Chapter 19 Stars

notes: Both luminosity and surface temperature of stars can very widely- luminosity from less than 0.0001L to over 1000000L, and surface temperature from 3000K to 40000K. As a result, both scales in the HR diagram are normally logarithmic plots

what are nebulae

gigantic clouds of dust and gas ( mainly hydrogen )

what are nebulae often referred to as

stellar nurseries

how are nebulae formed

what happens after star birth 

how does a protostar become a star? 

• nuclear fusion needs to start in its core

• many protostars never reach this stage 

• fusion reactions produce energy in the form of kinetic energy 

• extremely high pressures and temperatures inside the core are needed in order to overcome the electrostatic repulsion between hydrogen nuclei in order to fuse them together to form helium nuclei 

• in some cases, as more and more mass is added to the protostar, it grows so large and the core becomes so hot that the kinetic energy of the hydrogen nuclei is large enough to overcome the electrostatic repulsion

• hydrogen nuclei are forces together to make helium nuclei as nuclear fusion begins

• a star is born

why do stars remain in a stable equilibrium with almost a constant size when they form?

• gravitational forces act to compress the star, but the radiation pressure from the photons emitted during fusion and the gas pressure from the nuclei in the core push outwards

what are stars called when they are in their stable equilibrium phase

main sequence 

how long a star remains stable depends on?

the size and mass of its core

how are the cores of large, massive supergiant stars different to those of small stars 

they are much hotter, releasing more power and converting the available hydrogen into helium in a much shorter time 

really massive stars are only stable for…

a few million years, whereas smaller stars like our sun are stable for tens of billions of years

planets

dwarf planets 

asteroids

planetary satelites 

comets

solar system

galaxies

stars with low mass

• since the core of stars with low mass is cooler than that of more massive stars, they remain on their main sequence for much longer 

• however, eventually, often after billions of years, they run low on hydrogen fuel in their core

• at this stage, they begin to move off the main sequence into the next phase of their lives 

what is solar mass

what will happen with stars between 0.5M and 10M?

they will evolve into red giants

red giant phase 

eventually most of the layers of the red giant around the core drift off into space as a…

planetary nebula, leaving behind the hot core as a white dwarf 

white dwarf qualities

• very dense 

• often with a mass around our sun, but with the volume of the Earth 

• no fusion reactions take place within it 

• emits energy only because ti leaks photons created in its earlier evolution 

• surface temperature - 30000K 

Pauli exclusion principle 

2 electrons cannot exist in the same energy state 

what is electron degeneracy pressure and why does it happens 

when the core of a star begins to collapse under the force of gravity, the electrons are squeezed together, and this creates a pressure that prevents the core from further gravitational collapse

what is the Chandrasekhar limit

the electron degeneracy pressure is only sufficient to prevent gravitational collapse if the core has a mass less than 1.44M. This limitis the maximum mass of a stable white dwarf star. if the core is more massive than this, the star’s life takes a more dramatic turn 

more massive stars

inside red supergiants the temperatures and pressures are high enough to fuse even…

massive nuclei together, forming a series of shells inside the star 

for more massive stars, at a critical point ( depending on the mass of the star ) the nuclear fusion taking place in the core suddenly becomes unable to withstand the crushing gravitational forces so… what happens? 

the star collapses in on itself, leading to a supernova 

neutron star

black hole 

supernovae create…

all the heavy elements, everything above iron in the periodic table was created in a supernova

what is the hertzsprung-russel diagram

what is the luminosity of a star

the total radiant power output of the star

the luminosity of a star is related to…

its brightness - in general the greater the luminosity, the brighter the star

in the HR diagram, luminosity is often plotted in units relative to

when stars are plotted on the HR diagram a pattern appears. the hottest, most luminous stars are in the top left at A, with the coolest, least luminous stars at B. Most stars on their main sequence form part of a curved line from A to B. Our Sun has a surface temp of around 6000K, and so sits near the middle of this line

very hot, dim stars like white dwarfs appear along a different line at C. Their surface temperatures can be many times greater than our Sun’s. However, they are much smaller and less luminous 

