Stars are astronomical objects and represent the most fundamental building blocks of galaxies. The history, dynamics and evolution of the galaxy is shown from the stars age, distribution and composition. The stars are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen and oxygen. Astronomy is the study of the birth, life and death of stars.
Stars are formed from clouds of dust and scattered throughout most galaxies. The orion nebula is a examble of a dust cloud. Inside of these clouds turbulences gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction. When these clouds start to collapses the centre begins to hear up this is known as a protostar. Inside of its hot core at the heart of the collapsing cloud this will one day become a star. Most stars in the milky way are paired in groups of multiple stars this may be because when star form the spinning clouds of collapsing gas and dust break up into two or three blobs.
When the cloud collapse, a dense, hot core forms and begins to gather dust and gas. The remaining dust can become planets, asteroids or comets or may remain as dust. Sometimes the cloud does not collapse at a steady pace. A amateur astronomer James McNeil discovered a small nebula that appeared unexpectedly near the nebula messier 78 in the constellation of orion. When observers looked at this nebula they found that the brightness appears to vary this might be because of the interaction between the young stars magnetic field and the surrounding gas causes this increase in brightness.
Main Sequence Stars
A star that is the size of our sun requires about 50 million years to mature and the sun will star in the mature phase for about 10 billion years. The stars are fueled by the neclear fusion of hydrongen to form helium deep in their interiors. The energy within the star provides enough pressure necessary to keep the star from collapsing under its own weight and the energy by which it shines. The main sequency of stars span a wide range of luminosities and colours. The small stars are known as red dwarfs which may contain as little as 10% of the mass of the sun and emit only 0.01% as much energy glowing feebly as temperatures between 3000-4000k. These red dwarf stars are the most common in our universe and have lifespans of tens of billions of years. Massive stars which are known as hypergiants, may be 100 more times more massive than our sun and have temperatures of about 30,000k, these hypergiants emit hundreds of thousands more energu than the sun but only has a few million years lifetime. These stars are extremely rare and only a handful exist in our milky way.
Stars and Their Fates
Most of the time the larger the star the shorter life it has. When all the hydrogen has been fused inside of a star the nuclear reactions cease. When deprived of the energy production needed to support the star the core begins to collapse into itself and becomes much hotter. Since hydrogen is still outside of the core it continues to shell surrounding the core. This hot core also pushes the couter layers of the star outward causing them to expand and cool this transforms the star into a red giant. If a star is massive then the collapsing core may become hor enough to support the more exotic neclear reactions that consume helium and make a variety of heavier elemenets up to iron. Eventually the stars internal nuclear fires and comes unstable this sometimes means burning furiously other times dying down. This can cause the star to pulse and throw off its outer layers. When that happens the star enshrouds itself in a cocoon of gas and dust.
In general, the larger a star, the shorter its life, although all but the most massive stars live for billions of years. When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core. The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant.If the star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the star's internal nuclear fires become increasingly unstable - sometimes burning furiously, other times dying down. These variations cause the star to pulsate and throw off its outer layers, enshrouding itself in a cocoon of gas and dust. What happens next depends on the size of the core.
White Dwarfs are the dimmest stars in the universe. They have commanded the attention fo astronomers since the first one was discovered in the middle of the 19th century. The main interest is that white dwarfs represent an intriguing state of matter and that most of the stars in our galaxies including our sun will become a white dwarf when it reaches its final burnt out collapsed state
When the hydrogen is used up the stars core collapses and since its a non renewable energy source the shell of hydrogen will be compressed and heated. The nuclear fusion of the hydrogen will produce a new surge of power that will cause the outer layers of the star to expand until it has a diamter a hundred times its value this is when is called the red giant phase.
Hundred of millions of years after the red giant phase all the stars available energy resources will be used up. The wolf-rayet type star is the exhausted red giant that will puff off its outer layers leaving behind a hot core. This star has a surface temperature of about 50000 degress celsius and is furiously boiling off its outer layers in a fast wind traveling 6 million kms per hour.
