Posted on by Steve Nerlich

Astronomy Without A Telescope – Forbidden Planets

The theorised evolution of the circumbinary planet PSR B1620?26 b. Credit: NASA.

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Binary star systems can have planets – although these are generally assumed to be circumbinary (where the orbit encircles both stars). As well as the fictional examples of Tatooine and Gallifrey, there are real examples of PSR B1620-26 b and HW Virginis b and c – thought to be cool gas giants with several times the mass of Jupiter, orbiting several astronomical units out from their binary suns.

Planets in circumstellar orbits around a single star within a binary system are traditionally considered to be unlikely due to the mathematical implausibility of maintaining a stable orbit through the ‘forbidden’ zones – which result from gravitational resonances generated by the motion of the binary stars. The orbital dynamics involved should either fling a planet out of the system or send it crashing to its doom into one or other of the stars. However, there may be a number of windows of opportunity available for ‘next generation’ planets to form at later stages in the evolving life of a binary system.

A binary stellar evolution scenario might go something like this:

1) You start with two main sequence stars orbiting their common centre of mass. Circumstellar planets may only achieve stable orbits very close in to either star. If present at all, it’s unlikely these planets would be very large as neither star could sustain a large protoplanetary disk given their close proximity.

2) The more massive of the binaries evolves further to become an Asymptotic Giant Branch star (i.e. red giant) – potentially destroying any planets it may have had. Some mass is lost from the system as the red giant blows off its outer layers – which is likely to increase the separation of the two stars. But this also provides material for a protoplanetary disk to form around the red giant’s binary companion star.

3) The red giant evolves into white dwarf, while the other star (still in main sequence and now with extra fuel and a protoplanetary disk) can develop a system of orbiting ‘second generation’ planets. This new stellar system could remain stable for a billion years or more.

4) The remaining main sequence star eventually goes red giant, potentially destroying its planets and further widening the separation of the two stars – but it also may contribute material to form a protoplanetary disk around the distant white dwarf star, providing the opportunity for third generation planets to form there.

How a binary system might give birth to generations of planets: a) First generation planets - small and close-in - might be possible while both stars are on the main sequence (MS) and in close proximity to each other; b) Eventually one star evolves from the main sequence to the Asymptotic Giant Branch (AGB) - in other words, it goes red giant. c) The two stars spread further apart while stellar material blown off from the red giant builds a protoplanetary disk around the other star and second generation planets form; d) the second star eventually goes red giant giving the first star (now a white dwarf - WD) a protoplanetary disk which could create a third generation of planets. Credit: Perets, H.B.

The development of the third generation planetary system depends on the white dwarf star sustaining a mass below its Chandrasekhar limit (being about 1.4 solar masses – depending on its rate of spin) despite it having received more material from the red giant. If it doesn’t stay below that limit, it will become a Type 1a supernova – potentially lobbing a small proportion of its mass back to the other star again, although by this stage that other star would be a very distant companion.

An interesting feature of this evolutionary story is that each generation of planets is built from stellar material with a sequentially increasing proportion of ‘metals’ (elements heavier than hydrogen and helium) as the material is cooked and re-cooked within each stars’ fusion processes. Under this scenario, it becomes feasible for old stars, even those which formed as low metal binaries, to develop rocky planets later in their lifetimes.

Further reading: Perets, H.B. Planets in evolved binary systems.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Through A Lens Darkly

Gravitational lensing in action - faint hints of an "Einstein ring' forming about an area of space which has been 'lensed' by the warping of space-time. If the galactic cluster has been orientated in aplane the lay faceon directly towards earth - the ring would be much more apparent. Credit: HST, NASA.

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Massive galactic clusters – which are roughly orientated in a plane that is roughly face-on to Earth – can generate strong gravitational lensing. However, several surveys of such clusters have reached the conclusion that these clusters have a tendency towards lensing too much – at least more than is predicted based on their expected mass.

Known (to some researchers working in the area) as the ‘over-concentration problem’, it does seem to be a prima facie case of missing mass. But rather than just playing the dark matter card, researchers are pursuing more detailed observations – if only to eliminate other possible causes.

