Apparent magnitude definition. Magnitude
Astronomers measure the brightness, or more precisely, the brilliance, of stars in stellar magnitudes. A rather original term, introduced back in the second century BC by the Greek astronomer Hipparchus.
Hipparchus divided stars according to brightness into six degrees, into six magnitudes, calling the brightest stars stars of the first magnitude, and the weakest, barely visible to the eye, referred to the sixth magnitude. Stars of intermediate brightness were distributed by magnitude subjectively, “by eye,” so that the “steps” of stellar magnitudes were approximately the same.
Later it turned out that subjectively uniform “steps” from one magnitude to the next correspond to exponential growth physical brightness (luminous flux). In other words, the visible shine increases on step, and the physical brightness - V repeatedly. This is the property of any physiological sensations; they obey the logarithmic law: the intensity of the sensation is proportional to the logarithm of the intensity of the stimulus.
It is pleasant that a difference of 5 stellar units (denoted 5 m) corresponds to a hundredfold change in the luminous flux. Accordingly, one magnitude is a change in the luminous flux of approximately two and a half times. The star Vega was chosen for zero magnitude, but the most bright stars do not fit into the scale and have a negative magnitude, these are Sirius, Canopus, Alpha Centauri and Arcturus.
The greater the magnitude, that is, the dimmer the stars, the more of them there are. Analysis of the Catalog of Bright Stars, which includes all stars brighter than 6.5 m, gives a good relationship: with an increase by one magnitude, the number of stars increases by 3 times. Please note: an exponential dependence appears here too! Many processes in nature are described by exponentials.
To see this exponential relationship, it is convenient to use graphs with a logarithmic scale, which is what I do in the second figure. The stars of the Almagest catalog of Ptolemy (2nd century AD), the oldest surviving catalog, and the Ugulbek catalog were also added there. In them, stellar magnitudes are determined by the hipparchian method “by eye”; nevertheless, it is clear that they are, in general, consistent with modern ones. The excess of magnitude 3 and 4 stars is explained by an overestimation of the brightness of dim stars. In addition, it is clearly visible that ancient astronomers omitted a huge number of the faintest stars of 5th and 6th magnitude.
Description
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Description of the table
Table includes the number of stars brighter than a certain magnitude.
Stellar magnitude Limiting stellar magnitude.
Catalog of Bright Stars The number of stars brighter than the specified magnitude from the Catalog of Bright Stars. Almagest The number of stars brighter than the indicated magnitude from the Almagest catalog.
Ulugbek The number of stars brighter than the specified magnitude from the Ulugbek catalog. First chart
| shows the dependence of the number of stars brighter than magnitude on magnitude. | Second graph | shows the dependence of the number of stars brighter than magnitude on the magnitude in the logarithmic scale for various catalogs. | Magnitude |
|---|---|---|---|
| -1.0 | 1 | ||
| -0.5 | 2 | ||
| 0.0 | 4 | ||
| 0.5 | 10 | ||
| 1.0 | 15 | 14 | 15 |
| 1.5 | 23 | ||
| 2.0 | 50 | 54 | 50 |
| 2.5 | 93 | ||
| 3.0 | 174 | 249 | 252 |
| 3.5 | 287 | ||
| 4.0 | 518 | 726 | 678 |
| 4.5 | 904 | ||
| 5.0 | 1630 | 961 | 934 |
| 5.5 | 2887 | ||
| 6.0 | 5080 | 1010 | 1013 |
| 6.5 | 8404 |
Catalog of bright stars
Almagest
Ulugbek Even people far from astronomy know that stars have different brightnesses. The brightest stars are easily visible in the overexposed city sky, while the faintest stars are barely visible under ideal viewing conditions.
To characterize the brightness of stars and other celestial bodies (for example, planets, meteors, the Sun and the Moon), scientists have developed a scale of stellar magnitudes. Apparent magnitude(m; often called simply “magnitude”) indicates the radiation flux near the observer, i.e., the observed brightness of the celestial source, which depends not only on the actual radiation power of the object, but also on the distance to it.
This is a dimensionless astronomical quantity that characterizes the created celestial object illumination near the observer. Illumination
– light magnitude, equal to the ratio the luminous flux incident on a small area of the surface, to its area.
Illumination is directly proportional to the luminous intensity of the light source. As the source moves away from the illuminated surface, its illumination decreases in inverse proportion to the square of the distance (inverse square law).
