Chapter 19 - Stars

• A planet is an object in orbit around a star which has a mass large enough for its gravity to give it a round shape, has no fusion reactions and has cleared its orbit of most other objects
• A dwarf planet is a planet which has not cleared its orbit of other objects
• Asteroids are objects that are too small or uneven to be planets, usually having near circular orbits around the sun without the presence of ice
• Comets are small irregular bodies made up of ice, dust and small pieces of rock that orbit the sun with highly elliptical orbits (as they approach the sun, some comets develop spectacular tails) They range from a few hundred meters to tens of kilometres wide.
• Galaxies are collections of starts and interstellar dust and gas on average containing 100 billion stars
• (1) Nebulae are huge clouds of gas and dust formed over millions of years due to the tiny gravitational attraction between particles which causes the particles to pull together. Denser regions form which pull in more mass becoming denser and hotter as gravitational energy is transferred to thermal energy. This forms protostars
• (2) For a protostar to become a star, nuclear fusion must begin in the core. Fusion produces kinetic energy. Extremely high temperatures and pressures are required to overcome the electrostatic revulsion between hydrogen nuclei to start fusion so it is rare that a protostar gains enough mass that its core is hot enough to produce enough kinetic energy to overcome the repulsion. If this does occour a star is born.
• (3) Once a star is formed it will remain stable for some time. Gravitational forces act to compress the star but the radiation pressure from the photons emitted during fusion along with the gas pressure in the core, push outwards balancing the forces allowing stars to remain stable.
• Smaller stars are able to remain stable for tens of billions of years as they have cooler cores so fusion occurs at a slower rate
• More massive starts remain stable for millions of years as they have hotter cores so fusion will occur at a faster rate and the core will run out of hydrogen quicker
• (4a) Stars with low mass evolve into red giants. The reduction in energy produced by fusion once they have run out of hydrogen means that the gravitational force is now greater than the force produced radiation and gas pressure so the core begins to collapse. As the core shrinks, pressure increases enough for fusion of helium to begin in a shell around the inert core. This causes the periphery of the star to expand and the layers begin to cool
• Stars will solar masses between 0.5 and 10 evolve into red giants
• (5a) Eventually the layers of the red giant around the core drift into space as planetary nebulae leaving behind a hot dense core where fusion no longer takes place. when the core of a star begins to collapse, the electrons are pressed together and Due to the Pauli exclusion principle they create a electron degeneracy pressure preventing the core from further collapse. This pressure is only sufficient until the Chandrasekhar's limit, <1.4 solar masses
• (4b) Stars with a mass greater than 10 solar masses have much hotter cores which run out of hydrogen is a smaller amount of time. Once hydrogen runs out, the remaining heat of the core leads to fusion of helium into heavier elements causing the star to expand into a red supergiant. Fusion of more massive nuclei also takes place forming a series of shells inside the star with an inert iron core. Fusion no longer takes place so there is not enough pressure to counteract the gravitational collapse leading to an unstable star and eventually a supernova.
• (5b) After a supernova if the mass of a supergiant's core is greater than Chandrasekar's limit the gravitational collapse continues forming a neutron star. If the mass of the core is greater than 3 solar masses the gravitational collapse continues to compress the core resulting in a black hole.
• A neutron star is made almost entirely of neutrons, with a size of around 10km, a typical mass of 2 solar masses and densities similar to that of an atomic nucleus
• A black hole have a gravitational field so storng that an object would need an escape velocity greater than the speed of light to escape
• The luminosity of a star is the total radiant power output of the star
• A Hertzsprung-Russel diagram shows temperature against luminosity> It shows the lifecycles of stars
• Electrons which are bound to their atoms in a gas can only exist in one a discrete set of energies, energy levels
• An electron cannot have energy between two energy levels
• The energy levels an electron exists in are negative as external energy is required to remove an electron from the atoms
• An electron with zero energy is free from an atom
• When an electron is excited by providing it with external energy, it moves to a higher energy level.
• For an electron to move up or down energy levels it must absorb or emit the exact energy required for the transition
• When an electron moved down an energy level it emits a photon
• Emission line spectra, the wavelengths emitted are shown as colour
• Absorption spectra, the wavelengths absorbed are shown as colour
• Starlight can be analysed using emission and absorption spectra to determine the composition of gases in its atmosphere
• To accurately determine the wavelength of monochromatic light, the double slit experiment can be altered by using a grating with a large number of slits to create stronger maxima
• The altered double slit experiment uses the equation, $d\sin\theta=$$n\lambda$where d is the grating spacing, n is the maxima and $\theta$ is the angle between the normal and the incident ray
• At any given temperature above absolute zero, an object emits black body radiation
• A black body is an idealised object that absorbs all the electromagnetic radiation that shines onto it and emits a characteristic distribution of wavelengths at a specific temperature
• Weins displacement law states the absolute temperature of a black body is inversely proportional to the peak wavelength at which the intensity is a maximum
• Many objects can be modelled as black bodies allowing scientists to determine the temperature of objects by analysing the em radiation they emit
• Stefan's law states that the total power radiated per unit surface area is directly proportional to the fourth power of the absolute temperature of the black body. Total power is luminosity$L=$$4\pi r^2\sigma T^4$