# Laniakeya and 10 more terms from astronomy that you need to know

We present to your attention several terms with which your knowledge of astronomy will become deeper.

## Apparent magnitude

The number of stars in the night sky available to the naked eye is not as great as it seems. If you have good visual acuity and get out of the city, away from street lighting, then about 6,000 stars will be available for observation. Moreover, half of them will always be hidden from the observer over the horizon. But even this amount is enough to notice how the stars differ in their brightness. Ancient scholars also noticed this. The ancient Greek mathematician and astronomer Hipparchus, who lived in the II century BC, divided all the stars he observed into six magnitudes. He attributed the brightest to the first magnitude, the dimmest to the sixth.

In general, this principle is still used today. But today the possibilities of astronomy make it possible to observe countless stars, most of which are so dim that it is impossible to observe them with the naked eye. And the very concept of stellar magnitude is applied not only to distant stars, but also to other objects - the Sun, the Moon, artificial satellites, planets, and so on. Therefore, it is believed that the magnitude is a dimensionless numerical characteristic of the brightness of an object.

As follows from the above, the apparent magnitude of the brightest objects will be negative. For comparison, the stellar magnitude of the Sun is –26.7, and the stellar magnitude of the star Proxima Centauri, closest to our star, but not visible to the naked eye, is +11.1. The maximum stellar magnitude of Mars is? 2, 91. The "Mayak" satellite, which was created and is planned to be sent into orbit by young Russian scientists, should have a stellar magnitude of no more than? 10, as planned. And if everything succeeds, it will for some time become the brightest object in the night sky, unless, of course, we count the full moon (? 12, 74).

## Absolute magnitude

Deneb is one of the largest stars known to science, has a magnitude of +1, 25. Its diameter is approximately equal to the diameter of the Earth's orbit and is 110 times greater than the diameter of the Sun. The distance to this giant is 1,640 light years. Although scientists are still debating this issue, it is too far away. Most stars at this distance can only be seen through a telescope. If we were closer to this star, then the brightness of Deneb in the sky would be much higher. Thus, the apparent magnitude depends on both the luminosity of the object and the distance to it. To be able to compare the luminosity of different stars with each other, use the absolute magnitude. For stars, it is defined as the apparent stellar magnitude of the object if it were located at a distance of 10 parsecs from the observer. If the distance to the star is known, then the absolute magnitude is easy to calculate.

The absolute stellar magnitude of the Sun is +4.8 (visible, recall, -26.7). Sirius - the brightest star in the night sky - has an apparent magnitude of 1, 46, but the absolute value is only +1, 4. Which, however, is not surprising, because the diamond of the night sky (as this star is called) is close to us: at a distance of only 8, 6 light years. But the absolute magnitude of the already mentioned Deneb is -6.95.

## Parallax

Ever wondered how scientists determine the distance to a star? After all, this distance cannot be measured with a laser rangefinder. In fact, everything is simple. During the year, the position of the star in the sky changes due to the revolution of the Earth in its orbit around the Sun.This change is called the star's annual parallax. The closer a star is to us, the greater its displacement against the background of stars that are farther away. But even for the nearest stars, this displacement is extremely small. The inability to detect parallax in stars at one time was one of the arguments against the heliocentric system of the world. It was possible to do this only in the 19th century. At the present time, special space telescopes are being put into orbits to measure parallaxes and, consequently, distances to stars. The Hipparcos Telescope of the European Space Agency (named after the very same Hipparchus who classified stars by brightness) has measured the parallaxes of more than 100,000 stars. In December 2013, its successor, Gaia, was launched into orbit.

Actually, parallax (and this is not only an astronomical concept) is a change in the apparent position of an object relative to a distant background (in our case, more distant stars), depending on the position of the observer. It is also used in geodesy. Matters for photography. Parallax is measured in arc seconds (arc seconds).

## Light year

It is not at all convenient to measure distances in outer space in kilometers. For example, the distance to the nearest star Proxima Centauri? 4, 01:1013 kilometers (40, 1 trillion kilometers). It is rather difficult to imagine this distance. But if you measure this distance in light years, a unit of length equal to the distance traveled by light in one year, you get 4.2 light years. The light from this red dwarf reaches us for about 4 years and 3 months. It's simple.

## Parsec

But with another unit of length used in astronomy, it is not so simple. The distance to the star Proxima Centauri, measured in parsecs, is 1.3 units. The word "parsec" itself is formed from the words "parallax" and "second" (meaning the angular second, equal to 1/3600 of a degree, remember the school protractor). The same parallax, thanks to which we can measure distances to stars. Parsec (denoted by "pc")? this is the distance from which a segment of one astronomical unit length (the radius of the earth's orbit), perpendicular to the line of sight, is seen at an angle of one arc second.

## Galactic arm

Our Milky Way is 100,000 light years across. It belongs to one of the main types of galaxies. The Milky Way is a barred spiral galaxy. All the stars that we see in the sky with the naked eye are in our Galaxy. In total, the Milky Way contains, according to various estimates, from 200 to 400 billion stars. How do you navigate and find out where the Sun is among these billions of stars?