Subjectively visible stellar magnitude is perceived as brightness (for point sources) or brightness (for extended sources).
In this case, the brightness of one source is indicated by comparing it with the brightness of another, taken as a standard. Such standards usually serve as specially selected fixed stars.
Magnitude was first introduced as an indicator of the visible brightness of stars in the optical range, but later extended to other radiation ranges: infrared, ultraviolet.
Thus, the apparent magnitude m or brightness is a measure of the illumination E created by the source on the surface perpendicular to its rays at the observation location.
Historically, it all began more than 2000 years ago, when the ancient Greek astronomer and mathematician Hipparchus(2nd century BC) divided the stars visible to the eye into 6 magnitudes.
The most bright stars Hipparchus assigned the first magnitude, and the dimmest ones were barely visible to the eye, – the sixth, the rest were evenly distributed among intermediate values. Moreover, Hipparchus made the division into stellar magnitudes so that stars of the 1st magnitude seemed as much brighter than stars of the 2nd magnitude as they seemed brighter than stars of the 3rd magnitude, etc. That is, from gradation to gradation the brightness of the stars changed by one and the same size.
As it turned out later, the connection between such a scale and real physical quantities logarithmic, since the change in brightness in same number times is perceived by the eye as a change of the same amount - empirical psychophysiological law of Weber–Fechner, according to which the intensity of sensation is directly proportional to the logarithm of the intensity of the stimulus.
This is due to the peculiarities of human perception, for example, if 1, 2, 4, 8, 16 identical light bulbs are lit sequentially in a chandelier, then it seems to us that the illumination in the room is constantly increasing by the same amount. That is, the number of light bulbs turned on should increase by the same number of times (in the example, twice) so that it seems to us that the increase in brightness is constant.
The logarithmic dependence of the strength of sensation E on the physical intensity of the stimulus P is expressed by the formula:
E = k log P + a, (1)
where k and a are certain constants determined by a given sensory system.
In the middle of the 19th century. English astronomer Norman Pogson formalized the magnitude scale, which took into account the psychophysiological law of vision.
Based on real results observations, he postulated that
A STAR OF THE FIRST MAGNITUDE IS EXACTLY 100 TIMES BRIGHTER THAN A STAR OF THE SIXTH MAGNITUDE.
In this case, in accordance with expression (1), the apparent magnitude is determined by the equality:
m = -2.5 log E + a, (2)
2.5 – Pogson coefficient, minus sign – a tribute to historical tradition (brighter stars have a lower, including negative, magnitude);
a is the zero point of the magnitude scale, established by international agreement related to the choice of the base point of the measurement scale.
If E 1 and E 2 correspond to the magnitudes m 1 and m 2, then from (2) it follows that:
E 2 /E 1 = 10 0.4(m 1 - m 2) (3)
A decrease in magnitude by one m1 - m2 = 1 leads to an increase in illumination E by approximately 2.512 times. When m 1 - m 2 = 5, which corresponds to the range from the 1st to the 6th magnitude, the change in illumination will be E 2 / E 1 = 100.
Pogson's formula in its classical form establishes a relationship between visible stellar magnitudes:
m 2 - m 1 = -2.5 (logE 2 - logE 1) (4)
This formula allows you to determine the difference in stellar magnitudes, but not the magnitudes themselves.
To use it to construct an absolute scale, you need to set null point– brightness, which corresponds to zero magnitude (0 m). At first, the brilliance of Vega was taken as 0 m. Then the null point was redefined, but for visual observations Vega can still serve as a standard of zero visible magnitude (according to the modern system, in the V band of the UBV system, its magnitude is +0.03 m, which is indistinguishable from zero to the eye).
Usually, the zero point of the magnitude scale is taken conditionally based on a set of stars, careful photometry of which has been carried out using various methods.
Also, a well-defined illumination is taken as 0 m, equal to the energy value E = 2.48 * 10 -8 W/m². Actually, it is the illumination that astronomers determine during observations, and only then it is specially converted into stellar magnitudes.
They do this not only because “it’s more common,” but also because magnitude turned out to be a very convenient concept.
magnitude turned out to be a very convenient concept
Measuring illumination in watts per square meter is extremely cumbersome: for the Sun the value is large, and for faint telescopic stars it is very small. At the same time, it is much easier to operate with stellar magnitudes, since the logarithmic scale is extremely convenient for displaying very large ranges of magnitude values.
