Polaris is probably the most famous star in the sky. What do astronomers know about it today? Let's consider several facts about Polar, some of which are well known to astronomy lovers, and some of which may become a discovery.
Unlike other stars, which at different times of the day are visible in the east, then in the south, then in the west, then rise above the horizon and set below the horizon. The North Star hardly changes its position in the sky. In winter and summer, it points north, towards the Earth's pole. It is because of its position in the sky that the North Star has gained such popularity.
Fact #1: Polaris is not the brightest star in the sky.
The claim that Polaris is the brightest star is one of the most common astronomical misconceptions. Many people who are not very familiar with the starry sky believe that the North Star is very bright, if not the brightest star in it. But that's not true. In the list of the brightest stars in the sky, Polaris is only in 46th place, far inferior in brilliance to stars such as Sirius, Vega or Arcturus.
Fact No. 2. The North Star is located near the north pole of the world.
It is obvious to everyone that the picture of the starry sky changes depending on the time of day. The constellations that were visible in the south in the evening shift to the west at night, and in the morning they completely go beyond the horizon. And other constellations, which rose in the east in the evening, are already high in the southern part of the sky at night. The stars rotate from east to west with a period of a day, from east to west and reflect the rotation of the Earth from west to east.
The earth rotates like a top - around one axis. The points where the Earth's rotation axis intersects with the surface of the planet are called the North and South Poles of the Earth. By extending the Earth's rotation axis into the sky, we get the north and south poles of the celestial sphere, or, as astronomers call them, the celestial poles. Now let's imagine that we are at the North Pole. During the day, all the stars describe circles in the sky, but the closer the stars are to the zenith, the smaller the radii of the circles they describe. This is understandable, because at the zenith is the north pole of the world, around which all the stars revolve. Therefore, near the zenith, the stars are almost motionless, describing very tiny circles around it.
The uniqueness of the North Star is that it is located closer than other stars visible to the naked eye to the north pole of the world, at a distance of about 1°. Because of this, she is practically in the same place all the time, regardless of the time of day or year. The immobility of the North Star has made it a true guide for sailors, cattle breeders and travelers for fifteen hundred years now.
It is clear that the North Star has a special position only in the Earth’s sky. In the sky of other planets, the role of the polar star is played by other stars, since the axes of rotation of these planets are inclined differently relative to the planes of their orbits. It is interesting that in our sky the North Star was not always like this. Many thousands of years ago, the Earth's rotation axis was directed in the other direction and Vega played the role of the polar star. The fact is that the Earth’s rotation axis is in constant motion, describing a circle in the sky over a period of 25,800 years. This phenomenon is called precession, which is translated from Latin as the anticipation of the equinoxes. The reason for precession is the slight flattening of the globe. Because of this, the Moon and other bodies of the solar system slowly rotate the earth's axis by the force of their gravity. Where is the axis of the world moving today? So far, everything is still in the direction of the North Star, which is (as mentioned above) not exactly at the celestial pole, but at a distance of about 1° from it. By 2100, this distance will be halved, after which the celestial pole will begin to slowly move away from the star, continuing its movement in the direction of the constellation Cepheus.
Fact No. 3. Polaris is the guiding star.
How to navigate by the North Star? First of all, you need to learn to find it in the sky. The easiest way to do this is to start from the Big Dipper bucket - the most famous star pattern. Let's take the two outermost stars in the bucket and mentally draw a straight line through them. The polar star is located at five times the distance of Merak-Dubge (the names of the stars in the Big Dipper bucket). The color of the star is yellowish-white and its brilliance is approximately equal to these stars.
Having found the North Star, draw a line from it perpendicular to the horizon. The intersection of the line with the horizon will point north. Finding the other cardinal directions is already easy: south is in the opposite direction, east is on the right, and west is to the left of the North Star. Another interesting and important characteristic of the North Star is its height above the horizon. Due to its proximity to the celestial pole, Polyarnaya is practically motionless for this location. However, the star's height above the horizon may vary depending on your geographic location. So, being at the North Pole, we would see Polaris not at the zenith, but at the equator (that is, on the “side” of the Earth). Polaris would be almost exactly on the horizon. Therefore, the height of the North Star above the horizon determines the latitude of our location. By measuring the angle between the North Star and the horizon each night in the direction of the north, past sailors could tell how far they had traveled north or south.
Fact No. 4. Polaris is the brightest star in the constellation Ursa Minor.
