Astronomy

• Visible light telescopes penetrate the atmosphere, radio waves and light
• There’s two kinds of telescopes reflecting vs refracting
• refracting telescopes require large lenses to be precisely manufacture
• Due to manufacturing restrictions of refracting telescopes, they relatively have smaller openings for viewing, which leads to diffraction and poor resolution
• Reflecting telescopes use large mirrors at the back of the telescope that can be easily supported from behind since light won’t pass through it. A larger opening means less diffraction and more resolution
• High energy EMR (UV, x rays and y rays) behave more like particles (bullets) and generally don’t demonstrate the properties of diffraction patterns
• Imagine the difference between “reflecting” a tennis ball of a wall vs a bullet

  • the ball would bounce off; the bullet would go through
  • however it is possible to “redirect” bullets if the angle of incidence is shallow enough
  • x ray telescopes use shallow angle mirrors to redirect x rays
• The EMR we “see” using telescopes can tell more than what an object simply looks like

  • Temperature from continuous spectra
  • composition from absorption or emission spectra
• Spectroscopy is the study of light after it has been “split” by either a prism or grating
• When a gas is very hot (like in a star), it causes the electrons to bounce around their energy levels and give off different amounts of energy

  • more heat = more transitions = more colors
• If a substance is hot enough, it will emit visible light at all wavelengths

  • this is called a continuous spectrum
• If a star is very hot, it emits more blue light

  • it appears to be blueish-white
• Our sun is a medium hot star

  • it appears more yellowish
• Cooler stars will emit more red light

  • they appear more reddish
• If we could observe the light coming from the sun‘s core it would show a continuous spectrum
• As light moves through the sun’s cooler atmosphere, certain wavelengths are absorbed by the gases found there

  • remember to how ozone absorbs UVC
• When the sun is looked at through a spectroscope, we see a continuous spectrum with some wavelengths missing

  • the missing wavelengths appear as dark lines called absorption lines
• If lights passes through a cooler gas, then only certain wavelengths will be absorbed. the absorption spectrum for each element is unique
• Absportion spectra are like finger prints for various elements
• It’s possible to easily generate the opposite of an absorption spectrum by applying an electric current to a gas and observing which wavelengths are emitted

  • this is an emmision spectrum
• The emission and absorption spectra have lines at identical wavelengths. this means spectroscopes can be used to examine starlight and see what elements they’re made of
• the Doppler shift

  • the reason for the shift is because the star is moving away from us
• When an object is moving away from you, any emitted frequencies are more spread out (longer wavelength [smaller frequencies])
• When an object is moving towards you, emitted frequencies are closer together (shorter wavelengths [longer frequencies])
• This well know phenomenon (Doppler shift) is the same reason that sirens have a higher pitch when coming towards you and a lower pitch when travelling away
• When we look at any star, its spectra are always red shifted. Remember that the universe is always expanding
• What makes some stars hotter than others? Why are some stars rich in helium but others have lots of heavier elements like carbon or iron?


  the answer lies in the original mass of gas and dust that forms the star at birth
• A low density cloud of gas and dust in space is called a nebula
• All stars start out as a nebula
• As a nebula collapses, the immense heat and high pressure produces fusion reaction (stars shine because of the nuclear fusion)
• larger star will produce larger elements and have different life cycles
• Low mass star (significantly smaller than our sun) become white dwarfs

  Less than 1.4x the mass of the sun

  The process:

  1. gas and dust (nebula)
  2. low mass star
  3. red giant
  4. expanding shell of gas
  5. white dwarf
• Intermediate mass stars (like our Sun) become a neutron star

  1. gas and dust (nebula)
  2. intermediate mass star
  3. Supergiant star
  4. supernova
  5. neutron star
• High mass stars (significantly bigger than our sun) become dense black holes

  8x the mass of the sun

  1. Gas and dust (nebula)
  2. high mass star
  3. supergiant star
  4. star collapses
  5. dense black hole