Astrognome 100 Great Stars No. 4 Agena

 Agena

Beta Centaurus sometimes known as Agena which is a relatively modern name from the 17^th century, with no indication as to a meaning. The star is sometimes known as Hadar, however rather confusingly this name is also sometimes used to refer to alpha Centaurus as well as beta. So I will stick with Agena for now.

Agena is the second of the pointers to the Southern Cross the other being alpha Centaurus and although alpha appears brighter in the sky that is simply down to distances because alpha is just over 4 light years away while Agena is around 390 light years away.

Like many stars Agena is not a single star it is a triple star system, Agena the main star is a blue giant star much hotter than the Sun with a surface temperature of around 25,000 degrees centigrade. If Agena was as close as alpha it would be so bright that it would cast a shadow.

Agena with alpha Centaurus were known to the bushmen of South Africa as two men who were once lions, while the Australian Aborigines were regarded as two brothers.

Agena is not visible from Britain but from the southern hemisphere it is circumpolar from New Zealand which means from there it can be seen all year round.

Astrognome 100 Great Stars No. 3 Adhara

 

Adhara

Winter in the northern hemisphere and looking to the south, the sky is dominated by Orion the Hunter and Sirius the Great Dog Star in Canis Major, the brightest star in the sky. Although Sirius is the object everyone looks at Adhara or epsilon Canis Major is also worth seeking out, its name means ‘the Virgins’ which of course has nothing to do with the mythological dog in the sky. Adhara is in fact part of an old Arabic constellation.

Unfortunately as seen from Britain Adhara is very low in the sky meaning that any haze makes seeing the star difficult. It lies at a distance of around 430 light years away and can be seen as a star with a magnitude of + 1.5 it is in fact the brightest of the second magnitude stars. First magnitude stars are brighter than magnitude +1.5. Around 5 million years ago it would have been the brightest star in the sky with a magnitude of -3.9, Sirius by comparison has a magnitude of -1.4. No other star will appear as bright as Adhara in the sky for at least another 5 million years. It only appears fainter than Sirius because it is very much further away. Sirius is only about 8 light years away.

Adhara does have a companion star which was discovered at the Cape Observatory in South Africa in 1850 but can only be seen with fairly large telescopes because the light from Adhara is so bright that it drowns the fainter light from its companion which could be seen using binoculars if it was not so close to Adhara.

It is much hotter than the Sun with a temperature of around about 22,000 degrees centigrade compared to the Sun’s 5,800 degrees and if your eyes could see in ultra violet light then Adhara would be the brightest object you would see in the sky.

When people talk about the spectrum they are usually referring the optical spectrum the colours of the rainbow. Scientists study the electromagnetic spectrum which covers not only visible light, but also infra red, ultra violet and radio.

In winter while looking for Sirius and Canis Major and you have a really clear dark night try looking for Adhara low in the sky in the south.

Astrognome 100 Great Stars No. 2 Achernar

 

Achernar

Achernar is the brightest star in the long winding constellation of Eridanus the River which begins close to Orion and ends in the far south. Achernar is so far south that it cannot be seen from Britain.

Achernar lies around 140 light years away and is much hotter than the Sun, and is the 9^th brightest star in the sky. Its name is derived from Arabic and is referred to as the ‘Star at the end of the River’.

We normally think of a star as a sphere of hot spinning gas but Achernar spins so quickly that it is pushed out of shape so it appears more like an oblate spheroid. It is quite possible the least known spherical star in our galaxy.  Achernar is around 10 times the diameter bigger than our Sun.

There is something of a mystery here because the star Acamar which is also  in Eridanus and which lies north of Achernar was referred to as the Star at the end of the river by the Greeks. Ptolemy who is often regarded as one of the last Greek astronomers does not mention it suggesting that he never saw Achernar. He could have seen it from Alexandria which further suggests that Ptolemy’s star charts were based on those produced by Hipparchos who live on the Island of Rhodes further north from where Achernar was not visible.

