June 2, 1999
"Eta Carinae is at its brightest in visible light since 1864," said Dr. Theodore Gull of NASA's Goddard Space Flight Center, Greenbelt, Md. "An inner cone-shaped nebula has tripled its brightness since we first discovered it in March 1998. There is a lot going on inside the larger Eta Carinae nebula, and we do not understand all of the activity at this point. Some of the brightness increase may be due to the excitation of gas in the nebula by light from Eta Carinae. However, we may also be witnessing the start of another massive eruption."
The observations will be the subject of a press conference June 2 at 11 a.m. EDT during the 100th Anniversary meeting of the American Astronomical Society in Chicago.
Eta Carinae is about 7,500 light-years from Earth and appears in the southern sky in the constellation Carina. At about 100 times the mass of the Sun, Eta Carinae is a rare luminous blue variable star, a class that includes the brightest and most massive stars in the universe. Luminous blue variables are prone to violent displays because of their instability -- they shine so brightly that their gravity can hardly hold them together.
In the 1840's, Eta Carinae flared brightly and, over a 20 year period, ejected enough material to create three Suns. The result of this eruption can be seen today as a spectacular double-lobed nebula of gas and dust, lit from within by an intemperate star. The STIS researchers recently found a cone-shaped nebula inside of the larger nebula.
"Eta Carinae is unquestionably the most mysterious and powerful star that we can see with the unaided eye," says Professor Kris Davidson of the University of Minnesota. "We basically understand how all of the naked-eye visible stars work -- except Eta Carinae. Its new brightness increase surprised us for a couple of reasons. First, we thought we had good reasons not to expect a major outburst in the next few decades. Second, when an eruption does occur, it shouldn't look like the recent behavior -- the spectrum should change, and its color should get cooler. Both are contrary to the data," said Davidson.
The extremely high resolution of the STIS instrument made the discovery possible. "We could not separate the activity of Eta Carinae from its surrounding inner nebula without the very high resolution of the STIS instrument on Hubble. Its one tenth of an arc-second resolution is like viewing a baseball from across Lake Michigan. STIS showed us the new, cone shaped inner nebula as well as revealing the brightness increase before it was obvious to astronomers on the ground," said Gull.
Astronomers in Chile, South Africa and Australia confirmed the change in April, but the effect is less dramatic for them because ground-based telescopes cannot separate the star from the surrounding outer nebula, which is half a light year across," said Prof. Roberta Humphreys, also of University of Minnesota.
Eta Carinae is one of the brightest known luminous blue variables, a mysterious category of extremely massive, extremely powerful unstable stars. The explosion that ends the life of such a massive star is even more violent than a normal supernova explosion.
"Only one star in a billion is as massive as Eta Carinae. Some theorists speculate that such an object ends its life as a hypernova, not a supernova explosion, and may produce a gamma ray burster, which occurs about once every 100 million years for each galaxy. A gamma ray burster is such an cataclysmic event that it might even be hazardous at a distance of 7500 light years," said Davidson.
The research group is planning more observations of Eta Carinae in the next two years. "The STIS instrument combines high resolution imaging with the ability to get velocity information. We plan to use this powerful combination to build a three dimensional map of the Eta Carinae nebula, which will help us understand the beat within its fiery heart," said Gull.
Images to support this story are available on the internet at:
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
11 January 1998
"It appears to be brightening and fading over a cycle of many years," said CSIRO's Dr Bob Duncan, co-leader of the observing team.
"Since 1992 it's become four times brighter in the radio, then last year it began to drop dramatically."
Eta Carinae is gigantic -- 100 times more massive than the Sun and perhaps the heaviest star in our Galaxy. Prematurely aged, the star has sloughed off its outer layers, cocooning itself in a vast dumbell-shaped cloud of gas and dust.
Radio telescopes such as the Australia Telescope can pierce this camouflage, seeing through to the radio-emitting gas that lies near to the star.
Eta Carinae puzzled the Australia Telescope team by brightening steadily for four years. Then from infrared studies Brazilian astronomer Augustus Damineli suggested that the star was brightening and fading on a five-and-a-half year cycle. He predicted it would fade around 1997.
Such slow cycles of brightening have been seen in a handful of other stars.
The radio source around the star shrinks and swells. Like twiddling the dimmer switch of a light, this leads it to put out more, then less radiation.
Eta Carinae has other bizarre behaviour. Two years ago astronomers found that one of the gas clouds around the star pours out a powerful ultraviolet laser beam, several times brighter than our Sun. Eta Carinae is the only star known to produce a natural ultraviolet laser, although a handful of natural infrared lasers have been found.