The Milky Way is a spiral galaxy, and it has spiral galactic arms located in the plane of the disk. The Galactic Arm is a structural element of a spiral galaxy. Most of the stars, dust and gas are contained in the galactic arms.

There are several such sleeves, but the main ones are the Sagittarius sleeve, the Cygnus sleeve, the Perseus sleeve, the Centauri sleeve and the Orion sleeve. They received such names by the name of the constellations in which the main array of arms can be observed. The Orion Arm is small in comparison with others. Sometimes he is even called the Spur of Orion. It is only about 11,000 light years long. But for us this sleeve is notable for the fact that the Sun and the small Blue Planet revolving around it and being our home are located in it.

## Apocenter and pericenter

Most of the known orbits of artificial satellites and celestial bodies are elliptical. And for any elliptical orbit, you can always specify the point closest to the central body and the most distant from it. The closest point is called the periapsis, and the farthest is called the apocenter.

But, as a rule, instead of the word "center", after "peri" or "apo", they substitute the name of the body around which the movement takes place. So, for the orbits of artificial satellites of the Earth (Gaia - in ancient Greek) and the orbit of the Moon, the terms apogee and perigee are used.For a circumlunar (Moon - Selene) orbit, apolunctions and perilune are sometimes used. The closest point to the Sun (Helios) is the point of the orbit of our planet or other celestial body of the solar system - perihelion, the farthest - aphelion or apogelium. For orbits around other stars (an astronomer is a star) - periastron and apoaster.

## Astronomical unit

The perihelion of the orbit of our planet (the closest point of the orbit to the Sun) is 147,098,290 km (0.983 astronomical units), aphelion - 152,098,232 km (1.017 astronomical units). But if you take the average distance from the Earth to the Sun, you get a convenient unit of measurement in space. For those distances where it is already inconvenient to measure in kilometers, but in light years and parsecs it is still inconvenient. This unit of measurement is called an "astronomical unit" (denoted by "a. E.") And is used to determine the distances between objects in the solar system, extrasolar systems, as well as between the components of binary stars. After several refinements, the astronomical unit was found to be equal to 149597870.7 kilometers.

Thus, the Earth is removed from the Sun at a distance of 1 AU. That is, Neptune, the planet farthest from the Sun, is at a distance of about 30 AU. e. The distance from the Sun to the planet closest to it - Mercury - is only 0, 39 AU. e. And at the time of the next great opposition of Mars and Earth, on July 27, 2018, the distance between the planets will decrease to 0.386 AU. e.

## Roche limit

Nothing is permanent in space. It just takes millions of years to change the order we are used to. So, if a certain observer in a few million years will observe Mars, then he may not find one or even two of its satellites. As you know, the largest of the satellites of the red planet - Phobos - is approaching it by 1.8 meters per century. Phobos moves only about 9,000 km from Mars. For comparison, the orbits of the navigation satellites are at an altitude of 19,400-23,222 km, the geostationary orbit is 35,786 km, and the Moon, a natural satellite of our planet, is at a distance of 385,000 km from the Earth.

Another 10-11 million years will pass, and Phobos will pass its Roche limit, as a result of which it will collapse. The Roche limit, named after Edouard Roche, who was the first to calculate such limits for some satellites, is the distance from a planet (star) to its satellite, closer to which the satellite is destroyed by tidal forces. It was found that the planet's gravity is compensated by the centrifugal force only at the satellite's center of mass. At other points of the satellite, there is no such equality of forces, which is the reason for the formation of tidal forces. As a result of the action of tidal forces, the satellite first acquires an ellipsoidal shape, and when it passes through the Roche limit, it is broken by them. But the orbit of another satellite of the red planet - Deimos (orbital altitude of about 23,500 km) - is farther and farther each time. Sooner or later, he will overcome the gravity of Mars and set off on an independent journey through the solar system.

## Laniakeya

Can you tell where our planet is in the universe? Of course, planet Earth is in the solar system, which in turn is in the Orion Arm, the small galactic arm of the Milky Way. Well, what about next? Our Galaxy, the Andromeda galaxy closest to us, the Triangle galaxy and more than 50 galaxies are included in the so-called Local Group of Galaxies, which is a constituent of the Virgo supercluster.

And here is the Virgo supercluster, also called the Local Supercluster of galaxies, the Hydra-Centauri and Peacock-Indian superclusters, as well as the Southern Supercluster form a supercluster of galaxies called Laniakea. It contains about 100 thousand galaxies. Laniakei is 500 million light years across. For comparison, the diameter of our Galaxy is only 100 thousand light years. Translated from Hawaiian Laniakea means "immense skies." Which in general accurately reflects the fact that in the foreseeable future we will hardly be able to fly to the edge of these "heavens".

You can learn a little more about astronomy here.About distances in the nearest space - in this article. And about what stars are and how they live their lives - in this one.