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Stars in the universe: Classification, evolution, quasars and pulsars and black holes


The stars

Although most of the space that we can observe is empty, it is inevitable that we look at those dots that glow. It is not that the empty space lacks interest. Simply, the stars attract attention.
Because of the gravitational pull, the star commodity tends to concentrate in its Center. But that increases its temperature and pressure. From certain limits, this increase causes reactions nuclear that they release energy and balance the force of gravity, so the size of the star remains more or less stable for a time, emitting large amounts of radiation, including, of course, the luminous space.
However, depending on the amount of matter in a star and the moment of the cycle which is, very different behaviors and phenomena can occur. Dwarfs, Giants, doubles, variables, quasars, pulsars, black holes,... In this chapter we will give an overview about the stars, their types, their behaviors and its evolution.

Stars in the universe

The stars are masses of gases, mainly hydrogen and helium, which emit light. They are at very high temperatures. In its interior there are nuclear reactions.
The Sun is a star that we have very, very close. We see the other stars as a very small bright spots, and only at night, because they are at enormous distances from us. They seem to be fixed, keeping the same relative position in the heavens every year. In fact, all these stars are fast moving, but at such distances that their changes in position are only perceived through the centuries.
The number of observable stars with the naked eye from Earth has been estimated at about 8,000, half in each hemisphere. During the night can not see more than 2,000 at the same time, the rest are hidden by atmospheric fog, especially near the horizon, and the pale light of the sky.
Astronomers have calculated that the number of stars in the milky way, the Galaxy that belongs to the Sun, amounts to hundreds of billions.
As our Sun a typical star has a visible surface called the photosphere, an atmosphere full of hot gases and, above them, a more diffuse Crown and a stream of particles called stellar wind. The colder areas of the photosphere, which in the Sun are called sunspots, probably are in other common star. This has been proven in some great upcoming stars using interferometry.
The internal structure of the stars cannot be observed directly, but there are studies that indicate currents of convection and density and temperature increase up to the nucleus, where thermonuclear reactions take place.
The stars are composed mainly of hydrogen and helium, with variable amount of heavier elements.

The star nearest to the Solar system is Alfa Centauro

The individual stars visible in the sky are closer to the Solar system in the milky way, our galaxy. The nearest is Proxima Centauri, one of the components of the triple star Alpha Centauri, which is about 40 billion kilometers from Earth.
It is a three-star system located 4.3 light years from the Earth, which is only visible from the southern hemisphere. The brighter, known as "Alpha Centaur A" has a real brightness of our Sun.
Alpha Centauri, also known as Rigil Kentaurus, is in the constellation of the Centaur. At first glance, Alpha Centauri appears as a single star with an apparent magnitude of - 0.3, making it the third brightest star in the southern sky.
When observed through a telescope are cautioned that the two brightest stars, Alpha Centauri A and B have apparent magnitudes of - 0.01 and 1.33 and revolve one around the other over a period of 80 years.
The weaker star, Alpha Centauri C, has an apparent magnitude 11.05 and tour around their peers during a period of approximately one million years. Alpha Centauri C also receives the name of Proxima Centauri, which is the nearest to the Solar System star.

The star rating

Started the photographic study of stellar spectra in 1885 the astronomer Edward Pickering at Harvard College Observatory and his colleague Annie J. Cannon concluded it. This research led to the discovery that the spectra of the star are arranged in a continuous sequence, depending on the intensity of certain absorption lines. The observations provide data of the ages of the different stars, as well as their degree of development.
The various stages in the sequence of the spectra, designated by the letters O, B, A, F, G, K and M, allow a full of all types of star rating. Subscripts 0 through 9 are used to indicate the inheritance model within each class.
Class O: Lines of helium, oxygen and nitrogen, in addition to the hydrogen. It very hot stars, and includes both showing bright line of hydrogen and helium Spectra showing dark lines of the same elements.
Class B: Helium lines reach maximum intensity at B2 branch and gradually pale in higher subdivisions. The intensity of the lines of hydrogen increases steadily in all subdivisions. This group is represented by the star Epsilon Orionis.
Class A: This includes the so-called stars of hydrogen Spectra dominated by hydrogen absorption lines. A typical star of this group is Syrian.
Class F: In this group include the so-called lines H and K of the calcium and characteristics of hydrogen lines. A notable star in this category is Delta Aquilae.
Class G: It includes star with strong lines H and K of the calcium and less strong hydrogen lines. The spectra of many metals, especially of iron are also present. The Sun belongs to this group and that is why the stars G are called "solar-type stars".
Class K: Stars that have strong lines of calcium and others that indicate the presence of other metals. This group is typified by Arthur.
Class M; Spectra dominated by bands that indicate the presence of metallic oxides, especially those of titanium oxide. The violet end of the spectrum is less intense than the K stars. The star Betelgeuse is typical of this group.

