Previously at HAS...

short summaries of our past society meetings


 

Tuesday 2nd May 2017: "Nuclear Astrophysics" - Prof. Alex Murphy

Supernovae are some of the biggest and most energetic explosions in the universe, marking the transition from the star burning its fuel during its lifetime to the star’s dramatic death. Core collapse supernovae are very important as they are probably responsible for all the heavier elements in the universe beyond iron which, along with lighter elements, are spread throughout space during their explosive deaths. These elements then go on to make planets and ultimately, us.

(1)

There have been between seven and eleven supernovae recorded as seen by mankind. They are thought to occur once every 30-50 years in our Galaxy. There may be more that are not seen because they are too far away and/or obscured by dust. We were then shown images of a few supernova remnants, many of which have pulsars in the centre. The Crab Nebula (1) has a pulsar at its centre.

(2)

Supernova 1987A occurred in the Large Magellanic Cloud and was originally a 20 solar mass blue supergiant. This is the nearest supernova since the invention of telescopes. (2)

The Hubble Space Telescope took many photos as 1987A evolved. Material had been thrown off near the end of its life and then, after the explosion, the shock wave slammed into this material and lit it up as the “ring of fire”. (3).

(3)

When the star runs out of hydrogen in the core, it will contract and heat up and will then begin to fuse helium to carbon and oxygen. Once helium has been used up, contraction occurs again with a subsequent rise in temperature and carbon starts fusing to neon and magnesium. This continues with each element being used up faster than the last one. Eventually the core consists of iron with layers of other elements around it like an onion.

Iron cannot be fused as energy needs to be put into the system to achieve its fusion, so the core of the star, now several thousands of kilometres thick, collapses under gravity at a third of the speed of light in about 1/40th of a second, producing as much energy as our Sun in its entire lifetime.

As the density of the material reaches that of nuclei, it can’t collapse any further so the material bounces off the core. The shockwave moves through the outer layers of the star and the enormous heat and energy produced during the explosion creates the heavier elements. The collapse happens so fast that the outer layers of the star don’t know about it until they are thrown out into space by the shock wave.

Material tends to continue falling onto this dense hard core generating heat and radiation, but at the same time the shockwave radiates outwards giving rise to some interesting physics which are so extreme that it is not possible to reproduce them in the laboratory.

How can we make progress? We need a probe that can tell us about the underlying physics thousands of km deep under the surface of the star. The probe must be something we can measure. The probe that can be used is an isotope of Titanium called Ti 44, which is an unstable form and has a half-life of about 60 years, and we can see this isotope in the latter stages of the light curve.

It is one of the nuclei produced in supernova ejecta and requires very high temperatures to make it so it must be produced in the deepest layers near the ‘mass-cut’ that separates the remnant (material that gets stuck inside the star) from the ejecta (material that bounces outwards in the explosion – the shockwave). The Ti 44 is produced in the region of the mass-cut so some gets stuck inside the star and some is thrown out of the star. If there is a more energetic explosion then more Ti 44 gets thrown out and vice versa. Ti 44 also emits gamma rays as it decays which can be detected by space telescopes such as the Integral satellite which replaced the Compton in 2002 then the NUSTAR which was launched in 2012.

The best data we have comes from Cassiopeia A and 1987A and the amount of Ti 44 produced in a supernova is around 1.4 - 1.5 x 10 –4 solar masses. NUSTAR can also image supernovae remnants. This is Cassiopeia A (4) – the blue shows radioactive Ti 44 and can give a more direct look at the heart of the explosion. The green shows silicon and magnesium in X-rays as seen by Chandra. It was thought the blue and green should have been correlated but they are not – this was a surprise.

(4)

The amount of Ti 44 measured in observations has shown much more Ti 44 than the best models predict. To find out why there is this difference, it is important to measure the amount of Ti 44 destroyed during the supernova explosion by looking at its decay product of Vanadium 45 after it has fused with an alpha particle.

To do this, they had to go to the Paul Scherrer Institute in Switzerland for radioactive waste from particle accelerators, which contained enough Ti 44, which after extraction enabled them to run an experiment in the Isolde accelerator at Cern. They fired a beam of Ti 44 into a helium filled gas chamber to recreate the explosive energy of a supernova to see how quickly Ti 44 is destroyed. They didn’t actually find anything to measure, but it still proved very interesting because the limit that they put on this is much smaller than the rate was thought to be – less Ti 44 was destroyed, which means more was left over.

