• Doug
• September 5, 2024

Exploding Stars

August 10, 2024 Presentation To The TCAA Astronomy Club!!

Abstract:

This paper explores the lifecycle of stars from creation within condensing areas of vast molecular clouds. Ultimately, a star’s self-destruction occurs in the final stages of its life, resulting from the conversion of hydrogen to helium, and finally to even heavier elements which cause atomic fusion to become endothermic, requiring more energy to fuse the heavy elements than is produced by the fusion. Eventually, the core can no longer support the weight of its own mass and the inevitable collapse occurs. In some cases the core can withstand the collapse and produce a supernova event. In other cases the core is overwhelmed, being compressed into an infinite singularity and forming a black hole. In the case of a supernova event, the resulting shockwave distributes materials across the interstellar medium, with a newly formed neutron star reaching a brightness exponentially greater in the night sky than even our brightest star, Sirius. The newly formed supernova remnant then has the potential to become a new star-forming region where the cycle of creation will begin again.

Star Formation

Exploding Stars – Star Birth – Molecular Clouds

Vast clouds of aggregated molecular matter exist within galactic discs. In these regions of matter, there are smaller areas of condensing matter. These areas of matter can be caused by random density fluctuations in the surrounding molecular cloud or by external factors such as shockwaves from supernovae or collisions of galaxies. As the condensing matter flows inward a protostar begins developing in the central core of the surrounding gas and dust, which is supported by the developing pressure of the collapse. Within this period of star formation, there is a constant inward flow of gas and dust surrounding the protostar in the form of an accretion disk who’s inward flow increases the central density and temperature as a result of the increasing compression of the inflowing matter at the core formation area of the protostar. A tipping point eventually occurs where the pressures and temperatures reach a level where nuclear fusion of the matter is ignited. _{NASA/IPAC Extragalactic Database. (2011).}

Low and High Mass Stars

Low mass stars are defined as having eight or less solar masses. It is thought that the understanding of low mass star formation is well understood while high mass stars, greater than eight solar masses, is not understood as well. This is due to the emissions of a massive star that would have great enough pressure to push the accretion disk away, clearing the area of the gas and dust needed to continue increasing mass. In one model it is thought that massive stars are formed as a result from a jet of incoming material which forms a cavity that allows a directed flow of radiation outward without displacing the accretion disk. Bannerjee and Pudritz.(2007). In other models, the merging of two or more low mass stars or from a combination of low and high mass stars within the same gravitational potential accrete from the same molecular cloud. _{Bonnel IA, Bate MR. (2006).}

Stellar Nucleosynthesis: Nuclear Fusion Process

Classifications

The initial mass distribution of a star, as calculated against the galactic mass, is known as the Initial Mass Function. In order for a star to sustain fusion, it must have a solar mass of at least 8%. Stars with smaller mass, less than 8% of the solar mass, do not have enough gravitational binding energy to create the pressure needed to generate temperatures required to ignite nuclear fusion. Stars of this type (not technically stars) are known as Brown Dwarfs. Stars formed with greater than 8% and up to 40% of solar mass are able to ignite nuclear fusion, yet due to their lower mass and temperatures emit less energy than our Sun, having fainter emissions and cooler temperatures. Red Dwarf stars fall into this classification.
Red Dwarfs give off more light in the red wavelengths and are the most common type of star found in the Milky Way. They comprise upwards of 78% of the 200 billion stars in the galaxy. Stars are designated by type, defined by attributes such as temperature and color. Stars falling on a main distribution plot, including our Sun, are known as Main Sequence stars. These distributions are shown on the Hertzsprung–Russell (HR) diagram (fig. 1). Main sequence stars (fig. 2) have classes which include types, classified under the Morgan–Keenan (MK) system. From hottest to coolest, the MK system defines type O, B, A, F, G, K, and M stars by classifications of spectral and related temperature characteristics. For example, our Sun is a type G yellow star with a surface temperature of around 5,800 K° (5526 C°). Figure 1 illustrates where the Sun is positioned on the Main Sequence distribution. _{NASA, ESA, and Z. Levy (2020), NASA/IPAC Extragalactic Database. (2011).}

