Two views of AR11476 sunspot group at different times on 5 May 2012 (UT). Sketches and copyright Harry Roberts ©, all rights reserved

A big spot group emerging on the sun can host a wide range of amazing phenomena when viewed in hydrogen-alpha. While attention is mostly on any big flares there is a lot more to record that, over a two-hour session say, can be almost overwhelming. To show this let’s review the logs for May 5 and May 9, 2012.

May 5 summary: First views of AR11476 (476 for short) showed remarkable surges erupting from the group (Fig1). These arose as narrow jets near 476’s central spots and bent almost horizontal to travel north for 60Mm. There they turned upwards for a further 40Mm – in a flattened ‘S’ shape 100Mm in length! New surges joined the earlier ones and engaged in an “out there and back again” display of plasma physics. Several records of these were made and two are shown. Some surges were dark (in absorption) against the disc, while a smaller surge at the site was bright against the disc; some were both bright and dark against the disc: a rare display!

Flare M1.3: At 23:02 the surface (i.e. chromosphere) between the large preceding (p) spot (+9, 189) and the intermediate spots was lit-up in a scatter of brilliant points – a flare that, I later found, was a short-lived M1.3 that peaked just sixty seconds earlier as I switched from WL back to H-alpha (!).

White light: This showed the group stretched across 14 degrees of longitude from a large double (p) spot sited at +9, 189 (twenty degrees onto the disc) to a single following (f) spot at +10, 175, just six degrees from the sun’s limb. Smaller spots lay between the two. This was a very big group and contained ten spot umbrae. Helio freeware gave an aggregate area of 500 units – it was another northern ‘supergroup’ arising (cf 11429 in March)

Magnetic class: at this stage 476 was a relatively simple Beta-preceding group, with a well-defined separation of violet spots in front and red in the rear.

White light image of the whole Sun on 10 May 2012 at 0:13 UT. Photo and copyright Nick Lomb, all rights reserved

May 9 summary: From the fifth to the ninth the logs shows a slow increase in magnetic complexity. On the 7th (not shown) a single red polarity spot arose in the large “violet” (p) spot on the south side, promoting the entire group to Hale class Gamma-Delta (i.e. opposite polarities in a single penumbra: the most complex type).

Sunspot group AR11476 on 9 May 2012 (UT). Sketches and copyright Harry Roberts ©, all rights reserved

By the 9th this was well advanced (Fig3) with the huge and complex “violet” (p) spot sprouting “red” spots on its south side. As expected the group produced a burst of GOES Class M flares that peaked with three on the 9th.

The fit between the WL spots and the Mt Wilson magnetograph on 9 May 2012. © Regents of University of California

Fig 4 suggests the fit between the WL spots and Mt Wilson’s magnetograph of the ninth of May. The possible inversion line is marked in black with triangle arrows.

Flaring: No large flares were logged on the 9th during the two-hour session but three small one were. These are coloured in the figure; the brightest, a C1.5 at 22:45, is orange. All were small with only the latter being ‘bright’, and they arose near the “inversion line” cited – the boundary between opposite polarities in the group (outlined, Fig4).

Surges: as on the fifth surges were very active. The largest (Fig3, partly shown upper left) was 60Mm long and showed Doppler ‘blue-shift’ in approach at 21:30, presumably during retraction. On the west side of the (p) spot are several smaller surges (arrows in Fig) that emerged and retracted during the session. Some faint active region filaments were also present, captioned ‘arf’, but such filaments were unusually faint in Group 476. Why?

Summary: These two records of AR11476 are not meant to be a comprehensive history of the group. Developments in the huge preceding spot, as well as the fantastic surges of the fifth, will need to be treated in more detail (Ed willing!)

At 80 magnification the sun’s disc is a little larger than the eye-piece FOV, and the image is full of detail that changes moment by moment. Recording everything is at times impossible – but it sure is fun trying. Keep your h-alpha ‘scopes at the ready!.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers

Two southern sunspot groups. Sketches and copyright Harry Roberts ©, all rights reserved

Activity on the sun can be very unevenly distributed, both in location on the disc, as well as in time. Waves of activity come – and go. For the whole of SC24 (so far) northern hemisphere spots have greatly outnumbered southern ones. Why is this so?

Recently a burst of southern activity produced some interesting spots, interesting due to their differences rather than their similarities. Let’s look at three southern groups, 11459, 62, 65 and one northern, 11467. These groups were all so different it is hard to believe the same process produced them; and all were on the sun during the last half of April 2012.

AR11459 arose mid-month as an open grouping of scattered nuclei with very little penumbra, stretched across a large solar latitude as well as longitude. Emerging spots mostly spread east-west (due to the Hale-Nicholson force) with little north-south spread; but this group covered more than 5 degrees of south latitude. While it grew somewhat, it remained by far the most open and scattered of our examples (Fig1, lhs).

Umbral fields in its main spots were weak, with R21 (red 2100G) in the preceding spots, and V21 and V20 in the following ones. Fields > 2000G are needed to form penumbrae – and this group’s penumbrae were faint and hard to see. Almost 40 tiny spots could be counted in this group.

AR11459 looked like the “skeleton” of a major spot group, one that needed stronger fields to put “flesh” on its scattered “bones”.

Two views of sunspot group AR11462 with the second showing the group as it reached the edge of the Sun. Sketches and copyright Harry Roberts ©, all rights reserved

AR11462 by contrast, looked much healthier (Fig2, lhs). This was a classic bipolar group with fields in the range R22 to V23, resulting in large penumbrae with multiple umbrae, some elongated, in both the preceding (p) and following (f) spots. It emerged on the 18th and grew to its impressive size in little more that 24 hours. Despite this growth and strong umbral fields flaring was modest, no more than GOES C2.

This group was a fine sight at the SW limb April 23(Fig2, rhs) with several bright surges above it. Surges a and b are the type that appear near large penumbrae where emerging fields turn almost 90 degrees, and c is perhaps also a surge, tightly collimated, in more vertical fields of the following spot. Prominences x and y may be ejecting filaments unrelated to the spot group.

AR11465 emerged on April 19 and by 23rd had the appearance of a major active group (Fig1, rhs). It was compact with a dark penumbra holding many elongated umbrae and chains of smaller spots. And by the 24th the following V20 violet spot to the NE had joined with the main mass – promoting the group to Hale delta class: a sure predictor of strong flares (Zirin “Astrophysics of the Sun” Cambridge Uni Press. P402). Yet they did not occur. The strongest flare for this group was C2.5, only a bit stronger than those of tiny group AR11467, below.

A flare from the northern sunspot group AR11467. Sketch and copyright Harry Roberts ©, all rights reserved

AR11467, a northern group, was one that flared during the writer’s “watch”. This group is the easiest to describe: it was just a tiny dark speck or two, almost without penumbra. Despite its puny size it had three GOES X-ray class C flares (all

The flare had some ribbons and two bright flare loops (Fig). The scale bar shows the loops were <20”arc in length: typical for a small event. The flare peaked at 22:14 and by 22:19 began fading. Only one tiny spot was seen- showing that small groups can flare like big ones, at times.

A preview of May’s activity – the large southern sunspot group AR11471 on the morning of 4 May 2012. Photograph and copyright Nick Lomb ©, all rights reserved

Overview. These active regions give a sense of April’s activity. There were many new groups, often short-lived, and they remained magnetically simple. AR11465, April’s only delta group, hosted some modest flares, but nothing like the month before. The contrast with March’s multiple X and M class flares was striking- but giant northern group AR11429 had dominated March’s activity. What will the month of May reveal?

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers

The entrance to Stardome Planetarium and Observatory in Auckland photographed on 28 April 2012. Image and copyright Nick Lomb ©, all rights reserved

Last weekend, 28-29 April 2012, I attended the annual meeting of the Australasian Planetarium Society at Auckland Stardome Planetarium and Observatory in New Zealand. The agenda for the meeting included the showing of many exciting planetarium shows and two talks to which members of the Auckland Astronomical Society (AAS) were also invited. One was a talk by me on the forthcoming transit of Venus and the other was by Dr Grant Christie of Stardome and the president of the AAS on the detection of exoplanets, that is, planets around distant stars.

Here I report on Dr Christie’s fascinating talk that was held in the very comfortable surroundings of the planetarium dome. Of course, any errors in the report are my own and possibly due to those comfortable surroundings.

Astronomers, like almost everyone, else are curious to know if there is life elsewhere in the Universe. Judging by our own situation on Earth, good places to search are on planets around stars other than the Sun. The first step in this quest is to find those planets. Since the mid 1990s many exoplanets have been discovered, initially by looking for a small wobble in the motion of stars due to planets circling around them. More recently, the Kepler spacecraft has been finding numerous candidate planets with the transit method, which is looking for the slight dimming due to a planet moving in front of a star.

There is, however, a third method involving gravitational microlensing, which is particularly useful in finding planets in the Goldilocks or habitable zone, that is, at a distance from its parent star that is neither too hot or too cold for water to exist in liquid form. Liquid water is likely a necessity for life.

Any star can act like a lens increasing the brightness of another star that happens to pass behind it. Such events are, of course, so rare that calculations suggest that the probability of it happening for any star is one in a million. In spite of this low probability, astronomers are finding hundreds of such events a year by monitoring areas near the centre of the galaxy where many millions of stars are bunched together.

If the lensing star is a single star it forms is a symmetrical lens so that it appears to brighten and then fade smoothly. If, however, the lensing star has an orbiting planet there are distortions in the shape of the lens and in the brightness curve as the source star passes behind. Auckland Observatory is part of an international collaboration called MicroFun – Micro Lensing Follow-Up Network – that picks out microlensing events that could be suitable for detecting a planet and then arranges for intensive 24-hour coverage through the various observatories that are part of the network.

The research telescope at Auckland Stardome is a Meade 40-cm telescope. Stardome also has another, larger telescope for public use. Image and copyright Nick Lomb ©, all rights reserved

Auckland Observatory uses a Meade 40-cm telescope on a solid Paramount mount. Once the observatory receives a request it uses the telescope on every available clear night to continually take images of the target star. At the end of each night of observing the images are sent to Ohio State University, the headquarters of MicroFun, where the images are processed to yield brightness measurements, merged with data from other observatories and the brightness curve is put together.

MicroFun has now detected a number of planets including one in April 2005 with a mass three times that of Jupiter and soon afterwards another with a mass similar to Neptune. This is cutting edge science and it is highly admirable for a small institution like Auckland Stardome, with its relatively small telescope, to have a major involvement.

To help you learn about the southern night sky, Sydney Observatory provides an audio guide/podcast, transcript of that audio, and a sky map or chart each month. This month’s guide is presented by Melissa Hulbert, an Astronomy Educator at Sydney Observatory.

Mel points out constellations to look out for this month (Orion the Hunter, Scorpius the Scorpion, and Crux or the Southern Cross), planets (Venus, Saturn, Mars and Mercury) and tells us about the Eta Aquarid Meteor Shower which should be visible until 27 May, with the peak on 5 May.

Mel also gives a preview of the rare astronomical event on 6 June this year – the transit of Venus. The following one won’t be until 2117! You can buy the book, ‘Transit of Venus: 1631 to the present’, by Dr Nick Lomb, which is beautifully designed and full of fascinating information about this historically important astronomical event. Also, keep posted for news about our iPad version of the book which will be available in the iTunes store soon. We’ll let you know when it’s available and we also provide more information about the transit of Venus on our web pages.

All this and more in the audio and transcript below.

HEAR THE AUDIO
You can subscribe with iTunes or upload the (12 mins 36 secs) audio to your iPod or mp3 player, or listen to it on your computer.

SEE THE SKY CHART

We provide an embedded sky map (below) and a May 2012 night sky chart (PDF) which shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.

May 2012 night sky chart

BUY THE BOOK
Our annual book, ‘The 2012 Australasian sky guide’, by Dr Nick Lomb has more information and star maps for months from December 2011 until December 2012 inclusive, plus information about the Sun, twilight, the Moon and tides, and a host of other fascinating astronomical information. You can purchase it ($16.95) at Sydney Observatory and Powerhouse Museum shops or other good bookshops, or online through Powerhouse Publishing (additional packing/postage costs apply).

READ THE TRANSCRIPT (after the jump)

(more…)

The northern sky over Anzac Cove at 3:00 am EET on 25 April 1915. Calculated with the Stellarium planetarium program

On 25 April each year in Australia we commemorate the landing by Australian and New Zealand troops at Anzac Cove in Turkey. To try to gain some advantage of surprise over the enemy the landing had to be carefully coordinated with the time of moonset and sunrise. Here we look at how those times matched the events of the landing.

All calculated times are in Eastern European Time (EET) which is two hours east of Greenwich. That is the appropriate time zone and, as far as I can ascertain, that is the time zone used by the military for the landing. Note though that back in 1915 watches were not coordinated amongst the navy and army personnel and, in any case, would not necessarily be running exactly on time.

That night the Moon was gibbous, two and a half days after first quarter phase, so it was fairly bright. It set at 2:57 am.

The first report on the landings was by war correspondent Ellis Ashmead-Bartlett. It appeared in the Hobart Mercury on 12 May 1915. Here are a few extracts with inserted comments in square brackets:

“As the moon waned [he meant was setting], the boats were swung out. The Australians received their last instructions, and these men, who only six months ago were living peaceful, civilian lives, began to disembark on a strange, unknown shore, and in a strange land to attack an enemy of a different race.”

“At 3 o’clock it was quite dark [the Moon had set], and a start was made towards the shore with suppressed excitement. Would the enemy be surprised, or be on the alert?”

“Not a sound was heard, not a light seen, and it appeared as if the enemy had been surprised. In our nervy state the stars were often mistaken for lights ashore.”

No wonder that the stars were mistaken for lights as they would have been unfamiliar constellations and stars for the Australians. As indicated in the diagram above, in the northern sky they could see Ursa Minor or the Little Bear as well as Ursa Major or the Great Bear plus the ‘W’ of Cassiopeia. These are all well-known star groups in the northern hemisphere, but either not seen or not seen well from Australia.

Nautical twilight – that is the time when the horizon starts becoming visible – began at 4:21 am. Civil twilight – when lights can be switched off for outdoor activities and possibly dawn in this context – was at 4:55 am. The Sun rose at 5:24 am. Thus the opportunity for surprise only lasted until shortly after 4:00 am though at the same time the light started becoming sufficient for the landing.

“The progress of the boats was slow, and dawn was rapidly breaking at 4.50 when the enemy showed alarm for a light which had flashed for ten minutes then disappeared. The boats appeared almost like one on the beach. Seven torpedo-boat destroyers then glided noiselessly towards the shore.”

You can read more of Ashmead-Bartlett’s report here. We will finish with an extract from Laurence Binyen’s famous poem:

They shall grow not old, as we that are left grow old:
Age shall not weary them, nor the years condemn.

At the going down of the sun and in the morning,
We will remember them.

A bright prominence at the edge of the Sun that reached a height of 149 000 km on 3 April 2012 (UT). Image and copyright Monty Leventhal OAM ©, all rights reserved

Serious observers of the Sun like Monty Leventhal OAM of the Sydney City Skywatchers use special filters called hydrogen alpha filters. These are safe to use as they cut out all light from the Sun except for the red light of hydrogen atoms. Hence these filters emphasise features that radiate at that wavelength, which are those composed of hot hydrogen atoms. Features on the Sun that can be seen with a hydrogen alpha filter includes prominences, filaments and flares.

A prominence at the edge of the Sun reaching 93 000 km on 19 March 2012 (UT). Image and copyright Monty Leventhal OAM ©, all rights reserved

Prominences are hot clouds of gas travelling along lines of magnetic field. They can exhibit all sorts of shapes such as arches and loops and can sometimes stay above the edge of the Sun for days. Others can detach from the Sun’s visible surface and float away.

It should be noted that the Sun’s visible surface is not solid. Nothing on the Sun is solid as it is a gas even towards its central regions. The visible surface is a region of surface temperature around 5500°C with the deeper regions beyond it too hot and opaque to be visible.

A long filament viewed in the red light of hydrogen atoms stretching 297 000 km across the Sun on 3 April 2012 (UT). Image and copyright Monty Leventhal OAM ©, all rights reserved

When prominences are seen against the bright solar disc instead of the darkness at the edge of the Sun they appear as filaments – long dark lines snaking across the Sun.

The most exciting and the rarest events on the Sun are flares. These are explosions on the Sun that can be seen as the brightening of regions of the Sun near sunspot groups. They can last from for just a few minutes to a few hours. Satellites such as the GOES satellites provide continuous measurements of the X-rays emitted by the Sun and so provide complementary information to what can be seen visually.

Words can be so confusing, well for some of us at least. I even confuse myself occasionally trying to explain certain things. You can therefore imagine my trepidation when it comes to interpreting what others are saying about a strange event like “crossing the centre of the galaxy in December 2012”. This one has come up a few times recently as we head towards the “END” of the world this December 21st, so I thought I’d have my say to try and explain what is going on.

Rest assured the world will not end this year or anytime soon. Nonetheless we appear to cross the plane of the galaxy this December, as we see it. In fact we do it twice a year at the winter and summer solstice because of the Earth’s tilt on its axis, which incidentally also causes the seasons. This is however a purely visual effect as our entire Solar System is presently something like 75-100 light years above the plane of the galaxy.

It gets worse. Yes we do pass through the plane in a complicated gravitational dance that takes about 33 million years to complete one cycle of above and below. We last crossed the plane roughly 3 million years ago and are not expected to do so again for another 30 million years, certainly not in 8 months’ time.

Will we ever really pass through the centre of the galaxy? No, and I can say with absolute confidence that is never going to happen. The Milky Way galaxy is very large at about 100,000 light years side to side. Our small solar system is about 28,000 light years from the massive black hole that lurks at the centre. We won’t ever go through the centre but we do “fall around” or orbit the centre of the galaxy about once every 250 million years.

In summary, this December like all Decembers we will appear to cross the plane of the galaxy as we see it, but we will really cross only in 30 million years’ time and we will never pass through its centre.

The dark filament at the same site where the sunspot group AR11429 had been seen during the previous rotation of the Sun. A small part was ejected on 3 April 2012. Sketches and copyright Harry Roberts ©, all rights reserved

Sunspot group AR11429 first appeared at the sun’s east limb on March 3 (2012) as a compact “Island Delta” group that, much evolved, finally passed behind the western limb on the 15th – after strong flaring. At the time we noted that “Island Delta” groups were the most active sunspots – but were also short-lived ones. Would it return at the east limb in late March?

During its two weeks behind the sun several CME’s erupted at the spot‘s location – suggesting it might reappear. March 30 showed bright faculae at the east limb – but no spots. March 31 however, showed more faculae at the site (now 30º onto the disc) and two dark filaments stretching N-S through it. Finally, with high magnification, two tiny spots were detected (Fig 1, LHS). AR 11429 had survived a full rotation of the sun – but only just! NOAA promptly dubbed the returnee AR 11451.

Filaments: While the tiny spots were gone by April 3, the filaments had grown into the most striking feature on the disc (Fig1, RHS), compared to the other small groups present (not shown).

For amateurs, filaments are only visible in H-alpha (and in satellite EUV bands). They are magnetic “channels” between surface fields of opposite polarity where cooler material collects; they come in two kinds: active region filaments and quiet region filaments. This filament was a mixture of both!

Helio freeware sited the darkest part of the big filament between +20º, 302 and +11º, 311 with a “tail” northwards. These points were very close (~5º) to the inversion line that bisected spot group AR11429 a month before: the stage was set for spotless flares (Zirin, H. “Astrophysics of the Sun” PP 221 and 333).

The Mt Wilson daily magnetogram on 4 April 2012. Copyright Regents of the University of California ©

AR11429 had been (briefly) an “Island Delta” class with mixed and reversed polarities in one penumbra, but when it evolved into a larger simpler group it remained fully reversed. Mt Wilson’s daily magnetogram (Fig 2) now showed that regions of reversed polarity persisted at the old sunspot site, little changed from the previous rotation (Fig2. Black line is the filament site). And the low power fields still showed concentrations of stronger field where the main spots had been during the previous transit.

Although now spotless, the old AR11429 site did have more flares – but not big ones, mostly along the filament channel that a month earlier had separated the spot group’s opposite polarities.

The magnetogram also showed that the whole active area now stretched across 45º of longitude with three attached active regions; new “normal polarity” groups AR 11450 and AR 11452 (both small), with reversed AR11451 (old 11429) in between. Some unusual coronal links between new and old spots were seen in SDO EUV images of the region.

Filament Ejection: A small ejection at the south end of the big filament erupted on April 3rd. While stable from 21:48 to 22:56, at 23:02 a small globule ejected that was only seen ‘off central band’ H-alpha (Fig1, detail). This was tracked as it moved westwards– always ‘off band’, and invisible ‘on band’. This was likely part of the large filament that, while showing small lateral motion (6º in total, and dividing at b, 23:16UT), was ejecting along the line-of-sight at high velocity. The filter’s tuning range is stated to be ~10Å; assuming a -5Å shift the ejection velocity was 250km/s in approach. Meanwhile, the big filament remained stable. Since it seems to be the product of active and quiet region fields the potential is for a big ejection of this filament – spectacularly at the limb perhaps. Here’s hoping!

AR11429 (aka 11451) was by now only a ghostly manifestation of its younger self – with no spots, just the wraith-like filament and its recurrent flares – yet there in the magnetogram were signs of the once majestic spot group: the largest reversed group yet of SC24.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers

The gibbous Moon shining serenely on the evening of 3 April 2012 with the large crater Tycho on the top right and Mare Crisium on the centre left. Image and copyright Nick Lomb ©, all rights reserved

As a young postgraduate student I gave my first professional talk on the possible origins of the Moon. At that time there were three main possibilities: the fission theory that somehow the still molten Earth split into two and the Moon flew off; the twin planets theory that the Earth and the Moon somehow formed separately at the same time and at the same place; and the capture theory that the Earth captured a passing asteroid that became the Moon. All of these theories had difficulties in matching the observed facts. For instance, to capture a passing body a huge amount of energy would have to be dissipated. I spent much of the talk arguing, probably unconvincingly, that the energy dissipation could happen under some very special and improbable circumstances.