The result therefore is in line with what the observations show and brings it into closer agreement with the models once the new figures have been taken into account. Prof Murphy says this is a convoluted way of saying that a non-measurement in a very complicated scenario helps to explain the observations that are seen in astrophysics.

It is still a mystery as to why core collapse supernovae occur; why do they explode? A chap from Aberdeen who works in quantum gravity has a theory, which suggests that in very dense environments (as in the beginning of the universe) a new particle might exist which undergoes a phase transition of potential energy that releases energy. Prof Murphy also wondered if the theory could also apply to supernovae, which are also dense environments. A new particle, like another kind of Higgs Boson, could solve inflation, provide a late universe explanation of dark energy and provide an extra energy boost for incredibly dense environments like supernovae.

LIGO will be releasing new information in two to three weeks time which will include five or ten more observations of gravitational waves, one of which quite possibly could be from a core collapse supernova. Thank you Prof Murphy for a stimulating and challenging talk about your work with Ti 44 and its importance in helping to understand why core collapse supernovae explode.

 


 

Tuesday 4th April 2017: "Burning Questions: understanding exoplanets by studying the hottest worlds" - Dr. Eric Lopez

The first extrasolar planet to be discovered was 51 Pegasi-b in 1995; since then we have found thousands (1) most of which have been discovered by the Kepler mission which was launched in 2009. Kepler is a space telescope with a mirror just less than 1m across which studies 100 sq degrees of the sky off the galactic plane in the summer triangle between Cygnus and Lyra. Its mission is to discover hundreds of Earth-size and smaller planets in or near the habitable zone and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.

(1)

The extrasolar planets are found by the transit method (2) which involves looking for a tiny reduction of light as a planet passes in front of its star. Kepler spent three and a half years of its primary mission staring at about 150,000 stars monitoring them continuously.

(2)

There appear to be a lot of planets small enough to be rocky, however, most are Neptune sized and there are relatively few ‘hot Jupiters’ close to their star. It is thought that about one and a half times the size of the Earth is the point where a planet would transition from rocky to non-rocky, and the non-rocky maybe water worlds or perhaps more like Neptune - an ice giant with a gaseous atmosphere.

The transit method can give the size of a planet whereas the radial method, also called Doppler shift (3), can give the mass of a planet. The radial method is how the first extrasolar planets were found. It is particularly useful to combine these two methods because when the size and the mass are known it is possible to get an idea of the composition. The instrument used to determine the precise measurement of planet mass when using the radial velocity method, is called HARPS-N. Many of the rocky planets we are finding seem to have abundances of rock and iron similar to Earth, i.e. 30% iron.

(3)

Most extrasolar planets are found close to their star, and planets close to their star orbit very quickly. A planet with an orbit of one day would be so close that the star’s radiation would be incredibly fierce, and any atmosphere around the planet would be stripped off. This is called photo-evaporation and is perhaps what happened to Venus early in its history.

(4)

(5)


 

If an extrasolar planet blocks a lot of starlight this can be due to escaping gas; GJ 436b (4) has been found to have an extended tail of atmosphere. Kepler 1520b is on an eight-hour orbit and can be likened to a disintegrating Mercury gradually spiralling into its star. The light curve (5) shows the difference between a normal transit (red line) and the transit of a planet that is disintegrating and material is escaping.

Studying these hot planets will help us to understand how planets form and evolve in general and eventually we should be able to work out how the planets in our solar system formed. (6)

(6)

Earth-like planets need to be in a habitable zone so that liquid water can exist on their surface. It is difficult to detect Earth-size planets around Sun-like stars as they orbit so far out. Some of the most exciting discoveries last year occurred when using the radial velocity method. The first concerned a planet orbiting Proxima Centauri in 11 days, with a similar mass to Earth and it was in the middle of the habitable zone. Proxima Centauri is a red dwarf star only 4.2 light years away, a star not much bigger than Jupiter with just 12% the mass of our Sun.

