WHAT Is…?

While the data are public domain, the entire textual content and layout of the articles on this page are Copyright © Brian Timmins 2008, et seq. All rights reserved. Should you wish to use any of it for personal research or educative purposes, you may make one hard copy for your own use without further permission or charge. Teachers and educationalists may make as many copies as they need for classroom/lecture purposes - an eMail to say you are doing this would be appreciated.

Except where noted, all images courtesy of either NASA or ESA.

Newcomers to Astronomy please note that, except where annotated, the colours in images are used to show different properties of objects and may be, but are not necessarily, a true representation of their visual aspect.

Any mistakes will hopefully be mostly typographical. However if there are any real mistakes Brian takes the blame and cannot, in conscience lay any fault with the myriad of experts who have spent their valuable time trying to drum the history, theory and practice of Astronomy, Astrophysics and Planetology into his thick head.

Anyone who finds any errors, please mail me and I will correct where applicable. As time passes and new discoveries are made, some (if not all) of these articles will need modifying, changing or adding to. If anyone notices one that needs such attention, please be so good as to eMail me. Each article is dated and was based on the best information available at that time.

1. A Meteor, or shooting star, is a rock that burns up in a planets atmosphere(if it has any) and does NOT reach the surface of the planet. These are extremely common, the planet Earth gets thousands every year.
2. A Meteorite is a Meteor which does not burn away completely and hits the planet's surface. These are not uncommon, the planet Earth gets quite a few every year.
3. A Bolide is a Meteorite (or Comet) which had sufficient mass to reach the planet's surface without burning away but explodes before it gets there. Big ones are very rare, thank goodness, see Tanguska, Siberia, 1908.
4. A Meteoroid is any of the above three but has not been attracted to a planet and is still wandering through space
5. An Asteroid is a cataclysmically big Meteoroid and we don't want any thank you very much. The last real biggie was 65 million years ago - remember what happened to the dinosaurs? Unfortunately calculations show we are overdue one and likely to receive such a visitor sometime between tomorrow and 100,000 years hence.
6. An active Comet (one that shows a tail as it get close to the sun) is quite a large conglomerate of rock, ice, mud and organic compounds that originates far outside the planetary solar system. The short period comets mostly originate in the "Kuiper Belt". Long period comets which appear once and are never seen again (or have periods in the thousands of years) come from much further out, from that area called the "Oort Cloud". This "dirty snowball" structure was first coined by Fred Whipple (1906-2004). Like the planets, they orbit the sun and like the planets their orbits are elliptical - but VERY eccentric and huge, they spend a few weeks visible at most when they get inside the orbit of Jupiter, round the sun and then speed off into the far reaches of the solar system. Their orbits can take tens or hundreds of years to complete. An inactive Comet, which is one where all the volatile compounds and water have been "boiled off" in its approaches to the sun; therefore there is no tail. It is probable that most Bolides are inactive comets - or the remains thereof - since they mostly are conglomerates and more likely to disassociate explosively.

In general terms, a Black Hole is a point in space that has so much mass concentrated in it that there is no way for a very close object to escape its gravitational pull, not even light.

As anything approaches a Black Hole there comes a point where it cannot escape the pull of its gravity - this is known as the Event Horizon. How far out from the centre of the Black Hole the Event Horizon is, obviously depends on the massiveness of the Black Hole. This distance is known as The Schwarzschild Radius. The centre point of a Black Hole is sometimes referred to as a Singularity

Now "Escape Velocity" is that velocity which an object (rocket, probe, whatever) needs to achieve to escape from a planet or other astronomical body. In the case of the Earth, this is approximately 25,000 miles per hour. In the case of a Black Hole the Event Horizon is that point where the escape velocity equals the velocity of light. Outside of the horizon, the escape velocity is less than the speed of light, so in theory, if you have a powerful enough propulsion system you can escape from just outside this horizon. At or inside the Event Horizon... no chance. So far as is thought, in principle there is no limit to the mass of a Black Hole

For the scientifically inclined amongst you, the equation that gives the radius is:

