Great Balls Of Fire! Hubble Discovers A Real ‘Shooting’ Star

An artist's view of the red giant star V Hydrae and its small companion. The companion orbits through the outer envelope of the star, gathers up material and then shoots it cannonball-style into space every 8.5 years. Credit:NASA, ESA, and A. Feild (STScI)
An artist’s view of the red giant star V Hydrae and its small companion. The companion orbits through the outer envelope of the star, gathers up material and then shoots it cannonball-style into space every 8.5 years. Credit:NASA, ESA, and A. Feild (STScI)

I loved Roman candles as a kid. You know the firework. You light one end and it shoots out balls of fire from the other. I never imagined we’d find the stellar equivalent of this July 4th favorite, but a team of astronomer’s using the Hubble Space Telescope did just that. In a recent paper they report the detection of super-hot blobs of gas, each twice as massive as the planet Mars, being ejected near a dying star. Instead of fire, these balls are made of heated, glowing plasma, hot electrified gas similar to lightning or what makes a neon sign glow.

The plasma balls are zooming so fast through space it would take only 30 minutes for them to travel the 239,000 miles from Earth to the moon. This stellar “cannon fire” has continued once every 8.5 years for at least the past 400 years according to best estimates.

This is how astronomers think V Hydrae shoots its fireballs 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 expanded 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 material 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 eight years, the time it takes for the orbiting star to make another pass through the bloated red giant's envelope, shown in panel 4. Credit: NASA, ESA, and A. Feild (STScI)
This is how astronomers think V Hydrae shoots its fireballs of plasma into space. A companion star’s close orbit takes right through the the giant’s outer envelope where it gobbles up stellar gas. The buildup of material reaches a tipping point and is eventually ejected as blobs of hot plasma along the star’s spin axis. The ejection process is repeated every eight years, the time it takes for the orbiting star to make another pass through the bloated red giant’s envelope. Credit: NASA, ESA, and A. Feild (STScI)

The volleys are coming from an a colorful, red-orange star named V Hydrae in the constellation of Hydra the Water Snake. A small telescope will show just before the start of dawn this month. V Hydrae is a bloated red giant 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 heat up as they plunder their remaining nuclear fuel reserves, expand in size and shed their outer layers into space.

Red giants do a lot of huffing and puffing, but they don’t launch plasma balls. At least not on their own. Instead astronomers believe they’re being blasted by an unseen companion star. According to this theory, the companion would have to be in an elliptical 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 layers, it gobbles up material via its gravitational pull. 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.

A selection of planetary nebulae photographed by the Hubble Space Telescope. Credit: NASA / ESA
A selection of planetary nebulae photographed by the Hubble Space Telescope. Complex shapes seen in some of them may be due to closely-orbiting companion stars similar to the situation in V Hyd. Planetary nebulae get their name because early astronomers likened their rounded shapes to planets. Credit: NASA / ESA

This star system could be the archetype to explain the incredible variety of glowing gas clouds, named planetary nebulae, seen around dying stars. A planetary nebula is an expanding shell of glowing gas expelled by a star late in its life and set to glow like neon in a sign by the light of the dying stellar ember called a white dwarf.

Astronomers observed V Hydrae and its surrounding region over an 11-year period, first from 2002 to 2004, and then from 2011 to 2013 using a spectrograph, a tool that decodes light from an object, revealing information on its velocity, temperature, location, and motion.

V Hydrae, shown here in a drawing by Jeremy Perez, is one of the reddest stars in the sky. Credit: Jeremy Perez
V Hydrae, shown here in a drawing by Jeremy Perez, is one of the reddest stars in the sky. Credit: Jeremy Perez

The data showed a string of monstrous, super-hot blobs, each with a temperature of more than 17,000° F (9,400°C) or almost twice as hot as the surface of the sun. The researchers compiled a detailed map of the blobs’ location, allowing them to trace the first behemoth clumps back to 1986. “The observations show the blobs moving over time,” Sahai said.

The blobs expand and cool as they move farther away, and are then not detectable in visible light. But observations taken at longer sub-millimeter wavelengths in 2004, by the Submillimeter Array in Hawaii, revealed fuzzy, knotty structures that may be blobs launched 400 years ago, the researchers said.

A surprise from the STIS observation was that the disk does not fire the monster clumps in exactly the same direction every 8.5 years. The direction flip-flops slightly from side-to-side to back-and-forth due to a possible wobble in the accretion disk. “This discovery was quite surprising, but it is very pleasing as well because it helped explain some other mysterious things that had been observed about this star by others,” said Raghvendra Sahai of NASA’s Jet Propulsion Laboratory.

Two of the Atacama Large Millimeter/submillimeter Array (ALMA) 12-metre antennas gaze at the sky from the Chilean Andes. Astronomers hope to use these dishes, which can "see" in submilliter light (just beyond the infrared) to probe the plasma balls of the past around V Hydrae. Credit: Iztok Bončina/ESO
Two of the Atacama Large Millimeter/submillimeter Array (ALMA) 12-metre antennas gaze at the sky from the Chilean Andes. Astronomers hope to use these dishes, which can “see” in submilliter light (just beyond the infrared) to probe the plasma balls of the past around V Hydrae. Credit: Iztok Bončina/ESO

The team hopes to use Hubble to conduct further observations of the V Hydrae system, including the most recent blob ejected in 2011. The astronomers also plan to use the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to study blobs launched over the past few hundred years that are now too cool to be detected with Hubble but but still show up in longer wavelengths of light