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SCIE 1121
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Dec 21, 2024
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THE HISTORY OF NEUTRON STARS; How do they form? And why do they behave the way they do?SCIE1121 | Our Universe
Major Assignment – The History of Neutron StarsHow do they form? And why do they behave the way they do?Neutron stars stand as enigmatic remnants of stellar death, their formation and behaviour has captivated astronomers for decades and still continue to do so. From their theoretical prediction in the mid 1930’s to the first observational confirmation, the journey of understanding this small yet powerful object has been as exhilarating as it has been enlightening. Neutron stars were first hypothesised by Walter Baade, and Fritz Zwicky who suggested that neutron stars may be the remains of supernovas (Lattimer, 2005). This later turned out to be true with neutron stars being classified as a category of star that has reached the end of its life and undergone a supernova. What’s left is a core of neutrons packed extremely closely together, hence neutron star.To understand how neutron stars form, we first must understand why stars don’t collapse under constant gravitational pressure. For most of their lives, stars are mainly made up of hydrogen atoms, which consist of a singular proton and an electron orbiting it. However, stars are so hot that the protons and electrons have so much kinetic energy that they are unbounded from their atomic bonds. The heavier of the two particles, the proton, moves at speeds so high that it collides with other protons creating a repulsive pressure, keeping the star from collapsing in on itself (Lang, 2013). This however is only a short-term solution to the problem. The main way in which a star survives the persistent force of gravity is via nuclear fusion. The abundant hydrogen atoms fuse together to form deuterium. The mass difference in this reaction is converted into energy according to Einstein’s theory of special relativity, which states that energy and mass are equivalent. The energy released in this reaction can be calculated using the following formula.E= mc2 (1.1)This process continues, creating heavier and heavier elements such as carbon, neon, and oxygen. This balanced formula is what keeps the star from collapsing in on itself. When it runs out of fuel however, it can no longer repel the persistent force of gravity, causing the star to implode and only leaving the core of the star behind. As gravity further constricts the stars core, the electrons are squeezed together to such a degree that they form their own type of repulsive force known as electron degeneracy pressure (Buongiorno, 2023). According to (Lattimer & Prakash, 2004), if a star’s mass exceeds 8 Solar Masses (1.6x1031kg), the force of gravity is so strong that it overcome this repulsion and further constrict the star (Sloan, 2021). Because of this, electrons are forced to occupy higher and higher energy levels and as the density of star surpasses 4 × 1014kg/m3the electrons combine with the protons in the star in a process called inverse beta decay. This reaction produces neutrons and neutrinos as shown in the equation below. 𝑝+𝑒→𝑛+𝜈𝑒The neutrinos are sent flying out of the star and what’sleft is a core consisting of approximately 90% neutrons with the average neutron star weighing over 1.44, solar masses and has a radius of ~20km (Alfredo, 2023). Figure 1: The proton-proton sequence which shows the fusion process in stars (Lang, 2013).
In 1965 a phased array radio telescope proposed by British astronomer Antony Hewish was built at Cambridge University in order to study interplanetary scintillation (fluctuations in radio waves). This led to one of Hewish’sbrilliant students, Jocelyn Bell to discover the first ever recorded radio-waves from a specific type of rapidly rotating neutron star. According to (Hewish et al., 1968) these pulses of radio-waves repeated every 1.337s and lasted ~0.33s. This is not common in stars and man-made objects such as space probes, leading Bell, and Hewish to name the source Little Green Men (LGM-1). This discovery took the world by storm with the daily telegraph publishing this article on it. “Anentirely novel kind of star came to light on Aug. 6 last year and was referred to, by astronomers, as LGM (Little Green Men). Now it is thought to be a novel type between a white dwarf and a neutron [star]. The name Pulsar is likely to be given to it. Dr. A. Hewish told me yesterday: '... I am sure that today every radio telescope is looking at the