Size and brightness of the stars

The biggest stars that are known are the canterell, with diameters about 400 times larger than the Sun, while the stars known as "white dwarfs" can have diameters of only one-hundredth of the Sun. However, giant stars tend to be diffuse and may have a mass just about 40 times larger than the Sun, while the white dwarfs are very dense despite its small size.
There may be star with one mass of 1,000 times larger than the Sun, and smaller-scale, balls of hot gas too small to trigger nuclear reactions. An object that could be such (a brown dwarf) was observed for the first time in 1987, and since then others have been detected.
The brightness of the stars is described in terms of magnitude. The brightest stars may be up to 1,000,000 times brighter than the Sun; White dwarfs are about 1,000 times less bright.
The classes established by Annie Jump Cannon are identified by colors:
-Blue, as I Cephei star
-White-blue, like the star Spica
-White color, as the star Vega
-White-yellow color, as the Procion star
-Yellow color, as the Sun
-Orange, as Arcturus
-Red color, as the star Betelgeuse.
Often the stars are named using the reference to their size and their color: red giants, white dwarfs,...

Visible stars a-l

Alcor: little bright star belonging to the Big Dipper, which forms, together with Mizar, a dual system visible to the naked eye.
Aldebaran: Star of the constellation of Taurus which, with an apparent magnitude of 1.1, is one of the brightest in the sky. Also known as eye or heart of the bull, is a 53 year old light-years from Earth and has a 90 times the Sun's luminosity.
ALGOL: Star b of the constellation of Perseus. With a period of rotation of 69 hours, it is a double system that provides aspect of variable, but it is actually a binary eclipsante, i.e., their periodic variations in brightness are due to the mutual filing of its components.
Arthur: Star of Bootes, situated in the extension of the tail of Ursa Major. Spectral type K0 and visual magnitude 0.2, it has a diameter of 22 times that of the Sun.
Betelgeuse: Star of the constellation of Orion, the brighter and red, whose magnitude oscillates between 0.2 and 0.9. It is a semiregular variable, with 2.07 days.
Goat: brightest star in the constellation of the Coachman, of spectral type G, and the fourth in the sky by its apparent luminosity of 0.2.
Wavelets: visible group of Pleiades Stars.
Canicula.: brightest star in the greater Can, call SIRIUS today.
Capella or Capela: main star in the constellation of the Coachman, of magnitude 1.
Beaver: Star of the constellation of Gemini. It is a double, with a period of 350 years star, and its components have magnitudes of 2 and 2.9, respectively.
Deneb: Star of the constellation of the Swan. It is a supergiant, magnitude 1.3, located 1,000 a.l. of the Earth.
Denebola: second most important (b) star in the constellation of Leo, of magnitude 2.
Spike: main star in the constellation of Virgo. It's a dual system with a period of 4 days. Located about 160 a.l. of the Earth, it has a magnitude of 1.21 and belongs to the spectral type B2.
Polaris: Star located within 1 ° of the Northern celestial pole and is a useful reference to locate the direction of North. It is currently a star of magnitude 2 located in the constellation of Ursa minor. However, because of the precession, towards the 13,000 year this position will be occupied by the star Vega.
Formalhaut: main star in the constellation of the southern fish. Located 23 a.l., it has a magnitude of 1.3 and belongs to the spectral class A3. It is visible from the northern hemisphere in autumn.
Lynx or Lynx: (Alpha Lyncis) star of the third magnitude, the brightest in the constellation of the same name, located in the northern hemisphere, between the driver and the Big Dipper, South of the giraffe and North of Cancer

