PX158 - F3 - colours

johnson-cousins system

• $U:$ ultraviolet
• $B:$ blue
• $V:$ visible
• $R:$ red
• $I:$ infrared

difference in magnitudes

• from here:

• this system is calibrated using vega such that:

- a number greater than 0 means a temperature less than that of vega
- most stars will have colour $>0$ as vega is a hot star

• the colour is given as: $bluefilter-redfilter$

  • eg: $(B-R)$, $(V-R)$, $(V-I)$

• larger magnitude means lower flux

• eg $(B-V)=1.83$ for antares, $(B-V)=0.65$ for the sun. which one is hotter? which one is more lumious?

  • $(B-V{)}_A>(B-V{)}_{\odot }⟹$ larger magnitude of bluer light from antares
  • smaller flux in $B$, more ${\lambda }_V$ from antares
  • ${\lambda }_V>{\lambda }_B$ and $T\propto \frac{1}{\lambda }⟹$ antares is colder
  • $L=4\pi R^2\sigma T^4:$ radius of star is needed to determine the relative luminousity