Harry observes the strange morphing sunspot group AR11089

The sunspot group AR11089 as it first appeared on the east limb or edge of the Sun. Drawing. Drawing and copyright Harry Roberts ©, all rights reserved

AR11089 is not the name of an underground rock band, but the label applied by the US NOAA organisation, charged with keeping track of sunspots – as counting spots is still the key way of comparing solar activity across the centuries. Was the sun really spotless during the 17th century? Was its activity in the mid 20th century a four hundred year peak? Such questions are answered mainly by looking at the sunspot count.

The second half of the 20th century was the age of satellites; they imaged the sun in a range of wavebands particularly EUV (extreme ultraviolet). But satellites have short lives (one was even ‘shot down’ by the USAF) and normalising their data with the classic spot count isn’t easy. In fact, the best view of any sunspot is still the one in your trusty 3” or 4” amateur ‘scope with an aperture filter!

While this is titled AR11089 I want to briefly recall AR11085 – the spot group that “nobody saw” (except NOAA and the lucky writer). This small bipolar group perhaps lasted less than ten hours, and then disappeared – but when its locality next returned to the sun’s east limb, there was a large active-looking spot group at the exact site. And now everyone could see it, renumbered AR11089.

There are several odd things about this new group, apart from its Lazarus like reappearance. It’s roughly triangular in layout while most spot groups are bipolar and align E-W on the sun. And AR11089 sits in a large area of faculae (WL) and plage (Hα) that has another such activity patch only a few degrees west of it; this latter area is void of spots. Are they related in some way?

Solar Dynamics Observatory views of sunspot region AR11089, annotated by Harry Roberts

They are; the new SDO satellite views show magnificent arches (field transition arches) connecting both plage regions – the whole being one huge magnetic entity (Fig 3, main spots arrowed). And yet while fields within AR11089 briefly reached a ‘strong-ish’ 2500G the group has produced no flares >GOES C1. Curiously it also has few of the usual dark active region filaments (arf) that attend even the smallest sunspots.

The writer first saw the group at the east limb early in its development amid brilliant faculae – having six spots, the largest sited at –21/204 (Fig 1). The Fig shows changes from 19th to 21st (UT) – the group developing substantial penumbrae and ten or more umbrae, still in a triangular layout.

Sunspot group AR11089 on 23 July 2010. Drawing and copyright Harry Roberts ©, all rights reserved

Twenty-four hours later saw an increase to 20+ spots, with many tiny ones in short chains. The preceding spot (p) had a field of R25G (red 2500 gauss) and the larger following (f) spot to the SE had V24. Both are strong fields; but the trailing spot of the ‘triangle’ was now fading, with a few small spots in a chain. Overall the group seems to be ‘morphing’ into a more conventional E-W arrangement, and the remaining (p) and (f) components with 6º N-S separation may soon feel the pull of the Hale-Nicholson force and undergo some dramatic rearranging.

Harry Roberts is a regular contributor to this blog and a member of the Sydney City Skywatchers.

Harry observes a long-lived sunspot AR 11084 and considers if it is the return of a previously seen spot

Two sketches of sunspot AR11084, drawing Harry Roberts

While many Cycle 24 (C24) sunspots are hard to see and short lived, AR11084 was easy to see and long-lived. In fact, it is probably the return of AR11078 that first appeared in the sun’s SW quadrant on June 7th, when it emerged as a pair of tiny spots preceding (p) a pair of larger spots with some penumbra around them – a bipolar spot group.

It was next recorded on June 11 close to the solar SW limb – now both (p) and following (f) spots had grown larger and developed penumbrae – Helio freeware sited the group at –19/140 (i.e. solar latitude 19º south, longitude 140º) Bright faculae threaded between the (p) and (f) components. Umbral fields showed the bipolar group “broke” the Hale Nicholson Law – having reversed polarity for a C24 group, with violet polarity preceding red – the second reversed group of C24. Would the group survive a transit of the sun’s far side? Partly, it did!

