Astronomy Without A Telescope – Strange Stars


Atoms are made of protons, neutrons and electrons. If you cram them together and heat them up you get plasma where the electrons are only loosely associated with individual nuclei and you get a dynamic, light-emitting mix of positively charged ions and negatively charged electrons. If you cram that matter together even further, you drive electrons to merge with protons and you are left with a collection of neutrons – like in a neutron star. So, what if you keep cramming that collection of neutrons together into an even higher density? Well, eventually you get a black hole – but before that (at least hypothetically) you get a strange star.

The theory has it that compressing neutrons can eventually overcome the strong interaction, breaking down a neutron into its constituent quarks, giving a roughly equal mix of up, down and strange quarks – allowing these particles to be crammed even closer together in a smaller volume. By convention, this is called strange matter. It has been suggested that very massive neutron stars may have strange matter in their compressed cores.

However, some say that strange matter has a more fundamentally stable configuration than other matter. So, once a star’s core becomes strange, contact between it and baryonic (i.e. protons and neutrons) matter might drive the baryonic matter to adopt the strange (but more stable) matter configuration. This is the sort of thinking behind why the Large Hadron Collider might have destroyed the Earth by producing strangelets, which then produce a Kurt Vonnegut Ice-9 scenario. However, since the LHC hasn’t done any such thing, it’s reasonable to think that strange stars probably don’t form this way either.

More likely a ‘naked’ strange star, with strange matter extending from its core to its surface, might evolve naturally under its own self gravity. Once a neutron star’s core becomes strange matter, it should contract inwards leaving behind volume for an outer layer to be pulled inwards into a smaller radius and a higher density, at which point that outer layer might also become strange… and so on. Just as it seems implausible to have a star whose core is so dense that it’s essentially a black hole, but still with a star-like crust – so it may be that when a neutron star develops a strange core it inevitably becomes strange throughout.

Anyhow, if they exist at all, strange stars should have some tell tale characteristics. We know that neutron stars tend to lie in the range of 1.4 to 2 solar masses – and that any star with a neutron star’s density that’s over 10 solar masses has to become a black hole. That leaves a bit of a gap – although there is evidence of stellar black holes down to only 3 solar masses, so the gap for strange stars to form may only be in that 2 to 3 solar masses range.

By adopting a more compressed 'ground state' of matter, a strange (quark) star should be smaller, but more massive, than a neutron star. RXJ1856 is in the ballpark for size, but may not be massive enough to fit the theory. Credit:

The likely electrodynamic properties of strange stars are also of interest (see below). It is likely that electrons will be displaced towards the surface – leaving the body of the star with a nett positive charge surrounded by an atmosphere of negatively charged electrons. Presuming a degree of differential rotation between the star and its electron atmosphere, such a structure would generate a magnetic field of the magnitude that can be observed in a number of candidate stars.

Another distinct feature should be a size that is smaller than most neutron stars. One strange star candidate is RXJ1856, which appears to be a neutron star, but is only 11 km in diameter. Some astrophysicists may have muttered hmmm… that’s strange on hearing about it – but it remains to be confirmed that it really is.

Further reading: Negreiros et al (2010) Properties of Bare Strange Stars Associated with Surface Electrical Fields.

Astronomy Without A Telescope – Brown Dwarfs Are Magnetic Too


I feel a certain empathy for brown dwarfs. The first confirmed finding of one was only fifteen years ago and they remain frequently overlooked in most significant astronomical surveys. I mean OK, they can only (stifles laughter) burn deuterium but that’s something, isn’t it?

It has been suggested that a clever way of finding more brown dwarfs is in the radio spectrum. A brown dwarf with a strong magnetic field and a modicum of stellar wind should produce an electron cyclotron maser. Roughly speaking (something you can always depend on from this writer), electrons caught in a magnetic field are spun energetically in a tight circle, stimulating the emission of microwaves in a particular plane from the star’s polar regions. So you get a maser, essentially the microwave version of a laser, that would be visible on Earth – if we are in line of sight of it.

While the maser effect can probably be weakly generated by isolated brown dwarfs, it’s more likely we will detect one in binary association with a less mass-challenged star that is capable of generating a more vigorous stellar wind to interact with the brown dwarf’s magnetic field.

This maser effect is also proposed to offer a clever way of finding exoplanets. An exoplanet could easily outshine its host star in the radio spectrum if its magnetic field is powerful enough.

