Tag: supernova

Of White Dwarfs, "Zombie" Stars and Supernovae Explosions

Artistic view of the aftermath of a supernova explosion, with an unexpected white dwarf remnant. These super-dense but no longer active stars are thought to play a key role in many supernovae explosion. (Copyright Russell Kightley)
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White dwarf stars, the remnant cores of low-mass stars that have exhausted all their nuclear fuel, are among the most dense objects in the sky.
 
Their mass is comparable to that of the sun, while their volume is comparable to that of Earth. Very roughly, this means the average density of matter in a white dwarf would be on the order of 1,000,000 times greater than the average density of the sun.
 
Thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star — a category that includes the sun and over 97% of the other stars in the Milky Way — they are dim objects first identified a century ago but only in the last decade the subject of broad study.
 
In recent years the white dwarfs have become more and more closely associated with supernovae explosions, though the processes involved remained hotly debated.  A team using the Hubble Space Telescope even captured  before and after images of what is hypothesized to be an incomplete white dwarf supernova.  What was left behind has been described by some as a “zombie star.”
 
Now a team of astronomers led by Stephane Vennes of the Czech Academy of Sciences has detected another zombie white dwarf, LP-40-365 , that they put forward as a far-flung remnant of a long-ago supernova explosion.  This is considered important and unusual because it would represent a first detection of such a remnant long after the supernova conflagration.
 
This dynamic is well captured in an animation accompanying the Science paper that describes the possible remnant.  Here’s the animation and a second-by-second description of what is theorized to have occurred:
 
 
00.0 sec: Initial binary star outside the disk of the Milky Way galaxy. A massive white dwarf accreting
material through an accretion disk from its red giant companion star. The stars orbit around the center of
mass of the binary system.
 
14.6 sec: The white dwarf reaches the Chandrasekhar mass limit and explodes as a bright Type Ia
supernova. However, the explosion is not perfect; a fraction of the white dwarf shoots out like a shrapnel to the left. The binary system disrupts.
 
18.0 sec: The supernova explosion again, at an edge – on view.
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Supernovae Give, And Can Take Away

What is likely the brightest supernova in recorded human history, SN 1006 lit up planet Earth’s sky in the year 1006 AD. The expanding debris cloud from the stellar explosion, still puts on a cosmic light show across the electromagnetic spectrum. The supernova is located about 7,000 light-years from Earth, meaning that its thermonuclear explosion actually happened 7,000 years before the present day.  Shockwaves in the remnant accelerate particles to extreme energies and are thought to be a source of the mysterious cosmic rays. NASA, ESA, Zolt Levay (STScI)

We live in a dangerous universe. We know about meteor and comets, about harmful radiation that could extinguish life without an electromagnetic shield, about major changes in climate that are both natural and man-made.

There’s another risk out there that some scientists assert could cause large-scale extinctions even though it would occur scores of light-years away.  These are supernovae – explosions of massive stars that both create and spread the heavy elements needed for life and send out high energy cosmic rays that can travel far and cause enormous damage.

As with most of these potential threats, they fortunately occur on geological or astronomical time scales rather than human ones. But that doesn’t mean they don’t happen.

At the recent Astrobiology Science Conference (AbSciCon) a series of talks focused on that last threat – starting with a talk on “When Stars Attack.”

And together five different presenters made a persuasive case that Earth was on the receiving end of a distant supernova explosion some two to three million years ago, and probably around 7 or 8 million years ago as well. The effects of the cosmic ray bombardment have been debated and disputed, but the evidence for the occurrences is based on the rock record and is now strong.

“The evidence is there on the ocean floor:  in rocky crusts, nodules and sediment,” said Brian Fields, professor of astronomy at University of Illinois.  “We’ve been able to date it and provide some idea of how far away the star blew up.”  The answer is between about 90 and 300 light-years.

 

Supernova 1994D exploded on the outskirts of disk galaxy, and outshines even the center of the galaxy. Supernovae may expel much, if not all, of the material away from a star,  at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-moving shock wave into the surrounding interstellar medium that, if close to Earth (or any other planet) can have dire consequences. 

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