Super Sized
Finding out the nature of that phenomenon, however, has proven difficult. Although astronomers regard Eta Carinae as one of the largest and brightest stars in the Galaxy, it has never been viewed directly, hidden as it is within the gigantic cloud that surrounds it. In fact, the star's dusty lobes prevent scientists from even knowing the exact duration of the Great Eruption, which might have continued long after the star had faded to 7th magnitude in 1868. As the lobes were spewed outward they drew a curtain on the eruption, making it appear as if the fireworks had ended, when in truth they could have continued for years.

What astronomers do know about Eta Carinae is that it belongs to a rare class of stars called Luminous Blue Variables, or LBVs, objects whose temperature and mass approach the absolute maximum believed possible for a star. Eta Carinae appears to tip the scales at a mass 100 to 120 times larger than the sun while its surface broils at a temperature that ranges between about 22,000° and 50,000° Fahrenheit. Compare this to the surface temperature of the sun, which comes in around 10,000° Fahrenheit.

At its coolest, Eta Carinae also becomes most compact — though its size would still be large enough to swallow the orbit of Mercury if it replaced the sun. When the star's temperature rises, its radius swells and would reach the orbit of Mars or beyond. During the Great Eruption, the star probably expanded to a diameter of two billion miles, the size of Saturn's orbit.

Moreover, astronomers think that Eta Carinae has one of the densest solar winds known, blowing off about 0.003 solar mass per year, some six trillion trillion tons, or two Earth masses each day. At this rate the sun would evaporate in a little more than three centuries. But Eta hardly notices.

Such intensely large, hot objects must periodically shed additional large amounts of mass to remain stable. What causes these larger eruptions remains a mystery, though astronomers suspect the incredibly high mass and temperature play key roles. The most popular hypothesis says that the star's luminosity is so great that it occasionally overpowers the gravity that holds the star together. The star becomes unstable; its outer layers pulse in and out as if unsure whether they wish to remain in place or gush into space. Eventually an eruption occurs and the outer layers are flung away.

With the loss of this shell of hot gas, the star cools and its surface temperature drops to a relatively low 13,000° Fahrenheit. At the same time, its electromagnetic output shifts from the high-energy ultraviolet to less energetic optical radiation, so though the star is now cooler, it radiates more brightly at wavelengths our eyes can see. Hence, while the star seems brighter, its overall output undergoes no intrinsic change.

After an eruption, these strange stars become stable for long periods, with their visible luminosity generally holding steady (though small, irregular fluctuations are not unusual). Whether the huge outbursts happen more than once is simply not known. If so, they are separated by thousands of years.

It is also far from clear whether the war between gravity pulling inward and the pressure of radiation pushing outward is the sole cause of these giant eruptions. Some scientists think that turbulence and convection on the star's surface either contributes to or causes the outbursts. Others believe that no current theory can be correct because none seems to explain adequately what instigates the eruptions as well as what makes them stop.

Grand and powerful, luminous blue variables such as Eta Carinae are among the rarest types of stars known. Fewer than three dozen have been identified, with five belonging to our Galaxy and the rest scattered among eight nearby galaxies in the Local Group. A good example is the first LBV discovered, P Cygni, which was unknown until it brightened to 3rd magnitude several times during the 17th century. It then stabilized at around 5th magnitude, where it remains today.

The significance of luminous blue variables is that they are thought to be a short-term transitional stage in the life of stars, spanning a mere 25,000 years or so. "Most stars above 40 to 50 solar masses will eventually go through an LBV instability phase," explains Humphreys. As the star sheds mass it slowly evolves from a blue supergiant to a Wolf-Rayet star, thought to be one of the last stages that stars reach before they die. Such stars have lost more than half to two-thirds of their original mass, have exhausted their hydrogen fuel, and now burn other, more complex atoms in their nuclear furnaces. Their next and final act, astronomers believe, is to explode as supernovae.

The warm, dusty surroundings of Eta show up best at infrared wavelengths. Elisha Polomski (University of Florida/CTIO)

More Mysteries
Ironically, Eta Carinae's recent behavior has punched a few holes in the theories that try to explain luminous blue variables. From 1993 to 1998, the star's light output first dipped and then rose about half a magnitude, an event attributed at the time to Eta Carinae's continuing irregular fluctuations. The star, however, then began to brighten precipitously, rising one full magnitude in less than a year. As of June 1999, the star and the surrounding Homunculus glistened at about 5th magnitude, the brightest it has been since the Great Eruption in 1843. "Without question, the central star has brightened since 1997 by roughly a factor of two," notes astrophysicist Jon Morse of the University of Colorado, whose Hubble images in June 1999 confirmed this brightening. And if the brightening trend continues, Morse adds that "in two years we will not be able to take an image with Hubble without saturating the camera. [Eta Carinae] will be too bright."

While astronomers had expected the star might return to higher magnitudes sometime next century, no one had anticipated it happening so abruptly. Kris Davidson points out that "if you had asked us we would have told you it probably wasn't going to happen."