June 26 showed a fair sized single spot at the sun’s SE limb with very bright faculae following 8º behind (Fig 1) – there was only the one penumbral spot present. H-alpha showed a low bright prominence above the ‘new’ spot and filaments and plage surrounding it. The ‘new’ spot was sited at –19/146. It slowly “dawned” on the writer that the new spot was the return of AR11078!

Two clues prompted this: one is positional. AR11078’s preceding component had a longitude of ~143º, (as well as the same latitude as the new spot 19ºS). Allow a degree or two of western drift that all (p) spots undergo and we see that the ‘new’ spot matches 11078’s western component exactly.

Another clue was polarity – the two parts of 11078 had reversed polarity, with the (p) spots having violet polarity (as stated) and the single monopolar spot of 11084 had violet polarity also, with bright faculae following. This makes AR11084 Hale class “Alpha preceding” – the bright faculae having persisted since first seen in AR11078 14 days earlier. Mt Wilson magnetograms showed that it was the following spots of the earlier group that had disappeared, leaving the (p) spot and faculae intact.

An extreme ultraviolet view of AR11084 from NASA’s Solar Dynamics Observatory, annotated by Harry Roberts

Alpha class sunspots have only one polarity with no other visible spots nearby. Hale first suggested (1920’s) they were bipolar BUT the spots of opposite polarity were invisible. This is in fact the case; and field transition arches (fta in Figs) from the single spot fan out and connect to multiple small polarities in the associated faculae; polarities too “warm” to be visible as spots in any ‘scope. A fine image from the SDO satellite shows exactly this in the corona above AR11084 (Spaceweather Archive for July 2nd – take a look. Note: the fta are NOT visible in H-alpha unless a flare has occurred when they show as post flare loops. SDO can detect them in EUV)

A sketch of spiral filaments around sunspot AR11084, drawing Harry Roberts

AR11084 was remarkably stable – growing a little to just over 100 area units as it crossed the sun’s face, followed by extensive dark filaments and plage –at one stage dubbed the “spiral” sunspot due to filaments that like the arms of a galaxy encircled the spot (Fig 2). The spot produced very weak flares– despite its umbral field slowly rising to V24 on June 30 15:00UT. The spot will surely survive to pass behind the sun’s SW limb a second time – and may yet reappear at the SE limb around July 23.; who knows?

Keep a close watch on the sun – anything can happen!

Harry Roberts, a frequent contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.

Harry sees a short-lived sunspot group – AR11085

Sketches of short-lived sunspot AR11085

These are strange days for sun watchers: many emerging spots are faint and short-lived – some last less than 24 hours; here today, gone tomorrow! ‘Old hands’ suggest we are still in sunspot minimum – but most measures of activity show a slow rise from the deep deep minimum of 2008 –2009.

A good example is AR11085, that was briefly seen on June 29 when the 4”’Mak.’ (white light) was pointed sunwards. The large single spot AR11084 was clear to see on the sun’s disc – and well west of it a small bipolar spot cluster was visible too –“AR 11085?” my log suggests. Nobody else reported this new group – Monty saw nothing at the site only 5 hours earlier; Mt Wilson’s 150’ ‘scope had nothing 18 hours before. Timings sited the new group at –23/203; I expected the Net would soon bristle with reports.

I was to be disappointed; apparently nobody else saw the new group. And on June 30 I was surprised when it got a NOAA active region number AR11085 – clearly someone had seen it! (Sadly, NOAA does not reveal its sources.) Next day Mt Wilson reported one tiny spot at the 11085 site, with a weak umbral field of R17 (i.e. red 1700 gauss. Remember, around 1800G is the minimum for visibility).

Since the group had not been seen by Monty L six hours earlier, and when logged by Mt Wilson ten hours later, had almost gone, it seemed that AR11085 had had a very short life indeed.

Timeline of observations of short-lived sunspot AR11085. Drawing Harry Roberts

To get an Active Region number a spot has to last at least 24 hours – shorter-lived groups being so much statistical ‘noise’. How long did AR11085 actually last? Presumably NOAA knew it had a life of over 24 hours – at this point a timeline was created to find the probable lifetime of the group (Fig).

