the life of a star?

Question

briefly describe the order of events in the evolution of the star

if u don't know, please don't answer like the first person to answer the question

Answer 1

A star starts out as a cloud of gas and dust which begins collapsing (for example, from a nearby supernova explosion). As the core continues to accumulate mass it reaches the point where nuclear chain reactions in the star keep it from collapsing any farther. The core is now a star burning hydrogen to create helium. Depending on the mass of the star, this can go on for millions to billions of years.

Finally, the core runs out of hydrogen and starts burning helium creating oxygen and heavier atoms This heats up the star causing its outer layers to expand and it goes through a red giant phase.

As the nuclear reactions in the core make heavier and heavier atoms, the star continues to burn until it is mostly iron. Iron can not be converted to heavier atoms through nuclear reactions so the core cools off and contracts.

Again, depending on the initial mass of the star, it becomes a white dwarf or explodes in a supernova, creating atoms heavier than iron. Left over from the explosion of a supernova is a neutron star or a black hole.

Answer 2

The path a star takes during its lifetime is determined by its mass.
A star such as our own Sun was born when enormous clouds of dust and gas, stretching over 2 light years, collapsed under the force of gravity. As this material was compressed it gradually warmed up to temperatures of 15 million degrees. At this point nuclear fusion began in its core and energy in the form of heat and light travelled out from the centre. As long as there is still material within the star the nuclear fusion process can continue with hydrogen being fused to form helium.

When, however, it has used up its core hydrogen fuel, the star starts to fuse helium and eventually other elements. During this time the star will expand into a red giant or super-giant, becoming tens or even hundreds of times larger. Eventually, no more fuel will be available and the star will collapse under gravity to form a white dwarf star only a few thousand kilometres in diameter.

For stars more massive than the Sun, the end is much more dramatic. Each time one type of the nuclear fuels is consumed, the core contracts a little further, the temperature rises and a new region is ignited. After a series of such steps with the star producing increasingly heavier elements the core is turned into iron. At this point the core can no longer support itself and it collapses violently with the outer layers of the star being torn apart in a supernova explosion.

The core continues to collapse to a neutron star which is only a few tens of kilometres wide but with its material so tightly packed that a teaspoon of it would have a mass of about a billion tonnes.

For stars that have an initial mass of more than 8 times that of the Sun their final destiny is to fall so far under gravity that they turn into black holes, from which nothing, not even light can escape

Answer 3

Stars are born in nebulae, giant clouds of dust and gas left over when a dying star explodes. The matter coalesces over a long period of time, and eventually grows in mass, and grows, and grows, until it has enough hydrogen under enough pressure to spark a fusion reaction, and then it burns for a while (3 to 10 billion years) and when its fuel runs out, there can be a 'red giant' phase where the star expands dramatically, but then gravity takes over and squashes the remnants down into a white dwarf. That glows for a while and eventually cools and becomes a brown dwarf. Bigger stars turn into other interesting things like neutron stars or black holes when they die.

Answer 4

Hi. Condensation, Accretion, Ignition, Fusion, and depending on it's mass, Explosion, White Dwafdom, Neutron Star, or Black hole. My synopsis.

Answer 5

whilst on the region of roaches. I additionally heard as quickly as that the female basically has to get impregnated as quickly as and he or she will become fertile consistently! Now you recognize why there are such a lot of of the little buggers!

Answer 6

http://en.wikipedia.org/wiki/Star

There you go. You're going to have to put a little work into your own homework.

Answer 7

umm.. nebula, something that doesnt come to me, um.. explosion, white dwarf, black dwarf.

Answer 8

i dont remember

Answer 9

Ok... I want it to be cool and as brief as i can get it so here you go......Stars come to life, they live over a very long period of time - and finally they die. The life of the stars can be very varying; the same applies to their death.

Our SUN is one of the small stars. The smallest stars are one tenth of the mass of our sun and the biggest stars are maybe 20 times larger than our Sun.

The stars consist of matter that is already present in the vast outer space. From the scientific purely physical point of view stars are enormous nuclear furnaces, performing nuclear fusion. The stars produce heavier chemical elements out of hydrogen. The enormous distance between us and the stars - together with the atmosphere of our planet - is the only factor that protect our bodies from painful death out of exposure to radiation.

Below is an outline of evolution of the stars, from the beginning until the spectacular death.


Stage 1: The Inter-stellar Medium.

