Filters are used on telescopes to determine the brightness of an object in a specific color. One use of this information is to estimate the color of stars. Astronomers generally use a set of standard filters, meaning that the color of light each filter lets through is very well known. This is so one observer can compare data with another observer.
To determine the color of a star, a combination of filters that will show a sharp distinction between stars must be used. The diagram on this page shows plots of brightness versus color for three stars. Vertical boxes are drawn to show the approximate position of red, yellow and blue filters. Astronomers generally use the yellow and blue filters to measure the color of stars because the difference of light through these filters changes significantly for different stars.
B–V index = (magnitude through B) – (magnitude through V)
The B–V index uses magnitudes, which are units astronomers use to quantify brightness. For converting magnitude to brightness use the Brightness Conversion Table In the investigation Measuring the Color of Stars in the next few pages.
Magnitude Tip #1: The magnitude scale is an inverse scale, meaning that brighter stars have lower magnitudes than dim stars. One tip to remembering this is to think about replacing the word “magnitude” with “class.” One might expect a first class star to be brighter than a second class star, just as a first magnitude star is brighter than a second magnitude star.
In the 17th century, Isaac Newton discovered that white light, when passed through a glass prism, can be seen to be made of a spectrum of colors--red, yellow, green, blue, violet. This ultimately led to use of prisms and then grooved glass or plastic (diffraction gratings) to build instruments called spectroscopes, which are used to analyze colors of light from stars.
Age of Stars—Stellar Evolution
We already spoke of the birth of stars from gravitational contraction of nebulae, or gas clouds. In the next chapter, we’ll speak of the most dramatic and violent possible death of stars. But in between birth and death, stars change slowly, and by examining millions of stars all at different stages in their lifetimes, we can put together a picture of what the stages of a single star’s lifetime must be.
Hertzsprung-Russell diagram (Using the HR Diagram)
Between 1911 and 1913, two astronomers were working independently on the classification of stars and came up with very similar results. A Danish astronomer, Ejnar Hertzsprung, plotted stars according to their absolute magnitudes and spectral classes. An American astronomer, Henry Norris Russell, created a plot of luminosity vs. temperature for many stars. Their investigations were seen as roughly equivalent, and the Hertzsprung-Russell (HR) diagram is a result of their findings. Their goal was to clarify understanding of the life cycle of stars.
The HR diagram below is called a general HR diagram because it is based on stars of all different types from many different regions of the sky. The objective is to show the distribution of various types of stars and their relative quantities. To create a general HR diagram, many stars are observed at a given time, their luminosity and temperature are determined and those values are plotted. The HR diagram can be thought of as a snapshot plot of these stars at one time. A star’s position on the HR diagram is determined by its luminosity and temperature at the time of observation. Since HR diagrams of many different stars, in many different regions, observed at many different times all yield similar distributions, it can be assumed that the general HR diagram describes an average distribution of stars. More specific HR diagrams of a single star cluster are used to determine factors about that cluster such as the type of stars in the cluster and the distance or age of the cluster.
After its hydrogen fuel is depleted, a star contracts and begins to fuse helium in its core. This can occur rapidly or gradually depending on the mass of the star, but in either case it causes the star to expand to a greater radius than that of the main sequence star. During the expansion the star cools considerably. A low mass star that was a yellow or orange main sequence star evolves to a red giant during this expansion period. It is red because it is cool, and it is a giant because it has such a large radius. Similarly, a high mass blue or white main sequence star evolves into a yellow or orange supergiant.