Sounds of Space

Over the years, I’ve been involved in a lot of projects featuring sounds from space: Wonder with the composer Alan Williams and the BBC Philharmonic Orchestra, the art installations Pulsar Polyphon and Silsila, even collaborations with Sigur Ros and New Order.

This week’s edition of the Sky at Night is all about the Sounds of the Universe. The team came to film at Jodrell Bank and Maggie Aderin-Pocock, Chris Lintott and I discussed a few space sounds: the rhythmic thud of pulsars, the cacophony of the Jovian Chorus, the sound of Voyager 1 leaving the heliosphere, and the sound of an exploding star.

Before I describe the Sky at Night sounds in a bit more detail, it’s important to emphasise (as we explained a few years back in an episode of our Jodcast) that most of what you hear in these space noises was not originally a sound.

In space no one can hear you scream

The tagline from Ridley Scott’s Alien (1979) captured the movie’s atmosphere of claustrophobic terror. It also referenced the generally-held view that space is a vacuum and, since sound waves need a bunch of molecules to bang into each other in order to travel, clearly sound cannot travel in space.

Not quite true. Space is not a vacuum, although admittedly it’s pretty close. There is stuff between the stars, just not very much of it. The density of matter in space varies over a wide range but a typical density is about 1 hydrogen atom per cubic centimetre i.e. about 0.0000000000000000000000017 grams of matter in each little cubic box one centimetre on a side (that’s 1.7×10^-24). By comparison, the typical density of air at sea level is 0.0012 grams per cubic centimetre, about one thousand million million million times higher!

Achieving extremely high vacuums is difficult. For example, in the Large Hadron Collider ATLAS detector, the extremely low gas densities are equivalent to about 200,000 hydrogen molecules per cubic centimetre i.e. about 400,000x denser than interstellar space, but still a thousand million million times less dense than the air at sea level!

So, sound waves can travel in space. There are even sonic booms (shock waves) from stellar explosions. But the densities and pressures are so low they would not be directly audible. So many sounds from space are modified in some way e.g. their frequencies are brought into the range of human hearing, or they are “sonifications” of data i.e. scientific data turned into a sound in some way.

The sound of pulsars

Pulsars were discovered in 1967 by Jocelyn Bell-Burnell as regularly flashing sources of extra-terrestrial radio waves. In fact, the flashes were so regular that they seemed artificial, leading the team to dub the first pulsar LGM-1 – Little Green Man 1. However, it was soon realised that rather than messages from aliens, the pulsars were something astrophysical but also rather exotic – neutron stars, the spinning, collapsed cores of exploded stars.

These neutron stars weigh about one and a half times as much as the Sun but are only about the size of a city, twenty kilometres across. They spin as fast as hundreds of times a second and shine beams of radio waves from their magnetic poles. These beams sweep across our line of sight producing an effect like a cosmic lighthouse.

Measuring the brightness of the radio waves reveals a peak each time the beam sweeps past. These peaks can vary in strength and structure but they arrive with extreme regularity. So much so, that over many periods the spin period of the neutron star can be measured to extreme accuracy . For example that of Pulsar B1937+21 is 0.001557806468819794 +/- 0.000000000000000200 seconds. The behaviour of pulsars as very stable natural clocks enables their use in many areas of astrophysics, in particular in tests of Einstein’s theory of gravity, General Relativity.

Rather than simply look at graphs of the radio power against time, we can feed the digitised signal into a sound card and listen to the sound of a pulsar. What we hear is the regular thud of the pulsar flash superimposed on the background hiss or static made up of receiver noise plus astrophysical background (emission from the interstellar medium plus cosmic microwave background). But remember this wasn’t a sound originally, it’s the brightness of a radio signal turned into a sound.

For the Sky at Night the pulsar research group at Jodrell Bank developed a way of listening live to data from the Lovell Telescope. We used to have an analogue sound system at Jodrell Bank that hasn’t been in use for some years. However one of our academic staff, Patrick Weltevrede had some software he’d written previously to take a stream of data and feed it out through a sound card. Working with Sam Bates and Cees Bassa, Patrick got the system up and running streaming data from one of the pulsar backend computers. We tried it for the first time on the Lovell whilst recording the piece for the Sky at Night. It worked perfectly!

