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It all depends on how big the star is.


The larger the star the shorter the life span. This is due to convective currents. In a small star convective currents allow helium to disperse and not accumulate at the core. This results in more complete fusion giving the star a life span longer than the age of the universe. As such for small stars (red dwarfs) we don't know exactly what will happen. It is possible that they just cool off.

Slightly larger stars like our sun will eventually explode and then compact. When they compact there will be no space between electrons and protons. This super compact star is called a white dwarf. It is white because it is super hot, which is odd because it is no longer creating heat. As it cools over billions (maybe a trillion) years it is theorized that it will eventually lose all heat and turn into a black dwarf. This image shows how much the sun will compact when it turns to a white dwarf.

enter image source here


Stars that are at least 8 times bigger than our sun up to about 30 times bigger than our sun will turn into neutron stars. A neutron star is even more dense than a white dwarf because not only is there no space between the sub atomic particles but protons and electrons begin to fuse together forming neutrons. The smallest neutron star is around 50 miles across but has twice the mass of our sun.

Like other celestial bodies, stars spin. When a neutron star forms, due to the conservation of angular momentum it will increase the speed of its spin. This is like a figure skater doing a spin and then pulling her arms in. She speeds up but there is no change in force. In the case of a neutron star, this increase in speed can be so great that the star can rotate at speeds of several revolutions per second. This generates energy that due to magnetic field will produce a burst of energy coming from the poles. If the angle of the rotation is such that the beam sweeps across the solar system, like an airport beacon, it can appear like the star is actually pulsing. This is a pulsar, and it looks like this.

enter image source here


Finally if a star is too big to be a neutron star it will compact even more, to the point that the center of it will reach a density approaching infinity. At this point any matter that gets close enough will get sucked in too. The neutrons all fuse together so that you get a single point (called a singularity) and the gravity is so high not even light can escape. This is a black hole. Since light can't escape we cannot sense then except when a body passes behind the black hole so that the hole will distort what we see. This is called gravitational lensing.

enter image source here



Different fusion reactions occur in small stars, large stars and supernovae.


In a smaller star such as the Sun the main process of fusion from Hydrogen to Helium is the proton-proton chain reaction. This is where two protons combine under the strong nuclear force to form a diproton.

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

The Helium 2 or diproton is very unstable and usually breaks down into two protons. Occasionally transforms into deuterium by the weak nuclear force.

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

Then a proton is added to form Helium 3.

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

There are several reactions which depend on the temperature of the star which lead to the most stable #""_2^4He#.

Larger stars use the CNO fusion reactions. This creates Helium from Hydrogen by a fusion process converting Carbon to Nitrogen to Oxygen and back to carbon. There are several reactions.

#""_6^12C + ""_1^1H -> ""_7^13N + gamma#
#""_7^13N -> ""_6^13C + e^+ + nu_e#
#""_6^13C + ""_1^1H -> ""_7^14N + gamma#
#""_7^14N + ""_1^1H -> ""_8^15O + gamma#
#""_8^15O -> ""_7^15N + e^+ + nu_e#
#""_7^15N + ""_1^1H -> ""_6^12C + ""_2^4He#

In the case of supernova explosions, vast quantities of free neutrons are released when the core collapses. A process called neutron capture creates elements heavier than Iron. Many elements heavier athan Iron and up to Plutonium are created in this way.


Pulsars, white dwarfs, neutron star and black holes are the remains of dead stars, quasars are powered by black holes.


When a star less than about 8 solar masses runs out of hydrogen and helium fuel, its core isn't hot enough to start carbon fusion. The core which consists of mainly carbon and oxygen collapses under gravity to form a white dwarf. Gravitational collapse is stopped by electron degeneracy pressure.

If the star is larger than about 8 solar masses it is able to fuse heavier elements up to iron. As iron fusion required energy rather than releasing it the fusion reactions stop and the stellar core collapses under gravity. It the core is more massive than the Chandrasekhar limit of 1.44 solar masses gravity overcomes electron degeneracy pressure atoms can no longer exist. Protons become neutrons and large numbers of neutrinos are emitted causing a supernova explosion. The star's core become a neutron star.

If a neutron star is spinning and has a strong magnetic field it emits radiation. As it spins at a precise rate the beam of radiation its the Earth periodically with a period of milliseconds to seconds. This is a pulsar.

If the stellar core is more than about 4 solar masses gravity overcomes neutron degeneracy pressure. Once the core collapses below its Schwarzschild radius, spacetime is curved to the point where not even light can escape. This is a black hole.

Most large galaxies have a supermassive black hole at their centres. These are in excess on hundreds of thousands of solar masses. If there is a good supply of gas and dust in the vicinity of a supermassive black hole it forms an accretion disc of material falling into the black hole. Material falling into the accretion disc gets superheated by friction and gravity to the point where it emits huge amounts of energy. This is a quasar.

