Cosmic Distance Ladder

Distances in Space

Space is vast and to try and measure it we must use distance measurement units unfamiliar on Earth.  Astronomers decided to use the speed of light as the basic unit of measure since it is a universal constant: 186,282 miles (~300,000 km) per second in a vacuum.  That would put the Moon at 1.3 light seconds away from Earth.  Light travels 11-million miles in one minute.  The Sun is 8.45 light minutes from Earth.  Neptune is 4.26 light hours away from the Sun.  Light travels 6-trillion miles in one year.  Proxima Centauri, the closest star is 4.22 light years away (~25-trillion miles).  The Milky Way galaxy has a diameter between 100,000 and 130,000 light years.  The M31 Andromeda Galaxy is 2.54 million light years away.   The Virgo Cluster of galaxies are about 54 million light years away.

Space Units of Measure

Miles and Kilometers are so tiny in space that we have crafted better units based on observed phenomena.  The Earth’s Orbital Radius (Semi-Major axis of the elliptical orbit of Earth and Sun) is called one Astronomical Unit (1 AU = 93 million miles).  Using that distance as the base line to measure parallax to a star results in another empirically measured unit, the parsec.  1 parsec = 3.26 light years.  A star at 3.26 light years distance would appear to move 1 arc-second in position for the Earth’s movement of 1 AU as we orbit the Sun.  Proxima Centauri is 1.29 parsecs from the Sun.  We multiply the parsec by 1,000 for the kiloparsec, Kpc (3,260 light years) and by 1 million for the megaparsec, Mpc (3,260,000 light years).  The M31 Andromeda Galaxy is 0.78 Mpc from the Milky Way.  Gigaparsec is 3,260,000,000 light years, Gpc. These units are derived directly from parallax measurements of stars and galaxies based on the Earth’s orbital motion about the Sun.  They can be converted easily to light years and miles, but are a shorthand for these otherwise huge resulting numbers.

The Cosmic Distance Ladder

We use AU to measure planetary systems dimensions.  Jupiter is about 5 AU from the Sun (486 million miles).  Parsecs are used to measure distances inside of the Milky Way.  We have the technology to use parallax up to 100,000 light years utilized in the Gaia Satellite.  Beyond 100,000 light years, parallax is too small to measure.  So, we then use standard candle stars: Cepheid variables.  Cepheid variables vary in brightness in a regular way that is directly related to their absolute brightness.  By comparing their apparent brightness against their absolute brightness, we can use the inverse square law to measure their distance.  This method is used to measure distances to galaxies about 5 Mpc away since we must be able to resolve individual stars in them to measure the variability of stars.  The local group of about 40 galaxies are all measured by this method.  Beyond 5 Mpc, we use another standard candle: Type 1a Supernovae.  These are extremely bright and useful out to the limits of the observable Universe.  But, Supernovae are unpredictable and not always available.  Edwin Hubble discovered the expansion of the Universe.  Space itself is expanding and moving galaxies apart at a certain rate.  We can see this movement via red-shift in the spectrum of a galaxy.  The further away a galaxy is, the higher the red-shift.  This is how we measured the distance to the Virgo Cluster of galaxies.  In fact, we use red-shift for the majority of galaxy distances outside of our local group.  Quasars are active black holes in the cores of galaxies and their red-shift can be measured quickly.  All Quasars are billions of light years away.  The distances to Galaxies are usually expressed in megaparsec or gigaparsec units that easily convert to light years.  The Virgo Cluster is 16.5 Mpc or 54 million light years away.

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