Deep Sky Objects - Time, Distance, Size

It’s difficult for me to comprehend how far away and how vast deep sky objects are.  As you likely already know, a lightyear is the measure of distance light travels in 1 year.  It’s an incomprehensibly large number, ~5.9 trillion miles.  A more manageable number is the distance light travels in 1 second, 186,000 miles. 

The following provides a little more context for the intersection of time and distance.

A Lightyear – How Far? How Long Ago?

Nebulae within our own galaxy, the Milky Way, are much closer to us (hundreds to tens of thousands of lightyears) than other galaxies which are millions to hundreds of millions of lightyears away.

What does that mean, really?

In the Blink of an Eye - Round Trip from Boston to San Francisco - 3 Times 

The blink of an eye is about 1/10 of a second (100 ms); light travels 18,600 miles in that time.   The distance from Boston to San Francisco is approximately 3,000 miles.  In the blink of an eye, one could travel from Boston to San Francisco - round trip - 3 times at the speed of light.

1 Second - Around the Earth – 7½ Times 

The circumference of our planet is about 24,900 miles. A beam of light travels around the earth about 7½ times in 1 second.

8 Minutes - The Distance to the Sun 

The sun is about 93 million miles away. Light from the sun takes about 8 minutes to reach us. So, the sun is a distance of 8 light minutes away.

Historical Milestones - What was Happening Then?

Now imagine traveling at that speed for an hour, a day, a lifetime, a century, a thousand years, ten thousand years, millions of years … hard for me to comprehend. Another way to think about these vast distances is to figure out what was happening on earth when the light left the deep sky object.  This helps me understand how old the light is when it reaches my camera sensor.  It is, in a word, Ancient Light.

What was happening on Earth when light that fell on my camera sensor left the Nebulae or Galaxy?

         2,000 years ago (2K lightyears away – e.g. the Veil Nebula in Cygnus): 

Early construction, early pottery, agricultural civilizations

5,000 years ago (5K lightyears away – e.g. the Trifid Nebula in Sagittarius): 

End of the Stone Age, start of the Bronze Age (tools)

10,000 years ago (10K lightyears away – e.g. the Pac Man Nebula in Cassiopeia): 

Neolithic Period – first rudimentary farming

    2,400,000 years ago (2.4 million lightyears away – e.g. Andromeda Galaxy):

                  Homo sapiens had not yet evolved.  Our ancestors, homo habilis, lived.  The ice ages began.