Double Stars - Mizar and Alcor

At the time of writing, there’s a bright moon in the sky. As a rule of thumb, if the moon's up then it's the only thing worth looking at. But I’m not looking the moon this month, I’m saving that up. Instead we’re going to take a closer look at a star we used last week in our search for Polaris.

Naked Eye

Take a second look at the Plough. The middle star of the “Handle” is the one we’re interested in.

It doesn’t take a dark moonless night to see that we’re not looking at one star here, we’re actually looking at two. Mizar and Alcor are, perhaps, the most famous double star in the sky. In reality they’re quite far apart and don’t orbit each other. In fact all but two stars in the Plough are moving in the same direction in space, suggesting that they are all related. This is unusual for a constellation, but the Plough isn’t a constellation, it’s an asterism, a grouping within a constellation. If you look around the area of the Plough, most nights you’ll be able to pick out the rest of Ursa Major, the Great Bear.

Unusually it pretty closely resembles what it’s supposed to depict – you can see the legs, the head, the snout and… the tail. Yes, the Great Bear has a great big long tail, which doesn’t make a lot of sense, but nearby in the sky is a constellation called Lynx that most certainly doesn’t look in any way, shape or form, cat-like. So a long-tailed bear is pretty close in astronomical terms.

Binoculars

Through binoculars there are, quite clearly, two stars. The ancients used it as a test of eye-sight. Through the bins you may be able to see a third star in between the two. It was named Sidus Ludoviciana after a German nobleman and was originally thought to be a planet. In the end it turned out to be a star in the same line of sight, but much further away. To see a proper binary system here, you need a telescope.

Telescope

With the greater magnification that a ‘scope brings, you can finally reveal the secrets of Mizar. In fact this bright star is a double with the two stars very close together (14 arc seconds). Each component of Mizar is itself double making the star a quadruple system. Because of the proximity of each pair, the only way to tell they’re double is by looking closely at the light produced by the stars – the science of spectroscopy. It transpires that double stars are quite common in the universe and our own sun is unusual for being on its own.

Next time it’s going to get even more exciting because we’re going to be making babies. Baby stars that is…

Pole Position - Polaris

Finding your way around the stars can be quite a bewildering prospect. To make things easier the stars visible in the night sky are divided up into patterns – the constellations. It’s not the only way of navigating around, but recognising constellations does make finding things easier. This week we’re going to use one of the most famous constellations in the sky to find the most important star of all to humans – Polaris.

Actually, I say most important, really there is one other star that is far more important than all of them. Our own sun is a star, and it’s the only star we can see close enough to work out what stars really are.

 

Credit: NASA/JPL

The sun is, in effect, a giant nuclear explosion in space. It’s so enormous that it has about 10 million years worth of its primary fuel inside it. Inside stars, atoms of hydrogen are pushed together at enormous temperature and pressure so that they fuse. This nuclear fusion process generates heat and light, and as a by-product helium.

Not all stars are the same – the sun is classed as a dwarf star, pretty small in the grand scheme of things. There are much larger stars out there and also much smaller – many of these we may meet over the coming months. They also come in a variety of colours – looking around now you might spot bright red Betelgeuse, yellow Capella or blue-white Rigel.

Stars are grouped by us into constellations. These groupings aren’t physically associated with each other or even close to each other in space – often they’re separated by many thousands of light years. But they’re a useful way of dividing up the night sky and learning them is both fun and useful. There are a few that are little more than a couple of faint stars but many are very bright and recognisable.

This week we’re going to use one of the most recognisable patterns – the Plough (Actually an asterism, a group of stars within a constellation) – to help us find the North Pole, handily marked by Polaris, the pole star.

Naked Eye

Finding Polaris is one of the most useful astronomical skills you can learn. The star is at the North Celestial Pole so being able to find it allows you to find out which way is north.

To do it you first have to locate the Plough – this is a group of seven stars that forms most of the constellation of Ursa Major. Once you’ve found the Plough, look for the two stars at the front of the shape. If you take a line through these two, from bottom to top, and keep following it, they point to Polaris.

 

Finding Polaris

Polaris is the brightest star in Ursa Minor, the little bear. It’s a faint constellation of seven stars – the number of other stars visible can give an indication of how dark the night is.

Binoculars and Telescope

There’s not a whole lot to see of Polaris through binoculars, and the situation is the same through a 3’’ telescope. However at high magnification Polaris yields a surprise. Next to it, you may be able to make out a faint star. This is because Polaris isn’t one star, it’s two. Double stars are actually fairly common, although Polaris is maybe not an ideal example: the faint companion star is drowned out by the glare from Polaris. We’ll look at a better example of one in a bit more depth next week.

n.b. I had originally envisaged using sketches to illustrate this blog. Unfortunately my scanner is not yet working, so we'll have to stick with these simple paint images for now.

By Jove! - Jupiter

There are so many people who want to find out more about the universe but are put off astronomy for one reason or another.Well re-write your resolutions because, by the end of this year, if you follow this weekly blog you’ll have grasped the basics of observational astronomy. You don’t need a telescope (Binoculars are handy), you can do this from a city and all you need is half an hour a week and some warm clothes.

Dipping your toe into the wonders of the universe can be daunting. The sky is a bewildering place, and finding your way around does take practise. People are also often put off because they’re expecting to see an image that looks something like this:

Credit: NASA/JPL

Of course through even a decent sized Earth-based telescope you'll never get that level of detail, but there's still lots to enjoy out there.

It happens that we’re going to start off with Jupiter. There are two reasons for this – firstly because it’s easy to find, and secondly because it’s pretty damned cool, even if it doesn’t look quite like the image above.

So, how to find it:

Naked Eye

At 9.30pm, look up high towards the West – Jupiter is the brightest thing you can see. In fact it’s the fourth brightest thing in the sky, after the Sun, Moon and Venus. Its colour is striking with the naked eye, a vivid yellow.

There are two ways to recognise something bright and shiny in the sky as being a planet:

1 – Stars twinkle, due to the movements of our atmosphere. Planets do not.

2 – Planets move around the sky, relative to the stars. That’s why they’re called Planets; the word means “Wanderer.” As a consequence of this, if you read this article after January 2014, Jupiter won’t be in the same position – it moves by roughly one zodiacal constellation per year. At the moment it’s in the constellation of Gemini, but in 2015 it’ll be moving through Cancer and Leo. We won’t worry too much about constellations for now – Jupiter is bright enough that it’s hard to mistake for anything else.

Binoculars

Look at Jupiter through a pair of binoculars and things get interesting. You’ll see four small pin-pricks of light strung out in a line beside it. These are the four brightest moons of Jupiter: Io, Europa, Ganymede and Callisto.

Every time you look at Jupiter they’ll have shifted around slightly with some of them occasionally going behind or in front of the planet. The Voyager space probes revealed a wealth of information about these worlds, but they were first discovered by Galileo using a telescope.

Telescope

Jupiter is by far my favourite planet for telescopic observation. The moons are clearly there and, with a big enough telescope, it is possible to watch them silhouetted against the disc of the planet when they pass across it – a transit.

The disc itself reveals a lot of detail – even relatively small instruments show a series of dark bands running across the planet’s surface - the cloud belts. In my 3’’ refractor the largest of these, the north and south equatorial belts, are most easily seen. The bigger the ‘scope the more detail you get, and it’s possible to see the wisps and curls of the clouds of gas as they interact with each other, and the famous Great Red Spot, an enormous storm the size of our own planet.

Jupiter is the largest planet in our solar system. In fact it weighs two and half times as much as all the other planets combined. Landing there would be tough – it’s mostly made up of a mixture of gases, all of which are rather unpleasant. For example its clouds are full of hydrogen sulphide, the gas that gives rotten eggs and flatulence their characteristic smell. Even if you went a little way into the clouds, your body would swiftly be crushed by the weight of gas above you. The gravity of the planet is so intense that it squeezes the core of its closest moon, Io, causing massive vulcanism. It's not a place to consider for a holiday either - the intense radiation would bake you if you were to get too close to it. Probably worse for you even than sunbeds.

That's it for this week - next time I'm going to describe how to find the most useful star in the entire night sky: Polaris.