The Sunyaev-Zel’dovich (SZ) effect is a novel way of scanning the sky for massive objects like galactic clusters – which distort the Cosmic Microwave Background (CMB) via inverse Compton scattering – where photons (in this case, CMB photons) interact with very energized electrons which impart energy to the photons during a collision, shifting the protons to a shorter wavelength frequency.

The SZ effect is largely independent of red-shift – since you start with the most consistently red-shifted light in the universe and are looking for a one-off event that will have the same effect on that light whether it happens close by or far away. So, with equipment sensitive to CMB wavelengths, you can scan the whole sky – detecting both close objects, which might be directly observable in optical, as well as very distant objects which may have been red-shifted into the radio spectrum.

The SZ effect causes CMB distortions in the order of one thousandth of a Kelvin and the effect does require really massive structures – a single galaxy is not sufficient to generate the SZ effect on its own. But, when it works – the SZ effect offers a method to measure the mass of a galactic cluster – and does it in a way that is quite different to gravitational lensing.

The SZ effect is thought to be mediated by electrons in the inter-cluster medium. This means that the SZ effect is solely the result of baryonic matter, since it is a consequence of the inverse Compton effect. However, gravitational lensing is the result of the warping of space-time – which is partly due to the presence of baryonic matter, but also of dark (i.e. non-baryonic) matter.

Gralla et al used the Sunyaev-Zel’dovich Array, an array of eight 3.5 meter radio telescopes in California, to survey 10 strongly lensing galactic clusters. They found a consistent tendency for the Einstein radius of each gravitational lens to be around twice the value expected for the mass, determined from the SZ effect, of each cluster.

A distant, actually double, Einstein ring captured by the Hubble Space Telescope. Many more Einstein rings are visible from distant galactic clusters - although they are generally only 'visible' in radio wavelengths.

The Einstein radius is a measure of the size of the Einstein ring that would be formed if a cluster was exactly orientated in a plane that was exactly face-on to Earth – and where you, the lens and the distant light source being magnified, are all in a straight line of sight. Strongly lensing galaxies are generally only in close approximation to this geometry, but their Einstein ring and radius (and hence their mass) can be inferred easily enough.

Gralla et al note that this is work in progress, for now just confirming the over-concentration problem found in other surveys. They suggest one possibility is that the amount of inter-cluster medium may be less than expected – meaning that the SZ effect is underestimating the real mass of the cluster.

If, alternatively, it is a dark matter effect, there would be more dark matter in these clusters than the current ‘standard model’ for cosmology (Lambda-Cold Dark Matter) predicts. The researchers seem intent on undertaking further observations before they go there.

Further reading: Gralla et al. Sunyaev Zel’dovich Effect Observations of Strong Lensing Galaxy Clusters: Probing the Over-Concentration Problem.

And just for interest, Einstein’s letter on lensing and rings: Einstein, A (1936) Lens-like Action of a Star by the Deviation of Light in the Gravitational Field. Science 84 (2188): 506–507.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Black Hole Evolution

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While only observable by inference, the existence of supermassive black holes (SMBHs) at the centre of most – if not all – galaxies remains a compelling theory supported by a range of indirect observational methods. Within these data sources, there exists a strong correlation between the mass of the galactic bulge of a galaxy and the mass of its central SMBH – meaning that smaller galaxies have smaller SMBHs and bigger galaxies have bigger SMBHs.

Linked to this finding is the notion that SMBHs may play an intrinsic role in galaxy formation and evolution – and might have even been the first step in the formation of the earliest galaxies in the universe, including the proto-Milky Way.

Now, there are a number of significant assumptions built into this line of thinking, since the mass of a galactic bulge is generally inferred from the velocity dispersion of its stars – while the presence of supermassive black holes in the centre of such bulges is inferred from the very fast radial motion of inner stars – at least in closer galaxies where we can observe individual stars.

For galaxies too far away to observe individual stars – the velocity dispersion and the presence of a central supermassive black hole are both inferred – drawing on the what we have learnt from closer galaxies, as well as from direct observations of broad emission lines – which are interpreted as the product of very rapid orbital movement of gas around an SMBH (where the ‘broadening’ of these lines is a result of the Doppler effect).

But despite the assumptions built on assumptions nature of this work, ongoing observations continue to support and hence strengthen the theoretical model. So, with all that said – it seems likely that, rather than depleting its galactic bulge to grow, both an SMBH and the galactic bulge of its host galaxy grow in tandem.