The space between the stars out in the great Cosmos is almost empty. By the standards of reference that we have on Earth, it would be called perfect vacuum. While the normal air contains slightly below one quadrillion of particles per cubic centimetre, the best vacuum that can be achieved in laboratories is about a billion of particles per cubic centimetre. The vacuum in outer space contains from 0,00001 up to less than 1 particle per cubic centimetre, which is far, far better than the best vacuum achieved on Earth.

Note: one quadrillion = 1024 = 1 000 000 000 000 000 000 000 000.

The materia in the inter-stellar space has a name: the Inter-stellar Medium, consisting mainly of hydrogen and helium.

When the condensation of particles exceeds 1 particle per cubic centimetre, we start talking about an inter-stellar cloud. And think now: if the inter-stellar cloud is that empty, then there is no mass inside it then. Both yes, and no..

A cubic centimetre or even a cubic kilometre of inter-stellar cloud contains a very limited number of particles and weighs extremely little. But the inter-stellar space is immense, so immense that the volume is no longer calculated in cubic centimetres or kilometres, but in years of light. And... a cloud of a few hundred cubic years of light will contain an extremely vast number of particles and will have a very substantial mass.

All particles in the cloud attract each other by gravitational force. According to calculations of scientists, a cloud having the mass comparable to the mass of our Sun can be self-sustained in the meaning that the gravitational force will keep it together. The cloud itself is very cold, somewhere around a hundred degrees Kelvin, that is far below -150ºC.


Stage 2: The Contraction of the Cloud.



The inter-stellar cloud will start contracting if it is sufficiently large (and sufficiently heavy). There are also other factors that can make such a cloud contract, like influence from the gravity of surrounding stars etc.

The contraction of the cloud causes the particles of the Inter-stellar Medium pick up speed towards the centre of gravity of the cloud. The speeds of particles are not very high, a few kilometres per second - the process is very long. The increase of speed of the particles means also that the temperature of the cloud increases: the temperature of an object is directly related to the speed of particles within that particular object - whether it is a bowl of water or a piece of iron, or an inter-stellar cloud.

Finally, the temperature inside the cloud reaches around 50 000K. This means that the matter inside the cloud degenerates. The atoms normally consist of nuclei and electrons in orbit around the nucleus; at high temperatures the atoms collide so violently that they get stripped off their electrons. The matter turns into a mixture of two gases:

* the nuclei gas composed mostly of hydrogen nuclei (protons)


the gas of electrons.

The atomic nuclei repel each other. So do the electrons. The gas is stable. We call it PLASMA.

Up to this moment, the original size of the cloud has also diminished from trillions of kilometres down to around 200 millions kilometres or less, which is roughly 30 000 times the diametre of the Earth. Given the speed of particles, this contraction of the original cloud takes a tremendous amount of time, from millions to billions of years. The contraction speed depends on the mass of the cloud: the larger the mass, the fastest the contraction.


Stage 3: The Proto-star.
The Trapezium in the Orion Nebula, another region of starbirth

The compression of the cloud generates heat and that heat gives the cloud own light. Now it is no longer a dark inter-stellar cloud but of a glowing proto-star.

The gravitational force causes further contraction of the cloud, giving an internal temperature of the proto-star of around 150 000 K and the surface temperature around 3500 K. Of course, there can be hardly any "hard" surface, as the proto-star is still a cloud of gas. Due to the huge size of the proto-star, the surface - and thus the radiation - is enormous.

The glow of the proto-star is only caused by release of energy due to the contraction of the proto-star. No actual nuclear reaction takes place, yet. At this stage the proto-star can evolve in two directions:

* if it is massive enough, it will continue the process of contraction; finally the nuclear reactions will fire, giving the proto-star the status of star


if it is not massive enough, the fate will be different. The proto-star will eventually transform into a giant planet made of gas, with some heat and some emission of light, but that heat eventually will cool down. The planets of Jupiter and Saturnus are good examples of that. If the planets had been hundreds of times more massive, they would be able to start a nuclear fire, making them tiny stars. But they don't have the enough mass and their fate is to end as brown dwarfs.

Image of Gliese 229B and Brown Dwarf Gliese 229B

The celestial bodies that do not fire up as stars are of no interest to us, so we will leave them alone.

The proto-star continues the contraction until the temperature at the centre mounts to around 15 000 000 K, at which point a very important event occurs...


