Helioseismology, simply put, is the study of the Sun’s core through the observation of wave propagation that is visible on the surface. This is possible due to both the continual movement of plasma to the sun’s surface, as well as the constant radiation from the heat, that forms convection cells resulting in the investigable convection currents or “waves”. As each cell and consequently current, have varying fluctuations in pressure, the Sun is left with its uneven surface. As well as this, sound waves also drive fluctuations to the surface creating a more unsteady surface. In this way, we are able to study the sun much like how we study the earth’s interior via seismic waves during earthquakes, in turn being called ‘Helioseismology’.
With the knowledge that waves resonate radially to the surface, we are able to investigate temperatures, chemical composition as well as the changes and variances between each layer of the sun through an analysis of the amplitude of and periods, which is usually five minutes, of the convection currents. Once these waves reach the surface, or “Photosphere”, they are reflected and refracted, with their direction of motion shifting constantly until reaching the surface once again, where the process is repeated. These mysterious oscillations were in 1975, proven to be sound waves that are generated by the Sun and then confined within.
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As previously stated, sound waves by the chaotic movement of convection cells in the sun’s core, this causes the sun to undulate in an innumerable distinct “P-modes”. The “P” refers to the notion that it pressure causes the vibrations, and mode conveys the variance of possibilities. By identifying them, an analysis of inner sun conditions is made possible. For example, a reduction of pressure of sound energy towards the solar centre, conveys nuclear reactions of hydrogen to helium, or that the percentage of helium is quite similar from core to surface. Providing the notion that heavier elements are attracted to the centre of the solar entity, thus leaving a helium deficit in the convection zone, which is the outermost inner layer of the sun.
Another form of helioseismology is one that focuses more on more localized areas of the sun rather than the entire object and utilises 3D imaging to recreate the subsurface levels of the sun to better understand unusual occurrences such as ‘sunspots’. We now know that these relatively darker and cooler regions of the sun are caused by certain areas or ‘spots’ of strong magnetic energy which affects the flow of convection currents, causing it to last for, at times, weeks. This is just one of the few phenomena that could affect the Earth. Others include coronal mass ejections and solar flares caused by, once again, magnetic disturbances that could lead to danger for astronauts or shuttles that are in space, or even power outages on our planet. The detection of these events or irregularities is key in preparing for worst case scenarios.
To acquire the specific information on wave properties, scientists utilise the “Doppler shifting of light”, where the apparent changes in wavelength which would thus demonstrate different oscillation points and allow scientists to receive a better idea of the interior solar conditions. Thus, showcasing how helioseismology is key in understanding the inner workings of our sun.
Solar Neutrinos
Solar Neutrinos, as the name suggests, are neutrinos from the sun. These neutrinos are created through the process of nuclear fusion within the core of the sun where hydrogen atoms fuse to make helium. This atomic reaction leads to the release of an almost massless, neutral particle that travels at the speed of light, which carries a small amount of energy of which was produced in the initial reaction. Although there are many other particles that are ejected during this thermonuclear process, many of these are stifled by surrounding matter. However due to the neutrino’s lack of a charge, it is able to travel to the earth where it can be detected by specific instruments. Since, it is directly from the Sun’s core it provides a key insight into the inner make up of the Sun itself.
The incalculable number of solar neutrinos that flow to the earth from the sun every moment, are resultant from the vast amount of protons reacting in the solar core. We were unable to detect solar neutrinos for a long time due to their chargeless and almost massless nature, being first detected in the 1960’s. However, the result was much less than what scientists had predicted, this was then followed by a slew of other experiments that ended in the same way. The fact that experimental flux was less than half of the theorised flux, put into question the understandings of the sun as well as the Standard Model (SM) of Particle Physics.
Until the 2000’s when distinct variations of neutrinos were found, electron, tau and muon. The reasons for the previously unsuccessful tests happened to be due to the fact that electron neutrinos changed types during their journey to the earth with “Neutrino Oscillation”. These were undetected at the time as only the electron neutrinos were collected, thus solving the ‘Solar neutrino problem’.
Since it has not even been two decades since the solving of the ‘solar neutrino problem’, the discoveries made past that have not been too plentiful. Nevertheless, the following provide further insight into the inner solar environment than ever before. The international Borexino experiment, which has been ongoing since 2007, discovered neutrinos that were birthed from different reactions than what was originally theorised, noting that these involved heavier elements such as boron. Although as of now, the experiment is not that precise, it can still infer the temperatures of the core, thus quantifying the number of various elements within the sun. Thus demonstrating the multitude of practical applications, the study of solar neutrinos has for understanding the solar core.
Life Cycles of Stars
Simply put, stars are massive exploding balls of gas that contains mostly hydrogen and helium, which through nuclear fusion release large amounts of heat and light. It is thought that stars are originally created from the contraction of hydrogen and helium clouds, caused by the pull of gravity. Due to gravity being the only attracting force, it begins as a weak adhesion, which slowly grows stronger as the density of the clouds increase there by increasing the gravitational energy. Thus leading to particles accelerating to the centre of the mass, in turn starting nuclear fusion, which has such a tremendous outward force it acts against gravitational contraction, thus resulting in a stable celestial body.
This then thrusts the star into the ‘Main Sequence’ which is a series of categories the majority of all stars belong to, represented by the band in the Hertzsprung–Russell diagram. This is where stars spend about ninety percent of their life. The category or ‘type’ of star can be decided based on age, mass and composition, and is what separates a low-mass star like the sun from one twenty times the size.
The sun is currently within the main sequence and is made up of mostly hydrogen at around 70%, helium at about 28%, and the remaining 2% coming from nitrogen, carbon, oxygen, iron, neon, silicon, magnesium and sulfur. As its life cycle progresses, a collapse of the core is imminent due to the loss of helium, as this occurs, outer layers are discharged, and particles radiate away. Since there is no longer fusion occurring, the core falls victim to the incredibly powerful inward gravitational force, becoming denser and hotter releasing heat as light energy. This luminesce is why, in this stage of life it is aptly called a ‘white dwarf’. Then eventually as it cools down it becomes a ‘black dwarf’ thus thrusting it to the end of its cycle, which will take billions of years.
A star that is about twenty times the mass of our sun, would have a drastically different path. The higher the mass the higher its tendency to burn out quicker due to their more rapid use of its hydrogen, this also means that large stars have higher luminosity due to the higher temperatures to counteract the more powerful gravitational contraction. An example of such a star that is twenty times the mass is Betelgeuse, a red supergiant in the Orion constellation, it is a thousand times the size of our sun, if it replaced our sun, it would almost reach Jupiter. It is called a ‘red giant’ due to the red glow that emanates from it, this is due to it being further on in its life cycle than our sun, meaning nuclear fusions have ceased and the outer layers have begun to expand. As it is expected to die in the next thousand years, it has already been fusing helium into carbon. From here the gravitational energy continues to coalesce carbon atoms through fusion and forms oxygen, nitrogen and iron, which is the stage Betelgeuse is currently at. The core ceases to fusion when the core is just iron due to the stable properties of the element, eventually causing an implosion, called a supernova, this may then leave a dense neutron star, or even collapse into a singularity creating a black hole.
In conclusion, through the study of the sun and its inner core, via helioseismology and solar neutrinos allows for us to understand not just our solar system but other celestial objects in the vast expanse of the universe, even those twenty times larger. This then grants us the ability to comprehend the differences and various changes that occur during a star’s lifespan. The recency of most of these major discoveries, affords the idea that we are just at the tip of the iceberg, that is the universe.
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