H-R diagrams for star clusters

H-R Diagram

Relation between luminosity L, temperature T and radius R: L = 4  R 2  T 4

H-R diagram for closest stars

Stellar evolution on H-R diagram

Theoretical isochrones

Observational diagrams H-R Instead of luminosity L – observed flux in V filter: V = -2.5log 10 (F V /F V0 ) Instead of temperature – difference of fluxes in filters B and V: B-V where B = -2.5log 10 (F B /F B0 )

Middle-aged open cluster NGC 7789

Young open cluster M46 (NGC 2437)

Globular cluster 47 Tuc (NGC 104)

Problems Calibration Standard stars with known values of V s i B s should be observed; values of reference fluxes F V0 i F B0 can be calculated from the formula: V s = -2.5log 10 (F s /F V0 ) Information about standard stars can be found at: Interstellar extinction (reddening) Hint: observe clusters with b +10  Stars in clusters may vary in brightness by a few orders of magnitude One should make observations of different length or – better – make several observations and combine the data

Strategy Choose suitable clusters Information about open clusters can be found at: Choose suitable field with standard stars Observe standard stars and the cluster Using SalsaJ software make photometric measurements of stars Export results to Excel and create H-R diagram