The Pogson formalization subsequently became the standard method for estimating stellar magnitude.
True, the modern scale is no longer limited to six magnitudes or only visible light. Very bright objects can have a negative magnitude. For example, Sirius, the brightest star in the celestial sphere, has a magnitude of minus 1.47 m. The modern scale also allows us to obtain values for the Moon and the Sun: the full moon has a magnitude of -12.6 m, and the Sun -26.8 m. The Hubble orbital telescope can observe objects whose brightness is up to approximately 31.5 m.
Magnitude scale
(the scale is reversed: lower values correspond to brighter objects)
Apparent magnitudes of some celestial bodies
Sun: -26.73
Moon (full moon): -12.74
Venus (at maximum brightness): -4.67
Jupiter (at maximum brightness): -2.91
Sirius: -1.44
Vega: 0.03
Faintest stars visible to the naked eye: about 6.0
Sun from 100 light years away: 7.30
Proxima Centauri: 11.05
Brightest quasar: 12.9
The faintest objects photographed by the Hubble telescope: 31.5
Stellar magnitude is a numerical characteristic of an object in the sky, most often a star, showing how much light comes from it to the point where the observer is located.
Visible (visual)
The modern concept of apparent magnitude is made to correspond to the magnitudes assigned to stars by the ancient Greek astronomer Hipparchus in the 2nd century BC. e. Hipparchus divided all stars into six magnitudes. He called the brightest stars of the first magnitude, the dimmest stars of the sixth magnitude. He distributed the intermediate values evenly among the remaining stars.
The apparent magnitude of the star depends not only on how much light the object emits, but also on how far it is from the observer. Apparent magnitude is considered a unit of measurement shine stars, and the greater the brilliance, the smaller the magnitude, and vice versa.
In 1856, N. Pogson proposed a formalization of the magnitude scale. The apparent magnitude is determined by the formula:
Where I- luminous flux from the object, C- constant.
Since this scale is relative, its zero point (0 m) is defined as the brightness of a star whose luminous flux is equal to 10³ quanta /(cm² s Å) in green light (UBV scale) or 10 6 quanta /(cm²· s·Å) in the entire visible range of light. A star 0 m outside the Earth's atmosphere creates an illumination of 2.54·10 −6 lux.
The magnitude scale is logarithmic, since changes in brightness by the same number of times are perceived as the same (Weber-Fechner law). Moreover, since Hipparchus decided that the magnitude of topics less than a star brighter, then the formula contains a minus sign.
The following two properties help to use apparent magnitudes in practice:
- An increase in luminous flux by 100 times corresponds to a decrease in apparent stellar magnitude by exactly 5 units.
- A decrease in stellar magnitude by one unit means an increase in the luminous flux by 10 1/2.5 = 2.512 times.
Nowadays, apparent magnitude is used not only for stars, but also for other objects, such as the Moon and Sun and planets. Because they can be brighter than the brightest star, they can have a negative apparent magnitude.
The apparent magnitude depends on the spectral sensitivity of the radiation receiver (eye, photoelectric detector, photographic plate, etc.)
- Visual magnitude ( V or m v ) is determined by the sensitivity spectrum of the human eye (visible light), which has a maximum sensitivity at a wavelength of 555 nm. or photographically with an orange filter.
- Photographic or “blue” magnitude ( B or m p ) is determined by photometrically measuring the image of the star on a photographic plate sensitive to blue and ultraviolet rays, or using an antimony-cesium photomultiplier with a blue filter.
- Ultraviolet magnitude ( U) has a maximum in the ultraviolet at a wavelength of about 350 nm.
Differences in magnitudes of one object in different ranges U−B And B−V are integral indicators of the color of an object; the larger they are, the redder the object is.
- Bolometric the magnitude corresponds to the total radiation power of the star, i.e., the power summed over the entire radiation spectrum. To measure it, a special device is used - a bolometer.
absolute
Absolute magnitude (M ) is defined as the apparent magnitude of an object if it were located at a distance of 10 parsecs from the observer. The absolute bolometric magnitude of the Sun is +4.7. If the apparent magnitude and distance to the object are known, the absolute magnitude can be calculated using the formula:
Where d 0
= 10 pc ≈ 32.616 light years.
Accordingly, if the apparent and absolute magnitudes are known, the distance can be calculated using the formula 
The absolute magnitude is related to luminosity by the following relationship: 
where and are the luminosity and absolute magnitude of the Sun.
