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# Lesson 4 - Measuring Distance

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## Luke Bohni

on 19 February 2013

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#### Transcript of Lesson 4 - Measuring Distance

Lesson 2.2 Measuring Distance Space is really Big. An example... The Astronomical Unit (AU) But what about even bigger distances? If we were to use to the measures of distance that we are familiar with everyday (meters, kilometers, etc.) we would struggle to be able to truly understand the vast distances we deal with in astronomy. Proxima Centauri is the closest start to our Sun. It is 39,900,000,000,000 km away from us The crab nebula however is 568,000,000,000,000,000 km away All those 0's make it really hard to know which one is further away and how much further away they are In comparison, the Crab Nebula is actually 1500 times further away from us than Proxima Centauri.

That's like comparing a sheep... 1 Au is equivalent to the average distance from the Earth to the Sun.

It is equivalent to 149,598,000 km The Lightyear (ly) is perhaps the most well known measure of large distances. It is the distance that one would travel if they were travelling at the speed of light for one whole year.

1ly is equivalent to
9,460,000,000,000 km To the world's tallest building Astronomers also use the distance measure known as a Parsec. It is in fact larger than a light year (3.26 times to be exact).

1 Parsec is the distance from the Sun to an object that would result in a parallax angle of 1 arc second....

... huh? The Parsec The Problem is that even though 1 parsec is a truly massive distance, most objects in the Universe are many, many, MANY parsecs away and so we can't measure their distance using parallax.

So how do we know how far away they are? Cepheid Variable Stars A Cepheid Variable Star is a special type of star that changes brightness on a regular interval.
In fact, Cepheids are unique because if we know their period we know precisely how bright they are As such we can observe the period and apparent brightness of these stars in other galaxies and determine how far away they actually are
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