The timeline plots observations both negative (not sighted) and positive (sighted) for the spot group by the following: MtW is Mt Wilson’s 150’ solar ‘scope; ML is Monty Leventhal; HR is the writer; and NOAA the well-known US Authority.

Although Solarmonitor’s website asserts the group persisted through June 30th, the evidence (MtW) shows it was almost gone by 14:15UT on the 29th – and its persistence through the 30th is unlikely. Unless other records can be added we may conclude that AR 11085 lasted less than 24 hours – say 18 hours. Perhaps the group should not have gained a NOAA active region number – and it illustrates one of the challenges now facing sun watchers: recording current sunspots before they disappear!

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

Harry observes the white dwarf star 40 Eridani B and considers if it really is degenerate

Sketch of 40 Eridani A & B, drawn by Harry Roberts

Degenerate! was an unflattering term common in the 1970’s – and when I came across it in Kaler’s Stars and their Spectra I realized the word had another usage.

When stars exhaust their nuclear fuel via the proton-proton reaction they have other fusion reactions available to keep burning – but many finally run out of juice. Such stars are termed degenerate – that is, they no longer have any way of generating energy.

Contraction accompanies the failing of fusion, and titanic explosions or the rapid shedding of matter can result – it depends on the star’s initial mass. Some stars simply fade away having shrunk from a million km diameter to only 10,000 km – or 1% of their initial size! Such stars are called white dwarfs, and their average density can increase from 1 to 100,000 (!) with a similarly huge rise in their magnetic fields – up to millions of gauss!

How abundant are white dwarfs? is a tricky question (for example, see Napiwotzki R. The galactic population of white dwarfs Journal of Physics; Conference Series 172 (2009)) with complex answers: broadly speaking they are very abundant. We learn from the University of Texas that Although abundant, as they cool, white dwarfs fade and become difficult to detect with telescopes.

For amateurs there are probably only two white dwarfs visible – Sirius B and 40 Eridanus B – the latter star being quite easy to see, while Sirius B is almost impossible. The Table of White Dwarfs above lists the thirteen brightest stars of the type – six of which are too far north for us to see. Of the remainder the best are Sirius B, 40 Eri B, Procyon B or W1346 in Cygnus – the latter being Mv 11.5 is a pretty faint star.

What about Procyon B? As the Mv10.9 companion to bright Procyon A (Mv 0.35) is currently only 2.5 “arc away from the primary it’s impossible for amateurs to see.

40 Eridani B is the best white dwarf for amateur ‘scopes and it’s part of a triple star system with members of contrasting spectral types and colours that is both beautiful and interesting to view. The sketch at the top shows my impression of the 40 Eri system. To the naked eye it is one of a wide pair of stars also named Omicron1 Eri and Omicron2 Eri (40 Eri being O1) The Omicron pair is easy to see just one hour (of RA) west of Rigel. In the ‘scope O1 is seen to be an orange type K1 star Mv 4.5 with companion B at PA 105º and 83”arc separation – a bluish star Mv9.7 – the easiest white dwarf visible. As the table shows this star’s diameter is only 1½ times Earth! The 40 Eri system is only 16Ly away – the main reason we can see this tiny white dwarf – there’s lots of white dwarfs; we just can’t see them!

We have considered 40 Eri A and B – there is also star 40Eri C, a fainter (Mv10.8) red dwarf in orbit about B, and now ~7”arc separation NW of B (period 248y). I have not yet seen this red dwarf, and as the group is too far west now, it’s on the agenda for next summer!

WIKI says 97% of all stars will (it’s believed) become white dwarfs eventually. Another site says that since the number of fossilized stars is almost as great as the number of living stars, white dwarfs are abundant in our galaxy. i.e. the number of white dwarfs is roughly the same as the number of all other stars, or ~50% of the total. But a recent paper suggests their mass totals only 10% of the mass of all stars in our galaxy.

Harry Roberts, a regular contributor to this blog and a member of the Sydney City Skywatchers.

