# Astronomy Chapter 3

Basic Star Info
Have much on common with the sun
Form great clouds of gas and dust

Stars begin life with
the same chemical composition as the sun
– 3/4 of star’s mass at birth is hydrogen
– 1/4 is helium
Differ in size ,age, brightness, and temperature

Measuring Stellar Luminosity
Brightness of a star depends on:
-its distance
-how much light it emits

Apparent Brightness
brightness of a star as it appears to our eyes

Luminosity
Total amount of power a star emits into space
-This is how bright stars are regardless of their distance

How is a lightbulb related to apparent brightness and the sun?
same watt is same amount of light

stellar temperatures
we measure temperatures of stars
determined by either star’s color or it’s spectrum

color of stars?
stars come in almost every color of the rainbow

blue star
hottest stars

red star
coolest star

Why do stars come in different colors?

cooler stars look red because
they emit more red light than blue light

hotter stars look blue because
they emit more blue light that red light

how is temperature measured?
By comparing star’s brightness in blue light and red light

determine star’s temperature from spectral lines
most accurate way to measure star temperatures

types of spectral lines present provide a :
direct measure of the star’s surface temperature

stars displaying lines of highly ionized elements are:
fairly hot because it takes a high temperature to ionize atoms

stars displaying lines of molecules must be:
cooer because molecules break apart into atoms unless they are really cool temperatures

spectral type
astronomers assign each star a spectral type determined from the spectral lines present in a star’s spectrum

hottest stars with blue colors are:
spectral type O, followed by B, A, F, G, K, and M in decreasing temperature.

OBAFGKM
Oh be a fine girl/guy kiss me

measuring stellar masses
mass is more difficult to measure than luminosity or surface temperature

mass is more difficult to measure than:
surface than luminosity or surface temperature

most dependable method for “weighing” a star depends on:
Keplers Third Law

Keplers third law:
can be applied only when we observe one object orbiting another.
-Requires we measure both the orbital period and the average orbital distance of the rotating object

orbital period:
time to complete one orbit

average orbital distance
distance between two objects

binary star systems
star systems in which 2 stars continually orbit each other

members of binary star systems
half of all stars orbit a companion star and are members of binary star systems

measuring orbital period is
easy

mass ranges of stars
0.08X Sun’s Mass- 150X sun’s mass

white dwarfs
exposed core of a star that has died and shed it’s outer layers in a planetary nebula

size of white dwarfs
high in mass but small in size

white dwarf composition
reflects products of star’s final nuclear burning stage

neutron stars
ball of neutrons created by the collapse of the iron core in a massive star supernova

size of neutron star
small in size, but very massive

black holes
a bottomless pit in spacetime

can anything escape from a black hole?
No, we can never agin detect of observe an object that falls into a black hole

how are black holes formed?
form when the gravity in a stellar corpse becomes so strong that nothing can prevent the corpse from collapsing under it’s own weight.

event horizon
boundary between the inside of a black hole and the universe outside

spacetime
the inseparable, 4D combination of space and time

Singularity
place at the center of a black hole where gravity crushes all matter to an infinitely tiny and dense point.