Summer Triangle Of Stars (click for larger)

The “Summer Triangle” is a startling asterism of three bright stars. Even non-astronomers notice this prominent triangle pattern, during the Northern hemisphere summer.

The star pattern is made more noticeable by the generally light sky at this time of year (it doesn’t get properly “nautical” dark, if you are more than about 52 degrees of latitude North).

I have enhanced the image below, by drawing the “triangle sides”. Hopefully, this will help you to spot the Summer Triangle, when you look up at this time of year. (Ignore the aeroplane trail!)

Enjoy the summer!

Summer Triangle of Deneb, Vega, Altair (click for larger)

The stars are Deneb in the constellation of Cygnus, Vega in Lyra and Altair in Aquila.

This is something of a relief…  

For the past three weeks, I’ve been making maps of Lulin’s projected position for literally thousands of visitors.

Lulin close to Regulus, plus "streak", Feb 27th

What if I hadn’t seen it myself?

Anyway, I have now.  :)

The image on the right shows Lulin close to the star Regulus, in the constellation of Leo.

Between them is an orange  ”streak”.  I’m not sure exactly what it was, probably an aircraft.  But it’s interesting.

(PS. It has since been pointed out… if you look carefully, you can see the periodically flashing white and double-red lights, of a high-flying aircraft)

The image was captured with my Canon 1000D dslr camera, through the ED80 600 mm focal length refractor.  

It’s a single frame of 2 minute exposure at ISO 800.

I actually took about 15 frames with different exposure times.  I shall try my hand at stacking some of the best, to see if I can produce better images after processing.

Here’s the best single frame shot of Lulin I have found, so far.  It was a 3 minute exposure.  You can see from the star shapes, there is some tracking error.  I need to work on a better North alignment of the mount.

Comet Lulin, February 27th (click for larger)

We are familiar with occultations of the Sun and the Moon. We give these occultations a special name – eclipses – solar eclipse for the Sun and lunar eclipse for the Moon.

Lunar Eclipses

A lunar eclipse occurs when the Earth, Sun and Moon are aligned, with the Earth between the Sun and the Moon.

The Earth is three times larger in diameter than the Moon, so it easily blocks out the Sun’s light and casts a shadow on the Moon.

The Moon of course, does not produce light itself, only reflecting sunlight, so it looks dark during an eclipse.

In fact the degree of darkness does vary due to the condition of Earth’s atmosphere. If Earth’s atmosphere is clean, it will bend sunlight to some extent and this gives the Moon a slight illumination.

If however Earth’s atmosphere is dusty, as happens after major volcanic eruptions, it does not bend sunlight so much and a lunar eclipse can be a very dark one.

As the Moon orbits around the Earth once every month approximately, you might expect there to be a lunar eclipse each month, but this does not happen.

The reason is that the orbit of the Moon is inclined a few degrees to the orbit of the Earth. This ensures the Moon does not pass into the Earth’s shadow every month.

However, lunar eclipses are more often seen than solar eclipses because when they do occur, they can be seen from everywhere on Earth where the Moon can be seen at that time.

When a solar eclipses takes place, it can only be seen from a narrow strip of places on Earth. The path of totality tracks across the Earth’s surface, with a strip of partial totality on either side.

Solar Eclipses

WARNING… Never, ever look at the Sun through binoculars or a telescope, as you will certainly damage your sight. Observe an eclipse only by projecting the Sun’s image onto a sheet of card.

Solar eclipses happen when the Moon is aligned between the Sun and Earth, so that sunlight is blocked-out and a shadow is cast on the Earth.

This gives a remarkable effect here on Earth, as the Sun is obscured and then reappears some minutes later.

From a particular point on Earth, a solar eclipse is sometimes partial, and occasionally total, meaning that the Sun is completely covered by the Moon.

During a total eclipse, the Moon just about exactly covers the Sun. This is remarkable in itself.

