- 2656 - GRAVITY - in supernovae explosions. Is the force of gravity instantaneous across the Universe? Or does gravity have a speed limit to how fast the force can travel.? This is not as simple a question as it seems on the surface. We know how fast light travels, and if the Sun were to suddenly wink out of existence, we’d still receive light from it for just over 8 minutes after it disappeared!
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--------------------- 2656 - GRAVITY - in supernovae explosions
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- What happens with gravity in a supernovae explosion? What about gravity controlling the Earth’s orbit? Would the Earth simply fly off in a straight line if gravity suddenly stopped?
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- Or, would it continue to move in its planetary orbit for some time. According to Newton, you have two masses separated by a distance, and that determines the force. You take one of those masses away, and the force goes away. Instantly.
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- It isn’t just mass, that causes gravity. All forms of energy (including mass) affect the curvature of space. So for the Sun and the Earth, the incredibly large mass of the Sun dominates the curvature of space, and the Earth travels in an orbit along that curved space, just like all the other bodies in the Solar System.
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- If you simply removed the Sun, causing it to wink out of existence, what would happen? In general relativity, it’s true that space would go back to being flat, but it wouldn’t do so right away at every point. In fact, just like the surface of a pond when you drop something into it, it snaps back to being flat, and the disturbances send ripples outward!
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- In Einstein’s theory of gravity, these ripples move at the speed of light, not instantaneously. This tells us that the distortion of spacetime due to matter and energy propagate at “c“. The speed of gravity ought to be equal to the speed of light in a vacuum.
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- This is a really amazing idea. Think about it; if the Earth were stationary, it would feel the ripples in one way, but if the Earth were moving over the surface of space, it feel the ripples differently?
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- While Newton doesn’t care what your velocity is, Einstein does. The Sun, as it is right now, won’t have its gravity affect Earth for another 8 minutes, and the gravity that the Earth feels right now pulling it towards the Sun is actually pulling it towards where the Sun was 8 minutes ago!
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- The changes in the gravitation that Earth experiences is due to the fact that the positions and momentum of all the objects in the Universe are changing over time, changing the curvature of space in our vicinity.
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- The Earth, since it is moving, “rides” over the ripples of the gravity wave traveling through space, so that it comes down in a different spot from where it was lifted up. It looks like we have two effects going on: each object’s velocity affects how it experiences gravity, and so do the changes that occur in gravitational fields.
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- The changes in the gravitational field felt by a finite speed of gravity and the effects of velocity-dependent interactions cancel almost exactly! The inexactness of the cancellation is what allows us to determine, observationally, if Newton’s “infinite speed of gravity” model or Einstein’s “speed of gravity = speed of light” model matches with our Universe.
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- The speed of gravity should be the same as the speed of light. But the Sun’s force of gravity out here, by us, is far too weak to measure this effect. In fact, it gets really hard to measure, because if something moves at a constant velocity in a constant gravitational field, there’s no observable affect at all. What we’d want, ideally, is a system that has an object moving with a changing velocity through a changing gravitational field. What would that take?
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- Something intense, like a neutron star orbiting another stellar-mass object extremely close together!
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- Occasionally a neutron star emits very regular blips of light, pulsing with incredible precision: this makes it a pulsar! In very rare cases, we even have two neutron stars orbiting one another! If one of these neutron stars is a pulsar aimed at us, we can test whether gravity moves at the speed of light or not! Incredibly enough, we’ve discovered multiple independent binary pulsars with this exact configuration!
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- Not only is the gravitational source star moving, but the other star is changing its velocity, as it changes its direction in orbit around the gravitational source! Remarkably, this effect causes the orbit to ever-so-slowly decay, which leads to time changes in the pulses!
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- The predictions from Einstein’s theory of gravity are incredibly sensitive to the speed of light, so much so that even from the very first binary pulsar system we have constrained the speed of gravity to be equal to the speed of light with a measurement error of only 0.2%!
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- In 2002, when a chance coincidence lined up the Earth, Jupiter, and a very strong radio quasar all along the same line-of-sight. As Jupiter moved between Earth and the quasar, the gravitational bending of Jupiter allowed us to measure the speed of gravity, ruling out an infinite speed and determining that the speed of gravity was between 2.55 × 10^8 and 3.81 × 10^8 meters-per-second, completely consistent with Einstein’s predictions.
