Stars are like the sun, or rather the sun is a normal star. They
generate their energy by thermo nuclear reactions, converting H to
He. Have a large range of L, M, R, T etc
Need to know how far away the stars are. Use a technique called
parallax: d = 1/p with d in parsec's and p in arc seconds. Measure
angle shift over the space of a year. All know parallax are < 1'' as
closest star > 1pc away. Proxima Centaui d = 1.30pm, p=0.772. Can use
out to 100pm (Hipparcos out to 500pc). Very important step on the
distance ladder. Only an apparent motion.
However, stars do move. We can measure their radial motion via the
Doppler shift and their transverse motion by seeing how ot moves
across the sky. Then phythagoras therum gives v.
Barnards stars has the largest proper motion known.
We need to know the distance to stars to know how bright they are. A
faint star may be faint or a bright star just a long way
away. Brightness follws an inverse square law b = L/(4 pi d^2).
If we know the luminosity of stars we can construct a luminosty
function. We find the sun is very average. Big range of luminosity.
We really use magnitude rather than luminosty or brightness. This is
wierd as the more -'ve the brighter the star. We'll stick to
brightness in this course.
We want to know the temperature of stars - use Wien's law. See where
the spectrum peaks T = 0.0029/lambda with T in K and lambda in m.
Stars look to have different colours depending on peak lambda.
Taking a spectra is time consuming - use pphotometry and filters to
get colour ratios which imply temperature.
Colour ratios Vs T
Spectra give us more information though - chemical composition for
example. Harvard astronomers decided to classify stars on the basis of
their spectra. A-P depending on strength of hydrogen emission. Got rid
of many classifications - too fine. Later rearranged interms of
T. FOund sequence OBAFGKM. O stars are too hot to have Balmer
(hydrogen lines) - the atoms are all excited. At T<10000 K not enough
energy to make things happen hence M stars low T.
Final property is radius. Distance + brightness gives L. Speactra gives T. L +
T gives R. L = 4 pi sigma r^2 T^4
Herzprung Russell diagram. Very important. Found stars didn't just
cover all of the diagram, lay in certain places. Main sequence -
normal stars buring H to He. Other bits discussed later.
Luminosity affects stellar spectrum. Changes widths of lines. Narrow
implies more luminous.
Star with T=5800 K can be like the sun or a large giant star.
The luminosity class tells us where the star lies on the HR diagram
and hence what it's luminosity is. This is called the Spectroscopic
parallax as if know brightness can get the distance to the star this
way. Only accurate to 10% or so. Works for 10^5 stars.
Different types of stars.
Measure brightness. Spectra get T, L class and composition. T, L class
plus HR diagram = L. L and b give d, L and T give R.
Still not considered mass. Only way to get it is from binaries.
The orbits gives the relative masses along with the center of
mass. Can get M's.
Find that there is a mass luminosity relation. L propto m^3-3.5
Reveals that HR diagram just a sequence in mass. High mass stars high
on the HR diagram, low to the bottom left.
Binaries can be found in various ways. Spectrocopy - see shift in
doppler lines.
Also get eclipsing binary - light curves change with time.
These techniques are used to search for extra solar planets.
