Bronze-level articleStar

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Many stars are hotter than the sun.  Those stars look blue in the sky. A blue appearance can also be due to a "blueshift", or the light waves being pushed together as the star moves closer towards us. Other stars are cooler than the sun, and look red in the sky. Reddish appearance can also be due to the "redshift" of stars that are receding from us at a rapid rate. This is called the Doppler Effect.  
 
Many stars are hotter than the sun.  Those stars look blue in the sky. A blue appearance can also be due to a "blueshift", or the light waves being pushed together as the star moves closer towards us. Other stars are cooler than the sun, and look red in the sky. Reddish appearance can also be due to the "redshift" of stars that are receding from us at a rapid rate. This is called the Doppler Effect.  
  
There are no green stars. This is due to the fact that, as an object gets hotter, its radiation ranges from red, to orange, to yellow, to blue, to white.  Stars work the same way. Similarly, there are no purple, brown, black, or pink stars.  No striped, polka-dotted, or chequered ones either.<ref>[http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/ BadAstronomy]</ref>
+
There are no green stars. This is due to the fact that, as an object gets hotter, its radiation ranges from red, to orange, to yellow, to white, to blue-white.  Stars work the same way. Similarly, there are no purple, brown, black, or pink stars.  No striped, polka-dotted, or chequered ones either.<ref>[http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/ BadAstronomy]</ref>
  
 
==Death ==
 
==Death ==

Revision as of 01:01, 6 July 2011

A star is a localised aggegation of matter which, by gravitational compression, is heated to a temperature at which fusion takes place. They are big and really quite hot.

Stars are suns and the sun is a star. Stars only look small and faint because they are incredibly far away. Unless there is an undiscovered dead star closer the nearest star to the sun is Proxima Centauri which is just over 4 light years away. That means Proxima Centauri is so far away that it takes light over 4 years to reach the Solar System from there.[1]

Contents

Color

Many stars are hotter than the sun. Those stars look blue in the sky. A blue appearance can also be due to a "blueshift", or the light waves being pushed together as the star moves closer towards us. Other stars are cooler than the sun, and look red in the sky. Reddish appearance can also be due to the "redshift" of stars that are receding from us at a rapid rate. This is called the Doppler Effect.

There are no green stars. This is due to the fact that, as an object gets hotter, its radiation ranges from red, to orange, to yellow, to white, to blue-white. Stars work the same way. Similarly, there are no purple, brown, black, or pink stars. No striped, polka-dotted, or chequered ones either.[2]

Death

Stars are incredibly hot and shine because nuclear reactions in their cores keep heating them. Stars eventually "die" by either exploding as a supernova or merely cooling to a point where they cannot support reactions. The ultimate fate of stars can be black holes, neutron stars, white (or black) dwarf stars.

Black hole

Main article: Black hole

A black hole is an astronomical body so dense that the escape velocity[3] is greater than the speed of light.

Existence and formation

There are almost certainly black holes of many (thousands of?) solar masses at the centre of our and most other galaxies[4][5] . These may have been created by the compression caused by the enormous central mass of stars mutually attracting each other until the gravity exceeded the radiation pressure. These supermassive black holes are so gravitationally powerful that their effect on the real motion of stars in close orbit around them can be (easily - with the right equipment) detected.

They have been conjectured to form through the collapse of large stellar objects when the gravity of the object becomes sufficiently greater than the radiation pressure [6] or by the huge compressive forces inside (exploding) supernovas.

It has been hypothesised that black holes might have been created at or about the time of the big bang. These black holes could be tiny in comparison with stellar black holes and would probably have decayed to ordinary matter by now[7].

