AT 2018 COW- the mysterious Supernova
It suddenly came out from nowhere, the mysterious entity pulled out the attention of many astronomers. It all started on June 16 2018, 10:35:02 UTC at Haleakala Observatory of Hawaii. A bright entity from Hercules constellation near CGCG 137-068 galaxy attracted astronomers. Before getting into AT 2018 cow, let us first understand supernova
What is a Supernova? And how it is formed?
Supernova is a transient astronomical event or simply a transient, occurring at the last stellar evolutionary stage of a star’s life. Stars have a cycle of life, from the remnants of a supernova that is a dead star birth of a new star can take place. Supernova in simple words is a phenomenon where star dies with a titanic explosion. On the way, this transient occurs supernovas are classified into two types which is type I and type II.
Type I supernova always occurs in a binary star system, where it is compulsory that one of the stars is a White Dwarf star. A binary star system is a system in which two stars are orbiting one another. Other star can be anything from a red giant to even a small white dwarf. The white dwarf collects matter from its companion star. When stars collect matter there is a change in mass of the star. The Star is stable till its mass is equal to Chandrasekhar mass i.e. 1.4 solar mass, but as soon as it crosses this threshold there’s a catastrophic collapse resulting in the creation of the supernova. Stars in this categories have converted all hydrogen into helium and helium into carbon forming a dwarf star.
Type II Supernova occurs with the star much larger than our sun at the red giant stage. The star in this category has mass at least 8 times to the mass of the Sun. The star having the mass less than this end up with a carbon core, forming a dwarf star. But in this type, the star goes through different end stage of their life. In the core of a star, pressure and temperature are high enough that atomic nuclei can be squeezed together and fuse. This releases energy and creates heavier elements. Hydrogen fusion makes helium and helium fusion makes carbon. Stars in type I ends up with carbon core because it is enabled to create high temperature required to fuse carbon, but in the case of a star having the mass equal to or greater than 8 time mass of sun the star is capable to create a high temperature in its core. The star can produce can produce 500,000,000 °C i.e. 500 million degrees temperature such a huge number! Imagine what will happen if we take water there as water boils at 100°C! So in this type of star at different ages, the star fuses an element, create a heavier one, then the heavier one builds up until the core contracts and heats up enough to start fusing it. So carbon fusion makes neon, contracts and heats up enough to start fusing it. The carbon fusion makes neon, magnesium and sodium. When the core reaches about a billion degrees then Neon fusion creates Magnesium as well as some oxygen now these elements shrinks, heats up to about 1.5 billion degrees and then oxygen fuses creating silicon. Then it builds up until next suitable temperature for fusion is there. At 2-3 billion degrees silicon fusion occurs, silicon fusion creates iron and this is the biggest trouble. Once silicon fusion starts the star is a ticking time bomb which can explode anytime.
Why is it so dangerous to have a star building an iron core?
In the previous cases, during fusion energy was created that transformed energy into heat but it’s different in the case of iron. Iron does not create energy, rather absorbs the energy and because of that, the core tends to lose temperature. What keeps the iron core from collapsing is the repulsive force of electrons. As more and more silicon convert into iron, the core of star expands and once it crosses the Chandrasekhar’s limit i.e. 1.4 times the mass of sun then the balance of forces ends and could not sustain the electron degeneracy pressure. As this happens the core collapse in a fraction of second with speed of 10% to the speed of light. This event occurs in a very fraction of seconds. So now the temperature of the core goes to 9999999 degrees Celsius. The temperature is so immense that now at this stage photodisintegration occurs, which means that the photon will come in and break up iron into smaller nuclei. So now what happens is that the pressure is so high due to which electrons are forced to combine with proton forming neutrons. In the formation of neutrons, gamma rays and neutrinos are formed. When this happens the core is neutral but the formation of gamma rays and neutrinos tends to push the stars outer layers with a high velocity but the impact of neutrinos on the core increases the temperature. Because of these two things the outer layer are blown out forming a type II supernova.
