This interactive figure was built using the MSUM Stellar catalog of about 1000 stars with measured B-V colors and known distances. Recall that to measure the B-V color of a star, we have to have measured the brightness (also called "flux") of the star in the B and V filters. To turn a brightness measurement into a luminosity estimate, we also need an estimate of the distance to the star.
If you select a region on the HR diagram, you will see the same stars will be highlighted on the Brightness diagram. This lets you see that the same stars can appear on very different regions in these two kinds of plots. Why is that?
The checkbox and slider on the far right allow you to highlight stars in a given distance range on the brightness diagram. Examine the appearance of a group of stars in a narrow range of distances (like 100 pc) on the brightness diagram. Select another group of stars at another distance but similar range.
Can you explain why a group of stars close to each other on the HR diagram can appear at such varied positions on the Brightness Diagram?
The interactive figure below shows the previous HR diagram on the left and the observed brightness diagrams of several star clusters on the right.
Most stars we observe lie on the Main Sequence of the HR diagram, which means if we construct a brightness diagram for stars in a star cluster, we can compare the brightness of the star cluster's main sequece to the KNOWN luminosity of the main sequence stars to estimate the distance.
This interactive allows main sequence fitting" of the distance to a star cluster in two steps:
The HR diagram has blue-green line representing a main sequence on the HR diagram. Initially this line has not been 'fit' to the main sequence. Click and drag the blue-green dots until the blue-green line representing the main sequence matches the observed main sequence. You are essentially defining the luminosity of the main sequence for a series of B-V colors.
Did you notice the corresponding blue-green line on the brightness diagram representing the brightness of your main sequence at 10 parcsecs changed as your made your previous adjustments? Adjusting the distance slider will change its brightness to correspond to the new distance. Adjust the distance until you match the brightness of the main sequence to the observed main sequence for your selected star cluster. When you do this, you will have estimate the distance to the star cluster via main sequence fitting.
Note: The Pleiades and 47 Tuc data is fairly 'clean' whereas the Hyades and M53 data contains foreground stars contaminating the data. This is meant to allow you to see a little of what real data looks like that astronomers have to deal with when attempting this method of distance estimation.
The figure below is allows estimation of the age of star cluster by using stellar evolution models.
If you first set the distance to the cluster to the distance you previously estimated in Interactive Figure 2, then you can allow the cluster to age and use this interactive to determine both the distance and age of the star cluster.
Note: This interactive starts very early on, before the star cluster has really left the gas cloud it formed in. We never see star clusters only a few hundred thousand years old.