It is speculated that the earliest galaxies, which formed in a smaller, denser universe, may have started with the rapid aggregation of gas and dust, which evolved into massive stars, which evolved into black holes – which then continued to grow rapidly in size due to the amount of surrounding gas and dust they were able to accrete.

Distant quasars may be examples of such objects which have grown to a galactic scale. However, this growth becomes self-limiting as radiation pressure from an SMBH’s accretion disk and its polar jets becomes intense enough to push large amounts of gas and dust out beyond the growing SMBH’s sphere of influence. That dispersed material contains vestiges of angular momentum to keep it in an orbiting halo around the SMBH and it is in these outer regions that star formation is able to take place. Thus a dynamic balance is reached where the more material an SMBH eats, the more excess material it blows out – contributing to the growth of the galaxy that is forming around it.

The almost linear correlation between the SMBH mass (M) and velocity dispersion (sigma) of the galactic bulge (the 'M-sigma relation') suggests that there is some kind of co-evolution going on between an SMBH and its host galaxy. The only way an SMBH can get bigger is if its host galaxy gets bigger - and vice versa. The left chart shows data points derived from different objects in a galaxy - the right chart shows data points derived from different types of galaxies. Credit: Tremaine et al. (2002).

To further investigate the evolution of the relationship between SMBHs and their host galaxies – Nesvadba et al looked at a collection of very red-shifted (and hence very distant) radio galaxies (or HzRGs). They speculate that their selected group of galaxies have reached a critical point – where the feeding frenzy of the SMBH is blowing out about as much material as it is taking in – a point which probably represents the limit of the active growth of the SMBH and its host galaxy.

From that point, such galaxies might grow further by cannibalistic merging – but again this may lead to a co-evolution of the galaxy and the SMBH – as much of the contents of the galaxy being eaten gets used up in star formation within the feasting galaxy’s disk and bulge, before whatever is left gets through to feed the central SMBH.

Other authors (e.g. Schulze and Gebhardt), while not disputing the general concept, suggest that all the measurements are a bit out as a result of not incorporating dark matter into the theoretical model. But, that is another story…

Further reading: Nesvadba et al. The black holes of radio galaxies during the “Quasar Era”: Masses, accretion rates, and evolutionary stage.

Posted on by Steve Nerlich

Astronomy Without A Telescope – So Why Not Exo-Oceans?

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Earth's saline ocean

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Well, not only may up to 25% of Sun-like stars have Earth-like planets – but if they are in the right temperature zone, apparently they are almost certain to have oceans. Current thinking is that Earth’s oceans formed from the accreted material that built the planet, rather than being delivered by comets at a later time. From this understanding, we can start to model the likelihood of a similar outcome occurring on rocky exoplanets around other stars.

Assuming terrestrial-like planets are indeed common – with a silicate mantle surrounding a metallic core – then we can expect that water may be exuded onto their surface during the final stages of magma cooling – or otherwise out-gassed as steam which then cools to fall back to the surface as rain. From there, if the planet is big enough to gravitationally retain a thick atmosphere and is in the temperature zone where water can remain fluid, then you’ve got yourself an exo-ocean.

We can assume that the dust cloud that became the Solar System had lots of water in it, given how much persists in the left-over ingredients of comets, asteroids and the like. When the Sun ignited some of this water may have been photodissociated – or otherwise blown out of the inner solar system. However, cool rocky materials seem to have a strong propensity to hold water – and in this manner, could have kept water available for planet formation.

Meteorites from differentiated objects (i.e. planets or smaller bodies that have differentiated such that, while in a molten state, their heavy elements have sunk to a core displacing lighter elements upwards) have around 3% water content – while some undifferentiated objects (like carbonaceous asteroids) may have more than 20% water content.

Mush these materials together in a planet formation scenario and materials compressed at the centre become hot, causing outgassing of volatiles like carbon dioxide and water. In the early stages of planet formation much of this outgassing may have been lost to space – but as the object approaches planet size, its gravity can hold the outgassed material in place as an atmosphere. And despite the outgassing, hot magma can still retain water content – only exuding it in the final stages of cooling and solidification to form a planet’s crust.

