Observational Methods Flashcards
Cosmic rays
High energy particles from space (supernovae, sun etc) detected at high altitude
Most atrononomical information is deduced from
Observations of EM radiation
EM waves
Oscillating electric and magnetic fields
Travel at c
C=v lambda
Or considered particles, E=hf
Atmospheric windows
Only certain parts of the EM spectrum can be viewed from the ground
Some parts completely opaque
Radio window completely transparent
Atmospheric windows - Main bands
Visible ~300-1100nm
Microwave/radio 10^-2 - 10m
Parts of infrared also observed from Earth
Effect of the Atmosphere
Gamma and x-rays absorbed by atoms and nuclei
UV mostly absorbed by O2 and O3
IR absorbed in different bands by H2O and CO2
Radio>10m blocked by charged particles in ionosphere
Luminosity
Power radiated by an object
Bolometric luminosity
Refers to power radiated at all wavelengths
Lbol= integral between 0 and infinity of Lmono d labda
Monochromatic luminosity
Measured over a small wavelength window
Power
Shaded area of curve monochromatic L x d lambda
Total power = integral (area under whole graph)
Flux
Power radiated per unit area received a distance d from an object (units Wm^-2)
F=L/4pid^2 (=L/ area)
Monochromatic flux
Exactly the same but using monochromatic luminosity in equation instead
Magnitude
Describes an object’s brightness
Apparent magnitude, m
How bright an object appears
Absolute magnitude, M
Intrinsic brightness of an object
Sirius
Brightest star in sky has m=-1.46
Apparent magnitude zero point
Apparently brighter star has smaller apparent magnitude than fainter star
m=0 point chosen to be vega
Apparent magnitude of sun
-27
Parallax
Difference in apparent position when an object is viewed along different lines of sight
Annual parallax
Difference in apparent position when a star is viewed from Earth and from the sun
Earth’s orbit gives different viewing positions
Calculate annual parallax from positions 6 months apart
Equation for annual parallax
Draw triangle to get tanP=SE/d = 1au/d
Since p very small, tanP~sinP~P (in rad)
So P~ 1au/d
Parsec
The distance at which an object has a parallax of one second of arc
Need P in radians
Absolute magnitude
Compares luminosities accounting for distance
M is the apparent magnitude a star would have at a distance of 10 parsecs
Two functions of telescopes
Make objects appear brighter and bigger
Collect a lot of light to enable faint objects to be seen and magnify objects to allow detail to be resolved