Shall we start on our way?
Why not, indeed. One of the best reasons for doing anything. So here we are at one of my favourite stars, Deneb. Stars are grouped together into what are called constellations and named after animals or mythical beings. Our star is part of the constellation Cygnus or the Swan. For most constellations the pattern of the stars doesn't really look like anything, let alone an animal. On a clear summer night our constellation is almost overhead, a huge majestic cross with outstretched arms. So it really does look like a swan in full flight.
Well sort of like a swan, in the same way a stick drawing of a man looks like a man. Our star Deneb is the tail of the Swan and a huge distance from the Earth, 18,970,843,745,070,000 miles to be precise. It’s hard to imagine what that kind of distance really means but in car journey terms it would take over thirty billion years at a steady 70 miles an hour.
"Wait a minute, how can we get there in 3000 years if it would take thirty billion years to drive there?"
But we’re not going to drive, we’re going to fly through space. We are going to cover such a huge a distance in such a short time because we're travelling at the speed of light. We are light particles or photons now. Not just any light, the very best type of light – starlight. So travelling at light speed comes naturally to us. Even though this sounds like a long journey, it’s really a fairly short stroll through this galaxy, the Milky Way, a journey of a mere 3,227 light years in a galaxy that is over 70,000 light years across.
“Is a light year the same as a normal year?”
Even though it sounds like it ought to measure time, a light year is a measure of distance rather than time and is how far light can travel in a year. At 186,000 miles a second and with over 31,000,000 seconds a year that makes a distance of almost 6,000,000,000,000 miles or six trillion miles. To give you some idea how far a light year really is, the Earth’s moon is only a light second away. Now think how much longer a year is than a second.
Deneb, is one of the largest, brightest stars in the whole galaxy, that’s why I like it. It's 160,000 times brighter than the sun and much bigger. So big that in the Earth's solar system it would stretch all the way from the Sun to the Earth. If the Earth were going around Deneb rather than the Sun it wouldn't be floating in space but skating along the star's surface and cooked to a temperature of over 8,000 oC. Deneb only looks like a very normal star because it is so far away. If Deneb was as close as Earth as the nearest star, Proxima Centauri which is only 4 light years away, it would be bright enough to cast shadows at night and be visible day and night.
“So what would the sun look like from Deneb?”
Not very impressive. If you looked in the direction of the sun from Deneb with just a pair of eyes you wouldn’t even be able to see it. You’d need a very powerful telescope to make it out as a faint very ordinary looking star. If today you had a powerful enough telescope to see what was happening on earth all the way from Deneb what would you see?
“Is that possible?”
No, but try and imagine it.
“OK, well you’d see all the people doing what people do these days I suppose.”
Not at all, you’d see what people were doing three thousand years ago. If light takes that long to travel from Deneb to the Earth, it will take that long to go the other way. So the light from earth reaching your telescope would be three thousand years out of date. If little green men from the Deneb solar system pointed a huge telescope towards the Earth today they would see the Earth around the time of the Trojan horse.
“So is all starlight old?”
When you look up at a star filled night the light reaching your eyes has been traveling for tens, hundreds or even thousands of years. For each star the light you are seeing started its journey at some moment in history. Imagine there are stars whose light started towards Earth before the time of the ancient Greeks and Romans and light that has been traveling since before
“I thought that people claim to tell the future from looking at the stars?”
Oh astrologers think that but when you look at the stars you are looking at the past not the future. Don’t confuse astrology with astronomy. Astrology is merely the art of telling people what they want to hear. Astronomy is the science of working out what stars are and what is happening out here in the universe.
"So why is our star called Deneb?"
Well not all stars have names but where there is one it usually comes from the ancient Arabic astronomers. The Romans named most of the planets after their own Gods but didn't seem to bother so much with the stars. After the Greeks and Roman empires died out and before
“No, but now you mention it that’s an amazing thought.”
Well, that because the way we all write numbers comes from one place, Arabic mathematicians in places like
"I thought our constellation was a swan."
Well things have gotten a bit mixed up over time, but all things considered we've been quite lucky with our names. Take Betelgeuse, the big red star in the constellation Orion, the Hunter. This star started with the Arabic name Yad al-Jauza, or Hand of Orion. It somehow ended up as Bait al-Jauzah which over time became Betelgeuse which apparently means the ‘Armpit of the Mighty One’.