Mathematical modelling suggests that if planets accrete from materials with 1 to 3% water content, liquid water probably exudes onto their surface in the final stages of planet formation – having progressively moved upwards as the planet’s crust solidified from the bottom up.

Otherwise, and even starting with a water content as low as 0.01%, Earth-like planets would still generate an outgassed steam atmosphere that would later rain down as fluid water upon cooling.

As the Earth formed, water contained in rocky materials either 'outgassed' or just exuded onto the surface - as magma solidified, from the bottom up, to form the Earth's crust. And OK, this is just a nice image of a deep sea volcanic vent - but you get the idea. Credit: Woods Hole Oceanographic Institution.

If this ocean formation model is correct, it can be expected that rocky exoplanets from 0.5 to 5 Earth masses, which form from a roughly equivalent set of ingredients, would be likely to form oceans within 100 millions years of primary accretion.

This model fits well with the finding of zircon crystals in Western Australia – which are dated at 4.4 billion years and are suggestive that liquid water was present that long ago – although this preceded the Late Heavy Bombardment (4.1 to 3.8 billion years ago) which may have sent all that water back into a steam atmosphere again.

Currently it’s not thought that ices from the outer solar system – that might have been transported to Earth as comets – could have contributed more than around 10% of Earth’s current water content – as measurements to date suggest that ices in the outer solar system have significantly higher levels of deuterium (i.e. heavy water) than we see on Earth.

Further reading: Elkins-Tanton, L. Formation of Early Water Oceans on Rocky Planets.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Necropanspermia

Exogenesis
A new instrument called the Search for Extra-Terrestrial Genomes (STEG) is being developed to find evidence of life on other worlds. Credit: NASA/Jenny Mottor

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The idea that a tiny organism could hitchhike aboard a mote of space dust and cross vast stretches of space and time until it landed and took up residence on the early Earth does seem a bit implausible. More likely any such organisms would have been long dead by the time they reached Earth. But… might those long dead alien carcasses still have provided the genomic template that kick started life on Earth? Welcome to necropanspermia.

Panspermia, the theory that life originated somewhere else in the universe and was then transported to Earth requires some consideration of where that somewhere else might be. As far as the solar system is concerned – the most likely candidate site for the spontaneous formation of a water-solvent carbon-based replicator is… well, Earth. And, since all the planets are of a similar age, the only obvious reason to appeal to the notion that life must have spontaneously formed somewhere else, is if a much longer time span than was available in the early solar system is required.

Opinions vary, but Earth may have offered a reasonably stable and watery environment from about 4.3 billion years until 3.8 billion years ago – which is about when the first evidence of life becomes apparent in the fossil record. This represents a good half billion years for some kind of primitive chemical replicator to evolve into a self-contained microorganism capable of metabolic energy production and capable of building another self-contained microorganism.

Half a billion years sounds like a generous amount of time – although with only one example to go by, who knows what a generous amount of time really is. Wesson (below) argues that it is not enough time – referring to other researchers who calculate that random molecular interactions over half a billion years would only produce about 194 bits of information – while a typical virus genome carries 120,000 bits – and an E. coli bacterial genome carries about 6 million bits.

A counter argument to this is that any level of replication in a environment with limited raw materials favors those entities that are most efficient at replication – and continues to do so generation after generation – which means it very quickly ceases to be an environment of random molecular interactions.

Put the term panspermia in a search engineand you get (left) ALH84001, a meteorite from Mars which has some funny looking structures which may just be mineral deposits; and (right) a tardigrade - a totally terrestrial organism that can endure high levels of radiation, desiccation and near vacuum conditions - although it much prefers to live in wet moss. Credit: NASA

The mechanism through which a dead alien genome usefully became the information template for further organic replication on Earth is not described in detail and the case for necropanspermia is not immediately compelling.

The theory still requires that the early Earth was ideally primed and ripe for seeding – with a gently warmed cocktail of organic compounds, shaken-but-not-stirred, beneath a protective atmosphere and a magnetosphere. Under these circumstances, the establishment of a primeval replicator through a fortuitous conjunction of organic compounds remains quite plausible. It is not clear that we need to appeal to the arrival of a dead interstellar virus to kick start the world as we know it.