Neutron star

A speculative diagram of a neutron star. The "atmosphere" of the star is roughly one meter thick, below which one encounters a solid "crust". This crust is extremely hard and very smooth because of the extreme gravitational field. Neutron star core material could be a superfluid mixture of neutrons with a few protons and electrons, or it could incorporate high-energy particles like pions and kaons in addition to neutrons, or it could be composed of strange matter incorporating quarks heavier than up and down quarks, or it could be quark matter not bound into hadrons.[8]

A neutron star is a compact object that is created in the core of massive star during supernova explosion.[9] As their name suggests, neutron stars are composed almost entirely of neutrons. Though they are dead stars, they are still very hot. They are extraordinarily smaller than the original star from which they originated with a radius of about 12 km. In contrast, the Sun's radius is about 60,000 times that. They typically have a mass between 1.35 and about 2.1 solar masses.[10]

How the hell?

After the core of an aging massive star ceases to generate energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole, releasing gravitational potential energy that heats and expels the star's outer layers.

Neutron stars can form during two types of supernova: core-collapse supernova and stripped core-collapse supernovae supernova. In each, when the progenitor star is below 20 solar masses (depending on the strength of the explosion and the amount of material that falls back), the degenerate remnant of a core collapse may be a neutron star. Above 20 solar masses, the remnant collapses to form a black hole.

If the collapse produces a mass above 2 to 3 solar masses (the Tolman-Oppenheimer-Volkoff limit), a quark star might be created. If the compact star that results is less than 1.44 solar masses (the Chandrasekhar limit), it will form a white dwarf. If the solar mass of the compact star is 1.35 to 2.1 solar masses, it will form a neutron star.[11] White dwarfs can also become neutron stars if they obtain enough mass to push over the Chandrasekhar limit.[12]

In a typical core-collapse supernova, the newly formed neutron core has an initial temperature of about 100 billion kelvin (100 GK); 6000 times the temperature of our Sun's core. Much of this thermal energy must be shed for a stable neutron star to form (otherwise the neutrons would boil away), and this is accomplished by a further release of neutrinos.

Characteristics

As the core of a massive star is compressed during a supernova, and collapses into a neutron star, it retains most of its angular momentum. Due to its drastic reduction in size, its moment of inertia is sharply reduced. Because of this, when formed they have very high rotation speed (neutron stars are known to have rotation periods between about 1.4 ms to 30 seconds) which gradually slows down due their rotating magnetic fields radiating energy. The small radius and high mass mean that neutron stars possess a very high surface gravity, more than 1011 times of that of Earth's. One measure of such immense gravity is the fact that neutron stars have an escape velocity of about 33% of the speed of light (note: if the mass and therefore e.v. increases through acquisition of matter the star might become a black hole). Matter falling onto the surface of a neutron star would be accelerated to tremendous speed by the star's gravity. The force of impact would likely destroy the object's component atoms, rendering all its matter identical, in most respects, to the rest of the star. Due to the high gravitational field on the surface of neutron stars, radiation emitted by the star is bent. This results in parts of the typically invisible rear surface becoming visible.

A neutron star is so damn dense that just one teaspoon of its material would have a mass over 5×1012 kg. The force of gravity is so strong that if an object were to fall from just one meter high it would hit the surface of the neutron star at around 4.3 million miles per hour. The temperature inside a newborn neutron star is from around 1011 to 1012Kelvin. However, the huge number of neutrinos it emits carries away so much energy that the temperature falls within a few years to around 1 million Kelvin. Most of the light generated by a neutron star is in x-rays, even at 1 million Kelvin. Some science textbooks will illustrate neutron stars as black or dark, and white dwarfs as white, in order to differentiate the two in the illustration. This can be misleading however, because both white dwarfs and neutron stars are visible. In visible light, neutron stars probably radiate approximately the same energy in all parts of visible spectrum, and therefore appear white.[13]

Pulsars

Some neutron stars are known to emit radio waves that pulse on and off. These neutron stars are called pulsars. The "off" and "on" emission that is characteristic of pulsars is due to the star's rotation. The radio waves only escape from the North and South magnetic poles of the neutron star. If the spin axis is tilted with respect to the magnetic poles, the escaping radio waves sweep around like the light beam from a lighthouse. On Earth, radio astronomers pick up the radio waves only when the beam sweeps across the range of the Earth. The first pulsar was detected in 1967 and for very briefly the regular signal was thought to be evidence of extraterrestrial life.

