- Planets - An object in orbit around a star with three important characters:
- A mass large enough for its own gravity to give it a round shape
- It has no fusion reactions
- It has cleared its orbit to most other objects
- Planetary satellites - A body in orbit around a planet
- Comets - Small irregular bodies made up of ice, dust, and small pieces of rock. All orbit the sun. Can develop tails as they near the sun. Range from a few hundred metres to tens of kilometers across
- Solar systems - Contains the sun and all objects that orbit the sun.
- Galaxies - A collection of stars, and interstellar dust and gas. On average it will contain 100 billion stars.
- Universe - A collection of all the galaxies
Nebulae are gigantic clouds of dust and gas. They are formed over millions of years as the tiny gravitational attraction between particles of dust and gas pulls the particles towards each other forming vast clouds. The gravitational collapse accelerates as the dust/gas gets closer together and denser regions form which pull in more dust and gas, gaining mass and getting denser. They also get hotter as gravitational energy is eventually transferred to thermal energy. In one part of the cloud a prostar forms - this is a very hot, very dense, sphere of dust and gas but is not yet a star.
Nuclear fission needs to occur for a prostar to become a star. Fusion reactions produce kinetic energy. Extremely high pressures and temperatures inside the core are needed in order to overcome the electrostatic repulsion between hydrogen nuclei in order to fuse them together to form helium nuclei. Sometimes, as more and more mass is added to the prostar, it grows so large and the core becomes so hot that the kinetic energy of hydrogen nuclei overcomes the electrostatic repulsion and hydrogen nuclei are forced together to make helium nuclei - it is here that a star forms.
The star now remains in a stable equilibrium with almost a constant size since gravitational forces compress the star but the radiation pressure (from the photons emitted during fusion) and the gas pressure (from the nuclei in the core) push outward. The forces balance so an equilibrium is maintained. This is known as the stars main sequence. How long a star remains stable depends on the size and mass of it's core. The cores of large, massive supergiant stars are hotter than those of small stars so they release more power and convert the available hydrogen to helium in a shorter time meaning they are only stable for a few million years. Smaller stars (e.g. our sun) are stable for tens of billions of years. As stars run out of hydrogen in their core (because it's all fused to form hydrogen), the star begins to move off it's main sequence.
Stars with a mass between 0.5 - 10 times our sun (M☉)
These stars eventually evolve into red giants. At the start of this phase the reduction in energy released by the fusion in the core means the gravitational force is greater than the radiation/gas pressure force and the core shrinks. Pressure increases enough to start fusion in the shell around the core. Red giant start have inert cores (fusion no longer takes place). This is because very little hydrogen remains and also the temperature is not high enough for the helium nuclei to overcome the electrostatic repulsion between them. Hydrogen fuses to helium in the shell around the core. This causes the periphery of the star to expand as the layers slowly move away from each other. These layers cool as thy expand making the star go red.
Most of the layers of the red giant will drift away as planetary nebula. The core then becomes a very dense hot (30,000K) core as a white dwarf. No fusion reactions occur here, it only emits energy because it leaks photons created in its earlier evolution. According to the Pauli exclusion principle, no two electrons can exist in the same energy state. When the core of a star begins to collapse under the force of gravity, the electrons are squeezed together. This creates a pressure that prevents the core from further gravitational collapse. This pressure created by the electrons is known as the electron degeneracy pressure.
It is important to realise that the electron degeneracy pressure is only sufficient to prevent gravitational collapse if the core has a mass less than 1.44M☉. This is the Chandrasekhar limit. This limit is the maximum mass of a stable white dwarf.
Stars with a greater mass of 10M☉
Since their mass is much greater their cores are much hotter meaning they use up their hydrogen supply in a shorter amount of time. As with smaller stars, when the hydrogen in the core runs low it begins to collapse under gravitational forces. As the cores of these larger stars are much hotter the helium nuclei (formed from fusion of hydrogen) are moving fast enough to overcome electrostatic repulsion and the helium nuclei fuse into heavier elements. These changes in the core cause the star to expand which forms a supergiant (sometimes known as a super red giant).Temperatures and pressures are high enough to fuse even massive nuclei forming a series of shells inside the star. This process continues until the star develops an iron core. Iron nuclei cannot fuse as the reaction would produce no energy. This makes the star very unstable and the star dies by supernova (type 2) - a catastrophic implosion of the layers that bounce off the solid core leading to a shockwave that ejects all the core material into space. Supernovae create all the heavy elements (everything above iron in the periodic table was created in a supernova (such events distribute these heavier elements throughout the universe.
After a supernova the remnant core is compressed into either a:
- Neutron star - if the mass of the core is greater than the Chandrasekhar limit, the gravitational collapse continues, forming a neutron star. These are almost entirely made up of neutrons and are very very dense.
- Black hole - if the core has a mass greater than 3M☉ the gravitational collapse continues to compress the core resulting in a gravitational field so strong that an object must be travelling greater than the speed of light to escape it.
The last thing we need to know in this section of the spec is the Hertzsprung-Russell diagram. This is a graph of stars in our galaxy showing the relationship between luminosity and their average surface temperature. The luminosity of a star is the total radiant power output of the star. The luminosity and temperature of stars can both vary widely so the HR (Hertzsprung-Russell) diagram scales are logarithmic.
Lower mass stars evolve into red giants moving away from their red sequence. Then they lose their cooler outer layers and slowly move across the diagram crossing the main sequence line ending up as white dwarfs. Higher mass stars start at point X before rapidly consuming their fuel and swelling into red supergiants at Y before going supernova.
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