Further reading: Wesson, P. Panspermia, past and present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Indigenous Australian Astronomy

The Homunculus Nebula arising from the Eta Carinae star system - thought to be stellar material blown off by this massive star system during a 'supernova impostor' event that occurred around 1840.

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Eta Carinae is a massive binary system – of which the dominant member is an eruptive luminous blue variable star. The system’s last significant eruption – also known as the ‘great outburst’ – made Eta Carinae briefly the second brightest star system in the night sky after Sirius over the period of 1837 to 1845, after which it faded again. The great outburst left behind the Homunculus Nebula – and also left an impression on the indigenous Aboriginal people of Australia who observed it at that time.

Hamacher, with research interests in Australian archaeoastronomy – and Frew, an astrophysicist with research interests in the light curves of variable stars over long time periods, have collaborated on a paper which draws on historical records to build a case that the Boorong people of northwest Victoria incorporated the observation of Eta Carinae’s great outburst into their oral traditions.

This is of general interest as the only known observation of the Eta Carinae outburst by indigenous people – and of particular interest to Hamacher to support his assertion that Australian Aboriginal oral traditions are dynamic and evolving – and often incorporate transient astronomical events.

The Boorong clan apparently no longer exists as an entity and much of their traditional knowledge may have been lost. However, William Stanbridge published records of his encounters with them around 1860, particularly detailing their astronomical knowledge. His records include Aboriginal star names and stories associated with them – against which he either wrote down the relevant European star name or otherwise at least indicated the general vicinity of the star in question.

Of particular interest here is the star named Collowgullouric War by the Boorong – described as a ‘large red star in Rober Carol, marked 966’ by Stanbridge. In Boorong oral tradition at that time, Collowgullouric War was the wife of War – which Stanbridge directly identified as the star Canopus – and which today we consider the second brightest star in the night sky.

There are other examples of husband and wife pairings in Aboriginal astronomy – where the stars are generally closely associated in the sky and of similar apparent magnitude. Stanbridge noted Collowgullouric War as the third brightest star in a list that included Sirius as brightest, Canopus as second brightest and Alpha Centauri (or Rigil Kent) as fourth brightest. Today we would agree with most of that statement, except that Alpha Centauri is the third brightest star – and what the heck is Collowgullouric War?

The reference “large red star in Rober Carol, marked 966” refers in short-hand to a now-defunct constellation Rober Carolinum – and 966 is almost certainly a designation drawn from one of the first southern sky star catalogues, produced by La Caille in 1763. Lac 966 is actually the Carina nebula, while the Eta Carinae star is Lac 968 – but since it’s unlikely Stanbridge had his own copy of the rare La Caille catalogue, there is the possibility of a transcription error. And, in any case, in referring to a star associated the Carina Nebula, it seems reasonable to assume he really meant Eta Carinae.

Argo Navis (the ship Argo) was one of Ptolemy's 48 constellations - since split into the modern constellations Vela (the sails), Puppis (the stern) and Carina (the keel). Another now-defunct constellation, Robur Carolinum (the Oak of King Charles) introduced by Edmond Halley, also overlies this region of the sky. Around the 1840's, Eta Carinae (red arrow) might have been classified as a star of the Robur Carolinum constellation - but is now considered part of the Carina constellation. Canopus (or Alpha Carinae) is the large, bright star to the right of the drawing of the ship's rudder. Credit: Johannes Hevelius' star catalogue Firmamentum, circa 1690 - as sourced from Hamacher and Frew. And... for reasons unknown, Hevelius did his star catalogues from the point of view of an outsider looking in, so this map is kind of back the front. The same approach is used on the flag of Brazil - for reasons unknown. What a long caption this is.

So for a brief period of a decade or so – Eta Carinae rivalled Canopus in brightness, during the period of its variable brightening from 1837 to 1845.

On this basis, it is reasonable to assume that an indigenous people with an interest in the night sky would certainly have noted the Eta Carinae outburst – and might well have developed a story based on its close association with the similarly bright star Canopus, which was present in the sky nearby.

It remains to be discovered what other southern sky events the Indigenous Australians may have gained a privileged view of during their 40,000 year colonization of the Australian continent.

Further reading: Hamacher and Frew An Aboriginal Australian Record of the Great Eruption of Eta Carinae

Posted on by Steve Nerlich

Eyes On The Solar System

Eyes on the Solar System - a 3d environment browser application that operates in real time, letting you see what our robot spacecraft are up to.

