Original: c. August 2020
Types of stars
Average and massive stars have different life cycles due to the large difference in mass. Larger stars have a shorter lifespan as they burn through their fuel disproportionately faster compared to less massive stars. When their hydrogen fuel runs out, nuclear reactions can no longer continue, resulting in the core to contract due to its gravity.
Stars spend the majority of their lives fusing hydrogen into helium. Once the hydrogen runs out, helium is fused into carbon. The elements would fuse into heavier elements up till iron. The more massive the star, the heavier the elements it could fuse.
Formation of stars
Stars start off at a gravitational instability within molecular clouds caused by regions of higher density. Most of it is caused by radiation, expanding bubbles of low-density areas. The unstable gravity collapses the clouds, increasing their density. With more mass in the same area, there is more gravity there, pulling in more gas and ultimately creating a star.
Main sequence stars
Main sequence stars spend around 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. the proportion of helium in a star's core will steadily increase along with the rate of nuclear fusion in the core and the star's temperature and luminosity.
O-type and K-type stars are the extremes of the main-sequence stars.
O-type stars
O-type stars are blue-white coloured stars with a luminosity of V. Its mass is 15-90 times that of the sun and surface temperatures reach between 20000K to 50000K. The star's luminosity is between 40000 to 1000000 times that of the sun's.
K-type star
K-type stars have a luminosity of V. They range between M-type main sequence stars and yellow G-type main-sequence stars. They have masses between 0.5 and 0.8 times the mass of the sun and surface temperatures range between 3900 and 5200K. They provide similar conditions to that of the sun and are thus of interest to the search of extraterrestrial life.
Red giants
Red giants are stars that evolved from main-sequence stars. They have a hydrogen-burning shell instead of a core. Stars would start to become red giants once they are 5 billion years ago.
Helium flash
As a red-giant burns hydrogen, it reaches a temperature up to 100000000K, allowing helium to fuse into carbon and begin thermonuclear reactions.
Stellar-mass loss
Stellar-mass loss is a phenomenon observed in some massive stars. It occurs when a large portion of a star's mass is ejected. This could result in the reduction of mass of red-giant stars. This could be caused by the gravitational attraction of a binary star.
Planetary nebulae
Planetary nebulae refer to the ejection of the outer mass of a red giant. It occurs when a star ends its red-giant phase. They are a relatively short-lived phenomenon. Once the red giant's atmosphere dispersed, ultraviolet radiation from the core ionises the ejected material, making it visible.
White dwarf
A white dwarf is an extremely dense star. However, it is formed from a main-sequence star. Electron-degeneracy allows the white dwarf to not collapse under its own gravity. It occurs after a planetary nebulae event.
Supernovae
The luminosity of supernovae could be compared to an entire galaxy's. The star can no longer fuse atoms together as it has reached the iron stage. The core is crushed by the weight of its own gravity. The core would become the size of a city. It causes electrons and protons to become neutrons as they are crushed, essentially turning it into a large atomic nucleus.
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| By NASA/ESA, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=407520 |
Neutron stars
A neutron star's radius is around 30km. They are the result of the gravitational collapse of a massive star, provided that the star is not large enough to become a black hole.
Electron-degeneracy matter
Electron-degeneracy matter make up white dwarfs due to the extreme pressures within them. They prevent the white dwarf from collapsing in on to itself.
Reflection
The lesson seems interesting. The presenter was active and made an interesting quiz. The lesson was content-heavy and was done better compared to the previous one. When I present, I would try to emulate this one. The two videos at the end were also interesting. I learnt a lot about the evolution of stars and their eventual fate, including our sun. They would end up as either white dwarves, neutron stars or black holes.
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