Pulsars”(Daily Telegraph, 1968.) Prior to the first recorded observation of the pulsar, research from astrophysicist Franco Pacini proposed that neutron stars were likely to be rapidly rotating and have extremely strong magnetic fields, magnitudes higher than anything else in the universe (Pacini, 1967). Austrian American physicist Thomas Gold expanded on Pacini’s hypothesis by suggesting that Pacini’s rotating neutron star could be the cause of the pulses seen by Bell and Hewish (Gold, 1969). This is now the best explanation we currently have for the emission of pulses but the exact cause for radiation emitting neutron stars is still unknown with physicist Werner Becker stating:"The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work." (Becker et al., 2006)Another type of neutron star is known as a Magnetar. This type of neutron star has extremely large magnetic fields which is responsible for most of its energy unlike pulsars. They have magnetic fields with magnitudes of 1013 - 1015 G. (Mereghetti et al., 2015) which is over 1.5x1015times that of Earth making it the most magnetic object in the universe. There are currently 24 confirmed magnetars as of writing this paper but there is an estimated 30 million magnetars in total within the milky way galaxy (Olausen & Kaspi, 2014). Perhaps the greatest observation of a magnetar occurred in 1979 when two Russian space probes orbiting Venus detected a massive burst of gamma rays jumping from 100 counts all the way to 40,000 (Kouveliotou, 2003). Many other probes within our solar system detected this gamma radiation too. The source of this gamma ray burst was found to be located within the Large Magellanic Cloud (A dwarf galaxy orbiting the Milky Way) and is the result of radiation release from a magnetar formed in a supernova 5000 years ago. In conclusion, the exploration of neutron stars, from their theoretical prediction to their observational confirmation, represents a captivating journey of scientific research. Initially postulated as remnants of supernovas by Baade and Zwicky, neutron stars have emerged as fascinating stellar objects marking the pinnacle of stellar evolution. Formed through the cataclysmic collapse of massive stars, these dense cores have challenged our understanding of physics and still continue to do so.Figure 2: Light curve from the 1979 gamma ray burst, recorded by the Venera 12 space probe (Kouveliotou, 2003).
References:Lattimer, J.M. (2005). Neutron stars. SLAC Stanford, 1. https://www.slac.stanford.edu/econf/C0507252/lec_notes/Lattimer/Lattimer.pdfLang, K.R. (2013). The life and death of stars. Cambridge University Press. https://doi.org/10.1017/CBO9781139061025Buongiorno, C. (2023, December 27). What are neutron stars? The cosmic gold mines, explained. Astronomy. https://www.astronomy.com/science/what-are-neutron-stars-the-cosmic-gold-mines-explained/Lattimer, J.M., Prakash, M. (2004). The physics of neutron stars. Science, 304(5670), 1. https://doi.org/10.1126/science.1090720Sloan, G. C. (2021, January 11). The death of stars. UNC. https://users.physics.unc.edu/~gcsloan/fun/death.htmlAlfredo, C.J. (2023). Structural characteristics and physical properties of neutron stars: theoretical and observational research. Arxiv. https://doi.org/10.48550/arXiv.2303.08734Hewish, A., Bell, J.S., Pilkington, H. D. J., Scott, F.P., Collins, A.R. (1968) Observations of rapidly pulsating radio source. Nature, 217, 1-3. https://doi.org/10.1038/217709a0Unknown. (1968, March 5). Daily TelegraphPacini, F. (1967). Energy Emission from a Neutron Star. Nature. 216, 567-568. https://doi.org/10.1038/216567a0Gold, T. (1969). Rotating neutron stars, and the nature of pulsars.Nature221, 25–27. https://doi.org/10.1038/221025a0Mereghetti, S., Pons, J.A., Melatos, A. (2015). Magnetars: Properties, origin, and evolution. Space Sci Rev, 191, 315-338. https://doi.org/10.1007/s11214-015-0146-yOlausen, S.A., Kaspi, V.M. (2014) The McGill magnetar catalog. The Astrophysical Journal, 212(1), https://doi.org/10.1088/0067-0049/212/1/6Kouveliotou, C., Eichler, D., Woods, P.M., Lyubarsky, Y., Patel, S.K., Gogus, E., Van der Klis, M., Tannant, A., Wachter, S., Hurley, K. (2003). Unravelling the cooling trend of the soft gamma repeater, SGR 1627-41. The Astrophysical Journal, 569. https://doi.org/10.48550/arXiv.astro-ph/0309118Becker, W., M. Krämer, A. Jessner, Taam, R. E., Jia, J. J., Cheng, K. S., Mignani, R., A. Pellizzoni, A. De Luca, A. Słowikowska, & Caraveo, P. A. (2006). A Multiwavelength Study of the Pulsar PSR B1929+10 and Its X‐Ray Trail. The Astrophysical Journal, 645(2), 1421–1435. https://doi.org/10.1086/504458