Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

A white dwarf strips its companion of material (Image: Mark Garlick/SPL)

Object type: Stellar pair

Location: 500, no, 370 light years from Earth

Surveillance of the police variety is strictly for professionals. But humble backyard stargazers have carried out a cosmic stakeout on an unpredictable star system, SS Cygni, which periodically explodes. The result is a measurement of its distance from Earth that bests one made by NASA’s Hubble telescope – and shores up the leading mechanism for the process that lights up the most common type of black hole.

“Because there are people all over the world we get better coverage in time than if we just focus on observations from one location,” says astronomer Gregory Sivakoff of the University of Alberta in Canada, who called on the hobbyists for assistance. “Professionals just can’t monitor objects over long periods of time when competition for time on large instruments is fierce, so citizen astronomers are crucial when it comes to monitoring the transient universe.”


Tucked away in one corner of the constellation Cygnus, SS Cygni consists of an Earth-sized white dwarf – a remnant of a now-dead sun-like star – plus a companion. The strong gravity from the dwarf strips material from the companion, forming a whirling, flattened accretion disc. As this material accumulates, some regularly ignites, forming outbursts that occur every 49 days or so.

Tied-up telescope

But the distance to SS Cygni as measured by Hubble in 1999 – 520 light years – suggests an inherent disc brightness that, when plugged into the leading model of accretion disc formation, would put it in a permanent state of explosion. “Until this study, the observational distance from Hubble simply did not match up with what the theoretical model was saying it should be,” says Sivakoff.

To solve the conundrum, he and his colleagues enlisted 280 stargazers from the American Association of Variable Star Observers (AAVSO), which has been observing the system since it was discovered in the late 19th century. They tipped off Sivakoff’s team whenever SS Cygni started to ignite, so that two high-end radio telescopes could be pointed at the baffling stellar pair.

“No one can accurately predict when the next eruption will occur and professional astronomers can’t tie up telescopes, satellites or dish arrays waiting for something to happen,” says AAVSO’s Mike Simonsen, a veteran of more than 80,000 observations of variable stars. “They need someone to monitor these systems and alert them when they begin to behave in the manner they want to observe, in this case, a narrow time window at the beginning of an outburst.”

Disc relief

Unlike Hubble, which had to use unpredictable stars in the Milky Way as a stationary reference point, Sivakoff was able to use a more reliable, distant galaxy, leading to a much more precise measurement. This put the distance to SS Cygni at 370 light years – much closer than Hubble’s measurement. It also implies a lower inherent brightness for SS Cygni, which, when plugged into the disc model, produces the observed, periodic outbursts.

This suggests the leading model for accretion discs is correct – a relief for astronomers, who use it to explain all kinds of exotic phenomena, including the similar discs that form around the most common type of black hole. Indeed, the only reason we know that so-called stellar black holes exist is the radiation emitted by their accretion discs, which form when they suck material from a companion star.

“By understanding the detailed physics of what’s going on around these white dwarf binaries we are getting a better feel of the mechanisms of black holes in galaxies across the universe,” says Sivakoff.

Journal reference: Science, doi.org/mmv