The brightening is even more puzzling because it is occurring across all wavelengths. If the star were merely ejecting an outer shell in response to radiation pressure, the brightening would happen only at visible wavelengths, with the star's outer layers cooling and its output shifting from the ultraviolet to the optical. An increase in brightness across the entire electromagnetic spectrum implies that Eta is growing intrinsically brighter, a possibility no theory predicted. Moreover, this brightening implies that the star's luminosity may soon overwhelm its gravity, once again causing the star to become unstable and erupt as it did in the last century. "And Eta Carinae can't brighten very much," notes Humphreys, "or else it will go boom."

Adding to Eta Carinae's baffling story is the recent discovery by Augusto Damineli of the University of Sao Paulo in Brazil that the star's light fluctuations are not as irregular as once thought. Damineli's analysis of the star's spectrum reveals that every 5.5 years, Eta's ultraviolet and x-ray output undergoes a short-term change. In x-rays the flux increases steadily, then plunges, followed by a slow recovery. In the ultraviolet, several dark emission lines on the star's spectrum disappear. Damineli was able to trace this cycle back through the last 50 years. He also found that all the major outbursts of the previous century, in 1827, 1832, 1838, and 1843, seemed to line up with it. Finally, he successfully predicted an event in December 1997, at which time the star's x-ray output plummeted and the ultraviolet emission lines faded, as they had in previous cycles.

Based on this cycle, a number of astronomers immediately proposed that Eta Carinae was actually a binary system with both stars hidden by the system's dense surrounding nebula — and that all previous eruptions might have been caused by the orbital interaction of two stars.

Unfortunately, no proposed binary system has yet explained Eta's behavior completely. Furthermore, a bewildering wave of new facts has swamped the theorists and left them scrambling to recover. "We've got so much data from so many wavelengths it's difficult to get one model to explain everything," notes Michael Corcoran of the Universities Space Research Association. For example, recent observations have discovered an additional 85-day cycle in x-ray emissions and a 200-day cycle in the ultraviolet, with hints of a 58-day cycle in the optical.

Although there's still much disagreement about the existence of all these cycles, most astronomers now attribute the star's powerful x-ray emission to the collision of two dense stellar winds, emanating either from the two stars of a binary system or from the fast and slow stellar winds of a single star. Astronomers suspect that the 5.5-year cycle occurs when the region where these winds collide is periodically eclipsed, either when one star moves behind the other or when the rotation of a single star moves the region out of our line of sight.

While most scientists today favor the binary hypothesis, others argue that a single star could still produce all the observed phenomena. Some have even considered the possibility that the system is comprised of a barely stable three-star system, whereby the Giant Eruption of the last century was caused when two of the three stars switched positions in an orbital dance of grand proportions.

In general, astronomers remain skeptical of all these theories — including some of their very advocates. No hypothesis is yet able to explain all the known facts, among them the star's inexplicable and stunning jump in brightness since 1998. As Davidson notes, "We really, truly do not understand what is happening, even at a fairly rudimentary level."

	Numerous stars, gas, and dust lie in the immediate vicinity of Eta. ©1984, Anglo-Australian Telescope Board

The Life and Death of Stars
When you point a telescope at Eta Carinae, you are looking down the length of our Galaxy's Carina-Sagittarius spiral arm, which traces a corkscrew around the galactic center and is home to some of the Milky Way's most massive stars and most glorious nebulae. In such spiral arms stars are born. Eta Carinae lies embedded in a dense giant molecular cloud only a few million years old, about 425 light-years across, and containing the mass of 10,000 suns. Moreover, surrounding Eta Carinae's immediate environs are vast clouds of nitrogen and dust processed from hydrogen by the star's pile-driving nuclear engine.

"What's past is prologue," wrote Shakespeare. Astronomers now firmly believe that the formation of more commonplace stars is intrinsically linked to the feverish life and death of massive stars like Eta Carinae. These kingly objects not only process huge amounts of hydrogen fuel into more complex atoms such as carbon, oxygen, nitrogen, and iron, they also spray it out into the interstellar medium, accounting for a large percentage of the material found in the dense molecular clouds that surround them. Then, when such majestic stars go supernova, the explosion sweeps up this surrounding debris, forming a shock wave within which eventually flower the longer living but less massive ordinary stars — stars such as our sun.

Corcoran recently said that "The death of Eta Carinae is likely to be one of the most explosive events ever experienced in the Galaxy." When such massive stars go supernova, the energy released can be so powerful that it equals and possibly exceeds that of the rest of the Galaxy. Such explosions might also be the progenitors of gamma-ray bursts, events so energetic that even from a distance of several thousand light-years, the explosion could seriously damage life on Earth, possibly even wiping it out.

So while the explosion of lordly stars like Eta Carinae leads to the birth of new stars, it concurrently causes the destruction of life in nearby solar systems. Let us hope that our solar system is just far enough away so that we can enjoy the show without any tragic consequences.