This week in pictures: Astronomers have watched auroral storms at Jupiter, imaged strands of the cosmic web, and seen stellar fireworks off the sword of Orion.

Juno Watches Dawn Storm at Jupiter

In the video above, NASA's Juno spacecraft offers us a polar view of Jupiter's auroral oval, usually seen only side-on in images from the Hubble Space Telescope or ground-based observatories.

As on Earth, Jupiter always has an oval of auroras around its poles, where ionized particles interact with the planet's uppermost atmosphere. But while the charged particles raining down around Earth's poles come directly from the solar wind, at Jupiter most of those particles originate from its highly volcanic moon Io.

Juno's polar view offers a unique chance to understand dawn storms, during which the aurora brighten dramatically. These storms start on the nightside, which is out of view from Earth. Bertrand Bonfond (University of Liège, Belgium) and his colleagues looked at data from Juno's first 20 orbits around Jupiter, piecing together the details about the storms' typical progression.

The researchers conclude that dawn storms at Jupiter resemble substorms on Earth, in which the magnetic field in Earth's magnetotail pinches off, sending a bunch of particles hurtling toward our planet's poles. It's possible the same process is at work at both planets, despite their differences.

Read more details in the study, published in AGU Advances, or in NASA's press release.

The Silken Strands of the Cosmic Web

A piece of the cosmic web
An image of some two billion years after the Big Bang in the constellation Fornax (Oven). Each point of light is an entire galaxy. The blue silk of the cosmic web was discovered with MUSE. The gas extends over a distance of 15 million light years. That's roughly equivalent to 150 times our Milky Way placed back to back.
ESO / NASA / Roland Bacon et al.

Astronomers have discovered new filaments of the cosmic web between 8 million and 13 million light-years long.

Simulations of the universe's evolution show that galaxies and groups of galaxies tend to form along filaments that line vast cosmic voids, forming a large-scale structure that resembles a three-dimensional spiders' web. The gas in these filaments is so spread out, it emits almost no light. Astronomers can only see the parts of the web nearest galaxies, where stars' ultraviolet radiation lights up the filaments. In this way, some teams have recently managed to image parts of the cosmic web directly.

In the latest of such efforts, Roland Bacon (CNRS, Centre de Recherche Astrophysique de Lyon, France) and colleagues pointed the Multi Unit Spectroscopic Explorer (MUSE) instrument at a section of the Hubble Ultra Deep Field for six nights last August. MUSE captured emission from ionized hydrogen gas roughly 2 billion years after the Big Bang.

Most of this hydrogen is associated with tiny galaxies, themselves unseen, but it could be that some of it extends between the galaxies, belonging to the cosmic web. In any case, the tiny galaxies seem to trace the filament, even if we're not seeing the filament itself.

This study will appear in Astronomy & Astrophysics (preprint available here). In the meantime, read more in the NOVA press release.

Fireworks Off the Sword of Orion

Infant stars in Orion
These Hubble images of the regions around newborn stars in the Orion Complex have a resolution of about 80 astronomical units, so the finest details visible in these images are roughly the size of the solar system.
NASA / ESA / STScI / N. Habel and S. T. Megeath (University of Toledo)

The newborn stars seen above are growing in the Orion Complex, which at roughly 1,000 light-years away is the nearest nursery of massive stars. Nolan Habel (University of Toledo) and colleagues used data from NASA's Hubble and Spitzer Space Telescopes as well as the European Space Agency's Herschel observatory to investigate the areas around 304 protostars at different stages of evolution. Their goal was to piece together the stars' effects on their environment.

Massive stars are generally thought to drive such powerful winds and jets as they grow that they clear out a cavity around themselves, ultimately driving away the fuel of their own formation. However, when Habel's team studied the cleared-out spaces around stellar newborns in the Orion Complex, they found that those cavities didn't widen over different stages of evolution as predicted.

"Our observations indicate there is no progressive growth that we can find, so the cavities are not growing until they push out all of the mass in the cloud," Habel explains.

That's puzzling, though, because astronomers already know that star formation isn't terribly efficient — only 30% of the gas in a star-forming cloud will actually end up in stars. "There must be some other process going on that gets rid of the gas that doesn't end up in the star," Habel adds.

The study will appear in the Astrophysical Journal (preprint available here); more information is available in the Hubblesite's press release.


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