Patrick, Sam and Cees have since recorded several more pulsars – you can hear them and read more about them at the updated Jodrell Sounds of Pulsars webpage.

You can also read more about the project to use pulsars to detect gravitational waves on the European Pulsar Timing Array website.

The Jovian Chorus

There is a strong interaction between an electromagnetic wave and the charged particles in a plasma (particularly the electrons as they are least massive). Waves triggered by lightning travelling along the Earth’s magnetic field give rise to radio emissions known as “whistlers” because of their characteristic sound when played through a speaker.  The individual chirps sound a little like the tweeting of birds.

For the Sky at Night, we played a recording of bursts of these whistler-mode waves in the magnetosphere of the planet Jupiter. Known as the Jovian Chorus, these sounds were recorded by Professor Don Gurnett of the University of Iowa group using instruments on board Voyager 1 as it flew by Jupiter.

Voyager 1: Our first interstellar messenger

The rate of low-energy cosmic rays (>0.5 Mev/nuc) as detcetd by Voyarger 1 during the year 2012.

The rate of low-energy cosmic rays (>0.5 MeV/nuc) as detected by Voyager 1 during the year 2012.

On August 25th 2012, the Voyager 1 spacecraft left the region dominated by the influence of the Sun, the so-called heliosphere, and entered interstellar space. You may have heard sonifications from its plasma wave instruments (the same detectors used to produce the recordings of the Jovian chorus), but its exit from the heliosphere was also clearly shown by data from Voyager’s cosmic ray detectors.

For the Sky at Night, I took Voyager’s measurement of the rate of impact of low energy cosmic rays (charged particles which originate in the Sun and flow out in the solar wind) and turned them into a sound. By simply converting the number of particles per second into the pitch of a note in cycles per second, it’s possible to sonify the data. The point at which the spacecraft enters interstellar space is marked by, first of all, a wavering in the pitch, and then a sudden drop to a lower note as the number of low energy cosmic rays drops suddenly.

X-rays from an exploding star

For many years I’ve studied stellar explosions, in particular the novae in which an explosion takes place on a white dwarf in a binary star system.  Gas from the companion star falls onto the white dwarf and builds up on its surface. Eventually a critical pressure is reached at the base of this accreted layer of gas. Thermonuclear fusion reactions begin and rapidly runaway into an explosion which blasts material off the white dwarf, causing the star to suddenly brighten. Once a nova outburst is spotted we observe its progress with telescopes working right across the spectrum from radio waves to gamma rays.

On February 6th 2014 a star called V745 Sco was seen in outburst by Rod Stubbings in Australia. Telescopes around the world swung into action, as did a space observatory called Swift. Designed primarily to observe much more massive explosions called gamma-ray bursts, Swift also observes the X-ray emission resulting from nova explosions.

Since the white dwarf is not destroyed in the explosion, the whole process can repeat and when more than one outburst has been seen we call it a recurrent nova. V745 Sco has now been seen to explode in 1937, 1989 and 2014.

V745 Sco is also a member of a class of novae in which the companion to the white dwarf is a red giant star. This makes the aftermath of the explosion particularly interesting as the material ejected from the white dwarf slams into the dense wind form the red giant and sets up a system of shock waves. These shocks heat the gas to millions of degrees which then glows brightly in the X-ray part of the spectrum. Sure enough, V745 Sco was detected by Swift as a high-energy X-ray source within just a few hours of the nova being discovered.

The first two weeks of X-ray emission from V745 Sco as detected by Swift.

The first two weeks of X-ray emission from V745 Sco as detected by Swift.

However, as the shock expands, we see through to the hot white dwarf where hydrogen fusion still proceeds in the aftermath of the explosion. This is itself a brighter source of X-rays but of rather lower energy. This so-called “super-soft” X-ray source appeared just 3 or 4 days later. This component brightens, dominating the “harder” shock emission, before fading away a week or so later.

I thought this X-ray emission might lend itself to being turned into sound. Kim Page of the University of Leicester works on the Swift observations of novae and she provided me with the latest data on V745 Sco. I took the ratio of the mean X-ray energies of the soft and hard bands of the Swift data and used this as the ratio of the pitches of two notes. The high note representing the high energy hard X-rays and the low note the low energy soft X-rays. The volume of each tone is then given by the changing brightness (or count rate) as measured by Swift.

