FYFD

Tag: astrophysics

  • Escaping the Sun

    Escaping the Sun

    One enduring mystery of the solar wind — a stream of high-energy particles expelled from the sun — is how the particles get accelerated in the first place. The sun frequently belches out spurts of plasma, but without further momentum, that material simply falls back to the sun’s surface under the star’s gravity. Mechanisms like shock waves can further accelerate particles that are already moving quickly, but they cannot explain how the particles get going in the first place.

    A recent study used supercomputers to tackle this challenging problem in turbulent plasma physics. Each simulation tracked nearly 200 billion particles, requiring tens of thousands of processors. The results showed that turbulence itself provides the necessary initial acceleration and serves as the first step to getting particles moving fast enough to escape the sun. (Image credit: NASA SDO; research credit: L. Comisso and L. Sironi; via Physics World)

  • Turbulence in Accretion Disks

    Turbulence in Accretion Disks

    Accretion disks form everywhere, from around young, planet-building stars to massive black holes. As matter circles in the disk, it slowly loses angular momentum and falls inward toward the central gravitational body. But the details of this process have long vexed astronomers. The low-viscosity environment of gas and dust in accretion disks simply is not sufficient to account for the level of angular momentum lost. Turbulence is expected to provide a boost to the effect, but neither astronomical observations or Taylor-Couette experiments have shown how.

    A new study uses a liquid metal, confined in a disk using radial and vertical electrical fields. Unlike prior experiments, this set-up creates a more gravity-like force to rotate the liquid. With it, researchers can tune both the rotation speed and the level of turbulence. They found that turbulence is, indeed, responsible for the loss of angular momentum that transports mass inward — even at the limit of zero intrinsic viscosity.

    Unfortunately, the apparatus isn’t a perfect analog for astrophysical disks; in the experiment, the turbulence originates from secondary flows that aren’t present in real systems. So while the team demonstrated that turbulence can drive the accretion disk’s behavior, it can’t pinpoint where that turbulence originates in real accretion disks. (Image credit: NASA; research credit: M. Vernet et al.; via Physics World)

  • Witch’s Broom

    Witch’s Broom

    Known by many names — including the Witch’s Broom Nebula — NGC 6960 is part of a supernova remnant visible in the constellation Cygnus. The wisp-like filaments of the nebula are shock waves moving through the cloud of dust and ionized gas. Based on observations using the Hubble Space Telescope, the nebula is expanding at around 1.5 million kilometers per hour. When the original supernova exploded thousands of years ago, astrophysicists estimate it would have been bright enough to see during daytime! (Image credit: K. Crawford)

  • Betelgeuse’s Flickering

    Betelgeuse’s Flickering

    Between November 2019 and March 2020 Betelgeuse, the red supergiant star in the constellation Orion’s left shoulder, experienced what’s being called the Great Dimming. Usually, the star is one of the ten brightest stars in the sky, often visible even in the suburban sprawl. But as of February 2020, it had dimmed by a factor of 2.5.

    Observers speculated all sorts of causes, including the idea that this was a precursor to a supernova explosion. Instead, it’s a relatively normal occurrence for a star like Betelgeuse. The image above is from a numerical simulation of the star, and it shows approximately what it would look like to the human eye over a 7.5 year time span. As you can see, its brightness varies noticeably, and its surface seems almost to boil. This has to do with convection in the star. Compared to a star like our sun, Betelgeuse has fewer — and much larger — convection cells.

    With a little more time and data, astronomers pinned down the exact source of Betelgeuse’s flickering during the Great Dimming. The year before the star belched an enormous bubble of gas into space. Then, when part of the star cooled in the aftermath, that gas condensed and formed a dust cloud which partially obscured the star. You can see an artist’s conception of the situation in the video below. (Image and research credit: B. Freytag; research credit: M. Montargès et al.; video credit: ESO/L. Calçada)

