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The Oort cloud explains the existence of long period comets.


Long period comets with periods of over 200 years have to come from somewhere. One problem with such comets is that they will eventually either fall into the Sun or be expelled from the solar system.

The comets had to have been around from the early solar system, so they must have been somewhere for a long time before becoming a comet.

The Oort cloud explains this. The comets would have been in the Oort until some gravitational perturbation caused them to fall into cometary orbits.

Also, models of the early solar system predict that the large outer planets, particularly Jupiter, would have ejected material as they moved to their current positions.

The problem with actually detecting the Oort cloud is that it is between 2,000 and 50,000 AU away. It is possible that modern space telescopes, such as Kepler, may be able to detect Oort cloud objects transiting a star.



The decadal oscillations cause temporary changes in world-wide climate, both global warming and global cooling.


One theory is that the present observed trend in global warming is at least partially the result of the decadal oscillations. The effects of the oscillations can be seen in the arctic ocean and the glaciers of Greenland.

During the 1930s and 1940s there was a global warming effect due to the decadal oscillations. During this time the Arctic Ice melted and the Northwest passage was open for a time in 1939 and 1940. Planes landed on the glaciers of Greenland were later covered with very thick layers of ice. These planes have recently been uncovered by the present trend in global warming.

Similar oscillations have been seen in the past. Vikings settled in Greenland, raised crops, grapes and cattle during a warming trend.
A later cooling trend forced the Vikings to abandon their settlements on Greenland and closed the Northwest passage that had allowed the Vikings to settle in what is now Russia.

Another theory is that the oscillations have nothing to do with the present trends in global warming. The oscillations are by definition are always moving back and forth. While the oscillations have a temporary affect on world climate the present trends are thought to be independent of the temporary affect of the oscillations. The theory is that the present trend in global warming is linked to the increased production of Carbon Dioxide.



Planets orbits are defined by conservation laws.


Johannes Kepler discovered by observation that planets follow elliptical orbits. A few decades later Isaac Newton proved that by applying the law of conservation of energy that a planet's orbit is an ellipse.

When two bodies orbit around each other, they both always orbit about the centre of mass. This centre of mass is called the barycentre. The Moon doesn't orbit around the Earth. In fact both the Earth and Moon orbit around the Earth-Moon Barycentre (EMB).

When it comes to something more complex like the solar system a similar principle applies. None of the planets etc actually orbit around the Sun. In fact the Sun, planets, asteroids, comets and other bodies all orbit around the centre of mass of the solar system which is called the Solar System Barycentre (SSB).

The SSB is in constant motion and can be anywhere from near the centre of the Sun to over a Su radius outside of the Sun. So, everything in the solar system is orbiting around a point which is in constant motion.

The diagram shows the path of the SSB over several decades. The points where the SSB is furthest from the Sun occur when the planets are aligned.

enter image source here



Antimatter is matter made up from particles with the opposite charge to normal matter.


Most particles have an anti-particle. In particular charged particles have an anti-particle with the opposite charge. Some uncharged particles are their own anti-particle.

Normal matter consists of mainly protons, neutrons and electrons. The proton is positively charge and the antiproton is negatively charged. The electron is negatively charged and the antielectron, or positron, is positively charged. The neutron and the anti-neutron have neutral charges.

Antimatter is made up of antiprotons, antineutrons and positrons.

When a particle meets its antiparticle they annihilate each other to form photons.

When the universe was formed there should have been equal amounts of matter and antimatter. For reasons unknown, matter was more dominant. Although it is possible that there may be antimatter galaxies.

We don't know much about the properties of antimatter. We do know that the antiparticles exist and behave in the same way as the normal particles except for charge. It is now possible to form small quantities, upt to a few hundred atoms of anti-Hydrogen, in the laboratory and more experiments can now be conducted.

Richard Feynman showed that mathematically a positron is indistinguishable from an electron travelling backwards in time. This doesn't mean that antimatter travels backwards in time, it is just a consequence of the mathematics.



There are many types of galaxies such as spiral, lenticular, elliptical and irregular.


Elliptical galaxies form through galactic collisions . We have found evidence of this by actually observing two galaxies collide.
NGC 3597 (image below) is an image of two galaxies that are colliding to form an elliptical galaxy.
Courtesy NASA/ESO Hubble Space Telescope

Edwin Hubble predicted that Elliptical galaxies evolve into Spiral galaxies, but this was proven wrong because stars found in Elliptical galaxies were found to be much older. Elliptical galaxies contain between 10 million to 100 trillion stars. The largest known galaxy is actually an elliptical galaxy known as IC 1101. This behemoth contains nearly 100 trillion stars.

Elliptical galaxies mostly contain metal-rich stars that appear orange and red in colour. There is nearly no interstellar medium. and thus very little star formation. They also may also harbour enormous super-massive blackholes which may range from a million to several billion solar masses. Many Globular Clusters orbit the centres of elliptical galaxies.

These galaxies tend to be spherical or ovoid and have diameters ranging from 30,000 light years to more than 700,000 light years.

