Intro to Stars

Textbook chapter 19

What is a nebula?

Gigantic clouds of dust and gas, mainly hydrogen

What is a 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, forming a dense cloud

• Gravitational potential energy is transferred to thermal energy, causing this cloud to be very hot

• This means we have a very hot and dense sphere of gas and dust - a protostar

How does a protostar become a star?

• Nuclear fusion needs to start in the core

• This only happens if the protostar gains enough mass that the kinetic energy of the hydrogen nuclei is large enough to overcome the electrostatic repulsion between them

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

What is gas pressure (in the context of stars)?

the pressure of the nuclei in the star's core pushing outwards

What is radiation pressure (in the context of stars)?

pressure from the photons in the core of a star

What is the main sequence on a HR diagram?

• The main period on an H-R diagram in a star's life, during which it is stable

• It goes from top left to bottom right of an HR diagram

What is a 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