From the December 2013 issue

Why is one of the three filters in the Hubble palette ionized sulfur? Is Sulfur really that prevalent in the universe?

Dennis Demcheck, Baton Rouge, Louisiana
By | Published: December 30, 2013 | Last updated on May 18, 2023
Soul Nebula (Sharpless 2-199)
The Soul Nebula (Sharpless 2-199) contains open clusters with young stars. The hot forming suns emit ultraviolet energy, ionizing nearby sulfur atoms. The photographer processed this image so that the sulfur gas glows red, while hydrogen shows up as green and oxygen as blue.
Bob Franke

Sulfur is not terribly abundant in the universe. It shows up, however, near high-energy regions and objects such as supernova shock fronts and hot massive stars.

We typically see this element near areas where high-speed gases are colliding. The gaseous remains of exploded stars — called supernova remnants — tend to have relatively strong sulfur emission. We also see it in the gas ejected from dying Sun-like stars and their remnants, planetary nebulae. Hot young stars emit ultraviolet light, which kicks electrons off nearby atoms, thus ionizing them. Sulfur is one of the ionized gases we see near these young suns. Sulfur is useful to astronomers as a diagnostic tool that shows us where shocked gases and high amounts of ultraviolet radiation are.

Thus, to capture images of such regions, the Hubble Space Telescope’s Wide Field Planetary Camera 2 (1993–2009) included a narrowband sulfur filter. This filter let only a small range of light around a wavelength of 672 nanometers pass through to the camera.

Howard E. Bond
Pennsylvania State University,
University Park