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Lab 8 – The Distance to the Pleiades
Open or Galactic Cluster
M13 - Globular Cluster
Distance-23,000 ly
Size-150 ly
Stars in a Cluster
• Common Properties
• Distance
• Age
• Chemical composition
• Different Properties
• Spectral Types (temperature)
• Luminosity Class (size)
• Mass
Distance Modulus
Star Cluster
HR Diagram
10 pc
Main-sequence
O
B
A
F
G
K
M
D >10 pc
{
Color-Magnitude
Diagram
DM
D <10 pc
D <10 pc
10 pc
Distance Modulus = m - M
The difference between the absolute magnitude and the
apparent magnitude can be used to find the distance to a star
cluster.
If m-M > 0 then the distance to the cluster is > 10 pc.
If m-M = 0 then the distance to the cluster is = 10 pc.
If m-M < 0 then the distance to the cluster is < 10 pc.
O
B
A
F
G
D >10 pc
K M
Distance Modulus
(m  M )  5
log d 
5
d  10
 ( m  M ) 5 


5



Interstellar Dust
– Reddens Light (makes stars appear cooler)
– Dims Light (makes stars appear further away)
Pleiades - Open Cluster
The End.