Stage 4: The Young Star.
The Pleiades in Taurus, a young open cluster

The temperature in the centre of the proto-star makes the nuclei collide more and more violently. At some point, when the gas temperature in the centre of the star has reached around 15000000 K, the collisions between nuclei become so violent that the repulsive forces are too weak and the nuclei merge together. The merging process of two nuclei generates heat; that heat rises the temperature even more.

The merging of two atomic nuclei can be compared to a collision between two cars: with low speeds, you can still see the two different cars. With very high speeds, you just see one single piece of unidentified junk.

In physics, the reaction, when two nuclei merge (fuse), is called a nuclear reaction or a nuclear fusion.

Once ignited, the nuclear furnace in the centre of the star will burn for the greater part of the rest of the life of the star.


Stage 5: The Mature Star.


The mature star is not very fascinating. It contains mostly hydrogen. It burns... and it burns... and it burns... The hydrogen inside the star is converted into helium by the means of nuclear fusion.

The contraction of the star slows down and finally ceases, as the heat inside the star compensates the gravitational force of the star. The star is in a state of equilibrium.

The star radiates yellowish or white light, depending on its surface temperature, which in turn is connected to the star's size. Warmer surface gives whiter light of the star, cooler surface gives yellowish light of the star. Very warm stars may even appear blue in their shine.

The big stars consume fuel at rapid pace, the bigger the star - the faster the fuel consumption. Our Sun has probably the life span of around 10 billions of years. A star with a mass 100 times bigger than the mass of the Sun will probably burn fuel 10 000 times as fast as the Sun, which means that it will exhaust the nuclear fuel in about 100 millions of years. A star with a mass of one tenth of the mass of our Sun will probably live 10 times longer than the lifespan of our Sun.

The star stays in the mature state for the most of the lifetime; observing changes in a mature star's glow and size is about as exciting as watching a light bulb...


Stage 6: The Red Giants

Betelgeuse,
the only star (other than our Sun) for which it's possible to image the surface -
because it's so big (size of Earth's orbit) and close enough
Eventually the star will burn around 10% of the original hydrogen supply and increase brightness with around 50%.

The helium nuclei are 4 times heavier than the hydrogen nuclei, which means that they "sink" into the centre of the star. Finally the number of helium nuclei in the core of the star becomes so large that there is little chance for hydrogen nuclei to collide with other hydrogen nuclei. The nuclear reaction in the centre of the star comes to a gradual halt, which decreases the temperature of the core of the star. At the same time the surface of the star cools down to around 3000 K, which makes the star reddish in appearance.

With the nuclear reaction stopped - and the heat still emanating from the star - there is no longer balance between the heat released inside the star and the gravitational force. The core of the star starts contracting once more, which increases the temperature of the core to the point where the outside of the core is around 15 000 000 K hot. This once again starts the nuclear fusion of hydrogen nuclei. But this time, the burning of hydrogen occurs NOT in the core of the star, but in the envelope surrounding the core of the star. It is one of the last breaths of the star.

The heat generated by hydrogen burning in the envelope surrounding the core of the star makes the outer layers of the star expand. As opposed to contraction - the gas becomes cooler as the result of expansion. And that is exactly what is happening here. The outer layers of the star expand 50-100 times and they become cooler.

The star is now a red giant, emanating tremendous amounts of light because of the great surface. The light is red because the surface is comparatively cool.

To put things in the appropriate context: the day our Sun expands into a red giant it will probably expand to the planet of Venus and it will occupy most of the sky during daylight. Needless to say, everything on the Earth will be burned to ashes.



Stage 7: The Helium Flash


The helium core of the star will continue to contract. As there is no nuclear reaction inside it, the gravity works unchecked. But at some point, the plasma combined of nuclei and electrons ceases to behave like an ideal gas. After all, the plasma is concentrated into extreme density, equal to many tons per cubic centimetre. Under that tremendous pressure the gas of electrons begins to behave like solid matter, i.e. increase of temperature causes only very moderate expansion.

The temperature of the core of the star rises steadily, making the hydrogen burn even more vigorously in the regions adjacent to the core. At some point the temperature of the core of the star reaches 100 000 000 K, which starts burning the nuclei of helium. The new nuclear reaction fuses helium nuclei into heavier elements, like carbon.

The temperature of the core of the star rises as the result of the new nuclear reaction. The core of the star - though - behaves like solid matter; the core cannot expand very much to compensate the extra heat source. This means that the inside of the core of the star becomes even hotter, which adds even more to the pressure, which again means even more heat in the core of the star and that means an even faster nuclear reaction.