Schwarzschild Radius = 2 × G₀ × m_p ÷ c²

• G₀ is the universal gravitational constant
• m_p is the mass of the singularity in Planck Units of mass
  (1 Planck Unit of mass = 2.17645 × 10^-8 kg ± 1.6 × 10^-12 kg)
• c is the speed of light

Purely as a point of historical interest, the name "Black Hole" was invented by John Archibald Wheeler, which was a much catchier term than the previous name..."Frozen Star". Ironically, Wheeler did not initially think that the formation of a Frozen Star was possible until he was presented with some convincing evidence by a colleague. Once converted to this new idea, he was the first person to use the phrase "Black Hole" for this phenomena. He also coined the phrase "Black Holes have no hair" and "Worm Hole". The former statement is true because they should be a perfect shape and not have any sorts of projections out of them.

How the theory developed

A Black Hole is initially formed when a massive star collapses. In the 1920s a physicist called Subrahmanyan Chandrasekhar deduced that any star greater than 1.44 times the mass of our sun (The Chandrasekhar Limit) would collapse under Einstein's General Theory of Relativity, but another physicist called Arthur Eddington believed that some property would prevent this. They both had bits of the theory correct. In 1939 Robert Oppenheimer predicted that a supermassive star could collapse, thus forming what was then referred to as a "frozen star" in nature, rather than just in mathematics. We had to wait until 1967 when physicists Stephen Hawking and Roger Penrose showed that not only were Black Holes a direct result of general relativity, but also that there was no way of halting such a collapse. A notable addition to the theory of Black Holes is the theory of Hawking Radiation, developed by British physicist Stephen Hawking in 1974, wherein it is hypothesized that there is an extremely slow leaking of the matter in a Black Hole via matter-antimatter pairs.

The image on the right is thought to be one of the few locations of a Black Hole actually observed by Astronomers

• The outer white area is the core or centre of the galaxy NGC4261.
• Inside the core there is a brown spiral-shaped disk, oval in shape because we cannot see it face on. It is likely that this is stream of cosmic gas, dust and debris, trapped by and spiralling into the Black Hole.
• It weighs one hundred thousand times as much as our sun.
• Because it is rotating we can measure the radii and speed of its constituents, and hence weigh the object at its centre.
• This object is about as large as our solar system, but weighs just over a million times as much as our sun.
• Meaning, of course, that its gravity is around a million times as strong as on the sun.
• It is thought certain that at the centre of the spiral is a Black Hole.

That Dark Matter exists is generally accepted but what it is, why it exists and what it does... the jury is still out. Dark matter was proposed by Swiss-American astronomer Fritz Zwicky (1898-1974) in 1933 to explain why the Coma galaxy cluster (Abell 1656) appeared to have a stronger gravitational field than expected if it were composed of matter that interacts with light (i.e. visible).

When you look through a telescope you cannot see it, but the matter you can see (stars, nebula clouds etc) moves in such a way that the indications are that there must be more mass present. It is currently thought that about 10% of the matter in the universe is visible, the rest is something we don't understand.

There are many candidates for dark matter including undetected brown dwarf stars, white dwarf stars, black holes, or neutrinos with mass (the "neutrino", is a fundamental nuclear particle that is electrically neutral and of much smaller mass, if any at all, than an electron), or indeed exotic subatomic particles, such as WIMPs (Weakly Interacting Massive Particles) or MACHOs (MAssive Compact Halo Objects). Physicists are currently searching for such particles in laboratories deep underground (to prevent interference) and ways to detect them.

Dark matter does not emit enough energy to be directly detected. But indirectly, researchers note its presence. Anything that has a mass exerts the force that we call gravity. Dark matter - or something that we have yet to find - exerts a gravitational pull on objects in and around distant galaxies, and even on light emitted by those objects. By measuring these mysterious effects of gravity, researchers determine how much "extra" gravity is present, and hence how much extra mass, or dark matter, must exist. In large clusters of galaxies, for example, scientists say that five to 10 times more material exists than can be accounted for by the stars and gas they find.

As a rich and compact galactic supercluster, Coma shows a strong central condensation and spherical symmetry, unlike the closer Virgo (which includes the Local Group and the Milky Way) and Perseus superclusters. Coma's visible mass, however, has long been known to be insufficent to maintain its strong symmetry. Hence, the cluster's existence provides strong evidence of the existence of unseen, dark matter that is presumed to be providing the gravitational pull for holding the cluster together.

