Type 1a Supernovae - Essay
A disclaimer before anyone gets into it: due to the complete idiocy of my instructor, I have been banned from using the verb "to be" in any form for any reason - including quotations, idioms, helping verbs, etc. (Amongst other words also, but none are quite so annoying.) Each word on his "banned word list" ducts me one point, no exceptions. Considering I don't want to be failed on account of not playing his ridiculous little game that supposedly prepares us for college essay writing, I had to write it like this. Sorry. I have to get at least a C on the paper to even graduate, and it's like half my grade for this particular class, so... :P
I haven't done too much editing on it. I still have to go through still and add in all my citations (I typed this mostly from memory :P) A lot of the phrasings are pretty convoluted, forced by his "guide to better writing", and I don't know if it's even possible to fix some of them. Also, I know this is pretty technical stuff (he might fail me for doing a topic of relative complexity compared to, oh, the history of baton twirling, knowing him *kicks*) so pointing out any places where I could try and be clearer and stuff would be really helpful also. (I cut about 500 words of the more technical yabbering already. :P) I don't really expect anyone to be able to correct any of this, but if anyone *coughs* FH! *coughs* has any suggestions as to things I might have omitted or should mention... that'd be sweet too.
I'd love you forever if you could add suggestions in any way. Seriously. Like... I'd-bake-you-cookies-and-write-you-bardic-odes love you. Not to mention a "edit an obscenely long essay of mine free" card. :P It's due Friday, so there's a week if you can find the time.
Version 2.0, edited one early Thursday morn. Wouldn't mind more concrit, but this is probably what I'll turn in. I'll run around and thank peeps later, what with the cookies and bardic odes and all.
Without further ado:
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Type 1a supernovae number among the brightest, most spectacular explosions in the universe. They have proven important in virtually every topic within astrophysics, from determining distances to distant galaxies to calculating the rate of expansion of the universe to discovering the triggers of star formation. Though in many ways the phenomenon of the type 1a supernova remains an enigma, scientists have also come closer and closer to understanding the mechanisms and consequences of these explosions.
The current theory explaining the mechanisms that cause type 1a supernovae proposes that the progenitors orbit in a common binary system. One star must have evolved to become a white dwarf, a hot, dense, but rather small star that fuses hydrogen and helium in its core. The companion star in the binary system varies greatly, potentially any main sequence or red giant star, as long as the white dwarf can accrete matter from it. ‘Double degenerate systems’ refer to binary systems consisting of two white dwarfs; ‘single degenerate systems’ refer to binary systems with only one white dwarf (Koester 54).
The white dwarf slowly accretes matter from the companion star as they orbit each other. As the white dwarf becomes more and more massive, the core heats up due to the increase in gravitational pressure. Eventually the core becomes hot enough to fuse yet heavier elements, a point called the Chandrasekhar Limit at 1.4 solar masses.
This “deadly stellar tango” appears unstable as the white dwarf approaches this limit. For example, RS Ophuichi, a potential type 1a supernova in the making, has flared up several times in the recent past. Theoretically, these flares occur when the “thermonuclear flame has swept across the face of the star without quite catching hold” (Pease).
Most white dwarfs never even reach this point: perhaps summing the masses of the two stars results in less than the Chandrasekhar Limit, or perhaps the stars drift apart and no longer accrete matter from each other. Regardless of the reason, type 1a supernovae have proven exceedingly rare. Out of all the trillions of stars in a Milky Way-size galaxy, perhaps one type 1a supernova in 500 years will grace the skies (Leibundgut 184). Fortunately, from Earth we can observe billions of galaxies from the furthest reaches of the universe, so generally scientists observe about 200 supernovae in any given year (Leibundgut 180).
Once the heavier elements have begun to fuse in the core, the white dwarf explodes. The star quickly rises to maximum luminosity by almost half a magnitude per day for up to twenty days (Leibundgut 185). Type 1a supernovae briefly outshine even their parent galaxies, which typically consist of trillions of stars. Observations of type 1a supernovae during the rise to maximum have proven rare because while the stars remain dim and thus inconspicuous, astronomers rarely have telescopes pointed at the right patch of sky. However, the occasional happy coincidence has allowed for a few early peeks at type 1a supernovae.
After maximum magnitude, the luminosity begins to decline in a characteristic curve. After about 150 days, the star has dimmed to about five magnitudes below peak brightness, often disappearing entirely into the haze of the parent galaxy (Leibundgut 188). Due to the near impossibility of observing such dim, distant objects, observations past that point remain virtually nonexistent. No remnant appears to remain: the star, after one great flash of light, becomes nothing more than scattered gas and photons slipping through the cold silence of space.