Another extraordinary discovery is the Trappist system that has seven planets around a low mass red dwarf star as close to their star as the Galilean moons are to Jupiter. Three of these planets are within the habitable zone and may even be rocky. The new James-Webb Telescope should help us to understand these planets. It has such sensitivity and spectral resolution that it should be possible to decide, not only if an extrasolar planet has an atmosphere, but also the composition of this atmosphere, and this is what makes this mission exciting. It’s still on course for launch in 2018...

 


 

Tuesday 7th February 2017: "Recent Curiosities on Mars" - Pauline Macrae (HAS)

Mariner 4 took the first close up photographs and these showed a disappointingly dry and dusty planet until Mariner 9 and the Viking Orbiters went into orbit around Mars and discovered a world of giant volcanoes, an immense canyon system and indications that Mars was once wet with possible rivers.

 

The other landers have confirmed that Mars was once wet, so Curiosity was to go a step further and look for places where micro-organisms could have lived.

(1)

Curiosity (1) is a big lander and carries a suite of ten instruments mostly designed to determine the minerals of the rocks and soils – after all Curiosity is taking the place of a geologist. There are also cameras to take photographs. The landing took place in August 2012 after a very complicated set of procedures to set it down on the surface.

 

Gale crater (2) was chosen because it contained a layered mountain, called Mount Sharp (formally known as Areolis Mons). The Mars Reconnaissance Orbiter satellite had determined that the lower layers had been made of clay minerals, haematite and sulphate minerals, which have all been either laid down by water or altered by water in some way. If searching for habitable environments it’s important to choose somewhere that was once wet to give microorganisms the best chance of survival.

(2)

Close to its landing site was an alluvial fan – a deposit of water transported materials – and possible clay minerals. A good place to start exploring, and very quickly Curiosity came across an ancient streambed (3) with sedimentary conglomerate rock. The pebbles cemented in sandy material were too big to have been transported by wind, so must have been moved by water moving at walking pace, for perhaps a few kilometres starting beyond the rim of the crater, and across the floor of the crater in the form of streams or rivers ankle to hip deep.

Further on, Curiosity drove over mudstone, confirmed by taking a sample, which proved to be clay of a type that on Earth is common in lake deposits and found in biologically rich environments. They also found organic molecules, chemicals that are required by life, and are an energy source similar to the one used by rock eating microbes on Earth. The clay was made from eroded basaltic rock (basalt is a volcanic rock which is the most common rock found on Mars) and it is believed that the water would have been fresh water – not the acidic water that may have been in the area where Opportunity is exploring. Mineral veins in the rocks showed that groundwater would have been circulating with a different chemical mix.

(3)

Curiosity drove along the edge of Mount Sharp through some interesting terrain (4). It first explored an area that has been called Glenelg, then came across hills of sandstone, which are called buttes and mesas. At one point it found itself looking at what was most probably a lakebed with sandstone layers at the edge where rivers deposited their load of sediment, and the finer clays would have then gradually fallen to the bottom of the lake to form mudstone.

(4)

The many layers of sand are astonishing (5). They are capped by erosion resistant sandstone that was the same material Curiosity found itself driving over and which necessitated checking its wheels for damage.

(5)

In summary, it is thought that Gale crater was formed around 3.8 billion years ago and the impact cracked the basement rock allowing ground water carrying various chemicals to circulate. Rivers brought sediment from beyond the rim of the crater and gradually began to erode the north rim and wall of the crater. Water poured down the wall bringing in pebbles, sand and fine clay which was deposited in a lake. The lake level rose and fell numerous times and sediment gradually built up at the edge of the lake. During a drier time, sandy material was brought into the crater to form sand dunes around Mount Sharp, and these were subsequently buried by more sandy, dusty material squashing the sediments present to form the conglomerate, sandstone mudstone and petrified dunes. Water circulated within cracks formed in the sediments driving chemical reactions, dissolving some minerals and precipitating others.

Curiosity is still to reach the haematite ridge, clay-rich bedrock and sulphate rich hills beyond. The light rock in this photo is probably sandstone that has not been altered by water. We have been exploring Gale Crater with Curiosity to look for habitable conditions and have found them. Hopefully our curiosity will eventually enable us to find life on a world other than our own.

This was a really interesting talk to research and I find it fascinating just how much our knowledge of Mars has grown in the last 40 years, when the first landers, Vikings 1 and 2, reached the surface. Perhaps one day, we will actually find fossilised remains of microorganisms.