For those of you who have 8"+ telescopes and wish to have a look at the Coma supercluster...

• Cluster: Abell 1656
• Constellation: Coma Berenices
• Right ascension: 12h 59m 49s
• Declination: +27° 58' 50"
• Number of galaxies: 484

The cluster's mean distance from Earth is 321 million light years. Its ten brightest spiral galaxies have apparent magnitudes of 12-14. The central region is dominated by two giant elliptical galaxies: NGC 4874 and NGC 4889. The cluster is within a few degrees of the north galactic pole.
[Cite: Data - NASA/IPAC Extragalactic Database ]

As per the ruminations of the International Astronomical Union (IAU), the definition for a planet (from the Greek planetes, "wanderers") at their General Assembly in August 2006.

A planet is a body that:

1. Is in orbit around a star
2. Is massive enough that its self-gravity gives it a nearly-spherical shape
3. Has cleared the neighborhood around its orbit

A body that fulfills the first two criteria but not the third is a dwarf planet, provided that it is not a satellite of a body already defined as a planet.

But...

As per the further musings of the International Astronomical Union (IAU) at their General Assembly in June 2008, the descriptive term "Plutoid" was coined thus... 'Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit.'

There is a school of thought - to which your webmaster subscribes - that considers all this to be a tad silly and that Kuiper Belt Objects (KBOs) which are bigger than the norm should be just called what they are, "big KBOs".

Galaxies (from the Greek 'galaxias', meaning "milky", a reference to the Milky Way) are massive, gravitationally bound systems consisting of stars and their planetary systems, interstellar gas & dust, and dark matter. They range from dwarf galaxies with as few as ten million stars up to giants with one trillion stars. All galaxies rotate around a common center of mass and can also contain many multiple star systems, star clusters, and various interstellar clouds. The astronomer Edwin Hubble devised a descriptive scale for placing Galaxies in categories:

• Elliptical Galaxies, with eight categories, named E0 - being spherical or nearly so, up to E7 - being strongly elliptical
  
• Spiral Galaxies, with three categories, named Sa - being a simple, two-armed, open Spiral, up to Ac - being multi-armed, closely packed spiral
  
• Barred Spiral Galaxies, all with a central bar and having the same categories as Spiral
  
• The two types of Spiral Galaxies are collectively known as Lenticular Galaxies all of these have a bright central core, most of them probably having a Back Hole at the centre
• There is one other type of Lenticular Galaxy, elliptical in shape and with the required core but no spiral arms
  
• Irregular Galaxies - just what the name implies. They typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.

There are estimated to be more than 100 billion galaxies in the observable universe with most galaxies in the range 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs. This space is filled with a tenuous gas of an average density less than one atom per cubic meter. Most of the galaxies in the observable universe clump together in clusters.

To show the relationship of the galactic shapes, Edwin Hubble arranged the main types of galaxies and the sub-types into a chart that has come to be called "The Tuning Fork Diagram".

A cluster is merely a discrete globular group containing from a few millions to a few tens of millions of stars. They can be within or just outside of a galaxy but are gravitically bound to the "parent" galaxy. Good examples are the Magellenic Clouds.

Light does not always travel in straight lines. Einstein predicted this in his Theory of General Relativity that massive objects will deform the fabric of space itself. When light passes one of these objects, such as a galaxy, its path is changed slightly. This effect, called gravitational lensing, is only visible in rare cases and only the best telescopes can observe the related phenomena.

There are three different types of lensing...

1. Strong Gravitational Lensing: where a single background source is distorted into multiple images, giant arcs and complete rings. This is obviously the most spectacular effect of the three different types. Shown here is an example each of an Einstein Ring and and an Einstein Cross.
   
   
2. 
   
   
   Weak Gravitational Lensing: where we see the weak distortions of many distant background galaxies by intervening matter. Good examples can be seen here in the Abell 2218 lens cluster.
   
