New Giant Stars Found Outside Our Galaxy

(Inside Science) -- The most luminous and massive system of stars within 10,000 light-years of Earth, known as Eta Carinae, is surrounded by a veil of gas and dust that makes it unique in the Milky Way. Now Eta Carinae is no longer a lonely anomaly — astronomers have for the first time detected five twins of the system in other galaxies. This finding could shed light on the violent ends of massive stars and the way they can shape their galaxies, scientists added.

Located about 7,500 light-years away from Earth in the southern constellation Carina the Keel, Eta Carinae is 5 million times brighter than the sun. The binary system is made up of two massive stars — the larger star is estimated to be 100 to 150 times more massive than the sun, while its smaller companion, detected in 2005, is 30 times the sun's mass.

In a rare celestial outburst later dubbed the "Great Eruption," the larger of Eta Carinae's stars blasted out at least 10 times the mass of the sun into space in 1838, an explosion that made it the second-brightest star in the sky for a decade. This eruption left behind an expanding veil of gas and dust known as the Homunculus Nebula that still envelops Eta Carinae, the only shroud of its kind known in the Milky Way.

One possible explanation for the Great Eruption is that when massive stars age, they start burning the heavier elements inside them, leading to dramatic spikes of energy that make the stars unstable. "These instabilities can result in thick stellar winds, or even ejection of the outermost portion of the star," said study lead author Rubab Khan, an astrophysicist at NASA Goddard Space Flight Center in Greenbelt, Maryland.

"An alternative idea is that the less massive star in the Eta Carinae binary got too close to the more massive star, and it literally ripped off a portion of the higher mass star," Khan added. "Very recently, one group even proposed that Eta Carinae originally consisted of three stars, and two of those merged, causing the eruption."

Still, while Eta Carinae is probably the best-studied massive star in the sky, "a working model for how this star erupted still eludes us," said astronomer Nathan Smith at the University of Arizona at Tucson. "Its eruption is not something that fits into the standard picture we have from models of stellar evolution."

Solving the mystery of why Eta Carinae erupted could help scientists better understand the lives and deaths of massive stars, where elements heavier than helium are forged. "By better understanding massive star evolution, we can learn about the process of making all the elements and the dust that then made the world we now live in," Khan said.

To find out why the Great Eruption happened and how common such explosions are in the evolution of massive stars, astronomers need more examples of such events. However, such massive stars are rare because their size makes them unstable and relatively short-lived, and the aftermath of such explosions are similarly brief, making the discovery of such eruptions needle-in-a-haystack levels of difficult. "It is a rare phase in the lives of the rarest stars," Khan said.

Now, for the first time, astronomers have detected potential matches for Eta Carinae dubbed "Eta twins."

"Eta Carinae is not a one-off freak of nature — rather, its present state represents a condition that occurs in nature possibly for most or even all very high-mass stars," Khan said.

One in a Few Hundred Billion.

The researchers analyzed visible and infrared light from Eta Carinae. Its shroud absorbs ultraviolet and visible light and reemits it as mid-infrared rays. The researchers then combed through data from the Hubble and Spitzer Space Telescopes to find similar patterns of dimming and brightening.

The scientists focused on four nearby galaxies — M83, NGC 6946, M101 and M51, located 15 million 18.6 million, 21 million and 26 million light-years away from Earth, respectively. These galaxies display high rates of star formation, suggesting they might possess a lot of massive stars.

While an initial search of seven galaxies from 2012 to 2014 failed to find any Eta twins, highlighting their rarity, a follow-up 2015 hunt discovered five candidate Eta twins in four different galaxies — two in M83, and one each in NGC 6946, M101 and M51. These likely each contain high-mass stars hidden behind five to 10 solar masses of gas and dust.

"We expected to first find just one, and then another after some time, and so on," Khan said. "So when we found five in one sweep, after so many years and so many galaxies of not finding any, we were quite startled. In fact we spent quite some time checking, double-checking, and double-double-checking to make sure that everything was in order."

"Khan's recognition of some possibly similar analogs in other galaxies may shed some light on the question of how rare Eta Carinae-like events are," said Smith, who did not take part in this research.

By comparing what researchers know about Eta Carinae and will learn about Eta twins, they can begin solving a number of mysteries surrounding massive stars. "Do all very high mass stars lose a significant portion of their material as they age?" Khan said. "Do all of them lose that mass through a single giant eruption, or through many small eruptions, or through a heavy wind? How long do they live after these eruptions?"

"The next step is to further study the Eta twins with present generation telescopes, and then with the James Webb Space Telescope when it is launched in 2018," Khan said. "We would also like to find more Eta twins, especially in galaxies that are unlike the ones in which we have found them now, to figure out if it takes special physical conditions to produce these objects."

Khan and his colleagues detailed their findings in the Dec. 20 issue of Astrophysical Journal Letters and on Jan. 6 at the American Astronomical Society meeting in Kissimmee, Florida.