Astronomical DistancesDistances in astronomy are huge

The Moon is 240,000 miles away

The Sun is 93, 000,000 miles away

Neptune is 2,810,687,500 miles away

Proxima Centauri, the next closest star, is 25,300,000,000,000 miles away

The center of our Milky Way Galaxy is16,500,000,000,000,000 miles away

Andromeda, the next closest galaxy is 17,000,000,000,000,000,000 miles away

The Universe is166,000,000,000,000,000,000,000

miles across

Miles and kilometers are too small of a unit tomeasure such huge distances

Astronomers use the following units:

Astronomical Unit (AU), the average distance ofthe Sun to the Earth, 1 AU = 93,000,000 miles

Light-year (ly), the distance light travels in oneyear, 1 ly = 5,900,000,000,000 miles

Parsec (pc), the distance when parallax equals1 sec of arc, 1 pc = 19,000,000,000,000 miles

To measure distances to nearby stars, “parallax” isused.

The angle of movement a starmakes in a 6 month period across the sky is measured.

This gives you the angle of a right triangle, you already know the opposite side (distance to the Sun), so thedistance to the star can be calculated (1 AU / tan).

Because the angle gets extremely small to measure farther and farther stars, distance can only be determined out toabout 150 ly (the nearest 5900 stars).

The closest star, besides the Sun is Proxima Centaui. It has a parallax of .76 seconds of arc.

One degree

60 minutes in 1 degree

60 seconds in 1 minute

Astronomers can also determine the distance to stars bycomparing how bright it looks, to how bright it wouldappear at some standard distance.

A car headlight in the distance appears dimmer than one close by. If you know how bright a headlight actually is, you can calculate the distance to a distant one.

Color is an indication of energyHigher frequencies have more energy

Usually, red stars give off less energy than blue stars,so you calculate its actual brightness by its color

So, you can measure how bright it appears, calculate how bright it should be, and calculate the distance to the star

by its color

Certain stars regularly change their brightness andare called “variable stars”. One kind are called

Cepheid Variables.

Cepheids pulsate in direct relation to how bright theyare. If you time the period of pulsation, you knowtheir true brightness, and thus, distance.

When stars end their lives in an explosion, it is called asupernova

A type II supernova is when a giant star blows up.They have hydrogen visible in their spectra andwidely different brightness of their explosions

Type I supernovas were first predicted in 1930, andhappen when the dead core of a medium star like theSun (white dwarf) explode. They have no hydrogen

in their spectra and always explode with the exact samebrightness. So distances to other galaxies can be calculated.

Most galaxies are moving away from us. The farther away they are, the faster they are moving. Becausethey are moving away from us, the light they give off is shifted to the red end of the color spectrum. Thefaster it is going, the more the “redshift”.

You can calculate distance by measuring theredshift of a galaxy.

Ways to find distance1.parallax2.comparing brightness3.measuring color4.Cepheid Variables5.type I supernovas6.galactic redshift