Astronomy & Astrophysics

Information compiled and edited by Veronica PV (3vil) Student of Theoretical Physics

Astronomers discover a 140-year-old supernova, the youngest in the Milky Way.

The finding will help determine how often they explode. Until now, the most recent identified dated to 1680. Each century about three supernovae can explode in the Galaxy

A group of astronomers has discovered the youngest supernova of the Milky Way, only 140 years old and which was being tracked for more than two decades, according to a press conference.

Until now, the most recent supernova they had identified dated from 1680, according to studies on the expansion of the remains of Cassiopeia A.

The discovery, which has been under investigation since 1985, will help determine more accurately how often supernovas explode in the Galaxy.

The discovered supernova, the scientists said, had not been seen in these 140 years because it exploded near the center of the Galaxy and was embedded in a dense field of gas and dust.

This made it three million times more imperceptible than if it had been in the dark, but thanks to the new X-ray systems and the radio waves that were used, they easily penetrated it.

Stellar explosion

This discovery has been made possible by the NASA Chandra Telescope and the National Radio Astronomy Observatory ( NRAO ). Astronomers estimate that about three supernovas can explode in the Milky Way every century, although these forecasts have a wide margin of error.

The supernova is a stellar explosion that produces very bright objects in the celestial sphere and usually appears where nothing was observed before. This discovery is essential to calculate more accurately the age of the supernovae of our galaxy.

 A supernova (Latin: nova,’new’)? it is a stellar explosion that produces very bright objects in the celestial sphere, hence they were initially called Estrella nova or simply Nova since many times they appeared where nothing was observed before. Subsequently, the prefix “super-” was added to distinguish them from another phenomenon with similar but less luminous characteristics, the novas.

Supernovae give rise to intense flashes of light that can last from several weeks to several months. They are characterized by a rapid increase in intensity until reaching a peak, then decrease in brightness more or less smoothly until it disappears completely.

Fundamentally they originate from massive stars that can no longer fuse their depleted nucleus, unable to sustain themselves either due to the degeneracy pressure of electrons, which leads them to suddenly contract and generate, in the process, a strong emission of energy. There is also another process even more violent, capable of generating even more intense flashes. They happen when a white dwarf companion of another star, still active, adds enough mass of it to exceed the limit of Chandrasekhar and proceed to the instant fusion of its entire nucleus, which generates a thermonuclear explosion that expels almost everything, if not all , the material that formed it.

Supernovae cause the expulsion of the star’s surface layers in the form of huge shock waves, filling the surrounding space with heavy elements. The remains eventually compose clouds of dust and gas. When the explosion wavefront reaches other nearby clouds of gas and dust, it compresses them and can trigger the formation of new solar nebulae that originate, in a certain time, new star systems (perhaps with planets, as they are enriched with the elements coming from the explosion).

Supernovae can release 1044 joule several times. This has resulted in the adoption of foe (1044 joules) as a standard unit of energy for the study of supernovae.

In the arrange to perceive supernovae, astronomers have classified them in line with the absorption lines of various chemical components that seem in their spectra.

The first key to division is the presence or absence of hydrogen. If the spectrum of a supernova does not contain a hydrogen line it is classified as type I, otherwise it is classified as type II.

Within these two main groups, there are also subdivisions according to the presence of other lines in the light curve.

Type Ia supernovae lack helium and instead have a silicon line in the emission spectrum. The most accepted theory regarding this type of supernovae suggests that they are the result of mass accretion by a white carbon-oxygen dwarf of a companion star, usually a red giant. For a very close binary star systems, this can happen. Both stars are the same age and the models indicate that they will almost always have a similar mass. But usually, there is always one more massive than the other and slight differences in this aspect make the most massive die before the smaller star. If the stars have less than 8 solar masses they will form white dwarfs.

This cover, basically of hydrogen and helium, is little gravitationally cohesive, so it is easily captured by the white dwarf. Around each star there is a perimeter of influence in which the gravitational force of one or the other overcomes. This is the lobe of Roche and, if part of the envelope of the red giant invades the lobe of the white dwarf (usually greater than that of its companion), all the matter contained in its area of ​​influence will be attracted to it.

