Located about 7,500 light-years away in the southern constellation of Carina, Eta Carinae comprises two massive stars whose eccentric orbits bring them unusually close every 5.5 years. Both produce powerful gaseous outflows called stellar winds, which enshroud the stars and stymy efforts to directly measure their properties. Astronomers have established that the brighter, cooler primary star has about 90 times the mass of the sun and outshines it by 5 million times. While the properties of its smaller, hotter companion are more contested, the star has about 30 solar masses and emits a million times the sun’s light.
The stars are so massive and hot that they emit a never-ending stream of charged particles called stellar winds, which travel at velocities of up to ten million kilometers per hour. As they approach one another, their winds collide and form hot plasma that can emit X-rays. This time, a Hiroshima University team confirmed the collision also forms shock waves. The energy of the particles shooting back and forth gets higher and higher, and they accelerate to nearly the speed of light. Then, these ultrahigh-energy particles, or cosmic rays, escape the shock region and scatter throughout space.
The power of the Eta Carinae binary pair creates dramatic phenomena. A “Great Eruption” in the system was observed by astronomers in the 1830s. We now know that this was caused by the larger star of the pair expelling huge amounts of gas and dust in a short amount of time, which led to the distinctive lobes, known as the Homunculus Nebula, that we see in the system today. The combined effect of the two stellar winds as they smash into each other at extreme speeds is to create temperatures of millions of degrees and intense deluges of X-ray radiation. The central area where the winds collide is so comparatively tiny –a thousand times smaller than the Homunculus Nebula (shown at the top of the page).
An international collaboration operating NASA’s NuSTAR satellite has revealed that two of the biggest stars in the galaxy are capable of creating cosmic rays. Their results were published in Nature Astronomy this month.
In the time it takes you to read this sentence, hundreds of cosmic rays have pummeled through our bodies. Cosmic rays are mostly made of protons and electrons, with the smallest fraction made of X-rays and gamma rays. These jets of high-energy particles not only make up a sizable portion of radiation astronauts and airplane pilots receive, but they also can reach the ends of the galaxy.
Scientists have found that cosmic rays can come from places like supernova remnants, neutron stars, or solar flares from the sun. Still, their origins are a mystery. They could come from other parts of outer space. However, it has not been easy figuring out where.
“Since these particles are electrically charged, they wander when in the presence of magnetic fields,” Hiromitsu Takahashi said. “This means we could not tell precisely where they are coming from when observed them from Earth.” Takahashi is an astrophysicist at Hiroshima University and coauthor on this study.
Takahashi and his team were interested in Eta Carinae, a binary star system 7,500 light years away from Earth. The two stars are massive – one is thirty times heavier than our sun, the other ninety – and thought to be a source of cosmic rays. The team built upon findings from an experiment conducted with the Fermi Gamma-ray telescope, in which they discovered a source of gamma rays coming from around the area of Eta Carinae.
However, the resolution of the images they collected was not clear enough to confirm whether these rays were coming this star system or somewhere else. “We had to come up with a different way of finding their source – by measuring X-rays and gamma rays with a more sensitive detector,” Takahashi said.
For this experiment, Takahashi and his colleagues observed Eta Carinae through NuSTAR, a recently launched X-ray satellite from NASA. Fermi can resolve images at about one degree, or twice as large as the angular diameter of a full moon as viewed from Earth. NuSTAR, on another hand, has a one-twentieth more precise angular resolution of Fermi – similar to that of the human eye.
Because of NuSTAR, Takahashi’s team not only confirmed that the gamma rays are coming from Eta Carinae – as they suspected in the Fermi mission – but they also deduce how cosmic rays come out of Eta Carinae.
In 2015, a long-term study led by astronomers at NASA’s Goddard Space Flight Center, used NASA satellites, ground-based telescopes and theoretical modeling to produce the most comprehensive picture of Eta Carinae to date. New findings include Hubble Space Telescope images that show decade-old shells of ionized gas racing away from the largest star at a million miles an hour, and 3-D models that reveal never-before-seen features of the stars’ interactions.
“We are coming to understand the present state and complex environment of this remarkable object, but we have a long way to go to explain Eta Carinae’s past eruptions or to predict its future behavior,” said Goddard astrophysicist Ted Gull, who coordinated a research group that has monitored the star for more than a decade
At closest approach, or periastron, the stars are 140 million miles (225 million kilometers) apart, or about the average distance between Mars and the sun. Astronomers observe dramatic changes in the system during the months before and after periastron. These include X-ray flares, followed by a sudden decline and eventual recovery of X-ray emission; the disappearance and re-emergence of structures near the stars detected at specific wavelengths of visible light; and even a play of light and shadow as the smaller star swings around the primary.
During the past 11 years, spanning three periastron passages, the Goddard group has developed a model based on routine observations of the stars using ground-based telescopes and multiple NASA satellites. “We used past observations to construct a computer simulation, which helped us predict what we would see during the next cycle, and then we feed new observations back into the model to further refine it,” said Thomas Madura, a NASA Postdoctoral Program Fellow at Goddard and a theorist on the Eta Carinae team.
Eta Carinae’s great eruption in the 1840s created the billowing Homunculus Nebula, imaged here by Hubble. Now about a light-year long, the expanding cloud contains enough material to make at least 10 copies of our sun. Astronomers cannot yet explain what caused this eruption.
Both of the massive stars of Eta Carinae may one day end their lives in supernova explosions. For stars, mass is destiny, and what will determine their ultimate fate is how much matter they can lose — through stellar winds or as-yet-inexplicable eruptions — before they run out of fuel and collapse under their own weight.
For now, the researchers say, there is no evidence to suggest an imminent demise of either star. They are exploring the rich dataset from the 2014 periastron passage to make new predictions, which will be tested when the stars again race together in February 2020.
The Daily Galaxy via Hiroshima University and NASA/Goddard Space Flight Center