1.1 Recap on Stars Flashcards
Define a galaxy
A group of stars bounded by gravity
What are the two observational characteristics of stars?
Brightness and colours as measured by us
What are the two physical characteristics of stars?
Luminosity (J/s)
Flux (W/m^2)
Can also get flux densities
What is important about the apparent magnitude equation?
m = - 2.5 logF constant
- Logarithmic so matches our vision
- Negative scale (brighter stars have a lower magnitude)
What is colour?
The difference between two magnitudes measured in 2 regions of the spectrum
c = -2.5log(F1 / F2)
What is absolute magnitude?
Magnitude of source if placed at 10pc
What is the distance modulus?
The apparent magnitude - the absolute magnitude
DM = 5log(d/10pc)
Is the spectrum of a star a black body?
No
How do we deal with spectra that aren’t radiating as a BB?
Aproximate it as a BB then modify it by accounting for absorption/emission by molecules and atoms in the atmosphere
What is Wein’s law a relation between, and state the equation
Relationship between the wavelength at which a black body peaks and its temperature
Hotter the star, the bluer the spectrum
λ = 2900 / T
State the properties of a HR diagram
Axes are Luminosity and surface temp(right to left)
Most stars align on main sequence
Different diagram regions equal different stages of star lifetime
When are stars a main sequence star?
During Hydrogen burning
What is the relationship between luminosity and mass?
L is prop. to M^3.5
What is the equation for the lifetime of a star, and what is significant about this?
τ = Fuel available / Rate of consumption = M / M^3.5 = M^-2.5
- More massive stars have shorter lifetimes
What does the colour of a galaxy depend on?
The balance between the old, red, faint stars and the young, blue, luminous stars
