Eta Carinae is a binary stellar system approximately 7,500 light-years away in the constellation Carina. Also known as HD 93308 and Hen 3-481, the system was first cataloged by the English astronomer Edmond Halley in 1677, as a star of fourth magnitude. In 1838, Eta Carinae underwent a cataclysmic outburst called the Great Eruption, quickly escalating to become in 1844 the second brightest star in the sky by April of that year. The star has since faded, but the new ultraviolet image from the NASA/ESA Hubble Space Telescope shows that the spectacular display is still ongoing, and reveals details that have never been seen before.
Astronomers have monitored the Eta Carinae binary system for more than two decades. It has been prone to violent outbursts, including an episode in the 1840s during which ejected material formed the bipolar bubbles seen here. Now, using Hubble’s Wide Field Camera 3 (WFC3) instrument to probe the nebula in ultraviolet light, they have uncovered the glow of magnesium embedded in warm gas (shown in blue) in places they had not seen it before. The luminous magnesium resides in the space between the dusty bipolar bubbles and the outer shock-heated nitrogen-rich filaments (shown in red). The streaks visible in the blue region outside the lower-left lobe are a striking feature of the image. These streaks are created when the star’s light rays poke through the dust clumps scattered along the bubble’s surface. Wherever the ultraviolet light strikes the dense dust, it leaves a long, thin shadow that extends beyond the lobe into the surrounding gas. Image credit: NASA / ESA / Hubble / N. Smith, University of Arizona / J. Morse, BoldlyGo Institute.
Eta Carinae is one of the most massive binary systems astronomers can study in detail. The smaller star is about 30 times the mass of the Sun and may be as much as a million times more luminous. The primary star contains about 90 solar masses and emits 5 million times the Sun’s energy output.
Violent mass ejections are not uncommon in Eta Carinae’s history. The system has been blighted by chaotic eruptions, often blasting parts of itself into space, but the Great Eruption was particularly dramatic.
The resulting surge of light was outshone only by Sirius, which is almost one thousand times closer to Earth, and for a time made Eta Carinae an important navigation star for mariners in the southern seas. This close call stopped just short of destroying Eta Carinae, and the light intensity gradually subsided.
Astronomers have long known that the outer material thrown off in the 1840s eruption has been heated by shock waves generated when it crashed into material previously ejected from the star .
Dr. Nathan Smith of Steward Observatory and colleagues who captured the new image of Eta Carinae were expecting to find light from magnesium coming from the complicated array of filaments seen in the light from glowing nitrogen (shown in red).
Instead, a whole new luminous magnesium structure was found in the space between the dusty bipolar bubbles and the outer shock-heated nitrogen-rich filaments.
“We’ve discovered a large amount of warm gas that was ejected in the Great Eruption but hasn’t yet collided with the other material surrounding Eta Carinae,” Dr. Smith said.
“Most of the emission is located where we expected to find an empty cavity. This extra material is fast, and it ‘ups the ante’ in terms of the total energy of an already powerful stellar blast.”
The new data are important for understanding how the eruption began, because it represents the fast and energetic ejection of material that may have been expelled by the star shortly before the expulsion of the rest of the nebula.
Another striking feature of the image is the streaks visible in the blue region outside the lower-left bubble.
These streaks appear where the star’s light rays poke through the dust clumps scattered along the bubble’s surface.
Wherever the ultraviolet light strikes the dense dust, it leaves a long thin shadow that extends beyond the lobe into the surrounding gas.
“The pattern of light and shadow is reminiscent of sunbeams that we see in our atmosphere when sunlight streams past the edge of a cloud, though the physical mechanism creating Eta Carinae’s light is different,” said Dr. Jon Morse, of BoldlyGo Institute.
“We had used Hubble for decades to study Eta Carinae in visible and infrared light, and we thought we had a pretty full account of its ejected debris. But this new ultraviolet-light image looks astonishingly different, revealing gas we did not see in either visible-light or infrared images,” Dr. Smith said.
“We’re excited by the prospect that this type of ultraviolet magnesium emission may also expose previously hidden gas in other types of objects that eject material, such as protostars or other dying stars; and only Hubble can take these kinds of pictures.”