Finally the pressure is so great that the core of the star... EXPLODES. The explosion occurs inside the star and it is only visible by a sudden, although moderate, increase of the star's brightness. This is called the HELIUM FLASH. The time from the beginning of helium fusion to the explosion of the core of the star takes an extremely short time compared to the lifetime of the star: just a few hours.


Stage 8: The Helium Star


The helium explosion causes a great expansion of the core of the star. The expansion cools the extremely hot core of the star - and the surrounding hydrogen envelope. Because of the explosion, the helium plasma has no longer same characteristics as before. It behaves more like a gas.

The decrease of the temperature slows down the speed of burning of hydrogen; the decrease of the temperature causes also the expanded helium core of the star burn at a slower pace. For the first time in the lifetime of the star the luminosity of the surface decreases noticeably. The decrease of the temperature of the star causes contraction of the outer layers of the star.

About 10 000 years from that, a new state of equilibrium is achieved. The star has now two ovens:

* in the envelope, nuclei of hydrogen are being combined into helium at temperature of around 15 000 000 K


in the core of the star, nuclei of helium are being combined mainly into carbon at temperatures around 200 000 000 K.

Some oxygen is produced in this process as well. From this point in time the main part of the energy of the star comes from helium fusion. The helium core behaves the same way the hydrogen core did behave in the earlier period of the life of the star.

The star starts accumulating carbon in the centre of the core. And again, carbon being heavier than helium, it "sinks" down into the centre of the core of the star, where it forms an inner core inside the helium core of the star. Eventually, most of the star's helium is converted into carbon and oxygen.

Once again the outer envelope of the star expands, transforming it into a red giant once more. Only this time the process takes barely a few millions of years.


Stage 9: The Star that Dies: The White Dwarf and the Supernova


At some point all of the nuclear fuel in the star has been exhausted. A greater part of the hydrogen has been converted into helium and most of helium has been converted into carbon and oxygen. What happens now to the star depends on the mass of the star.

The carbon core of the star is extremely dense, one cubic centimetre of it weighing metric tons. The surface of the core of the star is also very hot, 50 000-100 000 K.

Small stars.

Smaller stars, about 4 masses of our Sun or less, cool down. Given time, the outer layers of the star become cool enough to leave the plasma state. The atoms reverse to their neutral state and capture electrons. The capture of electrons accelerates the expansion of the outer layers, which causes more atoms to leave the plasma state.

The envelope of the star becomes finally a transparent and extensive shell of atoms; this shell can only be seen from the side from very long distances, thus giving the surroundings of the star a peculiar appearance of a luminous ring. Once upon a time, astronomers believed those rings were the first stage of formation of planetary systems; because of that the rings were called "planetary nebulae" . We know today that there is no connection between the planetary nebulae and planetary systems, but the name remained.

The only remnant of the star is now the core of the star and it is a tiny and not very bright object in the middle of the nebula. In the beginning the core of the star is still glowing with a white glow, dissipating the heat from the nuclear fires, now extinct. It is called a"white dwarf" . A white dwarf weighs much less that the original star, for example a star four times heavier than the mass of our Sun gives origin to a white dwarf having 1½ of the mass of our Sun.



Medium sized stars.

Larger stars, between 4 and 8 masses of our Sun, encounter a more violent fate. The stopping of nuclear reactions makes them collapse more rapidly and more violently than the small stars. The core of the star consists now of solid carbon, which is not burning. However, the contraction of the star generates enormous amounts of heat. At the point when the temperature of the core reaches 600 000 000 K, the carbon starts a nuclear reaction, generating neon, helium, magnesium and some other elements.

But again, the core of the star is solid and cannot expand to release the internal pressure that builds up because of the nuclear reaction. The core becomes an uncontrollable nuclear reactor in alike manner as in the case of helium flash. But now, the temperature is higher, the pressure is higher and the result more violent.

The star explodes in a SUPER-NOVA. The explosion is so bright that a super-nova can even be seen on the Earth in daylight. The explosion shatters the star and may be so violent that it disperses all of the material of the star in outer space. If there are any remnants of the star, it will be a small compressed partt of the core of the star.



Large stars.

Really large stars, with masses greater than 8 masses of Sun, do not become a super-nova at this point of time. The stopping of nuclear reactions causes contraction, like for the smaller stars. But the core of the largest stars does never become as dense as the core of medium sized stars. This is probably caused by the intense radiation inside the core of the largest stars, giving an abundance of highly energetic photons that drive out matter from the centre of the star.

