Each year, we celebrate the April 24, 1990, launch of the Hubble Space Telescope with a special observation. This year, for the 29th Anniversary, we wanted to highlight the combination of imaging and spectroscopy that underlies the astronomical research results.
The target chosen to illustrate these ideas is the Southern Crab Nebula. This symmetric gas structure is the result of a dying star in a binary system. In the binary is an aging red giant star, a white dwarf stellar remnant, and a disk of material encircling them. The giant star is blowing off some of its outer layers of gas, and the dense disk restricts the outflow such that two opposing lobes are created. The result is an hourglass shape in three dimensions that, when seen in two dimensions, bears some resemblance to a crab.
[Note: The Southern Crab Nebula is a proto-planetary nebula, while its northern namesake, simply called the Crab Nebula, is a supernova remnant. Both are the result of the dying stages of stars, but the southern one is expelling its material in a wind, while the northern one did so in a titanic explosion.]
Hubble observed the nebula in four narrow-band filters that isolate the emission from oxygen, hydrogen, nitrogen, and sulfur. These glowing elements are at different temperatures and densities. Comparing their distributions helps astronomers dissect the physical structure of the nebula. Hubble also recorded a spectrum of the central region, which shows the emission across visible wavelengths, with significant enhancements at the featured spectral lines.
The science results from Hubble are roughly split 50/50 between those derived from images and those derived from spectra. While images show the grandeur and details of the objects, spectra reveal characteristics like temperature, composition, motion, and more. This montage showcases both to emphasize their complementary nature. Astronomy can not thrive without both types of observations!