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The Medusa Nebula Is a Spectacular Final Breath for a Dying Star

A technicolor funeral.
Image: ESO

The planetary nebula phase of a star's life is the beginning of the end. Lasting a mere tens of thousands of years—compared to a complete stellar lifetime, which might extend for billions—it's not a last gasp or wheeze, but a brilliant release of ionized gas. This gas, as it reaches ever outward from its source, forms a shell of sorts, a luminous rounded surface resembling a planetary sphere—at least according to the 18th century astronomer William Herschel, who coined the "planetary" misnomer. Much of the process remains something of a mystery.

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This new set of images, collected by the European Southern Observatory's (ESO's) Very Large Telescope in northern Chile as part of its Cosmic Gems program, is of the Medusa Nebula, which is located about 1,500 light-years away in the Gemini constellation. The nebula itself is about four light-years across, which, for reference, is about the distance between Earth and our nearest non-Sun star neighbor, Proxima Centauri. First spotted by astronomers at UCLA in 1955, Medusa was thought to be leftovers from a supernova explosion until Soviet astronomers determined otherwise in the 1970s.

Despite its size and relative nearness, Medusa is quite dim, as are most planetary nebulae. This is simply a feature of the structure's size relative to its total energy. In the image, Medusa's origin star isn't the bright one smack in the middle, but the dimmer blueish point off to the far right. A technicolor cloak for a humble star.

Generally, planetary nebulae are the hallmark of medium to small-sized stars. Stars greater than eight solar masses (eight times as massive as our Sun) usually end in supernovae explosions, spectacular events in which the very guts of a star are expelled in a brief, bright burst of radiation. Smaller stars, like our own Sun, will meet a different fate.

It goes something like this: As a star chews through its hydrogen supplies, creating helium through the process of nuclear fusion, it creates outward pressure, which balances out the inward gravitational pull of the star's mass. When this hydrogen starts to run out, things become unbalanced and the star begins to collapse in on itself. The resulting compression creates extreme heat, which ignites the star's helium ashes. The result is a second wind of extreme heat and a fusing of helium into carbon as the dying star finds a brief new life as a red giant.

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ESO's Very Large Telescope in Chile has captured the most detailed image ever taken of the Medusa Nebula. Image: ESO

"As stellar time goes, the helium won't last long," explains University of Washington astronomer Bruce Balick, using the Sun as an example. "Certainly less than a mere few hundred million years. With its helium transformed into unburnable carbon, the solar core shrinks suddenly—a few thousand years—until just over half the mass of the present Sun is packed into a hot (million degree), dense (a ton per teaspoon) ball the size of the Earth. This amazing stellar remnant is called a white dwarf."

The resulting star is essentially a husk, a furnace without fuel.

"The story shifts from the dying core to the star's distended outer layers. The core, their underlying foundation, now has all but imploded," Balick says. "The outer layers of the Sun fall inward toward the core. But the base material ignites on the way in, causing the outer surfaces to bounce and vibrate. Eventually the outer 40 percent of the Sun's mass will be spasmically 'coughed' into space, floating outward through the solar system and beyond in a concetric set of spherical bubbles … a cloud of smoke which escapes from a burning log as it collapses and crumbles into embers."

The cloud is energized, resulting in a glowing planetary nebula; the green tendrils are emissions from oxygen gas, while the red traces back to helium.

It's likely that planetary nebulae played a crucial role in the chemical evolution of the Milky Way, as elements created within collapsing stars—elements required for things like life—are delivered to the interstellar medium via this expanding surface of gas. We may have a Medusa relative to thank for our very Earthly existence.