Delay The radiance of the star that leans on me Was shining years ago. The light that now Glitters up there my eyes may never see, And so the time lag teases me with how Love that loves now may not reach me until Its first desire is spent. The star's impulse Must wait for eyes to claim it beautiful And love arrived may find us somewhere else. Elizabeth Jennings 1926-2001

Stellar types What different types of stars do you know of? How would their temperatures and luminosities differ?

Magnitude, luminosity and flux Recall that the apparent magnitude of a star is how bright is appears from Earth; absolute magnitude is how bright is would appear from a distance of 10ly The luminosity of a star is its output in Watts and is related to its temperature and size The flux of a star is the amount of energy per unit area that is received at a distance from it

Hertzsprung-Russel Diagram Different types of stars have different nuclear reactions (if they have any at all) occurring inside their cores. This means that the physical relationship between size, temperature and power will alter for each type of star. Classification of star is based solely on their temperature at the surface so is less instructive with regard what is happening in the star. The ranges of temperature and luminosity are very large; how can this be plotted on a graph? Luminosity ranges from 0.0001 to 10000 times the Sun’s power Temperatures range from 2500 to more than 40000 Kelvin Given that the key ingredients to understanding a star are Power (or luminosity) and Temperature, and given the ranges you can have for each, what sort of graph could be constructed?

Hertzsprung-Russel Diagram The usual method to show the different stellar types and properties is a log-log graph of luminosity against temperature.

Car Analogy for H-R Diagrams

Where would the sun be on this diagram?

Simple (to draw) H-R Diagram

Star formation and Protostars

Movements on the H-R diagram during a star’s lifetime Stars join the main sequence from a protostar from the right as they increase in temperature You should learn the track for a star similar to the sun

Movements on the H-R diagram during a star’s lifetime Stars join the main sequence from a protostar from the right as they increase in temperature You should learn the track for a star similar to the sun

Stellar evolution There is an area on the H-R diagram of unstable stars that vary in brightness. This is the path that stars are believed to take as they evolve from Main Sequence to Red Giant

What will happen to the Sun? Now - G class star with an absolute magnitude of 5 – Main Sequence In about 5 billion years – Hydrogen in core used up. Starts to fuse the hydrogen in its outer layers and expand. Becomes a giant star As it expands, the outer surface of the Sun will cool. As it uses up this fuel it will collapse and the increase in temperature and pressure in the core will initiate helium fusion. When this fuel is used up, helium shell fusion can then occur and the Sun will again expand. Eventually the outer material of the Sun will be pushed away to form a planetary nebula, leaving the extremely hot but small core as a white dwarf. Fusion has finished, and the white dwarf cools eventually to a black dwarf.

What will happen to the Sun?

Planetary Nebula (M57)

Nuclear Fusion in main sequence stars The main fusion reaction in a main sequence star is the p-p chain that you should already know There are other p-p chains that can take place; the percentage of each taking place in a star is controlled by the temperature of the core and the ratio of elements present

Other p-p chains

Large stars die young The size of a star (mass) has big consequences on how fast is fuses its material and what paths the fusion take High mass stars burn their fuel very quickly and do not last as long as low mass stars This means that the majority of stars in the sky will be lower mass stars; this is a selection bias

Summary The three main related measurements of a star are the magnitude, luminosity and flux The temperature and luminosity measurements can be plotted onto a Hertzsprung-Russell diagram This position of the star on this diagram will allocate it to a type of star, which in turn indicates where in its life cycle it is The tracks across the H-R diagram for stars like the sun must be known The scales of the H-R graph are logarithmic and students must be able to draw one and mark on the position of the Sun

Simple (to draw) H-R Diagram