Magnetism and the Lunar Eclipse

Being away from home and in a very difficult environment, I have had the opportunity to be mesmerised by the many loving and genuine young people that I have met, so peaceful and kind with some of the most heartbreaking stories I have ever heard. It is wonderful to see the awe in their eyes when I explained to them some of the upcoming events that our galaxy is offering. On Friday the 27th of July at 8.15pm, we will have the chance to witness the Lunar Eclipse begin, reaching full eclipse by 10.30pm and often known as ‘blood moon’ (it will go a deep burnt red due to refraction of light from our atmosphere much like a sunset) that will be the longest lunar eclipse of the century. In addition, the planet Mars will be at opposition when it will be at its most brightest and nearing its closest to earth in 15 years, accompanied with the International Space Station passing on what will be a clear summer’s evening that I will be spending on the rooftop of this dilapidated building in the heart of the Middle East. While light pollution is a significant issue, everyone will get a chance to witness naked eye these phenomenal galactic events that gives them a glimpse into a series of astronomical narratives that will no doubt provoke questions about our planet, orbits and space. These events arrive at the same time I hear the amazing news that my program working with refugees and asylum seekers back home in Australia will be funded, where I will take disadvantaged girls and young women out on hikes, camping and viewings over my telescope where I will teach them about stargazing. It has given me inspiration on my only afternoon off for the week to write this blog post!

I was asked today, “how does all this happen?” and I attempted – albeit due to language barriers rather awfully! – a brief explanation of the moon crossing the ecliptic where the Earth, Sun and Moon are aligned when the moon orbits through the shadow of Earth. Planets orbit around the sun, the gravity pulling and keeping them in orbit and gravity acts as a powerful force between two objects with a mass. You can read more on the eclipse in a previous blog post that I wrote. Magnetism is an entirely separate force despite similarities and it depends more on particular properties rather than simply mass, such as electrons and can both push and pull. Magnetism is present throughout the universe and we can experience it in many ways; when I am out hiking, my compass explains the pressure of magnetism and direction with the movement of the needle as it is attracted by the force.

There are a number of properties and varieties of magnetic forces that explain invisible fields that applies a force that influence objects or material from the magnetism. There are rules that confirm magnetic fields are dipolar and just like earth has both a north and south magnetic pole and the ‘magnetic flux’ explains how the force and attraction between the poles – usually represented by lines as visible in the image below – that can be averaged by the magnetic field and the perpendicular area the field infiltrates. Measurements of the force is determined by the mathematical formula F= qvB (Lorentz Force Law), which is the magnetic force, the charge, the velocity and the magnetic field and the unit of these field are measured in terms of Standard International (SI) units known as tesla.

Earth’s magnetic field is known as a geomagnetic field and magnetosphere the predominate reason for the magnetic field is the liquid iron core surrounding the solid inner core is the source of this phenomenon, the very ‘magnet’ where the electric currents produced by the flow of iron and other metals including nickel cause convection currents from the inertial force of the Coriolis Effect that ultimately splits the field into a surrounding force that envelops Earth and aligns back into the same direction. The changes in temperature and composition of the liquid core creating the currents that rise or sink matter all play a part in Earths magnetic field, that can be captured visually when solar winds collide with it (usually where the magnetic force is much stronger near the north and south poles) and the charged particles trapped by the magnetic field produce the aurora borealis or the aurora australis.

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The picture explains the rotational poles but that their alignment geographically differs from our north and south poles on earth, whereby the magnetic south poles resides further north of Antarctic’ South Pole and quite close to the south of Australia while the north magnetic pole is closer to northern Canada and thus south of the North Pole. The magnetic lines explain the streamlined flow of the magnetic field that makes it easier to ascertain the process mathematically. Jupiter has a number of powerful toroidal magnetic fields where the intensity is said to have formed from the dynamic movements of the metallic hydrogen within; the field on the surface of the clouds is almost ten times stronger than earth’s. The Milky Way also has magnetic fields as do galaxies and the universe contains some colossal magnetic fields, where observations of galaxy clusters have found magnetic fields extending millions of light years!