So far, searches for confirmed radio emissions from brown dwarfs or orbiting bodies around other stars have been unsuccessful, but this may become achievable in the near future with the steadily growing resolution of the European LOw Frequency ARray (LOFAR), which will be the best such instrument around until the Square Kilometer Array (SKA) is built – which won’t be seeing first light before at least 2017.

Geometrically-challenged aliens struggling to make a crop circle? Nope, it's a component of the LOFAR low frequency radio telescope array. Credit:

But even if we can’t see brown dwarfs and exoplanets in radio yet, we can start developing profiles of likely candidates. Christensen and others have derived a magnetic scaling relationship for small scale celestial objects, which delivers predictions that fit well with observations of solar system planets and low mass main sequence stars in the K and M spectral classes (remembering the spectral class mantra Old Backyard Astronomers Feel Good Knowing Mnemonics).

Using the Christensen model, it’s thought that brown dwarfs of about 70 Jupiter masses may have magnetic fields in the order of several kilo-Gauss in their first hundred million years of life, as they burn deuterium and spin fast. However, as they age, their magnetic field is likely to weaken as deuterium burning and spin rate declines.

Brown dwarfs with declining deuterium burning (due to age or smaller starting mass) may have magnetic fields similar to giant exoplanets, anywhere from 100 Gauss up to 1 kilo-Gauss. Mind you, that’s just for young exoplanets – the magnetic fields of exoplanets also evolve over time, such that their magnetic field strength may decrease by a factor of ten over 10 billion years.

In any case, Reiners and Christensen estimate that radio light from known exoplanets within 65 light years will emit at wavelengths that can make it through Earth’s ionosphere – so with the right ground-based equipment (i.e. a completed LOFAR or a SKA) we should be able to start spotting brown dwarfs and exoplanets aplenty.

Further reading: Reiners, A. and Christensen, U.R. (2010) A magnetic field evolution scenario for brown dwarfs and giant planets.

Astronomy Without A Telescope – Stellar Quakes and Glitches

The upper crust of a neutron star is thought to be composed of crystallized iron, may have centimeter high mountains and experiences occasional ‘star quakes’ which may precede what is technically known as a glitch. These glitches and the subsequent post-glitch recovery period may offer some insight into the nature and behavior of the superfluid core of neutron stars.

The events leading up to a neutron star quake go something like this. All neutron stars tend to ‘spin down’ during their life cycle, as their magnetic field applies the brakes to the star’s spin. Magnetars, having particularly powerful magnetic fields, experience more powerful braking.

During this dynamic process, two conflicting forces operate on the geometry of the star. The very rapid spin tends to push out the star’s equator, making it an oblate spheroid. However, the star’s powerful gravity is also working to make the star conform to hydrostatic equilibrium (i.e. a sphere).

Thus, as the star spins down, its crust – which is reportedly 10 billion times the strength of steel – tends to buckle but not break. There may be a process like a tectonic shifting of crustal plates – which create ‘mountains’ only centimeters high, although from a base extending for several kilometres over the star’s surface. This buckling may relieve some of stresses the crust is experiencing – but, as the process continues, the tension builds up and up until it ‘gives’ suddenly.

The sudden collapse of a 10 centimeter high mountain on the surface of a neutron star is considered to be a possible candidate event for the generation of detectable  gravitational waves – although this is yet to be detected. But, even more dramatically, the quake event may be either coupled with – or perhaps even triggered by – a readjustment in the neutron’s stars magnetic field.

It may be that the tectonic shifting of crustal segments works to ‘wind ‘up’ the magnetic lines of force sticking out past the neutron star’s surface. Then, in a star quake event, there is a sudden and powerful energy release – which may be a result of the star’s magnetic field dropping to a lower energy level, as the star’s geometry readjusts itself. This energy release involves a huge flash of x and gamma rays.

In the case of a magnetar-type neutron star, this flash can outshine most other x-ray sources in the universe. Magnetar flashes also pump out substantial gamma rays – although these are referred to as soft gamma ray (SGR) emissions to distinguish them from more energetic gamma ray bursts (GRB) resulting from a range of other phenomena in the universe.

However, ‘soft’ is a bit of a misnomer as either burst type will kill you just as effectively if you are close enough. The magnetar SGR 1806-20 had one of largest (SGR) events on record in December 2004.

Along with the quake and the radiation burst, neutron stars may also experience a glitch – which is a sudden and temporary increase in the neutron star’s spin. This is partly a result of conservation of angular momentum as the star’s equator sucks itself in a bit (the old ‘skater pulls arms in’ analogy), but mathematical modeling suggests that this may not be sufficient to fully account for the temporary ‘spin up’ associated with a neutron star glitch.

