The purpose of this paper is to develop a fundamental idea about the phenomenon of Supernovae and their significance in understanding the nature of our universe. This paper shall be trying to make an in-depth analysis of different types of supernovae that had been observed in universe and also the factors that cause them. Even though the paper does not go deeper into the technical aspects of observing supernovae and the characteristics of their each type, a significant amount of study has been conducted to make an understanding of how they are observed and the characteristics that make each of them unique. There is a detailed description on Type I and Type II supernovae and their further classifications. Their spectral characteristics as well as light curves are used to explain the theories of their origin and how the observed data support them. The study of supernovae has attained unprecedented prominence among Astronomers in recent years. This is due to the common belief that observing supernovae can help us explain the nature of our universe and ultimately, predict its future. This paper also has tried to make an understanding of such claims and make a credible viewpoint in introspection.
Supernovae
Supernovae are the cataclysmic explosion that marks the death of a massive star. Such enormous explosions that scatter tremendous amounts of energy into the surrounding medium can be observed in the night sky as the sudden appearance of a new brighter and a more vivid star that stays in the sky only for a few months. At the peak of its brightness, supernovae are known to emit more energy than that a solar like star would emit in its life time. The shockwaves from these explosions can travel faster than the speed of light (almost 10% faster) into the interstellar space surrounding the supernovae. The stellar materials and gases that are emitted and are later gathered from the surrounding space by the shockwave form a giant shell of gas and dust. This is termed as supernova remnants. After careful observations of supernova remnants from four different space telescopes, NASA has recently created an image of the oldest documented supernova explosion in the world’s astronomical history . The supernova that was responsible for these remnants, termed as RCW 86, was witnessed by Chinese astronomers as early as 185 AD. They had written about the appearance of a mysterious guest star that faded from the sky after eight months in some ancient astronomical scriptures.
There are mainly two types of supernovae 1. Type I: associated with low mass white dwarfs 2. Type II: associated with stars of mass ten times greater than solar mass . Both of them have a different origin story. Stars whose size are comparable to (or less than) that of solar mass cannot explode on its own. They, after thousands of years of evolution, become carbon-oxygen white dwarfs by ejecting their outer mantle. These white dwarfs, if situated in a binary system with a companion star, strips matter from the companion due to the dwarf’s gravitational pull. The accretion of matter caused by this serious gravitational pull can or may cause matter from the donor to reach Chandrasekhar Mass (1.4 solar mass), at which point (as the dwarf consists mainly of carbon and oxygen) the dwarf will simply incinerate until it finally leaves nothing else other than the donor star . This is the common type of supernova observed in the universe, Type I supernovae.
Type II supernovae are caused by stars of relatively higher mass than that of sun (almost 8 times solar mass). These stars simply do not die quietly. Upon reaching its end, these stars do not eject their mantle to continue as a white dwarf. They, during its last thermonuclear stages, shall continue to grow in mass (due to nuclear fusion) until finally surpasses the Chandrasekhar Mass. As these stars are now formed due to heavier metals (such as iron) they do not incinerate like the white dwarfs of lighter stars. As they grow heavier (than Chandrasekhar Mass) the electron degeneracy rate can no longer support the star from imploding. Within a fraction of a second, the core of these stars (almost the size of a planet) will implode into something very smaller (with size comparable to a few kilometers), causing the core to bounce back and launch a supernova explosion . These complicated processes take place within a fraction of a second. The remains of such explosions are usually a neutron star and in extreme cases, a black hole.
