April’s talk

Professor Martin Hardcastle of the University of Hertfordshire is one of those people we need to invite back in the future. His talk was about surveying the sky for radio galaxies. In 1933, Karl Jansky’s first radio survey showed the Milky Way and the Sun and that’s all. Since then, we know that galaxies have a wide range of radio profiles.

The first major radio surveys were done at the Cambridge Lord’s Bridge site. The old radio array (do they still call them Yagi?) is where the 3rd and 4th Cambridge Catalogue of radio sources was compiled (the 1st and 2nd were full of spurious readings). You will have heard of the Parkes radio source catalogue, from the radio observatory in Australia. There is also the B2 catalogue of around 10,000 sources from 1960 to 1975. Please google for further information.

Some of the sources were very bright, but visually very dim, so the 3rd Catalogue’s sources took 30 years to record at visual wavelengths; he mentioned one 19.5 magnitude galaxy seen between two bright radio lobes. These lobes had emitted from the nucleus of the galaxy. Now Prof Hardcastle started warming to his task!

It seems all reasonably large galaxies have a black hole in the centre, caused when orbiting stuff gravitates towards the centre over time. And when stuff starts building up gravitationally around it you also get a build up of magnetic fields. And, as it’s rotating, so the particles in the stuff get twisted around. He showed us a computer simulation on auto-replay showing how there is a massive flash then two blobs appear in the polar regions, which then become the lobes over 100million years. Mesmerising stuff. Depending on circumstances there may already be a halo of debris around the galaxy or the jets just spread out into the intergalactic medium. The strong magnetic fields are what makes some of these jets reach relativistic speeds near to the central black hole, i.e., half of the speed of light or even faster. The big elliptical galaxies are most exciting. My notes on M87 say it is an active old mess in the visual but far more active in radio wavelengths and has a huge radio envelope.

As a result of such activity you can imagine there is a wide frequency range needed to monitor all this radio output.

The Chandra (an orbiting x-ray telescope) website shows lots of x-ray and radio sources, and reveals that the envelope is hot gas interacting with the ‘intercluster medium’. Google Cygnus A.

The VLA (Very Large Array = 27 radio dishes which can be laid out in four different layouts for different resolutions) has a range of 75MHz (so a bit shorter wavelength than Radio Oxford at 95.2 MHz) down to 43 GHz. When working to GHz wavelengths its resolution is sub arc seconds. It does have a small field of view, however. It has recorded 100,000 sources and counting, from 1995 onwards. Its radio survey is called FIRST (Faint Images of the Radio Sky at Twenty cm) is a shorter wavelength study (1 to 4GHz). It has 5 arcsec resolution but again only has a small field of view.

More recently and increasing in size and resolution is the LOFAR (LOw Frequency Array) with a range of 30 to 300MHz (Anything less than 30Mhz is absorbed by the ionosphere). This area is mostly unexplored and FM radio sits in there too. These long wavelengths need very long base lines, so they are observed by lots of little antennae, like poles with strings attached. They are camped in fields in large units called Superterps. The HQ is in Groningen, Netherlands, but there are five stations in Germany, with others in France, Sweden, Poland and Ireland. We have one in Chilbolton, Hampshire. The data is coordinated via VLBI (Very Long Base-line Interferometry).

LOFAR is dishing out masses of data, and this is accessible to you (yes, you) via the galaxy zoo website for citizen science.

Another new kid on the block is the SKA, (Square Kilometre Array), which also has pole antennae but also dishes for shorter wavelengths to 14GHz. There are stations in South Africa and Western Australia. I googled SKA and on April 28th the Global HQ is now being built in the grounds of Jodrell Bank.

This data, along with data from Space telescope Herschel, is now creating quite a map of what is in the sky, and is able to separate the active galactic nuclei from those that are star forming, as a result of core activity in the past. Prof Hardcastle hopes we will end up with a Hertsprung-Russell diagram in the radio, with size versus temperature, just as there is with stars.

Jupiter and the Moon get close tomorrow

At 10pm on April 10th, Jupiter comes with about 1.5 degrees of the Moon. This will be a nice binocular or low power telescope view. You’ll see our own Moon and the moons of Jupiter at the same time!

moon and jupiter

The above image doesn’t show all of Jupiter’s moons so here’s a close up:

jupiters moons

Clear skies

Watch out for Mercury

Between now and April 20th Mercury should be visible to the naked eye in the evening twilight. Wait about an hour after sunset and you might catch a glimpse of this small world shinning in the evening sky. At the moment it is very low, only about 7 degrees above the horizon. The picture below shows Mercury at 20:15 BST for the 28/3/2017:

2017-03-28

Tomorrow you can use the Moon to guide you as Mercury will be 8 degrees to the right of a very thin crescent Moon (NB: the Moon is much larger in this picture than it will be in the sky):

2017-03-28 (1)

On April 1st the planet will be at its greatest elongation from the Sun, but it will still be low down in the sky. After that its orbital motion will bring it rapidly closer to the Sun and by April 20th it will be at inferior conjunction and no longer visible to us.

Clear skies.

