A stellar odd couple 700 light-years away is creating a chaotically beautiful display of colourful, gaseous filaments. The Hubble captured the pair, named R Aquarii, and their symbiotic interactions. Every 44 years the system’s violent eruptions blast out filaments of gas at over 1.6 million kilometers per hour.
R Aquarii consists of two dramatically different types of stars: a white dwarf and a particular type of variable star.
The white dwarf is a stellar remnant. It’s what remains of a main sequence star that’s reached the end of its life of fusion. It shines only because of its remnant heat. White dwarfs are extremely dense, so even though they’re about the same size as Earth, they have a mass similar to the Sun. That means for such a small volume object, they exert a powerful gravitational pull.
The variable star is a type of red giant called a Mira-type variable. It’s a complete opposite to its companion star. Rather than extremely compact and dense, the red giant is bloated and red. It’s more than 400 times larger than the Sun. It’s a pulsating giant star that’s more at home atop Sauron’s Dark Tower than it is in a catalogue of stars. As it pulses, it changes temperature and luminosity. Over an approximately 390-day period, its brightness changes by a factor of 750.
That means that when the star is at its peak brightness, it’s more than 5,000 times as bright as our Sun.
The powerful pulsing of this massive red star is enough to be a spectacle in itself. But it’s relationship with its binary partner creates an even more spectacular display. As the two orbit, the dense white dwarf draws hydrogen gas away from the red giant. The hydrogen accumulates on the white dwarf until the star can’t take it anymore. Then the hydrogen explodes in nuclear fusion on the surface of the small, dense star.
The nova explosion ejects the material into space in gaseous filaments. But the region around white dwarfs is dominated by the star’s powerful magnetic fields, which can be millions of times stronger than Earth’s. The force of the nuclear explosion and the magnetic fields twist the gaseous hydrogen filaments into trails and streamers, and eventually, they loop back on themselves and form spiral patterns.
We can only see this nebula of gaseous filaments because the radiation from both stars strips electrons from the hydrogen, turning it into ionized gas. The ionized hydrogen glows brightly and creates a beautiful natural display.
The central binary star’s brightness changes over time because of the pulsing of the red giant. The gas appears red to us, but not because of the red giant. R Aquarii is in a dusty region, and the dust absorbs all the blue light, with only red reaching us.
A Hubble timelapse consisting of five images of R Aquarii from 2014 to 2023 helps bring the dynamic interplay to life.
Looking at these images, it’s easy to misunderstand the scale of the stars, the nebula, and the brightly-lit, filaments of ionized hydrogen. However, the material blasted into space reaches as far as 400 billion kilometers (248 billion miles). For comparison, that’s about 24 times greater than our Solar System’s diameter.
R Aquarii was first observed by German astronomer Karl Ludwig Harding in 1810, when he was a colleague of Carl Friedrich Gauss at Gottingen Observatory. It’s one of the nearest symbiotic stars, and is an object that astronomers are very interested in observing. In the 20th century, Edwin Hubble and others studied it and recognized its complex interactions and the resulting nebula. R Aquarii and its brethren can teach astronomers a lot about stellar winds, accretion, and ionized nebula.