# Explainer: Light-years and units for the stars

##### August 21, 2013 by Michael J. I. Brown, The Conversation

"Space is Big. Really Big." The Hitchhiker's Guide to the Galaxy pretty much nailed space with those five words. And space is so really big that our earthly measures of distance struggle.

The distance to the sun? Just under 150 million kilometres.

The distance to the nearest stars? They are more than 4x1013 kilometres away. And if you aren't familiar with scientific notation, that's 40,000,000,000,000 kilometres, or 40 trillion kilometres.

Almost everyone struggles with numbers so unbelievably big, including astronomers. Could you memorise that the Large Magellanic Cloud is 1.5x1018 kilometres away? I start losing track somewhere around the 14th nought.

So, to cope with the overwhelming bigness of space, astronomers have devised a series of really big units.

Starting small?

The starting point is the Astronomical Unit, often abbreviated to AU. It corresponds to the (almost) comprehensible distance of 149.6 million kilometres. By construction, that's almost the exact distance from Earth to the sun.

Our solar system fits snugly within tens of Astronomical Units, with Neptune being roughly 30 AU away. The planet Venus, which is now the brightest "star" in the early evening sky, can be a mere 0.28 AU from Earth.

Saturn, the most we can easily see, is at a distance of 10 AU. Ten is a reassuringly small number, but remember 10 AU is 1.5 billion kilometres. Last month, the Cassini spacecraft took a photo of Earth from Saturn, and our home was reduced to a mere dot:

Truly astronomical distances

How do astronomers measure distances across the universe? With and .

The parsec is the unit astronomers use professionally, and is celebrating its 100th birthday this year. The light-year has long been used by astronomers when they are trying to communicate to a broad audience, perhaps with mixed success.

A parsec is 206,265 AU and is roughly the distance to the nearest stars. If we were to view a with a diameter of 1 AU at a distance of one parsec, it would appear to be just 1/3600th of a degree in angular size. For comparison, the sun and moon are both half a degree in angular size when viewed from Earth.

The parsec is what astronomers use every day but, being 206,265 AU and ending in "sec", it isn't the most easily understood unit of distance. Perhaps this is why Han Solo talks about parsecs as if they are units of time, not distance. And why Star Wars nerds have spent countless hours trying to explain away this broken piece of techno-babble dialogue.

The light-year is a more comprehensible measure of distance than the parsec – the catch being (and it is a big one) that you need to accept the finite speed of light and time machines.

We are comfortable with the finite speed of sound. There is a flash of lightning and some seconds later we hear the roll of thunder. The speed of sound is roughly 340 metres every second, or 1,200 kilometres an hour.

Light travels almost a million times faster than sound, a little under 300,000 kilometres every second, or just over a billion kilometres an hour.

That means the further we look, the further we look back in time. Our eyes and telescopes are time machines for looking into the past. When we look at the moon, we are seeing it as it was a second ago, because the light takes a second to travel from the moon to us.

The light-year is the distance light travels in a year in the emptiness of space, and is just shy of 10 trillion kilometres (or 1013 kilometres).

Despite a light-year being a vast distance, even the nearest stars are several light-years away. Alpha Centauri, the brightest of "The Pointers" near the Southern Cross, is over four light-years away. That means we are viewing Alpha Centauri as it was four years ago.

Across the universe

Even nearby galaxies are at vast distances compared to the stars we see at night. The Large Magellanic Cloud, which can be seen on very dark nights, is 160,000 light-years away!

Astronomers regularly observe galaxies that are billions of light-years away, and I have even seen such distant objects with my own eyes.

One evening, at Mount Lemmon Observatory in Arizona, I used an eyepiece and 1.5-m telescope to peer across the universe. Barely detectable to the human eye was the faint glow of the quasar APM 08279+5255. This quasar is more than 12 billion light-years from Earth.

The light from APM 08279+5255 took 12 billion years to reach Earth and started its journey when the universe was just a tenth of its current age. Billions of years before there were people, billions of years before there were dinosaurs, and even billions of years before Earth itself was formed.

Even to astronomers, the to APM 08279+5255 is truly "astronomical".

Explore further: How long does it take sunlight to reach the Earth?

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##### cantdrive85
2.8 / 5 (12) Aug 21, 2013
An amateur astronomer Robert Burnham wrote the book 'Burnham's Celestial Handbook' which gives a great explanation of the vastness of space. Chapter 2, The Celestial Survey;
http://books.goog...;f=false

On a 1AU to 1" (2.5cm) scale, the sun would be a dust mote 1/100" (.25mm) across, the earth an inch away. Pluto would be 39.5" (one meter) away. Voyager would be about 7 feet (2.15m) away. The nearest star from that dust mote of a sun would be 4 miles (6.5km) away! The MW would be 100,000mi (161,000km) across, 4 times around the equator. Kinda makes one feel awfully small...
##### rug
3 / 5 (4) Aug 21, 2013
Cantdrive, You found something I can agree with. The size of the universe does tend to make one feel a bit tiny.
##### cantdrive85
1 / 5 (10) Aug 21, 2013
Yep, and on that scale the binary pair of Alpha Centaurus exerts 153 trillionths of an earth "g" on the Sun.
http://www.thunde...t_id=177

For some reason it seems a stretch that the remarkably weak gravitational attraction between stars is what holds galaxies together. Well, except for adding 15 times more DM than what is visible.
##### rug
4 / 5 (8) Aug 21, 2013
and there you go....never mind I take back the complement.
##### Soylent_Grin
5 / 5 (9) Aug 21, 2013
For some reason it seems a stretch that the remarkably weak gravitational attraction between stars is what holds galaxies together.

