H-R diagrams for star clusters

H-R Diagram

Relation between luminosity L, temperature T and radius R:

L = 4R2T4

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 = -2.5log10(FV/FV0) Instead of temperature – difference of fluxes in filters B and V: B-V

where B = -2.5log10(FB/FB0)

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 Vs i Bs should be observed; values of reference fluxes FV0 i FB0 can be calculated from the formula:

Vs = -2.5log10(Fs/FV0)

Information about standard stars can be found at: http://cadcwww.hia.nrc.ca/cadcbin/wdb/astrocat/stetson/query  • Interstellar extinction (reddening) Hint: observe clusters with b < –10 or 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:

http://www.seds.org/~spider/ngc/ngc.html

http://www.univie.ac.at/webda//navigation.html    

• 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