Achernar lies in a very barren part of the sky which makes identifying it very easy and it does definetly mark the end of the river. It also does have the distinction of being the brightest close star to the south pole which is marked by the very faint star sigma octantis.

Astrognome 100 Great Stars No.1 Acrux

 Star 1

Acrux

Just as Ursa Major is the most famous constellation in the northern hemisphere then Crux or the Southern Cross is the most famous in the southern hemisphere. Yet surprisingly it is also the smallest of the 88 constellations in the night sky.

Acrux is the brightest star in the constellation of Crux, however the southern cross was actually part of the constellation of Centaurus. It is sometimes cited that it was in 1679 that the French astronomer Augustin Royer created Crux, however both the English astronomer Emery Mollineux in 1592 and the Dutch Flemish astronomer Petrus Plancius in 1598 had recorded Crux being a separate constellation.

Acrux is the 13^th brightest star in the sky but there is more to the star than meets the eye because there is not one but two stars here. They are both very hot blue / white stars with surface temperatures of around 23,000 degrees centigrade to 25,000 degrees centigrade. It is estimated that they take around 1,500 years to orbit each other. And there is more because Acrux 1 is itself also a double star and Acrux can be referred to as a triple star system.

Best of all Acrux leads us to one of the most glorious regions of our galaxy the Milky Way. To the east of the cross lies the ‘Coalsack’ a dense cloud of star forming interstellar gas and dust that blocks the background starlight. The coalsack is so dark and prominent that the Incas in South America made a constellation of it calling it the ‘Yutu’ a partridge like southern bird.

To the north east of Acrux is one of the finest star clusters the ‘Jewel Box’ it is so bright that it was named after the 10^th letter of the Greek alphabet kappa Crucis.

Acrux and the constellation of Crux is one of the few that is honoured in music with the tango by Richard Rogers “Beneath the Southern Cross” from the documentary ‘Victory at Sea’.

Acrux also appears on the national flags of Australia, New Zealand, Samoa and Papua New Guinea.

Once seen Acrux and its surroundings can never be forgotten.

Astrognome 100 Great Stars, Star 0 the Sun

 The 100 great stars will be presented alphabetically but I thought that I would make one exception and put the Sun as the first star.

The Sun

Our local star is the Sun, without it there would be no life on the Earth, so it is very important to us. The Sun is around 93 million miles away and is large enough to place around 1.3 million Earths inside it.

The Sun is around about 5 billion years old and formed out of a giant cloud of dust and gas, it was from that same cloud that the Earth and the other planets formed about 4.5 billion years ago. The Sun is made up of about 92% hydrogen, 8% helium and tiny amounts of everything else.

Astronomers classify the Sun as a yellow dwarf star, it is a fairly ordinary star, there are many stars smaller and some much larger than the Sun. The Sun has a surface temperature of about 5,800 degrees Centigrade, in the centre of the Sun this increases to an incredible 14 million degrees Centigrade.

The Sun is a stable star it is not what astronomers refer to as a variable star, that is a star that changes in brightness over a period of time. This is good for us as it has enabled life to evolve on Earth.

However it would be wrong to say that nothing happens on the Sun. Solar Flares come from features called sunspots which are cooler areas on the surface of the Sun. This causes them to appear darker compared to the disk of the Sun. These are areas of intense magnetic activity. If a flare from the Sun crashes into the Earth’s protective magnetic shield then we will see the Aurora Borealis or the Northern Lights. In the southern hemisphere this is the Aurora Australis or southern lights. The aurora can be seen at other times because there is a flow of particles from the Sun called the solar wind and when this become strong it can also cause aurora.

