This fusion staves off core collapse for a time—but only until the core is composed largely of iron, which can no longer sustain star fusion. In a microsecond, the core may reach temperatures of billions of degrees Celsius. Iron atoms become crushed so closely together that the repulsive forces of their nuclei create a recoil of the squeezed core—a bounce that causes the star to explode as a supernova and give birth to an enormous, superheated, shock wave.
Supernovae also occur in binary star systems. Smaller stars, up to eight times the mass of our own sun, typically evolve into white dwarves. A star condensed to this size, about that of Earth, is very dense and thus has strong enough gravitational pull to gather material from the system's second star if it is close enough. If a white dwarf takes on enough mass it reaches a level called the Chandrasekhar Limit. At this point the pressure at its center will become so great that runaway fusion occurs and the star detonates in a thermonuclear supernova.
A supernova can light the sky up for weeks, and the massive transfer of matter and energy leaves behind a very different star. Typically only a tiny core of neutrons, a spinning neutron star , is left to evidence a supernova.
Neutron stars give off radio waves in a steady stream or, as pulsars, in intermittent bursts. If a star was so massive at least ten times the size of our sun that it leaves behind a large core, a new phenomenon will occur.
Because such a burned-out core has no energy source to fuse, and thus produces no outward pressure, it may become engulfed by its own gravity and turn into a cosmic sinkhole for energy and matter—a black hole. All rights reserved. Star Fusion But massive stars, many times larger than our own sun, may create a supernova when their core's fusion process runs out of fuel.
Share Tweet Email. I try to reassure myself that our special effects expert just finished working on Transformers 3, directed by action movie hack Michael Bay. At least he has the most practice. Cameras roll. I arm the switch. A shock wave hits me like a wall, almost knocking me off my feet. I stumble backward to catch my footing. But the point of this shot is to catch my reaction, so the producer is waving his hands trying to quiet everyone else down. We scramble to catch the rising smoke ring on camera—a product of our own little mushroom cloud—as I try to switch from primal monkey to composed astrophysicist to explain it.
They respond that it was too powerful: The camera was saturated, and they just have white frames. Another balloon is filled. The mixture is tweaked, and the camera is stopped down.
When I hit the detonator again, the result is a gorgeous fireball—a work of explosive art. When the episode went to air, the beautiful explosion was paired with my expletive-laden, stumbling reaction to the first, more powerful one.
Figure 2. Will the next movie surpass explosions? Like Michael Bay, television producers know that more explosions lead to higher ratings Figure 2. But the ostensible reason was to dramatize supernovae, stellar explosions so distant that we never resolve the expanding fireball.
And yet it is wildly, completely wrong. The explosion we filmed released about million joules of energy. But a kind of supernova called a Type Ia supernova explodes with energy of 10 44 joules—an obscene factor of 10 36 more powerful see the sidebar for more about the different types of supernovae.
Explosions this powerful are incomprehensible to us on a basic level, because we seldom experience the ratio of any two quantities this extreme on Earth, where our brains evolved. Perhaps no performance in the history of cinema or television has failed to dramatize its subject by a greater margin. A supernova would easily eradicate any life unfortunate enough to be in the vicinity. They shine as bright as 6 billion Suns at once, and can be seen across nearly the entire observable universe.
There are other differences, too: the terrestrial explosion was powered by rearranging the chemical bonds in acetylene C 2 H 2 and oxygen O 2. Before it becomes a Type Ia supernova, a white dwarf star is an Earth-sized sphere of carbon and oxygen. In a split second, it becomes a hellish fireball of radioactivity and newly cooked elements, flying apart at a tenth the speed of light.
In the time it takes you to sneeze, a shock wave rips through the 8,mile-wide star, fusing the carbon and oxygen into heavier and heavier elements, climbing the periodic table to nickel. Above their heads, stars such as the Sun were converting hydrogen to helium, and helium to carbon.
And every second, somewhere in the universe, a supernova was exploding and synthesizing in a flash most of the basic ingredients for humans. Although many of these elements are also created in stars, most of the iron in your blood was created in supernovae. Figure 3. In a Type Ia supernova, massive amounts of radioactive nickel, 56 Ni, are synthesized, nearly as much as the mass of the Sun.
But this cosmic jackpot is fleeting; in a week most of the nickel decays into cobalt, specifically 56 Co. Within a few months, most of the cobalt turns into iron 56 Fe. For astronomers, the real windfall is photons. Every radioactive decay, from 56 Ni to 56 Co to 56 Fe, produces a gamma ray photon, the most energetic type of radiation.
But the supernova is initially so dense that the gamma rays are trapped. Instead of leaving, they dance around inside the rapidly expanding star and are converted to light we can see.