Subtypes of supernova
|Type I||Type IA||White dwarf||No H2/ No He|
|Type II||Type IB||Core collapse||No H2|
|Type IC||Core collapse||No H2/ No He|
|Type II||Type 2||Core Collapse||Strong Hα Line|
So supernova has two major types, Type I and Type II. Type I occurs due to the explosion of a dwarf star and Type II occurs due to core collapse of a giant star. Now type I has one more variant that is Type Ia. In type I the star ends up with a carbon core so when it explodes no hydrogen or helium is found. But when Type II collapses then the outer layers have hydrogen and oxygen but there are some subtypes of type II, namely type I B and type I C. In these two types when the Star does not any hydrogen in its outer layer then no hydrogen is detected and this lies in type IB. When both hydrogen and helium are not detected then it lies in type IC.
Naming a supernova
Supernova is named by International astronomical union’s Central Bureau for Astronomical Telegrams. Every supernova discovered is been reported to this department then IAU sends circular assigning the name to the supernova. The name is in the format SN Followed by some numbers and letters. here SN stands for SuperNova, the number followed stands for the year and the suffix letters stand for the number of supernovae discovered in that year. SN 2018cow represents a supernova discovered in the year 2018.
How is AT 2018 Cow different from a normal supernova?
- 10 and 100 times the brighter than the host galaxy
- Too bright to be fuelled by radioactive decay of nickel, the process that drives supernovas.
- A very rapid rise in brightness, a few days at most, as compared to a few weeks for supernovas.
- A smooth optical spectrum; no lines of absorption or emission reported, unlike a supernova.
- Brightest at all wavelengths from radio to x-rays, also not resembling the energy output of a supernova.
- A very rapid expansion into space is indicated, faster than expected in a supernova.
This Supernova occurred in a galaxy named CGCC 137-068 which is ~200 million light years away from us. Present in constellation Hercules
Malhar Kendurkar, Director of Prince George Astronomical Observatory, Canada defines the transient as, “The expansion speed of a Type Ia supernovae is measured by the rate at which Si II expands in the outer layers of the supernova photosphere. Si II velocities are typically between 11,000 and 12,000 km/s, but range as high as 30,000 km/s and as low as 5,000 km/s. Other supernovae exhibit a similar range of expansion velocities, although not as high as the fastest Type Ia supernovae. Most Type II and Type Ib/c (like SN 2018cow which we are studying) supernovae exhibit velocities are from the range of 5,000− 10,000 km/s. Depending on the explosion properties and environment of the supernova. We still need a lot more spectroscopic observations for SN 2018cow. We were observing but we are not sure about this transient. The type and everything.
It literally just appears from nowhere.” He also mentioned, “We will publish for this supernova again next week! It’s getting fainter and fainter every day.” Adding to it “Today I published one more photometry observations and found out how the change in amplitude is ~0.11 mag/day.
Before it was 0.35mag/day. (STRANGE!!)”
What impact do such events does on the daily lives of the common man?
Reading the title you must feel that I would be talking about some astrological impact of this event on you and how it will affect your future. But it’s very strange that I will talk nothing about astrology but still let you know how this event effects on all the common man or the user reading this article. The most pity thing and the truth about astronomical events are that non-astronomical background people only search for astronomical things to have a wallpaper or screensaver on their laptops, mobile or rooms. Here I would like to tell you that, (speaking with respect to a supernova) every single atom in your body or everything present around was once made from such a big explosion. Yes, we can say that everyone around you exhibits a relation between each other. Isn’t this fascinating that everything is in a relationship with each other unknowingly. Such a supernova explosion leads to create nebulas and after that stars, planet and solar system. So after some billion years this supernova may also exhibit life or may not we still don’t know what the future is but studying these strange events will help us to know more not only about supernovas but also our existence.
Images and light curve were taken by Malhar R. Kendurkar