For the first time ever, astronomers have witnessed a coronal mass ejection (CME) on a star other than our very own Sun. The star, named HR 9024 (and also known as OU Andromeda,) is about 455 light years away, in the constellation Andromeda. It’s an active, variable star with a strong magnetic field, which astronomers say may cause CMEs.
CMEs are an ejection of plasma and other material from the solar corona. They often follow a solar flare and are associated with active regions on a star’s surface. If the ejection of material is near the surface of the star, it’s called a solar prominence. If the material travels further than that, it’s called a CME. CMEs aren’t rare on our own Sun.
The new study outlining this work appears in the journal Nature Astronomy. The team behind the study is led by Costanza Argiroffi from the University of Palermo in Italy, who is also an associate researcher at the National Institute for Astrophysics in Italy. This CME detection on another star is significant because it’s the first one. They’re extremely difficult to detect, other than on the Sun, because of the spatial resolution required to see them.
CMEs are caused by the electromagnetic force lines of a star. When those lines become twisted into helical shapes, the energy becomes chaotic, and CMEs act like a kind of release for the energy. Astrophysicists think that without CMEs, stars would simply rip themselves apart.
The team used the Chandra X-Ray Observatory in this study, and the High-Energy Transmission Grating Spectrometer, or HETGS, aboard Chandra. That instrument is able to measure the motions of coronal plasmas with speeds of just a few tens of thousands of miles per hour, like this one from HR 9024. It’s the only instrument capable of seeing something like this. The CME wasn’t detected visually; it was observed when Chandra detected an extremely powerful flash of x-rays. The intense x-ray flash preceded the CME.
“The technique we used is based on monitoring the velocity of plasmas during a stellar flare,” said Costanza Argiroffi (University of Palermo in Italy and associate researcher at the National Institute for Astrophysics in Italy) who led the study. “This is because, in analogy with the solar environment, it is expected that, during a flare, the plasma confined in the coronal loop where the flare takes place moves first upward, and then downwards reaching the lower layers of the stellar atmosphere. Moreover, there is also expected to be an additional motion, always directed upwards, due to the CME associated with the flare”.
The CME coming from HR 9024 is much more powerful than anything our Sun can produce. It was about 10,000 times larger than the most massive ones ever seen from our Sun. The CME expelled about two billion billion (not a typo) pounds of material into space. But it’s not noteworthy just because of its strength. The observation of this CME aligns very well with theory, something that always excites astronomers.
The observations show some of the inner working of flares and CMEs. During the flare, extremely hot material, between 10 to 25 million degrees Celsius (18 to 45 million degrees Fahrenheit), rises then drops at speeds between 360,000 and 1,450,000 kmh (225,000 to 900,000 mph.) These measurements agree with predictions that stem from stellar theory.
“This result, never achieved before, confirms that our understanding of the main phenomena that occur in flares is solid,” said Argiroffi in a press release. “We were not so confident that our predictions could match in such a way with observations, because our understanding of flares is based almost completely on observations of the solar environment, where the most extreme flares are even a hundred thousand times less intense in the X-radiation emitted.”
“The most important point of our work, however, is another: we found, after the flare, that the coldest plasma — at a temperature of ‘only’ seven million degrees Fahrenheit — rose from the star, with a constant speed of about 185,000 miles per hour,” said Argiroffi in a press release. “And these data are exactly what one would have expected for the CME associated with the flare.”
The size of the CME revealed in the Chandra data dwarfed that of the Sun. The observations show that in very active stars like HR 9024, CMES are large-scale versions of CMEs we see in our own Sun. But the speed of the CME is much lower than expected. This suggests that the magnetic field in the active stars is probably less efficient in accelerating CMEs than the solar magnetic field.
HR 9024 itself is an interesting star. It’s a giant star, in stellar terminology, even though it’s “only” 2.86 solar masses, and 9.46 solar radii. It also has an unusually high rate of spin for a star its age. Some astronomers think that it may have engulfed a nearby Hot Jupiter, which gave it its high rate of spin. In contrast to our Sun, it exhibits almost constant flaring, an effect of its strong magnetic field.
HR 9024’s corona is dominated by strong, looping magnetic structures, and up to 30% of the star’s surface shows solar activity. As far back as 2003, astronomers hypothesized that these interacting looping structures cause flaring that’s responsible for heating the coronal material to such high temperatures.
Over time, the spin rate of HR 9024 is expected to decrease, which should reduce the power of its flares and CMEs. Maybe we’ll be around long enough to watch and see.
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