Ella reports on observing the stars from the South Pole

Dr Ella Derbyshire at the South Pole, image courtesy United States Antarctic Program

Alan Plummer: This winter Ella Derbyshire is at Amundsen-Scott South Pole Station where she is the station’s physician. The station is right under the South Celestial Pole at an elevation of 2,835 meters. On this trip – not her first – Ella had the thought to take binoculars with her for some stargazing. More to the point, Ella expressed an interest in doing some useful astronomy. As variable stars are the subject of active research and many are within the range of small binoculars I had the pleasure of sending of a set of charts and a choice of targets. Ella has since joined the VSS and the AAVSO and writes this report on what it’s like to observe where the Sun never sets. Over to Ella:

The bottom of the world is an interesting and challenging location for a novice variable star observer. Because the winter scientific research at the South Pole requires dark skies, we make conscious efforts to avoid light pollution. There are just enough thoughtfully-placed red lights outside to guide people from building to building as they walk through the dark. All our winter vehicles also shine only red lights. Once we arrive at astronomical darkness, our only two significant sources of light pollution are the moon, which rises monthly and then remains above the horizon for a fortnight, and auroras, which can be surprisingly bright and are far less predictable than the moon.

The map of the South Pole night sky is the map of the South Celestial Hemisphere. With the South Celestial Pole directly above us, and the Celestial Equator as our horizon, the star patterns are always in the same orientation, and so they are easy to learn. Stars do not rise or set here, they simple circle around us, inching a little bit westward with each passing day. With this simple arrangement, once I find a star in the sky, finding it again, assuming that it remains bright enough to find again, is easy.

I can choose any hour of the day for observations. Because target stars are always above the horizon, and the sky is as dark at noon as it is at midnight, the time of day that I choose to observe is not especially important. I just locate my guide stars, and then start star hopping.

Observing variable stars from the South Pole requires a bit of preparation. The coldest weather brings the clearest skies. Whenever it warms up, there is a persistent wind blowing, picking up the snow and tossing it in front of the stars. By the time that the temperature reaches -40ºC, the wind, which can reach 40 knots on warm days, is creeping up coat sleeves and under facemasks, threatening any inadequately covered skin with frostbite. Fortunately, staying outside to watch stars is pointless at such times because in polar wind storms, even Sirius and the moon are hidden. During cold, calm weather, with the temperature from -60 to -70ºC, dressed in a full set of extreme cold weather gear, I can stand still for almost an hour at a time, gazing up at the sky. If I get dark-adjusted before I venture out into the cold, I can make the most of these observing sessions.

In addition to chilling the observer, the cold affects whatever tools we bring outside. I record my results in pencil because ink freezes. Camera batteries die quickly, making astrophotography a real challenge. Camera lenses, eyeglasses and binoculars frost over with any misplaced breath, and must be warmed and dried to be useful again. It helps to be efficient at locating the target stars and comparison stars and at recording the results. After a while, even eyelashes frost up and will eventually stick together, providing an unambiguous clue that it is time to return inside.

Dr Derbyshire’s report was forwarded by Alan Plummer, prolific and expert variable star observer and a member of the Sydney City Skywatchers

Harry observes the weak sunspots of May 2010 and wonders if the Sun is “Hotting Up” or not

Four views of Sunspot Group AR11069 in May 2010. Sketch by Harry Roberts

May 2010 was greeted with shouts of “Callooh! Callay!” as no less than six new spot groups appeared on the sun at the one time. (Morris dancing broke out in solar observatories world-wide!)

But, oh dear me, something was wrong! The baby spots failed to thrive – and in a short time all but one had vanished! What had gone wrong? Readers, here is the melancholy tale.

Last year NASA confessed to the world that its attempts to model solar activity had completely failed, blaming the sun for “behaving unpredictably”. (Perhaps one of many reasons the new Administration has clipped their “wings”). Presumably the Universe (including the sun) always behaves unpredictably – we only ‘know’ about 5% of it – and that rather poorly!

NASA and most other modellers predicted that Cycle 24 (C24) would rapidly rise to a powerful maximum around 2010 after a short minimum – a much stronger maximum than that of C23. What happened in fact was that C24 was delayed by 18 months, and only in 2009 did a shaky start occur.

What went wrong with the nine new groups that arose in May? (There were nine groups on the disc in May, AR11063 to 11071 inclusive. Only 11069 was seen by the writer to have obvious spots. AR11067 and 11071 were seen briefly to have a few spots.) Basically it was the fact that their umbral fields failed to rise above 1800G, increasing field strength (i.e. energy) being crucial for the growth of sunspots (just as it is with humans). While the writer knew the small groups were on the sun’s disc, repeated scrutiny of the disc failed to show them, except for AR11069 and some faculae, plage and associated filaments at the various sites.

Last year I asked veteran Mt Wilson observer Tom Cragg how weak a field the 150’ ‘scope’s Babcock Magnetograph could detect, and was surprised to hear that values as low as 1000G were sometimes logged. The texts generally agree that sunspots become invisible at umbral fields below 1800G.

As stated above only one of the six new groups developed a field much above the 1800G threshold, this was AR11069, while the others quickly faded to lower values and vanished. AR11069 had a peak field of 2200G in the preceding (p) spot on May 5 (23:40UT), with 1900G in the following (f) spots. At these values only small penumbrae developed around some spots, since penumbra needs fields over 2200G to grow to any size. Accordingly, this group consisted of a chain of small spots stretching over ~9º of longitude at the high solar latitude of 41ºN where spots are seldom seen – the only example yet for C24. Moderate to strong flares occurred in this group, with a GOES M1 (or C9, authorities differ) on May 5 at 17:20 UT – but the writer recorded none. Also on the 5th the group had opposite polarities within the one (central) penumbra – making it Hale class Beta Gamma Delta, the reason for the flares perhaps. The group was last seen at the west limb on the 7th when only the central complex spot remained, with large filaments stretching northwards from the site, and bright faculae over the whole region.

Of the other small groups one of the more interesting was AR11063 that first arose on April 28 but disappeared on the 30th. You may think it would stay ‘gone’ – but it reappeared four days later on May 4 and remained visible for a few more days (in big ‘scopes, not mine). Presumably its umbral fields fluctuated around 1800G causing it to fade to invisibility, then reappear some days later. What do these weak and ephemeral sunspots indicate?

At least for the present they support “Ol’ Bill” Livingston’s thesis that “Sunspots may vanish”. Livingston you will recall is a respected solar astrophysicist, the one whose “Sunspots May Vanish” paper caused much controversy a few years ago. At least for the present the evidence supports his thesis – and perhaps, just perhaps, sunspots will become invisible by 2014.

Sun watchers, gather ye sunspots while ye may!

Harry Roberts, a regular contributor to this blog and a member of the Sydney City Skywatchers.

Harry locates and sketches a Moon crater named after the great star cataloguer – Annie Jump Cannon

A sketch of the area at the edge or limb of the Moon containing the craters Plutarch, Seneca and Cannon. Sketch by Harry Roberts

The lunar limbs are difficult to study; the craters crowd together, their floor features hidden by shadows or by their own rims. Ridges can look like craters and hundreds of kilometres of lunar surface is compressed into a narrow strip – it’s hard to identify any given crater. One such region is that between Mare Crisium and the Moon’s east limb where the dominant feature is Mare Marginis, the sea at the “margins”.

The above factors made for a confusing subject – and I dashed away with the pencil hoping to capture a particular crater in the area that had long been on my list: Cannon (see sketch above), named for indefatigable Annie Jump Cannon (see below). It seemed at the time that Cannon was the half lit deep crater left of centre – but next day with atlas and freeware it was clear that Cannon was the shallower crater at the right side on the terminator – only just in my field. Still it was a record at last – and one that may not recur soon.

Parts of eastern Mare Crisium were roughly sketched to give some context for finding Cannon – with Cape Agarum labelled cA near large crater Condorcet. While mapping Agarum I noted what looked like an elongate “vent” with rilles at either end – an interesting feature worth a closer look.

Ring ridges are perhaps the dominant landform in this area – Crisium being a multi-ring impact basin – and remnants of outer rings confuse the view, two such are tagged rX and rY – they may be parts of the “Geminus Ring” (C. Wood, “The modern Moon”, Sky Publishing, note his caution about the rings). Raised terrain north (i.e. left) of Cannon is brightly lit, but a jumble of smaller impacts confuses the view. Here and there light streams through gaps between the row of big craters on the terminator, illuminating higher ground beyond. The two craters north of Cannon are Plutarch and Seneca, named for thinkers from Greece and Rome respectively, but Cannon – well, she’s different in every way.

Who was Annie Jump Cannon? She was perhaps the greatest of the female “computers” [at Harvard College Observatory] who created the Draper Catalogue of stellar spectral types in the 1920s – devising the final (and modern) classification system while personally classifying 225,300 of the original catalogue of 359,000 stars; a gigantic feat! She was no mere “robot”, and in fact redesigned the entire system, supplanting the earlier models, and refining the scheme as her insight grew. We may look at her work in more detail in a future post on stellar spectra, but it’s true to say that her system is the essential key to modern astrophysics and astronomy. We may well cry “Oh! Brilliant Analytical Feat – Great. Key Maker!” [This in an “in” joke. The names of the stellar spectral classes in the system began by Annie Jump Cannon are O, B, A, F, G, K and M with O referring to the hottest stars and M the coolest. The usual mnemonic to remember the order of these classes is Oh Be A Fine Girl/Guy, Kiss Me. Harry is suggesting a clever alternative – Nick]

Harry Roberts is a regular contributor to this blog and a member of the Sydney City Skywatchers.

Harry examines the spectra of stars like the Sun – the G type stars

A low resolution view of the spectrum of the Sun (sunlight broken up into its component colours with the help of a prism or a grating). Drawing by Harry Roberts with colour information from Philippe Rousselle

Two amazing G type stars dominate southern skies –the Sun, and Alpha Centauri (α Cen).

We started our tour of star colours (i.e. spectral types) with the two brightest in the night sky, Sirius (type A) and Canopus (type F). The third brightest star is Alpha Centauri A (the brighter of the well known double) a type G2 dwarf (dwarf here means ordinary main sequence stars of luminosity class V. They are not tiny stars, but they are less luminous than the giants and supergiants) while its companion B is a cooler K type star. So of the spectral type sequence OBAFGKM the three brightest stars are good examples of types AFG and K respectively. In this piece we look at stars of spectral type G – the solar type stars.

Why did we not begin with the hottest O type stars or coolest type M? In “her” infinite wisdom the deity has put us well out of harm’s way, with none of the hot types nearby (and truly, we wouldn’t want any) – there are no type O or B among the first magnitude stars nor the nearest stars – so we began with the brightest of the naked eye stars, type A, Sirius.

The sun, type G, has dominated life on our planet – and our vision has evolved peak sensitivity in the band where G stars emit most strongly, the yellow part of the spectrum. Although it always looks white to me, if we viewed the sun from four light years away it would be yellow like α Cen A. Both the sun and α Cen A are main sequence dwarf stars, as distinct from more common giant stars – and are rarities in the bright star list.

Star colour indicates the star’s surface temperature; in G type the range is 5000 to 6000ºK, and light from the sun approximates a black body of that temperature. Because we live so close to a G2 dwarf we know more about its spectrum than any other – with so much light there is no limit to the detail seen at high resolution; the diagram shows a low-resolution spectrum.

Remember, the dark lines in a stellar spectrum reveal different elements in the star’s atmosphere, and of 92 natural elements about 70 are found in the sun’s spectrum. In A type stars (e.g. Sirius) we saw very strong hydrogen lines but they were weaker in F stars, and weaker still in the solar type stars – though they are still there; Hα (Fraunhofer C band) being the “window” through which amateurs watch violent eruptions on our star. Hydrogen lines β, γ and δ are there too but ‘high res.’ is needed to see them.