Visible star m-z

Markab: Star of the constellation of Perseus, belonging to the spectral type A and whose magnitude has a value of 2.6.
Menkar: Star of the constellation of the whale, which has a magnitude 2 and forms a triangular figure with Aldebaran, and Rigel.
Mira Ceti: Star of spectral type M, belonging to the constellation of the whale. It is the prototype of the variable stars of long period, with amplitudes and irregular periods.
Mirach or Mirak: Star of spectral type M and magnitude 2.4, belonging to the constellation of Andromeda.
Alpha Persei: Star of the constellation of Perseus. It belongs to the spectral class F and has a magnitude of 1.9.
Mizar: double star zeta Ursa Major, which along with Alcor forms a visible couple at a glance. It belongs to the spectral type A and has a magnitude of 2.4. It consists of two unequal components with a gap of 14.5 °.
Pearl: Star of the constellation of the Northern Crown, located 72 light years from Earth. It has a partner who turns to her around with a half-life of 17.4 days.
Pollux or Pollux: Star belonging to the constellation of Gemini, located at 35 light-years, with a magnitude of 1.2 and a luminosity about 34 times greater than the Sun.
Procion: Star of the constellation of the minor Can, located 11 light years from Earth and belongs to the spectral type F. With a magnitude of 0.5, it presents a notable movement (1.25 "per year) and forms a binary system with a magnitude 13.5 classmate.
Regulus: Star of the constellation of Leo, located 67 light years from Earth. It has a magnitude of 1.3 and belongs to spectral type B.
Rigel: Star b of the constellation of Orion, located 540 light years from Earth. It has a magnitude of 0.34 and belongs to spectral type B.
RR Lira: variable star, prototype of the kind of Star pulsating końca.
Rukbah: Star of magnitude 2.8 belonging to the constellation of Cassiopeia.
Scheat: Star of the constellation of Pegasus, of magnitude 2.6 b and belongs to the spectral type M.
Schedir, Shedar or Shedir.: Star of the constellation of Cassiopeia. It is a variable belonging to the spectral type K, whose magnitude oscillates between 2.1 and 2.6.
Sirio: Star of the greater Can, the brightest in the sky (magnitude 1.58). It belongs to the spectral type A and form a double set with another white dwarf star (Sirius B), 50 years.
Sirrah: Star of the constellation of Andromeda, of magnitude 2.2 and belongs to the spectral type A.
Tolimán: Star of the constellation of the Centaur. It is a dual system, in which one of the components is very similar to the Sun.
Trapeze: Star multiple q of the constellation of Orion, whose four main components have magnitudes 6, 7, 7 and 7.5, immersed in the great Orion Nebula (M 42).
Vega: Star of the constellation of the Lira, the brightest of the northern sky. Located at 26 light-years from Earth, it belongs to the spectral type A and has a magnitude of 0.14. It was Polaris 14,000 years ago and will again be within 12,000.

Evolution of the stars

Stars evolve over millions of years. They are born when accumulates a large amount of matter in a the space. The material is compressed and heats up until you start a nuclear reaction, consuming matter, converting it into energy. Small stars spend it slowly and last longer than large ones.
Theories on the evolution of the stars are based on evidence from studies of Spectra related to luminosity. The observations show that many stars are classified as either a regular sequence in which the brightest are the hottest and the smallest, the coldest.
This series of star formed a band known as the main sequence diagram known as a Hertzsprung-Russell Diagram temperatura-luminosidad. Other groups of stars that appear in the diagram include the aforementioned giant and dwarf stars.

The life of a star

The life cycle of a star begins as a great mass of gas relatively cold. The contraction of gas raises the temperature inside the star until reaching 1,000,000 ° C. At this point take place nuclear reactions, whose result is the nuclei of hydrogen atoms are combined with the deuterium to form helium nuclei. This reaction releases large amounts of energy, and stops the contraction of the star. For a while it seems it stabilizes.
But when you end the release of energy, contraction begins again and the temperature of the star returns to increase. Moment begins a reaction between hydrogen, lithium, and other light metals present in the body of the star. New energy is released and the contraction stops.
When lithium and other lightweight materials consumed, contraction resumes and the star enters the final stage of development in which the hydrogen becomes helium at very high temperatures thanks to the catalytic action of carbon and nitrogen. This thermonuclear reaction is characteristic of the main sequence of Star and continues until it consumed all the hydrogen there is.
The star becomes a red giant and reaches its largest size when all its central hydrogen has become helium. If it still shines, the core temperature must rise enough to produce the fusion of helium nuclei. During this process it is likely that the star becomes much smaller and, therefore, more dense.
When it has worn out all possible sources of nuclear energy, it contracts again and becomes a white dwarf. This final stage can be marked by explosions known as "novas". When a star is liberated from its outer exploding like nova or supernova, it returns to the interstellar medium elements heavier than hydrogen that has synthesized inside.
Future generations of stars formed from this material will begin its life with an assortment of elements heavier than previous generations. The star that shed its outer layers in a non-explosive manner become planetary nebulae, old star surrounded by spheres of gas that radiate in a multiple range of wavelengths.

