Variable stars

A star variable is a star whose brightness, sight from the Earth, are not constant. The variable stars include stars whose transmission of light really fluctuates (variable intrinsic) & others whose light is interrupted in its trajectory until the Earth by some external factor, that can be another star or an interstellar dust cloud (variable extrinsic). The changes in the luminous intensity of most of the intrinsic variables must to pulsations in the size of the star (variable pulsating) or to interactions between the components of a double star.

Some other intrinsic variables do not fit in any of these two main categories. The only frequent type of extrinsic variable is the eclipsante binary call . One is a double star formed by two next stars that pass one in front of the other periodically: Algol is the example more well-known, another example very known is the star b Lyrae whose curve of light is available in the Web. The binary eclipsantes almost constitute 20% of all well-known variable stars, being almost all the other intrinsic variables.

Of the stars that present an intrinsic variation in their production of light, most common they are the pulsating variables. Probably, the most famous examples are the cefeids variables , whose periodic pulsations provide an indication of their brightness, reason why constitute an important reference for the measurement of distances in the space, the name of cefeids comes to them because first that was discovered & located it was star d cephei, in the constellation of Cepheus & from his name it comes from his prototype or it stars there representative, that varies between magnitudes 3.5 & 4 in 5 cefeids days & 9 hours.

In the Web the curve of light of one of these cefeidas is available denominated h Aquilae . The cefeids ones are a type of star whose luminosity varies cyclical like result of regular variations (pulsations) of its size. They are stars giant or supergiant and, therefore, quite luminous; they are visible to long distances. Their periods of pulsation approximately vary between a day & about four months, & their variations of luminosity can be of 0.5 to 2 magnitudes (this is, a 600% & change of luminosity between 50 between the maximum & the minimum).

The importance of the cefeids ones is fundamentally in that they present a relation defined between his average luminosity & the period of pulsation; this relation was discovered in 1912 by the American astronomer Henrietta S. Leavitt, & period-luminosity is known like relation. Leavitt found that the luminosity of cefeid increases of proportional way to its period of pulsation. Thus, the astronomers can simply determine the intrinsic luminosity of cefeid measuring the period of pulsation. The apparent luminosity of a star in the sky depends on its distance to the Earth; comparing this luminosity with its intrinsic luminosity the distance can be determined to which it is. This way, the cefeids ones can be used as much like indicators of distances inside as outside the Milky Way.

Cefeid varies its luminosity as a result of alterations in its density & its size. When the star is smaller & compacts, she is more opaque; the radiation has difficulties to escape & the star is warmed up. The inner pressure increases, & the star expands. According to it does, becomes more transparent. The radiation escapes, the cefeid one cools off & the star becomes more luminous. Then it is contracted, & he becomes more shining in the process. The size of the star oscillates between 5 & a 10%. Most of stars they cross these variable phases towards the end of its lives.

The investigations have shown that exist two cefeids types of. Most common cefeids are called classic & the others, older & weak, are known as stars W Virginis . Both types have different relations period-luminosity.

A related type is the variables RR Lyrae , that usually are in globular clusters. Stars RR Lyrae are giant, less luminous than the cefeidas ones, & vary until in two daily magnitudes. When being all in the same evolutionary phase, all has the same luminosity approximately, reason why they constitute excellent references of distance once identified. She is one of the existing methodologies to find the distance in years light of the globular accumulations. Stars RR Lyrae are abundant & approximately constitute 20% of all known variable stars. In comparison, only around 1% they are cefeids.

The more common variable stars of all the types are the stars Mira, calls thus by their more outstanding representative, the Mira star or o Ceti, in the constellation of Cetus (Whale) . One is giants or red supergiants with enormous variations of intensity (up to 11 magnitudes, a factor of 25,000) throughout periods of months or years. Also variable stars of long period are called. Many other red giant or supergiant stars show certain degree of variation, but much less pronouncing than stars Mira. According to his interval of brightness & its greater or smaller regularity (if it is than they present regularity), they are classified like semiregular or irregular variables. In all the cases, the variations must to fluctuations in the size & the temperature of own stars.

