Ring nebula with an iron bar… a vapourised planet?

A mysterious iron 'bar' has been found in the heart of the Ring nebula, the remnant of an exploded star. Resisting any 'person walked into a bar and said ouch, because it was an iron bar' jokes, here is a little scientific summary of what has been found and why it is of interest.

The Ring nebula is one of the most iconic classic Hubble images, showing shells of glowing gas around the remains of a sun-like star, now in the white dwarf phase of its life. Indeed, our Sun will end its life in a similar fashion, surrounded by exquisite gas and dust in a colourful halo as it expels its outermost layers. Recently, astronomers have found a cloud of iron atoms in an approximate bar shape in the centre of the nebula. The composite photo below constructed from 4 different images by the William Herschel Telescope shows the bright outer ring which is made up of light emitted by oxygen, and a central ‘bar’ across the middle due to light emitted by a plasma of ionised iron atoms. For scale, the bar's length is around 500 times the distance of Pluto to the Sun. The colours are due to gas shells thrown off by the dying star being ionised by its intense heat.

A composite RGB image of the Ring Nebula constructed from multiple images by the William Herschel Telescope. The bright outer ring is made up of light emitted by oxygen, while the ‘bar’ across the middle is due to light emitted by a plasma of iron atoms. North is up and East is to the left in the image. Credit: University College London

The iron cloud was discovered due to a new instrument called the WHT Enhanced Area Velocity Explorer (WEAVE), installed on the 4.2m William Herschel Telescope. Incidentally, this is in La Palma where I will be on holiday next month to see an astrophysics friend! This new instrument has allowed astronomers to obtain spectra (where light is separated into its consitutent wavelengths) at every point in the whole face of the Ring Nebula for the first time. This has allowed for detailed mapping of chemical composition across the nebula, as seen in the image below, which highlights regions of intensity for iron, ionised oxygen, hydrogen, nitrogen, carbon and argon.

An illustrative set of 8 individual WEAVE LIFU emission-line images of the Ring Nebula. The colour in each panel tracks the brightness of emission, with brown-red being the most intense, shading through yellow and green to blue for the faintest emission. North is up and east, left. Credit: University College London

It is not yet known how this iron bar formed, so more detailed observations are required (this is a common theme in astronomy!) in order to see what exactly is happening in this nebula. A potential scenario is that the iron bar could be an arc of plasma caused by the vapourisation of a rocky planet caught up in the star's expansion! The study of shredded planets particularly around white dwarfs is a fascinating one, as it offers a rare glimpse into the interior composition of exoplanets.

It is hoped that further observations of similar nebulae may yield similar results, and the WEAVE survey will be looking into many fascinating objects over the next 5 years, from nearby white dwarf stars to distant galaxies.

Stay tuned for the next article featuring an inside out planet...