Other features of type G are the calcium II lines in the violet (Fraunhofer H and K) that are weakest in the hottest type stars (O and B). Molecular bands are stronger in cooler stars and the G band is one of these, due to the CH molecule (see diagram) – it’s strongest in types K and M. Several bands in the solar spectrum are caused by Earth’s own atmosphere; note the oxygen bands (O2, A and B) in the far red. While a homemade solar spectroscope shows hundreds of thin lines in the solar spectrum, the Baader spectroscope shows very little detail in type G stars – the thin lines needing higher resolution.

G type stars are about 13% of all stars – and we’ll look at spectral type abundances in a future piece (Ed. willing) – but note that solar type stars are not so common. A glance at any shots of Milky Way star fields confirms this, with blue stars everywhere and only a few yellow ones; you’ve probably noticed too that deep red stars are rarer still [surely that is only because the extremely bright blue stars are easier to see than the fainter yellow and the even fainter red stars - ed] . These are types K and M that we’ll look at later; meanwhile binoculars (best on a tripod) are all you need to confirm spectral types in our galaxy.

Enjoy autumn skies – and the colours of stars!

Harry Roberts, frequent contributor to this blog and member of the Sydney City Skywatchers

Alan talks about the Mira type variable star R Hydrae and tells how to observe it

A finder chart for the variable star R Hydrae, prepared by Alan Plummer from a Sydney Observatory star map.

This is the second in a series of posts on variable star observing, following the recent one on Eta Carinae.

Throughout this 2010 autumn and winter a star will be slowly brightening from the darkness only to become unobservable in the evening spring twilight just as it gets to naked eye brightness. The star is R Hydrae, and I’ve marked its location on the finder chart above made from the Sydney Observatory Sky Map. The coincidence of being in conjunction with the Sun at maximum light is temporary; because this star pulsates with a period of around 388 days, the blind spot drifts across the star’s cycle as the years go by.

R Hydrae is a Mira type variable, which is defined as being a pulsating red giant changing in brightness by more than 2.5 magnitudes over a period longer than about 90 days. R Hya is on the AAVSO ‘Legacy’ list of objects, meaning that they have a century or more of observations in the International Data Base. See the light curve pictured below. Such a span of observations can tell a story; but this one starts even further back than 100 years…

Observations of R Hydrae, courtesy of the AAVSO, used with permission

The first and best of the ‘modern’ European star atlas’ was Beyer’s Uranometria, compiled in 1603. This did not mark R Hya, but it was later plotted on Hevelius’ version in 1690. In 1669 a man called Montanari, who had Beyer’s 1603 atlas, noticed the star, and thinking the 1603 atlas in error, marked it on his own copy. It was a bit of bad luck he didn’t suspect a variable, as Montanari himself had already discovered one variable star – Algol (beta Persei). The nature of the star still remained undiscovered. To quote from Kerri Malatesta of the AAVSO:

Montanari’s marked atlas later came into the hands of Giacomo Maraldi…whose curiosity about the addition brought him to the field of R Hya in 1702. Using Montanari’s positional reference, Maraldi tried, without success, to identify the star. Intrigued by the mystery, Maraldi continued to monitor the area until 1712, noting maxima of the star in 1704 and 1708 and hence its variable nature.

Now to the story such an observational history reveals: a study has found that the pulsation period has declined from 495 to 380 days between around 1700 and 1950, and the amplitude has changed considerably. Recorded maxima have been observed as bright as 3rd mag and as faint as 6th. And the minima have been seen from 9th to 11th mag.

The most likely explanation for the changing nature of this pulsating star is that it had a so-called thermal pulse just before 1700. This is when a shell of burning hydrogen near the core accumulates enough helium in a shell below it to ignite that helium, in a flash, and in so doing extinguish the hydrogen shell. All this is wrapped around an inert carbon oxygen core. The idea is that changes within the star effect changes in the visible envelope, and how that behaves.

Chart and sequence for R Hydrae, excerpt from VSS RASNZ chart 12B, by Mati Morel

So if you want to join in the study of this star you’ll need 7×50 binoculars and/or a 4 inch telescope, and use the chart and sequence above in the same way that was described in the blog post on Eta Carinae. Observe this star once a fortnight and keep your observations on file; my next blog post will be about the agencies that collect and distribute the observations, and how you can use can those agencies. Or you can go to the AAVSO website now and do that yourself!