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NASA’s beta version of Eyes on the Solar System, built by JPL and Caltech, offers a neat way of tracking a range of current space missions – for example, as Nancy Atkinson mentioned yesterday, you can follow EPOXI’s flyby of comet Hartley 2. Reminiscent of Celestia, this browser application gives you a 3D environment running in real time and is updated regularly with NASA spacecraft mission data.

To get it operating, you can just click to the NASA link where you are prompted to install a Unity Web Player plug-in. This is fast and straight forward, from my experience. I did strike a problem with a certain small and squishy 64bit system that starts with X (where the menu text didn’t display correctly), but it ran fine on other systems. It is a beta version after all – and I feel obliged to note you should load at your own risk, yada, yada.

Anyhow, if you choose to proceed, you can then move around the solar system with left mouse click-hold and scroll wheel actions – or there’s the usual keyboard alternatives, or even on-screen controls. In default mode, a number of celestial bodies are shown and labeled, as are several spacecraft, which you can zoom over to by clicking on them. You can add more objects from the Visual Controls menu. Default settings have comets hidden, so you’ll need to add them to do an EPOXI-Hartley 2 encounter simulation.

There are some online tutorials you can take from the opening screen – which are short and useful – to get a quick run-through of the options available.

Eyes - in photo mode - showing EPOXI on approach to Hartley 2. If you're not a purist, you can also back-light an image. For example, to light up EPOXI in this image - where the Sun is not at the right angle to do it.

Like Celestia, you can speed up, slow down and move back and forth through time. This means you can replay EPOXI’s closest approach to Hartley 2 – or go right back to 1997 and zoom out to watch Cassini leave Earth and travel to Saturn via Venus and Earth flybys until it reaches Saturn in 2004 – all of which you can enjoy in about 5 seconds after cranking up the passage of time. You can also pick an ‘over the shoulder’ view to ride with Cassini through the F and G rings on its first approach to Saturn.

Unlike Celestia, because Eyes is mainly about spacecraft missions, its environment only covers the period from 1950 to 2050 and (curses) I couldn’t find any options to add in fictional spacecraft.

For a bit of edu-tainment you can access right-click controls which allow you to measure distances between objects – and monitor how those distances change as the objects move over time. For a bit of fun, you can also compare spacecraft to scale objects – with a choice between scientist, Porsche and football stadium. As one of the brief tutorials will explain, Voyager 1 is about the size of a Porsche.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Warp Drive On Paper

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The Alcubierre drive is one of the better known warp drive on paper models – where a possible method of warp drive seems to work mathematically as long as you don’t get too hung up on real world physics and some pesky boundary issues.

Recently the Alcubierre drive concept has been tested within mathematically modeled metamaterial – which can provide a rough analogy of space-time. Interestingly, in turns out that under these conditions the Alcubierre drive is unable to break the light barrier – but quite capable of doing 25% of light speed, which is not what you would call slow.

OK, so two conceptual issues to grapple with here. What the heck is an Alcubierre drive – and what the heck is metamaterial?

The Alcubierre drive is a kind of mathematical thought experiment where you imagine your spacecraft has a drive mechanism capable of warping a bubble of space-time such that the component of bubble in front of you contracts bringing points ahead of you closer – while the bubble behind you expands, moving what’s behind you further away.

This warped geometry moves the spacecraft forward, like a surfer on a wave of space-time. Maintaining this warp dynamically and continuously as the ship moves forward could result in faster-than-light velocities from the point of view of an observer outside the bubble – while the ship hardly moves at all relative to the local space-time within the bubble. Indeed throughout the journey the crew experience free fall conditions and are not troubled by G forces.

Standard images used to describe the Alcubierre drive. Left: Want to make the Kessel run in 12 parsecs? No problem - just compress the Kessel run into 12 parsecs. Right: The Alcubierre concept can be thought of as a spaceship surfing on a wave of space-time. Images sourced from daviddarling.info.

Some limitations of the Alcubierre drive model are that although the mathematics can suggest that forward movement of the ship is theoretically possible, how it might start and then later stop at its destination are not clear. The mechanism underlying generation of the bubble also remains to be explained. To warp space-time, you must redistribute mass or energy density in some way. If this involves pushing particles out to the edges of the bubble this risks a situation where particles at the boundary of the bubble would be moving faster than light within the frame of reference of space-time external to the bubble – which would violate a fundamental principle of general relativity.