The magnetic force is the attraction or the repulsion (as you experience when attempting to connect two magnets with equal poles) occurs from the magnetic field. The properties of electrical fields (pole) with a positive and negative charge differ with that of magnetic fields (dipole) despite a close correlation, because electromagnetism involves a magnetic dipole producing an electric field as it moves and conversely an electric field can produce a magnetic field meaning the difference is an elementary change in the field. A magnet does not have an electric charge as two separate poles, while a dipole interacts as a charge as visualised in the following image. What this means is that the electrical force itself behaves on a charged particle in the direction of the field and does not need motion while a magnetic force requires this motion and acts perpendicular to the magnetic field.

Gravitational fields also acts as force fields for mass and the gravitational force itself depends on the mass and the mass experiences the gravitational force. The gravitational field has a place in every direction and point in space and known by the formula g = F/m where F is the force of gravity. While this may be a brief example of the difference between magnetism and gravitational fields, it will be a wonderful experience with the full lunar eclipse and Mars showcasing the marvels of the universe over a hot, clear night here in Jerusalem!

Further Reading:

Maurizio Gasperini, Theory of Gravitational Interactions, Springer (2016) 115
Stephen Blundell, Magnetism: A Very Short Introduction, Oxford (2012) 106
Anupam Garg, Classical Electromagnetism in a Nutshell, Princeton University Press (2012) 83

The Cassini-Huygens Mission to Saturn and Titan

Following the painstaking experience watching Titan on Netflix last night (I highly recommend you don’t watch it) which was about the possibility of life on Saturn’ moon following an existential crises plaguing earth, out of sheer agitation I decided to look into this theme and find out more. The Cassini-Huygens mission was a seven year journey of the NASA Cassini Orbiter along with the European Space Agency’ Huygens Probe intended to travel to the Saturn system. Titan IVB-Centaur rocket was launched from Cape Canaveral carrying the orbiter and probe in 1997 and passing Venus, Earth and our moon, an Asteroid Belt and Jupiter, the Huygens Probe successfully landed on Saturn’s moon Titan in 2005. The Cassini spacecraft also managed to capture new and detailed information and images including a number of new moons such as Methone and Pallene, as well as the first spacecraft to orbit Saturn that showcased surprising activity on Enceladus, new rings around Saturn, and a plethora of additional information not previously known by scientists. The 2 hour 27 minute descent to the surface was matched with 72 minutes on the frozen ground until contact with Cassini was lost, and the probe managed to obtain images of Titan’s geology and meteorology to reveal astonishing similarities to Earth. But is Titan really similar to Earth?

The primary scientific goal of the mission was to explore Saturn’s interior and atmosphere, the ring system and the magnetosphere and plasma environment, as well as the magnetic field and origins of Saturn. In addition, a strong focus Saturn’ second largest moon, Titan when the Huygens probe was launched into the moon through the thick atmosphere during its descent to the surface. The probe was intended to study both the atmosphere and the surface composition and contain six instruments that enabled this, such as the Huygens Atmospheric Structure Instrument (HASI) designed to measure the electrical and physical properties of Titan.[1] It was additionally equipped with the following experiments:

Surface Science Package (SSP) aimed at determining the properties of the surface where the probe landed in order to ascertain further details of its composition and measure other aspects of the landing site.

Descent Imager Spectral Radiometer (DISR) that used sensors to uncover spectral measurements of the surface and as it descended toward the surface took images of the spectra.

Gas Chromatograph and Mass Spectrometer (GCMS) that attempted to identify gaseous atmospheric properties and captured material that it analysed as it descended to the surface.

Aerosol Collector and Pyrolyser (ACP) is a device that collected chemical and aerosol samples of the atmosphere and analysed the material taken from the atmosphere.

