After dusk the constellation of Orion the Hunter can be seen low in the eastern sky. As seen from the southern hemisphere, Orion is upside down. Rigel, the bright star at the top of Orion, represents Orion’s left foot. Betelgeuse, the bright reddish star at the bottom represents Orion’s right armpit.
The constellation of Orion with the correct orientation. Drawing by Johannes Hevelius in his 1690 star atlas
Early on Saturday and Sunday mornings we can view the Geminid Meteor Shower, one of the year’s best. The meteors or streaks of light can appear all over the sky, but those associated with the shower can all be traced back to a point in the north near the constellation of Gemini, the Twins.
A finding chart for the origin of the Geminids at about 1 am Saturday and Sunday mornings. Chart Nick Lomb
Some stars live longer than other ones. It all depends on their mass. Hot massive stars use up their fuel very quickly and die spectacularly after only a few million years. In contrast a cooler, less massive star like our own Sun is expected to keep shining for 10 thousand million years.
The Sun on 28 August 2012. Photo Nick Lomb
Pioneering female astronomer Annie Jump Cannon was born on this day in 1863 at Dover, Delaware in the United States. Working at Harvard College Observatory she classified almost 400,000 stars by their spectra and sorted them into spectral classes in a scheme of her own devising. Astronomers today still use her classification scheme.
A 1922 portrait of Annie Jump Cannon. Courtesy Wikimedia Commons
Flares stronger than class X1 during the current solar cycle plotted as bars. Some have been shifted a little to avoid overlapping. Sketch and copyright Harry Roberts ©, all rights reserved
Witnessing a ‘great’ solar flare is one of astronomy’s rare ‘treats’: they are uncommon at any time – and seeing one is a revelation!
If we define ‘great’ as any flare stronger than GOES X-ray class X1, there have been (as I write) only twenty-seven since solar cycle 24 (SC24) began five years ago.
Flare classes: There are five classes in the GOES system: they are A, B, C, M and X (for ‘extreme’?), each divided into steps 1 through 9, and class X describes any flare whose x-ray flux exceeds 10^-4 watts/m^2 at the satellite’s detector – the other classes are powers of ten weaker.
Flares are so hot they emit mostly in the x-ray band, and that flux is a good measure of total flare energy. In H-alpha they appear as irregular areas that brighten in a minute or so to at least twice the background brightness – often many times brighter. Most occur in sunspot groups.
Spot complexity. Class X flares occur in sunspot groups that have both strong magnetic fields and magnetic ‘complexity’. This ‘complexity’ may arise in various ways, usually when a new sunspot pair (or two) emerges within an existing group – particularly if one pair has its polarity reversed from normal. Field ‘helicity’, or degree of ‘twist’, plays a big role too – and the rearrangement or ‘reconnection’ of complex fields is what drives all flares.
These precursor events are uncommon, so ‘great’ flares occur only in brief periods during any solar cycle.
SC24 events. In SC24 the 27 X-class flares (so far) are ‘clumped’ into just a few short bursts of activity. Let’s consider events, year by year, since X-class flaring began in 2011. The graph, Fig 1, shows the SC24 flares >X1 plotted as bars, ‘shifted’ a bit to avoid overlapping.
2011. That year saw the first X-class flares of the new cycle (SC24) in February/March, with one each in AR11158 and 11166. Later that year, in August/September, we had a cluster of six X-class flares: one, an X6.9 in AR11263, was the strongest so far, with three lesser ones in the same group.
2011 had eight ‘great’ flares: a good start to SC24 flaring and hinting at stronger ones to come.
2012. Despite 2011’s strong start there were just seven class-X flares in 2012 though one, an X5.4 in AR11429, was second strongest of SC24 so far – with two lesser ones in that group also. The other four of 2012 were all
2013. Now most of the way through 2013 we have, so far, had 12 class-X flares: a big rise over earlier years.
Post flare loops associated with sunspot group AR11748. Sketch and copyright Harry Roberts ©, all rights reserved
The first four were in AR11748 (mid-May), when the writer managed to log an X3.2 (strongest of 2013). Fig2 shows post flare loops above this sparse group, with its odd mix of fields. It had three more class-X flares in the range X1.2 to 2.8 over just three days.
