Variable stars: About stars that change brilliance
During the life of an ordinary star, its brilliance changes very slowly, when compared, for example, with a human life or even the life of several generations. But the brilliance of variable stars varies in the range from several minutes to several years! Therefore, studying variable stars is a wonderful way to learn more about the processes occurring in the stellar interior. In modern astronomy, there are several dozen types of variable stars, and the number of known variables is close to one hundred thousand.
Spectral class of a star
However, before you begin to get acquainted with the amazing world of variable stars, you will have to introduce such a basic astronomical concept as the spectral class.
The spectral class allows you to immediately include three characteristics of the star - color, temperature and chemical composition. There are seven main spectral classes; they correspond to colors from blue through white to red: OBAFGKM. In order to remember them, English students came up with a saying: "Oh, Be A Fine Girl, Kiss Me." Everything is greatly simplified with classes: for example, instead of saying: “a blue star with a temperature of 20, 000 degrees and with the predominance of hydrogen lines in the spectrum, ” we can say: “a star of class O”. White and blue stars (classes O, A, B) are younger and hotter, and hydrogen and helium predominate in their spectra. With the “reddening” of the stars, they cool down, and in the atmosphere, hydrogen ceases to prevail and helium and carbon first appear, and then metals. It was previously believed that spectral classes also reflect the evolution of a star - a star is born blue and hot, then cools and passes through the whole chain of spectral classes. But this theory has not been confirmed.
In addition, stars vary in size. Supergiants, giants, subgiants and dwarfs distinguish stars here.
The scientists Hertzsprung and Russell constructed the following diagram: the luminosity of the star (the amount of energy emitted by the star per unit time) was plotted on the vertical axis, and the spectral classes on the horizontal. That is, for each star in this diagram there was a point.
Most of the stars appeared on line V, called the "main sequence". This means that almost any star in the process of its evolution spends most of its life there. Lines of supergiants and giants appeared at the top of the diagram, and dwarfs at the bottom. The evolutionary path of a star in this diagram depends on the mass and chemical composition of the star; it is a single star or it has a neighbor, and several other less significant factors. Usually it starts in the region of blue supergiants, from left to right, at some point the star sits on the main sequence and moves down it, then swells again and becomes a red giant, and then turns into a white dwarf.
As always, when studying a large number of objects (in our case, it’s several tens of thousands of variable stars!), It is necessary to systematize them by type and group them: eclipsing variables, pulsating variables and eruptive (incorrect) variables.
The most frequently mentioned in this class is Algol. After the ancient Arabs, its variability was rediscovered in the 17th century, and John Goodrake, an English astronomer, explained the reasons for the variability. Goodrayk made the following assumption: “If it were not too early to give reasons for the causes of variability, I could have assumed the existence of a large body orbiting Algol, which was confirmed a hundred years later.
Eclipsing variable stars are binary stars when one star revolves around the other or both revolve around a common center of gravity. When both stars are on the line of our vision, that is, an eclipse of one of the stars occurs, their visible brightness fades, and when they do not overlap, it increases.
When studying eclipsing variable stars, many questions arise. In fact, stars in the most diverse spectral classes are neighbors in binary systems. For example, the double star Sirius is an A2 class star and a white dwarf (their rotation period is about 50 years). The first of them, according to modern views, is a very young star, the second is at the final stage of evolution. How could it happen that these stars, so different in age, could form a single system? It is assumed that an important role in the evolution of binary stars is played by the mass exchange between the stars. When hydrogen burns out in the center of the star, the core contracts and the shell swells. The influence of the second component on the surface layers of the star is becoming more noticeable. And as soon as the star’s diameter reaches a certain critical value, the “transfer” of mass to another component begins. Calculations show that one of the stars can lose up to 80% of its initial mass, and not all of it will fall on a satellite star. It is possible that half or even two-thirds of this mass generally leaves the system, leaving for interstellar space. It is possible that in this way it is possible to explain the amazing combination of stars of the Sirius system.
Pulsating variable stars
In 1596, the German astronomer David Fabricius noticed a new bright star in the constellation Ceti, the brightness of which increased from third to second magnitude over 20 days, after which the brightness fell and the star became invisible to the naked eye (although it can be observed with a telescope) . Fabricius gave the star the name of the World, "miraculous." In 1784, our acquaintance Goodrayk discovered that the fourth brightest star in the constellation Cepheus (Cepheus delta) regularly changes its brightness from 3rd to 4th magnitude with a period of 5.37 days. All such pulsating stars are called Cepheids by the name of this star.
Both stars, Mira and Cepheus delta, are pulsating variables. So how, why do they change their brilliance? It was found that this is due to a change in the diameter of the star. The star expands - and shines as bright as possible, shrinks - and its brilliance falls. It makes the star expand and contract the ionized helium zone.
Explain a little more.
In a star, the temperature and density of matter increase toward the center. At a certain distance from the surface, hydrogen and helium gradually transform into an ionized state (that is, atoms lose their electrons).
