Shine on, crazy nano-diamonds

A cosmic radiation signature turns out to have been written in diamonds. Phil Dooley reports.

A star with diamonds. In the latest research, the diamonds are much smaller, and the stars much older.
A star with diamonds. In the latest research, the diamonds are much smaller, and the stars much older.
S. Dagnello, NRAO/AUI/NSF

Astronomers have solved a 20-year old mystery of strange microwave radiation coming from parts of the galaxy. The radiation has a signature, they say, that looks like a nano-diamond in the sky.

The glow was first discovered about two decades ago as an unwanted foreground in measurements of faint microwave emission from deep beyond our galaxy. The hunt has been on since then to explain its origin.

Jane Greaves at Cardiff University in Wales led a team that narrowed the source of the glow down to clouds of dust containing rapidly spinning nano-diamonds – molecules with a diameter 50,000 times smaller than a human hair, each made of between 10 and 700 carbon atoms.

In a paper in the journal Nature Astronomy the team reports microwave glow coming from three gas clouds around hot young stars that emit high levels of ultraviolet radiation and protons.

The energy pouring out of the stars excites the dust cloud. Nano-diamonds are formed as shock waves and high pressure bring carbon atoms together. High-energy protons then collide with them and attach to their surface, a process called hydrogenation, and set them spinning at speeds of tens of billions of rotations per second. The charged protons then act as tiny aerials that emit radiation.

Nano-diamonds can be found on Earth in meteorites that date from our solar system’s early days as a whirling cloud of dust. Coupled with the new observations, this suggests they may be common (given that there are many solar systems similar to ours) but like the sun, most stars are not bright enough to excite the microwave emission.

Greaves and colleagues used data from the Australian Compact Telescope Array and two telescopes at Green Bank, West Virginia, in the US, to study 14 stars. They found that only three of them were hot enough to create the emission.

The solution to the 20 year-old puzzle was a surprise.

“It was an unexpected result, not something we were looking for,” says Greaves.

“It was really amazing to find that the three stars with the radiation were the only three known to have the hydrogenated nano-diamonds – a really clean result.

“Astronomy studies things so far away that most of our results are harder to interpret or have missing pieces of information.”

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Phil Dooley is a freelance science writer based in Canberra.
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