The Moon covers the Sun in this way because the ratio of their diameters, happens to be very similar to the ratio of their distances from the Earth.

Putting it another way, the Sun and Moon have roughly the same angular size in the Earth’s sky.

This exact coverage gives rise to wonderful effects at the edge of the Sun during total eclipses such as the “diamond ring” and the “string of beads”.

It has also enabled astronomers to study phenomena very close to the Sun’s surface, such as solar prominences and the Sun’s corona.

Other Occultations

Occultations of other objects in the sky are always keenly observed and have been important to astronomers in the past.

Lunar occultations are when the Moon passes in front of a star or planet and this is interesting to observe. The Moon has no atmosphere, so stars suddenly disappear without any fading and then later, suddenly come back.

Double stars have been discovered through observing lunar occultations.

Sometimes, an star will “graze” the edge of the Moon, revealing details of hills and mountains on the Moon, as the star disappears and then reappears several times.

As recently as 1977, faint rings were discovered around the planet Uranus because a star flickered as it was occulted by Uranus.

Outside SOAS

I travelled to London on the train, to attend the quarterly meeting of the SPA.

These meetings are always held at SOAS (School of Oriental and African Studies), which is part of London University and located just off the splendid Russell Square in Bloomsbury.

In the morning, there was the SPA Council meeting, but at 2pm the main interest of the day began – the quarterly members’ (and their guests) meeting.

There were a number of entertaining and informative talks, but the highlight was Professor Derek Ward-Thompson from Cardiff University.  

He presented on the formation of Stars and Planets, using recent long wavelength (radio) evidence.

In Front Of SOAS - With SPA Vice President Taking A Photo

I left the meeting with the strong impression, the human race is beginning to understand our ancient origins.  

Quite astonishing.

Come to the next SPA meeting – I recommend it.

Most people will have heard the term “constellation” and will know that it relates to patterns of stars in the sky.

Many people will probably think of astrology and the “Signs of the Zodiac”, for example Aquarius and Pisces.

But how many people know what the constellations actually are?

The star constellations are really just a fairly arbitrary way of dividing up the night sky, in a memorable way.

But arbitrary or not, the constellations are very useful and help observers to find their way around.

Dividing the sky up in this way, probably started as long ago as humankind itself.

Early people certainly drew recognisable shapes representing star patterns and also began to associate these shapes, with Gods or important legends and stories.

The constellations became somewhat more precisely defined with the early Greek astronomers, who classified a total of 48 star patterns.

At this time, the notion of the “Zodiac” stars also began. The Zodiac is the 12 star patterns or constellations, that the Sun appears to pass through, during the 12 months of the Earth year.

Hence the idea of each constellation being associated with a specific month of the year.

Interestingly, there should probably be thirteen signs in the Zodiac, because the Sun does actually pass through the constellation of Ophiuchus, as it is now defined!

There is one very important point to make about the constellations. Each constellation is not a real grouping of stars – it is only the appearance of a group, from our viewpoint here on Earth.

In fact, the stars in any particular constellation are at greatly different distances from Earth. So if we were to see them from a different position in our galaxy, they would not look like a group at all.

The constellations as astronomers know them today, were defined properly in the 1930′s, by the International Astronomical Union (IAU).

There are now 88 constellations. They vary in size, but generally, each constellation extends beyond the recognisable pattern of naked-eye stars (called an asterism) into the surrounding sky.

So for example, the constellation of Ursa Major contains the highly recognisable shape of the “Plough” or the “Big Dipper”. This is called an asterism.

The Plough asterism is made up of seven stars that appear bright to us on Earth. But the constellation of Ursa Major is much larger and contains many, many more stars and indeed other galaxies, that we can observe.

Taken together, the 88 constellations map out the entire sky into 88 areas. This is very useful to astronomers, when they want to describe where an object may be observed.

However, it is rather humbling to realise that there are billions of stars in the universe. And even worse, our Sun is an unremarkable star.