Some of the most interesting phenomena must both to interactions between members of a double star. The most spectacular examples are you novate, in which a thermonuclear explosion in the surface of a white dwarfed star takes place that is accumulated hydrogen coming from a near companion. In the calls you nova dwarves (also well-known as stars U Geminorum) take place smaller but the more frequent tips of brightness. Like you nova, these stars are formed by a white dwarf & a normal companion; the gravity of the white dwarf drags gas coming from the other star, which forms a disc, the call accretion disc, that turns around the white dwarf. The increase of luminosity of one nova dwarf must to an increase of the brightness of the accretion disc, probably due to the liberation of gravitational energy on the part of the gas.

When one of the objects of the double system still more is condensed that a white dwarf, that is to say, if a black hole is a neutron star or black hole , its gravitational field is so intense that the gas catched by him is warmed up until extreme temperatures & emits X rays.

All the system denominates double X ray star, & can be observed by space satellites. The variations in the intensity of double X ray stars must to changes in the speed of transference of mass & to eclipses of the body that emits X rays .

The stars in formation process also are variable. The stars T Tauri or RW Aurigae are very young stars with similar masses to the one of the Sun as much present variations irregular, due to the activity of own stars like a the darkening caused by dust clouds that pass through.

Flashing stars ahead, also known as UV Ceti stars, are dwarfed red that experience sudden increases of brightness in a factor of up to 250, which minutes last only. One thinks that the cause is similar explosions to which they take place in the surface of the Sun. The flashing stars could be the following evolutionary stage of the T Tauri stars.

The stars R Coronae Borealis behave exactly of opposite form. They are supergiants that occasionally diminish their brightness up to eight magnitudes soon to recover it slowly. It thinks that these diminutions sudden must to expulsion of particles of carbon formed in atmosphere of star

Other stars which stars of cover expel matter are the calls , which they turn quickly & they throw gas from his equator, which produces a temporary diminution of the brightness. g Cassiopeiae is a cover star example.

The more spectacular variable stars are you supernovae, in which the star is destroyed in a gigantic explosion. The explosion of one supernovae is much more spectacular & destructive that the one of one novates & much more rare. These phenomena are little frequent in our galaxy, & in spite of their increase of brightness in a factor of thousands of million, only few can be observed simple view.

Until 1987 had only identified really three throughout the history, most well-known of which she is the one that arose in 1054 a.C. & whose rest are known like the nebula of the Crab.

Nebula of the Crab

One supernova that explodes leaves behind himself a cloud of gaseous material that expands cloudy call quickly. The nebula of the Crab was created when a star in our galaxy exploded. The light of the explosion was observed by Chinese astronomers in year 1054. In the center of the nebula one is to pulsar, a dense star that turns at great speed.

Hale Observatories/Science Source/Hale Observatories/Science Source/Photo Researchers, Inc.

The supernovae, like you nova, are seen with more frequency in other galaxies. Therefore, the most recent supernova, than appeared in the South hemisphere the February 24^th, 1987, arose in a galaxy satellite, the Great Cloud of Magallanes. This supernova, that exhibits some unusual characteristics, is today object of an intense astronomical study.

Great cloud of Magallanes

This radioimage of the Great Cloud of Magallanes was taken by the radio telescope of 64 ms Parkes, in Australia. One is an irregular & relatively small galaxy, located to about 150,000 years light of the Sun. The Clouds of Magallanes are the galaxies nearest the Milky Way.

Max-Planck-Institut für Radioastronomie/Science Source/Max-Planck-Institut für Radioastronomie/Science Source/Photo Researchers, Inc.

The mechanisms that produce you supernovae are known less than those of you novae, mainly in the case of the stars that more or less have the same mass that the Sun, the average stars. Nevertheless, the stars that have much more mass explode sometimes in the last stages of their fast evolution like result of a gravitational collapse, when the pressure created by the nuclear processes within the star no longer can support the weight of the outer layers.

To this supernova of Type II is denominated to him. One supernova of Type I is originated of way similar to one novates. It is a member of a binary system that receives the flow of pure fuel when capturing material of its companion.

The variable stars have a specific system of nomenclature. Those that they were sufficiently shining to have received a catalogue designation already (like Algol, denominated b Persei), they maintained it when its variability was detected. Nevertheless, to rest of variables assigned them name of one or two letters according to order of his discovery, beginning by R.

Once used all combinations of two letters in constellation corresponding (that is to say, when have been 334 stars variable), the stars receive a number preceded by letter V, beginning by V335.

Next is the variable star relation of which I have made to date observations of its curves of light & which the information are computerized. In all the indicated variables down it is possible at the moment to obtain in a new screen only the graph of the light curve, independently of the data base of the variables.