White dwarf

A white dwarf is a small star composed mostly of electron-degenerate matter. Because a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth, it is very dense. White dwarfs are faint in comparison to other stars because the matter within them no longer undergoes fusion reactions; their faint luminosity comes from the emission of stored heat. Over time, they become dimmer and give off less energy, turning into a theoretical "black dwarf". Because no white dwarfs are older than the universe itself, even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins, and no black dwarfs are thought to exist yet. Interestingly, our own Sun will more than likely become a white dwarf.

Formation

White dwarfs come about in numerous ways, and nearly every star meets this fate with the exception of only the largest.[14]

First, if the mass of a main-sequence star with very low mass is lower than approximately half a solar mass, it will never become hot enough to fuse helium at its core. Ever. It is thought that, over a lifespan exceeding the age of the universe, such a star will eventually burn up all of its hydrogen and end its life as a helium white dwarf composed chiefly of helium-4 nuclei.

Second, if the mass of a main-sequence star with low to medium mass is between approximately 0.5 and 8 solar masses, its core will become sufficiently hot to fuse helium into carbon and oxygen via the triple-alpha process, but it will never become sufficiently hot to fuse carbon into neon. Poor star. Near the end of the period in which it undergoes fusion reactions, such a star will have a carbon-oxygen core which does not undergo fusion reactions, surrounded by an inner helium-burning shell and an outer hydrogen-burning shell. It will then expel most of its outer material, creating a planetary nebula, until only the core of carbon and oxygen is left. This process is responsible for the carbon-oxygen white dwarfs which form the vast majority of observed white dwarfs.

Third and lastly, larger stars may create white dwarfs as well, though this is not thought to be very common. Some white dwarfs may be the product of a massive star's core-collapse supernova. If the compact star that results is less than 1.44 solar masses (the Chandrasekhar limit), it can be a white dwarf. In another model, a large star, of perhaps 8 to 10 solar masses, although sufficiently massive to fuse carbon to neon and magnesium, may be insufficiently massive to fuse neon. Such a star may leave a remnant white dwarf composed chiefly of oxygen, neon, and magnesium, provided that its core does not collapse into a neutron star or black hole, and provided that fusion does not proceed so violently as to blow apart the star in a supernova.

White dwarfs can collapse into neutron stars. White dwarfs in binary systems can accrete material from a companion star, increasing both their mass and their density. As their mass approaches the Chandrasekhar limit, this could theoretically lead to either the explosive ignition of fusion in the white dwarf or its collapse into a neutron star. It is also theoretically possible for a white dwarf to supernova. In one model, a carbon-oxygen white dwarf accretes material from a companion star, increasing its mass and compressing its core. It is believed that compressional heating of the core leads to ignition of carbon fusion as the mass approaches the Chandrasekhar limit which you ought to be familiar with by now. Because the white dwarf is supported against gravity by quantum degeneracy pressure instead of by thermal pressure, adding heat to the star's interior increases its temperature but not its pressure, so the white dwarf does not expand and cool as a result. Rather, the increased temperature continuously accelerates the rate of the fusion reaction. The thermonuclear flame consumes much of the white dwarf in a few seconds, causing a supernova explosion that obliterates the star. In another possible model, two carbon-oxygen white dwarfs in a binary system merge, creating a body with mass greater than the Chandrasekhar limit in which carbon fusion is then ignited.

Weird stars

A neat little animation, complete with indication of brightness change.
A Hubble image of the Sirius binary system. Sirius B is that tiny point of light to the bottom left. It is a white dwarf.

Stars aren't exactly what you could call homogenous. There are many, many different kinds of stars that stretch and bend at the imagination (and sometimes, the laws of physics).

