Free Astronomy Magazine November-December 2016

SPACE CHRONICLES Hubble detects giant ‘cannonballs’ shooting from star by NASA G reat balls of fire! NASA's Hub- ble Space Telescope has de- tected superhot blobs of gas, each twice as massive as the planet Mars, being ejected near a dying star. The plasma balls are zooming so fast through space it would take only 30 minutes for them to travel from Earth to the Moon. This stellar “can- non fire” has continued once every 8.5 years for at least the past 400 years, astronomers estimate. The fireballs present a puzzle to astronomers, because the ejected material could not have been shot out by the host star, called V Hy- drae. The star is a bloated red giant, residing 1,200 light-years away, which has probably shed at least half of its mass into space during its death throes. Red giants are dying stars in the late stages of life that are exhausting their nuclear fuel that makes them shine. They have expanded in size and are shedding their outer layers into space. The current best explanation sug- gests the plasma balls were launch- ed by an unseen companion star. According to this theory, the com- panion would have to be in an ellip- tical orbit that carries it close to the red giant's puffed-up atmosphere every 8.5 years. As the companion enters the bloated star's outer atmo- T his four-panel graphic illustrates how the binary-star system V Hydrae is launch- ing balls of plasma into space. Panel 1 shows the two stars orbiting each other. One of the stars is nearing the end of its life and has swelled in size, becoming a red giant. In panel 2, the smaller star's orbit carries the star into the red giant's expand- ed atmosphere. As the star moves through the atmosphere, it gobbles up material from the red giant, which settles into a disk around the star. The buildup of mate- rial reaches a tipping point and is eventually ejected as blobs of hot plasma along the star's spin axis, shown in panel 3. This ejection process is repeated every 8.5 years, the time it takes for the orbiting star to make another pass through the bloated red giant's envelope, shown in panel 4. [NASA, ESA, and A. Feild (STScI)] sphere, it gobbles up material. This material then settles into a disk around the companion, and serves as the launching pad for blobs of plasma, which travel at roughly a half-million miles per hour. This star system could be the archetype to ex- plain a dazzling variety of glowing shapes uncovered by Hubble that are seen around dying stars, called plan- etary nebulae, researchers say. A pla- netary nebula is an expanding shell of glowing gas expelled by a star late in its life. “We knew this object had a high-speed outflow from previous data, but this is the first time we are seeing this process in action,” said Raghvendra Sahai of NASA's Jet Pro- pulsion Laboratory in Pasadena, Cal- ifornia, lead author of the study. “We suggest that these gaseous blobs produced during this late phase of a star's life help make the structures seen in planetary nebu- lae.” Hubble observations over the past two decades have revealed an enormous complexity and diversity of structure in planetary nebulae. The telescope's high resolution cap- tured knots of material in the glow-