When each photon finally breaks free of its plasma prison, a few days or weeks after it was created, its journey has barely begun. It may travel intergalactic space for millions or billions of years before being absorbed by an eyeball or a digital camera. Each photon carries exactly one secret, one bit of information: its wavelength, which determines its color and energy.
Fortunately, a supernova emits 10 55 of these photons, in an ever-expanding sphere that, over time, alerts any beings within the observable universe that there was a catastrophe.
Even after this mind-bending geometrical dilution, many billions of photons still reach Earth. By a process of separately studying and averaging all that we can capture in a telescope, we can probe thousands of secrets about the inner workings of a supernova. By repeating this process for many supernovae, we can even deduce the entire expansion history of the cosmos and the dark energy underpinning it. Types of Supernovae There are two main types of supernovae: core-collapse and thermonuclear.
The first involve a kind of metamorphosis, as a single massive star is transformed into a neutron star, an extraordinarily dense star where nearly all subatomic particles have been crushed into neutrons, or a black hole, shedding its outer layers in the process. These happen when a star runs out of fuel, and thus its inner layers lose their eternal struggle with gravity.
They free-fall until they become dense enough to reach a new state of matter, and cause an explosion in the process. But supernovae used as standard candles for determining distances in the universe, known as Type Ia supernovae , are different: These are the complete thermonuclear destruction of a white dwarf star, the dense, burned-out core of a normal-mass star.
If such a white dwarf is left alone, it will cool and fade, never to explode. This is the fate of our Sun. But if the white dwarf is in a binary system, it can accrete mass from a companion and explode. There has long been debate over whether the second star is a normal star like the Sun or a large, puffy middle-aged red giant star, or whether it is another white dwarf that merges with the primary white dwarf.
Observations this year by my group and others have indicated that probably all these pairings lead to Type Ia supernovae. This double magic act is possible because atoms and the expansion of the universe influence photons in two different ways. Elements within a supernova and in any gas along the line of sight to it absorb photons of certain wavelengths. If one white dwarf collides with another or pulls too much matter from its nearby star, the white dwarf can explode.
In this illustration, a white dwarf pulls matter from a companion star. Eventually, this will cause the white dwarf to explode. Image credit: STScI. These spectacular events can be so bright that they outshine their entire galaxies for a few days or even months.
They can be seen across the universe. Not very. Astronomers believe that about two or three supernovas occur each century in galaxies like our own Milky Way. Because the universe contains so many galaxies, astronomers observe a few hundred supernovas per year outside our galaxy. Space dust blocks our view of most of the supernovas within the Milky Way.
Scientists have learned a lot about the universe by studying supernovas. They use the second type of supernova the kind involving white dwarfs like a ruler, to measure distances in space.
This is in the troposphere, where weather happens and where about 75 percent of the mass of the atmosphere resides. People have not been used to thinking about radiation from supernovae affecting the troposphere. So there is not much effect on the ozone layer unless, of course, the supernova is much closer than the ones that astronomers have documented. We expect the biggest effect to be on lightning.
Lightning starts when there is a big voltage difference between two regions, either within the atmosphere or between it and the ground. It must rely on a leader — a path of increased ionization in which an electric field can accelerate free electrons.
This sets up a growing cascade in which the accelerated electrons knock other ones loose, and you get a current that grows into a lightning bolt. But where does the leader come from? Atmospheric scientists think the main mechanism is paths of ionization left by cosmic rays. So, a twentyfold increase in tropospheric ionization should lead to a big increase in cloud-to-ground lightning because most of the cosmic ray tracks are more or less vertical.
The big change would be that ordinary storms would produce a lot more lightning. In normal conditions, lightning is the main ignition source for wildfires. Wildfires kill trees and other woody plants; more fires mean fewer trees and more grassland. The American Great Plains was largely kept as grassland by lightning-set wildfires. Native Americans set fires to renew the grass, which attracted bison.
Even today, ranchers there conduct controlled burns on their rangeland. During the past few millions of years, there has been a conversion in many places, including the Great Rift Valley in East Africa, from forest to grassland.
Finally, we can speculate on the consequences. The conversion from trees to grassland may have forced our ancestors out of trees and down to the ground, walking and using their hands. Once the cosmic rays and consequent lightning slack off, forest tends to replace grassland until a new burst of cosmic rays creates increased lightning. If this happened, it would require our predecessors to use their brains to adapt to a new environment. Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter.
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It would rain down devastating radiation that could alter life on our planet. What happened? The Local Bubble is an irregularly shaped region of hot million-degree but tenuous gas plasma in which our solar system and many other stars reside.
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