Astronomy 2022 October 3-5

Composed of plasma (electrically charged ionized gas), opaque surface emits continuous spectrum, convective zone, interface layer, radiative zone, core, envelope transfers energy from the core

gravity wants to collapse the core into a singularity. heat wants the star to expand, eventually blowing it apart, normally there’s a balance caused by gravity and gas pressure keeping a size constant

stars that vary in magnitude with consistent periods

Harvard computer, discovered the period-luminosity relationship

relates temperature to pressure, increase in temperature creates an increase in pressure, an increase in pressure creates an increase in temperature

ionized by increasing temperatures, electrons are expelled from neutral helium as energy is added through heat, singly ionized helium vs doubly ionized helium (most opaque)

pulsations are the result of changes to the opacity of the helium ions and the envelope’s density, the more ionized the dimmer

the He^2+ absorbs light and heat. Greatest opacity

star expands and cools, He^2+ becomes He^1+. Less opaque, more light escapes

Eventually the star cools and collapses until it returns to the original dim He^2+ state

supergiants, luminosities are 1k-50k (1 solar luminosity), magnitudes can change from a few % to 3 magnitudes, periods range from 1-50 days

we can see cepheid variables out to a distance of 100 Mpc

1 million parsecs

use the distance modulus m-M, absolute magnitude m can be observed, M can be determined using Henrietta’s discovery

a graph that shows how the brightness of a variable star changes over time, can determine the average apparent magnitude and its period in days

knowing the period of the cepheid, you can determine it’s mean absolute magnitude and mean apparent magnitude

knowing m and M gives you 10^((m-M+5)/5)

Used the period luminosity relationship in 1923 to determine that the “andromeda nebula” was actually another galaxy millions of light years away. Until then astronomers believed everything was located in the Milky Way

objects whose luminosity we know ahead of time, independent of distance

old low mass metal poor stars older than 10 billion years (50L), luminosity ranges 2-3, periods 12-24 hours, visible to about 1 Mpc using HST. found in globular clusters or galactic bulges and halos

color, temperature, luminosity, motion, life expetency, the extent of the gravitational collapse of the core, core temperature, luminosity

determines the temperature of the envelope, the surface, and the color of a star

determined by star’s mass, M^4

determined by star’s mass, 1/M^3

determined by star’s mass, whether it becomes a white dwarf, supernova, etc

not rare, approximately 65% of stars we see at night, or multiple star systems

apparent binaries, visual binaries, spectroscopic binaries, eclipsing binaries

not a real binary. Two stars with a random alignment but no gravitational interactions, are actually far apart in space but are lined up by chance

double stars, star system in which the companion stars are gravitationally bound and can be resolved into two stars with the aid of a telescope, both have the same proper motion

harmonic law, square of the planet’s orbital period is proportional to cube of its mean distance from the sun. P^2=a^3. Doesn’t work for objects that don’t orbit the sun

M1+M2=a^3/p^2 (masses must be measured in solar masses where one is 1.99 x 10^30, semi major aids must be measured in AUs 149,600,000 kilometers, orbital period must be measured in years)

balance point between two masses, barycenter

the ratio of the distances is the ratio of the masses. two bodies with a large difference in mass orbiting this causes star wobble. If two bodies have the same mass, this is directly in the middle

1. the periods p can be 10s=1000s of years, 2. the projection of the orbits against the sky…it’s orientation? 3. the measured size of the orbit a is distance dependent and distances are challenging