What you hear is the initial high pitch of the high energy hard emission from the shocked gas. Then the low pitch of the low energy soft emission from the white dwarf at the centre of the system starts to dominate. Finally, as this soft emission fades, the underlying high pitch of the shocked gas as it continues to expand can be heard more clearly again.

Aurora spotting near Manchester

Over the years I’ve missed seeing an aurora several times, but on the evening of Thursday 27th February 2014, I spotted an aurora for the first time and from virtually my own doorstep.

Way back in the 90′s, I was in Calgary, Canada, at an early conference about the Square Kilometre Array. At breakfast one day, the partner of a colleague who’d driven overnight through the Rockies to meet us said she’d seen a great aurora. I stayed up most of the next night…nothing.

Last solar maximum, about 12 years ago, Tom Muxlow arrived in the Jodrell Bank tearoom one morning regaling us with tales of how there’d been an aurora visible the night before. To see one so far south is very unusual and, of course, it didn’t appear again.

Last year, my wife and I went to Iceland for a holiday. Spectacular place and a great break. Of course we had hopes of seeing an aurora but sadly it was cloudy throughout most of our stay, but on those nights when there were gaps overhead, no aurora.

So, when last Thursday evening, stories of the aurora being visible from quite far south in the UK began to appear on Twitter, it was time to kick off the slippers, wrap up warm, grab the camera and head for the hills!

We live in Manchester , so light pollution would have killed any chance of seeing the aurora from home.  What’s needed is a dark sky and a clear northern horizon. We headed east out of Manchester, along the M67 and onto the Woodhead Pass through the Pennine hills towards Sheffield. Part way along the pass, a left turn on to the A6024 towards Holmfirth took us up onto the top of the Pennines where there is a transmitter station at Holme Moss.

I figured this would be a good spot from where to look out for the Northern Lights as there’s less light pollution to the north as this handy map shows. But, as it happens, it’s also a location with historical importance to Jodrell Bank.

In the 1950′s and 60′s astronomers at Jodrell Bank (including Hanbury Brown, Jennison, Das Gupta and Palmer) were investigating the angular size of radio sources – a project which helped lead to the identification of quasars. In order to increase the resolution of their observations they developed the technique of radio-linked interferometry, bringing signals from ever more distant radio antennas back to Jodrell for combination with various home antennas (including the 218-ft transit telescope and then the 250-ft Mark I, now the Lovell Telescope).

Eventually they moved their remote antenna to the far side of the Pennines with no direct line of sight back to Jodrell Bank. They needed a repeater station on the hills and so they borrowed the BBC transmitter site at Holme Moss (Elgaroy, Morris & Rowson, 1962).

Unfortunately, when we arrived at the Holme Moss transmitter station we realised that floodlights on the building at its base destroyed our dark adaptation. We drove on a little further down the far side of the hill. But from here the towns down below were visible, as was their light pollution. We parked up briefly and I took a couple of test photographs. No aurora could be seen by eye and I didn’t immediately see anything on the back of the camera so we decided to turn round and head back past the transmitter station, putting the hill between us and the direct light pollution. As it happens, when looking at the photos later, the green of the aurora is just visible through the light pollution, as is one of the red pillars above it.

The green glow of aurora is just visible above the light polluted clouds. To the left and above the green you might just be able to see a red pillar of auroral glow.

The green glow of aurora is just visible above the light polluted clouds. To the left and above the green you might just be able to see a red pillar of auroral glow.

From the second spot the transmitter station was above us, next to a dip in the hills. I started taking long exposure photos but these only showed thin cloud cover photos lit by the glow of streetlights below. However, after 15 minutes or so the skies began to clear and the green glow of the aurora appeared! At last!

The green aurora with the red aurora above and, in the foreground, the Holme Moss transmitter station.

The green aurora with the red aurora above and, in the foreground, the Holme Moss transmitter station. Cassiopeia can be seen in the upper middle and, for the eagle-eyed, to the lower left you can see the smudge of the Andromeda Galaxy.