First, a hydrogen ionization zone appears, where the loss of the only electron in this atom occurs. This zone is slightly overlapped by the zone of primary helium ionization (the helium atom has two electrons). Going even lower, the helium atom loses the second electron, forming a zone of complete ionization. It is this zone, which has a small thickness and mass, that sets in motion the entire star. Light in the zone of complete ionization is absorbed, the pressure increases and causes this layer to expand. As a result of expansion, a decrease in density occurs, so the opacity of the layer decreases, and the light stored in the layer is emitted. After achieving the greatest expansion, the outer layers under the influence of gravity begin to fall, slip through the equilibrium position and contract. The cycle starts over.
Calculations showed that only stars can behave in this way, in which the period of oscillations of the ionization zone is capable of reaching resonance with the whole star. And this is possible mainly for giants and supergiants. When moving along the types of stars from supergiants to ordinary stars and dwarfs, such an exact resonance tuning worsens, and instead of clear pulsations, more and more irregular fluctuations in the star’s brightness occur.
For Cepheids, a relationship was also derived between the period of brightness and the brightness of the star — the greater the brightness, the longer the period. This dependence is used to determine the distances to star clusters and galaxies in which Cepheids can be detected. From observations, the apparent brilliance and the period of its change are established. Knowing the period, one can determine the absolute brilliance of a star.
And knowing its visible brilliance and absolute, they find the distance to the star. The apparent brilliance (or apparent magnitude) depends on two factors: the luminosity and color of the star and the distance to it. It is difficult to compare the apparent brilliance, and the so-called absolute brilliance (absolute magnitude) is introduced for comparison. It is defined as the apparent brilliance of a star located at a distance of 10 parsecs from the observer.
Eruptive (incorrect) variable stars
All variable stars that do not belong to eclipsing and pulsating stars fall into this category - usually these are new and supernova stars.
The first mentions of supernovae are found already in the 2nd century BC. Then the first catalogs of stars appeared. Chinese astronomers observed in the XI century (1054) a supernova explosion (in its place now is the Crab Nebula - its gas shell scattered around a former star). Supernovae are characterized by the fact that they flare up unusually brightly. Compared to its usual light, their brilliance is amplified a hundred million times - the same galaxy emits the same amount of light. Supernova stars are divided into two main types (according to the explosion mechanism, which determines the luminosity, the nature of its change and spectrum). Type I stars quickly, in a week, reach their maximum brightness, which then weakens. Type II stars have a lower maximum brightness, shine for a longer time at a maximum and weaken faster. The flash of a supernova ends in its almost complete decay. In its place there remains a superdense star - the supernova nucleus (eventually turns into a neutron star or a black hole),
and the material of the stellar envelope is scattered into world space, forming a diffuse gas nebula.
In addition to supernovae, there are new stars that flare up less brightly than supernovae. For an observer, the difference between a supernova and a new one will be only in brightness - the supernova is brighter by tens of thousands of times, although the physical processes taking place in these stars are different (but this is perhaps the topic for a separate article). The outbreak of a new star (like a supernova) occurs suddenly. Her shine increases rapidly and reaches a maximum. After this, a gradual decrease in brightness begins, which occurs in different stars in different ways. In the end, the star’s brightness decreases to a “normal”, preflare state. At the end of the outbreak of a new star, a few years after the maximum, the gas nebula formed by the discharge of the envelope becomes visible, which gradually expands.
Astronomers also observed repeated new ones that flashed several times with an interval of several years. Like, for example, T of the Northern Crown. This is a double star consisting of a red giant (spectral class MH) and a hot star. During the flare of a repeated new star, its diameter increases. The star swells. The swollen shell becomes more thin and transparent, and then breaks up into separate clumps. The star is gradually fading.
Variable stars, like U Gemini, retain their minimal brilliance, as if accumulating energy for a subsequent sharp flash, which can last several days. Outbreaks do not occur periodically, but cyclically, so it is impossible to predict when the next outbreak will occur. The flash brightness depends on the duration of the cycle: the longer it is, the longer the cycle.
It would seem that an explosion of a star should always be followed by an increase in its brilliance. But for some stars this is not true. When the substance (carbon) worked out in thermonuclear reactions rises from the bowels of the stars and is ejected from the star, its brilliance weakens, since the ejected dust begins to absorb the light of the star itself. Gloss can fall by ten magnitudes, that is, tens of thousands of times. So it happens with stars of type R of the Northern Crown.
The richness of the world of variable stars has not yet been studied, and many discoveries are patiently waiting for their researchers and observers. After all, one successful observation of a variable star can make a greater contribution to science than years of theoretical research! Variable star observations are handled by many organizations bringing together amateur astronomers from around the world (for example, the American Association of Variable Star Observers, www.aavso.org).The article was published in the journal Popular Mechanics (No. 7, July 2004).