Good thing it isn't. What's holding the sun in the galaxy isn't Alpha Centauri. It's the billions of stars, thousands of black holes, countless brown dwarves and rogue planets AND all the DM on the other side combined that holds us near.
##### Eikka
5 / 5 (1) Aug 21, 2013
People don't have an intuitive grasp on distances anyhow, because we measure things not by how far apart they are in space, but by how long it takes to get there.

For an ordinary person, 5 miles by foot feels as distant as 50 miles by car, or 500 miles by airplane. People understand a foot, because they can see it, but they don't really understand a mile because it's beyond their immediate perception; by the time they've walked a mile they've already forgotten every single step they took but the last few.
##### Mayday
5 / 5 (2) Aug 21, 2013
Soy, that's what Cant was trying to say, I think. The vast distances between each of the large masses in a galaxy diminish their individual gravitational effect enormously. It is sobering to realize that the sum of these "subtle" effects can influnce those far-flung objects to become so reliably organized. The clockwork of it seems so fragile from that perspective, but it certainly is not. What it is... is inevitable.
##### rug
3.4 / 5 (5) Aug 21, 2013
@ Mayday - Sadly, what cantdrive was trying to do was push his own beliefs that have already been proven false.
##### Moebius
1 / 5 (1) Aug 21, 2013
Re Star Wars: What Solo said is perfectly logical. He was a smuggler which involves avoiding authorities which means you don't fly the crows path. If the Kessel run involves multiple stops and you know where the authorities are most likely to be, it's a zig zag path and distance equals time. He could have said he did it in light years or miles just as logically.
##### Soylent_Grin
5 / 5 (3) Aug 21, 2013
that's what Cant was trying to say

Unfortunately, what cantdrive was trying to say is that the scientific community has been selling us all a load of bunk, and can't be trusted to tell us the truth, and its obvious because look at this unimaginably tiny number, do you really think that's how things are?
##### Soylent_Grin
not rated yet Aug 21, 2013
He could have said he did it in light years or miles just as logically.

Except he was trying to tell Luke how fast the ship is, not how efficiently he could fly it. When Luke asked "Is it a fast ship?", he wasn't asking about the pilot's skill in evading detection; he was asking about speed, not distance.
Lucas or another writer made a goof and didn't know what a parsec is. People should stop trying to cover for their mistake. =)
##### L_Joyce
5 / 5 (1) Aug 21, 2013
The beginning of this - "The Hitchhiker's Guide to the Galaxy pretty much nailed space with those five words." was driving me crazy... what 5 words?? The start of the story was lost... http://scienceale...709.html reveals the 5 words to be "Space is Big. Really Big."

And on Han Solo - Parsec was correctly used as a distance measurement. The gist of it was that his ship was fast enough to fly closer than usual to a cluster of black holes, and therefore shave distance off a common smuggler's route.
##### barakn
5 / 5 (4) Aug 21, 2013
Yep, and on that scale the binary pair of Alpha Centaurus exerts 153 trillionths of an earth "g" on the Sun.
http://www.thunde...t_id=177

For some reason it seems a stretch that the remarkably weak gravitational attraction between stars is what holds galaxies together. Well, except for adding 15 times more DM than what is visible.

And so you think EM forces, which are equally likely to be repulsive as attractive, are a better explanation than gravity, an always attractive force? What have you been smoking?
##### Michael Brown
3 / 5 (1) Aug 21, 2013
I will chase up the missing quote ("Space is Big. Really Big") with phys.org.

Regarding the amount of dark matter within galaxies, the ratio of dark matter to normal matter varies as a function of radius within galaxies. At the centres of galaxies and within roughly 10 kiloparsec of the centre of Milky Way mass galaxies, conventional matter is dominant. This is to be expected if dark matter has relatively few interactions with itself and conventional matter apart from gravity.
##### Michael Brown
not rated yet Aug 21, 2013
Comparing astronomical distances and Earthly distances, using the scale model approach, has been used for over a century.

An example using light years, AU, miles and inches by Plummer published in Nature in 1890 can be found via DOI: 10.1038/043104d0
##### cantdrive85
1 / 5 (6) Aug 22, 2013
And so you think EM forces, which are equally likely to be repulsive as attractive, are a better explanation than gravity, an always attractive force? What have you been smoking?

Without a doubt, as has been shown repeatedly, the EM forces produced by a pair of parallel interstellar birkeland currents obeys the longest range force law 1/√ r. And that's long range attractive, short range repulsive. If the two stars are on parallel BC's, the EM force exerted will be magnitudes stronger than the gravitational pull between them.
Oh BTW, I live in CO and it's legal over here, however mine is strictly medicinal.
##### Michael Brown
5 / 5 (3) Aug 22, 2013
EM-Plasma cosmology ideas have been kicking around at the fringes for a long time and have been debunked many times. A particularly thorough debunking was done some time ago by Ned Wright and there is also some discussion on the Big Bang FAQ at talkorigins.org.