A planetary nebula is when the radiation from the hot star hears the slowly moving red giant atmospher and creates a complex and graceful filamentary shell. There are clouds of multimillion degree gas that have been compressed and heared by the fast stellar wind. This will eventually cause the star to collapse to form a white star.
In this state all the material from the star minus the amount blown off in the red giant phase will then be compressed into about 1 millionth of the size of the original star. To compare it will be like have a olive made of the same material the would have the same mass as a automobile. This phase is called 'white' because the surface temperatures will be about 20000 degrees celsius.
First white dwarfs were presented a paradox to astronomers if the white dwarf couldnt produce energy through nuclear fusion then it couldnt generate enough pressure to keep it from collapsing this reminded the scientists that maybe its their theories that are incorrect not the stars. Quantum theory helped resolve the paradox since it shows that the matter in the degenerate states of extremely high density could produce a new type of pressure. The reason is because quantum theory prohibits more than one electron from occupying the same state.
A massive star in our galaxy blows itself apart every 50 years or so to create a supernova explosion. These Supernovas are extremely violent and the force of the explosion generates a blinding flash of radiation and shock waves similar to sonic booms. These supernovas were first classified basic on their optical properties. Type II supernovas show visible evidence for hydrogen in the debris shooting out of the explosion while type Ia explosions do not. These types have been refined and a classification in terms of types of stars that give rise to super novas. Type II, Type Ib and Type Ic explosion, is produced by the catastrophic collapse of the core of a massive star. A type Ia supernova is produced by a sudden thermonuclear explosion that disintegrates a white dwarf star.
Type II supernovas are seen with alot of bright, young stars such as the spiral arms of galaxies. they apparently do not occur in elliptical galaxies. These bright young stars are typically about 10 times greater in mass of the sun and this leads to the conclusion that type II supernovas are produced by massive stars. Type i supernovas show many of the characteristics of type II supernovas. These supernovas called Ib and type Ic are somewhat different from type II since they have lose their outer hydrogen envelope prior to the explosion. This might be because the hydrogen was pulled away by a companion star.
Core-Collapse supernovas is when the nuclear power source at the center or the core of the star is exhausted this means the core collapses in less than a second. A neutron star is formed when this happens the star releases a enormous amount of energy in the form of neutrinos and heat which reverses the implosion. The central neutron star is then blown away at speeds of about 50 million kilometers per hour as the shock wave races through the now expanding stellar debris this fuses lighter elements into heavier ones and produces a brilliant visual outburst that can be as intense as the light of several billion suns.
A type Ia supernova or a termonuclear is produced by white dwarf stars that condense remnant of what used to be sun like stars. A white dwarf star a dense ball primarily comprimised of carbon and oxygen atoms is the most stable of the stars as long as the mass remains below the so called limit of 1.4 solar masses. If the matter from a companion star or the merger of another white dwarf is pushed over the limit of 1.4 solar masses the temperature in the core of the white dwarf will rise. This will trigger a explosive nuclear fusion that releases an enormous amount of energy. The star explodes in abotu ten seconds leaving no remnant. The cloud glows brightly for many weeks as radio active nickel produced in the explosion decays into cobalt and iron.
Since type Ia supernovas all occur in a star that has a mass of about 1.4 solar masses they produce the same amount of light. This makes them useful as a distance indicator. This means if one Type Ia is dimmer than another one it must be further away.
Because Type Ia supernovas all occur in a star that has a mass of about 1.4 solar masses, they produce about the same amount of light. This property makes them extremely useful as a distance indicator - if one Type Ia supernova is dimmer than another one, it must be further away by an amount that can be calculated. In recent years Type Ia supernova have been used in this way to determine the rate of expansion of the universe. This research has led to the astounding discovery that the expansion of the universe is accelerating, possibly because the universe is filled with a mysterious substance called dark energy.
Everything in space is mostly empty space even a rock is mostly empty since its matter is made of atoms. A atom is a cloud of electrons orbiting around a nucleus composed of protons and neutrons. This nucleus contains about 99.9 percent of the mass of an atom, Yet it has a diameter of only 1/100,000 of the electron cloud. This electrons take up very little space but the pattern of their orbit defines the size of the atom. which is about 99.99 percent open space.