The next most active spot group to host ‘great’ flares in 2013 was AR11890 that unleashed an X3.3, third strongest for the cycle and two X1.1 flares in November. The other ‘great’ flares of 2013 were the product of three separate spot groups. As the plot at the top shows activity was ‘clustered’ in May and in Oct/Nov.
November’s flares were mostly in southern spot groups – suggesting the southern hemisphere is playing ‘catch-up’ to northern activity that dominated in 2011 and 2012.
Trend. So far in SC24 the strongest flares were all in northern sunspots with an X6.9 and X5.4, while southern groups are yet to host a flare stronger than X3.3. No upward trend in the power of class-X flares is, as yet, seen (Fig1) – but with few such events, caution is needed: history suggests a rise in coming months.
Perhaps the last days of 2013 will unleash a ‘storm’ – not of bushfires we hope – but of X-class flares, under Australian summer skies. Remember, class-X flares are rare and always unexpected!
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 variable star Mira is high in the northern sky. It is a red giant star with a bloated atmosphere that is 200 times the width of our Sun. When the star is at its faintest its temperature is only (!) 2000 degrees Celsius. At those times Mira is one of the coolest stars known.
A Hubble Space Telescope close up of MIRA. Courtesy Margarita Karovska (Harvard-Smithsonian Center for Astrophysics) and NASA
In the early evening the star Mira, “the wonderful”, is high in north in the constellation of Cetus the Whale. Mira’s brightness varies over a cycle of about 11 months. At its brightest it is easily visible to the unaided eye while at its dimmest it is far too faint be seen even from a dark spot.
Mira was relatively bright on 3 October 2011. Photo Nick Lomb
On this day in 1908 the first large reflecting telescope since Melbourne’s Great Melbourne Telescope received its 60-inch or 1.5-metre wide mirror at Mt Wilson outside Los Angeles. The next evening was “first light” for the telescope, which astronomers still use to observe the activity of Sun-like stars.
The dome of the 60-inch (1.5-metre) telescope at Mt Wilson. Photo Nick Lomb
In the early evening we see Canopus the brightest star of Carina the Keel high in the southern sky. To its right or west is Achernar the brightest star of Eridanus the River. Further to the right, almost due west, is Fomalhaut the brightest star in Piscis Austrinus the Southern Fish.
Stars in the southern sky on 6 December. Chart Nick Lomb
Where to look for Nova Cen 2013 from Sydney at about 11:30 pm on 5 December 2013. Finding chart Nick Lomb
Australian amateur astronomer John Seach who lives on Chatsworth Island, which is between Grafton and Lismore in northern NSW, has discovered a star explosion known as a nova near one of the pointers to the Southern Cross. At last report Nova Cen 2013 was bright enough to be visible to the unaided eye from a dark country site and may still be getting brighter. From light polluted cities binoculars will be needed to see the nova.
The nova is easy to find but, unfortunately, this is a season of the year when the Southern Cross and the pointers are low in the sky after dusk. Hence the finding chart above is drawn for 11:30 pm and the nova will be higher in the sky as the night progresses.
Photography may also be possible. Put a digital camera on a tripod or other support, point it at the right part of the sky, turn off the flash, set a long exposure and then use the self-timer to to start the exposure. Hopefully, the nova can be found on the image by comparing with the finding chart.
There are different types of novae, but this one is believed to be the type when a lot of material has been dumped on a dead white dwarf star by a companion star. When enough material builds up a nuclear explosion takes place on the surface of the white dwarf. Generally, a nova likes this only brightens for a few days and then fades away.
Have look if you can and let us know if you had found the nova and seen an explosion on a dead star!