_{Fig. 1 NASA/IPAC Extragalactic Database. (2011)}

_{Fig. 2 Pablo Carlos Budassi}

Fusion Reactions

A star has two key zones related to fusion, the hydrogen envelope and the hydrogen burning core. The core is the location with which nuclear fusion occurs. Fusion is the result of two nuclei merging to become one atom. The hydrogen fusion reaction within stars fuses two hydrogen (H) atoms together, producing a helium (He) atom. The hydrogen atoms consist of two isotopes, one deuterium and one tritium. This reaction produces an isotope of lesser mass than the sum of those that were fused. We can explore this by having a look at the isotope’s protons and neutrons.
Deuterium has one proton and one neutron. Tritium has one proton and two neutrons. The difference being tritium has one more neutron than deuterium. When fused the resulting helium has two protons and two neutrons, one less neutron than the sum of the deuterium and tritium. This is known as a mass deficit. The mass deficit is made up as the conversion of mass to energy and the amount of energy created from this reaction is enormous in comparison to the energy required to fuse the atoms. _{US Department of Energy}

Stellar Evolution Of An Exploding Star

Stellar evolution is the process by which a star evolves over time, from its creation as a protostar to its eventual demise, which can result in several different outcomes. As mentioned earlier, stars are formed in galactic nurseries of vast molecular clouds. A star’s life cycle is directly dependent upon its mass, with truly massive stars having much shorter lifespans, in the order of millions of years, than that of typical main sequence stars. Low mass stars can have life spans of trillions of years.
The main factor which determines a star’s life span is the rate at which a star fuses hydrogen into helium. A star reaches a state of equilibrium, known as hydrostatic equilibrium, where the fusion core generates enough outward force energy to support the inward force of the star’s matter, thus preventing further collapse of material into the core. As the star ages and continues to fuse hydrogen into helium, an abundance of helium ash is deposited in the star’s core. The amount of helium ash continues to grow for the entire life of a star, while the hydrogen fuel for fusion is slowly spent over time. Helium is denser than hydrogen and the helium core contracts, creating even more heat (fig 3). The central temperature and pressure builds to a point where the energy output of the core overwhelms the outer hydrogen layers. In the case of red giant stars, pushes them out to sizes that can reach the equivalent of Jupiter’s orbit around the Sun. _{Prof. David Kipping. (2020).}

_{Fig 3 Prof. David Kipping}

The end game of the star’s life is the result of the central temperature and pressure becoming extreme enough that helium can fuse into carbon. This increases the energy output of the core dramatically and the loss of the hydrostatic equilibrium, pushing the un-fused hydrogen outward. The core continues to grow even more dense with carbon and gaining even more heat. Eventually, the core will produce enough pressure and heat to begin fusing carbon into iron and even heavier elements. This is the point at which the star’s demise is inevitable because the fusion reaction then becomes endothermic, taking more energy to fuse than it generates from fusion. This endothermic reaction consumes the energy of the star. Eventually, there is not enough energy generated to support the weight of the star’s mass and it falls in upon itself, imploding.
As the star core changes due to mass and temperature it continues to seek equilibrium and dramatically affects the star’s size and color. When the star is no longer able to maintain equilibrium a collapse occurs, the result of which depends on how massive the star is. There are two potential outcomes when the equilibrium is lost and a star collapses: _{US Department of Energy}.
A neutron star is formed when the mass of the star is not enough to fully collapse the star core. In this case the collapse results in a supernova with the hot core remaining surrounded by the formation of a nebula created from the explosion of the star’s outer layers of matter that remained. Tremendous shockwaves propagate out from the star carrying the remaining matter with it.
For truly massive stars, the result is more of a whimper where the collapse results in the core crushing into an infinitely dense singularity and the formation of a black hole. In some cases there is still nebulosity created following the implosion that then becomes an accretion disk for the newly formed black hole. Prof. David Kipping. (2020).

