What Does It Mean To Be ‘Star Stuff’?

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The Tycho supernova remnant. This type of structure is all that remains after a massive star dies, releasing the chemical building blocks of life and planetary systems into space. Credit: NASA/CXC/Chinese Academy of Sciences/F. Lu et al.

Courtesy of Vanessa Janek @ Universe Today:

At one time or another, all science enthusiasts have heard the late Carl Sagan’s infamous words: “We are made of star stuff.” But what does that mean exactly? How could colossal balls of plasma, greedily burning away their nuclear fuel in faraway time and space, play any part in spawning the vast complexity of our Earthly world? How is it that “the nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies” could have been forged so offhandedly deep in the hearts of these massive stellar giants?

Unsurprisingly, the story is both elegant and profoundly awe-inspiring.

All stars come from humble beginnings: namely, a gigantic, rotating clump of gas and dust. Gravity drives the cloud to condense as it spins, swirling into an ever more tightly packed sphere of material. Eventually, the star-to-be becomes so dense and hot that molecules of hydrogen in its core collide and fuse into new molecules of helium. These nuclear reactions release powerful bursts of energy in the form of light. The gas shines brightly; a star is born.

The ultimate fate of our fledgling star depends on its mass. Smaller, lightweight stars burn though the hydrogen in their core more slowly than heavier stars, shining somewhat more dimly but living far longer lives. Over time, however, falling hydrogen levels at the center of the star cause fewer hydrogen fusion reactions; fewer hydrogen fusion reactions mean less energy, and therefore less outward pressure.

At a certain point, the star can no longer maintain the tension its core had been sustaining against the mass of its outer layers. Gravity tips the scale, and the outer layers begin to tumble inward on the core. But their collapse heats things up, increasing the core pressure and reversing the process once again. A new hydrogen burning shell is created just outside the core, reestablishing a buffer against the gravity of the star’s surface layers.

While the core continues conducting lower-energy helium fusion reactions, the force of the new hydrogen burning shell pushes on the star’s exterior, causing the outer layers to swell more and more. The star expands and cools into a red giant. Its outer layers will ultimately escape the pull of gravity altogether, floating off into space and leaving behind a small, dead core – a white dwarf. Continue reading

The Sun’s Magnetic Field is about to Reverse

Don’t worry, its not an apocalyptic type situation, it happens every 11 years. Courtesy of Smithsonian Mag:

Sometime in the next two or three months, something special will happen: the magnetic field that emanates from the Sun and extends throughout the entire solar system will reverse in polarity.

“It’s really hard to say exactly when it’s going to happen, but we know it’ll be in the next few months, for sure,” says Andrés Muñoz-Jaramillo, a researcher at the Harvard-Smithsonian Center for Astrophysics who studies the Sun’s magnetic cycle. “This happens every solar cycle, and it’s a very special day when it does.”

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First, the basics: the Sun, like Earth, naturally generates a magnetic field. The massive solar magnetic field is a result of the flow of plasma currents within the Sun, which drive charged particles to move from one of the Sun’s poles to another.

Every 11 years, the strength of this magnetic field gradually decreases to zero, then emerges in the opposite direction, as part of the solar cycle. It’s as if, here on Earth, compasses pointed towards the Arctic as “North” for 11 years, then briefly wavered, then pointed towards Antarctica as “North” for the next 11 years (in fact, the Earth’s magnetic field does reverse as well, but it occurs with much less regularity, and takes a few hundred thousand years to do so).

Recent observations indicate that the next solar magnetic reversal is imminent—in August, NASA announced that it was three or four months away. The reversal, explains Muñoz-Jaramillo, won’t be a sudden, jarring event but a gradual, incremental one. “The strength of the polar field gradually gets very close to zero,” he says. “Some days, it’s slightly positive, and other days, it’s slightly negative. Then, eventually, you see that it’s consistently in one direction day after day, and you know the reversal has occurred.” His research group’s measurements of the magnetic field suggest this reversal is a few months away, but it’s impossible to say for sure which day it’ll occur.

Because the region that the solar magnetic field influences includes the entire solar system, the effects of the reversal will be felt widely. “The magnetic field flows out into interplanetary space, and it forms a bubble that encloses the solar system as it travels through the galaxy,” Muñoz-Jaramillo says.

One aspect of this bubble—formally known as the heliosphere—is an invisible electrically-charged surface called the current sheet pervades the solar system and resembles a twisted ballerina’s skirt, because the rotation of the Sun twists its far-flung magnetic field into a spiral. The reversal of the field will cause the sheet to become more rippled, which in turn will lead the Earth to pass through the sheet more frequently as it orbits the Sun.

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Passing through more often could cause more turbulent space weather, potentially leading to disruptions in satellite transmissions and telecommunications equipment. On the other hand, the current sheet also blocks high-energy cosmic rays that arrive from other areas of the galaxy, so a more wavy sheet could provide satellites and astronauts in space more robust protection from harmful radiation.

Additionally, the magnetic field reversal coincides with the maximum of other solar activity, which means a greater number of sunspots, more powerful solar flares, brighter aurorae and more frequent coronal mass ejections. Most of these events have little or no effect on Earth, but an especially powerful flare or plasma ejection aimed in the right direction could knock out Earth-based telecommunications systems. At the same time, this solar cycle has been especially weak—NASA solar physicist David Hathaway called it “wimpy” in an interview with Scientific American—so there’s not a ton to worry about with this particular reversal.

For Muñoz-Jaramillo, who spends his days monitoring and analyzing the Sun’s magnetic activity, the reversal will also have personal significance. “Because the cycle is such a long process, in terms of a human’s lifetime, a solar scientist is going to see maybe four reversals in a career,” he says. “That makes every turning point special—and this is the first time I’m seeing one of these since I started studying solar physics.”