Star to black hole

Stars with one mass much greater than that of the Sun have a more rapid evolution, of a few million years from his birth to a supernova explosion. The remains of the star can be a neutron star.
However, there is a limit to the size of neutron stars, more than which these bodies are forced to contract until they become a black hole, which can not escape no radiation.
Typical stars like the Sun can persist for many billions of years. The final destination of the Dwarfs from low mass is unknown, except that cease to radiate appreciably. It is likely that they become ashes or black dwarfs.

Double stars

The double (or binary) stars are very frequent. A double star is a pair of stars that are held together by the force of gravitation and rotate around their common Center.
The orbital periods ranging from minutes in the case of very close couples up to thousands of years in the case of distant pairs, depend on the separation between the stars and their respective masses.
There are also multiple-star systems in which three or four star revolve in complex paths. Lira seems a double star, but through a telescope you can see how each of the two components is a binary system.
The observation of the orbits of double stars is the only direct method that astronomers have to weigh the stars.
For close couples, its gravitational attraction can distort the shape of the stars, and is possible to flow gas from one star to another in a process called "transfer of masses".
Through the telescope is detectean many double stars that seemed simple. However, when they are very close, are detected only if we study its light through spectroscopy. Then the spectra of two-star look, and its movement can be deduced by the Doppler effect in both spectra. These couples are called binary spectroscopic.
Most of the stars that we see in the sky are double or even multiple. Occasionally, one of the stars of a dual system can hide the other when observed from the Earth, which leads to a binary eclipsante.
In the majority of cases, it is believed that the components of a double system originated at the same time, although sometimes, a star can be captured by the gravitational field of another in areas of high stellar density, such as star clusters, giving rise to the double system.

Variable stars

This concept encompasses any star whose brightness, seen from the Earth is not constant. They can be stars whose light emission actually fluctuates - intrinsexas-, or stars whose light is interrupted in their path towards the Earth, by another star or a cloud of interstellar dust, extrinsic variable calls.
Changes in light intensity in the intrinsic variables is due to pulsations in the size of the Star (pulsing variables) or to interactions between the components of a double star. Some other intrinsic variables do not fit into any of these two categories.
The only frequent type of extrinsic variable is the "binary eclipsante" call. It's a double star consisting of two upcoming stars who regularly spend one ahead of the other. ALGOL is the most famous example. The binary eclipsantes constitute nearly 20% of known variable stars.

Variable końca

The końca are oriented couples so, periodically, are eclipsed one to another. Probably the best-known examples are the variable końca, whose periodic pulse indicacan its brightness, therefore constitute an important reference for the measurement of distances in space.
Their pulsation periods vary between one day and about four months, and their brightness variations can be between 50 and 600% between the maximum and the minimum. Its name comes from its prototype or representative star, Delta Cefei.
The relationship between average brightness and the pulsation period was discovered in 1912 by Henrietta S. Leavitt, and is known as periodo-luminosidad. Leavitt found that the brightness of a cefeida increases in proportion to its pulse period.
Thus, astronomers can determine the intrinsic luminosity of a cefeida simply by measuring the pulsation period. The apparent brightness of a star in the sky depends on its distance from the Earth; Comparing this luminosity with brightness intrinsic distance which is can be determined. Thus, the końca can be used as indicators of distances both inside and outside of the milky way.
There are two types of końca. The most common are called classic końca and others, old and weak, are known as W Virginis star. The two types have different relationships periodo-luminosidad.