Spiral galaxies have many theories predicting how they were formed ranging from the collisions of smaller galaxies to the collapse of individual gas clouds early in the history of the Universe. Our own galaxy The Milky Way is a Spiral galaxy. Our Sun is located a third of the way down of one of the Milky Way's arms. However, we still do not know what it looks like as we are not facing it from the front but rather from the side. Here's an artist's rendition of the Milky Way.
Courtesy NASA/JPL-Caltech

Spiral Galaxies contain young blue stars that are hot and short-lived they are found in the spiral arms. Older stars such as Red Giants are located in the Bulge (the yellow part in the centre). Spiral Galaxies have a large amount of interstellar medium. Spiral galaxies also have Globular clusters which orbit the centre. The Super-massive blackholes of these galaxies are mid-range and have between 1 million to several million solar masses.

Spiral galaxies are rotating flat discs of star, dust and gases, they also have a distinct spiral with a nuclear bulge at its centre and spiral arms which emanate of the bulge. In diameter, spirals range from about 10,000 to over 300,000 light-years.

Lenticular galaxies are a hybrid between spiral and elliptical galaxies. They are formed when Spiral galaxies exhaust their supply of gases needed for star formation. They are deficient in the interstellar matter such as gas but lenticular galaxies may retain a considerable amount of dust in their disk. Here's an image of one.
Courtesy NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

They also have a shape nearly identical to spiral galaxies. They appear like pale discs of old stars that have faint spiral arms. The diameter ranges from 10,000 light years to 300,000 light years and over.

Lastly, Irregular galaxies. An irregular galaxy is a galaxy that does not have a specific shape, unlike a spiral or an elliptical galaxy. These galaxies are formed from external gravitational disturbances such as another galaxy passing near it. They have large amounts of interstellar medium which is scattered usually obscuring parts of the galaxy. The disruption of the gases that are needed in star formation causes rapid star formation called star bursts. Below is an image of an irregular galaxy M82 known as the cigar galaxy
NASA, ESA, The Hubble Heritage Team, (STScI/AURA)

Irregular Galaxies have a very chaotic structure and have no distinguishable features. They are 20,000 light years in diameter on average. They may have Super-massive blackholes that are mid-range.

Astronomers classify these galaxies using the Hubble Tuning fork diagram. Here is what it looks like.
Galaxies are the most important building blocks of the universe. Some are simple, while most are extremely complex both in composition and in structure. Edwin Hubble an American Scientist decided to classify galaxies. He created the Hubble Tuning fork diagram in 1926.

Ellipticals are classed from E0 to E7 the larger the number the more Elliptical.

The spirals were assigned letters from a to c which characterize the compactness of their spiral arms. Sa spirals, for example, are tightly wound whereas Sc spirals are more loosely wound.

Barred Spirals were also assigned letters a to c. The most notable difference between these two groups is the bar of stars that runs through the central bulge in barred spirals. Again SBa means that the spiral arms are tightly wound and SBc means that they're loosely wound.

"S0," or lenticular galaxies, are in the transition zones between ellipticals and spirals,

However please keep in mind that this is a General classification and that this is the simplest way to classify galaxies. In reality, there are many other types of galaxies such as Ring galaxies or Peculiar Galaxies.

Most of the Information about the galaxies are from my knowledge. I have however I have forgotten about the Tuning fork diagram so I got my information for the Hubble Tuning Fork diagram from

Hope this helps!



The gravitational parameter for a body #GM# is the gravitational constant #G# multiplied by the mass of the body.


When doing calculations involving gravity, the gravitational constant #G# is required. It is however difficult to measure the value of #G# to high degrees of accuracy.

The known value of #G=6.674 08 \times 10^-11 m^3 kg^-1 s^-2# but the uncertainty in the value is #0.000 31 \times 10^-11#. So, effectively we only know the value of #G# to four decimal places.

Calculating the mass #M# of a body, such as a planet or the Sun, is also difficult. To do so accurately would require knowledge of the body's volume and density which we can't know accurately.

Fortunately, in gravitational equations the quantities #G# and #M# are multiplied together to form the gravitational parameter #\mu=GM#.

Using data from measurements of the orbits of planets, moons and satellites it is possible to measure the value of the gravitational parameter for bodies with a high degree of accuracy. The values of the gravitational parameter for the Sun, Earth and other planets have been carefully measured.

So, if you want to calculate the orbital parameters of a satellite orbiting the Earth, you don't get the values for #G# and #M# and multiply them together. Instead you get the much more accurate gravitational parameter #\mu# for the Earth.



A photon isn't actually attracted by a black hole, but its geodesic may cause it to enter the black hole.


Photons of light have zero mass. This means that they travel at the speed of light. Interestingly photons do not experience time as a consequence.

Einstein showed through his General Theory of relativity (GR) that gravity is not a force. Mass, energy and momentum curve space time. Space time being 4 dimensional. Gravity is actually an acceleration caused by the curvature of space time.

The Einstein equations may look very simple. In fact they are most definitely not. The field equations are:

#R_{mu nu} - 1/2 Rg_{mu nu} = (8pi G)/c^4T_{mu nu}#

The left hand side describes the curvature of space time and the right hand side describes the mass, energy and momentum which cause the curvature. The equation is actually 10 second order partial differential equations!

