Facts about different star stages

Not all star types go through all of these stages. Consult your other resources to determine what stages your star goes through, then find

more info here.

Brown Dwarfs

Brown dwarfs are objects which are too large to be called planets and too small to be stars. They have masses that range between twice the mass of Jupiter and the lower mass limit for nuclear reactions (0.08 times the mass of our sun). Brown dwarfs are thought to form in the same way that stars do - from a collapsing cloud of gas and dust. However, as the cloud collapses, it does not form an object which is dense enough at its core to trigger nuclear fusion. The conversion of hydrogen into helium by nuclear fusion is what fuels a star and causes it to shine. Brown dwarfs were only a theoretical concept until they were first discovered in 1995. It is now thought that there might be as many brown dwarfs as there are stars.

Artist's rendition of a brown dwarf

Brown dwarfs are very dim and cool compared with stars. The best hope for finding brown dwarfs is in using infrared telescopes, which can detect the heat from these objects even though they are too cool to radiate visible light. Many brown dwarfs have also been discovered embedded in large clouds of gas and dust. Since infrared radiation can penetrate through the dusty regions of space, brown dwarfs can be discovered by infrared telescopes, even deep within thick clouds. Recently, 2MASS (Two Micron All Sky Survey) data revealed the coolest known brown dwarf. To the left is an infrared image of the Trapezium star cluster in the Orion Nebula. This image was part of a survey done at the United Kingdom Infrared Telescope ( UKIRT) in which over 100 brown dwarf candidates were identified in the infrared.

The discovery of objects like brown dwarfs will also give astronomers a better idea about the fate of our universe. The motion of the stars and galaxies are influenced by material which has not yet been detected. Much of this invisible dark matter, which astronomers call "missing mass", could be made up of brown-dwarfs. Our universe is currently expanding, due to the Big Bang. If there is enough mass, it is thought that the expansion of the universe will eventually slow down and then the universe will start collapsing. This scenario could mean that the universe goes through an endless cycle of expansions and contractions, with a new Big Bang occurring every time the universe ends its collapse. If there is not enough mass for the universe to collapse, then it will expand forever. We will only know the fate of the universe when we can accurately estimate how much mass the universe has in it. The detection missing mass objects, such as brown dwarfs will likely be a key to answering this question.

Artist's rendition by Robert Hurt,

IPAC

Brown Dwarfs were only a theoretical concept when the Spitzer Space Telescope was first proposed. Since the mid-1990s, various infrared telescopes and surveys have identified a few hundred of these objects. Spitzer will devote much of its time to the discovery and characterization of brown dwarfs. It is expected that Spitzer will study thousands of these objects, including those only slightly larger than Jupiter. This will provide astronomers with enough data on brown dwarfs for good quality statisical studies.

Red Dwarfs:

Stars in the universe come in all sorts of sizes, from comparatively small neutron stars to massive supergiants. By far the most abundant type of star, however, is the red dwarf. Smaller than our Sun but with a much longer lifetime, these balls of burning gas are extremely important in our understanding of the cosmos.

Red dwarf stars typically have a mass of between 7.5% and 40% of the Sun. Less massive stars are known as brown dwarfs, owing to their comparatively low luminosity, while more massive stars (including

our own star) are yellow dwarfs. Their reduced mass means that red dwarfs have a cooler surface temperature than the Sun, typically around 3,500 Kelvin (3,230 degrees Celsius) compared to over 5,750 Kelvin (5,475 degrees Celsius) for the Sun.

Energy is generated in a red dwarf in the same way that it is in the Sun, namely through the fusion of hydrogen into helium. Because of their lower mass and core temperature, though, the rate of nuclear fusion is much less, and thus they emit a smaller amount of light. Even the largest red dwarfs emit only 10% of the Sun’s light, while the smallest have just one ten thousandth of the Sun’s luminosity.

In all stars energy from the core is radiated out from the surface through a process known as convection, losing a large amount of mass in the process. Red dwarfs, on the other hand, are fully convective. This means helium does not accumulate at the core, and the stars can continue to burn hydrogen for a much longer time than other stars.

More massive stars, like O-type stars, are much less numerous than lower mass stars like red dwarfs.

So long is the process, in fact, that the lifespan of a red dwarf can be far longer than the expected age of the universe, thought to be about 14 billion years. More massive stars burn through their fuel much faster and thus have shorter lifespans, sometimes just a few million years, so the lower the mass of a red dwarf the longer it will live. A red dwarf with a tenth of the Sun’s mass will continue burning fuel for 10 trillion years. Therefore, there are no red dwarfs that we know of in the universe that are nearing the end of their lives, so we will likely never observe what happens in the

last throes of their lives.

