Astronomers at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, have produced the most comprehensive picture of Eta Carinae to date.
Eta Carinae’s great eruption in the 1840s created the billowing Homunculus Nebula, imaged here by Hubble. Now about a light-year long, the expanding cloud contains enough material to make at least 10 copies of our Sun. Astronomers cannot yet explain what caused this eruption. Image credit: Jon Morse, University of Colorado / NASA.
Eta Carinae is a stellar system located in the southern constellation Carina, about 7,500 light-years away. It 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.
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.
At closest approach, or periastron, the stars are 225 million km apart, or about the average distance between Mars and the Sun.
Astronomers observe dramatic changes in the system during the months before and after periastron. These include X-ray flares, followed by a sudden decline and eventual recovery of X-ray emission; the disappearance and re-emergence of structures near the stars detected at specific wavelengths of visible light; and even a play of light and shadow as the smaller star swings around the primary.
During the past 11 years, spanning three periastron passages, the Goddard group has developed a model based on routine observations of the stars using ground-based telescopes and multiple NASA satellites.
According to the model, the interaction of the two stellar winds accounts for many of the periodic changes observed in the Eta Carinae system.
The winds from each star have markedly different properties: thick and slow for the primary, lean and fast for the hotter companion.
The primary’s wind blows at nearly 1.6 million km per hour and is especially dense, carrying away the equivalent mass of our Sun every thousand years. By contrast, the companion’s wind carries off about 100 times less material than the primary’s, but it races outward as much as six times faster.
Computer simulations revealed the complexity of the wind interaction. When the companion star rapidly swings around the primary, its faster wind carves out a spiral cavity in the dense outflow of the larger star.
To better visualize this interaction, the Goddard scientists converted the simulations to 3-D digital models and made solid versions using a consumer-grade 3-D printer.
This process revealed lengthy spine-like protrusions in the gas flow along the edges of the cavity, features that hadn’t been noticed before.
The results will be published in the journal Monthly Notices of the Royal Astronomical Society.
_____