Supernovae Observation Methods
Supernovae events are usually seen in the observable night sky (using only our naked eye) as a bright star that lasts only for a few months. But this is not enough to make sufficient judgments about their origin, characteristics, and their age. Scientists use various kinds of telescopes that work at different wavelengths to record the emissions from these supernovae on a spectrometer. Each line observed on these spectrometers corresponds to a particular wavelength and each wavelength corresponds to a particular emission such as Hydrogen, Helium, Iron, Oxygen, etc. Understanding these emissions has helped astronomers discover the nature of the origin of supernovae and their distance from earth (actually the distance from their host galaxy). The emission spectra of these supernovae can also help in determining the characteristics of their parent star. For example, presence of Hydrogen lines in the spectroscope means that the star that caused this Supernova is possibly young. And similarly if there are prominent lines of Iron or similar heavier elements on the spectral data of a supernova, it is possible that the star that caused this is rather old.
But astronomers simply do not rely on one single tool to observe supernovae. Ordinary optical telescopes can also be used in this regard. Optical telescopes are helpful in observing the brightness (recorded through filters) as well as record their luminosity . Astronomers relied on light curves (that indicate luminosity) of supernovae to classify them in the early days. Careful observations of these data, both light curves and spectral data, are used by astronomers to determine the origin, nature, and causes of each supernova.
Categorization of Supernovae
With the advent of technology and availability of advanced analysis tools, more detailed classifications were made for all the supernovae observed in the universe. The most prominent way of categorizing supernovae is by using their spectral data. This is due to the fact that each variety of supernova yields unique characteristics in spectral charts. Quite withstanding with the theory of supernovae origin, the group of supernova that does not contain hydrogen lines in their spectrum are Type I supernovae. And in that, Type Ia group has a strong silicon line at 615nm . If Type I supernova has a strong helium line, it is called Type Ib and supernovae without such lines are classified as Type Ic. Actually, the absence of hydrogen in the spectra of Type I supernovae is consistent with the theory of their origin. The white dwarfs that are believed to cause Type I supernovae are usually devoid of any hydrogen (contains mainly Oxygen and Carbon).
Type II supernovae are quite different in that front. There is a strong hydrogen and helium spectrum observed in their spectral data. This is also consistent with the theory of Type II supernovae origin. It is believed that the massive stars that cause Type II supernovae has excess of hydrogen and helium in their outer shells as the heavier elements formation near the core usually do not affect the outer core. So even when the core is collapsing during Supernova explosion, the outer shell (with excess of Hydrogen and Helium) are affected last.
There is also another observable difference in the spectra of these two types of Supernovae. This was also the first observed difference in supernovae. Type I supernovae has sharp peaks of maxima in their light curves. These sharp peaks die rather slowly compared to the decay of Type II supernovae. Type II supernovae has rather smaller maxima and die away quickly compared to Type I supernovae. This is also consistent with their theory of formation.
Type I Supernovae
Type I supernovae are the most commonly observed category of supernova in the universe. They are differentiated from Type II supernovae with the help of spectral data and the absence of hydrogen lines in it. The maximum peak in light curves of Type I supernovae are recorded at almost 10 billion solar luminosities. This light curve shall show a slow or gradual decline over time. The spectral characteristics of Type I supernova can be used to classify within them 1. Type Ia: Silicon lines but no Hydrogen lines 2. Type Ib: Spectra is dominated by Helium lines but do not have Hydrogen lines or silicon lines 3. Type Ic: Lack both Hydrogen and Silicon lines and Helium lines are usually absent in alternate weeks.
Type Ia Supernovae: Type Ia supernovae are the most prominent among Type I supernovae occurrences. They are found in all galaxies including elliptical. It is believed that Type Ia Supernovae are caused by the thermonuclear disruption of Carbon-Oxygen white dwarfs in a binary system (with a companion star). Due to the difficulty in observing dim objects near bright Supernova, the properties of companion stars are still elusive. At early phases of Type Ia supernovae, broad profiles of Silicon II, Oxygen II, Sulphur II and Calcium II lines are observed in their spectral data. And as time passes these lines give way to heavier metal lines such as Iron and Cobalt. Approximately one month later, iron lines dominate the spectral charts . Another observable characteristic of Type Ia supernova is their steep rise to maximum (about 20 days after explosion) . Recent discoveries also suggest differences in the characteristics of many observed Type Ia supernovae. Even though these characteristics have not yet been fully identified it is possible that sub categories of Type Ia supernovae maybe introduced in the future.
