Color, Part II: Color in Astronomy

In my previous article, I discussed how important color is to humans in their everyday lives. Now, I shall discuss several ways in which color is perhaps just as vital to astronomers for understanding the universe.

"Color" can be something of a vague term in astronomy; however, it almost always means comparing the amount of light in one wavelength range (or filter) with the amount of light in another filter. Of course, the wavelength ranges used by astronomers do not necessarily (and often of necessity do not) correspond to those perceived by our human eyes. While our red and green human filters share a large fraction of wavelength coverage, astronomers usually try to choose filters which are mutually exclusive in wavelength coverage. Here is a typical example of a filter set (Sloan Digital Sky Survey Filters). The "u,g,r,i" names of the filters mean "ultraviolet, green, red, infrared." The z doesn't really mean anything; it's just longward of i. Notice that the filters have almost no overlap with each other, yet densely sample the optical wavelengths over similar widths of wavelength coverage. Finding just the correct chemical dyes to produce this filter system, needless to say, has taken a great amount of effort, though Mother Nature is very kind to give us plenty of optically useful chemicals. The Sloan's filter set is hardly the only one -- there are countless other filter systems that astronomers use. In fact, the Hubble Space Telescope has its very own filter set. However, the two things almost any good filter set has is little overlap but good coverage.

The prodigious effort devoted to creating these filter sets is easy to understand when one realizes the vital role color plays in astronomy. I'll discuss just a few of the many important color measurements filters allow.

• Stars--- One of the earliest applications of color was simply looking at the blue light versus the green light from stars. Early in this century, little was known about stars, and particularly, no one knew why some stars looked red, others white, others yellow, others bluish. In fact, many scientists of the day guessed that the color was an evolutionary sequence, that the bluest stars were the youngest and slowly cooled into old, red stars. One of the first systematic attempts to measure the color of stars was carried out independently by Hertzsprung (in 1911) and Russell (in 1913). In what now seems like an obvious to make, these astronomers were the first to plot the brightness of stars (Green magnitude) vs. their colors (Blue magnitude - Green magnitude). In doing so, they very quickly discovered that nearly all stars inhabit two sequences on
  The copyright of the article Color, Part II: Color in Astronomy in Astronomical Events is owned by Wesley Colley. Permission to republish Color, Part II: Color in Astronomy in print or online must be granted by the author in writing.

  Go To Page: 1 2 3

  Articles in this Topic    Discussions in this Topic