While the Italian volcano Stromboli spits fire on Earth and pyro-fireworks were ignited in the USA on Independence Day, the NASA/ESA Hubble Space Telescope captured a fireworks display that no firework on Earth could compete with. However, this extra-terrestrial firework exploded thousands of years ago.

The image taken by Hubble shows the expanding gases of the double star system Eta Carinae glowing in red, white and blue. The binary star is located in the Carina star system, about 7,500 light years away from Earth. Scientists have been observing the star and the “cosmic drama” that is taking place there for more than 25 years, but the new image is the first one to be taken with Hubble’s Wide Field Camera 3 “to map warm magnesium gas glowing in ultraviolet light”.

Big eruption almost 200 years earlier

The light of the first so-called “Great Eruption” of Eta Carinae reached Earth in 1838, making it the second brightest star in the sky after Sirius until 1844, also making it an important navigation star for mariners in the southern seas. In the meantime, its light has faded, but the Hubble image shows that the spectacle is not finished yet.

Eta Carinae

Picture Eta Carinae in 1994 © ESA

Violent mass ejections are not uncommon in Eta Carinae’s history, astronomers say. “The system has been blighted by chaotic eruptions, often blasting parts of itself into space.” The Great Eruption, however, was particularly dramatic. “The larger of the two stars is a massive, unstable star nearing the end of its life, and what astronomers witnessed over a century and a half ago was, in fact, a stellar near-death experience.”

Even today, the signature of the Great Eruption can be seen in the vicinity of the star: The dumbbell shape is created by dust, gas, and other filaments hurled into space in the expulsion. The hot, glowing clouds are known as homunculus nebulae.

Surprising insight

The ejected outer material was heated by shock waves generated when it collided with debris from the star that had previously been ejected into space. The scientists who took this new picture assumed that “light from magnesium coming from the complicated array of filaments seen in the light from glowing nitrogen. However, what they found was a completely new, a luminous magnesium structure in the space between the dusty bipolar bubbles and the outer nitrogen-rich filaments that had been heated by the shock waves.

“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,” explains Nathan Smith of Steward Observatory at the University of Arizona, lead investigator at the Hubble program. “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.” This new data would allow researchers to better understand 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. Astronomers need more observations to measure exactly how fast the material is moving and when it was ejected.”

Eta Carinae

Picture Eta Carinae in 2012 © ESA

The streaks visible in the image in the blue region outside the lower left bubble are caused by the star’s light rays penetrating the dust clumps along the surface of the bubble. “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,” explains team member Jon Morse of BoldlyGo Institute in New York.

Technology can also be used to explore other stars

According to the researchers, this technique, the search for ultraviolet light for warm gas, could also be used to study other stars and gaseous nebulae. “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,” Smith says. “But this new ultraviolet-light image looks astonishingly different, revealing gas that we did not see in either visible-light or infrared images.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.”

The cause of Eta Carinae’s Great Eruption remains a mystery to scientists. There are different theories. But the researchers are certain about its end: Eta Carinae will explode as a supernova in an even more gigantic fireworks – or has already exploded. The light of Eta Carinae’s death would take 7,500 years to reach Earth..

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