An interesting fact about Type Ia supernovae is that scientists have learned to use them to measure or survey the universe (by calculating distance between their host galaxies and our Milky Way). The concept here is to use these supernovae as standard candles (or beacons) and by measuring the difference between their absolute brightness and apparent brightness, we can accurately measure the distance from their galaxy to ours. Measuring absolute brightness and apparent brightness is possible by using large-aperture telescopes. These telescopes are capable of observing and taking measurements of dim objects whose dimness is caused by distance. Observations of Type Ia supernovae can also give us a verdict on the expanding universe theory and also the fate of our universe (explained later).
Type Ib and Type Ic Supernovae: Type Ib and Ic supernovae are comparatively rarer than Type Ia supernovae as they are only found in spiral galaxies (unlike Type Ia which is found in all types of galaxies). These supernovae are differentiated from Type Ia Supernovae by the absence of Hydrogen and Silicon lines in the spectral chart. There was a prominent Helium I line that separated Type IB completely from Type Ia supernovae. But soon, astronomers observed Type Ib supernovae without Helium I line in their spectral data. These supernovae were named as Type Ic.
Type Ib/Ic supernovae are usually associated with galaxies of large population of massive stars (some even bigger that the stars that usually cause Type II supernovae). In fact, Type Ib/Ic supernovae are believed to be caused by the core collapse of massive stars that have been stripped of their hydrogen (Type Ib) and helium (Type Ic) envelope before explosion . As they are originated in galaxies that host a large number of massive stars, it is expected that at some point these stars should interact with the interstellar material of other massive supernovae. This is why there is a prominent emission of radio waves from these supernovae. These radio waves are believed to be rising from the shock interaction of Type Ib/Ic supernovae with interstellar medium .
Type Ib/Ic supernovae are significantly dimmer than Type Ia supernovae. The light curves of them have been divided into subcategories of slow decliners and fast decliners. The early beliefs that Type Ib did not contain fast decliner subcategory were discarded by recent findings of Type Ib fast decliner supernovae. In recent years another spectacular observation has baffled astronomers worldwide. This was the spectral data of a Type Ic supernova with long duration of Gamma ray bursts. Indeed, Gamma ray bursts are usually observed along with Supernovae but long durations of these energy bursts can only be attributed to a very massive supernova with energies incomparable to others. These supernovae are believed to result in black holes rather than neutron stars. These high energy gamma ray bursts, if occurred nearby, could end all life on earth. But fortunately such threats do not exist anywhere near us.
Type IIb Supernovae
These are supernovae that show both the characteristics of Type I and Type II supernovae. In their early stages, Type IIb supernovae show a prominent Hydrogen line in their spectral characteristics. But as time flies, these hydrogen lines slowly vanishes and gives way to characteristics that are similar to that of Type Ib/Ic supernovae. Some of the examples of Type IIb supernovae are SN 1987K, SN 1993J, SNe 1996cb, 2001gd, and 2001ig. Among these the first Type IIb supernovae that was extensively studied and documented was SN 1993J.
The origin of these types of supernovae is believed to be from stars that had lost most of their masses before explosion . This is supported by some of the data observed in spectroscopes where the emissions from these supernovae show characteristics that are consistent with the idea of supernovae interacting with circumstellar material lost by other stars.
Type II Supernovae
Type II supernovae are recognized by the prominence of hydrogen lines in their spectral data. These stars are believed to originate from galaxies of where a recent star formation has occurred . The characteristics shown by these supernovae are consistent with the core collapse model of stars that still retain their hydrogen envelopes. According to their luminosity, Type II Supernovae has been categorized into Type IIL, Type IIP, and Type IIn.