White DwarfA white dwarf is the final stage of the evolution of a star that is between .07 and 1.4 solar masses. White dwarfs are supported by electron degeneracy and they are found to the lower left of the main sequence of the HR (Hertsprung Russel) diagram. White dwarfs represent a stable phase in which stars of less than 1.4 solar masses live out the rest of their lives. White dwarf stars got their name because of the white color of the first few that were discovered. They are characterized by a low luminosity, a mass close to that of our sun, and a radius only that of the earth. Because of their large mass and small area these stars are extremely dense and compact objects with average densities approaching up to 1,000,000 times that of water. White dwarfs have low luminosities. Because of this they can be observed only within a few hundred parsecs from the earth ( 1 parsec = 3.26 light years). 

Facts About White Dwarfs

All stars are burning at some point in their lives but eventually a star stops burning. When the stars stop burning the stars with less than 1.4 solar masses shrink in size. As they shrink they start to grow very faint. But regardless of their color they are called white dwarfs. The value of 1.4 solar masses is known as the Chandrasekhar limit. Chandrasekhar reasoned that something must be holding up material in white dwarfs against gravity, something known as electron degeneracy. When star contracts, electrons get close together and there is a continued increase in their resistance to being pushed even closer. This process is related to pressure. At great densities, pressure from the degenerate electrons is sufficiently great, it balances the force of gravity and the star stops contracting. So electron degeneracy stops the white dwarf form contracting and compresses the gas of the star. What this means is that a white dwarf is incredibly dense. A mass the size of the sun is compressed into a volume only the size of the earth. This is so dense that a teaspoon of white dwarf weighs ten tons.

What Is the Chandrasekhar Limit?

The Chandrasekhar limit is the maximum theoretically possible mass for a stable white dwarf star. The limiting value was named after the Indian-born astrophysicist Subrahmanyan Chandrasekhar, who formulated it in 1930. Using Einstein’s special theory of relativity and the principles of quantum physics, Chandrasekhar showed that it is impossible for a white dwarf star, which is supported solely by a degenerate gas of electrons, to be stable if its mass is greater than 1.4 times the mass of the Sun. If such a star does not completely exhaust its thermonuclear fuel, then this limiting mass may be slightly larger. For example, all direct mass determinations of actual white dwarf stars have resulted in masses slightly less than the Chandrasekhar limit. A star that ends its nuclear-burning lifetime with a mass greater than the Chandraskehar limit must become either a neutron star or a black hole.

What Happens In Time?

What happens in time? Pressure from degenerate electrons doesn’t depend on temperature so stars are stable even though no more energy is ever generated within them. Because of electron degeneracy they can’t contract further. However, they still have stored energy that will radiate for a few billion years. Once the star burns out completely or stops radiating the white dwarf has reached the final stage of evolution and it becomes a cold and inert stellar remnant sometimes called a black dwarf. Our Sun is destined to die as a white dwarf. But, before that happens, it will evolve into a red giant. When the sun becomes a red giant it will engulf Mercury and Venus in the process and at the same time it will blow away the earth’s atmosphere and boil its oceans. This will make earth uninhabitable but this process will take billions of years to develop.

Medium-size stars

  For stars heavier than 1.4 solar masses but lighter than about 3-4 solar masses (the calculations are still a bit uncertain), the electron pressure is not strong enough to balance gravity. The contraction then goes crushing the electrons together and braking apart the Iron nuclei into their constituents. These constituents, neutrons and protons, also detest being close to each other and, as mentioned above, produce a (degenerate) pressure which opposes gravity. For a star in the present mass range this pressure is sufficient to stop further collapse, but is effective only when the material is extremely dense which occurs only when the star has contracted to an object a few kilometers in diameter.

The contraction of these stars from their initial solar size to the size of a city is one of the most spectacular events in the heavens: a supernova. Imagine an object weighting 5 × 1027 tons (that is five thousand trillion-trillion tons, or about 2.5 solar masses), which contracts from a size of 106 (one million) kilometers to about 10 kilometers, and it all happens in a fraction of a second. During collapse the amount of energy generated is fantastic, part of it goes into creating all elements heavier than Iron, part into creating neutrinos and part is transformed into light.

Radioactive elements are also created during the collapse. These elements rapidly decay, and the resulting radiation is so intense it produces a fantastic flash of light. At this point the supernova will out-shine a full galaxy of normal stars (several billion or up to a trillion of them!).