 


 

Tuesday 10th January 2017: "Ephemeral Bubbles" - Paul Jenkins (HAS)

The name planetary nebula (1) was coined by William Herschel because of the resemblance to the planetary discs of Uranus and Neptune but actually they have nothing to do with planets. They occur when a main sequence star the mass of our Sun and up to eight times as massive, dies. Any larger and the star will explode as a supernova. Since most stars are within the range of one to eight times the mass of our Sun, most stars will become planetary nebulae, including our Sun.

In the lower mass stars helium burning can happen very suddenly and a gigantic, Sun-wide thermonuclear explosion known as the helium flash can occur. Stars with a mass greater than two and a quarter times more massive than our Sun do not experience helium flashes. The outer layers of the red giant are pushed away at speeds that take them beyond the reach of the gravity of the core (which is becoming a white dwarf) resulting in a planetary nebula. These nebulae are made of extremely rarefied gasses.

The white dwarf in the centre is tremendously hot and produces a lot of UV radiation, which ionises the gases in the nebula causing them to glow. The UV radiation emitted by the white dwarf is intercepted by atoms in the nebula and converted to visible line radiation.

They also produce radio, infrared and sub-millimetre radiation as well as optical and, because the gas can reach temperatures of a million degrees, they can also radiate in the X-ray part of the spectrum. Planetary nebulae come in many forms, but the straightforward ones tend to be spherical. This means we should see a ring. M57 is the well-known Ring Nebula (The Hubble photograph 2) shows the white dwarf in the centre.

Planetary nebulae often exhibit layers that can indicate more than one expulsion event such as seen in the Cat’s Eye Nebula in Draco (3). 10% of the original mass of a star is ejected so the white dwarf is smaller not just because of its contraction under gravity but also because there is less mass.

The expansion rates of the nebulae are generally 20 to 50 km per second, the temperatures are 10,000°C and the gas consists of several hundred to a million ionised atoms per cubic cm – thinner than any vacuum we can achieve on Earth. They are typically one light year across and there are around 3,000 planetary nebulae catalogued in our Galaxy but due to galactic dust, which tends to get in the way, there may be ten times that number. We can only see them for a short time – about 10,000 years, hence ephemeral bubbles and why we are privileged to be able to see the ones that are visible at the moment.

White dwarf stars are amongst some of the hottest stars in the universe with a temperature range of 25.000 to over 200,000°C. They are very luminous and radiate primarily in the UV. One of the effects of high temperatures in the core is that electrons in the hydrogen and helium separate from their atoms and become a gas in their own right – an effect called electron degeneracy.

The electrons in a degenerate state do not behave as we would expect. As the temperature of the gas is increased, instead of expanding and cooling, the pressure is unaffected. In a star, this means that when helium burning begins it spreads very rapidly throughout the core, this is unstable and results in an explosive release of energy that is known as the helium flash.

Looking at our solar system 5 billion years into the future, we will see a red giant star, no planets within the orbit of Mars and perhaps the start of the ionised outer layers of our Sun that have already been ejected – the start of the planetary nebula. In time, the nebula will have moved so far away it can no longer be lit up by the white dwarf and over billions of years the white dwarf will cool down until it becomes a black cold object that will probably be impossible to find.

Paul rounded off his talk with planetary nebulae that are local to us. He has viewed NGC 2438, which is a foreground object to M46 near Sirius, and although faint through the Society telescope, it is quite magnificent with the Hubble. (4). Others include M97, the Owl nebula, NGC 1535, Cleopatra’s Eye, NGC 2022, and NGC 2392, the Eskimo Nebula. M27 and M57 are easy to find but of course, they only last 10,000 years so you had better hurry up and start viewing them.

 


 

Tuesday 1st November 2016: "To Orbit and Back, Again and Again" - Arthur Milnes (HAS)

 

 

 

 

 

 

 

Arthur’s talk was about the Space Shuttle, the only reusable spaceship capable of taking human beings and cargo into near space, orbiting the Earth before returning safely, and then doing it all over again.

How did the Shuttle come to be built and prove to be so successful?