Doppler Wind Experiment (DWE) detected Doppler shifts that the probe experienced and caused by the atmosphere and winds was measured along with other properties through radio signals.[2]

The atmosphere of Titan contains the chemical composition mostly of nitrogen, but also a mixture of methane, ethane and other hydrocarbons, which makes the composition itself similar to that of Earth that contains 80% nitrogen,[3] particularly as it is the only other planetary body in our solar system that has evidence of liquid on the surface. Images from Cassini show how the surface of the moon contains rivers of ethane and methane and explains why it is the only moon that has clouds and a thick atmosphere. The orbital period of Titan around Saturn nears 16 days and is tidally-locked to Saturn, while the distance to the sun is almost ten times further than earth at 9.54 AU making the solar energy captured by the moon at a much lower rate, however the atmosphere enables Titan to capture solar energy. “It’s partial transparency to significant amounts of sunlight, and its high opacity to longer wavelengths… [s]ome of the energy from the Sun reaches the surface of Titan because solar radiation consists mostly of photons at near-visible wavelengths and the atmosphere is partially transparent at those wavelengths,”[4] that allows only about 10% of the energy to reach the surface. This energy is then absorbed by the surface that releases infrared heat out and back into the atmosphere that re-radiates by out and back down and the phenomenon of this greenhouse effect contributes to the temperature, which is at 92k (-180 degrees Celsius). The atmospheric pressure of Titan is 60% greater than Earth and the radius is almost half, but as the mass of Titan is 1.3452 x 10^23 kg compared to this size, the gravity cannot hold the gas and thus the atmosphere is much greater than that of Earth.

Seasonal variations to do exist on Titan and the Cassini mission aided researchers to ameliorate their understanding of the patterns of atmospheric changes, arriving on the northern hemisphere of Titan during winter.[5] Seasonal changes, however, are incredibly slow as it is driven by the eccentricity of Saturn’ 29.5 yearlong orbit around the sun, and the atmosphere responds to the effects of these changes in rising and falling temperatures as well as day to night variations. The orbital configuration has resulted in an imbalance of methane lakes and rivers in the northern and southern hemispheres of Titan that transports chemicals through evaporation and precipitation differently. The atmospheric pressure on the surface is at 1500mbar and researchers have confirmed the moon experiences cryovolcanism captured by observations from Cassini where the eruption style and composition is similar to volcanoes erupting liquid instead of lava.[6] Among the other important findings include the origins of Titan’ methane and the chemistry of the atmosphere as the gas increased during the descent of the probe, until it reached the surface when a spike was detected at almost 40% increase in methane suggest the possibility during the formation and accretion period, methane became trapped in the ice and reached the surface through cryovolcanism. In addition, radiogenic argon-40 (40Ar) was also detected that further confirms details of Titan’ interior since 40Ar can only form through the decay of potassium-40 (40K) that is found in rocks.[7]

HASI findings also enabled scientists to measure the density of the upper atmosphere and showed the thermosphere to be warmer and with a greater density. Atmospheric circulation and the transportation of heat was captured by the DISR by the “imbalance seen in the radiative flux measurements”[8] and it indicates winds verified by the probe as it encountered the flow from west to east and therefore the same direction as the rotation of Titan. Models have since showed that the captured data confirmed a reversal of direction and different points during the descent of the probe caused by temperature variations between the northern and southern hemispheres and within the Hadley cell that circulates from the north and south poles.[9] The super rotating winds measured the Doppler shift during the descent of the probe and measured large variations in wind speeds as it decreased the closer it reached the surface and the findings also suggest that Titan does not have a mesosphere as was predicted.

The atmosphere of Titan was found to be stratified comparatively to Earth’ troposphere where variations in temperature produce layers and while the amount of sunlight is minimal, the icy moon still contains wind and clouds, where clear by images by Cassini indicate massive dunes on the surface.[10] This provides greater details of the structure of the atmosphere including its temperature, as the DISR captured images of the surface and terrain and showed a plateau of dried river beds and lakes with narrow channels and dendritic networks that parallel the fluvial configuration you’ll find on Earth particularly erosion which DISR detected and indicative of such activity. Buried below the icy surface together with the discovery of what is known as Schumann resonance, very low radio signals in the atmosphere (around 36 Hertz) and for such signals to be reflected, “an ocean of water and ammonia which is buried at a depth of 55-80km below a non-conducting, icy crust”[11] could explain this.

