Astrophysics definitions:

  1. Planets: an object in orbit around a star which has 3 characteristics:
    • Mass large enough for its gravitational attraction to give it a round shape
    • No fusion reactions
    • Has cleared its orbit of most other objects, e.g. asteroids
  2. Planetary satellites: a body in orbit about a planet, including moons and man-made satellites
  3. Comets: small, irregular bodies made of ice, dust and small pieces of rock. Orbit the sun, many in highly eccentric elliptical orbits.
  4. Solar systems: a star and the objects orbiting it (planets, comets, asteroids etc.)
  5. Galaxy: a collection of stars and interstellar gas and dust (contain around 100 billion stars)
  6. Universe: everything that exists

Star birth:

  1. Interstellar clouds of dust and gas (nebulae) are drawn together by gravitational forces
  2. Due to tiny variations in the nebula, denser regions begin to form. These pull in more gas and dust, become denser and more massive
  3. Particles lose gravitational PE as it is converted into KE. The temperature increases. This is a protostar.
  4. Fusion of hydrogen nuclei into helium nuclei (releasing energy as KE and photons) begins. A star is born.
  5. The star is stable when radiation pressure and gas pressure equal the gravitational pressure. Stars in this phase are on their “main sequence”

Star evolution:

Low-mass stars (0.5 to 10 solar masses)


The star begins to run out of hydrogen so radiation pressure falls. The core shrinks, increasing pressure around it so hydrogen shell burning begins. 

Outer layers of the star expand and move away from the core. They cool so appear red. 

The red giant’s outer layers drift off into space as a planetary nebula, leaving behind the hot core as a white dwarf

  1. Red giants are very luminous because, although cool, they have a large SA

More massive stars (greater than 10 solar masses)

  1. Have hotter cores so hydrogen in core is consumed in less time
  2. Core begins to collapse and outer layers expand, forming a red supergiant
  3. Temperature and pressure is high enough for fusion of more massive nuclei, forming a series of shells
  4. This continues up to iron nuclei, beyond which further fusion is not energetically favourable. Nuclear fusion cannot withstand gravitational forces.
  5. Star becomes unstable so core collapses, leading to a shockwave that ejects core material into space (a supernova). This forms the elements heavier than iron and distributes them throughout the Universe.

Star death: the remnant of the core is compressed into one of two objects:

  1. Neutron star (core is 1.4 to 3 solar masses): the mass of the core exceeds the Chandrasekhar limit so electrons combine with protons to form neutrons and neutrinos. Neutron stars are very dense and small. Thy rotate fast and emit radio waves in two beams (pulsars).
  2. Black hole (core is more than 3 solar masses): neutrons cannot withstand gravitational forces so collapse to an infinitely dense singularity. The gravitational field is so strong that the escape velocity exceeds the speed of light.

Boundary is the event horizon.  

Characteristics of a white dwarf: 

  1. Remnant of a low-mass star
  2. Extremely dense, very hot, low luminosity
  3. No fusion reactions occur
  4. Further gravitational collapse prevented by electron degeneracy pressure up to the Chandrasekhar limit

Chandrasekhar limit: the maximum mass of a stable white dwarf before its gravitational attraction overcomes the electron degeneracy pressure and a supernova occurs. The maximum mass of a stable white dwarf star.

The Hertzsprung-Russel (HR) diagram: a luminosity-temperature plot (temperature direction is reversed).  It has distinct areas to sow the main stages in a star’s life cycle: the main sequence, red giants, super red giants, and white dwarfs. The stages are shown because they’re stable so stars exist in them for long periods of time. 

Low-mass stars begin on the main sequence then move away to become red giants at 1. They gradually lose their cooler outer layers as planetary nebula at 2 so their surface temperature rises. They then cross the main sequence and end up as white dwarfs at 3.

Higher mass stars begin at X before rapidly consuming their fuel and swelling into red supergiants. They then go into supernova and become a neutron star or black hole, depending on their mass.