While it is generally accepted by astronomers that such encounters between white dwarfs and "normal" companion stars are one likely source of Type Ia supernova explosions, many details of the process are not well understood.
One way to investigate the explosion mechanism is to look at the elements left behind by the supernova in its debris or ejecta. The blast wave and debris from an exploded star are seen moving away from the explosion site and colliding with a wall of surrounding gas.
Astronomers estimate that light from the supernova explosion reached Earth about 1, years ago, or when the Mayan empire was flourishing and the Jin dynasty ruled China. However, by cosmic standards the supernova remnant formed by the explosion, called MSH , is one of the youngest in the Milky Way galaxy. The explosion also created an ultra-dense, magnetized star called a pulsar , which then blew a bubble of energetic particles, an X-ray-emitting nebula.
Astronomers using NASA's Chandra X-ray Observatory have announced the discovery of an important type of titanium, along with other elements, blasting out from the center of the supernova remnant Cassiopeia A Cas A. This new result, as outlined in our latest press release , could be a major step for understanding exactly how some of the most massive stars explode. The different colors in this new image mostly represent elements detected by Chandra in Cas A: iron orange , oxygen purple , and the amount of silicon compared to magnesium green.
We are pleased to welcome Emanuele Greco as a guest blogger. Emanuele is the first author of a paper describing the possible discovery of a neutron star left behind by supernova A. He is now completing his PhD in Astrophysics at the same University, where he is expected to defend his thesis next June.
Imagine having a bright and small light bulb and putting it behind a thick wall made of elements like iron and silicon. No light stemming from the bulb would be observed, because it is completely obscured by the wall. SN A is the only naked-eye SN observed since telescopes were invented and offers a unique opportunity to watch a SN evolving into a supernova remnant SNR in this time of multi-wavelength and multi-messenger observatories simultaneously at work.
This event was particularly important because neutrinos emitted from an exploding star were detected on Earth for the first time. This discovery implies that the core of the progenitor star must have collapsed producing a shock wave — similar to the sonic boom from a supersonic plane — that ejected part of the stellar material into the surrounding environment.
As a result, a compact object such as a neutron star, a relic of the stellar core, should have formed in the very heart of SN A. However, despite the continuous monitoring performed at almost all wavelengths since the SN was detected, no clear indication for this compact object has been found so far.
Various hypotheses have been proposed to explain this non-detection, such as the formation of a black hole instead of a neutron star. Astronomers have found evidence for an unusual type of supernova near the center of the Milky Way galaxy , as reported in our latest press release.
Test your understanding before going on. Simulation of effect of a nearby supernova on a star like the sun.
Supernova a In February, , a star exploded as a supernova in the Large Magellanic Cloud, a small companion galaxy to the Milky Way. We could watch shock waves propagate outward and see the beginning of the formation of the supernova remnant nebula from Sugerman et al. This lovely composite image shows the infrared in red, optical in yellow, low-energy x-ray in green, and high-energy x-ray in blue. This animation based on a series of HST images shows energy from the pulsar whipping up the nebula around it and keeping it energized -- including with the energetic sea of electrons that makes it glow.
This animation starts with the image above rotated a bit and locates the pulsar and a shock front. Fowler Nobel Prize, Starting from the the upper left. The massive stars rapidly run through their main sequence lifetimes lower right , building up heavy elements, and then explode as supernovae bottom center. The heavy elements in their interiors escape in these explosions, and more are made during the explosions themselves.
The enriched remnants of these stars expand into the interstellar medium lower left and merge with the interstellar material to be swept up in the next cycle of star formation.
Rieke Click to go to the Milky Way. Supernovae generally arise from two types of sources. A Type Ia supernova is produced when a white dwarf accretes enough mass to make it unstable to gravitational collapse. White dwarfs are formed from low mass stars, and as such, they are generally very old and composed of relatively light elements, such as carbon and oxygen. On the other hand, a Type II supernova is the result of the gravitational collapse of a massive star one with a mass greater than about eight times the mass of the Sun after it forms an iron core and runs out of thermonuclear fuel.
Such massive stars have short lifetimes, and the core of these stars forms either a neutron star or a black hole. Supernovae only occur within the Milky Way at a rate of about two per century. While many extragalactic supernovae are detected every year, they are distant and can only be studied for a few years while they are bright.
On the other hand, SNRs within our Galaxy can stay visible for many tens of thousands of years and can thus give complementary information to supernova observations. For example, there are several ways to determine the type of supernova that gave rise to a particular SNR.
If the SNR is far from the Galactic plane, then the progenitor star was likely to be old, and hence the supernova was of Type Ia. Skip Navigation press 2. Imagine the Universe! What is a Supernova Remnant? How does a Supernova Remnant Evolve? Low Mass Stars In the usual scheme of things, we expect low mass stars to settle down, slowly fizzle out and become white dwarfs. Their masses are too low to bring about the collapse to a neutron star and the resulting spectacle of a supernova explosion.
They resist gravity's pull with electron degenerate pressure, where the electrons are squeezed until there is no more space between them. The gravitational pressure of the star is not enough to cause the conversion to neutrons and the star has reached a stable equilibrium. However, if the white dwarf is in a binary system with a red giant, it is possible for the dwarf to accrete matter from its companion.
If enough matter falls on it that it exceeds the Chandresekar limit for white dwarfs, the star is no longer stable, gravity will overcome the resistance of electron degeneracy pressure and the star will collapse. The collapse raises the temperature until carbon and oxygen in the core start to fuse, igniting a deflagration wave of runnaway nuclear burning which propagates through the core in seconds. The nuclear fusion reactions create about a solar mass of radioactive 56 Ni. The energy released is on the order of 10 52 ergs, and the white dwarf is completely disrupted in the process.
The star can outshine entire galaxies while the nuclear fusion proceeds. This is believed to be the mechanism for Type Ia SNe. How does the SNR Evolve? As the ejecta expand out from the star, it passes through the surrounding interstellar medium, heating it from 10 7 to 10 8 K, sufficient to separate electrons from their atoms and to generate thermal X-rays. The interstellar material is accelerated by the shock wave and will be propelled away from the supernova site at somewhat less than the shock wave's initial velocity.
This makes for a thin expanding shell around the supernova site encasing a relatively low density interior. This is known as the "free expansion" phase and may last for approximately years, at which point the shock wave has swept up as much interstellar material as the initial stellar ejecta. The supernova remnant at this time will be about 10 light years in radius.
0コメント