Hertzsprung-Russell Diagram
Hertzsprung-Russell Diagram
Netopil et al., 2017, MNRAS, 468, 2745
Color-Magnitude Diagram
Gaia Collaboration,
2018, A&A, 616,
A10
Color-Magnitude Diagram
Gaia Collaboration, 2018, A&A, 616, A10
Color-Magnitude Diagram
Color-Magnitude Diagram
Colour and Teff
• Measuring accurate Teff for stars is an intensive task – spectra
needed and model atmospheres
• Spectral Energy Distribution (SED) fitting, only useful if
measurements in the UV are available
• Magnitudes of stars are measured at different wavelengths
• Colours => Calibrations => Teff
• The Asiago Database on Photometric Systems (ADPS) lists
about 200 different systems
Colour and Teff
Various calibrations can be
used to provide the colour
relation:
(B – V) = f(Teff)
Remember that observed
(B - V) must be corrected for
interstellar extinction to
(B - V)0
Most of the calibrations are for
cool type stars
Absorption = Extinction = Reddening
• AV = k1 E(B-V) = k2 E(V-R) = …
• General extinction because of the ISM characteristics
between the observer and the object
• Differential extinction within one star cluster because
of local environment
• Both types are, in general wavelength dependent
Cardelli et al., 1989, ApJ, 345, 245
Important parameter:
RV = AV/E(B - V)
Normalization factor
Standard value used
is 3.1
Be careful, different
values used!
Depending on the
line of sight
Fitzpatrick, 1999, PASP, 111, 63
Wang & Chen, 2019,
ApJ, 877, 116
Absolute magnitude and bolometric
magnitude
• Absolute Magnitude M defined as apparent magnitude of a
star if it were placed at a distance of 10 pc
(V – MV) – AV = 5 log(d) – 5
where d is in pc. (V – MV) is also called distance modulus.
• Magnitudes are measured in some wavelength. To compare
with theory, it is more useful to determine bolometric
magnitude Mbol – defined as absolute magnitude that would
be measured by a bolometer sensitive to all wavelengths. We
define the bolometric correction to be
BC = Mbol – MV
Bolometric luminosity is then
Mbol – Mbol, = -2.5 log L/L; Mbol, = 4.75 mag
Bolometric Correction
BC from Flower, 1996, ApJ, 469, 355