Eta Carinae is inarguably one of the most incredible celestial objects humanity has ever observed. Composed of two stars located 7,500 light-years away from Earth, this truly massive stellar system radiates over 5 million times more energy than that of our own Sun. The expanding veil of erupted gas and dust shrouding Eta Carinae made it the only object of its kind in our galaxy for hundreds of years. Now, using archival data from NASA’s Spitzer and Hubble telescopes, five “twins," also with the same type of eruption-produced dust shroud, have been located within the Milky Way, as reported in Astrophysical Journal Letters.
Eta Carinae’s first star is roughly 90 times the mass of the Sun, whereas the second companion star is perhaps up to 30 solar masses. They orbit violently and eccentrically around each other, passing very close to each other once every 5.5 years. The most novel feature of this binary star system is that, between 1837 and 1856, it brightened significantly. Careful observation revealed that the star was erupting a vast amount of material into space.
This “Great Eruption” released approximately 10 percent of the energy that would have been released if the primary star went supernova. Even though the eruption has long ceased, astronomers can still trace its light echoes: the light that is still reflecting off dust particles in the nebula surrounding Eta Carinae. However, the eruption mechanism is poorly understood, and in order to properly comprehend it, star systems behaving similarly were required for study.
Eta Carinae imaged here by Hubble, shrouded by the billowing Homunculus Nebula. NASA/ESA/Hubble SM4 ERO
The team decided to search for these elusive star systems by trawling through the archive data of two of NASA’s most powerful telescopes, Hubble and Spitzer, the latter of which looks for energy in the infrared part of the spectrum. Fortunately, they came up trumps, finding five systems exhibiting the same dust shroud as Eta Carinae.
“We knew others were out there,” said co-investigator Krzysztof Stanek, a professor of astronomy at Ohio State University in Columbus, in a statement. “It was really a matter of figuring out what to look for and of being persistent.”
The way the dust shroud absorbs and emits both visible and ultraviolet light is very distinctive; thus, by looking at the characteristic energetic signature of Eta Carinae, its stellar “fingerprint” was determined and used to search for its twins.
The nearby spiral galaxy M83 contains at least two Eta Carinae twins. NASA/ESA/Hubble Heritage Team, R. Khan
Last year, two twins were found in the galaxy M83, located 15 million light-years away. Soon after, others were found in galaxies NGC 6946, M101, and M51, all of which are at a distance of between 18 and 26 million light-years away. Their energetic fingerprints are very similar to that of Eta Carinae, indicating that they each likely contain at least one heavy star shielded behind five to 10 solar masses of stellar gas and dust.
The authors of the study put forward several hypotheses as to why these eruption mechanisms occur, although no definitive answer can yet be given. One idea is that less frequent eruptions are triggered by the sudden onset of the burning of carbon in a later stage of the star’s life. This would cause the pulsation of the star – the expansion and contraction of its outer shell – to suddenly change, perhaps leading to a massive ejection event.
Another theory is that the eruptions are more frequent and episodic, and that later burning stages cause even more eruptions, but until the star systems are observed more carefully, the ultimate explanation will remain tantalizingly mysterious.