Astronomy

The sun could come from a large, loose-knit clan or a small family that’s always fighting.
New computer simulations of young stars suggest two pathways to forming the solar system. The sun could have formed in a calm, large association of 10,000 stars or more, like NGC 2244 in the present-day Rosette Nebula, an idea that’s consistent with previous research. Or the sun could be from a violent, compact cluster with about 1,000 stars, like the Pleiades, researchers report July 2 in the Astrophysical Journal.Whether a star forms in a tight, rowdy cluster or a loose association can influence its future prospects. If a star is born surrounded by lots of massive siblings that explode as supernovas before a cluster spreads out, for example, that star will have more heavy elements to build planets with (SN: 8/9/19).
To nail down a stellar birthplace, astronomers have considered the solar system’s chemistry, its shape and many other factors. Most astronomers who study the sun’s birthplace think the gentle, large association scenario is most likely, says astrophysicist Fred Adams of the University of Michigan in Ann Arbor, who was not involved in the new work.
But most previous studies didn’t include stars’ motions over time. So astrophysicists Susanne Pfalzner and Kirsten Vincke, both of the Max Planck Institute for Radio Astronomy in Bonn, Germany, ran thousands of computer simulations to see how often different kinds of young stellar families produce solar systems like ours.

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The main solar system feature that the pair looked for was the distance to the farthest planet from the star. Planet-forming disks can extend to hundreds of astronomical units, or AU, the distance between the Earth and the sun (SN: 7/16/19). Theoretically, planets should be able to form all the way to the edge. But the sun’s planetary material is mostly packed within the orbit of Neptune.
“You have a steep drop at 30 AU, where Neptune is,” Pfalzner says. “And this is not what you expect from a disk.”
In 2018, Pfalzner and her colleagues showed that a passing star could have truncated and warped the solar system’s outer edge long ago. If that’s what happened, it could help point to the sun’s birth environment, Pfalzner reasoned. The key was to simulate groupings dense enough that stellar flybys happen regularly, but not so dense that the encounters happen too often and destroy disks before planets can grow up.
“We were hoping we’d get one answer,” Pfalzner says. “It turned out there are two possibilities.” And they are wildly different from each other.
Large associations have more stars, but the stars are more spread out and generally leave each other alone. Those associations can stay together for up to 100 million years. Compact clusters, on the other hand, see more violent encounters between young stars and don’t last as long. The stars shove each other away within a few million years.
“This paper opens up another channel for what the sun’s birth environment looked like,” Adams says, referring to the violent cluster notion.
The new study doesn’t cover every aspect of how a tight cluster could have affected the nascent solar system. The findings don’t account for how radiation from other stars in the cluster could erode planet-forming disks, for example, which could have shrunk the sun’s disk or even prevented the solar system from forming. The study also doesn’t explain certain heavy elements found in meteorites, which are thought to come from a nearby supernova and so could require the sun come from a long-lived stellar family.
“I think [the research] is an interesting addition to the debate,” Adams says. “It remains to be seen how the pieces of the puzzle fit together.”
Pfalzner thinks that the star cluster would break apart before radiation made a big difference, and there are other explanations for the heavy elements apart from a single supernova. She hopes future studies will be able to use that sort of cosmic chemistry to narrow the sun’s birthplace down even further.
“For us humans, this is an important question,” Pfalzner says. “It’s part of our history.” More

Get out the marshmallows and toasting sticks. The closest images yet taken of the sun show tiny flares dubbed “campfires,” astronomers announced in a news conference on July 16.
The images are the first from Solar Orbiter, a new sun-watching spacecraft that’s a joint project between NASA and the European Space Agency.
“By looking from close by, we get so much sharper images,” said David Berghmans of the Royal Observatory of Belgium in Brussels in the news conference. The pictures were better than the science team expected. “When the first images came in, the first thought was, ‘This is not possible! It cannot be that good.’”   
These never-before-seen campfire flares are thought to be little relatives of larger solar flares, powerful magnetic outbursts that shoot bright spurts of radiation into space (SN: 9/11/17). Campfire flares are a million to a billion times as small as typical solar flares. The smallest ones in the Solar Orbiter images are a few hundred kilometers across, “about the size of a European country,” Berghmans said. It’s not clear yet whether the flickers are just scaled-down solar flares, or if the two phenomena have different driving mechanisms.

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Solar physicists think campfires could help explain one of the biggest solar mysteries: why the solar corona, the sun’s wispy outer atmosphere, is millions of degrees hotter than the solar surface (SN: 8/20/17). Together, the small but ubiquitous flares could be a source of energy to the corona that astronomers haven’t accounted for.
“These campfires are totally insignificant each by themselves, but summing up their effect all over the sun, they might be the dominant contribution to the heating of the solar corona,” said Frédéric Auchère of the Institut d’Astrophysique Spatiale in Orsay, France, in a news release.
Solar Orbiter captured these pictures of “campfire” flares (indicated with arrows) on the sun in extreme ultraviolet wavelengths of light. The newly spotted flares may help heat the sun’s outer atmosphere.Solar Orbiter/EUI Team/ESA and NASA, CSL, IAS, MPS, PMOD/WRC, ROB, UCL/MSSL
Solar Orbiter launched February 9 with a suite of scientific instruments to observe the sun and its surroundings (SN: 2/9/20). The new images were taken May 30 with the Extreme Ultraviolet Imager camera when the spacecraft was 77 million kilometers from the sun, about half the distance from Earth. Berghmans and Auchère are the principal investigators for the orbiter’s ultraviolet camera.
Other spacecraft have swooped closer to the sun. The Parker Solar Probe has gotten as close as 24 million kilometers, collecting data but no direct photos because it gets too close (SN: 12/4/19). It will eventually reach 6 million kilometers from the sun’s surface. Ultimately, Solar Orbiter will come within about 42 million kilometers of the sun, and will be the first spacecraft to fly over the sun’s poles. More

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