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Supernova eggs, Heaviest elements, South Pole octopuses

Q: In your answer about where oxygen comes from you show a picture of a "ring" of matter formed by a supernova. Shouldn't matter ejected by an explosion form a spherical "ball" or "shell" rather than a "ring"? (Claus, Tucson, Arizona)

The surrounding ring of matter existed before the explosion. The egg-shaped pink mass in the center is the debris from the blast. Supernova 1987A. [NASA, P. Challis; R. Kirshner, Harvard-Smithsonian Institute for Astrophysics; B. Sugerman, STScI]

A: The picture somewhat misleads us because the ring of matter existed before the star exploded.

The progenitor star has a story to tell. First, as a red giant star, it emitted a slow solar wind of ionized particles that streamed into space, as all stars do. Later, before going supernova, it became a blue supergiant star and, of course, kept sending out a solar wind but now a swifter one.

The ring is caused by the impact of the two solar winds — the high-velocity one of the blue supergiant with its earlier, slower red giant wind, says David N. Burrows, astronomer at Pennsylvania State University. The ultraviolet (UV) flash from the supernova explosion, which we first saw 18 years ago, ionized the ring and causes it to glow.

The debris from the explosion, on the other hand, is the egg-shaped red mass shown in the center of the ring. It forms a "ball or shell", as you say, of ejected material but the shape is ellipsoidal, not spherical as you might expect.

Astronomers, until recently, also assumed supernovas explode symmetrically. This results in a spherical blast wave that blasts out matter spherically. But that’s not what happened with this star — and others.

"When we see an egg-shaped blob of ejecta, that tells us that the explosion was not symmetrical. Unfortunately, it doesn’t tell us why. We still have to puzzle that out," says Burrows.

Even a greater puzzle, though, is the ring. "We don’t know why it is in the form of a ring, rather than a sphere." After all, solar winds radiate spherically. So why shouldn’t their collision be spherical?

Something made material "pile up in a ring, but what? Could it be due to stellar rotation? Or to a binary companion? Or even a planetary system? We just don’t know the answer," says Burrows.

By the way, the colliding solar winds are heating the ring fiercely. It’s about 50 million degrees Fahrenheit (28 million Celsius) now. Whereas the "glowing blob in the center is the coldest optically emitting object in the sky — about -300̊F (100̊K)," says Richard McCray, George Gamow Distinguished Professor, emeritus, of astrophysics at the University of Colorado.

Further Reading:

WonderQuest: Where oxygen comes from and "ring" supernova image

Burrows, David N. et al, "The X-Ray Remnant of SN 1987A." The Astrophysical Journal 243(2000):L149-L152

SolStation.com: Supernova 1987 A

Harvard University: SINS, the Supernova INtensive Study by Robert Kirshner

NASA: Hubble supernova 1987A scrapbook (1994 – 2003)

Q: In your Q&A about where oxygen comes from, you said the elements up to iron come from the thermonuclear furnaces of stars. Iron, however, doesn’t give up energy, which causes the star to collapse. Right? Well, where do all the other heavier elements (like uranium) come from? (Dick, Albuquerque, New Mexico)

A red giant star brewing heavier elements. (It’s Betelgeuse — the red star that marks Orion’s shoulder in the winter constellation.) [PRC96-04, ST SCI OPO, A. Dupree (CfA), NASA]

A: When a big star collapses (eight times or greater than our Sun’s mass), it sends a shock wave outward. The star explodes — a supernova.

"The enormous number of neutrons released in a supernova explosion manufacture the heavy elements," says Robert Massey, astronomer at the Royal Observatory Greenwich, London.

With heaps of neutrons around, a nucleus can capture two or more in quick succession and thereby build a heavier element. After chains of beta decay (electron emission), the decay product is a stable neutron-rich nucleus. This synthesizing of heavy elements happens extremely fast during the supernova explosion. The explosion then blasts the elements into interstellar space.

That’s one way we get a lot of heavy elements. Another way is generation within the atmosphere of red-giant stars. These are stars with "enormously distended atmospheres" large enough to encompass Jupiter if placed where our Sun is. Simply huge. Robert Massey provides the following explanation, which he credits to his ex-colleague, Robin Catchpole.

Red giants have gigantic convection cells (like a simmering "super-thin" stew) that carry surface hydrogen deep within the star to its core. Nuclear reactions in the core routinely release floods of neutrons and these neutrons combine with iron nuclei already there — somewhat like the nuclear synthesis that takes place in a supernova, except much slower. The resulting elements (such as, strontium, zirconium, barium, lanthanum, and lead) then decay into still heavier elements.

Further Reading:

WonderQuest: Oxygen comes from stardust.

Royal Observatory Greenwich: Supernova

Q: I read on a restaurant kids’ menu that some Antarctic octopuses have 40 legs? Is this true? (Minneapolis, Minnesota)

Octopus — only 8 legs. [NOAA]

A: An octopus (meaning "8 feet") is well named. Even exotic octopuses from Antarctica have only 8 legs.

Most of the 150 octopus species live in warm waters but a few, like the Parledone, glide in frigid waters of the Antarctic.

The Parledone grow extremely slowly in such cold. They also respire (breathe) much slower than those in the North Sea.

Further Reading:

British Antarctica Survey: Energy balance and cold adaptation in the octopus Pareledone charcoti

University of California at San Diego: Underwater field guide to Ross Island & McMurdo Sound, Antarctica

(Answered March 18, 2005)