The sunspots have an 11 year cycle when large numbers are seen, however rather curiously when there are very few sunspots the Earth appears to become cooler. There was a period during the late middle ages when the Earth experienced a cooling down and is referred to as the Little Ice Age this also coincided with a lack of sunspots. The same thing happened in the early 19^th century when a young man called Charles Dickens was growing up and he used his experiences as a child in the very cold weather as a back drop to some of his stories. The latest sunspot cycle has very few sunspots and experts cannot agree what this means for the future.

Astronomers are only really beginning to understand what makes the Sun tick and with this in mind astronomers have recently been sending space craft to study the Sun in more detail to try to understand how it works.

Jupiter seen from Manchester in 1869

 

The Planet Jupiter seen in 1869 from Manchester

Mr H. Ormesher of Manchester witnessed the partial transit of the 3^rd satellite on the night of August 14^th 1869. The satellite was visible as a bright spot 15 minutes after first contact.

On the night of the 24^th August 1869 the 2^nd satellite was observed during its ingress.

Mr Ormesher says “After first contact the satellite appeared to linger for some time” A somewhat analogous phenomenon is noticed on page 141 of Webb’s Celestial Objects,

Astrognome 100 Great Stars Introduction

 Astrognome 100 Great Stars Introduction

Following on from the Astrognome A-Z of constellations I intend to present the Astrognome 100 Great Stars. Many people I am sure could suggest an alternative 100 stars but hey this is my selection. I have tried to stick with the brighter and better known stars.

I thought it would be useful to present a very short introduction to the stars.

Look into the night sky there appear to be millions of stars, however on any clear night from anywhere really dark site away from city lights it is possible to see around 3,000 stars without a pair of binoculars or telescopes. The stars appear to be randomly spread across the sky.

The stars are actually grouped into patterns or constellations these tell stories from ancient Greece. However although the star stories are Greek most of the star names we use today are Arabic.

There are 88 constellations in the sky, 48 date back to the times of ancient Greece while the other 40 constellations were added in more modern times, by modern I mean the 15^th and 16^th centuries. Most of the modern ones are in the southern hemisphere and were added when European explorers went there, while some were added in the northern hemisphere to fill gaps between the classical constellations.

If you look at a star map or an app on your phone or use a tablet you will see the stars have strange looking symbols besides them. These are letters from the Greek alphabet. In 1603 Johann Bayer allocated the 24 brightest stars of each constellation a Greek letter. In theory the brightest star is alpha followed by beta, gamma etc until omega. However we will discover that this system does not always work.

The brightness of a star is measure by its magnitude with the brightest stars having minus numbers and the larger the magnitude number the fainter the stars. Assuming you are in a very dark site and have good eyesight we can see stars to magnitude 6 with our eyes without using binoculars and telescopes. You will notice that some stars are much brighter than others this can be because they are fairly close to us, or because they are genuinely very bright stars. However appearances can be deceptive because some stars only appear faint because they are a very long way away.

Astronomers use the speed of light to measure the distance to the stars, although we use the mile to measure distances on Earth it is too small a unit in space. Astronomers use the speed of light to measure the distances to the stars. Light travels at a speed of 186,000 miles per second or 300,000 km per second. In one year a particle of light will travel around 6 million million miles that is a 6 followed by 12 zeroes. Stars can be 10s, 100s 1,000s or even millions of light years away!

When you look at the stars it is possible to see that they are different colours and rather surprisingly blue stars are hotter than red stars. I know we say that things are red hot, meaning that's very hot, but if you look at a flame around the edge it will appear reddish while in the middle it is blue. This is the hottest part of the flame. And although stars don’t burn like fire they produce their energy through atomic reactions the colour principle is the same. Therefore when you look at a blue star it is much hotter than a red star. Astronomers divide the stars into different classes by using a series of letters with O class stars the hottest and M class stars the coolest.

Stars do come in different sizes giants and dwarfs and although our the Sun is very big by our standards, it is about 900,000 miles across some stars are much bigger.

That’s it for this very brief introduction to the stars, I hope you found this short section helpful.