In the ultraviolet the sky never turns black
The universe is permeated by a universal light. Don’t worry, it’s not getting esoteric here, and I’m not talking about the cosmic microwave background. It is about a completely different part of the spectrum, on the other side, the ultraviolet range. The so-called Lyman-alpha ultraviolet background was first discovered in the 1960s; its existence was confirmed in 1971. It is formed when light particles of a certain frequency (an excitation frequency of hydrogen) are scattered by neutral hydrogen atoms. Such photons emanate from the Sun in large numbers in the solar system. The light particles are invisible to the eye because they are ultraviolet. They are nevertheless quite useful because, with appropriate instruments, they can be used to image craters on the Moon, for example, into which ordinary sunlight never falls.
“The Lyman alpha background has already been studied in detail near Earth’s orbit and is so bright that if we could see it, the night sky would never get darker than twilight,” explains Dr. Randy Gladstone, lead author of a study that looked at the Lyman alpha distribution in the Milky Way using the New Horizons probe at the edge of the solar system. “It’s so bright due to solar Lyman alpha that we weren’t sure how much the Milky Way contributes to its total brightness. It’s like standing next to a streetlight on a foggy night. The fog scatters the light from the lamp and makes it hard to see anything else.”
In the outer solar system, however, where the New Horizons spacecraft is traveling, the scattered sunlight component of the Lyman-alpha signal is far less bright, and the fainter components from nearby regions of the Milky Way are easier to distinguish. “The Galactic Lyman-alpha background comes from hot regions around massive stars that ionize all the matter in their vicinity, especially hydrogen, the most abundant element in the universe,” Gladstone explains. “When the electrons and protons eventually reassemble or recombine, they almost always emit Lyman-alpha photons.” The hydrogen atoms between the stars scatter these photons into an approximately uniform glow throughout space.
Using the Alice UV imaging spectrograph aboard New Horizons, Gladstone has now been able to accurately measure the brightness of the Galactic component of the Lyman-alpha background for the first time, thanks to New Horizons’ exceptional position. “New Horizons has been moving away from the Sun for more than 15 years now,” Gladstone explains. “The farther it gets from the Sun, the less it is blinded by the solar component of the Lyman-alpha background.” As a result, the Lyman-alpha background in the Milky Way is about twenty times less bright than the Lyman-alpha background near Earth.
“That’s on the order of what astronomers have estimated for decades,” Gladstone said. “But now we have a much more precise number.”