By Bruce Dorminey

The star Eta Carinae (Eta Car), once one of the brightest in the southern sky, has long been shrouded in mystery. After a huge outburst of gas that occurred more than 150 years ago, it has largely been hidden by a dense cloud of dust--a strong indicator of sporadic eruptions.

Now, Eta Car, which sits in our part of the Milky Way some 2,300 parsecs (7,500 light years) from the Sun, is puzzling researchers and theorists all over again. A U.S.-based team has recorded a steep, inexplicable drop in its stellar wind--the outflow of gas from the star--measured as a change in the emission lines, or lines in the spectrum caused by the emission of light at particular wavelengths.

Researchers from the University of Minnesota in Minneapolis recorded the dip from the eruptive, luminous variable star using data from the Gemini South Telescope in Chile and the Hubble Space Telescope. Their results are detailed in a paper accepted for publication in Astrophysical Journal Letters.

"The collapse in the emission lines seemed almost too rapid and drastic to believe," says Kris Davidson, an astrophysicist at the University of Minnesota and a co-author of the latest study.

In January, Davidson's colleague and the study's lead author Andrea Mehner first noticed the effect in data from the Gemini South Telescope. The team then confirmed Mehner's findings through observations using Hubble.

Their findings have been independently verified by astronomers affiliated with Stony Brook University in New York and the Georgia Institute of Technology in Atlanta.

Supernova impostor

The root of Eta Car's oddness lies in the supernova-like eruption event that began in the late 1830s and lasted for some 20 years.

Huge stars like Eta Car can erupt for reasons that Davidson says are about 95 percent "mysterious." One theory is that once these massive stars near the end of their short lives--after some 2 million to 3 million years--perturbations in their cores may set off sporadic "supernova impostor" eruptions.

Supernova impostors are only one-hundredth the brightness of normal supernovae, but the eruptions can last for decades. And unlike normal supernovae, these impostors survive their eruptions. Eta Car, which is five million times more luminous than the Sun, was the first such impostor detected, although about a dozen are known to astronomers today.

During its mid-nineteenth-century eruption, Eta Car lost about ten times the mass of the Sun. Although this loss was equivalent to only 10 percent of its mass, the star's outer 50 percent was ejected. As a result, the star has still not returned to thermal and rotational equilibrium. Since 1858, it has been observed only as a massive gaseous outflow, losing the equivalent of one Jupiter-mass in gas per year.

"We still can't see the star itself," says Davidson, "but only an opaque dense wind outflow surrounded by muck--really bright ejecta."

Before 1700, Davidson notes that Eta Car was a fourth-magnitude star, recorded by astronomer Edmond Halley and only a few others. By 1843, it was almost as bright as Sirius before suddenly fading in 1858.

"After a supernova impostor explosion," says Davidson, "the star is in a state [of disequilibrium] that none of the theorists' computer codes are set up to deal with."

Nathan Smith, a postdoctoral researcher in astronomy at the University of California, Berkeley, who was not involved with the observations, says that Eta Car is a "key object for understanding the instability of the most massive stars."

Smith adds that these eruptions may provide glimpses of what to expect from the Universe's first stars, called population III stars.

Meanwhile, if Eta Car's current trend of decreasing winds continues, in a decade it will have very nearly emerged from its cocoon of dense gaseous outflow. That would return it to the state observed by Halley some 300 years ago, when it was seen as a hot, blue star, now known to have been of spectral class O.

The Minnesota team notes that this would finally enable contemporary observations of the radius and surface temperature of Eta Car, which is estimated to have the mass of well over 100 Suns.