After the collapse there is a violent overshoot before equilibrium sets in, at this time all the outer layers of the star are ejected at speeds close to that of light. When this material goes trough any planets around the star (if any) it vaporizes them. In the middle of this cloud the core of the original star remains, a rapidly rotating remnant, protected against further collapse by it neutron degenerate pressure.

The overshoot is so violent that the elements created will be strewn all over the region surrounding the star, part of this material will end up in dust clouds which will become stellar systems ( the shock produced by the supernova material colliding with a dust cloud may initiate the formation of a stellar system); this is how the Earth acquired all elements aside from Hydrogen and Helium. Every bit of tungsten used in our light bulbs came from a supernova explosion, as all the uranium, gold and silver. All the iron in your hemoglobin got there through a supernova explosion, otherwise it would have remained locked into the deep interior of some star.

 

After gravity is balanced, and after the exterior shells are ejected the star stabilizes forever. But not without some fancy footwork: the remains of the star usually rotates very rapidly (up to 30 times per second!) and it also possesses a very large magnetic field. These two properties cause it to emit X-rays in a directional fashion, sort of an X-ray lighthouse. Whenever the X-ray beam goes through Earth we detect an X-ray pulse which is very regular since the star's rotation is regular. This is called a pulsar. As time goes on the rotation rate decreases and the star dies a boring neutron star. Neutron stars are very compact objects having radii of about 10 km (6 miles) so that their density is enormous, a teaspoon of neutron-star material would weigh about 1012 (one trillion) tons on the Earth's surface

Red GiantStars spend the vast majority of their lives on what is known as the main sequence. Here they convert hydrogen into helium, mostly through the fusion process of the proton-proton chain. However, once this fuel has been depleted, the core will begin to contract.

As the core is collapsing the temperature is rapidly increasing. The resulting energy propagates from the core, pushing the outer envelope of the star outward. The star is now dramatically larger, and has become a red giant.

Properties of a Red Giant

Even if the star is a different color, say yellow like our Sun, the resulting giant star will be red. This is because as a star increases in size the average surface temperature decreases. According to Wien's law, the peak wavelength (color) will be determined by the temperature with cooler stars emitting most strongly in red, while the hottest stars will burn blue.

The red giant phase comes to an end once the core temperature reaches high enough that helium begins fusing into carbon and oxygen, resulting the in star shrinking in size (though not as small as it was on the main sequence) and becoming a yellow giant.

Not Everyone Gets to be a Giant

Not all stars will become red giants however. Only stars will with masses between about half and six times the mass of our Sun will eventually evolve into red giants.

Smaller stars do not have a radiative zone, but instead transfer energy from the core to the surface by a lone convection zone. The result is that helium produced by the fusion process in the core is redistributed through out the star, meaning that there is no possibility for this to be used for a further fusion process.

Additionally, because of their smaller size, the core temperature will be rise high enough to ignite helium fusion, and it is unclear that the evolutionary path for these stars is once they have exhausted the hydrogen fuel.

Usually, we ascertain the fate of stars by studying them at different evolutionary states and mapping out the probable life cycles. This was initially done by using Hertzsprung-Russell diagrams. Then, of course, these life cycles are compared to theoretical models of the physical interactions and mechanisms of the star to explain the evolution.

However, the smaller a star is the longer that it lives on the main sequence. And stars smaller than about a third of our Sun's mass would have lifetimes greater than the current age of the Universe.

Therefore it is not possible for us to observe what results once such a star has left the main sequence.

Planetary Nebulae

Low and medium mass stars, like our Sun, which follow the path described above will eventually evolve into a planetary nebula.

During the conversion of the core from helium to carbon and oxygen the star is highly volatile. even small changes in core temperature will have a dramatic effect on the rate of nuclear fusion.

Should the core temperature get too high, either by random dynamics in the core, or because of the amount of helium that has been fused, the runaway fusion rate that results will once again push the outer envelope of the star out into the interstellar medium creating another red giant; this one of even greater size than before.

However, because of the ever increasing core temperature at this juncture, and the fact that the star has become so large that the other layers have been gravitationally unbounded, the star begins to fade into the interstellar medium, creating a planetary nebula.

Eventually the outer envelope of the star will lose its energy and begin to fade, leaving behind only the core of carbon and oxygen. Fusion has ceased and the object, known as a white dwarf - still smoldering from the previous ordeal - will cool over time.

Eventually, the glow from the white dwarf will also fade, and there will only be a cool, dim ball of carbon and oxygen left behind.