Binary stars

Binary stars are stars that have been gravitationally bound together, both orbiting a central point. If a binary star is detected visually, it is called a visual binary. These are observed as double stars in telescopes. Binary stars can also be detected by looking at redshift and blueshift. This is caused by the star first moving towards us, then away from us, as it orbits around the center of mass. Each star's spectral lines will move up and down in wavelength, due to the Doppler Effect. A third kind of binary star is the eclipse binary- these star systems can be detected by the change in brightness. Even stranger are astrometric binaries, which are merely stars that appear to orbit around empty space, or maybe the flying spaghetti monster. The exact point can be expressed as m1 r1 = m2 r2 r1 + r2 = R where r is the total seperation between the two objects. The usual mathematics for computing the mass of a companion star can be used for these invisible companions, even though we can't see them.

Yum. . . star

Binary stars can be cannibalistic. Sometimes, a star will pull material from its companion star into it. For example, a white dwarf could have stellar material pulled from it towards a neutron star neighbor, in which case an accretion disk would form. For cannibalism, one star has to be significantly larger than the other, and be very close to each other. [15][16][17][18][19]

Brown dwarf

See the main article on this topic: Brown dwarf

A brown dwarf is a sub-stellar object. Typically its mass would be far above the range of what might be considered a gas giant planet. However, their mass is also insufficient for nuclear fusion to begin. The most common lay term would be a "failed star". According to models of the interior of brown dwarfs, typical conditions in the core for density, temperature and pressure are expected to be the following:

\rho_c \sim 10 - 10^3\,\mathrm{\tfrac{g}{cm^3}}
T_c \lesssim 3 \times 10^6\,\mathrm{K}
P_c \sim 10^5\,\mathrm{Mbar}

Brown dwarfs can either be confused with high-mass planets or low-mass stars. There are several ways to distinguish between the three.

Distinguishing brown dwarfs from other stuff

Brown dwarfs can be easily confused with other stellar objects. Fortunately, simple tests exist to determine whether it really is a brown dwarf.

  1. Lithium is generally present in brown dwarfs, but not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium when Lithium-7 and a proton collide, producing two Helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium spectral line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. This test can be flawed. Lithium can be seen in very young stars, as they have not had a chance to burn it off. Our sun also contains lithium in its outer atmosphere, as it is not hot enough for the process to take place. In addition, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than 65 M_J can burn off their lithium by the time they are half a billion years old.
  2. Unlike stars, older brown dwarfs are sometimes cool enough that over very long periods of time their atmospheres can gather observable quantities of methane (an example of a brown dwarf confirmed in this way is Gliese 229B).

Main sequence stars cool, but eventually reach a minimum luminosity which they can sustain through steady fusion. This varies

  1. from star to star, but is generally at least 0.01% the luminosity of our Sun. Brown dwarfs cool and darken steadily over their lifetimes: sufficiently old brown dwarfs will be too faint to be detectable.
  2. Brown dwarfs can be confused with gas giants like Jupiter, but have a much higher density.
  3. About all brown dwarfs, no matter what their mass is, are about the size of Jupiter.

[20][21]

Variable stars

Variable stars have brightnesses that change over time, or at least appear to. This can be due to either external or internal factors. Most stars, including our sun, have at least some change in luminosity, but are not classified as variable stars.[22] The sun has a sunspot cycle of 11 years which is associated with luminosity. When sunspots are at a minimum, the sun is cooler than normal. There are other longer cycles which are thought to be responsible for the ice ages.

Proxima Centauri

Proxima Centauri

Proxima Centauri is a red dwarf star and is the sun's closest neighbor. It is such a dim and weak star that even though it is relatively nearby, it cannot be seen with the naked eye. Proxima Centauri has a low mass for a star, so nuclear reactions happen more slowly than in more massive, brighter stars, and it uses up its nuclear fuel more slowly. Proxima Centauri should still be shining when the sun is dead. If it has a life sustaining planet, it will probably be like Aurelia [23] and could have a very long future. Any life near Proxima Centauri will have to evolve means of surviving unpredictable stellar flares[24] when the luminosity and radiation of the star increases suddenly.

Red dwarf stars are common in the galaxy. Because they are faint, astronomers can only detect them within about 60 light years of the Solar System. Proxima Centauri is a very ordinary faint red dwarf star. It is special only because it's so near to us, so astronomers can study it easily. Proxima Centauri may help us understand scientifically whether other red dwarf stars could be habitable and whether extraterrestrial life is likely to exist on planets orbiting them.