The star is doomed to explode "soon -- perhaps tomorrow or perhaps in a million years," said CSIRO's Dr Bob Duncan, a member of the observing team.
Eta Carinae can be seen only from the Southern Hemisphere. The Australia Telescope is the first radio telescope able to make a detailed picture of it.
Dr Stephen White of the University of Maryland, co-leader of the observing team, presented the Australia Telescope findings to the AAS meeting.
In other talks in the same session, Dr Damineli presented his latest observations of the current dip in the Eta Carinae's cycle and Dr. Michael Corcoran (Universities Space Research Association and NASA Goddard Space Flight Center) showed that the same cycle is seen in X-ray observations of Eta Carinae from space, which suggests that Eta Carinae is in fact a double star system.
1. Images of the star taken by the Hubble Space Telescope are available on the World Wide Web.
This is the more colourful version. Photo credit: J. Hester (Arizona State University), and NASA.
This is a more detailed but less colourful version. Photo credit: Jon Morse (University of Colorado), and NASA.
Please note that these images are governed by copyright conditions set down at http://www.stsci.edu/web/Copyright.html.
2. There is also a radio image showing brightening and fading:
Photo Credit: Dr. Stephen White, University of Maryland
Caption: Radio emission from the star Eta Carinae -- data and images taken with CSIRO's Australia Telescope. The graph shows how the star's total radio emission varied over 5.5. years. Except during the dip, Eta Carinae is almost certainly the brightest radio star in the night sky at these wavelengths.
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
January 7, 1998
The team, led by Dr. Michael Corcoran of the Universities Space Research Association, Columbia, MD, bases its conclusions on unusual variations in the intensity of X-rays emitted by hot gas near the star, called Eta Carinae, which is located about 7,500 light years from Earth. They believe that the variations are caused by the presence of a massive companion star in orbit around Eta Carinae.
The new work offers insight into the origin and evolution of a class of stars called luminous blue variables, which are the most massive stars known.
"Stars such as these shine so intensely that, sometimes, they become unstable and blow their outer layers off," said Corcoran. "That's what happened to Eta Carinae. During the mid-1800's, it blasted an amount of material equivalent to the mass of our entire solar system into space. The gas and dust that make up this material formed a shell that surrounds the star and now blocks it from direct view. We have taken what amounts to an X-ray of this shell and found that what's inside may really be two stars."
While using the Rossi Explorer to monitor the X-ray emission from Eta Carinae every week for a period of two years, the team found that X-rays emitted by hot gas near the star initially increased over a period of months and then rapidly diminished in intensity in a matter of days.
Such variability is highly unusual and has never before been observed for Eta Carinae. The simplest explanation is that the variability of the X-ray emission is due to the presence of a massive stellar companion orbiting the star, bound to each other by the force of gravity.
The presence of such a companion has recently been claimed based on variations in near-infrared and optical spectra by Dr. Augusto Damineli and collaborators at the University of Colorado at Boulder. However, the presence of the "companion" star remained controversial, since the spectrum (a means of measuring the properties of objects by splitting their light into its component colors) of Eta Carinae is notoriously variable, and since the spectral features originate in a very complex medium. As a result, the "binary model" for Eta Carinae has not yet been generally accepted by the astronomical community. The X-ray variations may help change this situation.
"We believe the orbit of the companion star is elongated into an ellipse," Corcoran said, "which alternately moves it closer to and further away from Eta Carinae over the five-and-a-half year orbital period. When the stars are close, the two stellar winds slam together, which creates a shock wave that heats the gas tremendously, to about 60 million degrees, and it emits large amounts of X-rays. When they are further away, this shock wave diminishes, along with the X-rays. This agrees pretty well with our RXTE X-ray measurements."
The spacecraft's data may have helped close one mystery, but at the same time, they have opened another.
"Strange peaks in the X-ray emission intensity seem to occur every 85 days," notes Kazunori Ishibashi of the University of Minnesota. "While the first peaks detected were relatively weak, their strength has recently risen as the overall X-ray emission from Eta Carinae has brightened."
"The most puzzling unknown is what causes the 85 day X-ray period," Davidson said. "It may be the rotation of the star, or the star may pulsate in that time, or it might even be the orbit period of a third object in the system, a possibility that makes some astronomers uncomfortable."
With at least 50 times more mass than the Sun, luminous blue variable stars like Eta Carinae are the most massive known. If Eta Carinae is really a double star system, each is estimated to be 70 times more massive than the Sun, according to Damineli's binary star model.
Images to support this story are available at the following Internet location: