Summer Sky Tour

Summer is a season of mixed blessings for astronomers. The nights are warm but short and often hazy. As if that weren’t bad enough, we have to contend with swarms of blood-sucking mosquitoes! Haze and insects aside, the summer sky is a veritable gallery of cosmic masterpieces. The Milky Way arches high across the sky, which is richer than its winter counterpart because we now look toward the heart of our galaxy. Dominating the evening sky are three 1st-magnitude stars forming the Summer Triangle. Vega is the brightest of the three and is located in a nifty little constellation called Lyra the Lyre.

Our second Summer Triangle star, Deneb, is the tail of Cygnus the Swan. Deneb and four other bright stars of the Swan form an asterism called the Northern Cross, which is immersed in the Milky Way. Deneb is at the top of the cross, the star Albireo is at the base. Albireo is a double star famous for its rich colours of golden yellow and sapphire blue. The star pair can be split with binoculars, but the colours can be seen only through a telescope.

Farther south on the Milky Way is the bright star Altair and its parent constellation, Aquila the Eagle. If you follow the Milky Way from Aquila toward the southern horizon, you should find a group of stars that looks like a teapot. This asterism is part of Sagittarius the Archer. This constellation marks the location of our galaxy’s centre. The area teems with deep-sky treasures especially bright nebulae and star clusters. To the right of the teapot is the ruddy 1st-magnitude star Antares, the “heart” of Scorpius the Scorpion. Like the winter star Betelgeuse, Antares is a red supergiant star in the last stages of its life. A fishhook-shaped row of stars trailing down and to the left of Antares forms the Scorpion’s tail and stinger, while an upright row of three stars to Antares’s right marks the location of its claws.

The view to the north

 

 

The view to the south

The Polar constellations

We start the Summer guide with the polar constellations, they’re easily recognised and they are a great starting point at any time of the year

The two most important Polar Constellations to recognize are the Plough and the Big W. The Big W is Cassiopeia – roughly on the opposite side of Polaris from the Plough, Cassiopeia is a Queen in her chair, and even though this isn’t the “official” way to look at her, I envision Cassiopeia’s head at the left side of the “W”, making the figure like a lounge chair with a foot rest. This is how I learned it as a kid, and it’s very useful because you can easily find the North Star by going “Up from the Seat” of Cassiopeia’s chair, in similar manner to going “Up from the Cup” of the Plough. Since the Big W is on the opposite side of the North Star, this gives you a way to find Polaris any time of the year, even if the Plough is below the tree line.

Cassiopeia got herself in trouble with Poseidon boasting she was more beautiful than the Nereids (sea nymphs) which leads to the story of Andromeda and Perseus and involves no less than six constellations in the sky (all fall constellations), the most involved in telling any constellation legend. That’s a story for another season.

The constellation Cassiopeia lies right smack in the path of the Milky Way, and is filled with galactic clusters, also called open clusters. There are several decent examples in Cassiopeia, like Messier object #52, or “M52”, one of many “M” objects named after a catalogue published in the late 1700’s by the Frenchman Charles Messier. Messier was a comet hunter of great renown who published a catalogue of fuzzy things that might be confused with comets. These ended up being some of the most interesting objects in the sky. When you find M52 you are looking at a “young” cluster of about 200 stars — young means that it is only about 100 million years old. The cluster is 10 light years in diameter and 3000 light years away from us, which is why you need the binoculars to see it.

Another interesting galactic cluster is NGC 663 (NGC stands for New General Catalogue — published in 1888 it’s not really that new). This cluster lies about half way between the two stars on the flattened side of the “W”, e and d Cassiopeiae. As you sweep the binoculars between the two stars, you will notice that NGC 663 is merely the largest and brightest of several clusters in this region. That may be because it is the closest, at about 3000 light years. The others in that area are 6000 – 8000 light years away. How many clusters can you spot between these two stars? You might be able to see as many as five, with NGC 663 at the centre and the other four marking the corners of a diamond around it.

The Plough points to the North Star — it also points to other important stars — you follow the arc of the Plough’s handle and “arc to Arcturus”, then “spike to Spica” – two very important stars that we will get to in a minute. So, the three stars of the handle are pointers as well.

Wait a minute is that really three stars in the Plough’s handle or is it four? Look there the middle star seems to have a companion — the bright star is Mizar and the companion is Alcor. Who can see the companion?

 

That was used by ancient Greek and Arab armies as an eye test. Some see them as a horse & rider. The Europeans saw the handle of the dipper as the tail of the Great Bear. Since bears don’t have tails they danced around it by explaining that when the gods lifted the bear to the sky, the tail got stretched out. Pretty lame. The Indians, who knew darn right well that bears don’t have tails, saw the three stars of the handle as hunters chasing the great bear (interesting that they also saw a bear). When the constellation Ursa Major sets in the fall, the Indians explained that the hunters catch up with him and shoot him with their arrows, which is why the leaves on the trees turn red. Anyway, the three stars are hunters and one of them brought his dog. Mizar is a hunter and Alcor is his dog. Or another story is that there are three hunters pursuing the bear and one brought a pot to cook the bear in (optimistic). So, Mizar is a hunter and Alcor is his pot. Yet another story involves the Pleiades. This is a star cluster in the constellation Taurus (a winter constellation). It is called “the Seven Sisters” and those with very sharp eyes can see seven stars but most people can only see six. So, the story is that Mizar is riding off with the Seventh Sister.

But wait there’s more! When we put the telescope on these two, you’ll see that Mizar is really a double star itself! These three form a triple star. But wait… that’s right… there’s MORE! In reality each of the two stars that make up Mizar is a double star, too close for us to see even with a big telescope, and for that matter, so is Alcor! Mizar & Alcor comprise a SIX STAR SYSTEM!!

Cepheus

The reddest star in the sky is m (Mu) Cephei, “The Garnet Star” in the constellation Cepheus. Cepheus is a king, and Cassiopeia is his queen.

 

Cepheus is sort of a house-shaped constellation, and m (Mu) Cephei is located halfway between the two stars at the bottom of the house. It is a red super-giant, 1,500 times the size of the sun. It was considered the largest star known to man until just recently when 3 other stars (which you can’t see without a big telescope) were measured about the same size but just barely edge it out. Placed where our sun is, the surface of m Cephei would extend out past Jupiter. To really appreciate how red this star is, it sometimes helps to use the binoculars (or the telescope).

Another star in Cepheus is of crucial importance to astronomy — d (Delta) Cephei. This star is a “variable”, meaning the star’s brightness varies over time — in this case it varies between that of z (zeta) Cephei and e (epsilon) Cephei over a period of five days. How bright is it now — as bright as z, e, or in between? We will assess again each night that we can during the week.

This star was the first of its type to be discovered, hence these variable stars are called ‘Cepheid’ stars. Their discovery, as it happens, rocked the astronomy world. So, what’s the big deal?

Cepheid stars have gotten to just the right mass to be unstable. The whole star is pulsating, the surface of the star is actually rising and falling, with a rhythm that is so precise you could set your watch to it. It was discovered in 1912 that this rhythm depends directly on the true brightness of the star — the brighter the star, the longer the time between peaks.

When a star is closer to us, it seems brighter. When it’s farther away it seems dimmer. If we know the true brightness of the star, and we measure its apparent brightness, we can figure out the distance of the star. If the star is part of a cluster or a galaxy, this tells us the distance to that entire body of stars. This has been used to find the distances to globular clusters, other galaxies and even our distance from the centre of our own galaxy — 28,000 light years.

In 1924, Edwin Hubble (yes, the telescope is named after him) used Cepheids to measure the distance to the Andromeda ‘nebula’ (2.3 million lightyears) and proved that it is not another solar system in formation but an ‘island universe’, another galaxy like our own. This was an extraordinary declaration about the structure of the universe back in 1924. Our whole system of measurement of the universe is built upon the Cepheids as our basic yardstick.

Then on top of all that, if we look in the telescope, we can see that bright yellow δ Cephei has a beautiful blue companion. This is both a variable star and a binary! The yellow star is the variable component, and once was a blue star like its companion. It is now expanding out as it goes through the end-of-life process, and is passing through an unstable phase as it does so. The pair is about a thousand light-years from us, based on the Cepheid period-brightness relationship.

Yep, that’s how we figured out how far we are from the centre of our galaxy. We’re looking in the wrong direction to see the centre of our galaxy. For that we need to swing around to the constellation Sagittarius — to the south — which is our next stop on the tour.

The centre of our galaxy is located to the South, in Sagittarius. You can’t see the entire constellation of Sagittarius but you can see the  asterism of the Teapot .

 

Sagittarius is our second Zodiac constellation. It is an archer that is a centaur — getting ready to shoot the giant scorpion next to him. The front of the teapot is his bow and the tip of the spout is the tip of his arrow. The rest of the constellation doesn’t look anything like an archer or a centaur. Not a problem – people only reference the Teapot in Sagittarius, just as they talk about the Plough, instead of Ursa Major.

 

The bright region of the Milky Way that looks like steam coming out of the spout of the Teapot marks where the centre of the galaxy is, about 30,000 light years away.

If you look at the top of the spout through binoculars then slowly scan up from there, you will come across the Lagoon Nebula (M8) and then the Trifid Nebula (M20). These are diffuse nebulae – clouds of dust and hydrogen gas – the stars you see in them are formed from the gas when it condenses together and compresses due to gravity then, under pressure, heats up and ignites. Nebulae like these, then, are the birthplaces of stars.

This region of the sky, Sagittarius and its neighbouring constellations, happens to be one that is dense with nebulae and young stars. The only other region of the sky with this much new-star formation activity is Orion and its neighbouring constellations.

If you put the binoculars right on the star at the top of the teapot, and slide to the left until the star is at the right edge of your field of view, then right in the centre you will see a smudge of light. This is M22, and in a telescope you can see this is a dazzling ball made up of a half million stars, 9600 light years away. It turns out that this is pretty close for a globular cluster.

 

Scorpius is a scorpion, a constellation that dominates the summer sky, and, unlike Sagittarius, looks exactly like what it’s supposed to be. This is your third Zodiac constellation.

 

Remember the red star Antares which is the heart of the Scorpion – Antares is a red super-giant and it is a first magnitude star. There is only one other red super-giant that is this bright and it’s on the opposite side of the sky, in the constellation that dominates the winter sky — the star Betelgeuse in the constellation Orion. Both stars are about 500 light years away in opposite directions. Se lie just about halfway between the two. Such stars are rare — there are only 200 known red super-giants and all are much dimmer, most are much farther away than these two stars.

Even though it is a red super-giant near the end of its life, Antares is actually a very young star, at least compared to the sun. Antares was born only about 20 million years ago, long after the Dinosaurs died out, and is nearly at the end of its life already. This is because Antares is so much more massive than the sun. The greater mass causes greater pressure and forces the star to burn hotter (bluer) and faster. Antares was once a blue supergiant with 12 times the mass of the sun, burning 6,000 times as fast as the sun. Its life as a star will only be about 2 thousandths as long the sun will live. With stars, the bigger they are, the harder they fall.

The scorpion used to be a much bigger constellation. The two brightest stars of Libra – your fourth Zodiac constellation, have Arabic names that mean “Northern Claw” (Zubeneschamali) and “Southern Claw” (Zubenelgenubi). Say those real fast five times. The Romans cut off the scorpion’s claws and created a new Zodiac constellation to fit their new, 12-month calendar. The constellation Libra represents a balance scale, with the top of the balance to the west (your right).

If you look at the Southern Claw in the binoculars you will discover it is actually a double star — a true double star — they are the same distance of 75 light years away and moving together. If you have good eyes you may be able to see both stars without the binoculars. Meanwhile if you look at the Northern Claw in the binoculars you see what many people call the only green star visible to the naked eye. What do you think? Is this star really green?

Tracing down the scorpion’s back with binoculars you can find a lot of great objects. M4 is a globular cluster about halfway between Antares and s (Sigma) Scorpii, the bright star just to the West. M4 appears in the binoculars as a faint ghostly presence, which almost looks painted onto the sky between the two bright stars.

m (Mu) Scorpii is a double blue star, a true pair almost a light-year apart. South of m is z (Zeta) Scorpii, a visual double, not a true double (very close to the horizon, difficult to see even with binoculars). The eastern, brighter star is an orange giant about 150 light years away. The western star is a blue super-giant estimated to be 5700 light-years away, one million times as bright as the sun – a candidate as possibly the most massive super-giant known. Just above z Scorpii is a beautiful open cluster, NGC6231. The bright star at the tip of the scorpion’s tail is Shaula, a brilliant blue star about 310 light years away.

 

NGC 6231 - Open Cluster in Scorpius | TheSkyLive.com

Above and to the east of Shaula you will find two “open clusters”, M6 and M7. Open clusters, also called ‘galactic’ clusters, are small groups of stars, maybe a hundred or so, that formed from the same (huge) cloud of gas and dust. They are very often young stars – blue ones which are large and very hot and don’t last long.

These two clusters are good examples, most of the brightest stars are blue ones. Groups like this eventually break up, mostly by random events, (our sun has left its original cluster). When they are still together like this it’s another sign that the cluster is young.

M6 is sometimes called the “Butterfly Cluster” because it looks like a butterfly with its wings open (flying toward the Northwest). Can you see it in the telescope? You may even be able to spot the butterfly’s antennae. Can you spot the orange giant among the hot blue stars? M7 is more spread out (it’s closer) – it can be seen perfectly well with binoculars and can even be spotted with the unaided (sharp) eye.

 

That great, empty part of the sky just above Scorpius actually is a constellation, it’s called Ophiuchus and it’s kind of coffee-pot shaped. It isn’t Ophiuchus the Coffee Pot, though, it’s Ophiuchus the Serpent Bearer, and the Serpent is there, too, with the name of “Serpens”. No, I’m not making this up as I go along. Serpens is an odd constellation, treated as a single constellation but it’s split into two parts — the head of the serpent, called Serpens Caput, and the tail of the Serpent, called Serpens Cauda. This has been recognized as a constellation for over 4,000 years, as a great giant wrestling a serpent. And by golly, with a little imagination you can just about see it. Well, I can.

 

The Milky Way

You can see the Milky Way on any clear summer night, but up in the north woods, on especially clear nights, the Milky Way glows like a neon light, from north horizon to south horizon, brilliantly reflected in the lake by our camp. Most people recognize the Milky Way as a band of light across the night sky, but what is that band of light – where does it come from?

It’s a bit like looking at a gravel road. Close to you, you can see individual stones, while off in the distance the road merges into a uniform grey. When you look at the Milky Way you are looking at the disk of our galaxy from the inside – the nearby stars you can see individually (most are less than 1,000 light years away), the distant stars merge into a hazy glow. When you see that band across the sky you are really looking way off into the distance, into the “billions and billions” of stars that make up the disk of our galaxy.

You’ll see a similar phenomenon later when you look at the M13 globular cluster in Hercules. Through binoculars you will see it as a hazy patch. When you see it through a telescope – that is, closer up – you can pick out individual stars. The better the telescope, the ‘closer’ you can get to M13, the more stars you can see.

We can find some more constellations — some of the best — by following the Milky Way up from Scorpius and Sagittarius. First, we come to Aquila the Eagle. The bright star is Altair, and the two stars on either side of Altair are called the Pilot Stars. Altair is 16 light years away. It’s about half again as big as our sun and nine times as bright. That shows how sensitive stars are to mass — just a little more mass causes the star to burn much brighter.

 

The star that is about halfway between Aquila’s eastern wingtip and his body is called h (Eta) Aquilae — and this is another Cepheid variable with a cycle of about 7 days. At its brightest it’s nearly as bright as the body star, d (Delta) Aquilae (actually it’s as bright as the eastern Pilot Star b Aquilae), and at its dimmest it’s about as bright as i (Iota) Aquilae. This is one of only three Cepheids you can see by eye – you already met d Cephei, the original, and the third one is in the winter constellation of Gemini.

If you look with the binoculars you can find the two stars of Aquila’s tail, then slide west (to the right) until they are at the far left of the field of view. To the far right of your field of view you can now see a fuzzy spot with a star at the centre – you have found the Wild Duck Cluster. In a telescope this looks like a dense cluster of tiny stars broken into groups – the central group is V-shaped and looked to early observers like a flight of wild ducks – hence the name.

The next constellation we reach is Cygnus the Swan. This constellation is also known as the Northern Cross and at Christmas time, in the evening, it has rotated across the sky so it is standing on its base on the north-western horizon, with the bright star Vega next to it. Very Christmas-y.

 

 

 

Deneb is the star at the tail of the Swan. Notice that it is about as bright as Altair, which is 16 light years away, how far would you guess Deneb is? As it happens… Deneb is 2600 light years away – about 150 times farther!! Deneb is actually 20,000 times brighter than Altair and 200,000 times brighter than our sun. If Deneb were as close as Altair, it would be visible in broad daylight and would cast a shadow at night. This star is one of the greatest super-giant stars known.

Notice in the picture above that Altair and Deneb are two of the three bright stars overhead, which form the “Summer Triangle”. You will be introduced to the third (Vega) in just a moment.

Albireo, the star at the head of the swan, is actually a beautiful double star, a yellow giant and a blue main-sequence star. You can just split this star with good binoculars held steady, and it looks better in a telescope.

You can use Cygnus (the Northern Cross) as a gauge of the sky, in similar fashion to Ursa Major, but with Cygnus at a higher elevation, you can check the sky at its very best. Use the western wing of Cygnus, starting with Deneb as a magnitude 1, and going around anti-clockwise to the body star at magnitude 2, the wingtip at magnitude 3, and a pair of stars on the back edge of the wing at magnitude 4. Just off the wingtip is a magnitude 5 star – if you can see it, you have an excellent clear, dark sky. Half-way between the wingtip and the pair of magnitude 4’s is a magnitude 6, which you can only see under the very best conditions.

I’ve mentioned in other tours that there are a few constellations that Look like there Earthly counterparts. I really can’t cover the Summer Triangle without mentioning one of my favourite constellations. If you look to the left of Aquila you can see a small, faint constellation comprised of five stars. All of the stars are around magnitude 4. The constellation is Delphinus, the dolphin, it’s not a giant leap, if you’ll pardon the pun, to picture a dolphin.

 

The Milky Way looks like a single solid band as you follow it from Cassiopeia through Cygnus to Sagittarius, but this is misleading. When you look at the “steam” above the spout of the Teapot in Sagittarius, you are looking toward the centre of the galaxy but you are not actually seeing the centre of the galaxy. You are actually looking at the great, massive spiral arm next to ours, the next one in toward the centre. You go through this spiral arm, past lanes of dust, and through two smaller spiral arms before you finally reach the centre. When you look at Cygnus, you are looking right down the beam of our local spiral arm, which is really just a tiny branch off the Sagittarius arm. Then when you look at Cassiopeia, you are looking at the great massive spiral arm that is just outside ours. Beyond that arm lies one more final wisp, and beyond that… the rest of the universe.

Astronomers Reveal Most Detailed Catalog of Milky Way Stars | IFLScience

Note that you can see how this all lays out, looking down on the galaxy, in the latest Milky Way diagram to come from NASA

 

The two birds, Aquila the Eagle and Cygnus the Swan, are flying straight at each other and, to enhance the suspense, Sagitta the Arrow flies between them, just missing both. What is most interesting about Sagitta, though, is not so much the constellation as the features for which it is a milestone. (Note: these two features, technically, are in the neighbouring and very faint constellation “Vulpecula” the Fox.)

If you locate the two tail-feather stars of Sagitta in binoculars then follow the line between them up (to the northwest), just one field of view, you will see the Coat hanger, an interesting cluster of stars.

 

 

If you are really good with those binoculars you can go up from the tip of the arrow, toward Albireo in Cygnus, and you will find a group of stars that just fill the field of view in the shape of an “M”. Right at the central point of the “M” is a faint fuzzy patch that actually is known as the “Dumbbell Nebula”. To really see its hourglass shape requires a telescope. This hazy patch once was a star, until it blew itself apart.

Messier 27 - The Dumbell Nebula - 2019 Version — Cosgrove's Cosmos

This is called a “planetary nebula”, because the typical disk shape (this one is less typical) suggested the look of a planet to early astronomers. In fact, it has nothing to do with planets at all. This is what’s left of a red giant star that finally did what all red giants eventually do. When the fuel at the core runs so low that the nuclear reactions can no longer hold up the weight of the star, it all collapses in to the centre, which in turn raises the temperature so high that the star blows off its outer envelope of gases, losing much of its mass. This exposes the core to outer space, or, more accurately, exposes outer space to the core. The intense radiation from the core causes the expanding shell of gas to light up like a neon light, and voila — the faintly glowing dumbbell that you see here. The white-hot core of this star is now a “white dwarf” — on some planetaries you can see the star at the centre. The Dumbbell’s central star is notoriously elusive, though, and we probably can’t see it in our telescope.

Diffuse nebulae, like the two we saw in Sagittarius, are the birthplaces of many new stars, and a planetary nebula is the deathbed of a single dying star. We will see another, even more famous, planetary nebula in the next constellation.

The constellation Lyra

Now as you look up at Cygnus the Swan you can’t help but notice a really bright star nearby that’s almost straight up. That star is the renowned Vega, the home of the aliens in the movie Contact. Vega is a zero-magnitude star, about the same (apparent) brightness as Arcturus. It is actually 25 light years away, nearly twice as far as Altair, 3 times the mass of the sun, and about 50 times as bright as the sun. Imagine having 50 suns in our sky. If Vega was our sun we’d be cooked.

Sow you have been introduced to all three stars of the Summer Triangle: Vega, Altair, and Deneb. See the triangle? Learning to recognize this can be a good way to orient yourself in the sky, especially early in the evening when the stars are first coming out.