Acknowledgement: This post has used Malatesta’s work R Hydrae, May 2002 Variable Star Of The Month, which is recommended for further reading.

Alan Plummer, Sydney City SkyWatchers

Harry looks for the return of sunspot group AR11057 while the Solar Dynamics Observatory returns breathtaking images

A still from a movie showing an explosive prominence at the edge of the Sun on 30 March 2010. One of the group of breathtaking images from NASA’s Solar Dynamics Observatory that have recently been released. Credit: SDO/AIA

The story of any sunspot group is not complete until we’ve asked, “did the spot group return?”

The sun’s rapid rotation (~27d) means groups developing on “our” side often disappear behind the western limb, where further changes occur unseen. It’s always useful to examine the site of an earlier spot group when it returns at the eastern limb for any clues as to what may have happened during those “missing” weeks. And the biggest groups can make several rotations of the solar disc before fading to only faculae and filaments.

Most surface features of the sun are shaped and maintained by magnetic fields with a range of strengths (in gauss, G): faculae and filaments at ~100G, bright H-alpha plages 1000G, and tiny sunspots around 1800G. Penumbra appears around spots at 2200G.

A still from a movie recorded by the Solar Dynamics Observatory showing a close up of the sunspot group AR11057 on 29 March 2010. Credit NASA/Goddard Space Flight Center Scientific Visualization Studio

AR11057 during its time on the sun’s nearside developed a large penumbra as umbral fields reached 2600G in the (f) spot and 2400G in the (p) spot. The long-lived (f) component was last seen at the west limb April 4th as a complex single penumbra with three umbrae and bright faculae, and a large filament.

On April 20 23:00UT (in white light) a large mass of bright faculae was seen spread over 16º of longitude, well onto the disc, in the sun’s NE region (see diagram below). In H-alpha a large faint prominence detached from the disc hung above the faculae – and a tangle of dark filaments lay near the faculae site – these features were logged using Helio software and their coordinates found.

Two days later the region was examined again. The faculae was no longer visible (WL) but dark filaments near the area, seen in H-alpha, had their coordinates calculated. These coordinates as well as those of the earlier faculae and prominence were all close to the earlier spot group AR11057, suggesting the site had returned, but without any spots (Monty Leventhal had a single small spot at the site on 21st UT that was coincidentally also seen by Nick at Melbourne Observatory a few hours later – a post on the visit to Melbourne Observatory is coming).

A sketch of the area where sunspot group AR11057 had been one solar rotation earlier. Sketch by Harry Roberts

The data for the two days was combined in a single image (see diagram above) showing the region at April 22nd but with the facula and prominence data from the 20th added. As Helio calculates heliocentric latitude and longitude, data from different dates can be related to any given point on the sun’s rotating surface.

The result left no doubt that the site of AR11057 had returned – but the strong sunspot fields (and spots) had gone – the earlier (p) and (f) sunspots are shown as + signs at the sites recorded three weeks earlier (with the spots outlines dotted). Interestingly, the dark filaments seen on 22nd, when plotted, closely match the active region filaments (arf) of the (f) spot mapped earlier (the Fig shows the latter as broken lines with block arrows). While the newer filaments would now be termed quiet region filaments (qrf) we can see they were once active region filaments leading north from the (f) spot – an interesting evolution.

The large faint prominence (of the 20th) closely matches these filaments in location and is assumed to show their true form when seen side-on. The features now at the site of AR11057 are all phenomena of weaker surface fields – the strong spot fields having faded– but they leave no doubt that a large spot group was once located at the site. It also means that one of the largest spot groups of cycle 24 (so far) and with the strongest fields, failed to survive for more than one rotation of the sun’s disc, though its bright faculae may persist for several rotations.

Harry Roberts, an avid solar observer, a frequent contributor to this blog and a member of the Sydney City Skywatchers