The material has to be deposited quickly enough so that the surface layer of hydrogen does not ignite, otherwise novas would be produced. If the accretion rate is adequate, the white dwarf will soon reach the Chandrasekhar limit, at which point degenerated electrons are no longer able to hold the object. The increase in pressure results in the collapse of the star whose temperatures soar until the carbon fusion in the star’s core ignites. This ignition is complete starting at its center and spreading rapidly to the outermost layers. Since they have very little hydrogen on their surface, it quickly ionizes, becoming transparent and undetectable when the spectra of these light flashes are read. The propagation of the energy of the explosion is still debated among scientists. Although it is assumed that the main source of energy would be generated in the center, it is unknown if there are other simultaneous ignition points that generate convergent crush waves thus enhancing the performance of the explosion. The turbulence generated by Rayleigh-Taylor’s instability seems to be the cause of a rapid spread of the ignition flame throughout its volume. It is unknown how such ignition transitions from a subsonic deflagration to a supersonic detonation. The turbulence generated by Rayleigh-Taylor’s instability seems to be the cause of a rapid spread of the ignition flame throughout its volume. It is unknown how such ignition transitions from a subsonic deflagration to a supersonic detonation. The turbulence generated by Rayleigh-Taylor’s instability seems to be the cause of a rapid spread of the ignition flame throughout its volume. It is unknown how such ignition transitions from a subsonic deflagration to a supersonic detonation.

During the detonation, in a matter of seconds, an amount of carbon burns a normal star that would take hundreds of years. This incredible energy releases a colossal shock wave that destroys the star by expelling all its mass at speeds of around 10,000 km / s. The energy released in the explosion also causes an extreme increase in its luminosity being this type of supernovae the brightest of all, around 1044joules are invested in light (1foe). Normally there is no trace of the star that caused the cataclysm, only remnants of gas and dust superheated in rapid expansion. The disappearance, therefore, of the gravitational field of the exploded star produces a change in the trajectory of the neighboring star, if it survived the detonation. Not being subjected to its force of attraction, It will be shot in the direction it was following at the time of the outbreak as if it were a sling. These launched stars could be detected as they should go much faster than those around them.

The mechanism of this type of supernovae is similar to that produced by the novae, according to which a white dwarf absorbs matter more slowly, turning it on before reaching the Chandrasekhar limit. In the case of a nova, the absorbed matter causes a fusion reaction of the newly accreted surface material but does not cause the star to collapse.

They are very rare phenomena since they require very strict requirements for their formation. In the first place they only occur in binary systems of stars of low middle mass. These systems are in principle quite common but there are still more restrictions. The sum of the masses of both stars must be greater than the mass of Chandrasekhar (1.44MSol). They must be close enough that their lobes of roche can be invaded by the expanding layers of the growing red giant. If possible, the giant’s mantle should engulf the white dwarf, which would ensure rapid absorption of the material and its braking due to friction with the star gas. This would lead her to increasingly closer orbits, which would increase accretion rates. If the absorption were too slow and slow,

There may also be a type Ia supernova generated by the encounter between two white dwarfs of the same binary system. It may be that neither of them alone managed to accrue enough mass to generate a thermonuclear supernova but together they would exceed the mass of Chandrasekhar. Two white dwarfs in rotation emit gravitational waves and, over time, their orbits approach and accelerate, which accelerates the emission of waves and feeds back the process. At one point, one of the two bodies (the least massive), breaks and forms a bull (donut), around the other star. The mass of that disk begins to fall on the surface. The pace should not be too slow or too fast either, since in any case it would cause burning of surface carbon.

