Our Milky Way if full of strange objects that blink on us. Recently, a weird signal coming from a strange star system decoded by astronomers. The object is called NGTS-7, and it looks like a single star for most telescopes.
Researchers at the University of Warwick in England began observing because it seemed to emit flares, but they realized that the starlight dims shortly every 16.2 hours after closer examination.
When the astronomers zoomed in, they noticed that in fact there are two similarly sized stars in the system and that only one of them dims slightly in this way. Suggesting that something dark circles on or just above the surface of the star.
Astronomers provide an explanation that a brown dwarf orbits one of the stars in such a narrow orbit that it only takes 16.2 hours to complete.
It’s impressive that the astronomers involved were able to parse this system’s complex signal. Disengaging where the brown dwarf intermixed light and the two tiny, youthful stars initially came from, said Hugh Osborn, an astronomer at the Laboratoire d’Astrophysique de Marseille in France who was not involved in the research.
The researchers used a similar approach to that used to detect exoplanets. Measuring how the light drops as the brown dwarf passed between its host star and Earth. This drop signifies a “transit” signal: a short, partial star eclipse by something too tiny to see directly, even thru a bright telescope.
“Detecting this system is probably the easy bit,” Osborn told Live Science. “Because the star is so small and the brown dwarf is comparatively large. The transit signal is effectively about ten times that of a typical exoplanet that appears in night sky surveys.”
But you need to make sense of it once you detect the transit signal. That’s difficult because the transit signals of brown dwarf are weird. For one thing, from internal heat and the heat of nearby stars, they tend to glow faintly.
“Typical brown dwarf temperatures are somewhere between lukewarm water that would appear black to our eyes and a campfire that would glow slightly red,” Osborn said.
“In case of this system, the brown dwarf is heated by the star it orbits, meaning that the object’s daytime would be glowing red hot. The night side would be darker, but some of that heat would be sucked by winds, heating it up.”
Accounting for all these different variables is challenging for astronomers to find out what you are truly looking at.
“It’s interesting to detect a brown dwarf. The objects are several dozen times bigger than Jupiter, or researchers typically detect large exoplanets, but not quite heavy enough to illuminate like a star with nuclear fusion. They should be easy to spot passing in front of stars because of their big size, Osborn said.”
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But they are rare. Less than 20 transiting stars like this have ever been discovered, and only about 1,000 were discovered elsewhere in the galaxy. In comparison, thousands of exoplanets have already been discovered by astronomers. That’s why astronomers speak about a kind of “brown dwarf desert,” we can obviously observe at least in the region of space.
“The fact that we have so few of them. Must be because they’re incredibly rare, not because we just missed them. This one is particularly strange, even for a brown dwarf, because of its close proximity to its host star.” Osborn said.
It seems to have been driven by gravity from the other star in the system into its tight orbit.
Now it is fully synchronized with its host star, with the two objects spinning and orbiting in such a way that one side of the planet always faces one side of the star as if a string linked them.
“It’s interesting that the brown dwarf’s orbit seems to have ‘spun up’ the star’s orbit. Satellites typically don’t have this impact on their host stars.” Osborn added.
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In this manner, the researchers can say the two objects are synchronized. Because in some observations, other shadows on the surface of that star, likely sunspots, appear to co-rotate on the same 16.2-hour cycle.
The researchers wrote over time that magnetic forces from the host star will slow the orbit of the brown dwarf, causing the orbit to shrink and even more regular transits to occur.
Eventually, the orbit of the brown dwarf should collapse completely in the not-too-distant future (at least in stellar terms) and it will fall into its host star. The resulting fireworks show— image a hot bowling ball slamming into a super-hot plasma huge water balloon — should be amazing for the astronomers alive when it happens.
Meanwhile, Osborn said, he would like to see researchers double-check that in their own wider orbits the two true stars in the system are actually locked together.
The research was published in arXiv.