Most obvious in the dip in the hills to the north and visible to the unaided eye, but not bright, the green colour was very apparent on photos. Also appearing in photos, but not visible to the eye alone, was the red glow above it. The photos also show structure in the green aurora.

The glow of the aurora comes from charged particles flooding down the Earth’s magnetic field (in this case, triggered by a solar coronal mass ejection two days earlier which caught the Earth a glancing blow) and colliding with atoms and molecules in the atmosphere. The collisions energise the atmospheric atoms which then relax back to their lower energy states, emitting photons with specific energies corresponding to the colours we see.

The green glow is from oxygen atoms 60 or more miles above the Earth’s surface. But what causes the red glow which very clearly sits above the green? It turns out that this is another energy transition in oxygen with a different energy gap and hence a different color of light is produced when it relaxes back.

As explained in this aurora FAQ, in the green transition, the oxygen atom sits at the upper energy level typically for 3/4 second before falling back and emitting a photon. Whereas in the red transition, the delay is far longer, about two minutes. The extra time for the red transition increases the chance that the energised atom will collide with another particle and so be de-energised without emitting a photon. At high altitudes where the density of atoms is low, there is a far greater chance that the energised atoms will last long enough to emit the red glow, explaining why the red is seen above the green.  At the lowest altitudes, even the green transitions are de-energised by collisions and all that is left is a purple colour caused by emission from nitrogen molecules. [You may also like this blog post about aurorae by Jim Wild of Lancaster University.]

As the aurora waxed and waned over the next hour or so, I amused myself with some photos to the west showing Jupiter, Orion and Sirius above the glow of the lights of Manchester.

Jupiter, Orion and Sirius above the glow of Manchester.

Jupiter, Orion and Sirius above the glow of Manchester.

Up on the hills there was no mobile phone signal but when we returned to Manchester, there was a voicemail message from the BBC inviting me on to the Breakfast TV programme the next morning to discuss the aurora and the amazing photos that viewers had sent in from around the country.

Not only did I get to see the aurora for the first time I also got a new job title – astronomist!

Discussing the aurora on BBC Breakfast with my new job title ;)

Discussing the aurora on BBC Breakfast with my new job title ;)

Clearly lots of people had gone out that Thursday evening to spot the aurora and take photos. Thinking back to my previous experiences of hearing about aurorae the day after and so missing my opportunity, this time I’d been alerted by Twitter. It struck me that this was the first solar maximum since the advent of social media and it was this that had enabled so many to get a glimpse of one of the world’s great natural phenomena.

Sunspot numbers over the last few solar maxima showing how the current one is the first to take place in the era of social media.

Sunspot numbers over the last few solar maxima showing how the current one is the first to take place in the era of social media.

Supernova 2014J and the upcoming deluge of discoveries

Like most  astronomers I was excited to hear about SN 2014J, the Type Ia supernova discovered recently in starburst galaxy M82, and I wanted to see it for myself.

The weather’s been shocking here for weeks but finally there was a clear evening on Sunday. Unfortunately I’d left my telescopes at work so thought I’d experiment with my SLR camera.

I live only a few miles from Manchester city centre so skies are light polluted to say the least. The Plough was visible from the backyard of our house sitting just above the rooftop. I set up my camera on a mini tripod, pointed it in roughly the right direction (somewhere above the line joining the Pointers to the Pole Star) and took a 30 second exposure.

With no clever mount to correct for Earth rotation, in 30 seconds all the star images trail left to right. However, a quick check with Stellarium let me spot the star patterns and sure enough there were the fuzzy blobs of galaxies M81 and M82.

My 30-second unguided backyard photograph of galaxies M81 and M82 (the fuzzy blobs at right and left respectively). The lower panel is a screenshot from Stellarium to help identify the stars and galaxies.

My 30-second unguided backyard photograph of galaxies M81 and M82 (the fuzzy blobs at right and left respectively). The lower panel is a screenshot from Stellarium to help identify the stars and galaxies.

Of course this is no claim for astrophotographer of the year but I quite enjoyed the back to basics challenge of city skies and minimal equipment. It’s possible to see the star towards the top end of M82, but unfortunately these images are not good enough to see SN 2014J. That will have to wait until the next clear night when I’m in close proximity to my telescopes.

Why the fuss?

