Answers for students: What is a light year?

Above: A nebula. The smallest part of this picture you can see in this picture is thousands of times bigger than our whole world - so normal measurements of distance just don't work. Unless you're a linguistic masochist who likes having to saying "billion trillion trillion trillion quadrillion" after everything.

This one comes up a lot - It comes up in exam questions, and everyone seems to stumble over it: What are these 'light years' that keep being mentioned by astronomers and sci-fi shows? Do you have to eat less calories, or keep at least one house light on nonstop for 365 days to make them?

And the answer is: No. I eat however many calories I like (to be fair I'm starting to get a bit fat and have to go for runs) and I leave all the lights on in my house all the time anyway.

I'm not scared of the dark. I never admitted to that. Ahem

Anyway...

A light year is NOT a measure of time. Yes, I know it has the word 'year' in it's name. Yes, that's a pretty dumb and confusing thing to name a measure of distance. No, I can't do anything about it.

The important bit is that we understand the dumbness: It's called a 'light-year' because it is how far a beam of light will travel in one year (if it doesn't hit anything). So, to the astronomers that first came up with it, 'light-year' seemed to fit.

So, how far is it? Well we can convert light years to meters like this: 

A beam of light travels at 300,000,000 meters per second. To work out how many meters are in a light year, we just need to work out how many seconds are in a year and multiply that number by 300,000,000 meters.

Seconds in a minute = 60
Minutes in an hour = 60
Hours in a day = 24
Days in a year = 365.25*

So the number of seconds in a year is 60 x 60 x 24 x 365.25 = 31557600. 
And, for our grand finale, the number of meters in a light year is 31557600 x 300,000,000 = 9,467,280,000,000,000 meters.

Or, in other words, a very, very long way. It's the absurdly big size of the distances out in space that makes astronomers use light years as their units of distance.

Above: This is the Pleides star cluster, which is 43 light years across. Work that out in meters, using the method above - how many millions of millions of millions is it? And how much longer does it take you to say "Captain the engines cannae take it for another (insert millions of meters)" than "Captain the engines cannae take it for another 43 light years"?

Where things get  kind of crazy is when you think about what those huge distances mean for how we see the Universe: We see stars with light, which is the fastest thing we've ever discovered. The nearest star to Earth (after the Sun) is four light years away. That means it takes light from it four years to reach us - when you see it in the sky you are seeing it as it was four years back. Most stars are much further away, hundreds or thousands of light-years. Which means that, if you look up at the night sky, you are not seeing those stars as they are today but as they were hundreds or thousands of years ago. 

When you look at a star you are, very literally, looking hundreds or thousands of years into the past.

So... yes Doctor: Time travel is possible - in this limited way at least.

A really good example of this is the star Betelgeuse, in the constellation of Orion. Betelgeuse is unstable, and in danger of going supernova. But, because it is 650 light years away, it might already have exploded in a supernova. Right now it could well be a huge cloud of debris. If that had happened anytime over the last 650 years we wouldn't know about it yet, because the light that makes up the image of  the explosion wont have reached us yet. 

When you see Betelgeuse in the sky, you might actually be looking at its ghost.

So cut astronomers a bit of slack. Yes they're a bit odd and out of this world. But if your job started at that level of weird (and it gets a lot worse from there, with hairy black holes, lenses made of empty space, and invisible matter that passes through us all the time like ghosts) you'd be a bit odd to by the time you hit retirement.


* The 0.25 is because each orbit of Earth around the Sun doesn't quite match a whole number of days, leaving us with a quarter of a day over. For this reason every four years we have a leap year, with an extra day, to compensate and keep the calenders in line with what Earth is actually doing.

Sound In Space: The Song of Jupiter

Above: The chaotic, Shoggoth inspired, swirl of massive storm systems at Jupiter's pole. Courtesy of NASA/JPL

The JUNO space probe has been wowing the Earth as it returns astounding images of the beautiful and titanic weather systems that shroud Jupiter - but there's another side to the exploration of the King of the Giant Planets: Jupiter's song. 

While space probes cannot pick up the actual sounds in Jupiter's clouds, (due to there being the vacuum of space between them and the planet), their radio antenna can pick up the natural radio transmissions from the giant world's immense magnetic field. 
So, for the full 'Jupiter experiance' try playing the two videos below - one a high def flyby of Jupier's cloud tops and one a recording of its eerie radio song - at the same time....

If you'd like to help us do things like podcasts, animations, and videos here why not...

Five of the strangest asteroids and dwarf planets...

Above: Pluto, a world half the size of the US but stranger than anything dreamed up by sci-fi writers.

Most people thinking of space exploration think of the planets - Mars, Venus, even Jupiter and its Moons. But that's missing something - hundreds of somethings in fact: The large asteroids and dwarf planets make up a collection of worlds as varied and compelling as the terrestrial planets. So here are a few of the oddest...

Ceres:
The king of the asteroid belt, Ceres is a dwarf planet nearly a thousand kilometres across. It's also one of the relatively few asteroids to have been visited by a human space craft - in fact the Dawn space probe is still in orbit, surveying this small world. Despite its small size Dawn has shown that there's still recent signs of geologic activity on that 4 billion year old surface, and perhaps even hints of subsurface water.

Above: A simulated flight over Ceres, assembled from data supplied by the Dawn probe.

Vesta:
The first small world visited by the Dawn probe, Vesta proved to be a world with a history of cataclysmic violence. Already suspected to have been a hot, churning, volcanic world in its youth, Dawn found that the tiny world’s entire South Pole had been obliterated, not once but twice, by gigantic asteroid impacts. The largest of these was so huge it left the 'ripples' caused by the shockwave permanently imprinted in the Vestan rock.

 

Above: Dawn's greatest finds at Vesta

Psyche:
Although we've only ever seen this world at a distance, through telescopes, it is due to be visited by a unique space probe (also called 'Psyche') in 2026. Why? Because it's a solid chunk of metal the size of the state of Massachusetts. The only sensible explanation anyone has come up with (so far) is that Psyche was once the core of a planet - a planet that suffered some terrible fate which stripped down to its metal core...

 Above: The bizarre metal asteroid Psyche, and the mission to explore it.

Sedna:
It's unlikely that Sedna, a world 8 billion kilometres from Earth with a year over 11,000 Earth years long, will get a visit from any spacecraft soon. Even so, it's playing a pivotal role in our exploration of the dim outer reaches of our solar system. The dwarf planet's odd shaped orbit could be the result of the gravity of a ninth major planet, far out in space. Or, perhaps even more intriguingly, it could be a sign that Sedna is not really a member of our solar system at all: It could have been a loosely bound dwarf planet of another star, kidnapped by our sun during a rare stellar flyby.

Above: Think you know how far a long way is? Well take a look at the orbit of Sedna...

Pluto:
I take it you've heard of this one? Although the debate still rages in parts as to whether its demotion from planet to dwarf planet was a fair one, I really don't think this immense and incredibly complex ice world cares. From it's weird hundred meter blades of ice, to its nitrogen ice glacier ocean on which mountains float like ice bergs, to the hints that rivers of liquid nitrogen once flowed on its surface and might well still run beneath its subsurface, Pluto has every reason not to give a damn what we teeny humans classify it as. But, thanks to the New Horizons space probe, this beautifully weird world has, been opened up to human eyes...

 
Above: A virtual fly over of Pluto, assembled using data from the New Horizons mission.

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