The space race began when Russia beat the Americans to the launch of the first satellite – Sputnik in 1957. NASA was born the year after. The Mercury and Gemini missions paved the way to developing the giant Saturn V (see above) and the technology to launch its starting weight of 2,950 tons into Earth’s atmosphere and to accelerate 45 tons of that rocket to a speed of 24,000 miles an hour to escape Earth’s gravitation pull. It stood 363 feet tall and had five huge engines generating a total of 3,750 tons of thrust.

Columbia was the first Shuttle, and it flew from Florida on the 12th April 1981. The whole assembly weighed 2,200 tons and stood 184 feet above the launch platform. The largest section of the tank was the hydrogen container holding 115 tons of liquid hydrogen; the smaller section of the tank contained 692 tons of liquid oxygen. The tank was jettisoned after the fuel had been used and burnt up during re-entry, the only non-reusable component of the Shuttle.

The Solid Rocket Boosters are unstoppable once lit. They will burn for two minutes providing 70% of the thrust required to propel the Shuttle to 24 miles above the Earth. The boosters were built from an incredibly strong steel tube in four main sections and when put together with 177 one inch steel bolts, measured 149 feet in length and 12 feet in diameter. The bolts had to withstand pressures of between 13 and 17 million pounds trying to blow the joint apart.

These boosters could control the direction of their exhaust blast, allowing them to rotate the Shuttle onto its back to take up orbit trajectory. After two minutes and at a height of about 24 miles, the boosters separated from the Shuttle continuing upwards on a ballistic path to 36 miles before falling into the Atlantic to be recovered, refurbished and reused.

Four Shuttles were built initially: Columbia, Challenger, Discovery, Atlantis, all named after famous American ships. After Challenger was lost, Endeavour was built as a replacement.

They were 122 feet long, had a wingspan of 78 feet and weighed 110 tons. They were designed to each fly 100 missions, but none did. The missions were designated STS followed by a number - STS stood for Space Transport System. Each one flew a distance equal to a flight from Earth to Mars. Even with the huge push of the solid rocket boosters, more thrust was required to take the Shuttle to a speed of 17,500 mph so it could escape Earth’s gravity.

The extra push was provided by the Shuttle’s main engines, which consisted of three large engines each producing a thrust of 156 tons that was controllable, vitally important for a safe flight. They could be throttled back to about 65% of power as the Shuttle accelerated through the sound barrier to reduce structural stresses from shock waves, and then could be throttled back up to 104% as the Shuttle climbed into thinner air. The Shuttle carried 46 smaller engines to help it to manoeuvre in space and glide back to Earth. There were two orbital manoeuvering system rockets, 16 smaller reaction control system rockets at the front and 28 of them at the rear. The 44 reaction control system engines were fired in short bursts to manoeuvre the Shuttle to successfully dock with the ISS.

The two orbital manoeuvering system engines were each capable of 6,000 pounds of thrust and were used to achieve final orbit and when boosting the ISS to a higher orbit. They also came into play to slow the Shuttle down so it could begin its descent. It had been flying upside down so the thrusters turned it 180 degrees so its 20,000 thermal protective system tiles covering the Shuttle met the atmosphere first to prevent it from burning up during re-entry.

There were disasters and some near disasters with plenty of heart stopping moments in the story of the Space Shuttle, but to complete the tale Arthur thought he would probably need another 50 minutes and, if we were willing, he would give us a further insight into this remarkable vehicle at a later date.

 


 

Tuesday 4th October 2016: "Is the Pale Blue Dot Special?" - Dr. Duncan Forgan

Duncan, a research fellow at the University of St. Andrews and founding member of SETI, began by asking if our planet is ordinary or unique, and wondered if Earth could be the only inhabited planet and the only place with intelligent life.

If there are alien societies out there, why haven’t we encountered them, considering it should be possible for them to have spread across our Galaxy within ten million years?

Drake’s equation suggests there are many planets around many stars that could support life, so there should be intelligent civilisations out there.

When looking for life elsewhere, a good place to start is to study known extrasolar planets – planets orbiting other stars.

The first was discovered over 20 years ago by the Doppler shift (radial velocity), when scientists look for a tiny wobble of a star caused by an orbiting planet.

There are now various other ways to detect extrasolar planets, including the transit method used by the Kepler spacecraft, which looks for a tiny dip in the light of a star caused by an orbiting planet. This method can also detect an atmosphere – if it exists.

