Scoping Out the Monster Star
Eta Carinae the biggest, baddest star in the Milky Way has started brightening again, and no one knows why or where it might lead.
by Robert Zimmerman

Eta
Carinae

Eta Carinae itself lies hidden among the rich gas and dust just north (above) of the bend in the dark wedge at center.
Luke Dodd

Like a king, this gigantic star imperiously ordains the life and death of all its neighbors. Some stars it feeds, providing the complex molecular material used by newborn stars to coalesce and form. Others it annihilates, exterminating any life that might dwell within nearby stellar systems.

The star shines so brightly that it would appear as bright as the sun does in our sky from 185 billion miles away some 50 times the distance to Pluto. With about 120 times the mass of the sun, it comes close to the theoretical limit of how big a star can be. And although it lies 7,500 light-years away, the star glittered more brilliantly than any other star in the Galaxy between approximately 1835 and 1855. During this 20-year Great Eruption, it ejected two to three suns' worth of material.

The star is Eta Carinae, considered by many astronomers to be the brightest object in the Milky Way Galaxy and among its heaviest. It is also one of the strangest and rarest of stellar bodies a gargantuan variable star that fluctuates in an irregular and completely unpredictable manner.

Today Eta Carinae remains luminous and unstable, surrounded by one of the most beautiful and distinct nebulae in the heavens. Moreover, astronomers were astonished recently when without any warning it suddenly began to brighten again at all wavelengths. In the past year its brightness has more than doubled, leaping upward so that the star and its surrounding nebula now glow around 5th magnitude, the brightest in more than a century.

One hundred and fifty years ago, the English amateur astronomer John Herschel asked: "What origin can we ascribe to these sudden flashes and relapses?" His question remains unanswered, no doubt in part because this regal star continues to be cloaked by dusty clouds. In fact, astronomers are not even sure if Eta Carinae is a single star. To paraphrase Winston Churchill, Eta Carinae remains "a riddle wrapped in a mystery inside an enigma."

Yet, Eta Carinae's place in stellar evolution is crucial. Its mysterious behavior challenges every theory that tries to explain the life and death of stars including our own sun. Its future activity might even tell us why, where, and when other stars go boom.

The Great Eruption
Kris Davidson of the University of Minnesota has said that Eta Carinae "is to the Southern Hemisphere what the Crab Nebula is to the north." Until the third decade of the 19th century, however, Eta Carinae was considered an unusual but undistinguished variable star, sometimes glowing at 4th magnitude, sometimes at 2nd. Then in the 1830s, John Herschel noticed that the star's light had risen steeply and, by December 1837, had reached 1st magnitude. Puzzled by this strange jump in brightness, Herschel researched the history of the star and discovered that it had also reached 1st magnitude in 1827 and again in 1832. After fading in 1838, the star blazed forth in 1843, zooming to a magnitude of 1 and becoming, for a short period, the second brightest star in the sky, eclipsed only by Sirius.

For the next twenty years, Eta Carinae remained among the sky's most brilliant stars, fading in fits and jerks until it finally dropped below naked-eye visibility in 1868, stabilizing at about 7th magnitude. In 1889 the star brightened for a short while, barely reaching 6th magnitude, then settled back down to a relatively quiet, stable glow around 8th magnitude for the next few decades.

In recent years, Eta had been growing slowly brighter, rising almost a full magnitude from 1950 to 1992. During this period scientists continued to puzzle over the star's nature. Though ground-based photographs showed that Eta was surrounded by a rapidly expanding cloud that could be traced back to the Great Eruption of 1843, the resolution of the images wasn't high enough to ascertain the exact structure or learn how the cloud interacted with the star. Because its shape resembled that of a human, Enrique Gaviola of the Cordoba Observatory in Argentina dubbed this nebula the Homunculus, which means "little man."

Then in 1994, the Hubble Space Telescope received a set of corrective lenses, and Eta Carinae became one of the first targets for the newly repaired telescope. The stunning image unveiled a star erupting. Suddenly, after centuries of unclear vision, scientists and ordinary citizens could look at the nebulosity surrounding the star and see its shape and detailed structure.

Homunculus Nebula

Hubble's view of the Homunculus Nebula reveals materials thrown off in the 1843 Great Eruption.
NASA/STScI

The Homunculus
The most obvious features in the Hubble images are the two large and grayish bipolar lobes, shaped somewhat like an hourglass. With a total mass somewhere around three times that of the sun, their glow comes mainly from starlight radiated by Eta Carinae that reflects off the ubiquitous dust in the lobes. Ejected from the star during the 1843 eruption, each lobe is expanding outward at the rate of about 1.5 million miles per hour. At this great speed fast enough for a spaceship to travel to the moon and back three times in an hour the lobes have expanded in 150 years to span about four trillion miles, or some 0.7 light-year.

In all likelihood, the lobes are mostly hollow, though astronomers have detected evidence of some dust within them. No one knows whether the lobes are shaped like spheres or cones. Either way, they apparently formed when matter was ejected from the star's polar regions. Perhaps this happened because Eta Carinae spins rapidly, possesses a powerful magnetic field, belongs to a binary star system, or some combination of these factors. Regardless, the ejecta from the 1843 eruption had a harder time escaping from near the equator and was forced to seek the path of least resistance, traveling outward from the poles.

Less obvious but also seen first in the Hubble images was the strange equatorial disk tilted between 52 and 60 to our line of sight and about 90 to the two lobes. Faintly resembling a ceiling fan with many blades, the disk consists of many curious objects moving at a wide range of speeds. Three mysterious blobs appear embedded within the disk only a few light-days (about fifty billion miles) from Eta. Flying outward from the star at about 100,000 miles per hour, all three seem arranged around the outside edge of the equatorial disk's largest fan (called the Paddle and visible as the triangular-shaped bright area above and to the right of the Homunculus's center). Astronomers don't yet know what caused these blobs to erupt from the star so asymmetrically, though their speed and distance from the star suggest they were ejected in 1889.

Within the Paddle itself, however, several small regions move at much slower speeds, as low as 30,000 miles per hour. These relatively sluggish speeds imply that the features were ejected from Eta several hundred years ago, long before the eruptions of the 19th century.

The equatorial disk also contains several mysterious and fast-moving jets, or "bullets" as some scientists have labeled them. The northern jet is shooting away from the star at a tremendous velocity, estimated as high as 3.4 million miles per hour. As it rockets outward it appears to be pushing its way through the interstellar medium of nitrogen gas that surrounds Eta Carinae and was ejected in a much earlier, unrecorded eruption. Although some scientists believe that the jet's origin is linked to the 1889 eruption, others contend that it along with most of the equatorial disk formed during the Great Eruption of 1843.

In fact, all the data gathered about the equatorial disk so far presents scientists with an exceedingly confusing picture of its origin. Depending on when and where they look, different astronomers get different results. Perhaps Ted Gull of NASA's Goddard Space Flight Center puts it best when he says "the disk appears to be the accumulation of many outbursts."

Even more baffling is the disk's odd radial appearance, with its fans, spokes, and jets all pointing toward the star. As Davidson and Roberta Humphreys, also at the University of Minnesota, have written, "We regard the radial streaks as warning arrows pointing inward toward some extraordinary phenomenon near the central star."

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.

Infrared image of Eta Carinae

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."

Eta and
surroundings

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.




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