Supernova Explosion:

Supernova Types:
Type Ia Supernovae: It is thought that type Ia supernovae result from a binary star system where a white dwarf is accreting matter from its binary companion. As the white dwarf’s mass increases as a result of the accretion, its core reaches a critical density. This critical density can result in an uncontrolled fusion of carbon and oxygen. This uncontrolled reaction results in the star’s detonation and resulting supernova.
Type II Supernovae: These supernovae mark the end of a truly massive star’s life. The star has exhausted its fuel and its energy can no longer support the weight of its own mass. When the star’s iron core is massive enough, it will deflect the imploding matter, exploding into a supernova. _{National Aeronautics and Space Administration. (2011).}

Supernova Explosion:

When a star explodes into a supernova, there is an explosive release of energy of approximately 1028 megatons. This forms a shockwave which propels the star’s outer matter into interstellar space to become a supernova remnant. _{National Aeronautics and Space Administration. (2011).}
The explosive release of energy during a supernova produces an incredible brightness, outshining even the brightest stars in the night sky. Supernovae are so bright, in fact, that their absolute magnitude can reach -19. This means a supernova approximately 32 light years distance from Earth would be 1.5*107 times brighter than Sirius, which is the brightest star in our night sky with an apparent magnitude of -1.44. _{Lindsay Clark. (2000).}
Shockwaves are created which ripple through the interstellar medium. As mentioned in Star Birth, these shockwaves can disrupt molecular clouds, causing areas of density that can then begin the process for new star formation. A supernova shock wave can cause compression and resulting heating of the interstellar materials, up to millions of degrees. This is truly the full circle of star creation, life, death, and re-birth. Without this process it is believed that there would be no new stars formed. _{National Aeronautics and Space Administration.}

Nebula Remnants:

Supernova Remnants:

Supernova remnants (SNRs) are simply the remains of a star that have been distributed across interstellar space following the supernova event. These remnants are most often observed as nebulae and are generally made up of hot, glowing, ionized gas and particles. These remnants can also contain materials that were caught in the supernova shockwave and brought along for the ride with the rest of the materials. The shockwave itself often forms the outer boundary of the SNR. The expansion of the shockwave into surrounding space interacts with the interstellar medium (ISM), creating beautiful and complex structures. Supernovae are responsible for the creation and dispersal of heavy elements, such as iron, gold, and uranium. Supernovae are also responsible for creating lighter elements such as hydrogen, helium, carbon and Oxygen, as well as amino acids and water. We are truly created from stardust, supernovae play a significant role in the formation of new stars, planets and the building blocks of life.

Remnant Classifications:
SNR are generally classified by their shape, formed as a structure resulting from the supernovae shockwaves that distribute the star matter, and swept up interstellar materials. There are three main classifications of SNR:
Shell Type: are a large distribution of heated material. This type often has a ring-like structure. Due to our perspective, we observe more heated gasses and debris along the edges than the center of the structure. This has the term “limb brightening”, describing the brightness of the outer edges as compared to the middle of the SNR structure.
Crab Type: are also known as pulsar wind nebulae or plerions. Crab type remnants are more blob-shaped as compared to shell type remnants. The Crab Nebula is among the most famous and responsible for the type name.
Composite Type: resemble characteristics of both the shell type and crab type SNR. Depending upon how they are being observed in the electromagnetic spectrum they may have characteristics of one, the other, or both types. There are two types of composite remnants:
Thermal: appear as shell type in the radio spectrum. In X-ray they appear as crab type. Thermal composite remnant’s spectra have spectral lines, indicating hot gas. True shell type SNR do not show these spectral lines.
Plerionic: appear as crab type in both radio and X-ray; but also have shells. Plerionic type X-ray spectra do not show spectral lines in the center but do near the shell shaped structure.

