dust
How are stars born?
- Stars are born in clouds of gas and dust usually left from previous supernovae
- The denser clumps of the cloud contract (very slowly) under the force of gravity
How are protostars formed? What happens to them?
- When the clumps of the cloud get dense enough, they form protostars that continues to contract and heat up
- When the temperature at the centre of the protostar eventually reaches a few million degrees, hydrogen nuclei can fuse into helium
When does a star reach the main sequence?
- An enormous amount of energy is released from fusion
- This creates enough pressure to balance the gravitational collapse
How long does a star spend as main-sequence stars?
Most of its life
What is core hydrogen burning?
When the pressure produced from hydrogen fusion in the core balances the gravitational force trying to compress the star
What happens when the hydrogen in the core of a main sequence star runs out?
- Nuclear fusion stops
- So does the outward pressure
- The helium core contracts and heats up under the weight of the star
- The outer layers expand and cool
- The star becomes a red giant
What happens in a red giant?
- The material around the core still contains plenty of hydrogen
- As the core contracts, this heats a layer (shell) around the core
- The shell gets hot enough to fuse hydrogen into helium
Why does the temperature increase as a star contracts?
Conservation of energy (gravitational potential energy is converted to thermal energy)
Why do red giants appear red?
The cooling of the outer layers of the star makes the star’s colour change to become redder
What is shell hydrogen burning?
When the shell surround the core of a star gets hot enough to fuse hydrogen into helium
What is core helium burning? How does this link to red giants?
- As the helium core of a red giant continues to contract, it eventually gets hot and dense enough to fuse helium into carbon and oxygen (core helium burning)
- This releases a huge amount of energy, which pushes the outer layers of the star further outwards
What happens when the helium in the core runs out?
- Fusion stops
- The forces in the core are unbalanced, causing the carbon-oxygen core to contract again
- This heats a shell around the core
- Shell helium burning occurs
What is shell helium burning?
When the shell surrounding the carbon-oxygen core of a red giant gets hot enough for the helium in this region to fuse
What happens in low mass stars after shell helium burning?
- The carbon-oxygen core won’t get hot enough for any further fusion
- It will contract until it is about Earth size
- As this point, electrons exert enough pressure to stop it collapsing further (core is stable)
What is electron degeneracy pressure?
The pressure exerted by electrons which stops the gravitational collapse of a star/core
How is a white dwarf formed?
- As the core contracts, the helium shell gets more and more unstable
- The star pulsates and ejects it outer layers into space as a planetary nebula
- Leaving behind a very hot, dense core (white dwarf)
- No more fusion is occurring
What happens to a white dwarf?
It will simply cool down and fade away
What is a supernova?
A star whose luminosity increases rapidly and enormously due to it exploding
How massive does a star have to be to become a planetary nebula/white dwarf?
Less than 1.4 solar masses
How massive does a star have to be to explode as a supernova?
More than 1.4 solar masses
What is one solar mass?
The mass of our Sun
How does a supernova occur?
- The core of a star runs out of fuel and starts to contract
- As the star is so massive, electron degeneracy can’t stop of core contracting
- The core continues to contract, and as it does, the outer layers of the star fall in and rebound off the core, setting up huge shockwaves
- These shockwaves cause the star to explode cataclysmically in a supernova
What happens when a star explodes in a supernova?
- It will experience a brief and rapid increase in absolute magnitude before fading over the next few weeks or months
- For some very massive stars, bursts of high energy gamma rays are emitted (lasting for minutes or very rarely, hours)
What type of core can be left behind after a supernova?
- Neutron star
- Black hole
Why do stars explode so suddenly in a supernova?
- A star the size of our Sun will only be able to fuse up to carbon and oxygen
- More massive stars can fuse heavier and heavier elements until they reach iron forming layers within the star to become red supergiants
- As nuclear fusion beyond iron isn’t energetically favourable, the core suddenly collapses before exploding
What is a neutron star?
A star with the density of nuclear matter
How massive must a star be to become a neutron star?
Between 1.4 and 3 solar masses
What happens to a star less than 1.4 solar masses when fusion stops?
- Core collapses
- The gravitational force of less than 1.4 solar masses is less than the force of electromagnetic repulsion between the electrons in atoms (electron degeneracy pressure)
- The core can only contract to the size of the Earth
- Results in a white dwarf
What happens to a star between 1.4 and 3 solar masses when fusion stops?