However, many observations challenge this basic theory, exceptions that scientists can only hazard guesses at as to the explanations. These especially worry astrophysicists, who often depend on type 1a supernovae observations to calculate distances and universal constants, because many of these exceptions do not appear immediately obvious as such.
Some explosions display the same light curve as the typical type 1a supernova-but appear to two to three times the theorized maximum luminosity (Shiga, Preuss). It seems unlikely that explosions with the same emission lines and the same shape and the same duration as type 1a supernovae would somehow form in any other conditions, so most scientists consider these a new variety of type 1a supernovae.
Most scientists agree that because these explosions appear brighter, it would merely mean that the progenitors had had more mass to explode with. Thus, the circumstances must have allowed for the white dwarfs to surpass the typical Chandrasekhar Limit, leading scientists to term these stars “Super-Chandrasekhars.” Observations that the ejecta from these explosions generally have low kinetic energy, meaning more gravity pulling inward, support this (Preuss).
One theory explains this circumstance by suggesting that the white dwarf rotates quickly enough that centripetal acceleration in part counters the gravitational pressure upon the core. This would allow for more accretion of mass before reaching the critical limit (Preuss). However, why a white dwarf might rotate at such high velocities remains anyone’s guess.
Another theory suggests that, instead of slowly accreting the required mass, the two stars collide, resulting in a net mass greater than the Chandrasekhar Limit. Binary systems where the stars orbit close enough to each other for this to occur only account for 2-20% of single or double degenerate systems (Koester 54). However rare, such collisions could account for some of the exceptions, in particular type 1a supernovae originating from globular clusters. Astronomers already suspect that a class of stars, termed ‘blue stragglers’, found those compact clusters may originate from white dwarf collisions (Murphy).
Yet another theory suggests that these explosions merely look like type 1a supernovae, actually exploding as ‘disguised’ versions of type II supernovae. This explains the extreme brightness. However, the reason for such extreme distortion of the light curve remains unexplained.
Astrophysicists have also noticed an age paradox amongst type 1a supernovae. White dwarfs typically form in old stellar populations; the complete absence of ‘simple’ elements, such as hydrogen and helium, and the strong presence of more ‘processed’ elements in the spectra strongly support this (Leibundgut 196). Type 1a supernovae, however, find themselves more commonly observed in younger elliptical galaxies than in older spiral galaxies. At that, type 1a supernovae also appear to have a preference for the intermediately aged star-forming spiral arms (Leibundgut 184). To say the least, scientists scramble for an explanation. Why would younger white dwarfs supernova more commonly? Does the differing proportion of heavier elements in the core between younger and older stellar populations have anything to do with it? Might younger binary systems structure themselves in a way that makes them more likely to accrete matter from each other? Some astronomers even suggest that the type 1a supernovae that originate from older populations could classify as a different type of supernova entirely. Few convincing theories have emerged from such anxious questioning, which will hopefully change soon.
Some type 1a supernovae display hydrogen lines in the spectrum. Hydrogen should have fused into heavier elements by the time the luminosity peaks. Indeed, the lack of hydrogen has proven one of the most reliable indicators of type 1a supernovae, considering most objects in the known universe contain this most common of elements. Some theories suggest that the hydrogen comes from the lighting of a circumstellar envelope during the explosion. However, considering the great age of white dwarfs, all significant circumstellar matter should have accreted onto the star by that point (Leibundgut 198). Other theories suggest that the explosion of the white dwarf might have triggered another explosion in the partner star, which might still have hydrogen. However, the dynamics of the companion star during the explosion remain relatively unknown, and that the companion star could burn enough hydrogen to interfere with the spectrum of the actual supernova seems unlikely.
Many type 1a supernovae display small, peculiar variations that have no apparent cause. Some display a ‘second maximum’ 21-30 days after the first in which the infrared spectrum peaks, but many do not. Of those that do, some appear to ‘plateau’ instead of displaying a more standard curve (Leibundgut 187). Some explosions with type 1a supernova curves appear to have no host galaxy, essentially making them random explosions in the emptiness of intergalactic space (Hecht). Some explosions have asymmetric ejecta patterns, others spherical, and others still bipolar. Some have displayed x-rays in the spectrum, which according to theoretical calculations shouldn’t even occur during the explosion (Siegfried).
Still more remain simply unknown. The dynamics of the companion star during the explosion remains unobserved in practice and unexplored in theory. Differences between the possible companion stars-whether the progenitor exists in a double or single degenerate system-remain unanalyzed in respect to the final explosion. What happens one, ten, one hundred years after the explosion remains unknown. The fate of the remnants of the white dwarf and the companion star seems lost in the haze of obscurity. The precise mechanism of the thermonuclear detonation has never had a successful computer model that can match observational data (Leibundgut 199).
