For centuries, the darkness of the night sky sparkling with stars and the glow of the moon has been an inspiration and wonderment to scientists, artists, philosophers and the casual observer. Gazing up at the night sky — just how dark is it?  

The question was the subject of recent studies by a team of researchers, including Darby Kramer, Tim Carleton, Rosalia O'Brien, Delondrae Carter and Rogier Windhorst at Arizona State University's School of Earth and Space Exploration, with Scott Tompkins at the University of Western Australia, Sarah Caddy at Macquarie University in Australia and the contribution of many other researchers. This artist's illustration shows the location and size of a hypothetical cloud of dust surrounding our solar system. Astronomers searched through 200,000 images and made tens of thousands of measurements from Hubble Space Telescope to discover a residual background glow in the sky. Because the glow is so smoothly distributed, the likely source is innumerable comets — free-flying dusty snowballs of ice. They fall in toward the sun from all different directions, spewing out an exhaust of dust as the ices sublimate due to heat from the sun. If real, this would be a newly discovered architectural element of the solar system. Photo courtesy NASA, ESA, Andi James (STScI) Download Full Image

The team's trio of research papers have recently been published in The Astronomical Journal and The Astrophysical Journal Letters.

To find out, astronomers decided to sort through more than 200,000 images from NASA's Hubble Space Telescope and made tens of thousands of measurements on these images to look for any residual background glow in the sky in an ambitious project called SKYSURF. This would be any leftover light after subtracting the glow from planets, stars, galaxies and from dust in the plane of our solar system, called zodiacal light.

When researchers completed this inventory, they found an exceedingly tiny excess of light, equivalent to the steady glow of 10 fireflies spread across the entire sky. That's like turning out all the lights in a shuttered room and still finding an eerie glow coming from the walls, ceiling and floor.

This faint glow — referred to as a “ghost light” — was detected in the SKYSURF analysis, and its source is still uncertain. The researchers say that one possible explanation for this residual glow is that our inner solar system contains a tenuous sphere of dust from comets that are falling into the solar system from all directions, and that the glow is sunlight reflecting off this dust. If real, this dust shell could be a new addition to the known architecture of the solar system.

Kramer, a lead author on one investigation and PhD student studying astrophysics, specifically looked at NASA's Hubble Space Telescope Ultra Deep Field image to see if the team could quantify how galaxies — faint galaxies specifically — could be hiding in that image.

To achieve this, Kramer and her team received the black-and-white deep space images and were able to manipulate the archived layers (the beautiful images the public sees) together to simulate how real galaxies block each other’s light in the real universe. But this requires writing code to handle the images and data; and it can take up to two weeks to get a well-processed image from Hubble. While writing code, Kramer finds it satisfying to overcome the challenge of finding the bug that prevented something from not working.  

"It's very relevant to extragalactic background light studies, which is what SKYSURF is focused on. There's a claim that there could be a significant amount of missing faint galaxies that could be causing this discrepancy that we have in the Extragalactic Background Light (EBL) measurements," Kramer said. "I dug into this investigation for my recent paper."

In these studies, the team used a "stacking process" — stacking images on top of each other to better understand what the data is they are looking at — to quantify how many galaxies could be hiding in these deep images. 

Carleton, an ASU postdoctoral researcher and a lead author on the papers, and his team analyzed hundreds of thousands of archival images. This team is unique, in that they are interested in something other than the bright, exciting objects in the Hubble image that everyone would expect.

Carleton is more interested in the background aspect of the image, taken out in one of those calibration steps. But because Hubble is above the Earth's atmosphere, that background level can reveal many interesting things; it's a valuable measurement because it constrains those models that help us understand of how bright that background should be.

"This has yet to be tested with this wide variety of data we have. There are a few measurements that look at this background level, but this SKYSURF project uses such an enormous amount of data that are all archived, but no one thought to look at," Carleton said. 

Video by Stephen Filmer/ASU Media Relations

Working with Carleton and Windhorst on the Hubble SKYSURF Project, co-author O'Brien, a third-year PhD candidate at ASU, studied images taken with the Hubble Space Telescope, which contains lots of galaxies, lots of stars and maybe even some dust clouds. She says they are looking past or ignoring all those discrete objects that astronomers usually study. Instead, they are studying what we call the sky's surface brightness.  

"When we look up at the night sky, we can learn a lot about the Earth's atmosphere. Hubble is in space," O'Brien said. "When we look at that night sky, we can learn much about what is happening within our galaxy, our solar system and on big scales as the whole universe."

Hubble veteran astronomer Windhorst first got the idea to assemble Hubble data to look for any "ghost light." 

"More than 95% of the photons in the images from Hubble's archive come from distances less than 3 billion miles from Earth. Since Hubble's very early days, most Hubble users have discarded these sky-photons, as they are interested in the faint discrete objects in Hubble's images, such as stars and galaxies," Windhorst said. "But these sky-photons contain important information which can be extracted thanks to Hubble's unique ability to measure faint brightness levels to high precision over its three decades of a lifetime."

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute in Baltimore conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Written by Ray Villard of the Space Telescope Science Institute with contributions by Kim Baptista of ASU's School of Earth and Space Exploration.