- Core collapses
- The gravitational force of between 1.4 and 3 solar masses is greater than the repulsion between electrons
- Electrons are forced into protons creating neutrons
- The gravitational force is not stronger than the repulsion caused by the strong nuclear force between these neutrons
- The core can only contact to about 20km in diameter
- Results in a neutron star
What happens to a star more than 3 solar masses when fusion stops?
- Core collapses
- The gravitational force of 3 solar masses is greater than the repulsion between electrons
- The gravitational force of 3 solar masses is greater than the repulsion caused by the strong nuclear force
- There are no other repulsive forces to stop gravity
- Nucleons are forced into nucleons
- Core collapses to an infinitely small and infinitely dense singularity
- Results in a black hole
What are neutron stars primarily made up of?
Neutrons
What is typically the diameter of a neutron star?
20 km
What are the three main features of a neutron star?
- 20 km in diameter
- Incredibly dense
- They can rotate very fast (up to 600 times a second)
How do neutron stars emit as they rotate?
Radio waves in two beams
What are pulsars?
- Pulsing neutron stars
- The beams of radio waves that neutron stars emit sometimes sweep past the Earth and can be observed as radio pulses
What are the three parts of a traditional black hole?
- The accretion disk
- The Schwarzschild radius (event horizon)
- The singularity
What is the accretion disk?
- A flat disc of matter that is spiralling into the black hole
- It consists of pieces of planets and stars that are being shredded
- It is extremely hot and bright
What is the Schwarzschild radius?
- The boundary at which the escape velocity is equal to the speed of light
- Outside the event horizon, the escape velocity < c so things can be seen
- Inside the event horizon, the escape velocity > c so things can’t be seen
What is the singularity of a black hole?
- This is the remains of the original star’s core
- It is now an infinitely dense point with a volume = 0 at the centre of the event horizon
- It is not visible
What were quasars thought to be when they were first discovered?
- First discovered in the late 1950s
- Originally thought to be stars in our galaxy
What was strange about quasars when they were thought to be nearby stars?
- They were producing very intense radio signals
- They sometimes shot out jets of material
- Their absorption lines didn’t match any known elements
- Their spectrums didn’t fit a regular black body curve
What was eventually noticed about the absorption lines of quasars?
The absorption lines were actually hydrogen-balmer lines that had been red shifted enormously
What did the fact that quasar’s absorption lines were enormously red shifted suggest?
Quasars aren’t stars in our galaxy, they’re the most distant objects ever seen
What are the current theories about quasars?
- The large red shift suggests they are very, very far away
- The fact they as bright as nearby stars suggests they are incredibly bright
- It is theorised that quasars are active galactic nuclei
What does the inverse square law show about quasars?
Quasars are extremely powerful and can have the same energy output as several galaxies
What are active galactic nuclei?
Supermassive black holes surrounded by a disc of matter which, as it falls into the black hole, causes jets of radiation to be emitted from the poles
What is an active galaxy?
A galaxy containing an active galactic nucleus
What mass must a black hole in an active galactic nuclei consume per year to produce the energy observed?
The mass of about 10 Suns per year
What is the Doppler effect?
The apparent change in frequency and wavelength of a wave as a result of relative motion between the source and the observer
What type of waves are affected by the Doppler effect?
All types of waves, including electromagnetic radiation
What happens to the light coming from stars to moving away from us?
Red shifted
What happens to light coming from stars moving towards us?
Blue shifted
What is red shift?
The increase in the wavelength of EM radiation due to a relative recessive velocity between the source and observer
How does recessional velocity link to change in frequency of a wave?
The greater the stars recessional velocity, the greater the change in the frequency of the wave
What is recessional velocity?
The speed a star is travelling away from Earth
How can red shift be observed?
By looking at the movement of absorption lines in a source’s spectrum and comparing them to the absorption lines for a particular atom or molecule in the lab
What is cosmological red shift?
- Red shift caused by the fact that the universe is expanding
- It is only noticeable over large distances
What is the cosmological principle?
On a large scale, the universe is:
- homogenous (every part is the same)
- isotropic (everything looks the same in every direction)
What links recessional velocity and distance?
Bubble’s constant
What is the Big Bang theory?
The universe started off very hot and very dense (perhaps as an infinitely hot, infinitely dense point) and has been expanding and cooling ever since
Why has the universe been expanding and cooling?
Because it’s a closed system
What are the three pieces of evidence for the big bang?
- Increasing red shift of distant galaxies (Hubble’s law)
- Cosmic microwave background radiation
- Relative abundance of hydrogen and helium
What does the Big Bang model predict was produced in the very early universe?
