A team of astronomers has just found a bright “spider” in the sky – a very unusual binary system, or a system in which two stars orbit each other, some 2,600 light-years away from us.

This binary system is unlike almost any other because one of the stars, about to become a white dwarf, is orbiting a neutron star… that has just turned into a pulsar! Astronomers nicknamed binary systems like these “spiders” because the pulsar usually “eats” the outer parts of its neighbour as it turns into a white dwarf. 

The system – called 4FGL J1120.0-2204 – is the first example of a cosmic “spider” discovered until now. It is the second brightest source of gamma-rays ever found and scientists did not know it was a binary system until recently.

Using the 4.1-meter SOAR Telescope in Chile, a team of astronomers could get closer to a mysterious source of gamma rays they knew before but did not know exactly what it was. They then found out that it is made of a millisecond pulsar (a pulsar that twirls extremely fast, about hundreds of times per second) and a star about to become a very low-mass white dwarf. 

The strong gamma-rays and X-rays gave away the pulsar, and astronomers analysing its visible light spectrum found that light would shift from red to blue – objects that are closer to us appear bluer, while objects that are moving away from us appear redder due to the Doppler effect. This is how the astronomers found a white dwarf was orbiting a massive pulsar.

A white dwarf is born from the death of stars with a mass equal or lower than our Sun. When a star like our Sun runs out of hydrogen, it starts “burning” helium to continue with the nuclear fusion that powers the star. It then heats up and contracts, to “swell” later into a red giant. As nuclear fusion ceases, the swollen star becomes a white dwarf about the size of Earth at blistering temperatures – over 100,000 °C (180,000 °F)! It will take some two billion years for the white dwarf in our 4FGL J1120.0-2204 spider to evolve to this point.

Millisecond pulsars spin about every 10 milliseconds – resulting in hundreds of twirls every second! They get their fuel by “swallowing” matter from a neighbouring star – in this case, the one about to become a white dwarf. Millisecond pulsars “spew” gamma rays and X-rays especially when pulsar wind hits material emitted by its neighbouring star. 

The discovery could be the “missing link” to our understanding of how binary systems evolve!

Image: Artist's impression of an evolving white dwarf (larger star) and a millisecond pulsar (smaller star at the right) in a binary system. Credits: NOIRLab/NSF/AURA/J. da Silva/Spaceengine. Acknowledgment: M. Zamani (NSF's NOIRLab)