During the middle of the 19th century, a star system known as Eta Carinae suddenly became the second-brightest star in the night sky, then gradually faded again. Known as the Great Eruption, this event released about 10 percent of the energy that would have been liberated if the star had gone supernova, and caused the star to shed approximately 10 Suns' worth of mass. Yet somehow, Eta Carinae survives to this day. Understanding the behavior of Eta Carinae (which is estimated to still hold at least 100 times the mass of our Sun) will provide astronomers with knowledge of the end-stages of very massive stars, and allow them to distinguish between eruptions and supernova explosions.

Even though the Great Eruption first became visible in 1838, astronomers are still able to observe its effects today through light echoes: light that has bounced off particles inside the nebula surrounding Eta Carinae for a while, and has reached Earth long after the initial eruption has faded. A new study of the light echoes, performed by A. Rest et al., reveals that Eta Carinae was relatively cool at the time of its brightening. While eruptions observed in other galaxies seem to be driven by thick, opaque clouds of matter being driven away from their progenitor star, the analysis published in the February 16 edition of Nature seems to show that the Great Eruption may actually have been triggered by a blast wave emanating from the surface of Eta Carinae.

Observing Eta Carinae's detailed structure has proven challenging: the system is swathed in a thick cloud known as the Homunculus Nebula. Hubble Space Telescope observations (like the one that produced the image above) show that the Great Eruption produced a dramatic lobed structure. The uneven distribution of matter is indicative of something very intriguing going on within the nebula. Astronomers agree that there are at least two stars inside the nebula, but confirming that has not been possible to date.

Despite these difficulties, astronomers are certain the main star in the Eta Carinae system is a luminous blue variable (LBV): an exceedingly bright, hot, and massive star. LBV stars occasionally experience rapid increases in size. Basic astrophysical principles dictate that this means there must be a subsequent drop in temperature and a rise in luminosity, with significant amounts of mass being shed at the peak of the expansion. In other words, though the star is blue in color for much of the time, during some periods it may appear to be a relatively cooler white or yellow.

Eruptions in LBV stars are not common: only two have been seen in our galaxy in the past 400 years, with only a handful of others in other galaxies for comparison. Because of their high energies, they are known as "supernova impostors," and understanding the mechanisms that produce them will help astronomers tell them apart from true supernovas, which involve the collapse of the core of a star and the explosion of the remainder of its envelope.

In a model commonly used to understand eruptions, a thick stellar wind (similar in principle to the one we get from our Sun, but involving far more particles) sweeps out from the surface of the unstable star, driven by the increased number of photons. The combination of temperature (about 7000 Kelvins) and density of this wind in this model mimics the appearance of an F-type (yellow-white) supergiant star.

However, the spectrum that has now been observed in the Eta Carinae light echoes is inconsistent with F-type stars. It corresponds more closely to G-type (yellow) supergiants, which have temperatures in the same range as our Sun's (approximately 5000 K), much cooler than the model described in the previous paragraph. In addition, the spectrum shows absorption by gas that is moving about 210 kilometers per second along the line of sight between Earth and the star, as determined by its Doppler blueshift. This means the gas flowing from the surface of Eta Carinae also contains a lot more kinetic energy than predicted by the stellar wind model.

Given the issues with the stellar wind model, the authors of the current study propose an alternate explanation for the eruption: an explosion within Eta Carinae. Unfortunately, they admit they don't yet know what could cause such an event. Typically, stellar explosions exhibiting such high energies are destructive—they are supernovas that leave no star behind—while Eta Carinae is obviously still there. The shape of the Homunculus Nebula is highly polar, with the two strong lobes of gas and an equatorial disc of material. These and other features may help determine whether an explosion actually occurred, or if some other mechanism is required to explain the strange nature of the Great Eruption.

Nature, 2012. DOI: 10.1038/nature10775 (About DOIs).