Lots of electromagnetic radiation
What happened to the EM radiation that was produced in the early universe?
- As the universe expanded, the EM radiation got stretched into the microwave region
- This is why space is not absolute zero but 2.73K
- It is found everywhere
How does the Big Bang theory explaining the large abundance of helium in the early universe?
- The early universe had been very hot and dense, so at some point it must have been hot and dense enough for hydrogen fusion to happen
- This time can’t have lasted long
What is the ratio of hydrogen to helium in the early universe?
3 : 1
Why have observations of type 1a supernovae led to controversy converting the behaviour of the Universe?
- The absolute magnitude of type 1a supernovae is known (-19.3)
- Using the magnitude equation or inverse square law, the distance to the supernovae can be calculated
- Type 1a supernovae are very bright so they can be seen in very distant galaxies
- If the supernovae are a long way away, the light has taken a long time to reach us, allowing us to make measurements from when the universe was younger
- Measurements of red shift and use of Hubble’s law show these supernovae are dimmer than expected, indicating that the Universe is expanding faster now than when the supernovae exploded as the light had to travel further to reach us than expected by the constant rate of expansion
- This theory has yet to be proven as there is no known energy source for this accelerating expansion
- The best theory is ‘dark energy’ but this have not yet been observed directly
What can supermassive black holes form from?
- The collapse of massive gas clouds while the galaxy was forming
- A normal black hole that accumulated huge amounts of matter over millions of years
- Several normal black holes merging together
What are the two types of supernovae?
- Type I: when a star accumulates matter from its companion star in a binary system and explodes after reaching a critical mass
- Type II: the death of a high-mass star after it runs out of fuel
What is a binary star system?
A star system where two stars are orbiting a common centre of mass
What are spectroscopic binary stars?
Binary star systems in which the stars are too close to be resolved by a telescope, meaning the only way to identify them is by using the Doppler shifts of each star
What are exoplanets?
Planets that exist outside of our solar system
Why can we not directly see binary stars as separate stars with out current telescope technology?
No telescopes have a high enough resolving power
Why are exoplanets difficult to see directly?
- Exoplanets are orbiting stars which are much brighter than the exoplanet
- They don’t produce their own light
- They’re too small to distinguish from nearby stars
What are the two methods astronomers can use to detect binary star systems and exoplanets?
- The transit method
- The radial velocity method (Doppler shift)
What is the least effective method of viewing exoplanets?
Direct observation
How can the transit method be used to detect a binary star system? When will the most light be blocked?
- In a binary star system, there is usually one brighter and one dimmer star system
- As they orbit each other, they will block some light
- So there will be dips in brightness as one star crosses the other
- The transit method measures how much light is received from a star system
- The most light will be blocked when the smaller/dimmer star is directly in front of the bigger/brighter star
How can the transit method be used to detect a exoplanets? How do the dips in brightness compare with those in a binary star system? How many drops in light will there be per orbit?
- When an exoplanet passes in front of the star, it blocks some of its light
- The drop is much smaller than with binary stars
- There will only be a single drop in light as the planet produces no light
What is the main issue with the transit method?
- Only works if the star/exoplanet passes between the star and Earth, blocking some light
- This means we are potentially missing seeing lots of exoplanets and binary stars based on the plane of their orbits
What does the transit method measure?
The change in apparent magnitude as an exoplanet passes in front of a star or a star passes in front of another star in a binary star system
What is the transit method?
- Involves observing the intensity of the light output of a star
- If a planet crosses in front of a star, the intensity dips slightly
- If the intensity of a star dips regularly, it could be a sign that there is an exoplanet orbiting it
How can the size and orbital period of the planet be determined using the transit method?
From the amount that the intensity falls by and the duration of the dip respectively
How can the radial velocity method be used to detect exoplanets?
- The star and the exoplanet orbit a common centre of mass, which causes the star to ‘wobble’ slightly
- This causes a periodic Doppler shift in the light received from the star
- The line spectrum of the star is blue-shifted when it moves towards the Earth, then red-shifted when it moves away
What is the issue with the radial velocity method?
The movement needs to be aligned with the observer’s line of sight
How can the radial velocity method be used to detect binary star systems?
- As binary starts orbit around each other, there are points where one star is moving towards Earth and the other is moving away from Earth
- When they are moving away, the light from that star will be red shifted
- When they are moving towards us the light will be blue shifted