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- To get more accuracy science needs to be able to do is detect these gravitational waves directly. The proposed Laser Interferometer Space Antenna (LISA) would have been sensitive to exactly these types of gravitational waves, and could have measured the speed of gravity directly.
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- The indirect measurements from very rare pulsar systems that give us the tightest constraints, and tell us that the speed of gravity is between 2.993 × 10^8 and 3.003 × 10^8 meters per second, which is an amazing confirmation of General Relativity and a terrible difficulty for alternative theories of gravity that don’t reduce to General Relativity!
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- No event in nature surpasses a supernova’s raw power. The flood of neutrinos accompanying the explosion of a single massive star releases as much instantaneous power as the rest of the visible universe combined.
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- Such blasts stir interstellar gas and dust, helping new stars form. Supernovae disperse most of the elements heavier than carbon, such as the iron in our blood, and create neutron stars and black holes.
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- Sound waves in a collapsing star’s heart could help kick-start a stalled explosion, while a white dwarf’s detonation may arise when the star’s gravity turns a thermonuclear conflagration back on itself.
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- By the 1930s, it was clear that some stellar flare-ups, called novae, were in a class by themselves. In 1933, astronomers suggested the explosions occurred when a massive star collapsed and created a neutron star.
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- In 1941 astronomers proposed supernovae come in two flavors based on the absence (type I) or presence (type II) of strong hydrogen spectral lines at peak brightness. Since then, the observational picture has become more complex as astronomers recognized new subclasses of both types. Nevertheless, astronomers generally agree that two scenarios likely account for most supernovae.
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- Type Ia supernovae occur in all galaxies among an older stellar population. All others, type II, plus types Ib and Ic associated with gamma-ray bursts, prefer galaxies sparkling with star-forming regions, which contain many hot, young, massive stars. Such stars explode when they use up their nuclear fuel and collapse.
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- Stars weighing more than about eight times the Sun’s mass burn through their hydrogen fuel quickly, but as a massive star runs low on one fuel, it taps into another. Its core contracts, growing hotter and denser until the previous nuclear reaction’s “ash” , helium, at first undergoes fusion itself. As each fuel runs out, the star’s core responds in the same way, running through a succession of fuels: hydrogen, helium, carbon, neon, oxygen, and silicon.
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- Each new fuel releases less energy, so the star burns through it even faster. Moreover, once carbon ignites and the core’s temperature approaches a billion degrees, neutrinos form and escape in greater numbers.
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- Formed in many nuclear reactions, neutrinos don’t interact easily with other matter and quickly exit the star. To compensate for the energy loss, the core burns its nuclear fuel even faster.
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- While such a star may take 10 million years or more to run through its “first course” of hydrogen fuel, it consumes its helium in 2 million years and its carbon in just 2,000 years. The last phase, when the core fuses silicon, lasts less than three weeks.
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- As silicon fusion ends, an Earth-sized iron-nickel core about 1.5 times the Sun’s mass resides in the star’s center. But iron-group elements have nature’s most tightly bound nuclei, so the core can’t resort to its old trick, fusing iron actually consumes energy. Neutrinos stream from the core.
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- The core’s central density is so high that it forces electrons, the star’s main pressure source, inside nuclei. The electrons transform some protons into neutrons. Both processes of streaming neutrinos and squeezing protons and electrons together remove pressure that supports the star. With pressure losses mounting and no new energy source to tap, the star’s battle with gravity is over.
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- The iron core collapses at about ¼ light-speed. In half a second or less, it transforms from an Earth-sized stellar core to a hot, dense proto-neutron star just 19 miles across. When the central density reaches about twice that of an atomic nucleus, the core stiffens and rebounds thanks to a repulsive component in the strong nuclear force. This core “bounce” acts like a spherical piston that drives into the star’s infalling gas.
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- As the shock moves out, it radiates lots of neutrinos, which saps its energy. The shock stalls a few milliseconds after it starts and simply sits there, heating the infalling gas. If nothing changed during the next second, the nascent neutron star would accrete a few tenths of a solar mass of matter and then become crushed into a black hole. No supernova.
