1. Basic Properties of Stars - 3
Luminosities
Fluxes
Magnitudes
Absolute magnitudes

2. Solid Angle
The solid angle, , that an object subtends at a point is a measure of how big that object appears to an observer at that point. For instance, a small object nearby could subtend the same solid angle as a large object far away. The solid angle is proportional to the surface area, S, of a projection of that object onto a sphere centered at that point, divided by the square of the sphere's radius, R. (Symbolically,  = k S/R², where k is the proportionality constant.) A solid angle is related to the surface area of a sphere in the same way an ordinary angle is related to the circumference of a circle.If the proportionality constant is chosen to be 1, the units of solid angle will be the SI steradian (abbreviated sr). Thus the solid angle of a sphere measured at its center is 4 sr,

3. For Fun 1) What is the angular size of the Sun as seen from Earth?
Radius Sun = 7.0 x 105 km
Distance to Sun = 1.5 x 108 km
2) What is the solid angle of the Sun as seen from Earth?
3) What fraction of the sky does the disk of the Sun then cover?

4. Luminosities and magnitudes of starsda r 

5. Luminosities and magnitudes of stars
Consider some source of radiation
Intensity I = energy emitted at some frequency , per unit time dt, per unit area of the source dA, per unit frequency d, per unit solid angle d in a given direction (,) (see p )
Units: w m-2 Hz-1 ster-1
d = da/r2  d = da/r2 = 4r2/r2 = 4

6. Luminosities and magnitudes of stars §3.2
Luminosity is energy passing through closed surface encompassing the source (units watts)
Luminosity L = IdAdd
If source (star) radiates isotropically, its radiation at distance r is evenly distributed on a spherical surface of area 4  r2
Flux is then F = L / 4  r2 (w m-2)
F falls off as 1 / r2
Inverse Square Law
Solar constant is w m-2
Fig 4.3. An energy flux, which, at distance r from a point source, is distributed over an area A, is spread over an area 4A at a distance 2r. Thus, the flux density decreases inversely proportional to the distance squared.

7. Brightness, the magnitude scale §4.2-3
In 120 BC, Greek astronomer, Hipparchus, ranked stars in terms of importance (ie. brightness)  “magnitude”
1st magnitude were brightest  6th magnitude faintest visible stars (later extended to 0 and -1)
Without realizing it, Hipparchus based his scheme on the sensitivity of the human eye to flux - logarithmic scale, not a linear one.
Perceived brightness  log (actual flux)

8. Rigel & Betelgeuse - 0th Magnitude Stars

9. Brightness and the magnitude scale
Magnitude scale later standardized so that mag. = 1 is exactly 100 x brighter than mag. = 6
Difference of 5 mag = factor 100 in brightness
Difference of 1 mag = factor in brightness i.e. (2.512)5 = 100
Note: smaller mag is brighter star
We can quantify this definition of magnitude scale: Ratio of two brightness (flux) measurements is related to the corresponding magnitudes by b1/b2 = 100 (m2-m1)/5
b1 and b2 are fluxes and m1 and m2 are magnitudes
NB that it is b1/b2 and m2 - m1

10. Brightness and the magnitude scale
This is usually expressed in the form:
m2 - m1 = 2.5 log10 (b1/b2)
Note that it is m2 - m1 on the left and b1/b2 on the right
ratio apparent mag. difference         brightness  (b1/b2) m2-m1
1 =
10 =
100 =
1000 =
10,000 =

11. Brightness and the magnitude scale

12. 1528 Latin translation of Ptolemy’s Almagest based on Hipparchus of 120 BC

13. Brightness and the magnitude scale
This is usually expressed in the form:
m2 - m1 = 2.5 log10 (b1/b2)
Note that it is m2 - m1 on the left and b1/b2 on the right
ratio apparent mag. difference         brightness  (b1/b2) m2-m1
1 =
10 =
100 =
1000 =
10,000 =

14. Brightness and the magnitude scale
Since brightness of a given star depends on its distance, we define:
Apparent magnitude, m (this represents flux) = magnitude measured from Earth
Absolute magnitude, M (this represents luminosity) = magnitude that would be measured from a standard distance of 10 parsecs (chosen arbitrarily)
m - M = 2.5log10 (B/b)
Where B is the flux measured at 10 pc and b is flux measured at distance d to the star

15. Brightness and the magnitude scale
Using inverse square law, B/b = (d/10 pc)2 we get
m - M = 2.5 log10 (d/10)2 = 5 log10 (d/10) = 5 (log10 d - log10 10 )
The last term is just = 1 so we have
m - M = 5 log10 d - 5 or m - M = 5 log10 d/10
m - M is called the distance modulus and will appear often.
d is distance to the star in parsecs.

16. Simple problems (a) What is the absolute magnitude M of the Sun?
(b) How much brighter or fainter in luminosity is the star Proxima Centauri compared to the Sun?
Needed data:
msun = -26.7; mproxima = 11.05
Parallax of proxima = 0.77”
1 pc = 206,265 AU
(c) Total magnitude of a triple star is 0.0. Two of its components have magnitudes 1.0 and 2.0. What is magnitude of the third component?