7 mins read 04 Jun 2024

Seeing The Stars By Day

Astronomers at Macquarie Univerisity have established a new method of studying celestial objects like stars and satellites during daytime hours - opening up the possibility to year-round astronomy, and increasing space situational awareness.

Betelgeuse as seen during the day by a reflecting telescope of higher magnification to illustrate the impact that the atmosphere has on the star. Credit: Sarah Caddy..

The blue sky we see every day is the result of sunlight interacting with the molecules and elements in our atmosphere. This familiar blue effectively blocks our view of the stars during the day, making it seem like we and our telescopes can't see beyond the sky to the celestial wonders beyond.

Or so we thought.

For years, amateur astronomers have managed to capture images of the bright planets in our Solar System during the daytime. Those with a sharp eye and knowledge of the planets' positions can even spot Venus in the daytime sky. But what if we could actually see some of the brightest stars in the sky during the day? What benefits could this bring to astronomy and the broader space industry?

New research led by Sarah Caddy, a PhD candidate at Macquarie University, shows that this is possible. Caddy and her team have successfully captured one of the brightest stars during daytime observations, opening up new possibilities for astronomy.

“The first time I saw stars during the day was actually whilst hosting a group of primary school students at Sydney observatory,” said Caddy. “The kids really wanted to look through the telescope, so despite it being midday, we pointed it at the brightest star in the sky - Sirius. You can imagine my astonishment - and their excitement - when we could actually see it!” 

“I guess it takes that sort of childlike curiosity to explore things we would have previously discounted as impossible.” 

Caddy and her team have shown that it's possible to observe some of the brightest stars during the day. In their latest paper, published in the Publications of the Astronomical Society of Australia, they describe their successful daytime observations of Betelgeuse, the bright and familiar red supergiant star.

The star Betelgeuse through the Huntsman Telescope at during the day. The star is a red supergiant, and can been seen as an orange point of light against the blue sky background. The lenses that make up the Huntsman Telescope are excellent at constraining light from the star to a single point of light, as compared to mirror based (reflecting) telescopes. Credit: Sarah Caddy.

“For about half the year, Betelgeuse is too close to the Sun in the sky to observe it with traditional telescopes that only operate at night,” she said. “It’s a really exciting target to study, because Betelgeuse is reaching the end of its life. Any time between right now and 100,000 years (that’s the blink of an eye in astronomical terms) it could go supernova and explode.” 

“If it did that when it was only visible during the day, we would miss one of the greatest light shows in the Universe. Now we can observe it all year round, and we can be sure that we won’t miss it.” 

“Betelgeuse is very close to us, only about 640 light years away,” she said. “That gives us a front-row seat in examining what happens when a star dies, which may help us understand what happens when our star, the Sun, eventually reaches the end of its life.” 

“As well as Betelgeuse, we’ve found that we can observe up to 900 stars in the sky during the day. That may seem like a lot, but it's only the very tip of the iceberg. Our next steps will focus on trying to increase that number to encompass many more fainter stars.”  

Discussing why, from an astronomical perspective, Caddy also said that this is an exciting and significant new parameter space, to study the variability of brightness of stars throughout the year.

“We can now begin to fill in the gaps in their light curves that we would have missed during the day. This helps us build up a more complete picture of their behavior, and what’s causing them to change in brightness in the first place.” 

“As an added bonus, by observing during the day, I’ve also enjoyed the luxury of regular working hours, as opposed to pulling those tough all-nighters,” she said. 

A Spider-Eye View of the Sky

The Huntsman Telescope, Siding Spring Observatory. Huntsman is a robotic telescope. It consists of an array of 10 lenses each with individual cameras that were originally designed for low surface brightness imaging of faint and diffuse objects in the Universe. It is now exploring new science cases such as detecting exoplanets, monitoring satellites and observing bright variable stars during the day. Credit: Sarah Caddy.