  • Eye of the Stellar Storm

    Eye of the Stellar Storm

    AG Carinae is a bright, unstable luminous blue variable star. This rare type of star lives fast and dies young (by stellar standards) over only a few million years. During that time, it will occasionally blow off its outer layers in a violent eruption as a result of the ongoing tug of war between its radiation pressure and gravity. That’s the source for the nebula we see surrounding the star in this image. The red areas of the image are a mixture of hydrogen and nitrogen gas; the blue clumps are cooler pockets of dust shaped by the hotter, faster-moving stellar wind. Zoom in on the image and you can see amazing structural detail in the nebula, evidence of turbulence on a scale of light-years. (Image credit: NASA/ESA/STScI; via Gizmodo)

  • Brown Dwarfs and Their Stripes

    Brown Dwarfs and Their Stripes

    Brown dwarfs are neither stars nor gas giants but something in between. Our two nearest brown dwarf neighbors are roughly equivalent to Jupiter in size but about 30 times more massive. Since these objects are so dim, little is known about their structure. Do they resemble stars in their atmospheric patterns or gas giants like Jupiter?

    To find out, a team of researchers studied two nearby brown dwarfs with the Transiting Exoplanet Survey Satellite. They were able to map the objects’ varying lightcurves and model an upper atmosphere consistent with those observations. They found that both dwarfs have high-speed winds running parallel to their equators, meaning that they likely have stripes like Jupiter. The similarities even extended to the brown dwarfs’ poles, where — like on Jupiter — the atmosphere became dominated by local vortices. (Image credit: NASA/JPL; video credit: Steward Observatory; research credit: D. Apai et al.; via Gizmodo)

  • Eyes on the Sun

    Eyes on the Sun

    Though it may look like the Eye of Sauron, this image is actually one of our best-ever glimpses of a sunspot. Captured by the Daniel K. Inouye Solar Telescope, this sunspot is larger than our entire planet, yet we can see details as small as 20km across. The dark central region of the image is the sunspot’s umbra, surrounded by the lighter, streakier penumbra. Along the edges of the image, you see a more typical pattern of bright convection cells. Compared to the rest of the sun’s surface, sunspots are cool — about 1,000 K cooler — due to their intense magnetic field flux inhibiting convection. (Image credit: NSO/AURA/NSF; via Bad Astronomer; submitted by Kam-Yung Soh)

  • Chaos in the Lagoon Nebula

    Chaos in the Lagoon Nebula

    Even on the scale of light-years, fluid dynamics plays a role in our universe. This photograph shows the Lagoon Nebula, where stars, gas, and dust are battling for supremacy. Jets from young stars push the dust left from supernova remnants into a chaotic patterns, and the high-energy particles streaming from the youthful stars illuminate interstellar gases, creating the nebula’s distinctive glow. This section of the nebula is about 50 light-years across, so every picture we capture is only the tiniest snapshot of the true scale of its turbulence. (Image credit: Z. Wu; via APOD)

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    Mimicking Supernovas

    The Hubble archives are full of incredible swirls of cosmic gas and dust, many of which were born in supernovas. Predicting the forms these massive explosions will generate is extremely difficult, thanks in large part to the complicated fluid dynamics generated by their blast waves. But new lab-scale experiments may help shed light on those underlying processes.

    Researchers mimic supernovas in the lab by launching blast waves through an interface between a dense gas (shown in white) and a lighter one (which appears black). As the blast wave passes, it drives the dense fluid into the lighter one, triggering a series of instabilities. Notice how any initial perturbations in the interface quickly grow into mushroom-like spikes that rapidly become turbulent. This behavior is exactly what’s seen in supernovas (and in inertial confinement fusion)! (Video credit: Georgia Tech; research credit: B. Musci et al.; submitted by D. Ranjan)

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    Shock Waves Drive Nova Brightening

    New observations of nova V906 Carinae have provided some of the first direct evidence that the observed brightening of these stellar objects is driven by shock waves. Novae form when hydrogen from a companion star settles onto a white dwarf. Once enough material accumulates, the white dwarf blows out the excess hydrogen in a donut-shaped shell moving about the speed of a typical solar wind.

    Next, another outflow — likely triggered by residual nuclear reactions on the dwarf’s surface — slams into the denser shell at about twice the speed. This collision triggers shock waves that emit light in the gamma and visible wavelengths. Weeks later, a third, even faster outflow expanded into the cloud, generating more shock waves and measurable flares. (Video credit: NASA Goddard; research credit: E. Aydi et al.)