Vega is part of the constellation Lyra the Lyre. It is easily recognizable as a parallelogram of four stars right next to the very bright Vega. This constellation represents a lyre, or harp, with Vega at the top of the lyre as one of the handles. (The other handle must have broken off.)

 

The star next to Vega to the northeast (or up and to the left), e (epsilon) Lyrae, is actually a double star when you look at it in binoculars. However, if we can get a good telescope on that star you will see that each of the stars in the double is itself a double – a very cool sight to see. These four stars form a set that is a true double-double star, formed from the same nebula, which was spinning so fast it first spun apart into two protostars. They in turn each spun themselves apart into two stars.

The parallelogram star to the southwest, the lower right-hand corner of the lyre, is b (beta) Lyrae, which is another variable star – it is usually about as bright as the southeast, or lower left-hand corner star g (gamma) Lyrae. Every 6½ days the star dims to half its brightness. This star is an “eclipsing binary” – two stars orbiting each other every 13 days. Every half orbit one of the stars passes in front of the other and instead of seeing two stars we only see one – the star drops to half its brightness, which is about a magnitude. This is different from a Cepheid variable, where the star itself is changing. Take a look at b Lyrae – do you think it’s eclipsing right now?

Now if we can get a telescope aimed right about half way between g and b Lyrae (a little closer to b) and look in the eyepiece, you’ll notice something a little funny – it doesn’t look like a star at all, it looks more like a – smoke ring maybe. This is the famous Ring Nebula – another planetary nebula – a shell of gas blown off by a dying star, and lit up by the radiation from the exposed core of the star, now a “white dwarf”.

 

The constellation of Hercules

Now as you go to the west, or to the right, of Lyra, you come upon another important asterism, “The Keystone”. Just like it says, it is four stars in the shape of a keystone, and like the other asterisms it is a part of a larger constellation – the constellation Hercules. The star that represents Hercules’ head, a Herculis, is another red supergiant like Antares – by most estimates it’s a little closer than Antares and since it’s not as bright, we presume it is smaller. With a good telescope you can see that this reddish-orange star has a blue-green companion.

 

One of the reasons for finding the Keystone is to help locate M13 – the brightest and best globular cluster in the Northern Hemisphere. Globular clusters are rare – only about 150 are known, and they are completely different from open clusters like the Butterfly Cluster that we saw in Scorpius.

 

 

The Butterfly is estimated to be about 100 million years old, M13 is estimated at about 10 BILLION years old, making it so old that it formed before the disk of the Milky Way galaxy formed! While the Butterfly cluster has maybe 100 stars in it total, M13 has no less than 1 million stars. The Butterfly cluster is about 1200 light-years away and about 20 light-years across, M13 is 20,000 light-years away and 160 light-years across.

These numbers are typical, globular clusters are much, much bigger than open clusters, and they are much, much older – some are nearly as old as the universe!

The constellation Draco (again)

Hercules is pictured (upside down) in a kneeling position, on one knee, with his foot on the head of Draco the Dragon. Draco starts off with two bright yellow eyes (actually one’s yellow, one’s orange), and then winds around Ursa Major with its tail between the two dippers. The lop-sided square that forms his head is known as “The Lozenge”, and makes a very distinctive shape that helps you to recognize Draco. Who’s got the binoculars? Use them to find Draco’s eyes, then go down to find the next star in the Lozenge closest to the eyes — n (nu) Draconis. Notice anything? That’s right, it’s a double star, and a very neat one in binoculars, tight together and exactly equal in brightness.

The third star from the end of Draco’s tail has its own name – Thuban – even though it’s not very bright. What makes Thuban special is that it once was the North Star – about 5,000 years ago. Our North Pole is actually moving through the sky because the earth wobbles on its axis, just like a top does, particularly just before it falls over (no – the earth is not about to fall over). With the axis wobbling like that, the North Pole is tracing a circle in the sky – it just happens to be passing by Polaris right now.

It is about a degree away from our North Star now and will get about a half degree closer, then will start moving away as it continues on its circle through the sky. That circle will take it almost exactly right down the centre of Cepheus, past Deneb in Cygnus, past Vega in Lyra, through Hercules leg and right by his knee, then down past Thuban and back to Polaris.

And speaking of coming full circle, we’ve come full circle. We started with the polar constellations and here we are, back at the polar constellations. Our summer sky tour is completed. For you die-hard stalwarts with a particularly high resistance to the night chill, there is still much to see. Some of which will be included in our Autumn guide.

 

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