Stellar Physics Flashcards
Method to find distance to nearer stars
Parallax
Luminosity of a star
It’s energy output per unit time
Measured in watts
If star radiates isotropically flux equal across sphere
L=4pir^2F
Visual magnitude
Magnitude in the visual part of the spectrum
If a star is hotter than its environment
It will cool down by re-radiating it’s energy
Rayleigh jean law for blackbodies
Good approximation at Lon wavelengths but radiance keeps increasing indefinitely at short wavelengths (UV catastrophe)
Failure of classical physics to explain thermal radiation
Wien’s law
Good approximation to the observed spectrum at short wavelength
Planck’s law
Assuming energy comes in discrete quanta
Fit data
What determines apparent colour of an object
Shape of spectrum and position of its peak
Atmospheric extinction
Light from sun obscured by atmosphere of Earth, which absorbs more than others.
Ignore in this course
Stefan Boltzmann law
Total luminosity per area is the spectral radiance integrated over solid angle and wavelength
j=L/A=sigma T^4
Total luminosity of a spherical star
Multiply Stefan Boltzmann law by surface area
L=4piR^2sigmaT^4
Stars have wide range of luminosity so helpful to use logs
Take logs of both side of luminosity equation
Plot graph of y=log10L against x=-log10T for some value r and get straight line
Lower radii lie on lines nearer bottom
HR diagram
80-90% of stars cluster in main sequence
Other branches of stars: white dwarfs, giants and supergiants
Temperature x axis decreases from left to right
Y axis is luminosity
Interpretation of HR diagram
Only certain combinations of L and T allowed
Most stars on main sequence
What tells us that stars move around HR diagram as they evolve
Clusters are stars at similar stages of their lives
Number of stars in each part proportional to duration of that stage of evolution
Main sequence
Most variation from top left to bottom right along a line of roughly constant radius
Top left blue stars hotter and more luminous
Two cut offs to the main sequence
At top: luminous stars blow material from their surface through radiation pressure naturally limiting their mass
At bottom: cool red stars not hot enough to begin nuclear reactions. Temperature in core too low
Estimate of time a star spends on main sequence
Lifetime=energy available/ luminosity
Most massive stars spend…
Least amount of time on main sequence
Giants and supergiants
Sit in top right of HR diagram
Large L but low T
Less populated so stars spend less time in this phase
Reach after main sequence
White dwarfs
Bottom left
Below main sequence so radius s smaller
None visible to naked eye
Are not powered by nuclear fusion
Photometric system
Divides spectrum into commonly used bands
Ultraviolet band centred 350nm, blue band 440nm, visible 550nm, red 600nm, near infrared 800nm
Filters placed over telescope to select a band
Colour index
Numerical difference in magnitudes between measurements made in two wavelength bands
Measurements made through two different filters eg B-V difference between magnitude in blue and visible band
The smaller the colour index (ie lower position on number scale that ranges from positive through zero into negative)
The more blue and hotter the star