   
3. Gravitational Microlensing: where we observe the time-changing magnification of a background source due to lensing. This not easily shown here as it requires multiple images taken over a period of time. When watching the video, there are two stars in the centre of the view, keep your eye on the one on the upper left of these two.
   [ Video (RealPlayer/WMP) ]

Put simply... The observer affects the observed. Or, in full...

It tells us that the observer can never exactly know both the position and momentum of a particle, because as every observation requires an energy exchange to create the observed 'data', some energy state of the observed object has to be altered. (i.e. it expends photons of light to create the image in your eye, or you have to affect its energy state by illuminating the phenomenon with photons of some form of electro-magnetic energy) Thus the fact of actual observation will have a distinct and discrete effect on what we measure, limiting how precisely we can determine both the position and momentum of the particle.

The "Kuiper Belt" was first described, in 1992, by Gerard P Kuiper (1905-1973), who discovered the belt while making spectroscopic studies of Uranus and Neptune. This is a trans-neptunian area around the solar system consisting of a huge cloud of small bodies orbiting the sun. There are at least 70,000 of these objects with diameters larger than 100 km in the radial zone extending outwards from the orbit of Neptune, 30-50 AU. Observations have shown that they are mostly confined within a thick band around the ecliptic, leading to the realization that they occupy a ring or belt surrounding the sun. Pluto, down-graded to the status of "Dwarf Planet" in 2006, and further down-graded to a "Plutoid" in 2008, lives in the Kuiper belt. The belt is home to most, if not all, of the short period comets.

Nebula (from Latin: "mist"; pl. nebulae or nebulæ)

This is an interstellar cloud of dust, hydrogen gas and plasma and some types are the birthplace of stars. Others are the remnants of massive explosions. In these regions of space the formations of gas, dust and other materials slowly accrete to form larger masses until gravity starts to have a significant effect. This attracts further matter which eventually will become big enough and collapses to form stars. The gravitational force of the newly born star will pull the remaining matter into a swirling disk where the same process will eventually form planets, and other system objects.

On the left is NGC1952, The Crab Emission Nebula, and on the right M42/43, The Orion Reflection Nebula.

Most nebulæ may be described as being diffuse. This being a descriptive term indicating that they are extended and contain no well-delineated edges. In visible light there are two varieties, Emission nebulæ and reflection nebulæ, these being descriptive terms for how we see the light that comes from them. Emission nebulæ contain ionized gas that produces its own emitted light. In contrast to emission nebulæ, reflection nebulæ do not produce significant amounts of their own light but are illuminated by the light from nearby stars.

Dark Nebulæ

The image shown on the left is part of the Eagle Nebula, sometimes called "The Pillars of Hercules". These are similar to diffuse nebulæ, but they are not seen by emitted or reflected light. Instead, they appear as dark clouds in front of more distant stars or in front of other nebulæ. Although all these nebulæ appear different at optical wavelengths, they all have strong emission at infrared wavelengths. This emission comes primarily from the dust within the nebulae.

Planetary Nebulæ

The image shown on the right is NGC6543, named The Cats Eye Nebula, for obvious reasons. These are nebulæ that form from the gaseous shells that are ejected from giant stars as they collapse into white dwarfs. These nebulæ are emission nebulae with spectral emission that is similar to the emission nebulæ found in star formation regions. Technically, they are a type of HII region because the majority of hydrogen will be ionised. However, planetary nebulæ are denser and more compact than the star forming emission nebulæ because they are caused, and therefore surround, just one star. Planetary nebulæ are so called because the first astronomers who observed these objects at that time thought that the nebulæ resembled the disks of planets.

Protoplanetary Nebulæ

The image shown on the left is CRL 618 - no alternate name has been assigned. This is one which is at the short-lived, intermediate stage between a star's rapid stellar evolution between the late giant stage and the subsequent planetary nebulæ phase. A protoplanetary nebulæ emits strongly in the infrared range, and is a kind of reflection nebulæ. A protoplanetary nebulæ changes to a planetary nebulæ at a point determined by the temperature of the central star.