As of the 1st October 2016, 3,532 confirmed exoplanets have been found orbiting 2,649 stars. Some of these are believed to be rocky, but we need to be able to sort out the Venus’s from the Earths – these solar system planets are the same size and each have an atmosphere, but only one is habitable.

Numerous new ways of searching for alien life will be possible in the future using various telescopes on Earth and in space. At the moment we can use radio telescopes to search for radio signals from space, including the large telescope at Aricebo which SETI uses.

Other ideas to look for intelligent civilisations include searching for laser light – one powerful enough could be seen from Earth. We could use infrared light to look for giant structures; there is one exoplanet with what appears to be debris around it and one thought suggests we are actually looking at alien structures.

However, at the moment, our solar system is not really represented anywhere as we are, as yet, unable to detect small planets. The only true Earth-like planet that we know is Earth itself. Duncan finished by saying the search for another pale blue dot is actually the search for ourselves.Duncan began by asking if our planet is ordinary or unique, and wondered if Earth could be the only inhabited planet and the only place with intelligent life.

If there are alien societies out there, why haven’t we encountered them, considering it should be possible for them to have spread across our Galaxy within ten million years?

Drake’s equation suggests there are many planets around many stars that could support life, so there should be intelligent civilisations out there.

When looking for life elsewhere, a good place to start is to study known extrasolar planets – planets orbiting other stars.

The first was discovered over 20 years ago by the Doppler shift (radial velocity), when scientists look for a tiny wobble of a star caused by an orbiting planet.

There are now various other ways to detect extrasolar planets, including the transit method used by the Kepler spacecraft, which looks for a tiny dip in the light of a star caused by an orbiting planet. This method can also detect an atmosphere – if it exists.

As of the 1st October 2016, 3,532 confirmed exoplanets have been found orbiting 2,649 stars. Some of these are believed to be rocky, but we need to be able to sort out the Venus’s from the Earths – these solar system planets are the same size and each have an atmosphere, but only one is habitable.

Numerous new ways of searching for alien life will be possible in the future using various telescopes on Earth and in space. At the moment we can use radio telescopes to search for radio signals from space, including the large telescope at Aricebo which SETI uses.

Other ideas to look for intelligent civilisations include searching for laser light – one powerful enough could be seen from Earth. We could use infrared light to look for giant structures; there is one exoplanet with what appears to be debris around it and one thought suggests we are actually looking at alien structures.

However, at the moment, our solar system is not really represented anywhere as we are, as yet, unable to detect small planets. The only true Earth-like planet that we know is Earth itself. Duncan finished by saying the search for another pale blue dot is actually the search for ourselves.

 


 

Tuesday 6th September 2016: "Aurora, in search of the Northern Lights" - Dr. Melanie Windridge

Melanie combined her love of exploration with studying the aurora and her book is about her travels through the auroral zone, meeting the people who live in there, listening to their experiences as well as discovering the science behind the aurora.

  • The aurora is an incredible, beautiful light show to watch, but it is also so much more than that – the aurora is a plasma. Plasma is the 4th state of matter – an ionised gas made up of charged particles.
  • The Earth’s magnetic field is like a bar magnet, and it protects us from the solar wind, which is also made up of charged particles. The Sun is very dynamic and will sometimes hurl out huge lumps of ionised gas outwards with masses of up to a billion tons! On occasions these Coronal Mass Ejections are directed towards the Earth.
  • Charged particles do find their way into the Earth’s magnetosphere on the day side of the Earth, but in order for us to see the aurora at night they must be coming from behind the Earth. Somehow the particles are being accelerated into the night side of our atmosphere.
  • Magnetic field lines cannot cross each other. The Sun’s field and the Earth’s field can connect (Sun’s field with the Earth’s field), but in so doing, they provide an opening for charged particles to enter – like a door. Then the field lines are pushed around to the back until the Earth’s field lines reconnect in the magnetotail, which is now full of charged particles. This closes the door.

 

JSL ObservatoryNext Observing Session

Solar Saturday
24th June
14:00-16:00

STATUS: Cancelled due to cloud and high winds at the observatory.

Come and observe the Sun safely with our specialist solar telescope! All welcome.