Polaris is part of the small constellation Ursa Minor. This constellation consists of 7 stars. Unlike the Big Dipper, the Small Dipper includes only three more or less bright stars. Therefore, it is much more difficult to distinguish it in the sky. Polaris is the outermost star in the handle of the Small Dipper. Kohab and Ferkad are the extreme stars in the Bucket itself. These two stars are often called the Guardians of the Pole. As is known, astronomers have long designated the brightest stars in the constellations with letters of the Greek alphabet. The letter "alpha" usually denotes the brightest star in the constellation, the second brightest - the letter "beta" and so on, up to the letter "omega". Therefore, the North Star is also the alpha (?) of Ursa Minor.
Fact #5: The North Star has many names.
Perhaps no other star has as many names as Polaris. Almost the entire set of names indicates its two main features: being at the pole and, as a consequence, its immobility. Many peoples considered the North Star to be a kind of stake driven into the sky, around which all the other stars revolve. This is where its names come from, such as Heavenly Stake, Joke Star, Iron Stake, Northern Nail - names that we find in the Turkic and Finno-Ugric languages. The name Polar came to Russia from the German language during the reign of Peter I. Before that, the Polar Star had another name, which, however, had the same meaning - Northern. The Khakass called Polar - Tethered Horse. (and here the name refers us to the immobility of this star.) And for the Evenks, the Polar Star was seen as a Hole in the sky. Names of Polyarnaya that are not related to its location are rare. The most famous of these names is, perhaps, the name Kinosura "dog's tail." This is what the ancient Greeks and Romans called the North Star; but in that era, Polaris did not yet perform the function of the polar star itself (2000 years ago, the star Kohab or Ursa Minor was closer to the north pole of the world). It is interesting that on ancient maps Polaris actually marks a tail, but not a dog’s tail, but a long one, non-existent in nature, the tail of the Ursa.
There is an interesting hypothesis about the origin of the name Kinosura. This is exactly what, according to Allen and other researchers of star names, the entire constellation Ursa Minor was called in ancient times. In one version of the myth about the birth of Zeus, the baby god was fed for some time in a cave by two bears - Helika (or Helis) and Kinosura, who were later ascended to heaven by a grateful Zeus. Gelika became Ursa Major, and Kinosura became Ursa Minor. Later this name began to refer only to the North Star.
Fact No. 6. Polaris is a supergiant star.
Now let's look at the physical characteristics of Polyarnaya. Already when observing through binoculars, its yellowish color is noticeable. The polar star is only slightly hotter than the Sun: its surface temperature is approximately 6000 K. But this is where the similarity with the Sun ends. The North Star, like the vast majority of stars visible in the sky with the naked eye, is much brighter than the Sun. Spectral studies have shown that the star belongs to the class of supergiant stars. Its radius is 46 times greater than the radius of the Sun, and its luminosity is approximately 2500 times that of the Sun. “Approximately”, since the exact luminosity of the North Star is unknown due to the fact that astronomers do not know the exact distance to it.
Stars with characteristics like Polaris make up only a fraction of a percent among the total number of stars in the Universe. Why are supergiant stars rare? The fact is that the supergiant stage in the life of a star is very short-lived and occurs only after the nuclear fuel in its core has been exhausted. Stars like Polaris are always old, highly evolved objects. This does not mean that the actual age of such stars is great - for example, the North Star is no older than 70 million years - but their life cycle, unlike the Sun, is already coming to an end. Knowing the physical characteristics of Polaris, we can guess what kind of star it was for most of its life. Most likely, Polaris was a bright blue star of spectral class? with a mass 5 times that of the Sun and a radius 3.5 times that of the Sun. The temperature of its surface was three times higher and amounted to about 18,000 K.
Fact #7: Polaris is the brightest and one of the most unusual Cepheids in the sky.
After almost all the hydrogen in the star's core is converted into helium, the star enters an instability phase. The radiation pressure in the core weakens, and it begins to shrink, while the outer layers of the star, on the contrary, increase in size - the star becomes a giant or even a supergiant. In addition, such a star begins to pulsate - to experience fluctuations in size, temperature and brightness. The North Star also pulsates, sometimes increasing and sometimes decreasing in volume. At the same time, its shine also changes slightly, although it is completely invisible to the eye. The oscillations of Polaris are very rhythmic and have a period of 3.97 days - the star works exactly like a clock. Astronomers have found many similar stars and identified them as a whole class of Cepheid stars. Cepheids have similar pulsations, but are not exactly alike. They have different masses, sizes, differ in temperature and, as a result, have different amplitudes and periods of pulsations. However, all Cepheids share an interesting relationship: their pulsation periods directly depend on their luminosity (the amount of light they emit). The longer the period, the greater the luminosity of such stars. The discovery of this dependence played a very important role in astronomy, as it made it possible to determine the distance to other galaxies.