Supernova Remnants

The image shown on the right is M16, the remnant of Kepler's Supernova, named after the astronomer who recorded the original explosion back in the 16th century. It is a composite image of visible, infra-red and radio images. A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion ends in the core of the star, the star collapses inward on itself. The gas falling inward either rebounds or gets so strongly heated that it expands explosively outwards from the core. The expanding shell of gas forms a supernova remnant. This is a special type of diffuse nebulæ.

Neutron Stars are the collapsed cores of some types of massive stars. A typical neutron star has a mass between 1.35 and about 2.1 solar masses, with a corresponding radius between 10 and 20 km. Models predict that Neutron Stars consist mostly of neutrons, hence the name. Such stars are very hot, and are one of the few possible conclusions of stellar evolution. These stars also have the strongest magnetic fields in the known universe. They vary between a hundred trillion and a hundred million times Earth's magnetic field. They are extreme in the matter of temperature as well as the cenre of a Neutron star is thought to reach 100 million degrees Kelvin.

The formation of Neutron Stars is thought to occur in supernovae - such as the one that formed the Crab Nebula. It is estimated that these stars start out with about 8 to 25 times the mass of our sun. These numbers will almost certainly change as supernova analysis and simulation becomes more accurate. It is currently believed that for initial masses much less than 8 solar masses the star becomes a white dwarf. The same school of thought says that stars which have initial masses a lot higher than 25 solar masses a black hole is produced. As the nuclear furnace in a star starts to "run down" the central part of the star gradually fuses its way to iron. There are good scientific reasons why it cannot fuse into heavier elements which we will not go into here. So, the iron core just accumulates until it reaches the Chandrasekhar Limit at about 1.4 solar masses. This is the point at which the forces holding the electron shells in place "give up the ghost" and the atoms collapse inwards into degenerate matter.

This collapse generates huge pressures and it has been calculated that it would be sufficient to force electrons and protons together to form neutrons and a spray of neutrinos. After this event the core settles down to become a neutron star. A burst of neutinos was detected here on earth just before the visible light from Supernova 1987a arrived, indicating that there should be a neutron star in there somewhere but to date, none has been discovered. the two favoured reasons are that 1) It is hidden by a Dark Nebula or 2) a lot more matter was sucked in causing it to collapse further into a black hole. The precursor of the 1987a Supernova was a blue giant star called Sanduleak -69° 202a. It was a magnitude 12 blue supergiant star, located on the outskirts of the Tarantula Nebula in the Large Magellanic Cloud. There was a problem with fitting a Blue Giant into Supernova->Neutron Star scenario which has not yet been resolved satisfactorily.

A Nova is a type of explosion, resulting in the sudden brightening of a star. The word comes from the latin for "new star", Nova, (pl. Novæ) because often a star, previously too dim to be seen with the naked eye, can briefly become the brightest object in the sky (besides the sun and the moon) when it becomes a Nova. The phenomenon occurs in binary star systems, where a white dwarf star absorbs a critical amount of matter from its companion, compressing hydrogen onto its surface and re-igniting in a nuclear explosion.

A white dwarf is the remains of a much larger star that has fused together much of its hydrogen into (usually) carbon and oxygen. Fusing carbon and oxygen requires so much enegy that it only occurs in the cores of very massive stars. If sufficient hydrogen is packed onto its surface, it forms a shell which eventually can reach the critical temperature and pressure for hydrogen fusion, around 20 million Kelvin. The hydrogen rapidly fuses like an extremely powerful nuclear device and a Nova in the result. The power of a Nova is insufficient to destroy the star so it can occur again and again with the same star.

Recurrent Novæ, type A (NRA): outbursts are a result of thermonuclear runaway on the white dwarf. T Pyxidis is an example of such a system.

Recurrent Novæ, type B (NRB): eruptions are driven by the accretion from a red giant on to the companion star. In such events, the outbursts may be generated by an instability in the cool companion or by a disk instability. T CrB, RS Ophiuchi, and V745 Sco are examples of this type.

If the study of variable stars interests you, there is a group within the astronomical community - The American Association of Variable Star Observers (AAVSO)- especially for people like you. Read more here.