There were a number of reasons for destroying the Cassini orbiter by incinerating it through the powerful atmosphere of Saturn. Not only will it provide additional information to scientists both gaining closer access to the rings of Saturn and the upper atmosphere that provide a glimpse of the interior structure and the opportunity to accurately ascertain the age and composition through closer inspection of the mass of the rings surrounding the planet, but also because of regulations vis-à-vis interplanetary contamination. The preservation of the integrity of the solar system in order to prevent and potentially damage both other planets and our own has led to all spacecraft to undergo sterilisation processes to avoid microbe contamination, particularly of planets or moons that may be capable of organic habitat. The Cassini spacecraft detected a number of new finds on Enceladus that verifies the possible conditions for contamination of microbes from Earth. Cassini itself captured vapour being released from Enceladus that confirmed an oceanic subsurface Knowing that the craft itself has a maximum output of fuel, to prevent any risk of it contaminating the integrity of the moon, scientists decided to plan the death of the device.[12]

While Titan does have similarities to Earth, particularly for having a thick atmosphere and a high percentage of nitrogen, but a number of other missing or differing constituents make these similarities marginal at best. The benefits of the Cassini-Huygens mission to Saturn and Titan enabled scientists to gain further insight to clarify these terrestrial and atmospheric differences.

[1]Fulchignoni, M., Ferri, F., Angrilli, F. et al. Space Science Reviews (2002) 104: 395. https://doi.org/10.1023/A:1023688607077:
[2] Patrick Irwin, Giant Planets of Our Solar System: Atmospheres, Composition, and Structure, Springer Science & Business Media (2003) 330
[3] http://www.lmd.jussieu.fr/~sllmd/pub/REF/2012Icar..218..707L.pdf
[4] Athena Coustenis, Fred Taylor, F. W. Taylor, Titan: The Earth-like Moon World Scientific (1999) 62
[5] Ingo Müller-Wodarg, Caitlin A. Griffith, Emmanuel Lellouch, Thomas E. Cravens, Titan: Interior, Surface, Atmosphere, and Space Environment, Cambridge University Press (2014) 215
[6] R. M. C. Lopes, Cryovolcanism on Titan: New results from Cassini RADAR and VIMS: https://doi.org/10.1002/jgre.20062
[7] Robert Brown, Jean Pierre Lebreton, Hunter Waite, Titan from Cassini-Huygens, Springer Science & Business Media (2009) 182
[8]Ibid., Titan from Cassini-Huygens, 345
[9] https://www.nasa.gov/mission_pages/cassini/whycassini/cassinif-20070601-05.html
[10] http://pirlwww.lpl.arizona.edu/~jani/radebaugh-titandunes-aeolian13.pdf
[11] http://sci.esa.int/cassini-huygens/55230-science-highlights-from-huygens-9-schumann-like-resonances-hints-of-a-subsurface-ocean/
[12] http://www.spacesafetymagazine.com/space-exploration/extraterrestrial-life/cassini-huygens-preventing-biological-contamination/

Alice in Wonderland: Inside a Black Hole

After a long morning on a charity walk before spending the afternoon fixing up my backyard, I decided that I would share the evening with a movie and well deserved dinner. ‘Event Horizon’ looked interesting, indeed it had the beloved Morpheus (Laurence Fishburne) as the main actor and certainly watching an action movie when your mind is incapable of processing anything can always work as a treat. But alas, the movie was rather tedious at best and I regretted not adhering to the temptation of re-watching Aliens with Sigourney Weaver who, admittingly, I have a huge girl-crush on. It would seem that the most mysterious in our universe tends to evoke the most interest, and indeed incredible levels of absurdity. The mystery of the existence of black holes is clearly one of them, from those who downright deny its existence to numerous suggestions about what happens to space and time when we enter a black hole that I felt compelled to ameliorate some details about black holes in this post and to hopefully reduce the likelihood of turning the science into a state of wild farcicality.