Red supergiants are very luminous because of their enormous size, but they have a relatively low surface temperature. They are found around . Smaller red giants are found in a line splitting from the main sequence at E 

lower mass stars like our sun evolve into red giants, moving away from the main sequence. They then gradually lose their cooler outer layers, and slowly…

move across the diagram, crossing the main sequence line to end up as white dwarfs

higher mass stars at C, before rapidly consuming their fuel and swelling into red supergiants at Y before they go supernova 

when electrons are bound to their atoms in a gas, they can only exist in one of a discrete set of energies, referred to as the…

energy levels ( or energy states ) of an electron 

an electron cannot have a quantity of energy between 

2 levels 

the energy levels are negative because…

external energy is required to remove an electron from the atom. The negative values also indicate that the electrons are trapped within the atom or bound to the positive nuclei 

an electron with 0 energy is… 

free from the atom

the energy level with the most negative value is known as the…

ground level or the ground state 

when an electron moves from a lower to a high energy level within an atom in a gas, the atom is said to be…

excited 

raising an electron into higher energy levels requires…

external energy for example supplied by an electric field( as in the nenon tube ), through heating, or when photons of specific energy ( and therefore frequency ) are absorbed by the atoms 

when an electron moves from a higher energy level to a lower one… what happens?

it loses energy. energy is conserved, so as the electron makes a transition between the levels, a photon is emitted from the atom. This transition between energy levels is sometimes called de - excitation 

in order for an electron to make a transition from -3.0eV to -6.8eV it must lose…

3.8eV. it emits this in the form of a photon with a specific energy 3.8 eV. The energy of the photon hf = 3.8eV 

in general, energy of any particular photon emitted in an electron transition from a higher to a lower energy level is given by 

E = hf where f is the frequency and change in E is the difference in energy between the 2 energy levels 

each element has its own unique set of…

energy levels, like fingerprints, so the energy levels of electrons in helium are different from those in hydrogen

worked example for emitting photons

emission line spectra

each element produces a unique emission line spectrum because of its unique set of energy levels

continous spectra

all visible frequencies or wavelengths are present. The atoms of a heated solid metal ( eg a lamp filament ) will produce this type of spectrum

absorption line spectra

this type of spectrum has series of dark spectral lines against the background of a continuous spectrum. The dark lines have exactly the same wavelengths as the bright emission spectral lines for the same gas atoms 

If the atoms in a gas are excited ( eg within the hot environment of stars ), then when the electrons drop back into lower energy levels

they emit photons with a set of discrete frequencies specific to that element. This produces a characteristic emission line spectrum 

each spectral line corresponds to…

photons with a specific wavelength. these spectra can be observed in a laboratory from heated gases 

each coloured line in figure 2 represents a unique wavelength ( or frequency ) of photon emitted when an electron moves between 2 specific energy levels 

Absorption line spectrum

detecting elements within stars

what is a diffraction grating 

an optical component with regularly spaced slits or lines that diffract and split light into beams of different colour travelling in different directions. These beams can be analysed to determine the wavelengths of spectral lines in the laboratory or from starlight 

the fringes produced by passing light through a double slit are not very sharp, so it can be difficult to determine the position of the centre of each maximum, to over come this limitation…

a transmission diffraction grating can be used in place of the double slit. The grating consists of a large number of lines ruled on a glass or plastic slide. There can be as many as 1000 lines in a millimetre. each line diffracts light like a slit. using a large number of lines produces a clearer and brighter interference pattern.

when light passes through a diffraction grating it is split into a series of narrow beams. the direction of these beams depends on the spacing of the lines, or slits of the grating and the wavelength of the light. therefore…

when white light is passed through a diffraction grating it splits into its component colours, making gratings especially useful in spectroscopy. 

Forming maxima

at any given temperature above absolute zero, an object emits…

em radiation of different wavelengths and different intensities

we can model a hot object as…

a black body

a black body is…

an idealised object that absorbs all the em radiation that shines onto it and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature. 

Figure 2 shows a graph of intensity against wavelength for em radiation emitted by a black body at 6000K 

Wien’s displacement law 

Stefan’s law