Sharpest-ever view of a star's disk reveals surprising details

A telescope can show a lot, but astronomers always want to see more detail in the sky. They look not only at stars, but also at the faint structures around them, including thin disks of gas.

A new trick with light has now helped capture the sharpest-ever measurement of a disk around a nearby star, beta Canis Minoris. The study revealed a strange, lopsided shape that had never been seen before.

The method was used with a ground-based telescope, which is impressive because telescopes on Earth have to peer through a shaky layer of air.

Even with that challenge, this technique pushed the view to finer detail than traditional cameras on the same size telescope can reach.

Sharpening our view of space

When a telescope gets larger, it gathers more light from faint and distant objects. A bigger mirror or lens also lets astronomers pick out finer structure.

Very large observatories use this technology to study things such as the rings of dust around young stars, or the gas near black holes.

Some projects go further and link several telescopes together. This makes a virtual telescope that can act as if it has a much bigger opening.

As a result, astronomers can hunt for very tiny features, such as spots on stars or the inner parts of disks that may hide new planets.

Viewing the star with a single telescope

A team led by researchers at UCLA and their collaborators tried a different strategy. They used a single telescope but changed how it handled the light.

Instead of sending the starlight straight into a standard camera, they fed it into a device called a photonic lantern.

“In astronomy, the sharpest image details are usually obtained by linking telescopes together. But we did it with a single telescope by feeding its light into a specially designed optical fiber, called a photonic lantern,” said study first author Yoo Jung Kim.

“This device splits the starlight according to its patterns of fluctuation, keeping subtle details that are otherwise lost. By reassembling the measurements of the outputs, we could reconstruct a very high-resolution image of a disk around a nearby star.”

The device was developed at the University of Sydney and the University of Central Florida. It now serves as the core of a new instrument called FIRST-PL, designed and led by the Paris Observatory and the University of Hawaii, and installed on the Subaru Telescope.

The potential of photonic technologies

“What excites me most is that this instrument blends cutting-edge photonics with the precision engineering done here in Hawaii,” said study co-author Sebastien Vievard.

“It shows how collaboration across the world, and across disciplines, can literally change the way we see the cosmos.”

The instrument uses photonic technologies, which guide light through tiny structures on chips and in fibers. These tools already sit at the core of high-speed internet and sensing devices. Now, they are becoming more common in observatories as well.

“This work demonstrates the potential of photonic technologies to enable new kinds of measurement in astronomy,” said Nemanja Jovanovic, a co-leader of the study at the California Institute of Technology.

“We are just getting started. The possibilities are truly exciting.”

The blur of Earth’s atmosphere

Even the best telescope has to deal with air that moves and swirls. That motion bends starlight in complex ways, so the image jitters and loses detail.

For this project, the team used adaptive optics at the Subaru Telescope, which constantly adjusts mirrors to cancel the turbulence and stabilize the wavefront of the light in real time.

“We need a very stable environment to measure and recover spatial information using this fiber,” said Kim.

“Even with adaptive optics, the photonic lantern was so sensitive to the wavefront fluctuations that I had to develop a new data processing technique to filter out the remaining atmospheric turbulence.”

That extra step let the researchers keep the fine structure in the light that would otherwise be blurred away.

Strange disk around beta Canis Minoris

To test the method, the group observed the star beta Canis Minoris, or β CMi, in the constellation Canis Minor. It sits about 162 light-years from Earth and is wrapped in a disk of hydrogen gas.

The gas races around the star so quickly that material moving toward Earth appears slightly bluer, while material moving away looks redder. This color change, caused by the Doppler effect, also shifts the apparent position of the light a little at each wavelength.

By applying their new computational approach to the photonic lantern data, the astronomers measured these color-dependent shifts with about five times greater precision than before.

The results confirmed that the disk is spinning, and also showed that the disk is not even.

“We were not expecting to detect an asymmetry like this, and it will be a task for the astrophysicists modeling these systems to explain its presence,” said Kim.

The future of astronomy

The method of splitting light into separate components and then rebuilding the image offers a fresh route to reach finer resolution than traditional cameras.

Study co-author Professor Michael Fitzgerald noted that for any telescope of a given size, the wave nature of light limits the fineness of the detail that you can observe with traditional imaging cameras.

“This is called the diffraction limit, and our team has been working to use a photonic lantern to advance what is achievable at this frontier.”

The same approach can be applied to many other targets. Disks around young stars, the surroundings of compact objects, and tight groups of stars that appear as single dots today could all come into clearer focus.

The full study was published in the journal The Astrophysical Journal Letters.

Image Credit: Yoo Jung Kim/UCLA

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