Gurl Horoscopes August 2014

Death and Rebirth

On July 4, 1054 A.D, a bright new star appeared in the sky. Although it was 6,500 light-years away from Earth, it shone brighter than whole galaxies and was visible in daylight for 23 days. Little did the astronomers of the day know, the “new” star was actually the violent death of an old star: a supernova explosion. Stars more than ten times the mass of our sun will eventually become supernovas when they die. For their whole lives, they battle to balance energy trying to get out and gravity trying to crush them in under their own weight—but when they run out of fuel to burn, gravity wins. The star’s core collapses and its very atoms are crushed, emitting an enormous shockwave that flings heavy elements out into space. The remnants of this particular supernova formed the enigmatic Crab Nebula, an energetic cloud spanning five light-years, with each different colour representing different chemicals: orange is hydrogen, red is nitrogen, green is oxygen… And at the centre of the nebula lies the remnant of the exploded star. Gravity has squashed all the empty space out of it, leaving an incredibly dense object called a neutron star—just 20 km across, but with the mass of our sun, so on Earth, one teaspoonful would weigh one billion tons. Rotating neutron stars are known as pulsars, and this one spins at a rate of 30 times per second, sending out violent jets of particles at nearly the speed of light.

(Image Credit: 1, 2)

1946 inflation in Hungary.

Stellar Evolution: How Stars Die

Depending on their mass, stars follow one of three evolutionary routes. When they end their lives they may become white dwarves, neutron stars, or black holes.

All stars fuse hydrogen into helium at their core, releasing energy that balances the force of gravity that pulls inwards. Helium is denser than hydrogen, so as it is created, it makes the star’s core hotter and more pressurised. Eventually, the temperature is high enough to fuse the hydrogen around the core, and as fusion reactions happen faster and faster, the force released pushes away the outer layers of the star. Since the star’s radius increases, its energy radiates away over a larger surface area and it emits cooler red light. The star is now a red giant. When it reaches temperatures of 100 million Kelvin and begins to fuse helium into carbon and oxygen, it may take three paths depending on its initial mass.

In light stars like our sun, all their fuel will have run out before temperatures get hot enough to fuse carbon and oxygen into heavier elements. All the excess gas and dust is blown off by stellar wind, forming a planetary nebula, and eventually the star simply becomes a hot, dense carbon and oxygen core: a white dwarf. To take this evolutionary route, a star must be less than 1.4 times the mass of our own Sun, otherwise known as the “Chandrasekhar Limit”. When a star stops producing energy, the only thing stopping gravity from collapsing the star is electron degeneracy pressure: when electrons are squeezed together, they repel each other, thus resisting gravitational collapse.

A star more massive than the Chandrasekhar Limit will collapse even further. As a red giant, its temperatures will become high enough to fuse oxygen and carbon into heavier elements. Fusion continues on and on until iron is created—and since fusing iron uses up energy instead of creating it, fusion stops. Electron degeneracy pressure is the only thing stopping the star from collapsing, and then that gives in too. Electrons are squeezed into the nucleus of the atom, protons turn into neutrons, and the core of the star is soon just a super dense ball of neutrons. An enormous amount of energy is released in the collapse—material hurtles down towards the core and bounces off, creating an immense shockwave that blasts away the rest of the star in a supernova. It’s so bright and energetic that it can temporarily outshine an entire galaxy, and can fuse even heavier elements, like uranium and gold. After the explosion, all that’s left is a neutron star: a stellar corpse 1.5 to 3 times the mass of the sun, but compressed to the size of a city in Earth (30 km in diameter).

But if the original star is more than 20 times the mass of our sun, something even wackier will form at the end of its life: a black hole. It goes from red giant to supernova to neutron star—but it doesn’t stop collapsing. Instead of forming a ball of neutrons, the star’s mass is so great that it collapses down and down to a point so small it must have infinite density. This is called a singularity. To quote Sir Roger Penrose, ‘Thou shalt not have naked singularities’, meaning that they cannot exist with an accompanying event horizon. Anything that crosses beyond this horizon can’t escape the immense gravitational pull of the singularity, not even light. Together, a singularity and its event horizon form a stellar mass black hole.

Denser and denser and denser, the corpse of a star and the grave in which is rests depends largely on its mass. These are the endpoints of stellar evolution, and will be the eventual fate of every point of light we see when we look up.

(Image Credit: Wikimedia Commons and Universe Today)

I was convinced these were mountains on Joy Division’s cover. *you learn something new every day*

The two high-speed jets of plasma move almost at the speed of light and stream out in opposite directions at right angles to the disc of matter surrounding the black hole, extending thousands of light-years into space. NASA

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1960’s(USA) Color Photography

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