Scientists struggle to answer these questions, often frustrated by their powerlessness. Like the rest of the human race, they suffer this paltry rock that sits eternities away from these fascinating objects. Indirect study remains the only thing possible for astronomers. However, the answers should become clearer with the passage of time, which should provide some satisfaction.
New technology, particularly advances in telescopes, will help astronomers see more, and the data that they glean will improve in quality. Basically, the bigger the telescope, the better; larger telescopes can take in more light, which results in more detailed observations to greater distances. Space-based telescopes have less interference from the atmosphere, resulting in clearer, sharper images, and can also observe wavelengths that the atmosphere otherwise absorbs. As those two qualities become more prevalent, the data with which scientists work will become better and more abundant.
Further empirical analysis will also help. As scientists find more and more trends between different type 1a supernovae, they can create a more precise picture of what a ‘standard’ type 1a supernova should look like. Also, empirical analysis can find patterns between the variations, which will help scientists identify different varieties of explosions and begin working out the explanations for those. Such separations will keep the data as pure as possible for the calculation of universal constants.
The mere passage of time, however, continues as the best way to ease the questions. As astronomers observe more and more type 1a supernovae with better and better telescopes, empirical analyses will reveal new correlations that fresh generations of astrophysicists will pour over. Perhaps this requires a bit more patience than what current scientists can satisfy themselves with, but they have a vast universe full of questions to try their hands at answering. No scientist could ever claim to feel bored.
Astronomers have little reason to feel bored about type 1a supernovae quite yet, however. Type 1a supernovae have become an important study focus in astrophysics. The measurements gleaned from observations have many different uses, and these explosions have also proven important to not only the creation of life but perhaps even the extinctions of the past-and future.
Due to the sheer radiation output from the explosions, they have proven detectable at distances greater than with any other single object. Combined with the uniformity of the absolute magnitude at maximum luminosity, they make for perfect distance indicators. Type 1a supernovae have proven especially useful for calculating distances beyond the range of more close-to-home, like Cepheid variables or trigonometric parallax.
In particular, type 1a supernovae measurements have helped scientists determine the rate of expansion of the universe. For example, type 1a supernova 1997ff occurred nearly 11.3 billion years ago, yet due to its great distance its light only arrived at Earth a few years ago (Preuss). The distortion of its spectrum, along with the spectra of other ancient type 1a supernovae, has allowed scientists to determine that during the time of the initial explosion the rate of the expansion of the universe decelerated. Once compared to the distortion of the spectrum of more recent type 1a supernovae, scientists have concluded that the expansion of the universe now accelerates. The repercussions of this conclusion has caused drastic rethinking of the nature of dark matter and energy-another hot topic in astrophysics today.
Measurements from type 1a supernovae have proven useful for the study of a wide variety of other topics and structures within astrophysics. For example, the interstellar medium can distort the light emitted by the explosion. This gives astronomers a chance to study the structure of the interstellar medium, which otherwise seems almost impenetrable to scientific study (“Flashes from the Past: Echoes from Ancient Supernovae”).
In the words of Evele, “We oughta thank supernovas because they made life possible.” Little seems truer. During the course of the explosion, hydrogen and helium fuses into heavy metals that could not otherwise exist in the universe-such as iron, found in virtually every living being (Koester 53). Almost all of the technology we use depends on these elements, from automobiles to refrigerators. The powerful shockwaves from the explosion may also trigger the collapse of nebulae and thus cause star formation. Without a star to heat the planet, a few twiggy proteins in the primordial soup would never have even had a chance to evolve to the complex creatures that might eventually read this paper.
However, scientists have correlated nearby supernovae explosions with some extinctions on Earth, including the one that ended the reign of the woolly mammoth (Krotz). The radiation output from a supernova explosion could easily kill anything within its immediate star system (Evele). The human race appears protected by not only a nice thick atmosphere, but also millions or billions of light years of distance from the explosion. If a star exploded near our planet, however, neither our amiable atmosphere nor our current technology could save us from certain extinction. Fortunately supernovae remain a rare event, and the stars that do show some potential for a supernova event probably won’t go for another few million years, which gives the human race ample time to think of defense (or escape) mechanisms.
Type 1a supernovae have become and will remain one of the most critical subjects of study in astrophysics today. Not only have they proven important to the dynamics of the universe, but they have also made themselves useful for human understanding of other objects of interest. Though many problems remain with the current theory, scientists have also gotten closer and closer to understanding the intricacies of these fantastic explosions.