Turn off the daylight and horizon displays. Open the "Display"
Palette on the vertical toolbar and turn on the NGCIC
display. Then click on the left side of the NGC-IC
button. This will open a window with options about displaying
NGC and IC objects. Under "Which objects to
draw" be sure Open Cluster is turned on (as indicated by a
check mark in its box). Click Apply and then OK. Use the
"Selection" button and click on "Find." In the window,
type in NGC 1981 and then click Find. The display will
move to the location of NGC 1981, an open cluster near
the Orion Molecular Cloud. Zoom in until the circle
around the cluster nearly fills the display. As you zoom in,
be sure to notice the Orion Nebula. Using the Selection
Tool, double click on the dozen or so brightest stars, one at
a time, to open the Info Window. Record the temperatures
of the stars. O and B stars are hotter than 10,000 K. What
fraction of the bright stars in NGC 1981 are O and B
stars? Repeat the process for two other open clusters, NGC
752 and NGC 1502. How does the fraction of O and B
stars you found in NGC 1981, near the Orion Nebula,
compare with the fraction in the other clusters? What do
you think these stars have to do with the visible light from
the Orion Nebula?