Stars that have managed to suck in enormous amounts of material at birth erupt into life as supergiants. These stars as well as being mammoths burn incredibly fiercely and after a relatively short lifetime will run out of fuel. As this happens the vast mass of the star collapses in on itself and the core becomes incredibly upset and volatile. This causes a tremendous reaction and the star blows itself apart. This explosion is called a supernova: nearly, but not quite, the most cataclysmic event in the universe.

Supernovæ fall into two different types whose evolutionary history is quite different. Type Is result from mass transfer inside a binary system consisting of a white dwarf star and an evolving giant star. Type IIs usually occur when a single massive star comes to the end of its life... in a rather violently, spectacular fashion.

The structure and luminosity of all stars is determined by the continuous battle between gravity and radiation pressure arising from the energy created internally. In the early stages this comes from conversion of hydrogen into helium.

Type II Supernovæ are associated with stars with masses of about 10 times that of the Sun where the "normal" burn continues for about ten million years, with part of this stage being the expansion into a Red Giant, until all the hydrogen is nearly used up and hydrogen "burning" can only continue in a shell around the helium core. This core shrinks under gravity until its temperature is high enough for helium to burn into carbon and oxygen. This phase lasts for about a further million years but eventually even the helium is nealy exhausted and we get another shell situation. Contraction occurs again until it is hot enough for the conversion of carbon into neon, sodium and magnesium. This lasts only for about 10,000 years. The cycle repeats - 12 years this time - as a neon to oxygen plus magnesium conversion. Penultimately there is a silicon to sulphur conversion lasting about 4 years and finally, the remaining silicon converts to iron taking only a week.

Because iron is so stable no further conversion takes place resulting in the loss of the expanding radiation pressure to balance the force of gravity. Everything starts happening when the mass of the iron core reaches 1.44 solar masses - The Chandrasekhar Limit. Compression heats the core to a point where it decays into neutrons giving off massive amounts of heat. The core collapses from half the Earth's diameter to about 100 kilometres in a few tenths of a second and in about one further second becomes a 10 km diameter neutron star. This releases an enormous amount of potential energy primarily in the form of neutrinos which carry 99% of the energy. A shock wave is produced which passes, in two hours, through the outer layers of the star causing fusion reactions to occur. These form the heavy elements. In particular the silicon and sulphur, formed shortly before the collapse, combine to give radioactive nickel and cobalt which are responsible for the shape of the light curve after the first two weeks.

When the shock reaches the star's surface the temperature reaches 200 thousand degrees Kelvin and the star explodes at about 15,000 km/sec. This rapidly expanding envelope is seen as the initial rapid rise in brightness. It is rather like a huge fireball which rapidly expands and thins allowing radiation from deeper in towards the centre of the original star to be seen. Subsequently most of the light comes from energy released by the radioactive decay of cobalt and nickel produced in the paroxyism that is a Type II Supernova.

Type I Supernovæ are even brighter objects than those of type II. Although the final explosion mechanism is somewhat similar the cause has similarities to an ordinary Nova. Their origin is an old, evolved binary system in which at least one component is a white dwarf star of less than 1.44 Solar Masses. White dwarfs represent the final evolutionary stage of all low-mass stars.

The binary system loses its angular momentum until the two stars are so close together that the matter in the companion star is is gradually accreted into a sphere around the white dwarf. This transferred mass increases the mass of the white dwarf to a value significantly higher than the critical value of 1.4, whereupon the whole star collapses and the relatively slow cycles described above under Type II Supernovæ occur so quickly that enough energy is suddenly created to blow the star to bits. The subsequent energy released is also as in the Type II case, from the radioactive decay of the nickel through cobalt to iron.

The last supernova to be seen in our galaxy, the Milky Way, was seen in 1604 by the famous astronomer Kepler. The brightest since then was supernova 1987A in the Large Magellanic Cloud, a small satellite galaxy to the Milky Way. The brightest supernova in the northern sky for 20 years is supernova 1993J in the galaxy M81 which was first seen on 26 March 1993.

A Hypernova is an awesomely powerful version of a Type II Supernova. They are the most destructive explosive forces in the entire universe. Whereas a main sequence star might live for 10 billion years and a "normal" giant that produces a Type II Supernova burns for about 10 million years, the star that drives a Hypernova is a viciously hot super giant that is here and gone in around 1 million years.