February’s talk

Dr Helen Walker is a familiar face from RAL, and runs the Satellite Operators’ Group. She gave us an update on planetary exploration.

She also gave us an update on the Moon, and that it was time to be a bit more adventurous and explore more exciting areas, rather than the safe, flat maria that we chose for the Apollo missions. The south pole, with its huge Aitken basin (1800km in diameter) has areas accessible with peaks in permanent sunlight and also ice trapped in its craters. There are proposals of creating a base there, such as an igloo-type affair, created out of bricks printed by a 3-D printer and covered by gravel.

Mercury is unlikely to ever be visited by us. Temperatures are too variable. Its surface composition is not as homogenous as that of the Moon, although it looks Moonlike.

Venus may still have active volcanoes as there are hotspots visible in its sky (according to the Venus Xpress orbiter). The Japanese Akatsuki orbiter (which arrived in 2015) is making infra-red observations of its clouds and notes they are very active at night, with a huge wave moving very slowly across the face of Venus as it rotates slowly. She thinks it is caused as the air moves over mountains underneath.

Mars is her first love. The rover Curiosity has now been there for over four years and is in the middle of Gale crater, which was once a lake and has lots of sedimentary rock. There is a crater near the north pole which has a permanent ice rink. One of the Mariner pictures had one white pixel in it. We now know that was the ice lake (and not a duff pixel?!). There are practice runs of a new Mars rover going on in Utah. Mars Utah Rover Field Investigations = MURFI.

Jupiter and its Galilean satellites have plenty going on for us. The Juno craft will be crashed into Jupiter once its mission was over. There are storms and hurricanes going on right to the pole. Does Callisto have rock and ice in an ocean under its surface? Europa’s insides are kept liquid by tidal stresses caused by Jupiter. We are treading very carefully where exploration of Europa is concerned, so we do not cause any possible contamination.

Likewise Saturn’s big moon Titan. Cassini is in its final year of Saturn exploration.

Uranus and Neptune. These two are so far away from us that they looked featureless from telescopes on Earth, but we now have had so many opportunities to see them that we have been able to see Uranus change through the seasons.

Pluto and Charon were passed by the wonderful New Horizons probe in 2015. Pluto’s icy surface is rock hard.

Comet 67p, which was visited so successfully by Rosetta and little Philae, is seen to consist of two different cometary bodies which came together slowly.

In the final stages of her talk she dismissed the possibility of seeing a planect direcitly, such as when Hubble was believed to have seen a very slow moving planet orbiting Fomalhaut. She postulates an icy ring.

Clear skies.

January’s talk

The Stan Cocking Memorial Lecture: Comets, Ferrets and Nebulae: Charles Messier and 18th Century Astronomy presented by Dr. A Chapman.

For those of you who are interested in the life and achievements of Charles Messier (1730-1817), you can easily do a search on the www, but you know that a talk on the subject by our local luminary, Dr Allan Chapman, is not going to be run of the mill.
When Charles Messier appeared on the scene, comets were still very much mysterious and misunderstood. They were monitored by Tycho Brahe, who had managed to observe some quite spectacular comets and supernovae. He finally had to admit that they were further than the Moon, and this challenged the whole of Greek physics.

Robert Hooke in 1664 observed that the comet had internal structure and produced its own light, so could not be reflected sunlight. But why did its tail get so much brighter near the Sun? Was it a braking effect? Was it disintegrating? Could a comet go behind the Sun and come out from the other side? Johannes Hevelius did lots of drawings of comets and the comet of 1682 was observed all across Europe. Edmond Halley worked out an orbit, which indicated it would come back in 1758. So it did, and this famous fellow that bears his name has been back a few times since.

So, when a 14 year old Charles Messier saw a six-tailed comet in 1744 he became hooked. He was already interested in Astronomy and by the age of 21 he was receiving a privileged education as a student of Joseph Delisle, who supplied all things astronomical to the French Navy and the palace of Versailles. He was based at the Hotel de Cluny in Paris and was lucky to be funded by the French aristocracy, who were happy to fund all his comet hunting as well as other astronomical phenomena, such as transits and other experiments to work out the shape of the Earth. (In the UK there was no public money being spent on astronomy at the time, part from a little at Greenwich.)

So, Messier was employed to look for comets, but was finding himself thwarted when he would be observing some nebula over several days and realised it was not a comet. He began making a list of them in order to save wasting time on them, and started his final list with the Crab nebula in Taurus.
The rest is history. Dr Chapman says Halley published a list of six objects in 1716, but I don’t know which ones they were. Messier never observed any comet on its subsequent return.

If you want to read his own descriptions of the objects in his catalogue, including those of his contemporary Pierre Mechain, they are copied from originals and are in the 1978 observer’s handbook called ‘The Messier Album’, by John H. Mallas and Evered Kreimer. There is also a lengthy description of Messier’s doings, including his very unfortunate fall through an icehouse door in November 1781 and fell 25 feet down onto the ice.

Messier was called the comet ferret by King Louis xv. He discovered 13 comets and 8 are co-discoveries.

Clear skies.