This picture depicts the center of the Great Nebula in Carina- one of the Milky Way's brightest and most prolific star forming regions. Over its estimated three million year history, this nebula has spawned several clusters of gigantic, hot, blue-white stars that litter the image like confetti at a New Year's celebration. The bizarre structure near the center of this view is actually a cooler cloud of dust and bright filaments of hot, fluorescing gas that is partially silhouetted against a background of brighter, even more energized material. Even though I imagine its distorted shape to resemble a partially melted hour glass, it is commonly known as the Keyhole Nebula. The size of the objects that have been captured in this picture are off our human scale for easy comparison. For example, it takes light, traveling at 681 million miles per hour, seven years to travel from one side of the Keyhole Nebula's spherical head to the other!
The bright star near the bottom of the image, however, was the inspirational germ for this project. It is the most massive nearby star that can be studied in great detail and many astronomers consider it to be one of the most massive stars in the universe! The star is named Eta Carinae (pronounced /ke 'rain i/), and some believe it may also pose a threat to our planet!
Eta Carinae is over one hundred times more massive than our Sun and it's over a million times more brilliant. In fact, this star releases so much energy that its gravity can barely hold it together- the outward flow of radiation from its interior is unbelievably strong. Vast portions of its outer layers are being blown off into space forming an ever expanding wispy shroud in a slow, non-stop eruption of incredible violence. Stars like this are extremely rare- only a few dozen exist within any island universe. Because of their high luminosity, very large stars, like Eta, burn through their fuel rather quickly and end their lives as a super or hyper nova explosions that out-shine the combined light of all the stars in their host galaxy. Where the luminous life of a star like our Sun will span about ten billion years , super massive stars, like Eta Carinae, only shine for a few million, at most.
Because of its prodigious mass, Eta Carinae is highly unstable and prone to violent outbursts that seem to occur in irregular, unpredictable cycles. This behavior first caught the attention of astronomers back in the 18th century.
Edmond Halley, who first predicted the return of a bright comet, cataloged Eta Carinae as a fourth magnitude star in 1677. That's about the same brightness as the star that's located at the position where the handle meets the ladle in the northern sky's constellation of the Big Dipper. Interestingly, over the next hundred years, observers reported that Eta brightened considerably before mysteriously returning to the same relative obscurity that Halley had previously noted. Around 1820, Eta again started to increase in brightness and by 1842, it had become the second brightest star in the sky- only Sirius, the brightest star seen from Earth, displayed more brilliance to earth bound eyes. To provide some perspective, Sirius is much closer to our planet, only 9 light-years away compared to Eta's 8,000 light-year distance. However, if both stars were located side-by-side, Sirius could be represented as a single candle flame with the combined output of over eight 100-watt light bulbs substituting for Eta! Over the next 100 years, Eta again faded until it was totally invisible to the naked eye by the 1920's. At the end of the 20th century and again, early in 2007, Eta's luminosity cycled through a period of increased brilliance. All of these episodes were unexpected, by the way.
Most astronomers concur that this star is a ticking time bomb already triggered to explode. Unfortunately, no one can confidently predict how much time remains until the countdown reaches zero- maybe, in a million years, possibly only a thousand. A few have suggested that Eta Carinae will self-destruct during our lifetime. But, nobody knows for sure.
Perhaps it's already been annihilated and the concussion, limited to traveling at the speed of light, has simply not reached us, yet. Remember, even though this picture is very recent, it portrays a view that dates back to the mankind's prehistory! Even the Sun's rays take eight minutes to traverse the Earth's distance. Distance and time are woven into a cloth that blankets the Cosmos. Very little we observe beyond the horizon is in real time. Regardless, sequestered at the bottom of our atmosphere, we would be safe from Eta's fury were it to strike us, say, tomorrow evening. Our orbiting satellites, our upper ozone layer and astronauts in the International Space Station, of course, would most likely be less fortunate.
Recent, high resolution images produced through the Hubble Space Telescope have revealed this star's immediate surroundings in jaw-dropping detail. Two gigantic lobes of material are expanding like giant balloons from both poles and its equator is ringed by an ever-broadening disc of far flung material. These structures are the suspected artifacts of the brightening observed during the 1800's.
One of the motivations for this image was a desire to see if the lobes surrounding Eta Carinae could be glimpsed with a modest telescope and commercially available astronomical camera. Chief among the challenges was the requirement to tame the brightness of Eta Carinae so that the surrounding lobes would not be lost in the star's glare.
Seven filters were used to produce this picture, for example, red, green and blue captured the hues needed to reproduce the natural colors of the scene. These are used for all of my projects, so their inclusion is nothing noteworthy. But, for the first time, a set of special astronomical filters were employed that are tuned to limit the light reaching the camera to those wave lengths associated with the natural glow emitted by atoms of hydrogen, oxygen and sulfur which are abundant in most interstellar nebulae. By carefully combining separate images produced with each of these six filters, it was possible to produce a picture that not only displays how this scene appears, but the colors also give an indication of what the scene is made of! Exaggerated blue hues indicates the presence of oxygen, red represents sulfur and turquoise reveals the signature of hydrogen atoms.
As a side benefit, the sulfur filter enabled control of the glare from Eta. Thus, the surrounding lobes were successfully resolved.