There are many stars that are bigger than our Sun and many are smaller. Some are older, others younger.

We also now know, through modern detection methods, that other stars have their own planets in orbit around them.

So what is a star and how does it come into being?

What Is A Star?

A star is just a large ball of gas, nothing much more. For all its apparent fury and violent energy generation, it is just gas.

The gas is mainly hydrogen and helium. If you have ever learned about the Periodic Table, you will know that these are the two simplest elements, with an atomic number of one and two, respectively.

The heat and light generated comes from nuclear fusion reaction, in which hydrogen atoms are combined to form helium, releasing enormous amounts of energy.

The star has so much mass and therefore so much gravitational force acting inwards towards its centre, that fusion processes keep going for millions of years, generating the heat and light that we are familiar with.

How Do Stars Form?

With telescopes, we can observe regions in our galaxy where there are vast clouds of gas.

It is believed that stars form in these clouds of gas, as it begins to coalesce. So these gas clouds are “stellar birthplaces”.

As the gas coalesces, denser clouds form and gravity increases. This leads in turn, to still greater density. The forming star begins to rotate.

Eventually, if there is enough material, temperature and pressure increase to the point where nuclear fusion begins and energy generation starts in earnest.

A “star is born”.

The star then settles into a long period of equilibrium, during which the energy generation that increases pressure and forces material outwards, is just balanced by the forces of gravity holding the material into a sphere.

The more massive the star at this point of equilibrium, the brighter it will be.

This is what we observe in the sky. Stars come in a range of sizes and brightnesses.

How Do Stars End?

Eventually, the hydrogen fuel for nuclear fusion begins to run out and the star generates less energy and cools.

What happens next, depends on how massive the star is.

With stars of low and medium mass like our Sun, the outer layers will expand in size and cool, forming a “Red Giant” star. This will be many times larger in diameter than before.

Eventually the outer layers will dissipate and the central core collapse into a “white dwarf” star.

Alternatively, for more massive stars, the outer layers may blow off to shroud the star in a nebula of gas, called a “planetary nebula”. A good example of a planetary nebula is M57 (the Ring Nebula), in the constellation of Lyra.

Much more massive stars may end catastrophically , with the star exploding as a “supernova“.

Part of this process involves the core of the star collapsing into a “neutron star” or a spinning, flashing “pulsar”.

An example is M1, the Crab Nebula in Taurus. This is the remainder of a supernova seen in the year 1054 and it has a pulsar at its centre.

In the case of very massive supernovae the core may collapse to form an incredibly dense “black hole”, from which no light or other radiation can escape.

Their brightness may alter by several magnitudes over a period of days, weeks or months.

One of the most famous variable stars is Algol, in the constellation of Perseus (Beta Persei). Algol is markedly red in colour and varies by about one magnitude, over the course of a few days.

Algol’s behaviour was first recorded in the 1600′s, but almost certainly it was known in ancient times. The clue for this is its name Algol, which means “the Demon” in Arabic.

There are two main types of variable stars, “intrinsic” and “extrinsic”, and the cause of the brightness changes are very different.

Intrinsic variables alter because of an actual change in brightness of their surface.

Extrinsic variables alter because of some outside factor such as another star getting in the way and blocking out light.

Intrinsic Variable Stars

Intrinsic variable stars produce an increase in brightness as a result of some physical change within them.

A dramatic example are Flare stars. This is the name given to stars that suddenly and unpredictably get much brighter over a short space of time, perhaps only minutes. They then fade, often taking much more time than the initial flare.

Proxima Centauri, the closest star to Earth is a well known example of a flare star.

The other main type of intrinsic variables are those that have a regular, predictable, brightening cycle.

These pulsating variables live up to their name and actually pulsate in size. Most of this type are red giants but there is a particularly important class called Cepheids.

