Steinn Sigurðsson Astro 475(W) Fall 2011

2

For this class read chapters 1 and 2 in "Galaxies...". you should know this stuff already.
yes, you should be worried if you don't.

Use magnitude system to classify stellar brightness.

Defined for a finite bandwidth filter, relative to some standard reference star. Note implicit assumption that star is not variable!

We can also define flux magnitudes.

The old standard ground based magnitude system is the Johnson UBVRI filter system.
For Johnson UBVRI system, reference star is Vega, which defines the zero point or normalization of the magnitude system, with V = 0.03.
Yes, it was meant to be 0.00, but there was a slight oopsie. See Johnson \& Morgan ApJ 117, 313 (1953).
Alternative systems: Gunn "griz" filters, see Schneider et al 1983, Wade et al 1979.
We also define an absolute magnitude system, for some ideal unfiltered object, to link the magnitude system to the SI system, and to convert relative brightness to absolute brightness.
AB magnitudes: m_(AB) = -2.5 log(f) - 48.60 , calibrates to absolute flux.
Remember, the luminosity is defined for objects at a distance of 10 pc.

For reasonable underlying spectrum can convert between magnitudes.
- think about the qualifier here.

• V = V_AB + 0.044 ± 0.004
• B = B_AB + 0.163 ± 0.004
• R = R_AB - 0.055 ± indef
• I = I_AB - 0.309 ± indef
• g = g_AB + 0.013 ± 0.002
• r = r_AB + 0.226 ± 0.003
• i = i_AB + 0.296 ± 0.005

This URL UBVRI gives the UBVRI transmission curves.

General photometric data is at URL Geneva Note that this site is biased towards stellar photometry! the order of ``importance'' is different if you observe different objects.

We can also convert between different filter systems, again with the implict assumption that the spectrum is ``normal''.

eg B = g + 0.51 + 0.6× (g-r);
or V = g - 0.03 - 0.42× (g-r).

For flux we use Jansky.

1 Jy = 10^-23 erg s^-1 cm^-2 Hz^-1.

This corresponds to:

1 Jy = 1.51 × 10⁷ photons s^-1 m^-2 (d lambda/lambda)^-1

From Bessel (1979) we can get flux expected (before atmopsheric absorption). For m_X = 0.

For example, the U band is at lambda = 360 nm and has width d lambda/lambda = 0.15. The flux at m_U = 0 is 1810 Jy.

For the V band, lambda = 550 nm, d lambda/lambda = 0.16 and the flux at m_V = 0 is 3640 Jy.

From G.D. Wirth, U. Victoria. URL Wirth

Also have extinction, which causes both reddening, E(B-V), and extinction A_V.

Note that both reddening and extinction are wavelength (or filter) dependent.
Rough rule of thumb: A_V ~ 3.2 × E(B-V), and A_K ~ 0.1 A_V !!!

Luminosity vs Brightness.

What is the range of stellar luminosity, approximately, and why?
What does this imply for range of brightness, and what does that tell us about the night sky.

Reminder: H-R diagram - we'll be getting back to that.

Last updated 08/11

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