Stars Flashcards
Planets
1) Objects with mass sufficient for their own gravity to force them to take a spherical shape, where 2) no nuclear fusion occurs, and 3) the object has cleared its orbit of other objects.
dwarf planets
Planets where the orbit has not been cleared of other objects.
planetary satellites
Bodies that orbit a planet
asteroids
Objects which are too small and uneven in shape to be planets, with a near circular orbit around the sun.
comets
Small, irregularly sized balls of rock, dust, and ice. They orbit the sun in very elliptical orbits.
Solar systems
The systems containing stars and orbiting objects like planets.
galaxy
A collection of stars, dust, and gas. Each galaxy contains around 100 billion stars and most galaxies seem to host a supermassive black hole at its centre.
nebulae
Gigantic clouds of dust and gas. They are the birthplace of all stars.
How are protostars formed?
In nebulae, there are regions that are more dense than others. Over time, gravity draws matter towards them, so these regions gain mass and density. They also get hotter because
GPE is converted into thermal energy. But the temperature at the centre is not hot enough for nuclear fusion to occur.
How are main sequence stars formed from protostars?
For a star to form, the temperature and pressure in the centre of the protostar must be high enough for hydrogen gas nuclei to overcome the electrostatic repulsion and undergo nuclear fusion to convert hydrogen into helium. When fusion begins, the protostar becomes a main sequence star, where the outward pressure due to fusion (radiation and gas pressure) and the inward force of gravity are in equilibrium.
Describe how a low-mass main sequence star becomes a red giant.
Low-mass stars have a mass between 0.5M☉ and 10M☉. As these stars have a smaller mass, they live for longer, because they burn fuel less quickly. Once the hydrogen fuel in the core runs out, the gravitational force inwards overcomes the radiation and gas pressure, so the core collapses inwards. The core becomes hotter and hotter (as GPE is converted into thermal energy) and begins to fuse helium into heavier elements (up to carbon). The outer layers on top of the core expand and cool down: this is when the star becomes a red giant. A red giant is burning He in the core and H in the outer layers around the core. It has a large luminosity (because the radius is large) and a cooler surface temperature. Because of the cooler surface temperature, it appears red.
Describe the evolution of a red giant into a white dwarf.
When the redgiant runs out of fuel, its outer layers expand into interstellar space, creating a planetary nebula. The left-over core contracts further, turning into a hot and dense ball of gas, that we call “white dwarf”. The white dwarf has a temperature of around 30 000K, and no fusion occurs. Photons which were produced earlier in the evolution of the parent star leak out, carrying away heat. This is what produces the luminosity of a white dwarf.
The electron degeneracy pressure (caused as two electrons cannot exist in the same state) prevents the core from collapsing.
Describe the evolution of a high-mass main sequence star into a red supergiant.
When the star’s mass is larger than 10 M☉, its evolution takes a different path. As hydrogen supplies deplete, the core contracts. Since the mass is greater, when GPE converts into thermal energy, the core gets much hotter than a red giant, allowing nuclear fusion of heavier and heavier elements to occur (up to iron). The outer layers expand and cool, forming a red supergiant.
Describe the process of the death of a high-mass star.
When all of the fuel in a red supergiant is used up, fusion stops (as iron fusion requires more energy than what it releases). Gravity is now the winning force of the tug of war between gravity and pressure. So the core collapses onto itself very rapidly and suddenly becomes rigid (as the matter can no longer be forced any closer together). The outer layers fall inwards and rebound off of the rigid core, launching them out into space as a shockwave. This shockwave is what we observe as a “supernova”, a stellar explosion. The remaining core of a supernova collapses either into a neutron star or black hole, depending on the core mass.
Describe the evolution of a red supergiant into a neutron star or black hole
If the remaining core mass is greater than 1.44M☉ but smaller than 3M☉, gravity forces protons and electrons to combine and form neutrons. This produces an extremely small, dense neutron star.
If the remaining core mass is greater than 3M☉, the gravitational forces are so strong that the escape velocity from the core becomes greater than the speed of light. This is a black hole, which even photons cannot escape.