Red SupergiantsA red supergiant is the bigger version of a red giant - so far no surprise. But with these stars with more than 8 to 10 solar masses (the exact value is still uncertain) the production of energy doesn't stop at helium or carbon.A red supergiant is made of several layers. The outer hull of red glowing hydrogen and helium is inactive. Below this is a layer in which hydrogen is fusioned to helium. In the next layer helium is fusioned to carbon. So it goes on until in the core iron is made. The supergiant shines extremely bright, but only for a short time (still several hundred thousand to million years). In the end the phase in which the star fusions sulfur and silicon to iron only lasts a few days to weeks.From iron no more energy can be made. The core cools down and implodes. The following supernova (of type II) disrupts the star and leaves a tiny neutron star or a black hole behind.

Red supergiants are frequently very unstable, pulsate and often have a strong stellar wind which blows away their hull.

Blue supergiant star

Blue supergiants are supergiant stars (class I) of spectral type O.

They are extremely hot and bright, with surface temperatures of between 20,000 - 50,000 degrees Celsius.

The best known example is Rigel, the brightest star in the constellation of Orion.

It has a mass of around 20 times that of the Sun and gives out more light than 60,000 suns added together.

Despite their rarity and their short lives blue supergiant stars are heavily represented among the stars visible to the naked eye; their inherent brightness trumps their scarcity.

Blue supergiants represent a slower burning phase in the death of a massive star.

Due to core nuclear reactions being slightly slower, the star contracts and since very similar energy is coming from a much smaller area (photosphere) then the star's surface becomes much hotter.

Blue Giant Star

Computer illustration of the star Rigel.

Stars come in many shapes and sizes and they come in many colors. Some of the hottest stars in the Universe are blue giant stars. You see, the color of a star is defined by its temperature; the coolest stars are red, while the hottest ones appear blue. And the temperature of a star

comes from its mass. The more massive a star, the hotter it’s going to be. Stars don’t get more more massive or hot than blue giant stars.

Blue giants blaze with a surface temperature of 20,000 Kelvin or more, and are extremely luminous. Just for comparison, a star like our Sun only has a surface temperature of about 6,000 Kelvin. A blue giant star can put out 10,000 times as much energy as the Sun. Astronomers categorize blue giants as type O or B stars, belonging to the luminosity class III. The can reach an absolute magnitude of -5 or -6.

The true monsters of the Universe are blue supergiant stars, like Rigel. These can be a blue star with surface temperatures of 20,000 – 50,000 Kelvin and can be 25 times larger than the Sun. Because they’re so large, and burn so hot, they use up their fuel very quickly. A middle-sized star like our Sun might last for 12 billion years,

while a blue supergiant will detonate with a few hundred million years. The smaller stars will leave neutron stars or black holes behind, while the largest will just vaporize themselves completely.

Type I supernovaeThe Vela Supernova Remnant - expanding clouds of gas from a supernova which occurred 10,000 years ago.(Image Credit: David Malin, Royal Observatory Edinburgh/Anglo-Australian Observatory.)

Type I supernovae are even brighter objects than those of type II. Although the explosion mechanism is somewhat similar the cause is rather different.

The origin of a Type I supernova is an old, evolved binary system in which at least one component is a white dwarf star. White dwarf stars are very small

compact stars which have collapsed to a size about one tenth that of the Sun. They represent the final evolutionary stage of all low-mass stars. The electrons in a white dwarf are subject to quantum mechanical constraints (the matter is called degenerate) and this state can only be maintained for star masses less than about 1.4 times that of the Sun.

The pair of stars loses angular momentum until they are so close together that the matter in the companion star is transferred into a thick disc around the white dwarf and is gradually accreted by the white dwarf. The mass transferred from the giant star increases the mass of the white dwarf to a value significantly higher than the critical value whereupon the whole star collapses and the nuclear burning of the carbon and oxygen to nickel yields sufficient energy to blow the star to bits. The subsequent energy released is, as in the Type II case from the radioactive decay of the nickel through cobalt to iron.

Type II supernovaeThe structure of all stars is determined by the battle between gravity and radiation pressure arising from internal energy generation. In the early stages of a star's evolution the energy generation in its centre comes from the conversion of hydrogen into helium. For stars with masses of about 10 times that of the Sun this continues for about ten million years.

After this time all the hydrogen in the centre of such a star is exhausted and hydrogen `burning' can only continue in a shell around the helium core. The core contracts under gravity until its temperature is high enough for helium 'burning', into carbon and oxygen, to occur. The helium 'burning' phase also lasts about a million years but eventually the helium at the star's centre is exhausted and it continues, like the hydrogen 'burning', in a shell. The core again contracts until it is hot enough for the conversion of carbon into neon, sodium and magnesium. This lasts for about 10 thousand years.

