Ultraviolet light is known for its use in detecting counterfeit money, fluorescing rocks and bug zappers, as well as causing sunburn. But UV light can also give us a different view of the universe via space telescopes designed to function in that part of the spectrum. At Columbia University’s Pupin Hall on November 2nd, Steven Mohammed summarized the history and science of ultraviolet astronomy. A graduate student in the university’s astronomy department, Mohammed’s primary research is galaxy evolution and structure, in which UV imaging plays a significant role.
Mohammed began by explaining why we need space telescopes to be able to observe in UV light. Ultraviolet astronomy can tell us a good deal about the chemical composition and temperatures of the interstellar medium as well as young, hot stars. Generally speaking, hotter objects emit light at shorter wavelengths, so UV observing is particularly useful for hot blue stars and blue galaxies which host many of them. In addition, because interstellar dust blocks UV light, UV astronomy gives us an indirect means to detect dust clouds. The practical problem is that the Earth’s atmosphere is opaque to wavelengths other than visible light and radio waves, so the UV light from distant stars can’t make it through to ground-based observatories. Hence, the need for space telescopes for UV astronomy. In fact, as Mohammed pointed out, the first UV telescope was operated from the Moon by the Apollo 16 crew!
Mohammed focused on the Galaxy Evolution Explorer, or GALEX, an orbiting UV telescope NASA launched in 2003, which operated until 2013, far exceeding its planned 2.5-year life. In fact, during its last year of operation, GALEX was controlled from a room in Pupin Hall! GALEX’s goals were to study star formation, observe thousands of galaxies, and if time, observe other UV phenomena. During its lifetime, GALEX produced the highest resolution and largest UV survey of the sky ever made, as well as deep UV images of galaxies from over 200 million sources, and 200,000 low-resolution spectra. The GALEX Ultraviolet Atlas of Nearby Galaxies illustrates the difference between blue galaxies, containing many hot, young stars, and red galaxies, with many old, cool stars.
Galaxies apart, UV imaging can show us aspects of other celestial objects which would not be observable under visible light. For example, many of us are familiar with the red giant star Mira, in the constellation of Cetus. A bright star at about 300 light-years from the Earth, Mira has been studied by astronomers for centuries. Mira has a very high proper motion, meaning it moves visibly against the background of more distant stars over a period of centuries. But before Galex, no one was able to see the 13-light-year tail Mira drags behind as if it were a giant stellar comet!
Finally, Mohammed showed us a picture from the Galactic Plane Survey (GAPS), which consisted of images of the Milky Way’s galactic plane taken under various wavelengths, using multiple space telescopes.
Unfortunately, GALEX is no longer in operation and no successor UV space telescope exists, or is even in the planning stages. For the time being at least, UV astronomy will be on hold.