The H-R diagrams you have made so far cover only stars in the area very close to our Sun – a small part of the whole galaxy. In fact, the volume of space you looked at to make your H-R diagrams of the nearest stars is less than 0.0001% of the volume of our galaxy. Do stars in the rest of the galaxy fall into the same groups as stars close to our Sun? To find out, you need to extend your H-R diagram to include more distant stars.
There is a problem, however. To make an H-R diagram, you need to know the star’s luminosity (or absolute magnitude). To find a star’s luminosity, you need to know its distance from Earth. SDSS records the apparent magnitudes of stars, but not their distances. If SDSS does not find distances to stars, how can you make an H-R diagram? You will need to collaborate with another project.
Collaboration is one of the most important aspects of science. Another group may have data that you would find useful but cannot obtain using your equipment. In this case, you need to know distances to stars.
To find the distances, you can “collaborate” with the European Space Agency (ESA). In 1989, ESA launched a satellite called Hipparcos. It made extremely accurate measurements of the brightnesses of and distances to 118,000 stars, and measured brightnesses of and distances to about 2.5 million stars with a slightly lower level of precision.
Hipparcos measured stars down to 12th magnitude. The SDSS uses digital cameras; if you take a picture of a very bright object, the cameras’ CCD chips will become saturated and you will not get a good image. For the SDSS telescope, very bright means about 12th magnitude. So the SDSS can’t give any information about stars brighter than 12th magnitude. Notice that the SDSS can’t observe stars that Hipparcos measured, and Hipparcos can’t observe stars that the SDSS measures! The SDSS and Hipparcos make a perfect collaboration – each data set has strengths that make up for the other set’s weaknesses.
Over the next few pages, you will make an H-R diagram of a famous star cluster using data from the Hipparcos satellite. After that, you will use what you learned to make an H-R diagram of a fainter, more distant star cluster using SDSS data.