Since 1975 a new theory has superseded the previous ones, solved most of their problems and has been widely accepted by the scientific community. According to this theory in the early days of the solar system there were still many Mars-size objects circling the Sun and one of those struck the Earth. This impact would explain the relatively fast spin of the Earth as well as its tilt of 23½° that is responsible for the seasons. In recent years the impactor has acquired the name Thea.

During the impact Thea’s metal core would have merged with that of Earth while the outer layers (mantles) of both objects would have been thrown into orbit around the Earth. Over a time of less than a century and possibly much quicker, this material would then have coalesced to form the Moon. This scenario would explain the Moon’s lack of the extensive iron core that most other similar and larger objects in the solar system possess.

Recently, a few problems have arisen with the current version of the impact theory. Calculations suggest that over 40% of the Moon’s material would have come from Thea, yet there are indications based on examining the oxygen in samples brought from the Moon that the lunar material is identical to that on Earth. To make the comparison scientists examined the ratios of different versions or isotopes of oxygen in lunar material and found that the ratios are identical to those from Earth.

Another study looking using titanium isotopes this time was published in Nature Geoscience on 25 March 2012. In a paper titled The proto-Earth as a significant source of lunar material five authors led by Junjun Zhang of the University of Chicago find that the titanium isotope ratios that they examined from lunar samples were the same as those from Earth to four parts per million. All this suggests that either that the Earth was the Moon’s sole parent and it was formed only from Earth material or that the material from Thea was so thoroughly mixed with that of the young Earth that it is now indistinguishable.

These results create problems for the current version of the impact theory. The theory is not dead, but scientists will clearly need to modify it considerably in the light of these new and intriguing results.

Venus passing close to the Pleiades star cluster with Jupiter nearby. The diagram is for 6:45 pm AEST on the evening of Tuesday 3 April 2012. Diagram Nick Lomb

The bright planet Venus often provides spectacular views as when it had a conjunction with Jupiter in March 2012. On the evening of Tuesday 3 April 2012 and on the following evening it provides the opportunity for another spectacular sight as it passes the famous star cluster Pleiades. Although, clouds permitting, the proximity of Venus to the star cluster will be noticeable to the unaided eye, binoculars are likely to provide a better view.

The Pleiades are known from Greek mythology as the Seven Sisters and are the most famous star cluster in the sky. They provide a good test of eyesight as most people can see six stars in the group while some sharp eyed people can see seven or more. Of course, through binoculars or a telescope many more can be seen. It is also advantageous to observe them from a dark spot instead of the light-polluted suburbs from where most of us experience the stars.

The stars of the cluster have been observed by different groups of people around the world for hundreds or thousands of years. Interestingly, many groups not just the Ancient Greeks refer to them as women. According to one group of Australian Aboriginal people they are the Kungkarungkara or Ancestral Women.

A colour image of the Pleiades put together from photographic images taken with the Palomar 48-inch (1.2-metre) Schmidt telescope between 1986 and 1996. Credit NASA, ESA and AURA/Caltech

Through a telescope hundreds of stars can be seen in the cluster together with blue dust clouds. At an age of about 100 million years the cluster is young in astronomical terms, so it would be easy to think that the dust is left over from the formation of the cluster. That is not the case, however, as astronomers have measured that the dust has a different motion through space to that of the stars. The blue colour of the dust is because it reflects and scatters the light from the stars and, as in the Earth’s atmosphere, blue light is preferentially scattered over other colours with longer wavelengths.

The brightest nine stars of the cluster all have proper names. They are the Seven Sisters, Alcyone, Asterope, Electra, Maia, Merope, Taygeta and Celaeno plus their mythological parents Atlas and Pleione. According to RA Allen in Star Names, their Lore and Meaning the last two were probably added relatively recently, possibly as late as the 17th century. There are a number of possible origins for the name of the cluster, but Allen suggests that Pleiades is probably derived from the name of the sisters’ supposed mother Pleione.

If clouds permit, dust off your binoculars and have a peek at Venus dropping in on the Seven Sisters.