To achieve this, Caddy and her team from Macquarie University used a unique telescope at the historic Siding Spring Observatory in central north NSW. This observatory houses many telescopes, including Australia's largest optical-wavelength telescope, the 3.9m AAT. 

However, the techniques in this study used a much smaller telescope with many "eyes," aptly named "The Huntsman" after the infamous Australian spider species. This innovative telescope can measure the light from stars, satellites, and other targets even when the Sun is high.

The Huntsman comprises of 10 highly sensitive 400mm Canon lenses, rigged together to observe the same patch of sky, covering a larger field of view. This setup is optimal for observing "transient" events, such as supernovae or neutron star collisions, which are short-term astronomical phenomena.

“To test out new experimental ideas like observing during the day, we use a smaller telescope called the Huntsman Pathfinder that is located at Macquarie University Observatory,” said Caddy. “It is nearly identical to Huntsman, except it has only 1 lens instead of 10, and it is always accessible - if anything were to go wrong, it’s much easier to fix the Huntsman Pathfinder, than travel 6 hours out to Siding Spring Observatory to debug Huntsman.” 

“Huntsman is built from lenses that are traditionally used for sports and wildlife photography - turns out they are also really good at minimizing stray light. This ensures that all the light from the star is focused in one place, and any light from unwanted sources like the Sun, is kept to a minimum. This is really useful during the day when the sky is so bright.” 

“Huntsman also makes use of the advancements in camera technology that has been driven by the development of mobile phones. Huntsman is equipped with detectors like those in your phone, which can take very fast images at high frame rates. This helps us to sample the rapidly changing starlight, due to the increased turbulence of the atmosphere during the day,” she said. 

Space Situational Awareness

The Huntsman Telescope Pathfinder at Macquarie University Observatory used for testing and in the telescope used in the 2024 study. Credit: Sarah Caddy.

The International Space Station taken by the Huntsman Telescope Pathfinder at midday, only a few degrees away from the Sun. Credit: Caddy et al. 2024.

Daytime observations of objects beyond Earth's atmosphere offer additional benefits. With thousands of satellites in orbit—the highest number in history—and more being launched as larger satellite constellations are deployed, the problem of space junk is growing. This debris poses significant risks, such as collisions that could trigger a chain reaction, known as the Kessler Cascade, famously depicted in the movie Gravity.

These collisions can have severe consequences, potentially disabling critical space infrastructure that societies on Earth rely on, including GPS systems, remote disaster observation, agriculture and mining operations, and the financial sector.

Astronomers use a technique called photometry, which monitors changes in the brightness of objects in space. This method could provide valuable information about the composition, age, condition, speed, and direction of objects in Earth’s orbit.

“[Using photometry], we measure the changing brightness of a satellite as it passes overhead, which forms a light curve. These are almost like satellite barcodes, which help to tell us what satellite is which and how they are orientated in space,” said Caddy.  

The brightness and trajectory of the ISS taken by the Huntsman Pathfinder. Credit: Caddy et al. 2024.

Across the space industry, ongoing monitoring, known as Space Situational Awareness, is crucial. Military, civilian, and commercial sectors rely on this to ensure the safety and operation of space-based assets.

“Optical space situational awareness has only really been performed extensively in a very small window of opportunity when the observing location is in darkness, but the satellites are still illuminated by the Sun. This generally occurs for an hour or so after Sunset, during twilight.”

“However, this severely limits how many satellites we can observe in such a short period. By removing this limitation and observing satellites all day, we can greatly increase the productivity of space situational awareness facilities which is critical in this approaching age of satellite mega constellations. There are lots of targets to monitor, and their numbers will only increase - the more time we can observe on sky the better.” 

“Hopefully with more daytime capable telescopes like Huntsman, we can help to monitor the increasing number of satellites and make Space a safer place for generations to come,” concluded Caddy.  

Read the paper in the Publications of the Astronomical Society of Australia