The reasons why stars twinkle are well understood. However, astronomers have been puzzled by why a few, but not all, quasars do something similar. A new explanation for this phenomenon – still a long way from confirmed – may have big implications for other areas of astronomy, from star formation to dark matter.
Quasars, or quasi-stellar objects, are the accretion disks around very active supermassive black holes in distant galaxies. Many have been observed to have slow fluctuations in brightness, but a small number change far more rapidly, in ways that cannot be attributed to variation in the quasars themselves. Instead, astronomers suspect that something in the line of sight between these quasars and Earth is getting in the way, but only erratically. The effect is similar to that seen when starlight is bent by variations in the temperature or density of the atmosphere.
However, the question of what it is that is doing the bending has remained a mystery, until a team of astronomers noticed that the fast-twinkling quasar PKS 1322-110 lies very close in the sky to the bright star Spica. Spica is only a 250 light-years away, while PKS 1322-110's distance is measured in the billions of light-years. However, the light from PKS 1322-110 has to travel very close to Spica to get to Earth.
Dr Mark Walker of Manly Astrophysics, a charitably funded independent research entity, remembered that another small group of strongly twinkling quasars, J1819+3845, is similarly close to Vega in the sky. He thought this seemed an unlikely coincidence. Further investigation revealed that a third twinkler, PKS 1257-326, is within minutes of a degree of Alhakim, another, albeit less famous, hot star.
In a paper in the Astrophysical Journal, Walker and co-authors calculate that the chances of the twinkling quasars being so close to hot stars is less than one in 10 million.
"We have very detailed observations of these two sources," said co-author Dr Hayley Bignall of Australia's CSIRO in a statement. "They show that the twinkling is caused by long, thin structures." The authors conclude that hot stars that are A-type and brighter are surrounded by filaments of hydrogen gas that get ionized by the powerful ultraviolet radiation these stars put out.
We have seen these filaments around very old stars, such as in the Helix Nebula, where globules of gas the size of the Solar System and with long comet-like tails are observed. It had been thought that these globules were a product of the end of the stars' lives, but the authors propose they exist much earlier in a star's evolution, but only get illuminated so we can see them when the star reaches old age.
Bignall told IFLScience we still don't know the source of these globules. They might be emitted by the star during formation, be associated with stellar winds, or form independently until being shaped by the star's radiation. Most intriguingly, the paper raises the possibility that they represent a significant portion of the galaxy's mass, possibly partly explaining the nature of dark matter.
The authors think very hot stars may have filaments extending out as far as six light-years, further than the distance between the Sun and the nearest stars. Bignall added that patches of hydrogen this size are so hard to detect, it is not surprising we have not seen them until now, but this won't stop the team trying to find ways to confirm their existence.