As soon as the discovery of 2014J hit the newsfeeds of the internet, telescopes around the world and in space were being directed towards it. But why the fuss?

SN 2014J is a Type Ia supernova. These are thought to be the explosions of white dwarf stars, either as a result of accretion of gas from a companion star, or from a merger with another white dwarf. Type Ia’s can be used as standard candles to measure distances in the Universe, famously resulting in the Nobel-Prize winning discovery in 1998 of the accelerating expansion of the Universe. Something that simply wasn’t expected.

The cause of the acceleration is unknown although we’ve dubbed it dark energy, in honour of the other major component of the Universe we don’t understand – dark matter. To find out more, astronomers need to see very distant (and hence faint) supernovae, far off in the Universe and far back in time. This will allow us to measure how the expansion has changed over time and so pin down the properties of dark energy.

But if we are to understand Type Ia supernovae themselves, then nearby examples which can be studied in detail are essential, and SN 2014J is the nearest Type Ia since 1972. [In fact, it was so bright it wasn't picked up on the automated searches for fainter, more distant objects and was discovered by chance by staff and undergraduate students in the University of London Observatory.] Our telescope and instrument technology has been revolutionised since the 1970′s, so 2014J promises to be a major source of information over the coming months and years.

A deluge of supernovae

The Central Bureau for Astronomical Telegrams (CBAT) designate confirmed supernovae using letters. The first 26 of the year are named with capital letters, so 2014J in galaxy M82 is the 10th supernova of 2014. After Z the designation moves to two lower case letters aa, ab, ac…az, then ba, bc, bd… etc. In 2013 the CBAT list got to supernova 2013hw, the 231st (26+7×26+23) supernova of the year. By the way, these supernovae are all in other galaxies – there hasn’t been one seen to explode in our Milky Way since Kepler’s supernova in 1604 (although the youngest remnant discovered dates from about 1870).

Of course you’d expect the rate of supernova discoveries to increase year on year as technology improves (each galaxy might have a supernova only every 50 years say, but there are billions of galaxies out there and automated searches are now the norm). It had been a while since I’d checked how many supernovae were discovered each year and I thought 231 last year seemed a bit low compared to what I remembered. So I took the CBAT list and plotted the numbers since 1980.

Number of supernovae discovered each year since 1950 and catalogued in the CBAT lists.

Number of supernovae discovered each year since 1980 and catalogued in the CBAT lists.

Now that seems a bit strange. Surely there can’t be fewer supernovae being discovered each year?

According to the Latest Supernovae website, maintained by David Bishop since 1997, at least part of the explanation is that some of the current supernova search projects are not submitting  all their discoveries to CBAT for naming. Bishop’s website aims to compile a complete list of all supernovae from various sources. Using the numbers for all supernovae from his annual statistics pages the discovery rate is, as one might expect, steadily increasing.

Supernovae discovered each year from the CBAT list and from the "Latest supernovae" list (All).

Supernovae discovered each year from the CBAT list and, since 1996, from the “Latest supernovae” list (All).

The automated surveys are indeed driving the increase in supernova discovery. The peak in numbers around 2006 was largely due to the Sloane Digital Sky Survey supernova search (SDSS-II SN). This operated from July 2005-July 2008 and was responsible for finding ~500 Type Ia’s and 80 core-collapse supernovae – all of which were reported and appear on the CBAT lists. For example, in the peak CBAT year of 2007, 573 new supernovae are listed of which 228 were down to SDSS.

Using Bishop’s list, 2013 had most supernovae with 1100. Out of these, 400 were discovered by the Catalina Real-Time Transient Survey, 146 by the OGLE-IV programme, 137 by LaSilla-QUEST and 117 by the Palomar Transient Factory.

Upcoming instruments will increase the discovery rate even further. For example:

The next decade is going to provide a deluge of supernova discoveries, hopefully bringing us closer to solving the problem of dark energy.

Stargazing 2014 : Spiral structure of the Milky Way

I thought I’d write something about a few of the topics we discussed last week during Stargazing Live. First up: the spiral structure of the Milky Way galaxy which we discussed in episode 3.