There are various work-around solutions proposed, involving negative energy, exotic matter and tachyons – although you are well down the rabbit-hole by this stage. Nonetheless, if you can believe six impossible things before breakfast, then why not an Alcubierre drive too.

Now, metamaterials are matrix-like structures with geometric properties that can control and shape electromagnetic waves (as well as acoustic or seismic waves). To date, such materials have not only been theorized, but built – at least with the capacity to manipulate long wavelength radiation. But theoretically, very finely precisioned metamaterials might be able to manipulate optical and shorter wavelengths – creating the potential for invisibility cloaks and spacecraft cloaking devices… at least, theoretically.

Anyhow, metamaterials capable of manipulating most of the electromagnetic spectrum can be mathematically modeled – even if they can’t be built with current technologies. This modeling has been used to create virtual black holes and investigate the likelihood of Hawking radiation – so why not use the same approach to test an Alcubierre warp drive?

It turns out that the material parameters of even so-called ‘perfect’ metamaterial will not allow the Alcubierre drive to break light speed, but will allow it to achieve 25% light speed – being around 75,000 kilometres a second. This gets you to the Alpha Centauri system in about seventeen years, assuming acceleration and deceleration are only small components of the journey.

Whether the limitations imposed by metamaterial in this test are an indication that it cannot adequately emulate the warping of space-time – which the Alcubierre drive needs to break light speed – or whether the Alcubierre drive just can’t do it, remains an open question. What’s surprising and encouraging is that the drive could actually work… a bit.

Further reading: Smolyaninov, I. Metamaterial-based model of the Alcubierre warp drive.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Blazar Jets

A 5000 light year long jet observable in optical light from the giant elliptical galaxy M87 - which is not technically a blazar, but only because it's jet isn't more closely aligned with Earth. Credit: ESA/Hubble.

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Polar jets are often found around objects with spinning accretion disks – anything from newly forming stars to ageing neutron stars. And some of the most powerful polar jets arise from accretion disks around black holes, be they of stellar or supermassive size. In the latter case, jets emerging from active galaxies such as quasars, with their jets roughly orientated towards Earth, are called blazars.

The physics underlying the production of polar jets at any scale is not completely understood. It is likely that twisting magnetic lines of force, generated within a spinning accretion disk, channel plasma from the compressed centre of the accretion disk into the narrow jets we observe. But exactly what energy transfer process gives the jet material the escape velocity required to be thrown clear is still subject to debate.

In the extreme cases of black hole accretion disks, jet material acquires escape velocities close to the speed of light – which is needed if the material is to escape from the vicinity of a black hole. Polar jets thrown out at such speeds are usually called relativistic jets.

Relativistic jets from blazars broadcast energetically across the electromagnetic spectrum – where ground based radio telescopes can pick up their low frequency radiation, while space-based telescopes, like Fermi or Chandra, can pick up high frequency radiation. As you can see from the lead image of this story, Hubble can pick up optical light from one of M87‘s jets – although ground-based optical observations of a ‘curious straight ray’ from M87 were recorded as early as 1918.

Polar jets are thought to be shaped (collimated) by twisting magnetic lines of force. The driving force that pushes the jets out may be magnetic and/or intense radiation pressure, but no-one is really sure at this stage. Credit: NASA.

A recent review of high resolution data obtained from Very Long Baseline Interferometry (VLBI) – involving integrating data inputs from geographically distant radio telescope dishes into a giant virtual telescope array – is providing a bit more insight (although only a bit) into the structure and dynamics of jets from active galaxies.

The radiation from such jets is largely non-thermal (i.e. not a direct result of the temperature of the jet material). Radio emission probably results from synchrotron effects – where electrons spun rapidly within a magnetic field emit radiation across the whole electromagnetic spectrum, but generally with a peak in radio wavelengths. The inverse Compton effect, where a photon collision with a rapidly moving particle imparts more energy and hence a higher frequency to that photon, may also contribute to the higher frequency radiation.