Type IIL Supernovae: These types of Supernova show a linear, uninterrupted decline in luminosity after their maximum. This is attributed to their relatively low mass envelope . Approximately 150 days after the explosion, these supernovae show an exponential decline in light curve. As in the case of Type II supernovae, Type IIL supernovae are dominated by the H Balmer lines in their spectral data. For most Type IIL supernovae, Helium lines are visible for a few days and then they disappear. Lines of heavier elements such as Sodium II, Calcium II, Iron II, Scantium II, Barium II lines are more prominent than Hydrogen lines later into their photospheric phase .
Type IIP Supernovae: They are distinguished from Type IIL supernovae by the difference in the shape and the luminosity of their light curves. The average absolute magnitude at the peak is fainter than that of Type IIL. Following the pattern of other Type II supernovae, there is a long period of constant luminosity (referred as plateau) in Type IIP as well. After this plateau the emission characteristics of Type IIP supernovae is almost similar to that of Type IIL supernovae. Scientists believe that these types of Supernovae are formed by large massive stars that did not lose much mass before the explosion. This gives an unsolicited advantage. The study of these types of supernovae is rather helpful in understanding the characteristics of the progenitor star since the emission data from these supernovae usually directly reflect the nature of their parent star.
Type IIn Supernovae: The ‘n’ in Type IIn denotes narrow lines. Their spectral characteristics are predominated by Hydrogen lines without their broad adsorptions . The early emission characteristics of these supernovae show the presence of Helium lines. There are also unresolved lines of heavy ionized elements such Iron VII and Iron X. Scientists claim that these emissions are caused when the supernovae interacts with the interstellar medium.
Supernovae and Imposters
Not all cataclysmic explosions found in universe are supernovae. There are a significant number of supernova imposters that astronomers find daily through their telescopes. This is because the energy emissions and spectroscopic characteristics of these imposters are very similar to that of ordinary supernova explosions. But the main difference between these imposters and actual supernovae is that the latter is caused by the death of a star while the former usually leaves the parent star unhurt. The light curves observed from these imposters also differ from that of supernovae. Usually erratic light curves are observed for these imposters whose peaks are much smaller than that of real Supernovae .
Supernovae and Fate of Universe
Observing supernovae has never been just a passion project for scientists all around the world. There is a shrewd practical reason behind their chase for more data regarding supernovae. The use of Type Ia supernovae in measuring distances between galaxies was already mentioned in this paper. As Type Ia supernovae are brighter and have a slow decline characteristics, these supernovae can serve as beacons (or candles) fixed at large distances. By observing their light curves and spectral data, scientists have successfully calculated the distances between the galaxy of supernovae and our Milky Way.
One interesting advantage to this measurement is that it can help us identify the speed at which the host galaxies (of the supernovae) are receding from us due to the expansion of Universe. And the speed of expansion of universe is crucial in calculating the amount of mass in it. According to Einstein’s theory of relativity, the total amount of matter in the universe shall determine the shape of the universe. As matter curves the space and time around it, all the matter in the universe should be curving our universe. Einstein postulated that if there is more matter in Universe, more will be the curvature of it. And by progressing in that scenario the current expansion will ultimately lead to the collapse of our universe due to gravity. But if there is not enough mass in our universe (that is, enough gravity) to cause this Big Crunch (as it is called), the universe would expand forever, without a halt . So ultimately, observing supernovae can shed more light into the future of our universe.
Conclusion
Supernovae observations have gained more momentum in the last few decades than ever before. This is partly because of the development of new technology that aid scientists to make an even more thorough supernova observation every day. But the most prominent reason for their growth in popularity is, as one would easily guess, curiosity. Scientists all over the world believe that supernovae observations are their key to understanding the nature of this universe better. As the world as we know it and the recorded universe around us are believed to have originated from such early supernovae explosions, there is no doubt that observing supernovae can significantly raise our knowledge in this regard. And as for the fate of our universe, it is yet to be seen.
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