Stellar evolution is primarily about the mass and luminosity of stars that over time evolves until it reaches the end of its life cycle with millions of years passing during this process. Initially forming from the nuclear reactions or stellar ignition within a nebulae where gravity pulls the clouds of gas and dust into dense and hot ‘cores’ until the collapse reaches a nuclear fusion, the star is finally born and as the temperature during this fusion increases, it provides the energy that enables continuous emission of light or luminosity. Depending on the size, such as our own sun, the star will quietly settle on the Main Sequence for most of its life as thermonuclear fusion is enabled by the temperature  (10mk) to thus burn hydrogen into helium until the former is completely depleted. To burn helium requires a greater temperature and this is enabled by the force of gravity following the end of nuclear fusion as it contracts and therefore becomes hotter that hydrogen burning is thus ignited as the outer layers expand to form into a red dwarf or giant. The helium nuclei fuse to convert into carbon and oxygen in this rather tumultuous and highly energetic process until helium has completely converted but if the size of the star is not large enough, the contraction at the core does not heat up to the high temperatures needed to burn carbon. The Chandrasekhar Limit is a limit of 1.4 solar masses that categorises the mass of white dwarfs, which are the final result of low mass stars that are held together through electron degeneracy pressure. The density and pressure is enough to prevent further gravitational collapse, however stellar remnants that exceeds this limit will continue to collapse further until it forms into a neutron star which, again, is held together by the neutron degeneracy pressure. To form a black hole, the force of gravity overwhelms the neutron degeneracy pressure and therefore there is nothing left in space that would prevent the continued collapse of the star and thus the continuous singularity where therein contains no volume and infinite density becomes a black hole.

So what would happen if we found ourselves falling into a black hole? The mathematical concept of escape velocity was the first introduction to the theoretical concept and force of a black hole by amateur astronomer Reverend John Mitchell in 1783, whereby equating the universal gravitational constant 6.67 × 10-11 N m2 kg-2 with the mass of the body creating the gravitational field and distance between the body and an object escaping the gravitational field – thus the gravitational potential energy and kinetic energy – one could calculate the required velocity an object would require in an attempt to escape the gravitational pull of the field it is near. Accordingly, the size and radius of this body would then mean that,“all light emitted from such a body would be made to return towards it” and therefore such density would mean that light could never escape. The boundary or radius of the region surrounding the black hole that would enable some form of ‘escape’ is called the event horizon and the distance between the black hole and the event horizon is called the Schwarzschild radius Rwhich is calculated by the escape velocity as equal to the speed of light:

$$R_s = \frac{2GM}{c^2}$$

Whatever falls inside the event horizon will never escape. So what would happen if one passed the event horizon and fell into a black hole? A plethora of postulations have been made, one of them being time dilation, whereby the person travelling into the black hole would experience time as we know it, however outside of the black hole we would never be able to see her cross into the event horizon because time is much slower and what would be a few minutes for the person within could be thousands of years for the observer. That is, as one approaches the event horizon, gravitational redshift would make us see increases in speed of the moving object and anything with strong gravitational fields or compact objects causes an increase in the wavelength while at the same time decreasing the energy output. Spaghettification is yet another, where the tidal force of the gravity would stretch the object as it gets pulled in and the friction would cause it to heat to an incredible temperature.

Steven Hawking has recently purported that it is possible to travel to an alternate universe through a black hole; that is a black hole has ‘soft hair’ or extremely low energy quanta and what passes and event horizon does not disappear into oblivion but can actually come back out, only it will no longer be the same place. It is assumed that a black hole contains only several properties and the ‘no hair theorem’ first expressed by John Wheeler is that whatever falls beyond the event horizon is permanently inaccessible. The speculation is that the conservation of time within the black hole is caused by low-energy quantum excitations or ‘soft hair’ that when a black hole captures information by the material entering it, it also releases this information back out as it evaporates. But with time dilation, the information that is released is released perhaps into somewhere billions of years into the future or even a completely different universe. Hawking studied the emission of thermal energy or blackbody radiation (Hawking Radiation), which is indicative that quantum matter must be entering the black hole and that the source of its parameters would also eventually dissipate. According to quantum theory, this is caused by subatomic particles that exist for a moment as two separate (positive and negative) charged particles before reunited into one another and annihilating that momentary separation, as though their existence relies on the other in a perpetuity and these particle/anti-particles are present all over space. If they separate at the time of reaching a black hole, the positive would have the necessary charge to escape – effectively becoming the blackbody radiation that we observe – while the negative is doomed to fall in and as such the black hole will lose mass. This changes the classical conversation laws as the state of the particles changes at quantum level.

