astrophysics

• • What is apparent and absolute magnitude?

  Apparent – The size an object appears to be.

  
  

  Absolute - The magnitude at a distance of 10 parsecs
• Why does an object further away appear smaller?

  Smaller angle subtended at the eye so smaller apparent size
• What is the condition for normal adjustment in refracting telescopes?


  Distance between objective and eyepiece lens is equal to the sum of their focal lengths
• Draw a labelled diagram of a refracting telescope in normal adjustment

  
• Describe the issues with refracting telescopes:

  1. Chromatic aberration: Different colors focus at different points leading to a blurry image
  2. Long telescopes: Objective focal length needs to be long for a large magnification
  3. Heavy lenses: Powerful telescopes need large lenses so glass at edges become distorted
• Formation of cassegrain reflecting telescope:

  
• What issues do reflecting telescopes suffer?

  Spherical aberration: Parallel light rays are reflected to different focal points leading to a blurry image
• How do you fix sperical aberration?


  Use a parabolic mirror

  
• Collecting power – The amount of light a telescope can collect

  Resolving power – A telescope’s ability to resolve fine details

  Minimum angular resolution – The smallest angle two objects can make with our eye or a telescope such that we can still distinguish between them.
• How can you just distinguish between 2 different stars?

  The central maximum of one star’s diffraction pattern must coincide with the first minimum of the other’s. This occurs when the angular seperation is equal to Rayleigh criterion

  
• • Some telescopes use charge-coupled devices (CCDs) to capture the images they produce
  • Quantum efficiency is measure of how effectively a CCD or an eye can detect light

  
• • Hipparcos scale – a discrete scale for the brightness of stars going from 1 to 6, where 1 is the brightest and 6 is the dimmest. We call these values apparent magnitude
• Intensity – power per unit area, I=P/A
• Brightness is subjective, while intensity is a measurable physical quantity
• For two stars, if the difference in apparent magnitude is n, the intensity increases by a factor of 2.51^n.

  
• Luminosity (or power output) – the total energy emitted each second in the form of electromagnetic radiation
• Absolute magnitude (M) – the apparent magnitude at a distance of 10 pc
• Binary star system – a system of two stars orbiting about their common centre of mass

  
• Eclipsing binary star system – a binary star system in which the stars pass in front of each other when viewed from Earth

  
• When stars pass in front of each other, they block each other’s light, causing a dip in the observed apparent magnitude. This dip occurs every half cycle.

  
• What is a parsec?

  distance at which radius of Earth (1AU) subtends an angle of 1 arcsecond

  
• spectral class:

  
• Exoplanets – planets outside of our Solar System
• Direct observation – Pointing a telescope at a star and seeing if there are any exoplanets orbiting it
• Doppler shift method – Looking for periodic changes in the Doppler shift of a star’s absorption lines over time.
• Transit method – Looking for dips in the intensity of light from distant stars.

   

  If the dips occur at regular intervals, then we can infer they’re caused by an orbiting exoplanet, blocking out some light as it passes in front of the star.

   

  The time between dips tells us the time period of the exoplanet’s orbit.
• A quasar is a supermassive black hole at the centre of a galaxy, surrounded by an accretion disk.

   

  Matter from the accretion disk falls into the black hole, heats up, and emits huge amounts of radiation.
• The intensity of radiation from a quasar follows an inverse square law

  • A typical quasar is about the size of our Solar System, but its power output is about 100010001000 times larger than all of the stars in the Milky Way put together!

  
• There are two methods we can use to find the distance to a quasar from Earth:

   

  Method 1

  • Use the Doppler shift equation  to find the quasar’s recessional velocity 
  • Substitute into Hubble’s Law to find the distance

   

  Method 2

  • Identify a nearby type 1a supernova, which is a standard candle with absolute magnitude M=− 19.5
  • Measure the apparent magnitude of the supernova
  • Substitute into this equation: m−M=5log⁡(d10)
  • Solve for the distance d (in parsecs)
• comparing telescopes:

  
• HR diagram

  x axis: temperature/spectral class (50,000-2500)

  y axis: absolute magnitude (+15 - -10)
• Sun has an absolute magnitude of 5 and a temperature of 5000K so is in G class

  HR:

  
• Stars bigger than the sun evolve into a red supergiant and end it's life in an explosion called a supernova.

  Type 1A always reach an absolute magnitude of -19.5

  During a supernova:

  • stars brightness rapidly increases
  • it emits gamma ray bursts along axis of rotation

  
• After a supernova a star becomes a neutron star or a black hole.

  Neutron stars are made up of neutrons and have extremely high density.

  Black Holes:

  • Incredibly dense
  • All mass concentrated at a single point called a singularity
  • Supermassive black holes are often found at the centre of galaxies
  • Escape velocity is greater than the speed of light

  Schwarzchild radius: the distance from the centre of a black hole to where the escape velocity is equal to the speed of light
• Quasar: a supermassive black hole at the centre of a galaxy, surrounded by an accretion disk.

  
  

  Matter from the accretion disk falls into the black hole, heats up, and emits huge amounts of radiation.
• Cosmic Microwave Background Radiation (CMBR)

  • Isotropic throughout the universe
  • Suggests it initially came from all parts of the universe
• CMBR wavelength

  1. Initially had a shorter wavelength

  2. Due to big bang expansion, wavelength redshifted into the microwave part of the spectrum
• Distant clouds of gas suggest the composition of early universe was 75% Hydrogen and 25% helium
• Initially universe had high enough pressures and temperature


  For hydrogen to fuse into helium
• As the universe expanded and cooled


  It became too cold for fusion