Type Ia supernovae have a characteristic light curve. Near the moment of maximum luminosity, the spectrum contains lines of intermediate mass elements ranging from oxygen to calcium (elements of the outer layers of the star). Months after the explosion, these elements have become totally transparent and the light that dominates is the one that comes from heavier elements coming from the nucleus. The light emitted by nickel-56 is concentrated in the emission peak. This is decaying by radioactivity to cobalt-56 also radioactive. At one point the emission of light is dominated by cobalt, whose emission of high-energy photons softens the curve of brightness decrease. The luminosity ends with the conversion of all cobalt to iron-56, which will emit the latest lines due to its ionized state.

Unlike other types of supernovae, type Ia supernovae are found in all types of galaxies, including ellipticals. They also do not show any preference for star formation regions. This is because the events that lead to a supernova Ia can last a long time in stellar terms, especially the approximation of the two bodies. In addition they do not originate from very massive stars, so they do not have to be located in young areas of recent formation (where the blue giants are found). So they can happen in the longest regions of galaxies. This peculiarity allows you to find them looking at any part of the sky, distributing homogeneously with a constant probability where there are galaxies.

The similarity in the shapes and in the magnitude of the light curves of all type Ia supernovae observed to date, has caused them to be used as a standard measure of luminosity in extragalactic astronomy, which in astrophysical terms is called a candle standard (can be calibrated with one tenth of magnitude). The advantages with respect to the other standard candles, such as the Cepheids, is that their high luminosity allows them to be detected in even farther galaxies, helping to infer distances from objects that would otherwise be impossible to calculate. The reason for the similarity in the luminosity curve is still a matter of debate but it seems to be related, in part, to the fact that the initial conditions in which these phenomena are generated are almost identical. These favorable properties have revolutionized cosmology,

In the Milky Way, the best known candidate for this type of supernova is IK Pegasi (HR 8210), located at a distance of only 150 light years. This binary system consists of a main sequence star and a white dwarf, separated only by 31 million km. The dwarf has an estimated mass of 1.15 times the solar mass. It is thought that several billion years will pass before the white dwarf reaches the critical mass necessary to become a type Ia supernova.

Types Ib and Ic

Types Ib and Ic do not have the silicon line present in type Ia and are believed to correspond to stars on the verge of extinction (such as type II), but which lost their hydrogen before, so hydrogen lines do not they also appear in their spectra. Type Ib supernovae are theoretically the result of the collapse of a Wolf-Rayet star with whose intense winds manage to get rid of hydrogen from the outer layers. Several of these supernovae are also known in binary systems and this is because the companion star can help to gravitationally detach the gas from the outermost layers of the other star which loses its cover without being so massive. In extreme cases not only hydrogen escapes but also helium leaving the carbon core bare, This is the case of Ic supernovae. These supernovae have an explosion mechanism essentially identical to that of typical gravitational collapse supernovae, type II.

Type II

Type II supernovae are the result of the impossibility of producing energy once the star reaches nuclear statistical equilibriumwith a dense core of iron and nickel. These elements can no longer be merged to give more energy. The potential barrier of its nuclei is too strong for the fusion to be profitable so that inert stellar core ceases to support itself and the layers above it. The definitive destabilization of the star occurs when the mass of the iron core reaches the Chandrasekhar limit, normally it takes just a few days. It is at that moment when it conquers the pressure that the degenerated electrons of the nucleus contribute and this succumbs. With the collapse of the nucleus, it becomes heated at around 3,000 million degrees at which time the star emits photons of such high energy that even iron atoms are able to split into alpha particles and neutrons in a process called photo-integration, these particles are in turn destroyed by other photons thus generating an avalanche of neutrons in the center of the star.

These reactions are endothermic so they do not help sustain the compact nucleus and it continues to collapse, emitting more and more neutrons every time. In fact they cause a cooling of the same, which translates into a lower pressure and, therefore, an acceleration of the process. The iron atoms themselves capture part of the immense neutron flux, transforming into heavier elements in a process called neutron capture, specifically the R-process.