Novas and supernovas

Novas and supernovas are stars that explode releasing part of their material into space. For a variable time, its brightness increases dramatically. It seems that a new star is born.
A nova is a star which greatly increases its brightness suddenly and then slowly pales, but may continue to exist for some time. A supernova, but the explosion destroys or alters the star. Supernovae are much rarer than novae, observed quite frequently in the photos.
Novae and supernovae provide materials to the universe that will be used to form new stars.

Novas, new stars?

Formerly, a star that appeared to hit where there was nothing, it was called nova, or 'new Star'. But this name is not correct, since these stars existed much before they could see at a glance.
Perhaps appear 10 or 12 novae per year in the milky way, but some are too far away to see or interstellar matter obscures them.
To novae occurs them more easily in other nearby galaxies which in ours. A nova increased in several thousands of times its original brightness in a matter of days or hours. After it enters a period of transition, during which pales, and cobra shine again; starting from there pales gradually until you reach your original brightness level.
Novae are stars in a late period of evolution. They explode because their outer layers have formed an excess of helium through nuclear reactions and expands with too much speed to be contained. The star says goodbye to explosively a small fraction of its mass as a layer of gas, increases its brightness, and then normalizes.
The star that remains is a white dwarf, the smallest Member of a binary system subject to a continuous decrease of matter in favour of the larger star. This phenomenon happens with dwarf novae, which arise time and again at regular intervals.


A supernova explosion is more spectacular and destructive of a nova, and much rarer. This is very rare in our Galaxy, and despite his incredible increase in brightness, few can be seen with the naked eye.
Up to 1987 only had identified three throughout history. The best known is which emerged in 1054 and whose remains are known as the Crab Nebula.
Supernovae, as well as the novas, are seen more frequently in other galaxies. Thus, the most recent supernova, which appeared in the southern hemisphere on February 24, 1987, emerged in a satellite Galaxy, the large cloud of Magellan. This supernova, which has unusual features, is the subject of an intense astronomical study.
The very large stars explode into the latter stages of their rapid evolution, as a result of gravitational collapse. When the pressure created by the nuclear processes, already cannot withstand the weight of the outer layers and the star explodes. It is called type II supernova.
A type I supernova arises similar to a nova. He is a member of a binary system which receives the flow of fuel by capturing material, fellow.
Of a supernova explosion are few remains, except for the layer of gas that expands. A famous example is the Crab Nebula; in its center there is a pulsar, or star of neutrons that rotates at high speed.


The quasars are distant objects that emit large amounts of energy, similar to the star radiation. The quasars are hundreds of billions of times brighter than the stars. Possibly, they are black holes that emit strong radiation when they capture stars or interstellar gas.
The light that we perceive occupies a very narrow range in the electromagnetic spectrum and not all the cosmic bodies emit most of their radiation in the form of visible light. With the study of radio waves, radioastronomers began locating powerful radio sources that do not always correspond to the visible object.
The word quasar is an acronym for quasi stellar radio source (almost stellar radio sources).

Identification of quasars

They were identified in the 1950's. Later saw that they showed a shift to larger than any other known object red. The cause was the Doppler effect, which moves towards the Red spectrum when objects move away.
The first studied quasar, 3 c 273 is 1.5 billion years Earth light. Since 1980, thousands of quasars have been identified. Some move away from us at speeds of 90% of the light.
Quasars have been discovered to 12 billion years Earth light. This is roughly the age of the universe. Despite the enormous distances, the energy that comes in some cases is very large. As an example, the s50014 + 81 is about 60,000 times brighter than all the milky way.
The most spectacular of the quasars is not its remoteness, but that they may be visible. Quasar must be as bright as 1,000 galaxies together so it can appear as a faint star, if light is thousands of millions of years. But even more surprising is the fact that this huge energy comes from a region whose size does not exceed a year light (less than one thousandth of the size of a normal Galaxy). The brightness of the quasars varies with periods of a few months, therefore, its size must be less than the distance that light travels in that time.
At first, astronomers did not see any relationship between the quasars and galaxies, but the gap between these two types of cosmic objects has been filling slowly to discover galaxies whose nuclei present similarities with the quasars. Today, it is thought that the quasars are very young galaxies nuclei, and activity in the nucleus of a Galaxy decreases over time, although it does not disappear at all.


