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Alnitak Region with the Horse Head and Flame Nebulae

by Warren Keller

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Autumn Colours

            ↳ Orange

Massive stars end their lives with a bang: exploding as spectacular supernovas, they release huge amounts of mass and energy into space. These explosions sweep up any surrounding material, creating bubble remnants that expand into interstellar space. At the heart of bubbles like these are small, dense neutron stars or black holes, the remains of what once shone brightly as a star.

Credit: ESA/XMM-Newton/ L. Oskinova/M. Guerrero; CTIO/R. Gruendl/Y.H. Chu

This image shows knots of cold, dense interstellar gas where new stars are forming. These Free-floating Evaporating Gaseous Globules (frEGGs) were first seen in Hubble’s famous 1995 image of the Eagle Nebula. Because these lumps of gas are dark, they are rarely seen by telescopes. They can be observed when the newly forming stars ignite, their intense ultraviolet radiation eroding the surrounding gas away and letting the denser, more resistant frEGGs remain. These frEGGs are located in the Northern Coalsack Nebula in the direction of Cygnus.

Credit: NASA, ESA, and R. Sahai (Jet Propulsion Laboratory); Processing: Gladys Kober (NASA/Catholic University of America)

COSMOS: Possible Worlds 2020

Black Holes Dine on Stellar Treats!

See that tiny blob of light, circled in red? Doesn’t look like much, does it? But that blob represents a feast big enough to feed a black hole around 30 million times the mass of our Sun! Scientists call these kinds of stellar meals tidal disruption events, and they’re some of the most dramatic happenings in the cosmos.

Sometimes, an unlucky star strays too close to a black hole. The black hole’s gravity pulls on the star, causing it to stretch in one direction and squeeze in another. Then the star pulls apart into a stream of gas. This is a tidal disruption event. (If you’re worried about this happening to our Sun – don’t. The nearest black hole we know about is over 1,000 light-years away. And black holes aren’t wild space vacuums. They don’t go zipping around sucking up random stars and planets. So we’re pretty safe from tidal disruption events!)

The trailing part of the stream gets flung out of the system. The rest of the gas loops back around the black hole, forming a disk. The material circling in the disk slowly drifts inward toward the black hole’s event horizon, the point at which nothing – not even light – can escape. The black hole consumes the gas and dust in its disk over many years.

Sometimes the black hole only munches on a passing star – we call this a partial tidal disruption event. The star loses some of its gas, but its own gravity pulls it back into shape before it passes the black hole again. Eventually, the black hole will have nibbled away enough material that the star can’t reform and gets destroyed.

We study tidal disruptions, both the full feasts and the partial snacks, using many kinds of telescopes. Usually, these events are spotted by ground-based telescopes like the Zwicky Transient Facility and the All-Sky Automated Survey for Supernovae network.

They alert other ground- and space-based telescopes – like our Neil Gehrels Swift Observatory (illustrated above) and the European Space Agency’s XMM-Newton – to follow up and collect more data using different wavelengths, from visible light to X-rays. Even our planet-hunting Transiting Exoplanet Survey Satellite has observed a few of these destructive wonders!

We’re also studying disruptions using multimessenger astronomy, where scientists use the information carried by light, particles, and space-time ripples to learn more about cosmic objects and occurrences.

But tidal disruptions are super rare. They only happen once every 10,000 to 100,000 years in a galaxy the size of our own Milky Way. Astronomers have only observed a few dozen events so far. By comparison, supernovae – the explosive deaths of stars – happen every 100 years or so in a galaxy like ours.

That’s why scientists make their own tidal disruptions using supercomputers, like the ones shown in the video here. Supercomputers allow researchers to build realistic models of stars. They can also include all of the physical effects they’d experience whipping ‘round a black hole, even those from Einstein’s theory of general relativity. They can alter features like how close the stars get and how massive the black holes are to see how it affects what happens to the stars. These simulations will help astronomers build better pictures of the events they observe in the night sky.

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What drives auroras on Saturn? To help find out, scientists have sorted through hundreds of infrared images of Saturn taken by the Cassini spacecraft for other purposes, trying to find enough aurora images to correlate changes and make movies. Once made, some movies clearly show that Saturnian auroras can change not only with the angle of the Sun, but also as the planet rotates. Furthermore, some auroral changes appear related to waves in Saturn's magnetosphere likely caused by Saturn's moons. Pictured here, a false-colored image taken in 2007 shows Saturn in three bands of infrared light. The rings reflect relatively blue sunlight, while the planet itself glows in comparatively low energy red. A band of southern aurora in visible in green. In has recently been found that auroras heat Saturn's upper atmosphere. Understanding Saturn's auroras is a path toward a better understanding of Earth's auroras.

Image Credit: NASA, Cassini, VIMS Team, U. Arizona, U. Leicester, JPL, ASI