When we bump into a sold rock it is really a ball of electrons moving through empty space so fast that we can see or feel the emptiness. If the matter wasnt empty and we crush the electron cloud down to the size of a nucleus the rock would be squeezed down to the size of a grain of sand and would still weigh 4 million tons.
These extreme forces in nature is the central part of a massive star collapses to form a neutron star. these atoms are crushed completely and the electrons are jammed inside the protons to form a star composed of neutrons. This creates a tiny star that is like a gigantic nucleus and has no empty space.
If you approached a neutron star the gravitational field would pull your space craft into pieces before you reached the surface. The magnetic fields around the neutron stars are extremely strong aswell. Even if you did make it within a few thousand miles above the surface of the neutron star you will face another problem. The neutron star rotates so rapidly and the strong magnetic fields combined with rapid rotation create a generator that can produce electric potential differences of quadrillions of voltes. These volts are about 30 million times greater than those of lightning bolts.
The high energy particles can produce beams of radiation from radio through gamma-ray energies. Like a rotating lighthouse beam the radiation can be observed as a pulsing source of radiation or pulsar. Some pulsars like the one in the crab nebula pulse in every wavelength- radio, optical, x-ray and gamma-ray but some pulsaras only pulse in X-rays and some pulse only in gamma-rays.
Neutron stars with magnetic fields that are about a quadrillion times greater than the magnetic field of earth are called magnetars. These magnetic fields are produced when a extremely rapidly rotating neutron star is formed by the collapse of the core of a massive star. This triggers a supernova explosion that expels the outer layers of the star at high speeds. These high rates of rotation intensifies the already strong magnetic field if strong enough a star quake is cause that can produce powerful x-ray flashs. These represent an intermediate type of supernova explosion. These magnetars are more energetic than orfinary supernovas but less than hyper novas and thought to be responsible for gamma ray bursts.
The strongest steady magnetic field produced on earth in a lab is about a million times greater than the earths magnetic field. This is beyoung the orfinary magnetic material. Only on a neutron star where graveity is more than 100 billion times as great as on earth can matter withstand the magnetic fields of a magnetar an even there the neutron stars crust can break apart.
When a star runs out of nuclear fuel it will collapse if the core has a mass that is greater than three suns no known nuclear forces can prevent the core from forming a deep gravitational warp in space called a black hole. The black hole does not have a surface in the sense but it is simply a region or boundary in space around a black hole beyond which we cannot see this is called the event horizon. Anything that passes beyond the event horizon is doomed to be crushed as it descends deeper into gravitational well of the black hole. No bisible light no x-rays nor any other form of electromagnetic radiation can escape. The radius of the event horizon is very small only 30 kilo meters. Astronomers cant see the black hole directly the only way to find one is find circumstantial evidence.
Large amount of matter must be compressed into a sufficiently small region of space so that no other explantation is possible as for stellar black holes this means that observing the orbital acceleration of a star as it orbits its unseen companion in a double or binary star system.
Its hard to search for black holes but one way to locate them is to study the x-ray binary systems these systems consist of a visible star in close orbit around an invisible companion star. Ehich might be a black hole the companion star then pulls the gas away from the visible star and as the gas forms a flattened disk. It swirls towards the companion. The friction then causes the gas to heat them to extreme temperatures and then x-rays are produce that flicker or vary in intensity within a second.
Many of these brigh x-ray binary sources have ben discovered. The rapid orbital velocity of the visible star indicates that the unseen companion is a black hole. Then the x-rays in these objects are produced by particles very close to the event horizon. They then disappear beyond the event horizon. Not all matter in the disk around the black hole is doomed. Sometimes some of the gas escapes as a hot wind that is blown away from the disk. This then causes a dramatic high energy jet which can move at nearly the speed of light.
Black holes do grow the mass of the black hole increases by an amount equal to the amount of mass it capture. The radius of the event horize also increases by about 3 kilo meters. A black hole in the center of a galaxy where stars are densely pack may grow to the mass of billions of suns and become a supermassive black hole.