The Milky Way

The Milky Way galaxy is made up of several hundred billion stars plus gas, dust and dark matter, all held together by their mutual gravitational attraction.  The stars, gas and dust are found mostly in a disc – like two Frisbees 100,000 light years across stuck face to face. The Sun lies in the disc about two thirds of the way from the middle.  From the Earth we see this disc side-on so it appears in the night sky as a band of hazy light stretching from horizon to horizon and threaded by dark dust lanes.

Milky Way

My photograph of the centre of the Milky Way taken from the South African Astronomical Observatory in Sutherland

Usually the closer objects are to us, the easier they are to study.  We can see more detail as they appear larger and we can detect fainter features in the light we collect from them with our telescopes.

However, although individual stars and nebulae may be easier to study in the Milky Way than in other galaxies, the overall structure of the Milky Way is very hard to discern. We are seeing it from the inside so it’s not possible to see it as a whole as it is for other galaxies. In particular we can’t easily see its spiral structure.

The 21-cm hydrogen line

On the Stargazing Live show we featured live observations from our 7-metre telescope (actually a misnomer since its actual diameter is 21 foot or 6.4 metres). This telescope, like our 42-foot pulsar monitoring telescope, is an ex-missile tracking dish from Woomera in Australia. Data collection is overseen by Christine Jordan at Jodrell Bank.

This timelapse video shows a day in the life of the 7-metre telescope. The Lovell Telescope and 42-foot telescope can be seen in the background.

The 7-metre is fitted with a receiver designed to pick up radio emissions from hydrogen atoms in interstellar space which occur at a wavelength of 21 centimetres. The famous 21-centimetre line (or simply the hydrogen line) is produced as a result of the change in energy level when the spins of the proton and electron in a hydrogen atom relax back to a low-energy configuration in which they point in opposite directions. This transition only happens every few million years for an individual atom, but there are so many atoms in space it is easily detected.

The existence of this line was predicted during World War II by Hendrik van de Hulst at Leiden Observatory in the Netherlands following a request from the Observatory Director Jan Oort (of Oort Cloud fame). It was detected in 1951 by Ewen & Purcell at Harvard University in the USA.

Radio astronomy was a brand new science in the 1940’s and Oort had recognised the fundamental importance of a spectral line (one in which the radio waves are produced with a very specific wavelength). In this case, any difference between the wavelength of the line received by our telescopes and the wavelength at which it should be emitted indicates a Doppler shift produced when the hydrogen atoms are moving relative to the telescope. Measuring this difference in wavelength immediately tells us the relative speed. Combined with the fact that radio waves travel more easily through interstellar dust, this gives a powerful way of studying the distribution and motion of the hydrogen gas clouds in the Milky Way and other galaxies.

Observations of the hydrogen line with radio telescopes in the Netherlands and Australia produced maps of our Milky Way revealing that the hydrogen clouds were distributed in arms with an apparent spiral structure.

Data from our 7-metre telescope

In the programme we showed a live spectrum of the hydrogen line pointing at a position with galactic longitude 132 degrees and latitude 0 degrees. The longitude is the angle around the disc (with zero towards the galactic centre) and the latitude is the angle above or below the disc – 0 degrees is pointing directly through the plane of the Milky Way. The spectrum is a plot of brightness versus frequency in the radio spectrum on the horizontal axis. It showed two peaks, a clear main peak and a fainter one just to its right. Since we were updating the spectrum in real time and using a relatively small telescope with room temperature receivers, the signal to noise wasn’t high – but this did emphasise the live nature of the observation.

Live H-line spectrum

Screenshot from Stargazing showing the live hydrogen-line spectrum on the right

Each peak in the spectrum is from a cloud of hydrogen somewhere along the line of sight of the telescope. If these clouds are moving at different speeds relative to us then they are Doppler shifted to different frequencies and so they appear at different points in the spectrum.

waterfall plot H-line

Full waterfall plot of hydrogen-line data from the Milky Way

Alongside the live spectrum I showed some data I‘d collected earlier. This was in the form of a “waterfall plot” where the horizontal axis is frequency (as in the live spectrum) but the brightness is now represented by colour. Each horizontal line in the waterfall plot is then a spectrum from a slightly different position on the sky. The image was made by driving the telescope across the sky along the plane of the Milky Way. In this case I collected data from a galactic longitude of 50 degrees through to about 165 degrees whilst holding the galactic latitude at 0.