Magnitudes of some objects
| An object | m |
| Sun | −26,7 |
| Full Moon | −12,7 |
| Iridium Flash (maximum) | −9,5 |
| Supernova 1054 (maximum) | −6,0 |
| Venus (maximum) | −4,4 |
| Earth (looking from the Sun) | −3,84 |
| Mars (maximum) | −3,0 |
| Jupiter (maximum) | −2,8 |
| International space station(maximum) | −2 |
| Mercury (maximum) | −1,9 |
| Andromeda Galaxy | +3,4 |
| Proxima Centauri | +11,1 |
| The brightest quasar | +12,6 |
| The faintest stars visible to the naked eye | +6 to +7 |
| Faintest object captured by an 8-meter ground-based telescope | +27 |
| Faintest object captured by the Hubble Space Telescope | +30 |
| An object | Constellation | m |
| Sirius | Big dog | −1,47 |
| Canopus | Keel | −0,72 |
| α Centauri | Centaurus | −0,27 |
| Arcturus | Bootes | −0,04 |
| Vega | Lyra | 0,03 |
| Chapel | Auriga | +0,08 |
| Rigel | Orion | +0,12 |
| Procyon | Small dog | +0,38 |
| Achernar | Eridanus | +0,46 |
| Betelgeuse | Orion | +0,50 |
| Altair | Eagle | +0,75 |
| Aldebaran | Taurus | +0,85 |
| Antares | Scorpion | +1,09 |
| Pollux | Twins | +1,15 |
| Fomalhaut | Southern fish | +1,16 |
| Deneb | Swan | +1,25 |
| Regulus | a lion | +1,35 |
The sun from different distances
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 visible to the naked eye is not as large 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 beyond the horizon. But even this amount is enough to notice how the stars differ in their brightness. Ancient scientists also noticed this. The ancient Greek mathematician and astronomer Hipparchus, who lived in the 2nd 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, astronomy allows us to observe countless stars, most of which are so dim that they cannot be observed with the naked eye. And the very concept of stellar magnitude is used not only for distant stars, but also for other objects - the Sun, Moon, artificial satellites, planets and so on. Therefore, it is believed that 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 magnitude of the Sun is –26.7, and the magnitude of the closest star to our star, but not visible to the naked eye, is +11.1. The maximum magnitude of Mars is? 2.91. The Mayak satellite, which young Russian scientists created and plan to send into orbit, is planned to have a magnitude of no more than ?10. And if everything succeeds, it will for some time become the brightest object in the night sky, unless, of course, you count the Moon at full moon (? 12.74).
Absolute magnitude
Deneb is one of the most big stars, known to science, has a magnitude of +1.25. Its diameter is approximately equal to the diameter of the Earth's orbit and 110 times greater than the diameter of the Sun. The distance to this giant is 1,640 light years. Although scientists are still arguing on this issue, this is too far off. 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 both on the luminosity of the object and on the distance to it. To be able to compare the luminosity of different stars with each other, absolute magnitude is used. For stars, it is defined as the apparent magnitude of an 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 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 an absolute magnitude of 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, it's simple. Over the course of a year, the position of a star in the sky changes due to the Earth's orbit around the Sun. This change is called the annual parallax of the star. The closer a star is to us, the greater its displacement against the background of stars that are further away. But even for nearby stars this shift is extremely small. The inability to detect parallax in stars was at one time one of the arguments against the heliocentric system of the world. It was possible to do this only in the 19th century. Nowadays, special space telescopes are launched into orbit to measure parallaxes, and therefore distances to stars. The European Space Agency's Hipparcos telescope (named after the same Hipparchus who classified stars by brightness) measured the parallaxes of more than 100 thousand stars. In December 2013, its successor Gaia was launched into orbit.
Parallactic displacement of nearby stars against the background of distant ones
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. Significant for photography. Parallax is measured in arcseconds (arcseconds).
Light year
Measure distances in outer space kilometers are not at all convenient. For example, the distance to the closest star to us, Proxima Centauri? 4.01?1013 kilometers (40.1 trillion kilometers). It is quite difficult to imagine this distance. But if you measure this distance in light years, a unit of length equal to the distance light travels in one year, you get 4.2 light years. The light from this red dwarf takes about 4 years and 3 months to reach us. It's simple.