In the absence of an external force all objects move in a straight line with constant, possibly zero, velocity. In curved space time the straight line becomes a geodesic which is the shortest distance between two points in space time.

So, as gravity is not a force, nothing is attracted by a black hole. Objects simply follow a geodesic. This also includes light photons.

If a photon passes near to a black hole its geodesic path will be bent around the black hole but it won't enter it. If the photon is headed towards the black hole the geodesic will intersect the event horizon and the photon will be consumed.



All bodies need to reach an equilibrium with gravity to stop further collapse.


Most small bodies, which includes comets and most asteroids, have insufficient gravity overcome the rigidity of their component materials to undergo collapse. This means that they can be any shape.

Larger bodies, such as most moons, all planets and all stars, have sufficient gravity to overcome material rigidity and undergo a level of collapse. This is why these bodies are almost spherical in shape.

Almost spherical bodies are in hydrostatic equilibrium where gravity is balanced out by internal pressure to prevent further collapse.

In the case of main sequence stars, the outward pressure generated by fusion reactions in the core keep the star at a relatively large size. All main sequence stars are in hydrostatic equilibrium.

When a star runs out of fuel for fusion reactions, then gravitational collapse of the core is inevitable.

If the stellar core is less than about 1.4 solar masses then gravitational collapse is stopped by electron degeneracy pressure. This is a quantum effect caused by the Pauli exclusion principle. No two electrons can be in the quantum state. This prevents atoms from having overlaying electron shells. The most degenerate state this allows is the white dwarf star.

If the stellar core exceeds 1.4 solar masses then electron degeneracy pressure fails and protons and electrons are forced to combine into neutrons forming a neutron star. This generates a huge supernova explosion in the outer layers of the star.

Neutron stars are held in hydrostatic equilibrium by neutron degeneracy pressure. This is another quantum effect which prevents two neutrons being in the same quantum state.

If the neutron star is more than about three solar masses then gravity overcomes it. This means that gravity can now collapse the stellar core to its extreme which is a black hole.



The Sun is a good example of nuclear energy as it utilises nuclear fusion, radioactive decay and particle annihilation.


The Sun's core is mainly Hydrogen under high temperatures and pressures. The Sun's main source of energy is the proton-proton chain reaction which actually involves three types of nuclear energy producing reactions.

First of all the temperature and pressure allow two protons get close enough for the strong nuclear force to overcome the electrostatic repulsion and fuse them into the highly unstable Helium-2.

#"_1^1H +## "_1^1H->##"_2^2He#

Most of the Helium-2 nuclei fly apart, but relatively rarely the weak force will turn a proton into a neutron a positron and an electron neutrino to for deuterium and releasing energy.

#"_2^2He->##"_1^2H + e^+ + nu_e#

The positron almost immediately annihilates with an electron releasing more energy.

A proton then fuses with deuterium to produce Helium-3 plus a high energy photon.

#"_1^2H##+ "_1^1H##->"_2^3He + gamma#

Finally two Helium-3 nuclei combine to form Helium-4 and two energetic protons.

#"_2^3He##+"_2^3He##->"_2^4He##+2 "_1^1H#

So, there are three nuclear reactions going on in the Sun, fusion, weak decay and electron-positron annihilation.



Hmmm. That rather depends on your definition, and on how you interpret a rather troubling finding at the opposite end of physics (quantum theory.)


Firstly, let’s clarify a little. We suspect the galaxy we are in (the Milky Way) contains many (some would say many times many) intelligent life forms. The answer to this is given by solving the Drake equation and I would strongly encourage you to do this yourself ( It isn’t too scary, just guess a series of probabilities and multiply the total by the number of stars in the galaxy. Most people get between 1 and 10,000 intelligent civilisations currently existing in just our galaxy.

Next, your main question was about other universes. This is considerably more difficult to answer as, by definition, no energy (which includes information) can enter or leave our universe. That’s what ‘universe’ means. Thus we can only speculate? Well, yes and no. It all depends on how you interpret the results of a very simple, mind-blowing experiment in quantum physics called the diffraction of electrons.

In essence, a stream of electrons pass through one of two narrow slits. If you do this with light you get an interference pattern as predicted by wave theory. We do not expect that particles, like electrons would do this too, but the weirdness of that branch of physics was only just becoming apparent.

If you repeat the experiment with just one electron at a time it also builds up the same pattern over time. It’s as if either the electrons interfere with each other in the past/future or a single electron passes simultaneously through both slits. Both are impossible to our understanding of particles in space and time as we know them.

We needed a new view of very very small things and this is what quantum physics provides. Importantly, if you cover one slit the pattern disappears, and if you “peek” to see where it goes, the pattern also disappears and the electrons behave like “normal” particles, passing through one slit or other but never both.

When the “probability function” collapses to form an ‘outcome’ in our universe some propose that our universe splits into two. One where the electron went through the right hand slit, and one where it passed through the left hand slit. This implies that every event or choice in our universe causes it to split in two, yielding a ‘multiple universe’ or multiverse.

Sorry for the long, long answer .... great question :D