01:750:109 Chapter Notes - Chapter 12.1: Observable Universe, Main Sequence, Solar Mass
Measuring Cosmic Distances
How do we measure the distances to galaxies?
●Measurements of astronomical distances depend on a chain of methods in which each step
allows us to measure greater distances in the universe
○Builds link by link from our solar system to the outermost reaches of the observable
universe
Radar Ranging
●First link:
Sun-Earth distance, or astronomical unit (AU)
●Precise value of AU determined through a technique called radar ranging
, in which radio waves
are transmitted from Earth and bounced off Venus
○Bc radio waves travel at the speed of light, the round-trip travel time for the radar
signals tells us Venus's distance from Earth
○Then use Kepler's law and some geometry to calculate length of an AU
Parallax
●Second link:
stellar parallax of nearby stars
●Can calculate a star's distance if we know the precise amount of the star's annual shift due to
parallax
○Aka measuring the star's parallax angle
○Parallax angle smaller if star is farther away
●Most accurate technique for measuring stellar distances, but only measurable for relatively
nearby stars
●European GAIA spacecraft is expected to provide parallax measurements for up to a billion stars
out to distances of tens of thousands of light-years
Standard Candles
●Third link:
other stars
●Standard candle
is a light source of known, standard luminosity
○Ex: light bulbs that are all 1000 watts
○But astronomical objects don’t come marked with a wattage
■Can serve as a standard candle only if we have some way of knowing its true
luminosity without first measuring its apparent brightness and distance
■EXXXX: any star that is a twin of our Sun (a main-sequence star with
spectral type G2) should have about the same luminosity as the Sun
■Can then use the inverse square law for light to estimate its distance
●Beyond the distances to which we can measure parallax, we use standard candles for most
cosmic distance measurements
○Always have some uncertainty tho
●The more confident we are about an object's true luminosity, the more certain we are of its
distance
Main-Sequence Fitting
●Main-sequence fitting
is using the entire main-sequence as a standard candle: bc all
main-sequence stars of a particular spectral type should have about the same luminosity, we
can calculate the distance to a cluster with unknown distance by comparing the apparent
brightness of its main sequence to that of the main sequence in a cluster whose distance we
already know
Now used less frequently tho bc we can extend the use of parallax to greater distances
Cepheid Variables
●Parallax measurements generally used within our own Milky Way
●For measuring distances to other galaxies, we need standard candles that are bright enough to
be seen at great distances and they are vvv
●Cepheid variable stars:
stars that vary in luminosity (and therefore also in apparent brightness),
alternately becoming dimmer and brighter with periods ranging from a few days to a few
months
●Cepheids obey a period-luminosity relation
: the longer the period, the more luminous the star
Document Summary
Measurements of astronomical distances depend on a chain of methods in which each step allows us to measure greater distances in the universe. Builds link by link from our solar system to the outermost reaches of the observable universe. First link: sun-earth distance, or astronomical unit (au) Precise value of au determined through a technique called radar ranging , in which radio waves are transmitted from earth and bounced off venus. Bc radio waves travel at the speed of light, the round-trip travel time for the radar signals tells us venus"s distance from earth. Then use kepler"s law and some geometry to calculate length of an au. Second link: stellar parallax of nearby stars. Can calculate a star"s distance if we know the precise amount of the star"s annual shift due to parallax. Parallax angle smaller if star is farther away. Most accurate technique for measuring stellar distances, but only measurable for relatively nearby stars.