So, all are similar in that they are formed from the remains of dying stars. Pulsars are a type of neutron star. Neutron stars and black holes behave similarly. The main difference between these objects is mass.


Pluto was downgraded from planet to dwarf planet because the definition of a planet was changed.


When Pluto was first discovered it was given the status of being the ninth planet.

Since then, other bodies have been discovered in the asteroid belt and beyond Neptune which arguably should be planets if Pluto is a planet.

There are three conditions which must be met to be a planet.

First is must be roughly spherical in shape. Most planets are shaped like a squashed sphere with an equatorial bulge.

Second it must be in orbit around the Sun.

If a body meets these two conditions it is a planet of a dwarf planet. Pluto and other bodies are dwarf planets using theses rules.

Thirdly the body must have cleared its orbit of other bodies. Planets are formed from a disc of protoplanetary bodies orbiting the Sun. These bodies collide and clump together to form a planet. For a body to be a true planet all of theses bodies must have either become par t of the planet or formed a moon of the planet.

Pluto doesn't meet the third condition as it hasn't cleared its orbit of other bodies. Hence the decision was made to downgrade Pluto to a dwarf planet.

Some people, myself included, still regard Pluto as the ninth planet from its historical context. In fact the NASA DE430 data which can be used to calculate the positions of the Sun, Moon and planets still includes data for Pluto.


The Sun and the planets orbit around the centre of mass of the solar system.


It is not actually true to say that the planets orbit the Sun and the Moon orbits the Earth. The Earth and Moon both orbit around their centre of mass which is called the Earth Moon Barycentre (EMB).

Likewise the Sun and the planets orbit around the centre of mass of the solar system. This is called the Solar System Barycentre (SSB). The SSB is in constant motion and moves between the centre of the Sun and about a solar radius outside the Sun. The diagram shows the motion of the SSB over several decades.

enter image source here

The Sun also orbits around the centre of the galaxy. This is even more complex. In principle all of the stars will orbit around the centre of mass of the galaxy. This will be in the galactic bulge which contains a supermassive black hole.

The motion of stars in the galaxy is complicated by the fact that some stars, particularly on the edges of the galaxy, move in a way which gravity can't describe. There isn't enough visible mass to account for their motion.

It is now thought that galaxies contain a lot of dark matter. This interacts only through gravity and is otherwise invisible. We would need to know the mass and distribution of this dark matter to define what the Sun actually orbits around.


We are made of star dust. See below how that's true:


To understand where everything we see on Earth comes from, we first have to understand what it's all made of. The answer that is common to all of it (the atmosphere, hydrosphere, biosphere, and the material that is the Earth) is atoms (and so structurally we've gone more basic than chemicals and molecules).

Some of the more common atoms we have on and around Earth are Oxygen, Hydrogen (these are the constituent parts to water), Carbon (this is the basic building block upon which all life is made - you may have heard the term "carbon-based life form"), Nitrogen (this is what roughly 78% of the atmosphere is made of), Iron (this is what our blood uses to carry oxygen), and a whole host of other elements. In fact, there are 92 elements that can be found in, on, or around Earth (all the elements on the periodic table, up to and including Uranium).


And so the question is - where did all these different types of atoms come from?

To get to that answer, we have to start at the absolute beginning of time - just after the Big Bang. This was a time when there were no atoms at all - they were too big and the early Universe too energetic to exist - the bits and pieces that make up atoms would smash together and then be ripped apart. However, as the Universe expanded and it became less energetic, those bits and pieces started to come together and they formed the most basic atom there is - Hydrogen. One proton, one electron.

As the Universe expanded even more and became even less energetic, the Hydrogen began to form bonds and formed #H_2# - a stable Hydrogen molecule. And this is what the Universe was filled with for a very long time.

But over time, as gravity pulled and nudged the Hydrogen into larger and larger groupings, as the temperature rose to many thousands of degrees within these vast groupings of Hydrogen, they would start a process called Nuclear Fusion - in essence, the temperature within the star caused the Hydrogen to fuse together, creating Helium, the second element on the periodic table, and also cause energy to be released, keeping the star very very hot.

Stars were born and as they continued to smash atoms together, as they continued to fuse, more and more elements were created, up to the element Iron, number 26 on the periodic table. But some stars weren't done yet.

Huge stars go through a process of nova - essentially exploding. Some stars go through an even more massive explosion called a supernova. It's in these explosions where the 26 elements that were created in the belly of a living star are smashed together one last time, creating all the 92 naturally-occurring elements we talked about earlier.


The various atoms combine to form molecules, chemicals, and all the stuff that make up the air, the water, the solid Earth, and the life on, in, and around it.

It's sometimes said that we are made of star dust - and this is why.

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