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 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 (AAVSO), which has . 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:

Object: A sneaky white dwarf
Behaviour: Short, ultra-bright X-ray flares

The white dwarf thought it was sending all the right signals. Embedded near a small satellite galaxy of the Milky Way, the star was emitting short, bright X-ray flashes that made it look like a feeding black hole. But after a multi-agency stakeout, cosmic detectives have blown the dwarf’s cover.

Behind the black hole façade, the white dwarf was stealing mass from a much larger companion, a process that occasionally causes a titanic thermonuclear blast. The discovery marks the first such binary system known, and hints that similar pairings may be hiding in plain sight across the universe.

White dwarfs had never before produced such enormous flashes, according to , an astronomer at the University of Southampton, UK. “That’s why they weren’t thought of as the natural explanation,” he says. “These findings indicate that binary stars comprising a massive hot star and a much less massive white dwarf are indeed possible, and also show why it has been hard to find them in the past.”

Playing it cool

The white dwarf mimic, aka MAXI J0158-744, made its revealing slip in November 2011. An X-ray camera aboard the International Space Station detected a super-bright flare lasting 92 minutes, in the vicinity of the Small Magellanic Cloud.

At first glance, the flare matched the behaviour of other ultra-luminous X-ray sources thought to be black holes firing off radiation bursts. But one piece of evidence did not ring true: despite its brightness, the flare’s observed temperature was much lower than that of other black hole outbursts.

Luckily, within 24 hours of the blast astronomers had NASA’s Swift space telescope and ground-based observatories in Chile and South Africa swinging into action to monitor the mysterious source. After combing through the signatures in its light, astronomers matched the flare to computer models of a white dwarf – the dense, burned-out core of a sun-like star – orbiting a much hotter and brighter companion star.

As the white dwarf orbits, it pulls matter from the larger star. When enough hydrogen builds up on its surface, the overload triggers a runaway nuclear reaction that ignites a flash of radiation. Astronomers have seen other white dwarf binaries flare before, but MAXI J0158-744 is the first to create an ultra-luminous eruption, giving the impression of a black hole drawing material from a companion.

Black-hole rethink

“Its X-ray luminosity is so similar to many black hole binary systems. Such a discovery will lead us to rethink other black-hole candidates in nearby galaxies,” says of the National Tsing Hua University in Hsinchu, Taiwan. “They may in fact belong to a whole new class of white dwarf binaries.”

Charles agrees that MAXI J0158-744 may be just the tip of the iceberg. “I actually think there are more out there, but we have been missing them because you need a particular combination of ground and space-based telescopes in order to pick them up,” he says.

Journal reference: Astrophysical Journal,

It’s no aqua-world, but the giant asteroid Vesta is surprisingly rich in one watery ingredient – hydrogen. The discovery, combined with its oddly pitted terrain, suggests that water arrived on young planets – including early Earth – during an intense round of meteor impacts.

The 530-kilometre-wide Vesta is unusual among asteroids because it’s thought to be the seed of a terrestrial planet that didn’t finish forming.

“Vesta is an example of such a world – as Earth once was – frozen in an embryonic state,” says , an astronomer at the Planetary Science Institute (PSI) in Tucson, Arizona, who wasn’t involved in the new studies.

The object therefore offers clues to the earliest stages of planet formation in our solar system.

Using data from NASA’s Dawn spacecraft, PSI’s and colleagues compared the chemical composition of Vesta’s surface with that of howardite-eucrite-diogenite, or HED, meteorites.

Dawn’s spectral maps show more hydrogen in regolith, near the asteroid’s equator, and less in relatively young impact basins, including the large crater Rheasilvia, from which many HED meteorites probably originated.

Hits and pits

“Vesta, like our moon, was thought to be bone dry, and yet we find this material that has been distributed all over Vesta’s surface,” says Prettyman. The HED meteorites, meanwhile, contain traces of carbon-rich chondrites, some of which hold significant amounts of water-bearing minerals.

The team thinks hydrogen was delivered to Vesta though a swarm of carbonaceous chondrite meteors. These rocks hit at slow enough speeds that their hydrous content was preserved on Vesta’s surface. Later, high-speed impacts ejected some of the hydrogen-rich surface material, leaving behind the erratic pattern.

The idea is supported by 30- to 500-metre-wide pits seen in Dawn’s images of some smaller craters, according to a second study led by of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

Denevi’s team thinks the pits formed when some of the impacts caused volatile compounds – also found in carbonaceous chondrites – to degas due to the high temperatures.

Journal reference: Science, DOI: and