From EarthSky: “Star’s death spiral into black hole”



March 22, 2017
Eleanor Imster

NASA said on March 20, 2017 that scientists used data from its Swift satellite to get a comprehensive look at a star’s death spiral into a black hole.

NASA/SWIFT Telescope

The star was much like our sun. The black hole contains some 3 million times the mass of our sun and lies at the center of a galaxy 290 million light-years away. As the black hole tore the star apart, it produced what scientists call a tidal disruption event. They’ve labeled this particular event – an eruption of optical, ultraviolet, and X-ray light, which began reaching Earth in 2014 – as ASASSN-14li.

Astronomers report the detection of flows of hot, ionized gas in high-resolution X-ray spectra of a nearby tidal disruption event, ASASSN-14li in the galaxy PGC 43234. This artist’s impression shows a supermassive black hole at the center of PGC 43234 accreting mass from a star that dared to venture too close to the galaxy’s center. Image credit: ESA / C. Carreau.

The scientists have now used Swift’s data to map out how and where these different wavelengths were produced, as the shattered star’s debris circled the black hole. The video animation above is an artist’s depiction of what these scientists believe happened. They said it took awhile for debris from the star to be swallowed up by the black hole.

Dheeraj Pasham, an astrophysicist at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, and the lead researcher of the study, said:

“We discovered brightness changes in X-rays that occurred about a month after similar changes were observed in visible and UV light. We think this means the optical and UV emission arose far from the black hole, where elliptical streams of orbiting matter crashed into each other.”

Their study was published March 15, 2017 in the Astrophysical Journal Letters.

A tidal disruption event happens when a star passes too close to a very massive black hole. ASASSN-14li is the closest tidal disruption discovered in 10 years, so of course astronomers are studying it as extensively as they can. During events like this, tidal forces from a black hole may convert the star into a stream of debris. Stellar debris falling toward the black hole doesn’t fall straight in, however, but instead collects into a spinning accretion disk, encircling the hole.

The accretion disk is the source of all the action, as observed by earthly astronomers.

Within the disk, star material becomes compressed and heated before eventually spilling over the black hole’s event horizon, the point beyond which nothing can escape and astronomers cannot observe.

The animation above, from NASA’s Goddard Space Flight Center illustrates:

… how debris from a tidally disrupted star collides with itself, creating shock waves that emit ultraviolet and optical light far from the black hole. According to Swift observations of ASASSN-14li, these clumps took about a month to fall back to the black hole, where they produced changes in the X-ray emission that correlated with the earlier UV and optical changes.

According to the scientists, the ASASSN-14li black hole’s event horizon is typically about 13 times bigger in volume than our sun. Meanwhile, the accretion disk formed by the disrupted star might extend to more than twice Earth’s distance from the sun.

Bottom line: A team of scientists used observations from NASA’s Swift satellite have mapped the death spiral of a star as it was destroyed by the black hole at the center of its galaxy.

See the full article here .

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