What you see in the waterfall plot is a series of vertical bands or ridges of enhanced brightness. Each of these ridges is a separate spiral arm marked by an increase in the density of interstellar hydrogen gas and hence its brightness. Since the stars and gas in the galactic disc are rotating, as we point the telescope in different directions through the disc the relative velocity of the gas changes and the ridges/arms meander from side to side across the waterfall plot. Some arms do not extend all the way round the disc and so are not visible at all galactic longitudes.

Debate on spiral structure

We mentioned in the programme that the spiral structure of the Milky Way is still debated. In 2008 it was suggested from observations of old stars with the Spitzer Space Telescope that the Galaxy had only two major arms rather than the four it had been thought to have. However, just before Christmas a new study of young stars was published which agreed with the original model. It seems this disagreement between observations using different data might be telling us something about the history of star formation in the arms of the Milky Way.

It’s worth noting that we still don’t know exactly how spiral arms are formed! It seems clear that waves of increased density travel through the disc, bunching up the stars and gas, but what drives these waves and how transient they are remain questions to be answered by continuing research.

2013 – A Nova Year to remember

For those interested in nova explosions, 2013 was a great year as two novae were discovered that became visible to the naked eye – V339 Delphini and V1369 Centauri. There’s a summary of what novae are at the bottom of this post.

[These novae are named by the constellation in which they appear – here Delphinus the Dolphin and Centaurus the Centaur – together with a catalogue number and the letter V for variable star].

Although I’ve carried out research into novae for almost thirty years (ouch), I’ve only ever seen one with my own eyes without the aid of a telescope. This was nova V1494 Aquilae which I spotted in 1999 from the Kerridge Ridge above Macclesfield. It’s quite unusual for a nova to get so bright and even for those that do, they are often only bright enough for a few days before they fade away and you need to get out your telescope.

As it turns out when the explosion of V339 Del was spotted on August 14th 2013, I was in an ideal place to see it before it faded – on holiday on a Greek Island with beautiful clear skies. But I didn’t have an internet connection and had no idea it had even exploded.

Even more frustratingly, just a few days before it exploded I’d been sitting by the pool at night taking some photos of the sky hoping to catch some Perseid meteors (as you do). So I have some lovely photos showing the Milky Way, a Perseid and the constellation of Delphinus (the Dolphin). Sadly the latter photos just prove that two days before the discovery was announced there was no new star at the position of V339 Del.

My shot of the region of sky where V339 Del (Nova Del 2013) was to explode only two days later

My August 12th photo of the region of sky where V339 Del (Nova Del 2013) was to explode only two days later (note all the stars appear as little lines because they are moving across the sky as the Earth spins during this 30-second unguided exposure)

When I returned to Manchester, the nova had faded below naked eye visibility but was still bright enough to see in photos taken with my SLR from the back garden in South Manchester (even through the light pollution).

My photo taken Aug 26th from Manchester showing V339 Del is now visible, although fading

My photo taken Aug 26th from Manchester showing V339 Del is now visible, although fading (note orientation rotated from the previous photo)

Leaving aside the fun of spotting novae with the naked eye, on the research front V339 Del has of course attracted a lot of interest. The nova-cv group has been monitoring it with the Swift spacecraft, following the transition from X-ray emission from shocked gas in the ejected material, to the emergence of brighter X-rays from the hot white dwarf remnant.  The nova was also detected in gamma-rays by the Fermi spacecraft and the radio nova group has been monitoring it using the Karl Jansky Very Large Array in New Mexico. If and when it becomes sufficiently bright we intend to image it with the e-MERLIN radio telescope operated from Jodrell Bank in an effort to measure the size and shape of the ejected shell.

The second bright nova of 2013 was V1369 Centauri discovered on December 2nd. Unfortunately for those of us northern types, this lies at a declination of -59 degrees. This means that it passes overhead for observers at latitudes of 59 degrees south. For me at 53 degrees north, only objects with declinations greater than -37 degrees (-37=53-90) get above the southern horizon. So sadly V1369 Cen is not visible from up here (you’d have to live at a latitude south of 31 degrees N to have any chance of spotting it, the farther south, the better of course). It’s looking lovely and pink in these recent shots by Rolf Wahl Olsen.