Cepheid variables are named after the original one of this type (Delta Cephei) that was discovered. They have the important property that the period of the pulsation is proportional to their absolute magnitude. Because of this, Cepheid variable stars have been used to estimate distances within the Universe.

Extrinsic Variable Stars

Variable stars of the extrinsic type are usually eclipsing binaries.

With an eclipsing binary pair, what appears to be a single star from Earth with the unaided eye, is actually a pair of stars that are orbiting around a centre of gravity. this can be seen through a telescope.

Algol, mentioned above, is a famous example of an eclipsing binary.

The stars of the pair orbit each other and when one star is in front of the other, as seen from Earth, the system is less bright then when the stars are side-by-side.

Optical Doubles

However, double stars are not always “real” – sometimes they are what is termed “optical” doubles.

This is when two stars just appear to be very close side-by-side when we see them from Earth, when in fact, they are at different distances from us and are not really close at all.

The double star appearance is nothing more than a line-of-sight effect.

Physical Doubles

However, most double stars we see are “physical” doubles. They really are a pair of stars that are close together and linked by gravity.

Each star orbits around a common centre of gravity, situated between the stars.

There are some well-known examples of double stars.

Mizar (zeta Ursae Majoris), a 2nd magnitude star in the Plough asterism, within the constellation of Ursa Major, has a 4th magnitude companion called Alcor. This may be easily seen with binoculars or a small telescope and can been seen with the unaided eye, if your eyesight is good.

Being able to see Mizar as a double, was used during the First World War as a test of eyesight for soldiers. I’m not sure whether it was a good or bad thing, to pass the test…

Another example and a beautiful sight in the sky, is Albireo (Beta Cygni) in the constellation of Cygnus. It is a 3rd magnitude orange giant together with a 5th magnitude blue-green companion. The colours make Alibireo a wonderful sight through a small telescope.

The separation between the components of double stars is measured in terms of angular separation, as viewed from Earth.

Many doubles have angular separations of a few seconds of arc (a second of arc is 1/3600th of a degree of arc). The closer the separation, the larger the telescope needed to “split” them, meaning that you can see the component stars.

Another good example is Epsilon Lyra in the constellation of Lyra. A small telescope or binoculars will easily show it as a pair of 5th magnitude white stars.

But look more closely with a powerful telescope, and each of the pair of stars can itself be seen as a double star. Epsilon Lyrae is sometimes called “The Double Double”.

Some double stars are so close together that the component stars cannot be seen through telescopes. Analysis of their light with a spectroscope however, reveals their double nature. These double star are called “spectroscopic binaries”.

This magnitude system was actually first devised by Hipparchus, the Greek astronomer, a long time ago in the second century BC.

It is based on the principle that the stars we can see with the unaided eye, should be classified between magnitude 1 for the brightest and magnitude 6, for the faintest.

Please note that the magnitude classification refers to the apparent brightness of the star as it appears to us, observing from the surface of the Earth. Magnitude figures usually have nothing to do with the actual brightness of the star.

A very bright star a great distance away, could easily appear less bright than a fainter star, much closer to Earth. Magnitude is (usually) not used as a measure of actual brightness.

However, sometimes books will quote actual magnitudes, although this should be clearly stated. You need to watch out for this so you do not get confused.

Back in the time of Hipparchus, magnitudes would have just been judged by eye.

In more recent times, sensitive scientific measurement has been applied to the classification of apparent brighness, but the fundamentals of the system have not really changed.

Magnitude 1 is now defined as exactly one hundred times brighter than magnitude 6. This means that each magnitude number is approximately 2.5 times brighter than the next. So a mag 1 star is 2.5 times brighter than a mag 2 star, and so on.

The faintest stars that people with good, but unaided eyesight can see, are still classified as magnitude 6.

As a consequence, some objects in our sky are so bright that negative magnitudes are needed, if the scale is to remain consistent.

So the brightest star we can see, Sirius, is magnitude -1.4. And the planet Venus is magnitude -4.