Parsec
But with another unit of length used in astronomy, not everything is 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 an arc second equal to 1/3600 of a degree, think of a school protractor). The same parallax thanks to which we can measure distances to stars. Parsec (denoted "pc")? this is the distance from which a segment of one astronomical unit (the radius of the earth's orbit) perpendicular to the beam view, visible at an angle of one arcsecond.
Galaxy sleeve
Our Milky Way has a diameter of 100,000 light years. 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 can 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 sleeve is structural element spiral galaxy. The bulk of stars, dust and gas are contained in the galactic arms.
Galactic sleeves Milky Way
There are several such arms, but the main ones are the Sagittarius arm, the Cygnus arm, the Perseus arm, the Centaurus arm and the Orion arm. They received such names from the names of the constellations in which the main array of arms can be observed. The Orion Arm is small compared to others. Sometimes it is even called Orion's Spur. 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, which revolves around it and is our home, are located precisely in it.
Apocenter and periapsis
Most of the known orbits of artificial satellites and celestial bodies are elliptical. And for any elliptical orbit you can always indicate the point closest to the central body and most distant from it. The closest point is called the periapsis, and the most distant point is called the apocenter.
Apocenter (right) and periapsis (left)
But, as a rule, instead of the word “center”, after “peri-” or “apo-”, the name of the body around which the movement occurs is substituted. Thus, 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 the cislunar (Moon - Selene) orbit, apopulations and periselenions are sometimes used. The point in the orbit of our planet or another planet closest to the Sun (Helios) celestial body The solar system is perihelion, the distant one is aphelion or apohelion. For orbits around other stars (astron - star) - periastron and apoaster.
Astronomical unit
The perihelion of our planet's orbit (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 measuring in kilometers is already inconvenient, and in light years and parsecs is still inconvenient. This unit of measurement is called an “astronomical unit” (denoted “au”) and is used to determine the distances between objects of the Solar system, extrasolar systems, as well as between components double stars. After several clarifications, the astronomical unit was recognized as equal to 149597870.7 kilometers.
Thus, the Earth is removed from the Sun at a distance of 1 a. That is, Neptune, the farthest planet from the Sun, at a distance of about 30 a. e. The distance from the Sun to the closest planet to it - Mercury - is only 0.39 a. e. And at the time of the next great confrontation between Mars and Earth, on July 27, 2018, the distance between the planets will be reduced to 0.386 AU. e.
Roche limit
There is nothing permanent in space. It just takes millions of years to change the order we are used to. So, if an observer observes Mars in a few million years, he may not detect one or even two of its satellites. As is known, the largest of the red planet’s satellites, Phobos, approaches it by 1.8 meters per century. Phobos moves at a distance of only about 9,000 km from Mars. For comparison, the orbits of navigation satellites are at an altitude of 19,400–23,222 km, the geostationary orbit is 35,786 km, and the Moon, natural satellite our planet, is located at a distance of 385,000 km from Earth.
Another 10–11 million years will pass, and Phobos will pass its Roche limit, resulting in destruction. The Roche limit, named after Edouard Roche, who first calculated 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. As it was established, the gravitational force of the planet is compensated by the centrifugal force only at the center of mass of the satellite. At other points on 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 passing the Roche limit, it is torn apart by them. But the orbit of the red planet’s other satellite, Deimos (orbital altitude about 23,500 km), is getting further and further each time. Sooner or later he will overcome the gravity of Mars and go on an independent journey through solar system.
Laniakea
Can you tell where in the Universe our planet is located? Of course, planet Earth is located in the Solar System, which, in turn, is located in the Orion Arm - a small galactic arm of the Milky Way. So what next? Our Galaxy, the closest Andromeda galaxy, the Triangulum galaxy and more than 50 other galaxies are part of the so-called Local Group of galaxies, which is a component of the Virgo supercluster.
Laniakea and the Milky Way
But the Virgo supercluster, also called the Local supercluster of galaxies, the Hydra-Centauri and Pavonis-Indian superclusters, as well as the Southern supercluster form a supercluster of galaxies called Laniakea. It contains approximately 100 thousand galaxies. The diameter of Laniakea is 500 million light years. For comparison, the diameter of our Galaxy is only 100 thousand light years. Translated from Hawaiian, Laniakea means “immense heaven.” Which, in general, accurately reflects the fact that in the foreseeable future we are unlikely to be able to fly to the edge of these “heavens”.