There are a number of methods currently being used to observe the existence of black holes, some indirectly particularly through binary systems – where a star is orbiting a black hole – and thus the emission of X-ray sources is stronger from the accretion disk’s spectrum, since it would imply that the star is orbiting a very dense object and thus a black hole. There are stellar black holes and then there are supermassive black holes, the latter containing millions and even a billion times more mass than its stellar counterpart. Supermassive black holes are said to be at the centre of our Milky Way and most large galaxies and observations of distant quasars that radiates incredible energy have enabled astronomers to conclude that the astounding levels of energy is only possible by a supermassive black hole. The formation of a supermassive black hole is unknown, though it is believed that the early stages of the universe assisted in their formation and as it consumed material over billions of years grew to its astounding size and power. It is also said that the supermassive black holes are the cause of active galactic nuclei that emit non-thermal energy such as quasars as well as galactic jets.

In 2014, NASA’ two telescopes detected an X-ray Flare from a supermassive black hole – Markarian 335 – that gave insight to astronomers about shifting coronas to an X-ray flare. The corona is a mysterious source of highly energetic particles or radiation found near the black hole accretion disk and they emit X-ray light, however details relating to their form and location of the black hole – since an event like a flare released near the event horizon would change our understanding of black holes including how fast it is spinning. There are two proposed suggestions of the position of the corona, with the first being Lamp Post Model where the corona is positioned on the axis above the rotating black hole, or the Sandwich Model where the corona is spread above and below the disc but the results suggest the former LP Model is likely. The disk around the black hole glows from the hot gas that is drawn around it and emits X-rays and as the material in the corona contracts as they are drawn closer together and the pressure launches the material out of the corona as it forms into a jet at ~20% speed of light. The brightness from the Doppler boosting or relativistic beaming where the concentration of superluminal motion of the jets remains somewhat mysterious.

The recent observation of the supermassive black hole Markarian 335 by NASA’ Nuclear Spectroscopic Telescope Array (NuSTAR) as well as the Swift Gamma-Ray Telescope – Markarian 335 being 324 million light years away – observed a large pulse of X-ray energy following the release of the corona away from the black hole. The observation enabled scientists to understand that the flare involves a process of release, that is a high-speed “launch” of the corona directly from the Black Hole that then causes the flare itself. The accretion disk of the black hole is incredibly hot where materials such as gas and space dust that has not yet been absorbed by the black spin around the event horizon and produce a glow in ultraviolet light. There are some explanations of the X-ray signals that NASA has detected, suggesting that as the heat around the accretion disk from the material glows ultraviolet and scatter above the disk which is further illuminated by X-ray energy that reflects off the disk, but there is also the theory that clouds block the visualisation of the mouth of the black hole and that shapes the X-ray spectrum that the detectors obtain with recent observations from the Gemini South Telescope in Chile that was able to measure the motions of gas around a supermassive black hole and zoomed in 10x closer to the galaxy core of NGC1097 and detected gas clouds ten light years from the nucleus. While flares are still mysterious, astronomers are taking steps closer toward understanding them.

Michael A. Seeds, Dana Backman, Stars and Galaxies, Cenage Learning (2015) 309
https://phys.org/news/2006-01-scientists-probe-black-hole-sanctum.html#jCp
Michal Dovciak, An XSPEC model to explore spectral features from black-hole sources – II. The relativistic iron line in the lamp-post geometry, arXiv:1412.8627  [astro-ph.HE]
Supermassive black hole corona and flare. A&G 2015; 56 (6): 6.5. doi: 10.1093/astrogeo/atv180
The Anatomy of a Black Hole, https://www.nasa.gov/image-feature/jpl/pia20051/the-anatomy-of-a-black-hole-flare
Gary T. Horowitz, Viewpoint: Black Holes Have Soft Quantum Hair, University of California, (June 6, 2016) Physics 9, 62
S. W. Hawking, M. J. Perry, and A. Strominger, “Soft Hair on Black Holes,” Phys. Rev. Lett. 116, 231301 (2016).