Pulsars are sources of radio waves that vibrate with regular periods. They are detected by radio telescopes.
The Pulsar Word is an acronym for "pulsating radio source", pulsating radio source. Required clocks extraordinary accuracy to detect changes of pace, and only in some cases.
Studies indicate that a pulsar is a small neutron star spinning at high speed. The best known is the Crab Nebula. Its density is so great that, in them, the subject matter of the extent of a ball pen has a mass of about 100,000 tons. They emit a large amount of energy.
Magnetic field, very intense, is concentrated in a small area. This accelerates it and makes it to emit a beam of radiation that we receive here, as radio waves through telescopes.
Pulsars were discovered in 1967 by Jocelyn Bell and Anthony Hewish in the Observatory's radio astronomy in Cambridge. Many pulsating stars are known, but only two, the press of the crab, and the press of sail, emit detectable visible pulses. It is known that these two also emit pulses of gamma rays, and one, the crab, also emits pulses of X-rays.
The regularity of pulses is phenomenal: observers can now predict the times of arrival of pulses in advance of a year, with a precision better than a millisecond.
The buttons are strongly magnetized neutron stars. The rapid rotation, therefore, makes them powerful generators, capable of accelerating charged particles to energies of millions of volts billion.
These charged particles are responsible for the beam of radiation in radio, light, X-rays, and gamma rays. Its power comes from the rotation of the star, which has therefore to be lowering speed. This decrease in speed can be detected as a lengthening of the period of pulses.

Where are pulsars?

Pulsars have been found mainly in the milky way. A full count is impossible, since you pulsars weak only can be detected if you are close.
Radio polls have already covered almost the entire sky. Their distances can be measured from a delay in the time of arrival of pulses observed in the radio frequencies; the delay depends on the density of electrons in the interstellar gas, and distance traveled.
Extrapolating from this small sample of pulsars detectable, it is estimated that there are at least 200,000 pulsars in our galaxy. Whereas those pulsars whose beams Lighthouse not sweep in our direction, the total population should reach one million.
Each pulsar emits for about four million years; After this time you have lost so much rotational energy that can not produce detectable radio pulses. If we know the total population (1,000,000), and life (4,000,000 years), we can deduce that a new strike should be born every four years, assuming that the population remains stable.
Recently found pulsars in a globular cluster. It is thought that they had been trained there by the accretion of matter in the white dwarf stars that are part of binary systems.
Other pulsars are born in supernova explosions. If all pulsars them were born in supernova explosions, we could predict that there should be a supernova in our Galaxy every four years, but this is not yet clear.

Black holes

The so-called black holes are bodies with a very big, huge gravitational field. Cannot escape no light or electromagnetic radiation, why are black. They are surrounded by a spherical "border" that allows light between but not exit.
There are two types of black holes: bodies of high density and low mass concentrated in a very small space, and bodies of low density but very large mass, as in the centers of galaxies.
If the mass of a star is more than two times that of the Sun, comes a time in your cycle that not only neutrons can withstand gravity. The star collapses and becomes a black hole.

Stephen Hawking and the light cones

The British scientist Stephen W. Hawking has devoted much of his work to the study of black holes. In his book History of time explains how, a star that is collapsing, the light cones that emit begin to sag in the surface of the star.
To be small, the gravitational field grows and light cones are inclined more and more, until they can't hide. The light turns off and turns black.
If a component of a binary star becomes a black hole, taking his partner material. When Eddy comes to the hole, he moves so fast that it emits x-rays. So, although you can not see, it can be detected by their effects on nearby matter.
Black holes are not eternal. Although no radiation does not escape, it seems that they can do it some Atomic and subatomic particles.
Someone who observed the formation of a black hole from the outside, would be a small, red star until, finally, it would disappear. Its gravitational influence, however, would remain intact.
As happened in the Big Bang, a singularity, i.e. physical laws is given also in black holes and the predictive power fail. As a result, any outside observer, if any, could see what happens inside.
The equations that attempt to explain a singularity, as it occurs in black holes, have to take into account the space and time. The singularities are always located in the past of the observer (such as the Big Bang) or in the future (such as gravitational collapses), but never in the present. This curious hypothesis is known by the name of cosmic censorship.
Published for educational purposes authorized by: Astronomía: Tierra, Sistema Solar y Universo


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