Anyhow, VLBI observations suggest that blazar jets form within a distance of between 10 or 100 times the radius of the supermassive black hole – and whatever forces work to accelerate them to relativistic velocities may only operate over the distance of 1000 times that radius. The jets may then beam out over light year distances, as a result of that initial momentum push.

Shock fronts can be found near the base of the jets, which may represent points at which magnetically driven flow (Poynting flux) fades to kinetic mass flow – although magnetohydrodynamic forces continue operating to keep the jet collimated (i.e. contained within a narrow beam) over light year distances.

Left: A Xray/radio/optical composite photo of Centaurus A - also not technically a blazar because its jets don't align with the Earth. Credit: X-ray: NASA/CXC/CfA/R.Kraft et al.; Submillimeter: MPIfR/ESO/APEX/A.Weiss et al.; Optical: ESO/WFI. Right: A composite image showing the radio glow from Centaurus A compared with that of the full Moon. The foreground antennas are CSIRO's Australia Telescope Compact Array, which gathered the data for this image.

That was about as much as I managed to glean from this interesting, though at times jargon-dense, paper.

Further reading: Lobanov, A. Physical properties of blazar jets from VLBI observations.

Posted on by Steve Nerlich

Astronomy Without A Telescope – No Metal, No Planet

The spiral galaxy NGC 4565, considered a close analogue of the Milky Way and with distinctly dusty outer regions. Credit: ESO.

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A Japanese team of astronomers have reported a strong correlation between the metallicity of dusty protoplanetary disks and their longevity. From this finding they propose that low metallicity stars are much less likely to have planets, including gas giants, due to the shorter lifetime of their protoplanetary disks.

As you are probably aware, ‘metal’ is astronomy-speak for anything higher up the periodic table than hydrogen and helium. The Milky Way has a metallicity gradient – where metallicity drops markedly the further out you go. In the extreme outer galaxy, about 18 kiloparsecs out from the centre, the metallicity of stars is only 10% that of the Sun (which is about 8 kiloparsecs – or around 25,000 light years – out from the centre).

This study compared young star clusters within stellar nurseries with relatively high metallicity (like the Orion nebula) against more distant clusters in the outer galaxy within low metallicity nurseries (like Digel Cloud 2).

The study’s conclusions are based on the assumption that the radiation output of stars with dense protoplanetary disks will have an excess of near and mid-infra red wavelengths. This is largely because the star heats its surrounding protoplanetary disk, making the disk radiate in infra-red.

The research team used the 8.2 metre Subaru Telescope and a procedure called JHK photometry to identify a measure they called ‘disk fraction’, representing the density of the protoplanetary disk (as determined by the excess of infra red radiation). They also used another established mass-luminosity relation measure to determine the age of the clusters.

Graphing disk fraction over age for populations of Sun-equivalent metallicity stars versus populations of low metallicity stars in the outer galaxy suggests that the protoplanetary disks of those low metallicity stars disperse much quicker.

Left image - The Subaru Telescope in Hawaii. Credit: NAOJ. Right image - the relationship between disk persistence for low metallicity stars (O/H = -0.7, red line) and stars with Sun-equivalent metallicity (O/H = 0, black line). The protoplanetary disks of low metal stars seem to disperse quickly, reducing the likelihood of planet formation. Credit: Yasui et al.

The authors suggest that the process of photoevaporation may underlie the shorter lifespan of low metal disks – where the impact of photons is sufficient to quickly disperse low atomic mass hydrogen and helium, while the presence of higher atomic weight metals may deflect those photons and hence sustain a protoplanetary disk over a longer period.

As the authors point out, the lower lifetime of low metallicity disks reduces the likelihood of planet formation. Although the authors steer clear of much more speculation, the implications of this relationship seem to be that, as well as expecting to find less planets around stars towards the outer edge of the galaxy – we might also expect to find less planets around any old Population II stars that would have also formed in environments of low metallicity.

Indeed, these findings suggest that planets, even gas giants, may have been exceedingly rare in the early universe – and have only become commonplace later in the universe’s evolution – after stellar nucleosynthesis processes had adequately seeded the cosmos with metals.

Further reading: Yasui, C., Kobayashi, N., Tokunaga, A., Saito, M. and Tokoku, C.
Short Lifetime of Protoplanetary Disks in Low-Metallicity Environments