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Nebulae
gigantic clouds of dust and gas
Protostar
A very hot, very dense sphere of condensing dust and gas that is on the way to becoming a star
How are protostars formed?
• gravitational attraction of dust and gas particles pull them towards each other
• gravitational potential energy is transferred to thermal energy.
How does a protostar become a star?
In some protostars, they gain so much mass that the kinetic energy of the hydrogen nuclei is large enough to overcome electrostatic repulsion, and fusion begins.
Which forces allow for stars to be in a stable equilibrium?
Gravitational forces compress the star, while radiation pressure and gas pressure push outwards to counteract gravitational forces
Gas pressure
the pressure of the nuclei in the star's core pushing outwards
Radiation pressure
pressure from the photons in the core of a star
Main sequence
The main period on an H-R diagram in a star's life, during which it is stable
Main sequence star
stars in the stable phase of their lives
Planet
an object in orbit around a star which:
• has a large enough mass for its own gravity to give it a round shape
• has no fusion reactions
• has cleared its orbit of most other objects
Planetary satellite
a body in orbit around a planet
comets
small irregular bodies made up of ice, dust and rock which orbit the sun, often in highly eccentric ellipses
Solar systems
system containing the sun and all objects that orbit it
Galaxies
Collection of stars, interstellar dust and gas held together by gravity
Universe
everything that exists in space and time
Black hole
the remnant core of a massive star after it has gone supernova and the core (which has a mass greater than ~3 solar masses) has collapsed so far that in order to escape it an object would need an escape velocity greater than the speed of light
Hertzsprung - Russell (HR) diagram
graph showing relationship between luminosity of stars (y axis) in our galaxy and their average surface temperature (temp increasing right to left)
Annotate the Hertzsprung-Russell diagram to show
the Sun’s evolution.
Slight curve upwards from red giant is important
Neutron star
the remnant core of a red supergiant after the it has gone supernova and the core (which has a mass greater than the Chandrasekhar limit) has collapsed under gravity to an extremely high density.
Planetary nebula
the outer layers of a red giant that have drifted off into space, leaving the hot core behind as a white dwarf
Red giant
an expanding star at the end of its life, with an inert core in which fusion no longer takes place, but fusion of lighter elements continues in the shell around the core
Red supergiant
a huge star in the last stages of its life before it explodes in a supernova
Supernova
the implosion of a red supergiant at the end of its life, which leads to ejection of stellar matter into space and leaving an inert, remnant core
White dwarf
a very dense star formed from the core of a red giant, in which no fusion occurs
What type of star becomes a red giant?
small stars between 0.5 and 10 solar masses
How do stars become red giants?
• the fusion process in the core slows down
• gravitational forces are greater than gas and radiation pressures
• the core begins to collapse
• as the core shrinks, the pressure increases enough to start fusion in a shell around the core
Fusion in a red giant
• no fusion takes place in the core since temperatures are not high enough
• fusion takes place in the shell around the core, causing the periphery of the star to expand
Electron degeneracy pressure
pressure created by electrons being squeezed together, preventing further gravitational collapse
Chandrasekhar limit
electron degeneracy pressure is only sufficient to prevent gravitational collapse if the core has a mass less than 1.44 solar masses
What type of stars become red supergiants?
large stars with mass greater than 10 solar masses
Fusion in red supergiants
temperatures and pressures are high enough to fuse massive nuclei together, forming series of shells inside the star, until an iron core is created
How do supernovas occur?
Iron nuclei cannot fuse, making the red supergiant unstable. The layers around the core implode and bounce off the solid core, ejecting all the core material into space
Why can't iron nuclei fuse?
their reactions need energy to be taken in, rather than releasing energy like the lighter elements.
Hydrostatic equilibium
The balance between inward and outward forces within a star; compressing force of gravity balanced by radiation and gas pressure which push outwards from the core
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