Ripples Through The Universe: A Stellar Find

Albert Einstein has become synonymous with term ‘genius’ and having set the foundations that strengthened the study of physics with his Theory of Special Relativity [SR] early in the twentieth century – that determined the speed of light is observed the same in any frame of reference and that the laws of physics is invariant for observers moving at a constant – it is easy to see why. This set to motion his General Theory of Relativity [GR], the most important step forward in scientific history that ameliorated our understanding of both gravity and of the curvature of space and time interwoven into a continuum. Unlike Newtonian physics where gravity is understood as a force and while space is influenced by this force, gravity instead was understood as a field within space-time and curved by the mass of objects like planets and stars. Thus gravitational fields are curved by matter and fastened to the geometry of space and time that responds by telling matter how it should move. Further studies by physicists continued toward the latter half of the twentieth century when Nobel laureates Russell Hulse and Joseph Taylor discovered a new type of pulsar, and the discovery enabled revolutionary studies into understanding gravitation. The survival of remnants from a supernova core is so massive that it collapses into a neutron star by condensing the protons and electrons into a neutron and into a very compact and dense space. The core of the neutron star can have the mass of one sun and only the diameter of around 15 km . Neutron stars spin very rapidly and emit radio waves that are detected as ‘pulses’ and their discovery of PSR 1913+16 binary pulsar – a radiating neutron star – helped ignite the prediction in GR of gravitational waves. The Hulse-Taylor Binary star system that has two neutron stars orbiting one another would lose energy through the radiation of gravitational waves and as the rate of the orbital timescale is decreasing, it confirmed gravitational radiation as predicted in GR must be the cause.

It is amazing that my studies in astronomy have allowed me to find out that there is evidence to prove gravitational waves! As a consequence, scientists attempted to build technology to study the possibility of gravitational waves and built the aLIGO – Advanced Laser Interferometer Gravitational-Wave Observatory – an upgrade to the initial iLIGO that is capable of observing great distances at almost 300 million parsecs. There are current discussions to prepare a LIGO detection device – eLISA – for space where detection could be far more accurate. These L-shaped devices known simply as interferometers are shaped like the letter L with each side of this L at the length of four kilometres, and contain laser lighting that measures the length of each side. This is done at a very meticulous rate at 1×10-15 meters as any gravitational-wave that enters LIGO would change the measurements and the light would stream out the interferometer. These interferometers are located in two observatories in Washington and Louisiana and when the gravitational-wave hit earth, both shifted synchronically by 0.0007 seconds. This was the first official confirmation that physicists detected gravitational waves and confirmed Einstein’ theory. The detection is said to have been formed by the collision between two black holes 1.3 billion years ago at a combined mass of 62 suns. However, individually, one binary black hole had the mass of 29 suns and the other 36, with the violence of the collision forcing three solar masses to be released as energy out as gravitational-waves across the fabric of the universe.

The first observation of gravitational waves known as GW150914 demonstrated binary black hole mergers that involve two black holes near one another. There are a number of ways that black holes can be formed and categorised depending on its mass and size, including the smallest primordial black holes, medium stellar black holes and the most common, as well as the largest supermassive black holes that contain the mass of one million suns that has been reported to exist at the centre of the galaxy. Stellar black holes begin its cycle during the end phase of the evolution of a star as it collapses in itself. To briefly ameliorate stellar evolution, when a protostar is small enough to heat and trigger nuclear fusion at its core, a star begins its life and the accumulation of particles continues to attract more as part of the accretion process until it generates a core temperature over 10,000K to enable it to sit on the Main Sequence, just like our sun. Otherwise, it will become a brown dwarf. When our sun – a yellow dwarf – becomes a red giant as hydrogen atoms are combined together to form helium atoms and along with the expansion of the surface area as the core continues to get hotter, the elements are transmogrified to heavy carbon and others until the helium stores are depleted. As the sun loses temperature, it really cannot do much with the carbon and thus gravity will enable it to expand and become unstable as outer layers disintegrate. The only remaining area of the star is the core – the white dwarf. At this point, the remnants of the red dwarf will cover the white dwarf with a planetary nebula and over time, it will cool down and into a black dwarf.

Hertzprung-Russel Diagrams [H-R diagram] contains details that explains the evolution vis-a-vis changes in the temperature and luminosity of a star and strengthens our understanding of where distinct group the brightest stars fall into [this includes supergiants, giants, subgiants and white dwarfs] and comparing it to our own sun that sits on the main sequence. Majority of our stars sit along the main sequence and all contain lumonsity scores that categorise them into spectral classes.

