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? More specific??? It’s HUGE! 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 light-years (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.)

The Universe is really big. 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 The same types of calculations can be done with light-years as well. Try these: Convert to m and km: a) 0.95 AU b) 25.5 AU Convert to AU: a) 3.4 x 1012 m b) 9.5 x 107 km Convert to m and km: a) 4.4 light-years b) 25 million light-years Convert to light-years: a) 2.7 x 1015 m b) 8.2 x 1020 km How many AU are there in 1 light-year?