Popular culture

In pop culture, "star" can mean "celebrity," which means "someone who appears regularly on Fox News."


Religion

In the Christian sacred scriptures Satan cast down a third of the stars of heaven with his tail as he fell to Earth (Revelation 12:4). This would have resulted in a mass-extinction event at least comparable to the small asteroid that struck the Yucatan peninsula 65 million years ago and wiped out the dinosaurs. In many religions, when the end of the world arrives (presumably in 2012), the stars will fall from heaven.

Throughout the Bible, the authors are apparently ignorant of the relationship between the sun and the moon, and the fact that the sun is a star. The Bible incorrectly refers to the sky as a dome over the earth, with the stars, the sun, and the moon being fixed to the dome. We now know that the stars are an astronomical number of light years away, with the light of many of them being the only thing left for us to witness as they have since exploded, shooting gamma ray bursts that incinerate other unfortunate bodies around them. But these passages are only metaphorical of course.

Islam also gets astronomy quite wrong. Muhammad describes the sky as being made of seven layers, and he arbitrarily placed the stars in the lowest layer, lower than the moon. This is, of course, incorrect, if taken literally. He also assumed that the stars are just lamps to adorn the lower heaven that are also used as missiles to shoot the intruding evil spirits (jinns). This is quite incorrect, as stars follow motions that are predictable and consistent.

Bullshit

Zeta Reticuli, where UFOs apparently come from.

Stellar Cartography

An historic map of the heavens, circa 1937.
Man has been mapping the sky since he adapted the ability to make representational images. Star cartography, celestial cartography, or uranography is the fringe of astronomy and branch of cartography concerned with mapping stars, galaxies, and other celestial bodies. Measuring the position and light of charted objects requires a variety of instruments and techniques that have developed from angle measurements with quadrants and the unaided eye, through sextants combined with lenses for light magnification, up to current methods which include computer automated space telescopes. Uranographers have historically produced planetary position tables, star tables and star maps for use by both amateur and professional astronomers. More recently computerized star maps have been compiled, and automated positioning of telescopes is accomplished using databases of stars and other astronomical objects.

See also

External Links


Footnotes, citations, random comments, and references

  1. The user Proxima Centauri, on the other hand, is much closer.
  2. BadAstronomy
  3. Escape velocity = the speed (strictly of course velocity) which anything needs to have in order to escape the gravitational effect of another object.
  4. Galactic black holes a likely source of cosmic rays
  5. Black hole 18 billion solar masses.
  6. Radiation Pressure = general photon and particle pressure caused by nuclear and subnuclear reactions (heat) it fluctuates over the lifetime of a star.
  7. Black holes were thought to be absolutely inviolable, but Stephen Hawking has shown that they are hairy and will, as long as there is space time, slowly decay through a phenomenon known as Hawking Radiation
  8. Neutron star from The Internet Encyclopedia of Science by David Darling.
  9. Neutron Stars, Imagine the Universe from the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center; April 5th, 2009.
  10. A solar mass is defined as the mass of the Sun, about two nonillion kilograms or about 332,950 times the mass of the Earth, or 1,048 times the mass of Jupiter.
  11. How they form, Eclipse.net by Chris Miller
  12. Introduction to neutron stars, M. Coleman Miller, Associate Professor of Astronomy, University of Maryland.
  13. Neutron star
  14. White dwarfs from The Internet Encyclopedia of Science, by David Darling.
  15. Accreting binaries
  16. Kopal, Z. (1989). The Roche Problem. Kluwer Academic.
  17. Wikipedia
  18. Binary Star at Wikipedia
  19. Visual binaries
  20. Theory of brown dwarfs at Wikipedia
  21. Brown dwarf
  22. Variable Star at Wikipedia
  23. Wikipedia on Aurelia
  24. See Wikipedia, Proxima Centauri is a flare star.
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