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Variable stars, however, are the least stable and often change luminosity and size rapidly both for intrinsic and extrinsic reasons. For instance, Rotating Ellipsoidal Variables is perhaps a simplified example of why the luminosity of stars change, whereby extrinsic variables implies that the fluctuations in luminosity are wholly external to the star’ dynamics. Brightness of stars is dependent on the surface area – hence why massive stars are more brighter – thus when a pair of stars appear side-by-side, their surface area increases [from where we see them] and thus a binary system where stars are close enough that their shapes become distorted. This shape appears somewhat egg-like rather than spherical. Changes to the stars shape and luminosity has enabled astronomers to measure the time it takes for the stars to rotate. Spica, the brightest star in the constellation Virgo, is considered a Ellipsoidal Variables but detection is difficult as fluctuations are minimal [0.92-1.04 magnitudes] though they have captured that the length of the rotation is four days. Spica A is 11 times greater than our sun whilst Spica B is 7, which is why it is one of the top brightest stars in the night sky.

AM Herculis is a part of a unique class of cataclysmic variable stars; a binary star containing a white dwarf where its magnetic field is so strong that it channels the flow or synchronisation of the rotational and orbital system with the red dwarf star. It is known as ‘AM Her Stars’ or ‘Polars’, magnetic cataclysmic variables that contain two stars – a dominate white dwarf together with a red dwarf star – where a circular polarization exists together with a strong magnetic field. Cataclysmic variables are distinguished between non-magnetic and magnetic and the case of AM Herculis, it is the latter (polars). Polars are also divided into two categories, intermediate DQ Her Stars [where the magnetosphere is not strong enough though an accretion disk is formed] along with AM Her stars, that contains a very strong magnetic field that it pulls the two stars together into a synchronous rotation where the magnetic field eventually dominates the entire system and does not form an accretion disk. That is the red dwarf loses material as it is channeled by the strength of the white dwarf’ magnetic field and prevents the formation of an accretion disk, instead forming a funnel or accretion stream toward the magnetic poles of the white dwarf that emits strong x-ray emissions. These emissions heat the area around the pole that the kinetic energy sources soft x-rays on the white dwarf’ surface. The flow of material is locked into this funnel preferentially to one magnetic pole of the white dwarf. Other variable star types can be found here.

The death of a star by supernova is a cataclysmic event or a cosmic explosion that releases remnants of gas and contain radio waves and X-ray emissions. However, sometimes as the star depletes nuclear fuel, it no longer contains the energy to resist gravity and therefore the elements or material in the core of the star is compressed by the force of the gravity until it collapses under the weight. An extremely large star – something around 3 solar masses or larger – may collapse into a black hole. While black holes are so dense that light cannot escape, astronomers have been capable of identifying the existence of them – such as Cygnus X-1  – through emitted X-rays from the hot accretion disks surrounding the black hole as it captures nearby gas from a star. A black hole, surprisingly, does not contain many properties, which are mass, spin and electrical charge  and it may be that the latter contains no ‘charge’ while theoretically the matter would continue to collapse until there was no longer a radius and thus infinite density. This compression is known as the singularity and discovered by Karl Schwarzschild following the release of Einstein’ GT as the curvature of spacetime becomes infinite.

The detection of gravitational waves confirms that Einstein was correct and will enable scientists to understand the early universe with more acumen. It will also set the stage for a new area of astronomy and I just had to write a little bit about it.

Hulse R A 1994 The discovery of the binary pulsar, Rev. Mod. Phys. 66 699 and Taylor J H 1994 Binary pulsars and relativistic gravity, Rev. Mod. Phys. 66 711
Otto Struve, Stellar Evolution: An Exploration from the Observatory, Princeton University Press (2015) 33
https://www.e-education.psu.edu/astro801/content/l5_p3.html
http://www.ligo.org/science/Publication-GW150914/
Beech, M, The Ellipsoidal Variable, Astrophysics and Space Science (ISSN 0004-640X), vol. 117, no. 1, Dec. 1985, p. 69-81.
Hessman, F.V., Gansicke, B.T., and Mattei, J.A., “The history and source of mass-transfer variations in AM Herculis”, Astron. Astrophys., 361, 952-958, 2000.