At some point the temperature of the core reaches the 600 000 000 K that ignite the carbon. The nuclear reaction will not be as violent as in medium sized stars, because the core of the largest stars is less dense. The carbon core burns at moderate pace; the temperature eventually increases, putting also oxygen on fire.

When carbon and oxygen are exhausted, the star cools down and shrinks again, which again heats the core of the star to higher temperatures. Those higher temperatures ignite the heavier elements produced from coal and oxygen, giving still heavier elements. After a while the star is a series of envelopes contained within each other; each of the envelopes burns different chemical elements. The heavier elements are in the inner envelopes, while the helium and hydrogen are in the outer envelopes.

A star at this stage of life can produce elements no heavier than iron. The nuclear reaction comes to an end with iron. Iron does not engage in nuclear fusion. Fusion of elements less heavy than iron releases energy, while fusion of iron and elements heavier than iron consumes energy.

The creation of iron extinguishes the nuclear fire inside the star. The star shrinks for the last time. The iron core of the star absorbs most of the heat generated by the contraction of the star, which accelerates the contraction even more. When the temperature inside the core reaches trillions of degrees and the neighbouring atomic nuclei touch each other, there can be no more contraction. Instead, the star rebounds in a great explosion.

This explosion is also called a SUPER-NOVA and may be as spectacular as for the medium sized stars. The star is billions times brighter than any time previously and it may even be as bright as an entire galaxy.

The explosion of the heavy super-nova shatters even the atomic nuclei to pieces; those pieces get captured by other atomic nuclei, forming elements beyond iron, like silver, gold and uranium. Elements beyond iron do not abound in nature - and that is attributed to their creation during the short super-nova blast.

Those heavier elements can later be captured into other clouds and become part of new stars and new planets. Because of that, heavy elements like uranium should also exist in stars. For many years it was only a theory; in the beginning of year 2001 the "European Southern Observatory" (ESO) in Chile discovered that the star called CS 31082-001 indeed has uranium in it. This was the first ever measurement of uranium outside of our planet.


Stage 10: The Remnants of the Stars: Black Dwarfs, Pulsars, Neutron Stars and Black Holes


The Black Dwarfs.

A white dwarf cools down slowly. The colour of the glow of the surface changes from white to yellow, to orange and red. Finally the remnant of the star becomes a cold dark lump of matter - the black dwarf. The black dwarf has the size of our planet and a gravity that is millions of times higher than the gravity we experience on the Earth.

The black dwarf is simply a quiet, desolate and dead remnant of a star, moving forever through the cold Universe.



Pulsars.

Some time ago it has been discovered that there are celestial objects that emit extremely regular radio signals, no longer than 1/100 of a second. In the beginning the scientists thought that it was a signal from an alien civilisation. But the signals were emitted over a very large band of radio frequencies, thus requiring tremendous amounts of energy.

By measuring signal distortion the scientists came to conclusion that the object emitting the signals was around 10-20 kilometres in radius, and yet as massive as the Sun. The signal's interval and duration came from the object's rotation. It resembles of a lighthouse with the light beam sweeping around.

But what are those objects?



Neutron Stars.

The existence of neutron stars has been predicted by theoretical astronomers. It has been pointed out that during the supernova explosion the star core (or the remaining part of it) can become so compressed that protons and electrons may be forced to merge. Merging protons and neutrons form together neutrons.

The neutrons of the star would form a very compact ball with a radius of maybe 10-20 kilometres and with most of the star's mass packed inside it. The matter in a neutron star would be so dense that a cubic centimetre filled with it weighs billions of tons.

There have been no direct observations of neutron stars. In the places where scientists predicted one would find neutron stars, pulsars have been found instead. Nowadays scientists are certain that pulsars and neutron stars are the same thing.



Black Holes.

A very massive star core, remnant of a super-nova explosion, can exert such a tremendous gravitational force that not only solid objects, atoms cannot escape from the star's surface. Also light "falls down" to the surface of the star. That kind of object is called a "black hole" .

The matter within the black hole probably shrinks to smaller and smaller volumes all the time. The star shrinks to a few kilometres, then a few centimetres and - finally - to a "singularity", which is one single point in space. Even though the matter inside the black hole collapses into sizes smaller than anything mankind has ever measured, the black hole itself does not change in size. After all, the name "black hole" applies to the radius around the degenerated core of the star, determining the line between place where we still can see into and the place where we can't see anything. This radius determines the size of "event horizon".