The nucleus falls so fast that it leaves an almost empty low-density space between it and the rest of the stellar material. The mantle, on the other hand, begins to fall on the nucleus braking by the flood of photons of extreme frequency that keeps that fall at bay photodesintegrating the innermost layers of the stellar cover. This destruction of nuclei not only transmits momentum but also produces a flow of neutrons and protons that will be captured by the following layers to form heavier elements. Simultaneously, the tremendous densities that are reached in the soup of heavy nuclei and electrons in which the supercompacted nucleus of the dying star has become, enable a new reaction. The electrons of the star nucleus begin to fall on the atomic nuclei, bonding with the protons to form neutrons in a process called electron capture, so that, little by little, the nucleus becomes a mass of hyperdense neutrons called neutronium. The processes of photointegration and electron capture accelerate the sinking of the star even more, since, in addition, now also the degeneracy pressure loses strength rapidly.

But electron capture not only results in the production of neutrons but also in that of neutrinos. The capture occurs at such a rate that an explosive flow of neutrinos is generated that is carried away by the collapse, until their increasing abundance causes them to degenerate and, thus, block the capture of new electrons. For brief moments the electrons cannot even continue to combine with the protons since there is no place in the phase space to place the resulting neutrinos, since they are already degenerated. But this does not take long to resolve since, as a result of this clogging, there is an escape of the neutrinos from the nucleus carrying large amounts of energy, which reactivates the captures and feeds back to the wave fronts of neutrinos that expand rapidly .

The outer layers of material that fall into the nucleus are on their way with the shock front of the neutrino avalanche, also called the neutrinosphere. Through a process that has not been fully disclosed yet, some of the energy released in the neutrino explosion is transferred to the outer layers of the star. It is believed that, as seen in the following formula, neutrinos are capable of generating photons by an inverse process to the generation of photoneutrinos (see: Thermal neutrinos). When the shock wave reaches the surface of the star several hours later, a massive increase in its luminosity occurs. If the mass of the collapsing nucleus is small enough, between 1.5 and 2.5 solar masses, the neutrons themselves can stop the collapse, if they do not continue to contract until all the matter is concentrated in a singularity, thus forming a black hole.

In the case of supernovae that generate neutron stars, the outer layers hardly collide with the surface of the compact nucleus. It is possible that they do not reach it and before they have been swept by the neutrino flow. For those who end up in black holes initially a neutron star is formed but the cover has so much mass and thrust that much of it falls on the neutron star causing it to exceed the maximum mass of about 2.5 solar masses, a limit that It is not known exactly.

that is not known exactly.

Light curves of SNII-P and SNII-L. The former have a plateau phase in which the ionized gas cools as it expands, recombining until it becomes transparent. This process compensates for the decrease in light and maintains the luminosity until it becomes neutral, at which point it decreases again. In the second case there are hardly any external layers surely lost by interaction with a neighboring star. It is also observed how it has a noticeably less pronounced peak than SNIa

The question of how supernovas manage to emit all that energy is not well understood. In fact, the models made by computer do not give any explosion or, if they do, it is very marginal. It has been speculated on a whole series of factors that could influence the power of the explosion or that could even be crucial for it to occur. First, there would be the centrifugal force that is maximum in the equatorial plane and that, without a doubt, has a positive contribution helping the material to escape. With the compression of the star, said force should be accentuated by conserving the angular momentum of the star. On the other hand there are the magnetic fields that should also help with their magnetic pressure.

Type II supernovae can be divided into subtypes II-P and II-L. Types II-P reach a plateau in their light curve while types II-L have a linear decrease in their curve. The cause of this is believed to be due to differences in the envelope of the stars. Type II-P supernovae have a large hydrogen envelope that traps the energy released in the form of gamma rays and releases it at lower frequencies, while those of type II-L, it is believed, have much smaller envelopes, making them smaller amount of gamma ray energy in visible light.

The masses of the stars that give rise to supernovae range from about 10 solar masses to 40 or 50. Beyond this upper limit (which is not known exactly), the final moments of the star are complete implosions in which nothing Escape to the black hole that forms, quickly and directly, swallowing everything before a single ray of light can leave. These stars literally fade to death.