Like V339 Del, V1369 Cen has also been causing quite a buzz amongst researchers. It too was detected as a gamma-ray source by the Fermi spacecraft. It’s now looking very much like all novae are gamma-ray sources and we just needed a good enough telescope to spot them. One of the favoured options is that the gamma-rays come from shocked gas. The shocks probably arise from variations in the speed of the ejecta so that faster gas catches up and runs into slower gas, or from the ejecta slamming into previously existing matter around the nova (e.g. from the wind of the companion star). This is a higher-energy manifestation of the same phenomenon that produces some of the X-ray emission seen with the Swift spacecraft in other novae, V339 Del for example. Although it’s too far south to be observed with the VLA or e-MERLIN, in December it was detected in the radio with the Australia Telescope Compact Array. We expect this radio emission to brighten as the ejected shell expands.

Although we have a fair idea of what novae are and the basic parameters of how they work there are many unanswered questions. For example, “Do some novae eventually explode as Type Ia supernovae?”, “Do novae eject jets?”, “What causes the gamma-ray emission?”, “Why do the X-rays show quasi-periodic oscillations?”, and “How much mass does a nova eject?”.

What is a nova?

Nova comes from “Stella Nova” – New Star. Every now and again a new star appears in the night sky. Originally thought to be new stars being born, we now know these mark cataclysmic explosions associated with stars near or at the end of their lives.

There are several types of nova.

Classical novae are explosions on the surface of white dwarf stars in binary stars systems. White dwarfs are the dead cores of lower-mass stars like the Sun. They are exposed after the star evolves beyond the red giant stage and its outer layers are lost to form a planetary nebula. If the white dwarf is in a binary, gas can fall from the companion star onto its surface. Imagine a white dwarf about the size of the Earth but weighing about as much as the Sun. It is in close orbit around its companion star, so close that the orbit would fit within the size of our Sun. The gas builds up on the white dwarf until the pressure reaches a critical point at which nuclear fusion begins at the base of the accreted layer. This results in a thermonuclear explosion expelling something like an Earth’s mass of gas into space at speeds of hundreds to thousands of kilometres per second. It brightens by a factor of 10,000 or so leading to the appearance of a stella nova.

Recurrent novae are simply classical novae which have been seen to outburst more than once. We actually think that all classical novae recur. The white dwarf is not destroyed in the outburst so the accretion process can soon begin again and another explosion can take place sometime later. For many novae this will not be for thousands of years so only one outburst is known. But in some cases the recurrence period can be much shorter (probably when the white dwarf is massive and the critical pressure is reached sooner). The shortest known is U Sco which explodes every 10 years or so, although recently a nova in the Andromeda Galaxy was discovered which appears to have outbursts every year or so.

Dwarf nova outbursts also take place in binary systems including a white dwarf. But here the brightening is not as extreme and appears to come from a sudden increase in the rate at which the gas is transferred onto the white dwarf through an accretion disc.

Supernovae, as the name suggests, are much brighter than the normal classical novae. There are several types. Type II supernovae are the explosions of massive stars (more than about eight times the mass of our Sun). The core runs out of nuclear fuel and collapses to a neutron star (or possibly a black hole for the most massive stars) whilst the remainder of the star is thrown off into space. This distributes into space the heavy elements created by nuclear fusion in the star and in the explosion, forming the raw materials for new generations of stars and planets. In contrast, Type Ia supernovae are thought to be the explosions of white dwarf stars which exceed their maximum mass of 1.4 solar masses (known as the Chandrasekhar mass) either by accreting matter from a companion star or by the merger of two white dwarfs.

We think there may be as many as 30-40 classical novae each year in the Milky Way but only around 10 of these are detected. Supernovae are much rarer with a galaxy like the Milky Way seeing one such explosion perhaps every 30-50 years or so. Again not all are detected. The last supernova in the Milky Way indisputably seen by observers was Kepler’s Supernova in 1604, although Cassiopeia A appears to be a remnant of a supernova which took place in about 1680 remnants and a recently discovered X-ray source G1.9+0.3 seems to be the remnant of a supernova dating from about 1870.