Theoretical model of a neutron star's interior. An iron crystal core overlies a region of neutron-enriched atoms, below which is the degenerate matter of the core - where sub-atomic particles are stretched and twisted by magnetic and gravitational forces. Credit: Université Libre de Bruxelles (ULB).

González-Romero and Blázquez-Salcedo have proposed that an internal readjustment in the thermodynamics of the superfluid core may also play a role here, where the initial glitch heats the core and the post-glitch period involves the core and the crust achieving a new thermal equilibrium – at least until the next glitch.

Astronomy Without A Telescope – Making Sense Of The Neutron Zoo

The spectacular gravity of neutron stars offers great opportunities for thought experiments. For example, if you dropped an object from a height of 1 meter above a neutron star’s surface, it would hit the surface within a millionth of a second having been accelerated to over 7 million kilometers an hour.

But these days you should first be clear what kind of neutron star you are talking about. With ever more x-ray sensitive equipment scanning the skies, notably the ten year old Chandra space telescope, a surprising diversity of neutron star types are emerging.

The traditional radio pulsar now has a number of diverse cousins, notably magnetars which broadcast huge outbursts of high energy gamma and x-rays. The extraordinary magnetic fields of magnetars invoke a whole new set of thought experiments. If you were within 1000 kilometres of a magnetar, its intense magnetic field would tear you to pieces just from violent perturbation of your water molecules. Even at a safe distance of 200,000 kilometres, it will still wipe all the information off your credit card – which is pretty scary too.

Neutron stars are the compressed remnant of a star left behind after it went supernova. They retain much of that stars angular momentum, but within a highly compressed object only 10 to 20 kilometers in diameter. So, like ice skaters when they pull their arms in – neutron stars spin pretty fast.

Furthermore, compressing a star’s magnetic field into the smaller volume of the neutron star, increases the strength of that magnetic field substantially. However, these strong magnetic fields create drag against the stars’ own stellar wind of charged particles, meaning that all neutron stars are in the process of ‘spinning down’.

This spin down correlates with an increase in luminosity, albeit much of it is in x-ray wavelengths. This is presumably because a fast spin expands the star outwards, while a slower spin lets stellar material compress inwards – so like a bicycle pump it heats up. Hence the name rotation powered pulsar (RPP) for your ‘standard’ neutron stars, where that beam of energy flashing at you once every rotation is a result of the braking action of the magnetic field on the star’s spin.

It’s been suggested that magnetars may just be a higher order of this same RPP effect. Victoria Kaspi has suggested it may be time to consider a ‘grand unified theory’ of neutron stars where all the various species might be explained by their initial conditions, particularly their initial magnetic field strength, as well as their age.

It’s likely that the progenitor star of a magnetar was a particularly big star which left behind a particularly big stellar remnant. Thus, these rarer ‘big’ neutron stars might all begin their lives as a magnetar, radiating huge energies as its powerful magnetic field puts the brakes on its spin. But this dynamic activity means these big stars lose energy quickly, perhaps taking on the appearance of a very x ray luminous, though otherwise unremarkable, RPP later in their life.

Other neutron stars might begin life in less dramatic fashion, as the much more common and just averagely luminous RPPs, which spin down at a more leisurely rate – never achieving the extraordinary luminosities that magnetars are capable of, but managing to remain luminous for longer time periods.

The relatively quiet Central Compact Objects, which don’t seem to even pulse in radio anymore, could represent the end stage in the neutron star life cycle, beyond which the stars hit the dead line, where a highly degraded magnetic field is no longer able to apply the brakes to the stars’ spin. This removes the main cause of their characteristic luminosity and pulsar behaviour – so they just fade quietly away.

For now, this grand unification scheme remains a compelling idea – perhaps awaiting another ten years of Chandra observations to confirm or modify it further.


Quintessence is one idea – hypothesis – of what dark energy is (remember that dark energy is the shorthand expression of the apparent acceleration of the expansion of the universe … or the form of mass-energy which causes this observed acceleration, in cosmological models built with Einstein’s theory of general relativity).

The word quintessence means fifth essence, and is kinda cute … remember Earth, Water, Fire, and Air, the ‘four essences’ of the Ancient Greeks? Well, in modern cosmology, there are also four essences: normal matter, radiation (photons), cold dark matter, and neutrinos (which are hot dark matter!).