Laniakea and the nearby Perseus-Pisces supercluster of galaxies
Each of these stars has a certain magnitude that allows them to be seen
Stellar magnitude is a numerical dimensionless quantity that characterizes the brightness of a star or other cosmic body in relation to the visible area. In other words, this value reflects the amount electromagnetic waves, body, which are registered by the observer. Therefore, this value depends on the characteristics of the observed object and the distance from the observer to it. The term covers only the visible, infrared and ultraviolet spectra electromagnetic radiation.
The term “gloss” is also used to refer to point light sources, and “brightness” to extended ones.
An ancient Greek scientist who lived in Turkey in the 2nd century BC. e., is considered one of the most influential astronomers of antiquity. He compiled a volumetric one, the first in Europe, describing the locations of more than a thousand celestial bodies. Hipparchus also introduced such a characteristic as stellar magnitude. Observing the stars with the naked eye, the astronomer decided to divide them by brightness into six magnitudes, where the first magnitude is the brightest object, and the sixth is the dimmest.
In the 19th century, British astronomer Norman Pogson improved the scale for measuring stellar magnitudes. He expanded the range of its values and introduced a logarithmic dependence. That is, with an increase in magnitude by one, the brightness of the object decreases by 2.512 times. Then a star of 1st magnitude (1 m) is a hundred times brighter than a star of 6th magnitude (6 m).
Magnitude standard
The standard of a celestial body with zero magnitude was initially taken to be the brightness of the brightest point in . Somewhat later, a more precise definition of an object of zero magnitude was outlined - its illumination should be equal to 2.54·10 −6 lux, and the luminous flux in the visible range should be 10 6 quanta/(cm²·s).
Ulugbek
The characteristic described above, which was defined by Hipparchus of Nicea, subsequently began to be called “visible” or “visual”. This means that it can be observed both with the help of human eyes in the visible range, and using various instruments such as a telescope, including ultraviolet and infrared ranges. The magnitude of the constellation is 2 m. However, we know that Vega with zero magnitude (0 m) is not the brightest star in the sky (fifth in brightness, third for observers from the CIS). Therefore, brighter stars may have a negative magnitude, for example (-1.5 m). It is also known today that among the celestial bodies there can be not only stars, but also bodies that reflect the light of stars - planets, comets or asteroids. The total magnitude is −12.7 m.
Absolute magnitude and luminosity
In order to be able to compare the true brightness of cosmic bodies, such a characteristic as absolute stellar magnitude was developed. According to it, the value of the apparent magnitude of an object is calculated if this object were located 10 (32.62) from the Earth. In this case, there is no dependence on the distance to the observer when comparing different stars.
Absolute magnitude for space objects uses a different distance from the body to the observer. Namely, 1 astronomical unit, while, in theory, the observer should be at the center of the Sun.
A more modern and useful quantity in astronomy has become “luminosity”. This characteristic determines the total radiation emitted by a cosmic body over a certain period of time. The absolute magnitude is used to calculate it.
Spectral dependence
As stated earlier, magnitude can be measured for various types electromagnetic radiation, and therefore has different meanings for each spectrum range. To obtain an image of any cosmic object, astronomers can use , which are more sensitive to the high-frequency part of visible light, and the stars appear blue in the image. This magnitude is called “photographic”, m Pv. To obtain a value close to visual (“photovisual”, m P), the photographic plate is coated with a special orthochromatic emulsion and a yellow filter is used.
Scientists have compiled a so-called photometric system of ranges, thanks to which it is possible to determine the main characteristics of cosmic bodies, such as: surface temperature, degree of light reflection (albedo, not for stars), degree of light absorption and others. To do this, photographs are taken of the luminary in different spectra of electromagnetic radiation and subsequent comparison of the results. The most popular filters for photography are ultraviolet, blue (photographic magnitude) and yellow (close to the photovisual range).
A photograph with captured energies of all ranges of electromagnetic waves determines the so-called bolometric magnitude (mb). With its help, knowing the distance and degree of interstellar absorption, astronomers calculate the luminosity of a cosmic body.
Magnitudes of some objects
- Sun = −26.7 m
- Full Moon = −12.7 m
- Iridium flare = −9.5 m. Iridium is a system of 66 satellites that orbit the Earth and serve to transmit voice and other data. Periodically, the surface of each of the three main apparatuses glows sunlight towards the Earth, creating the brightest smooth flash in the sky for up to 10 seconds.
