Solving the riddle at a nebula’s heart


UK research reveals why a fiery hole is smaller than it should be. Phil Dooley reports.


Rosette Nebula image is based on data obtained as part of the INT Photometric H-Alpha Survey of the Northern Galactic Plane.
Rosette Nebula image is based on data obtained as part of the INT Photometric H-Alpha Survey of the Northern Galactic Plane.
Nick Wright, Keele University

Astronomers have solved the mystery at the heart of the beautiful Rosette nebula, about 5000 light-years from Earth, in new research published in the Monthly Notices of the Royal Astronomical Society.

Although its striking red gas framing a central hole is pleasing to the eye, the proportions of the Rosette did not add up – the hole appeared to be too small compared to the predictions of astronomical models.

The problem shows up in X-ray images of the Rosette. Far from a peaceful rose, the centre of the nebula has several huge stars that burn nearly half a million times more brightly than the Sun and heat the surrounding gases to millions of degrees Celsius.

This is the energy that ionises the hydrogen atoms in the nebula and creates the red glow. But, according to the models, the power of the stars should have blown a much bigger hole in the gas cloud.

Astronomers believe the stars formed between two and six million years ago. But the fierce winds that the stars whip up are pushing back the edges of Rosette’s central hole at 200,000 kilometres per hour.

At that expansion rate, calculations show the hole should be at least 10 times larger.

A solution came from new models devised by a UK team of researchers from University of Leeds and Keele University, led by Christopher Wareing.

The team used a supercomputer to run sophisticated simulations of winds, radiation and magnetic fields in gas clouds. After half-a-million processing hours, the researchers found that of nine different scenarios, only one replicated the Rosette Nebula’s hole.

If the modelling is correct, the nebula isn’t shaped like a sphere or a thick disc – as images suggest – but rather a thin, sheet-like molecular cloud.

"It was the thin disc that reproduced the physical appearance – cavity size, shape and magnetic field alignment – of the nebula, at an age compatible with the central stars and their wind strengths,” says Wareing.

The new model reveals that in thin sheets of gas the magnetic field diverts winds away from the plane, which is why Rosette’s hole had not grown as big as expected.

The icing on the cake came from recent magnetic field data from the Planck space telescope, which agreed with the team’s simulations.

"To have a model that so accurately reproduces the physical appearance in line with the observational data, without setting out to do this, is rather extraordinary,” Wareing adds.

Contrib phildooley.jpg?ixlib=rails 2.1
Phil Dooley is a freelance science writer based in Canberra.
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