Published: 22:38, July 21, 2023 | Updated: 09:57, July 24, 2023
Ghostly action across time and space
By Quentin Parker

Sometimes as a scientist, and if you are very lucky, you come across something so crazy in your research that you have to step back and wonder, what on earth is going on? I never write about my own research in opinion pieces to avoid blowing my own trumpet but cover topical items in STEM education, space exploration, science, climate change, technology, and related matters instead. However, for once, I am making an exception. This is because I and a young just graduated HKU MPhil student, Shuyu Tan, have made a remarkable discovery that is extremely hard to explain. It will, however, have profound implications for understanding the formation and evolution of our own Galaxy and its beating heart. It is mysterious, perplexing, and very bizarre. How can I not share it with the wider general public as a researcher and educator?

It concerns objects known as planetary nebulae (PNe)  – the beautiful, glowing, ejected, gaseous shrouds of dying stars. Indeed, our own Sun will eventually become one in about 4 billion years  (so no need to panic just yet). These objects are often in the news as their mysterious forms are a photogenic magnet for general interest used by NASA, ESO, HST, and other institutes for education and public outreach. Many of them are shaped like butterflies with the formal name of Bipolar PNe due to their visible opposing lobes of ejected gas. They are also windows into the soul of stellar death, lasting for only a few thousand years before they dissipate into interstellar space. This is compared to the billions of years of life for low to intermediate-mass stars like our Sun.  Such short PNe lifetimes are like the cosmological equivalence of a blink of an eye, effectively an instantaneous snap-shot of stellar death (PNe are not to be confused with Supernova explosions which come from stars more than eight times that of our Sun). PNe are powerful astrophysical tools as their rich emission line spectra (where light is spread out like with a prism) give them their striking colors in public images. 

READ MORE: Chinese astronomers transform understanding of Milky Way

As it happens, HKU hosts one of the leading and strongest global teams working on PNe. Although we have had a few press releases over the last few years, this particular discovery is one of the weirdest we have ever made and may turn out to be the most important.

Our result indicates a constant process that has influenced these objects over billions of years and vast distances in the center of our Milky Way

We have just published a paper in the prestigious “Astrophysical Journal Letters”, a rapid publication, high-impact scientific journal that allows astrophysicists to publish short articles of significant, original research that are impactful, broadly understandable, and timely. Indeed, any interested reader can go straight to the source: 

So what did we find?

We found that a particular sub-class of PNe that have two stars at their center circling around each other in a short-period binary orbit (i.e., when the stars orbit their common center of mass closer than Mercury is to our own Sun) have their PNe shapes aligned in the center of our Galaxy to a very high degree of certainty.  

One of these stars in such binaries is at the stage in its evolution when it ejects its outer gaseous envelope to become a PN. The binary orbit causes the gas ejection directions to be constrained, and butterfly-shaped forms are the result. These bipolar PNe have a major axis and orientation we can measure. Alignments of PNe have been found at lower statistical significance before. However, as the perceived wisdom is that these orientations should be completely random, these results were often discounted. We have now not only confirmed the alignment at extreme statistical power but also discovered where the signal is coming from. Theorists will now have to sit up and take notice!

READ MORE: China's space telescope to survey Milky Way

Our result indicates a constant process that has influenced these objects over billions of years and vast distances in the center of our Milky Way. It is remarkable and difficult to explain. We suggest the binary stars formed in a strong magnetic field, which influences the axis of the binary star orbit and hence of the resulting bipolar planetary nebulae eject directions during its formation. However, the measurements of this magnetic field are three times weaker than needed to influence the binary star orbits when they are formed. Further studies are needed, but our robust and remarkable findings provide important evidence for a persistent, organized process that has influenced the formation and evolution of stars and their eventual PNe over billions of years and across vast distances. 

The author is a professor in the Faculty of Science at the University of Hong Kong, the director of its Laboratory for Space Research, and vice-chairman of the Orion Astropreneur Space Academy.