Conclusion:

Exploding stars, Supernovae are one of the most important natural events occuring in our universe. Critical to the formation of new stars and celestial objects, including planets, supernovae create the molecular clouds of gas and matter which become the birthing area for these newly formed stars and objects. Each star, though unique, goes through a life cycle with much similarity to other stars. However, depending upon a star’s mass, the life span of massive stars can be trillions of years less than that of the lowest mass stars. In the context of Exploding Stars, we are interested in the more massive stars, exceeding 8 solar masses, whose mass overwhelms the core’s ability to maintain hydrostatic equilibrium yet is not enough to fully collapse the core into a singularity, forming a black hole. Thus, a neutron star is instead created when the star’s hydrogen shell is blown off with unimaginable force, extending out into the interstellar medium with incredible brightness. The resulting celestial structures, known as nebulae, are areas of hot gas, dust, and a variety of elements which set the stage for the next cycle of creation.

About:

Douglas Reynolds is an amatuer astrophotographer with a passion for the amazing, interesting, unknown, and beautiful universe in which we live. He has explored the night sky in wonder with his eyes alone for the majority of his life. Became infatuated with deep sky objects upon seeing the first images from the Hubble Space Telescope, and as a result, began specializing in capturing high resolution images of deep sky objects. Douglas is a member of his local astronomy club, The Twin City Amateur Astronomers’ club located in Bloomington/Normal, IL. Joining your local club is a great way to experience the joys of the night sky, learn, and gain access to resources to help you observe. You are encouraged to reach out to the TCAA at: https://www.tcaa.club/. Douglas creates content around astrophotography and deep sky objects on his Youtube channel, is active in social media on Facebook and Instagram, and maintains his own astrophotography Website. Feel free to reach out and connect with Douglas on any of his content or social channels. High Resolution prints of Douglas’ works are available in his gallery https://astroprints.astroaf.space/
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Reference:

NASA/IPAC Extragalactic Database. (2011). Origin Of The Chemical Elements. https://ned.ipac.caltech.edu/level5/Sept16/Rauscher/Rauscher_contents.html
Bannerjee R, Pudritz RE. (2007). Massive Star Formation via High Accretion Rates and Early Disk-driven Outflows. https://ui.adsabs.harvard.edu/abs/2007ApJ…660..479B/abstract
Bonnel IA, Bate MR. (2006). Star formation through gravitational collapse and competitive accretion. https://ui.adsabs.harvard.edu/abs/2006MNRAS.370..488B/abstract
NASA, ESA, and Z. Levy. (2020). Comparison of G, K, and M Stars for Habitability. https://hubblesite.org/contents/media/images/2020/06/4618-Image
Pablo Carlos Budassi – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=92588077
US Department of Energy. Fusion Physics! A Clean Energy. https://www.energy.gov/sites/prod/files/2016/09/f33/Power%20Sources%20Lesson%20-%20Fusion%20Physics.pdf
Prof. David Kipping. (2020). Betelgeuse Explained. https://youtu.be/5bvuwTuGnkc?si=mXiD3SsgWxkB1NsE
US Department of Energy. DOE Explains…Supernovae. https://www.energy.gov/science/doe-explainssupernovae
National Aeronautics and Space Administration. (2011). Supernovae. https://imagine.gsfc.nasa.gov/science/objects/supernovae2.html
Lindsay Clark. (2000). Background: Supernovae. https://www.astro.princeton.edu/~dns/teachersguide/SNBkgd.html
National Aeronautics and Space Administration. The Dispersion of Elements https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-elements.html
National Aeronautics and Space Administration. Supernova Remnants. https://imagine.gsfc.nasa.gov/science/objects/supernova_remnants.html