The general process is exactly the same as for a Type II Supernova except the star is in excess of 20 Solar masses and they get this extra mass by being in the vicinity of what are called "Stellar Nurseries" where there are huge clouds of gas to feed them. When they do go nova, the residual mass is converted into a Black Hole and all the rest is converted into energy. It happens so quickly and with such devastating violence that it produces a catastrophic "Gamma Ray Burst" (GRB).

If such an event happened within 300 light years of earth it would appear 1 million times brighter than the sun. Not that we would notice it as the GRB would arrive at the same time as the light and it would destroy the ozone layer and the atmosphere would become super heated and trigger cyclones, hurricanes and tsunamis all over the world. We would also be hit by an electro-magnetic pulse so massive that every piece of electronic equipment on the planet would be fried. Earth would become a scorched, desolate, uninhabitable planet. There is absolutely no defence to any of this. We wouldn't even see it coming.

Hypernovæ occur frighteningly often. Every night more and more gamma ray bursts are located across the universe. They are so powerful we can see them 10 billion light years away and are constantly occurring all over the universe, one day our luck will run out, the one consolation being that if it happens close enough, we will know nothing about it.

Material ejected as energy, in Solar Masses

Nova	1/10,000
Type I Supernova	1.38
Type II Supernova	10-12
Hypernova	20+

The "Oort Cloud" is named after the Astronomer Jan Hendrik Oort (1900-1992), who first proposed the existence of huge spherical cloud of cometary matter left behind from the original formation of the solar system. Whereas comets with orbital periods less than 200 years are believed to come from the Kuiper Belt beyond Pluto. Longer period orbits are thought to originate in the Oort Cloud.

Before Oort started his studies in the 1950s, the following was known concerning comets...

• Comets "fall" towards the sun from just about any point, and orbital direction, with no one set of parameters being favoured
• The directions of their orbits indicate a high degree of randomness in their paths
• The semi-major component of their orbits fall in the range of being well outside neptune's orbit

Oort discovered that not only were the orbits concentrated towards the very large, but that a large number of the comets' orbits had been "perturbed" by our planets' gravity, Jupiter in particular. This could not have happened for the orbits observed, thus showing the comets to be new visitors. Oort conclusions were that the solar system is surrounded by a vast sphere of icy rocks at a mean distance of around 50,000 AU, that occasionally drop in to visit us.

A Pulsar is basically a Neutron Star which is spinning rapidly. Because a Neutron Star is made mostly from iron it has a very strong magnetic field and this field forces the radio waves emitted by the star to be squirted, like a lighthouse beam out of the north and south magnetic poles. Now if the axis of rotation is offset from the magnetic axis then these beams sweep across space like the light from a light house and to a person with fixed viewpoint the star appears to pulse, dropping from nearly zero to maximum in only a little over one second.

Anthony Hewish and Jocelyn Bell first discovered by accident, and subsequently proved and quantified, the existance of such bodies at the physics department of The Cavendisah Laboratories in Cambridge in 1967. This happened with a radio telescope built especially for a search for Quasars.
[Cite: Image courtesy of Cambridge Physics ]

A Quark Star is a superdense celestial object which is formed when the remnants of old stars collapse in on themselves, denser than a neutron star but not dense enough to become a black hole. Quark Stars were first hypothesized in the 1980s, but the first was not discovered until early 2002. Like Neutron Stars, Quark Stars are composed of degenerate matter but matter which has degenerated even further than neutrons. The pressure has become so immeasurably large that neutrons have dissolved into a mass of quarks and gluons. The up and down quarks of which neutrons are composed then change into strange quarks, with the resulting strange matter compacting into an even denser mass than a Neutron Star. They are also called Strange Stars. There are two theories as to how they are formed 1) A gradual process involving a Neutron Star getting denser and denser as a result of matter capture or 2) As the result of a particularily violent Supernova. Either, neither or both may be true, no-one knows yet.

Neutron Stars which have a mass of between 1.5 and 1.8 solar masses, and which have a rapid spin are hypothesised to be the best candidates for conversion to a Quark Star. It can be shown that this could amount to as much as 1% of the projected Neutron Star population. Data extrapolated on this indicates that up to 2 quark conversions may occur in the observable universe each day. Theoretically Quark Stars may be radio quiet, so radio-quiet Neutron Stars may be Quark Stars.

