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How does a Supernova become a Neutron Star?

Context: In 2011, the Nobel Prize was awarded to three scientists for discovering that the Universe is expanding at an ever-accelerating rate through observations of distant supernovae.Now a team of Indian astronomers observing such distant supernovae have narrowed down the possible mechanisms of explosion of such supernovae which provide key measures of cosmological distances. The explosive death of a star as a supernova is one of the most spectacular and catastrophic events in the Universe.

Analysis

  • When a massive star dies with a supernova explosion or condensed objects like neutron stars merge in a violent process, they let of bursts of photons lasting a few milliseconds.
  • These are known as gamma ray bursts.
  • In a few seconds the gamma-ray burst (GRB) emits more energy than the Sun will provide over its entire 10-billion year life.
  • Gamma ray bursts are distributed homogeneously on the sky.
  • Recently, researchers observed the most powerful gamma ray burst that has been recorded until now, as reported in a Nature paper.

Stellar Evolution – The Birth, Life, and Death of a Star

  • A star’s life cycle is determined by its mass. The larger its mass, the shorter its life cycle.
  • A star’s mass is determined by the amount of matter that is available in its nebula, the giant cloud of gas and dust from which it was born.
  • Over time, the hydrogen gas in the nebula is pulled together by gravity and it begins to spin.
  • As the gas spins faster, it heats up and becomes as a protostar.
  • Eventually the temperature reaches 15,000,000 degrees and nuclear fusion occurs in the cloud’s core.
  • Nuclear fusion is a nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones.
  • The difference in mass is converted to energy by Einstein’s famous equivalence “Energy = Mass times the Speed of Light squared”.
  • This is the source of the Sun’s energy.
  • The cloud begins to glow brightly, contracts a little, and becomes stable. It is now a main sequence star and will remain in this stage, shining for millions to billions of years to come. This is the stage our Sun is at right now.
  • As the main sequence star glows, hydrogen in its core is converted into helium by nuclear fusion.
  • When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts.
  • The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and glows red. The star has now reached the red giant phase.
  • It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward.
  • In the core of the red giant, helium fuses into carbon. All stars evolve the same way up to the red giant phase.
  • A typical star, such as the Sun, radiates small amounts of X-rays continuously and larger bursts of X-rays during a solar flare.
  • The Sun and other stars shine as a result of nuclear reactions deep in their interiors.
  • These reactions change light elements into heavier ones and release energy in the process.
  • The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight.
  • A star collapses when the fuel is used up and the energy flow from the core of the star stops.
  • Nuclear reactions outside the core cause the dying star to expand outward in the “red giant” phase before it begins its inevitable collapse.
  • The amount of mass a star has determines which of the following life cycle paths it will take from there.
  • If the star is about the same mass as the Sun (low-mass stars), a planetary nebula is formed by the outer layers and the core will turn into a white dwarf star.
  • The Chandrasekhar Limit of 1.4 solar masses is the theoretical maximum mass a white dwarf star can have and still remain a white dwarf.
  • If it is somewhat more massive, it may undergo a supernova explosion and leave behind a neutron star.
  • Supernova is the death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading.
  • At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy.
  • The hot material, the radioactive isotopes, as well as the leftover core of the exploded star, produce X-rays and gamma-rays.
  • But if the collapsing core of the star is very great — at least three times the mass of the Sun — nothing can stop the collapse. The star implodes to form an infinite gravitational warp in space — a black hole.

The brightest X-ray sources in our galaxy are the remnants of massive stars that have undergone a catastrophic collapse — neutron stars and black holes.

  • Other powerful sources of X-rays are giant bubbles of hot gas produced by exploding stars.
  • White dwarf stars and the hot, rarified outer layers, or coronas, of normal stars are less intense X-ray sources.
  • Thus, the rate of evolution and the ultimate fate of a star depends on its weight, or mass.
  • The cycle begins anew as an expanding supershell from one or more supernovas trigger the formation of a new generation of stars. Brown dwarfs have a mass of only a few percent of that of the Sun and cannot sustain nuclear reactions, so they never evolve.

What is a Supernova?

  • A supernova is a large explosion that takes place at the end of a star’s life cycle.

Fig. On the left is Supernova 1987A after the star has exploded. On the right is the star before it exploded.

  • A supernova is the explosion of a star. It is the largest explosion that takes place in space.

Where do Supernovas take place?

  • Supernovas are often seen in other galaxies. But supernovas are difficult to see in our own Milky Way galaxy because cosmic dust blocks our view.

What causes a Supernova?

  • A supernova happens where there is a change in the core, or center, of a star. A change can occur in two different ways, with both resulting in a supernova.
  • The first type of supernova happens in binary star systems.
  • Binary stars are two stars that orbit the same point.
  • One of the stars, a carbon-oxygen white dwarf, steals matter from its companion star.
  • Eventually, the white dwarf accumulates too much matter. Having too much matter causes the star to explode, resulting in a supernova.
  • The second type of supernova occurs at the end of a single star’s lifetime.
  • As the star runs out of nuclear fuel, some of its mass flows into its core.
  • Eventually, the core is so heavy that it cannot withstand its own gravitational force.
  • The core collapses, which results in the giant explosion of a supernova.
  • The sun is a single star, but it does not have enough mass to become a supernova.

Why do scientists study Supernovas?

  • A supernova burns for only a short period of time, but it can tell scientists a lot about the universe.
  • One kind of supernova has shown scientists that we live in an expanding universe, one that is growing at an ever increasing rate.
  • Scientists also have determined that supernovas play a key role in distributing elements throughout the universe.
  • When the star explodes, it shoots elements and debris into space.
  • Many of the elements we find here on Earth are made in the core of stars.
  • These elements travel on to form new stars, planets and everything else in the universe.

How do NASA Scientists look for Supernovas?

  • NASA scientists use different types of telescopes to look for and study supernovas.
  • Some telescopes are used to observe the visible light from the explosion. Others record data from the X-raysand gamma rays that are also produced.
  • Both NASA’s Hubble Space Telescope and Chandra X-ray Observatory have captured images of supernovas.
  • In June 2012, NASA launched the first orbiting telescope that focuses light in the high-energy region of the electromagnetic spectrum.
  • The NuSTAR mission has a number of jobs to do. It will look for collapsed stars and black holes. It also will search for the remains of supernovas.

Words to Know

  • White dwarf:a star near the end of its life that has used most or all of its nuclear fuel and collapsed into a size similar to Earth.
  • X-ray:a type of electromagnetic radiation with a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than gamma rays.
  • Gamma ray:the highest-energy, shortest-wavelength electromagnetic radiations.

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