Quintessence covers a range of hypotheses (or models); the main difference between quintessence as a (possible) explanation for dark energy and the cosmological constant Λ (which harks back to Einstein and the early years of the 20th century) is that quintessence varies with time (albeit slooowly), and can also vary with location (space). One version of quintessence is phantom energy, in which the energy density increases with time, and leads to a Big Rip end of the universe.

Quintessence, as a scalar field, is not the least bit unusual in physics (the Newtonian gravitational potential field is one example, of a real scalar field; the Higgs field of the Standard Model of particle physics is an example of a complex scalar field); however, it has some difficulties in common with the cosmological constant (in a nutshell, how can it be so small).

Can quintessence be observed; or, rather, can quintessence be distinguished from a cosmological constant? In astronomy, yes … by finding a way to observed (and measure) the acceleration of the universe at widely different times (quintessence and Λ predict different results). Another way might be to observe variations in the fundamental constants (e.g. the fine structure constant) or violations of Einstein’s equivalence principle.

One project seeking to measure the acceleration of the universe more accurately was ESSENCE (“Equation of State: SupErNovae trace Cosmic Expansion”).

In 1999, CERN Courier published a nice summary of cosmology as it was understood then, a year after the discovery of dark energy The quintessence of cosmology (it’s well worth a read, though a lot has happened in the past decade).

Universe Today articles? Yep! For example Will the Universe Expand Forever?, More Evidence for Dark Energy, and Hubble Helps Measure the Pace of Dark Energy.

Astronomy Cast episodes relevant to quintessence include What is the universe expanding into?, and A Universe of Dark Energy.

Source: NASA

High School Students Get Published in Astrophysics Journal

From the left: Klaus Beuermann (group leader), Jens Diese (back,teacher), and the high-school students Joshua Zachmann (front), Alexander-Maria Ploch (back), Sang Paik (front). JD, JZ, and AMP are from the Max-Planck-Gymnasium, SP is from the Felix-Klein-Gymnasium.

High school students from Germany have now done what many scientists strive for: had their research work published by a science journal. The Astronomy & Astrophysics science journal published a paper co-authored by three students who observed the light variations of the faint (19th magnitude) cataclysmic variable EK Ursae Majoris (EK UMa) over two months. Led by astronomer Klaus Beuermann from the University of Göttingen, and the students’ high school physics teacher, the team made use of a remotely-controlled 1.2-meter telescope in Texas. Astronomy & Astrophysics says the team “presents an accurate, long-term ephemeris,” and that “they participated in all the steps of a real research program, from initial observations to the publication process, and the result they obtained bears scientific significance.”

The students, Joshua Zachmann, Alexander-Maria Ploch, Sang Paik and their teacher, Jens Diese, made observations, analyzed the CCD images, produced and interpreted light curves, and looked at archival satellite data. Beuermann, the astronomer they worked with said, “Although it is fun to perform one’s own remote observations with a professional telescope from the comfort of a normal school classroom, it is even more satisfying to be involved in a project that provides new and publishable results rather than to perform experiments with predictable outcomes.”

Cataclysmic variable research is a field where the contributions of small telescopes has a long tradition. Cataclysmic variables are extremely close binary systems containing a low-mass star whose material is being stripped off by the gravitational pull of a white dwarf companion. Due to the transfer of matter between the stars, these systems vary dramatically in brightness on timescales in the whole range between seconds and years. This largely unpredictable variability makes them ideal targets for school projects, particularly since professional observatories are generally unable to provide enough observation time for regular monitoring.

An accurate ephemeris is needed to keep track of the orbital motions of the two stars, but none was available because EK UMa is faint in the optical range and requires a long-term observation of the light variations. The strong magnetic field of the white dwarf turns the light of the hot matter striking the surface of the white dwarf into two “lighthouse” beams. By measuring the times of the minimum between the beams, the group was able to determine an orbital period accurate enough to keep track of the eclipse that took place in 1985, over 100 000 cycles earlier. By combining their own measurements with those made by the Einstein, ROSAT, and EUVE satellites, they estimated the orbital period over 137 000 cycles to an accuracy of a tenth of a millisecond. Surprisingly, the orbital period is extremely stable, although the period of such very close binaries is expected to vary due to the presence of third bodies and magnetic activity cycles on the companion star.

The team’s paper: (not yet available) A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris, by K. Beuermann, J. Diese, S. Paik, A. Ploch, J. Zachmann, A.D. Schwope, and F.V. Hessman.

Source: Astronomy & Astrophysics