Studying stars with helioseismology and solar neutrinos

The Sun, the centre of our solar system, an object in which without life on Earth would cease to exist. The Sun provides energy to Earth which allows many important chemical reactions to take place in order to facilitate life. Our Sun is relatively hot and immense, conducting physical investigations to obtain information is impossible, therefore safe and long-distance techniques have been developed in order to study our Sun and then extend these techniques further to study other stars in the universe. There are two techniques that been proven effective in obtaining information from the interior of the Sun, these are known as helioseismology and solar neutrinos (Thompson, 2004).

How is helioseismology used to study the interiors of the Sun?

Helioseismology, a term that is quite similar to seismology, which is defined as the study of seismic waves that travel around the Earth through the events of earthquakes. Seismic waves can be used to study the interior of the Earth, data collected from seismic waves such as the speed of the waves at different time intervals can give details to the density of the areas that it travels through (Survey, 2019). Using this concept, the field of helioseismology was derived, using the data collected from the Sun’s waves, its interior can be studied. From this technique it has been discovered that the Sun does not move as a whole, that is, different areas of the Sun expand and contract at different time intervals, we know that from Edwin Hubble’s contribution to the red and blue shift phenomenon, light that is in the red shift indicates that objects are moving away from the observer and the opposite for blue (Learning, 2019). With reference to the image below:

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[image: Computer Simulation of Oscillations in the Sun. In this spherical cutaway diagram of the Sun, a triangular wedge shaped portion has been removed from the upper half of the sphere to expose the interior, with surface features shown in the lower half of the diagram. Moving radially outward from the center of the sphere to the surface are alternating regions of red and blue.]

It can be seen that there are many regions of the Sun that expand and contract, each with their own cycle, averaging a period of five minutes. By studying the waves and their properties astronomers are able to obtain information on the interior of the Sun, such as the layers that it is composed of and the temperature and density inside the Sun. This is one of the uses of helioseismology, confirming the existing solar models that past astronomers have used to predict the interior of the Sun (P. Demarque, 1999). To help illustrate this concept, the graphs below compares the results between models and data collected through helioseismology.

Not only does helioseismology confirm existing mathematical models, it also allows us to obtain more information on the connection between Sunspots and the dynamics of the convection zone. Sunspots are relatively cool patches on the surface of the Sun, one of the properties of a Sunspot is the possession of strong magnetic fields, these prevent the hot material from the sun’s core from reaching the photosphere. However, the material must still leave the core, this is done in the convection zone, where the material travels horizontally instead, exiting on the photosphere around the Sunspot, where the magnetic field no longer has any influence (SPACE.com, 2012). Figure 4 provides a visual aid to this concept.

What implications does this have on us?

Sunspots are associated with solar activities such as solar flares. The table below summarises the effects of solar flares. Being able to predict and study activities on the Sun, allows warnings to be given ahead of time, preventing fatal crashes and life-threatening situations.

Solar Flares

• The photosphere is a very complex region, energy, gases and particles from the Suns core move up and interact in this region.
• These gases are electrically charged, which in some cases generates magnetic fields – the basis for creating a Sunspot.
• As these gases move, the magnetic field also moves, these movements are often random and can cause the magnetic field to interact in all kinds of manner.
• As a result of these interactions sudden explosions can occur – these are called solar flares, they release immense amounts of radiation at the speed of light that could affect Earth.
• These affects include disrupting signal transmissions such as GPS navigations.
• And a burst of electromagnetic radiation that penetrate the Earth’s atmosphere, the propagating properties of these radiation could cause transformers to overheat as well as disrupt crucial data such as clocks that govern financial transactions. These bursts of electromagnetic radiation are known as coronal mass ejections (CME).

How can solar neutrinos assist in studying the interior of the Sun?

The Sun’s energy originates deep within its core through nuclear fusion, at its core the hydrogen gas is pressured and compacted tightly by the layers above the core. As a result, the hydrogen forms helium and in the process some of the hydrogen atom is converted into light, which allows it to shine and illuminate nearby planets (The Sun: Crash Course Astronomy #10, 2015). Neutrinos are created by various radioactive decays one of which is nuclear fusion taken place in the core of a star. By being able to get a sample of a neutrino, we are able to confirm that the energy source of the Sun is indeed nuclear fusion (Hallin, 1999). The following table summarises the events leading to the confirmation of the Sun’s energy source.

• In the late 1960’s Raymond Davis Jr placed approximately 400,000 litres of cleaning product deep in a gold mine at South Dakota, this was to prevent other forms of cosmic rays or radiation from interfering with the experiment, the neutrino however, is able to pass through as they do not interact with other matter unless it is through weak forces or gravity.
• The experiment was a success as neutrinos were detected, this was done through the cleaning products (large amounts of chlorine) being converted into argon through an interaction with a neutrino.
• Models of the Sun predicted that an argon atom should be produced each day, this was not the case, instead the experimental results found that only one argon atom was found each month.
• This was not consistent with the current models of the Sun and thus, called for a re-evaluation of the model.
• It was later theorised that there are three types of neutrinos, but only one type was detected in this experiment, the other two types changed forms as it travelled from the Sun’s core to the Earth.
• In the year 1999, this observatory was built to detect all forms of neutrinos, it was 2 kilometres underground and was a 12-metre diameter sphere with 1000 tons of heavy water.
• Interaction of neutrinos and heavy water results in the breakup of the heavy water nucleus.
• The sphere was surrounded with photomultipliers to detect when a neutrino would break up the nucleus of heavy water.
• The results found were now consistent with models of the Sun, directly confirming that the Sun’s energy originated from nuclear reactions.

What use does this information provide us?

Since it is now well understood that stars such as our Sun provide energy through nuclear fusion, the next natural thought would be, “what happens after the fuel runs out?”. Stars, like all life forms have a life cycle and depending on their size, the end result will differ (NASA: The Life Cycle of a Star - how are stars formed?, 2017). The following tables will account for the life cycle of different stars, one being an averaged sized star such as our Sun and another being twenty times the size of the Sun. The image below will illustrate the life cycle of different stars. Phases such as the main sequence and red giant will not be repeated for the table that accounts for stars 20 times larger than our Sun as they are almost identical, only differing by the size of the star.

Black Hole

• For completeness, a star large than 40 time the size of our Sun will become a black hole following a supernova.
• The remains of the star following a supernova will be compacted into an extremely dense object, this compaction occurs under the influence of gravity and is immensely powerful that nothing can escape its vicinity, not even light. Black holes can grow by consuming nearby stars or even other black holes.

Conclusion

Techniques such as helioseismology and solar neutrinos can be applied to other stars to extract information, giving us the capability to the inside of the stars. Our knowledge of the universe expands, providing scientific explanations for a variety of events that not only occur within our solar system but throughout the universe. Such explanations of events include the phenomenon of solar winds and the development and evolutions of different types of stars throughout their life.

References

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