This pattern of core exhaustion, contraction and shell 'burning' is repeated as neon is converted into oxygen and magnesium (lasting about 12 years), oxygen goes to silicon and sulphur (about 4 years) and finally silicon goes to iron, taking about a week.

No further energy can be obtained by fusion once the core has reached iron and so there is now no radiation pressure to balance the force of gravity. The crunch comes when the mass of iron reaches 1.4 solar masses. Gravitational compression heats the core to a point where it endothermically decays into neutrons. The core collapses from half the Earth's diameter to about 100 kilometres in a few tenths of a second and in about one 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 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 explosion.

Black Holes

Don't let the name fool you: a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area - think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City. The result is a gravitational field so strong that nothing, not even light, can escape. In recent years, NASA instruments have painted a new picture of these strange objects that are, to many, the most fascinating objects in space.

Intense X-ray flares thought to be caused by a black hole devouring a star. (Video) Read the full article

Although the term was not coined until 1967 by Princeton physicist John Wheeler, the idea of an object in space so massive and dense that light could not escape it has been around for centuries. Most famously, black holes were

predicted by Einstein's theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core's mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole.

Scientists can't directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation. We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby. If a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion. A similar process can occur if a normal star passes close to a black hole. In this case, the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them - emitting powerful gamma ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others.

One Star's End is a Black Hole's Beginning

Most black holes form from the remnants of a large star that dies in a supernova explosion. (Smaller stars become dense neutron stars, which are not massive enough to trap light.) If the total mass of the star is large enough (about three times the mass of the Sun), it can be proven theoretically that no force can keep the star from collapsing under the influence of gravity. However, as the star collapses, a strange thing occurs. As the surface of the star nears an imaginary surface called the "event horizon," time on the star slows relative to the time kept by observers far away. When the surface reaches the event horizon, time stands still, and the star can collapse no more - it is a frozen collapsing object.

Even bigger black holes can result from stellar collisions. Soon after its launch in December 2004, NASA's Swift telescope observed the powerful, fleeting flashes of light known as gamma ray bursts. Chandra and NASA's Hubble Space Telescope later collected data from the event's "afterglow," and together the observations led astronomers to conclude that the powerful explosions can result when a black hole and a neutron star collide, producing another black hole.

Superbubble/Supershell

The superbubble Henize 70, also known as N70 or DEM301, in the Large Magellanic Cloud [1]

A superbubble is a cavity hundreds of light years across, filled with 106 K gas blown into the interstellar medium by multiple supernovae and stellar winds. The solar system lies near the center of an old superbubble, known as the Local Bubble, whose boundaries can be traced by a sudden rise in dust extinction of stars at distances greater than a few hundred light years.

FormationThe most massive stars, with masses ranging from eight to roughly one hundred solar masses and spectral types of O and early B are usually found in groups called OB associations. Massive O stars have strong stellar winds, and all of these stars explode as supernovae at the ends of their lives.

The strongest stellar winds release kinetic energy of 1051 ergs (1044 J) over the lifetime of a star, which is equivalent to a supernova explosion. These winds can form stellar wind bubbles dozens of light years across.[2] Inside OB associations, the stars are close enough that their wind bubbles merge, forming a giant bubble called a superbubble. When stars die, supernova explosions, similarly, drive blast waves that can reach even larger sizes, with expansion velocities up to several hundred km s−1. Stars in OB associations are not gravitationally bound, but they drift apart at small speeds (of around 20 km s−1), and they exhaust their fuel rapidly (after a few millions of years). As a result, most of their supernova explosions occur within the cavity formed by the stellar wind bubbles.[3][4] These explosions never form a visible supernova remnant, but instead expend their energy in the hot interior as sound waves. Both stellar winds and stellar explosions thus power the expansion of the superbubble in the interstellar medium.

The interstellar gas swept up by superbubbles generally cools, forming a dense shell around the cavity. These shells were first observed in line emission at twenty-one centimeters from hydrogen,[5] leading to the formulation of the theory of superbubble formation. They are also observed in X-ray emission from their hot interiors, in optical line emission from their ionized shells, and in infrared continuum emission from dust swept up in their shells. X-ray and visible emission are typically observed from younger superbubbles, while older, larger objects seen in twenty-one centimeters may even result from multiple superbubbles combining, and so are sometimes distinguished by calling them supershells.

Large enough superbubbles can blow through the entire galactic disk, releasing their energy into the surrounding galactic halo or even into the intergalactic medium.[6][7]