Ch19 Stars Factual Learning

Nebulae

gigantic clouds of dust and gas

How are protostars formed?

• nebulae are formed over millions of years as the gravitational attraction between dust and gas particles pull them towards each other
• as the dust and gas gets closer together, gravitational collapse accelerates
• denser regions begin to form, which pull in more dust and gas and gain mass
• gravitational energy is transferred to thermal energy
• a protostar is formed

How does a protostar become a star?

Some protostars 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

Main sequence star

stars in the stable phase of their lives

Do larger or smaller stars have shorter lives and why?

Large stars have shorter lives. This is because the cores of larger stars release more power and fuse all the available hydrogen atoms in a much shorter time.

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 diagram

graph showing relationship between luminosity of stars (y axis) in our galaxy and their average surface temperature (temp increasing right to left)

Neutron star

the remnant core of a massive star after the star 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

Protostar

a hot, dense sphere of condensing dust and gas that is on its way to becoming a star

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 cannot produce any energy

Energy levels

a discrete amount of energy that an electron within an atom can possess

Ground state

energy level with the most negative value possible

Excited

electrons that have absorbed energy and have moved to a higher energy level

Emission line spectra

unique, coloured lines against a dark background

Continuous spectra

all visible frequencies are present

Absorption line spectra

dark spectral lines against a continuous spectrum

Diffraction grating

glass or plastic slide on which hundreds of lines per millimeter are ruled

Black body

an idealised object that absorbs all the EM radiation incident on it and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature

Wien's displacement law

the peak wavelength at which the intensity of radiation from a black body is a maximum is inversely proportional to the absolute temperature of the black body

Why do energy levels have negative values?

because external energy is required to remove an electron from the atom
the negative electrons are bound to the positive nuclei

De-excitation

when an electron moves from a higher to a lower energy level, it loses energy, and a photon is emitted from the atom

Which objects produce a continuous spectrum?

heated solid metals

How are emission line spectra produced?

when the electrons in an excited gas drop down to lower energy levels, they emit photons with a set of discrete frequencies

How are absorption line spectra produced?

• light from a source that produces a continuous spectrum passes through a cooler gas
• as the photons pass through the gas, those with energy equal to a difference between energy levels are absorbed by the gas atoms

Advantage of a diffraction grating over a double slit

a clearer and brighter interference pattern is produced

grating equation

d \= grating spacing
n \= order

Units of luminosity

Watt