It has been speculated that some exceptionally large stars could produce hypernovas upon extinction. The proposed mechanism for such a phenomenon would be that after the sudden transformation of the nucleus into a black hole of its poles, two relativistic plasma jets will emerge. These intense emissions would occur in the gamma ray frequency band and could be a plausible explanation for the enigmatic explosions of gamma rays.

The first phase of the supernova is a rapid collapse of the nucleus unable to sustain itself. This entails a strong emission of photons and neutrons that are absorbed by the inner layers thus slowing their descent. Simultaneously a shock front of neutrinos is generated during the neutronization of the compact nucleus. Finally, the neutrinosphere hits the roof and transmits its moment by expelling the layers and producing the supernova explosion 

Names of supernovae

Supernova discoveries are notified to the UAI (International Astronomical Union), which distributes a circular with the newly assigned name. The name is formed by the year of the discovery and the designation of one or two letters. The first 26 supernovae of the year bear letters from A to Z (eg Supernova 1987A); the following lead to a, ab, etc.

Featured Supernovas

X-ray image of the SN 1006 supernova, taken by ASCA, a NASA satellite for the study of cosmic rays.

Below is a list of the most important supernovae seen from Earth in historical times. The dates given indicate the moment they were observed. In reality, supernovae occurred much earlier because their light has taken hundreds or thousands of years to reach Earth.

  • 185- SN 185- references in China and possibly in Rome. Analysis of data taken in X-rays by the Chandra observatory suggests that the remains of the RCW 86 supernova correspond to this historical event.
  • 1006- SN 1006- Super bright supernova; references found in Egypt, Iraq, Italy, Switzerland, China, Japan and possibly France and Syria.
  • 1054- SN 1054- It was the one that originated the current Crab Nebula, reference is made of it by Chinese astronomers and, surely, by Native Americans.
  • 1181- SN 1181- Chinese and Japanese astronomers report about it. The supernova explodes in Cassiopeia and leaves the neutron star 3C 58 as a remnant, which is a candidate to be a strange star.
  • 1572- SN 1572- Supernova in Cassiopeia, observed by Tycho Brahe and Jerónimo Muñoz, described in the book of the first De Nova Stella where the term “nova” is used for the first time.
  • 1604- SN 1604- Supernova in Ophiuchus, observed by Johannes Kepler; It is the last supernova seen in the Milky Way.
  • 1885- S Andromedae in the Andromeda Galaxy, discovered by Ernst Hartwig.
  • 1987- Supernova 1987A in the Large Magellanic Cloud, observed a few hours after its explosion, was the first opportunity to test modern theories of supernova formation through direct observations.
  • – Cassiopeia A – Supernova in Cassiopeia, not observed on Earth, but it is estimated that it exploded about 300 years ago. It is the brightest remnant in the radio band.
  • 2005- SN 2005ap – This type II supernova is the brightest ever observed. It became up to eight times brighter than the milky way. This makes it surpass SN 2006gy almost twice.
  • 2006 SN 2006gy in the nucleus of the galaxy NGC 1260, is the second-largest that has been observed to date, five times brighter than the supernovae observed previously, its brightness was 50,000 million times that of the Sun. originated by the explosion of a star of 150 solar masses.

Galileo used supernova 1604 as proof against the Aristotelian dogma prevailing at the time, that the sky was immutable.

Supernovae leave a stellar remnant behind them; The study of these objects helps a lot to expand knowledge about the mechanisms that produce them.

The role of supernovae in stellar evolution

Supernovae contribute to enriching the interstellar medium with metals (for astronomers, metal is any element heavier than helium). Thus, after each generation of stars, the proportion of heavy elements increases. Higher abundances in metals have important effects on stellar evolution. In addition, only star systems with sufficient metallicity can develop planets. Greater metallicity thus implies a greater probability of planet formation but they also contribute to form smaller stars. This is because accretion gas is more sensitive to the effects of stellar wind the more heavy elements it possesses. Well, these absorb the photons better.

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