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- The central mystery of core-collapse supernovae is how this situation ever can turn itself around. Neutrinos eventually heat up the material behind the shock enough that you re-launch an explosion.
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- The large number of neutrinos departing the core makes up for the low odds that a single neutrino will interact with the star’s matter as it leaves. The action pauses for just a few hundred milliseconds.
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- In computer simulations, which assumed the collapsing star was spherically symmetric, even this process didn’t work. Such 1-D calculations gave way to more demanding 2-D models, which assume symmetry around the star’s spin axis. They revealed fluid instabilities and turbulence that promised to aid the stalled shock.
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- In 2005 science discovered a potentially important alternative energy source in collapsing stars: sound waves. In the team’s 2-D model, the stalled shock starts to wobble top-to-bottom along the star’s spin axis.
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- As matter streams onto the proto-neutron star, turbulence around the core sets it oscillating at around 300 hertz. Acoustic waves radiate back into the collapsing envelope.
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- While the energy from neutrinos is far greater, only a fraction of it becomes deposited in the stalled shock, whereas matter absorbs sound almost completely. There’s enough acoustic power to blow the star apart half a second after core bounce in these simulations.
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- The sound waves push streams of accreting matter to one side of the core while energizing the shock on the opposite side. So, by creating a path of least resistance, sound may help neutrinos revitalize a stalled the shock wave.
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- Large-scale computer simulations are also providing new insights into how white dwarfs, the end state of low-mass stars, destroy themselves as type Ia supernovae. Brighter and more uniform than core-collapse explosions, type Ia events are important probes of the distant universe. The discoveries of dark energy and cosmic acceleration add urgency to deciphering how they work.
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- A star similar to the Sun ends its days as a white dwarf, with the star’s carbon-oxygen-rich core crushed to Earth’s size. Most shine for billions of years, gradually cooling until they fade into dark stellar cinders.
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- Electron pressure prevents further collapse, but it works only if the dwarf weighs less than 1.44 Suns, called Chandrasekhar limit. Exceed that, and collapse resumes until the dwarf becomes a neutron star.
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- A white dwarf near this limit could be a giant thermonuclear bomb. Place a white dwarf in close proximity to a normal star, and the dwarf can gain mass until it nears the 1.44-Sun threshold and explodes.
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- The dwarf gobbles up hydrogen gas from its partner at a probable rate of about 1/30 of an Earth mass per year. If it’s much slower than this number, the dwarf’s stellar wind prevents the gas from reaching the surface; if it’s any faster, the gas will flash-fuse rather than accumulate.
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- As a white dwarf tips the scale toward 1.44 Suns, its carbon ignites somewhere inside.
The star, leaving behind a 10-billion-degree ash bubble. When this bubble broke through the dwarf’s crust, less than 10 percent of the star’s mass had been fused, too little to disrupt the dwarf or produce a strong explosion.
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- The 2-D simulations were used to see what happens after the bubble breaches the star’s surface. The nuclear ash erupts, moving at around 6.7 million mph , just shy of orbital speed.
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- The hot cloud hugs the dwarf’s billion-degree surface and rapidly spreads. As it does so, it plows up cooler, unfused surface material. The superheated ash-cloud wraps around the white dwarf and meets itself at the point opposite its breakout. The collision compresses all of the unfused surface material, which explodes and rips the star apart.
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- The model, called “gravitationally confined detonation,” is the most complete description of a type Ia supernova to date. More than 85 years after astronomers connected supernovae with stellar deaths, the universe’s most powerful explosions still tax astrophysicists.
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- But even the most complete simulations don’t yet capture the complex environment of an exploding star. Modelers are beginning to probe how neutrino emission, magnetic fields, and rotation affect the picture.
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- Astronomers watch and catalog new events, using them both as cosmic yardsticks and to find holes in current understanding. And new facilities designed to capture neutrinos and gravitational waves, signals that directly escape an exploding star’s core, one day soon may give us a glimpse of a supernova’s explosions.
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- March 8, 2020 2656
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