Quasars (QUASi-stellAR objects) These are extremely large luminous bodies, anything from 1 billion to 13 billion light years away. They are therefore very old. It is thought that they are active galactic nuclei with a central, supermassive Black Hole. The most luminous Quasars are 2 trillion times brighter than Sol. Their light output is continuous but they fluctuate in intensity on various timescales - years, months, weeks, days or even hours. This suggests that they are quite dense.

As recently as the 1980s astrophysicists were unable to agree as to exactly what Quasars really are. A consensus was arrived at when some Quasars were found to be surrounded by galaxies, bringing about the "Active Galactic Nucleus" theory. Calculations show that to generate the amount of light given off, Quasars must be powered by supermassive Black Holes that absorb between 10 and 1000 solar masses every year. As a result of this specatular absorption, superheated plasma is accelerated to close to the speed of light, releasing inordinately large amounts of energy waves throughout the electromagnetic spectrum. In such cases it has been calculated that about 10% of the absorbed matter is converted to energy, contrasting with the puny 0.7% of mass being converted to energy in fusion reactions within typical stars. It is thought that Quasars are believed to emit relativistic jets from their rotational poles, similar to their smaller cousins the pulsars.

Put simply, this is the distance from a primary that an orbiting body will break up and tend towards the formation of rings. The limit applies to all bodies of course, but for solid bodies a lot of other factors such as density, composition have to be taken into consideration, so it is simpler to consider a completely fluid body.

Imagine a sphere of water, well outside the Roche Limit (Fig 1) - it will remain more or less spherical.

If, for whatever reason, it moves closer to the limit (or for other reasons the limit moves closer to the sphere), then the sphere will become more and more oblate (Fig 2).

As the orbit of the sphere and the limit become coincidental the sphere starts to break up (Fig 3).

As you can see (Fig 4), globules of the sphere nearer the primary will start to speed up and move inwards. While those further away wil slow down and move outwards.

Finally, (Fig 5) the "bits" of the original sphere will form a continuous ring around the primary.

[Cite: Images courtesy of Wikipedia ]

Schrödinger's actual statement...

"One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small that perhaps in the course of one hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid.

If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The Psi function for the entire system would express this by having in it the living and the dead cat (pardon the expression) mixed or smeared out in equal parts."

This is an extremely difficult concept and I am not going to go into it in depth. Suffice it to say that Schrödinger created this mind game (for that is what it is) because he was unhappy with another mind game which is known as the "EPR Paradox" which was proposed by those luminaries Einstein, Podolsky and Rosen (hence EPR from the initial letters of their names).
[Cite: Image courtesy of Wikipedia ]

A sunspot is an area on the sun that can be seen as a small dark spot through a telescope. (WARNING: Do NOT try to look directly at the sun with a telescope, you WILL be blinded) Instead use the telescope - or a pinhole camera - as a projection device onto a white screen). Since their discovery by Galileo in 1609, astronomers have discovered that they are regions, varying in size but averaging about the size of the Earth, where powerful magnetic fields are concentrated. They frequently eject huge spurts of plasma and are often the site of solar flares and other storm activity.

These spots apear dark because the temperature of the solar gases inside them is about 2000°K cooler than the rest of the sun (5500°K). They appear black because they emit less light than the sun. If they were suspended alone in the night sky, they would actually glow a bright red color and be brighter than the full moon. The sunspot cycle has been noticed since about 1670 and has a period of about 11 years. Before 1670, no such cycles were noted

This time is in the middle of what was called the "Little Ice Age", 1645 to 1715, but more usually named "The Maunder Minimum" in astronomical circles, after the astronomer Edward W. Maunder (1851–1928) who discovered the dearth of sunspots during that period by studying records from those years. During one 30-year period within the Maunder Minimum, astronomers observed only about 50 sunspots, as opposed to 40,000–50,000 spots which would have been more typical. Scientists are now of the opinion that solar activity directly influences the Earth’s weather in some way.