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Space Unit!
Astronomy:
the study of matter and energy in outer
space (which includes Earth)
The universe:
All matter and energy that exists
How BIG is the universe?
It’s
HUGE!
More specific???
It’s infinite…….
(That means it has no beginning or end.)
Do we know that for sure?
As with almost everything in astronomy, we
need to evaluate indirect evidence and make
models using inferences from this evidence.
It’s all a matter of scale
Let’s start at a distance
of 1000 km.
Travelling at 1000 km/h,
it would take…
…1 hour to travel this
distance.
It’s all a matter of scale
Let’s go to a distance of
10000 km.
Travelling at 1000 km/h,
it would take…
10 hours to travel this
distance
It’s all a matter of scale
Let’s go to a distance of 100 000 km.
Travelling at 1000 km/h it would take…
just under 4 days to travel this distance.
It’s all a matter of scale
… 100 million
km!
Travelling at
1000 km/h it
would take…
…about 11.4
years!
It’s all a matter of scale
… 10 billion km!
Travelling at
1000 km/h it
would take…
…about 1141
years!
It’s all a matter of scale
Ten trillion km is just over one light-year.
 A light-year is the distance that light can travel in one
year.
The speed of light is 3 x 108 m/s. (300 000 km/s)
Distance = speed x time
1 year = 3.15 x 107 s (31 500 000 s)
So Distance = (3 x 108) x (3.15 x 107)
= 9.45 x 1015 m or about 10
trillion km!
It would take us over 1 million years to travel this
distance, and that would only bring us ¼ of the
way to the closest star!
It’s all a matter of scale
This is a typical galaxy.
 A galaxy is a
collection of stars
that revolve around
each other. There
are usually 100s of
billions of stars in a
galaxy.
The sun is 30000 lightyears (3 x 1017 km)
from the centre of
the Milky way
galaxy.
It’s all a matter of scale
… 10 billion km!
(1 x 1010 km)
Travelling at
1000 km/h it
would take…
…about 1141
years!
• At 1000 km/h it would take…
– 3 billion years to get out of the galaxy
– 30 billion years to get to the centre of the galaxy
– 100 billion years to cross the galaxy!
But we’re not done yet!!!
This is the Hubble Space Telescope’s Deep Field image
Each one of these fuzzy dots is a galaxy like our own,
each with billions of stars.
The red line represents about 100 000 light-years!
…and this is just one VERY small part of the sky! (You
would see nothing here even with a very good
telescope.)
Conclusion:
The Universe is really big.
Units of Distance in Space
Since the universe is so big, it is often easiest to
use relative units; units that we compare the
distance to something we are familiar with.
We have already seen the light-year (the
distance light travels in 1 year). We use this
when referring to the size of galaxies, or
distance to stars.
When dealing with smaller distances, inside the
solar system, we deal in astronomical units
(AU).
1 AU is the distance from Earth to the sun.
It is approximately 1.5 x 1011 m (or 150
million km)
So, if an asteroid is 2.5 AU from Earth, then
it is… 2.5 x (1 AU) = 2.5 x (1.5 x 1011 m)
= 3.75 x 1011 m, or
375 million km from Earth.
If a comet is 4.8 x 1012 m from the sun, then it
is… (4.8 x 1012 m) / (1.5 x 1011 m) = 32 AU
from the sun.
The same types of calculations can be done with light-years as
well. Try these:
1. Convert to m and km: a) 0.95 AU b) 25.5 AU
2. Convert to AU: a) 3.4 x 1012 m
b) 9.5 x 107 km
3. Convert to m and km: a) 4.4 light-years
b) 25 million light-years
4. Convert to light-years: a) 2.7 x 1015 m
b) 8.2 x 1020 km
5. How many AU are there in 1 light-year?