Back in the early 1970s, the brightness amplitude of Polyarnaya varied within 0.27m. This change in brightness is on the verge of being detected by the naked eye. After this, the amplitude of Polar, already small, began to decrease sharply. It was even assumed that by the beginning of the 21st century the star would cease to be a Cepheid altogether. Some astronomers have suggested that Polaris is the first recorded case of the cessation of Cepheid pulsations due to its leaving the instability band due to evolution. However, around 1993, the decrease in the amplitude of Polyarnaya pulsations stopped abruptly, and since then it has been 0.032m. At the same time, the brightness of the star increased by 15%. Some astronomers believe that the star does not stop pulsating, but rather is in the process of developing towards a main period of 5.7 days, becoming an ordinary Cepheid with a large fluctuation in brightness.
Fact No. 8. In the past, Polar was less bright.
The brightness of the North Star has increased by 15% over the past 100 years. This is an undeniable fact. How bright was Polar 2000 years ago? Villanova University graduate Scott Engle decided to re-analyze the Polaris brightness data listed in the catalogs of Ptolemy (137), Al-Sufi (964), Tycho Brahe and other astronomers. Bringing the data to a single scale, he found that Polar is currently 2.5 times brighter than in the time of Ptolemy. By a whole stellar magnitude. If Engle and his team's conclusions are correct, then the brightness of Polaris has changed over the past 2000 years by 100 times more than modern theory of stellar evolution predicts. It is not surprising that most scientists were skeptical about the astronomers' work. However, Engle and his team also found in ancient sources hints of variations in the brightness of Polar with a period of 4 days, which could indicate that in the past the amplitude was much greater than now. The study was published. in the authoritative journal Science, and this, of course, gives it a certain mark of quality. However, this does not mean that Engle's conclusions are final.
Fact No. 9. Polaris is a triple star.
We are all used to talking about Polaris as a star, although in fact it is a system of stars. Not content with either the status of a “polar” or the status of an anomalous Cepheid, Polaris is also a triple star, that is, it has two satellite stars connected to it by forces of mutual attraction. One of the satellites is already visible in small amateur telescopes. This is a star of almost 9 magnitude, located at a distance of 18 arc seconds from the North Star. It was first observed in 1780 by the famous 18th century astronomer William Herschel. Polaris "B" is an ordinary star of spectral class F3V: it is only 4 times brighter than the Sun and 1.4 times more massive. Next to the yellowish-white Polaris, its dim satellite looks greenish. In fact, the color of stars is not green - that’s how human vision works. In this case, we are observing, although a beautiful, but still an optical illusion caused by the proximity of the satellite to the North Star. The second component of the system (denoted Polaris Ab) is much closer to Polaris, so until recently its presence was indicated only by spectral observations.
Polar Ab was first seen only in 2006 using the Hubble Space Telescope. The angular distance between Polar A and Polar Ab turned out to be only 0.133?: at this angle a ruble coin is visible from a distance of 30 kilometers. In reality, the distance between Polaris A and its closest satellite is at least 2 billion kilometers. They complete one revolution around the common center of mass in about 30 years. Polar Ab is similar in its characteristics to Polar B. It belongs to the spectral class F6V, 1.26 times more massive than the Sun and 3 times brighter than our star.
Fact No. 10. Distance to the North Star.
It was said above that Cepheids play a vital role in astronomy. Thanks to the strict relationship between pulsation periods and luminosity, they are unique beacons of the Universe, allowing one to determine distances to other galaxies. The closest Cepheid is Polaris, the distance to it is estimated at about 400 light years. At this distance, parallaxes (changes in the apparent position of an object relative to a distant background depending on the location of the observer) give a large error. The most accurate polar parallax to date, determined by the Hipparcos satellite, has an error of 8 light years or about 2%.
Astronomer David Turner released a paper in which he showed that the current distance to the North Star is as much as 111 light years less than Hipparcos measured. For his research, the astronomer used the largest Russian telescope with a mirror diameter of 6 meters (BTA telescope). Dutch astronomer Floor van Leeuwen, “responsible” for the Hipparcos data, immediately wrote a response article in which he proved that the satellite’s data was correct, unlike Turner’s data. The dispute immediately spilled over into the vast expanses of the Internet, and many media outlets paid attention to it. Still would! After all, if we accept the new distance to the North Star, we will have to simultaneously admit that the true scale of the Universe has been greatly overestimated. You can go further and question the thesis about the rate of expansion of the Universe, although discoveries in this area were already awarded the Nobel Prize in 2011. So who is right...
