Hubble Space Telescope image showing Eta Carinae and the Homunculus Nebula. The Homunculus was partly created in an eruption of Eta Carinae. Eta Carinae itself appears as the white patch near the centre of the image, where the 2 lobes of the Homunculus touch.
Credit: Wikimedia
Eta Carinae (left) emitted light that researchers have detected reflecting off dust clouds (right).
Credit: A. Rest, STSI/NASA/NOAO
SYDNEY: 'Light echoes' have been detected from a giant cosmic eruption that occurred in 1838, shedding new light on the history of one of the most massive stars in our galaxy.
When the so-called Great Eruption occurred between 1838 and 1858, Eta Carinae, a massive system consisting of two stars, lit up and became the second brightest star in our sky for the following decade, before fading from view.
For years, scientists have puzzled over what caused the Great Eruption, but because telescopes of the 19th century lacked instruments such as sophisticated modern spectrographs, they could not record the physical conditions at the time of the eruption. Now, scientists have detected the light spectrum of this past eruption for the first time to discover that the Great Eruption was cooler, and stranger, than previously thought.
"Eta Carinae is probably the most studied object in our galaxy," said lead Armin Rest from the Space Telescope Science Institute in Baltimore, author of the paper published in the current issue of Nature. "It's an extremely interesting object and important to understand. The big missing piece was always that there was no spectra of the Great Eruption, we did not have the data."
A massive blue star
Discovered by English astronomer and mathematician Edmund Halley in 1677, the Eta Carinae star lies about 7,500 light-years from Earth in the constellation Carina. Scientists have suggested that it is a binary star, which is a system containing at least two stars, called Eta Car A and Eta Car B. Researchers suspect that if this is true, Eta Car B star has less mass than Eta Car A, which is a huge Luminous Blue Variable (LBV) star around 120 solar masses.
Due to its colossal mass, Eta Carinae is one of the most unstable stars known, and has gone through periods of brightness and dimness, burning its fuel faster than normal stars do. During the Great Eruption, it lost an amount of mass equivalent to that of about 20 times the mass of the Sun, and expelled the gas that now makes up the Homunculus Nebula. Eta Carinae is now swathed in the red haze of the nebula, making it very difficult to study.
"These really massive stars are really blue and not very stable. They sometimes go through these periods where they have eruptions," said Rest.
Previous wind models of the Great Eruption, based on visual estimates of Eta Carinae's brightness rather than the light echoes emitted from the eruption, implied that the minimum temperature during this eruption was around 7,000 degrees Celsius. Eta Carinae was thought to be the proto-type of these giant eruptions, so scientists thought the temperature could not go lower than this.
Light echo time machine
Light echoes have been used by scientists in the past to determine the characteristics of cosmic eruptions. When an eruption or explosion occurs in space, light will emit in all directions, and the light we see is the light that's travelling towards us. If the light that's travelling away from us happens to hit a dust cloud, it is reflected back towards us - like an echo - but will arrive at a later stage due to a longer path. "It's kind of like a time machine," said Rest. "You can look backwards in time and look at an event in the past."
"It's like you're chipping away at the base of a volcano, to find the lava from an eruption from 100 million years ago. It's pretty impressive," said Peter Tuthill from the School of Physics at the University of Sydney, who was not involved in the study.
But light echoes can be quite a challenge to pick up. "It's a catch-22 situation. You need something to reflect the light at you, and ideally you'd want a clean sheet of white paper, but [space] gives you a messy cloud with its own physics," Tuthill added. "Unravelling the light echo from all the stuff that's going on in the background is part of the challenge."
