Current Affairs / General Musings

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.

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