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Astronomical Distances
Distances 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 is
16,500,000,000,000,000 miles away
Andromeda, the next closest galaxy is
17,000,000,000,000,000,000 miles away
The Universe is
166,000,000,000,000,000,000,000
miles across
Miles and kilometers are too small of a unit to
measure such huge distances
Astronomers use the following units:
Astronomical Unit (AU), the average distance of
the Sun to the Earth, 1 AU = 93,000,000 miles
Light-year (ly), the distance light travels in one
year, 1 ly = 5,900,000,000,000 miles
Parsec (pc), the distance when parallax equals
1 sec of arc, 1 pc = 19,000,000,000,000 miles
To measure distances to
nearby stars, “parallax” is
used.
The angle of movement a star
makes 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 the
distance to the star can be calculated (1 AU / tanq).
Because the angle gets extremely small to measure farther
and farther stars, distance can only be determined out to
about 150 ly (the nearest 5900 stars).
The closest star, besides the Sun is Proxima Centaui.
It has a parallax of .76 seconds of arc.
60 seconds
in 1 minute
One degree
60 minutes in 1 degree
Astronomers can also determine the distance to stars by
comparing how bright it looks, to how bright it would
appear 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 energy
Higher 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 and
are called “variable stars”. One kind are called
Cepheid Variables.
Cepheids pulsate in direct relation to how bright they
are. If you time the period of pulsation, you know
their true brightness, and thus, distance.
When stars end their lives in an explosion, it is called a
supernova
A type II supernova is when a giant star blows up.
They have hydrogen visible in their spectra and
widely different brightness of their explosions
Type I supernovas were first predicted in 1930, and
happen when the dead core of a medium star like the
Sun (white dwarf) explode. They have no hydrogen
in their spectra and always explode with the exact same
brightness. 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. Because
they are moving away from us, the light they give off
is shifted to the red end of the color spectrum. The
faster it is going, the more the “redshift”.
You can calculate distance by measuring the
redshift of a galaxy.
Ways to find distance
1. parallax
2. comparing brightness
3. measuring color
4. Cepheid Variables
5. type I supernovas
6. galactic redshift