Monday, July 14, 2014

Astronomers bring the third dimension to a doomed star's outburst

A new shape model of the Homunculus Nebula reveals protrusions, trenches, holes and irregularities in its molecular hydrogen emission. The protrusions appear near a dust skirt seen at the nebula's centre in visible light (inset) but not found in this study, so they constitute different structures.  Credit: NASA Goddard Space Flight Centre, inset: NASA, ESA, Hubble SM4 ERO Team. Hi-res Image
Astronomers have created a detailed 3D model of the expanding cloud of debris produced in the outburst of a doomed star. The model can be reproduced at home by anyone with a 3D printer. The team report their findings in a paper published today in the journal Monthly Notices of the Royal Astronomical Society.

In the middle of the 19th century, the massive binary system eta Carinae (η Car) underwent an eruption that ejected more than 10 solar masses of debris and briefly made it the second-brightest star in the sky. The researchers used extensive new observations to create a detailed 3D model of the expanding debris cloud.

"Our model indicates that this vast shell of gas and dust has a more complex origin than is generally assumed," said Thomas Madura, of NASA's Goddard Space Flight Center and a member of the team. "For the first time, we see evidence suggesting that intense interactions between the stars in the central binary played a significant role in sculpting the nebula we see today."

Eta Carinae lies about 7,500 light-years away in the southern constellation of Carina and is one of the most massive binary systems astronomers can study in detail. The smaller star is about 30 times the mass of the sun and may be as much as a million times more luminous. The primary star contains about 90 solar masses and emits 5 million times the sun's energy output. Both stars are fated to end their lives in spectacular supernova explosions.

Animation of the 3D Homunculus Nebula model
Credit: NASA Goddard Space Flight Center Conceptual Image La
Between 1838 and 1845, eta Carinae underwent a period of unusual variability during which it briefly outshone Canopus, normally the second-brightest star. As a part of this event, which astronomers call the Great Eruption, a gaseous shell containing at least 10 and perhaps as much as 40 times the sun's mass was shot into space. This material forms a twin-lobed dust-filled cloud known as the Homunculus Nebula, which is now about a light-year long and continues to expand at more than 1.3 million mph (2.1 million km/h).

Using the European Southern Observatory's Very Large Telescope and its X-Shooter spectrograph over two nights in March 2012, the team observed the nebula in near-infrared, visible and ultraviolet wavelengths in 92 separate areas, making the most complete map to date. The researchers have used the spatial and velocity information provided by this data to create the first high-resolution 3D model of the Homunculus Nebula.

The shape model was developed using only a single emission line of near-infrared light emitted by molecular hydrogen gas. The characteristic light at 2.12 microns shifts in wavelength slightly depending on the speed and direction of the expanding gas, allowing the team to probe even dust-obscured portions of the Homunculus Nebula that face away from Earth.

"Our next step was to process all of this using 3D modelling software I developed in collaboration with Nico Koning from the University of Calgary in Canada. The program is simply called 'Shape,' and it analyses and models the three-dimensional motions and structure of nebulae in a way that can be compared directly with observations," said Wolfgang Steffen, of the National Autonomous University of Mexico and the lead author of the paper.
NASA astrophysicists Ted Gull and Tom Madura discuss eta Carinae and their new model of the Homunculus Nebula, a shell of gas and dust ejected during the star's mid-19th century eruption. Credit: NASA Goddard Space Flight Center. Video Youtube
The new shape model confirms several features identified by previous studies, including pronounced holes located at the ends of each lobe and the absence of any extended molecular hydrogen emission from a dust skirt apparent in visible light near the centre of the nebula. New features include curious arm-like protrusions emanating from each lobe near the dust skirt; vast, deep trenches curving along each lobe; and irregular divots on the side facing away from Earth.

"One of the questions we set out to answer with this study is whether the Homunculus contains any imprint of the star's binary nature, since previous efforts to explain its shape have assumed that both lobes were more or less identical and symmetric around their long axis," explained team member Jose Groh, of the University of Geneva. "The new features strongly suggest that interactions between eta Carinae's stars helped mould the Homunculus."

Every 5.5 years, when their orbits carry them to their closest approach, the immense and brilliant stars of eta Carinae are only as far apart as the distance between Mars and the Sun. Both stars possess powerful gaseous outflows called stellar winds, which interact most dramatically during closest approach. The faster wind from the smaller star then carves a tunnel through the denser wind of its companion. The opening angle of the cavity created closely matches the extent of the trenches (130 degrees) and the angle between the arm-like protrusions (110 degrees). This indicates that the shape of the nebula likely continues to carry an impression from a binary close approach around the time of the Great Eruption.

A 3D-printed model of the Homunculus Nebula is compared to a Hubble Space Telescope image of the object.  Credit: NASA Goddard Space Flight Center/Ed Campion. Hi-Res Image

Once the researchers had developed their Homunculus model, they took things one step further. They converted it to a format that can be used by 3D printers and made the file available along with the published paper.

"Now anyone with access to a 3D printer can produce their own version of this incredible object," said Theodore Gull, who is also at Goddard and a co-author of the paper. "While 3D-printed models will make a terrific visualization tool for anyone interested in astronomy, I see them as particularly valuable for the blind, who now will be able to compare embossed astronomical images with a scientifically accurate representation of the real thing."

Media contacts

Francis Reddy
NASA Goddard Space Flight Center
+1 301 286 4453

Dr Keith Smith
Royal Astronomical Society
+44 (0)20 7734 4582

Science contact

Prof. Wolfgang Steffen
National Autonomous University of Mexico
Please contact Francis Reddy (details above) in the first instance.

Images and captions

The 3D printing model is available, along with a custom stand.
Credit: Steffen, W., Teodoro, M., Madura, T., et al. (2014)
More media and captions are available from

Further information

This research has been published in Steffen W. el al., 2014, "The three-dimensional structure of the Eta Carinae Homunculus", Monthly Notices of the Royal Astronomical Society, vol. 442, p. 3316-3328, published by Oxford University Press.
Notes for editors

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