Each of these stars has a certain magnitude that allows you to see them.
A magnitude is a numerical dimensionless quantity that characterizes the brightness of a star or other cosmic body in relation to the apparent area. In other words, this value reflects the number of electromagnetic waves registered by the body 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 of electromagnetic radiation.
In relation to point sources of light, the term "brilliance" is also used, and for extended ones - "brightness".
An ancient Greek scholar who lived in Turkey in the 2nd century BC. e., is considered one of the most influential astronomers of antiquity. He compiled a volumetric, the first in Europe, describing the location of more than a thousand heavenly bodies. Hipparchus also introduced such a characteristic as a 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, the 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 a factor of 2.512. Then a star of the 1st magnitude (1 m) is a hundred times brighter than a star of the 6th magnitude (6 m).
Magnitude standard
The standard of a celestial body with zero magnitude was initially taken as the brilliance of the brightest point in. Somewhat later, a more accurate definition of an object of zero magnitude was presented - its illumination should be 2.54 10 −6 lux, and the luminous flux in the visible range is 10 6 quanta / (cm² s).
Apparent magnitude
The characteristic described above, which was identified by Hipparchus of Nicaea, later became known as "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 range. 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 (the fifth in brightness, the third for observers from the territory of the CIS). Therefore, brighter stars can have a negative magnitude, for example, (-1.5 m). It is also known today that among the heavenly 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 magnitude was developed. According to it, the value of the apparent stellar magnitude of the 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 in the center of the Sun.
A more modern and useful quantity in astronomy has become "luminosity". This characteristic determines the total that the cosmic body radiates over a certain period of time. For its calculation, the absolute stellar magnitude is just used.
Spectral dependence
As mentioned earlier, the magnitude can be measured for different types of electromagnetic radiation, and therefore has different values for each range of the spectrum. To obtain a picture of any space object, astronomers can use, which are more sensitive to the high-frequency part of visible light, and the stars turn out to be blue in the image. Such a stellar magnitude is called "photographic", m Pv . To get a value close to visual (“photovisual”, m P), the photographic plate is covered with a special orthochromatic emulsion and a yellow light 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, the luminary is photographed in different spectra of electromagnetic radiation and the subsequent comparison of the results. The following filters are most popular for photography: 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 (m b). With its help, knowing the distance and the degree of interstellar extinction, astronomers calculate the luminosity of a cosmic body.
Star magnitudes of some objects
- Sun = -26.7 m
- Full Moon = -12.7 m
- Flash Iridium = -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 vehicles reflects sunlight towards the Earth, creating the brightest smooth flash in the sky for up to 10 seconds.
(denoted by m - from English. magnitude) - a dimensionless quantity characterizing the brightness of a celestial body (the amount of light coming from it) from the point of view of an earthly observer. The brighter an object, the smaller its apparent magnitude.
The word "apparent" in the name only means that the magnitude is observed from the Earth, and is used to distinguish it from the absolute magnitude. This name refers not only to visible light. The quantity that is perceived by the human eye (or other receiver with the same spectral sensitivity) is called visual.
The magnitude is denoted by a small letter m as a superscript to a numerical value. For example, 2 m means the second magnitude.
Story
The concept of magnitude was introduced by the ancient Greek astronomer Hipparchus in the 2nd century BC. He distributed all the stars accessible to the naked eye into six magnitudes: he called the bright stars of the first magnitude, the naytmyanish - the sixth. For intermediate magnitudes, it was believed that, say, stars of the third magnitude, are as dimmer as the stars of the second magnitude, as they are brighter than the stars of the fourth. This method of measuring brilliance gained popularity thanks to the Almagest, the star catalog of Claudius Ptolemy.
Such a classification scale was used almost unchanged until the middle of the 19th century. The first who treated the stellar magnitude as a quantitative rather than a qualitative characteristic was Friedrich Argelander. It was he who began to confidently apply decimal fractions of stellar magnitudes.
1856 Norman Pogson formalized the magnitude scale, establishing that a first magnitude star is exactly 100 times brighter than a sixth magnitude star. Since, in accordance with the Weber-Fechner law, the change in illumination the same number of times perceived by the eye as a change by the same amount then a difference of one magnitude corresponds to a change in light intensity by a factor of ≈ 2.512. This is an irrational number that is called Pogson number.
So, the scale of stellar magnitudes is logarithmic: the difference in stellar magnitudes of two objects is determined by the equation:
, , are the stellar magnitudes of objects, , are the illuminations created by them.This formula makes it possible to determine only the difference in stellar magnitudes, but not the magnitudes themselves. In order to build an absolute scale with its help, it is necessary to set a zero point — illumination, which corresponds to zero magnitude (0 m). At first, Pogson used the North Star as a standard, assuming that it has exactly the second magnitude. After it became clear that Polaris was a variable star, the scale began to be tied to Vega (which was assigned a zero value), and then (when Vega was also suspected of variability), the zero point of the scale was redefined with the help of several other stars. However, for visual observations, Vega can serve as a standard of zero magnitude even further, since its magnitude in visible light is 0.03 m, which does not differ from zero by eye.
The modern magnitude scale is not limited to six magnitudes or just visible light. The magnitude of very bright objects is negative. For example, Sirius, the brightest star in the night sky, has an apparent magnitude of -1.47m. Modern technology also makes it possible to measure the brightness of the Moon and the Sun: the full Moon has an apparent magnitude of -12.6 m, and the Sun -26.8 m. The Hubble Orbital Telescope can observe stars up to 31.5 m in the visible range.
Spectral dependence
The stellar magnitude depends on the spectral range in which the observation is carried out, since the luminous flux from any object in different ranges is different.
- Bolometric magnitude shows the total radiation power of the object, that is, the total flux in all spectral ranges. Bolometer is measured.
The most common photometric system, the UBV system, has 3 bands (spectral ranges in which measurements are made). Accordingly, there are:
- ultraviolet magnitude (U)- determined in the ultraviolet range;
- "Blue" magnitude (B) — is determined in the blue range;
- visual magnitude (V)- is determined in the visible range; the spectral response curve is chosen to better match human vision. The eye is most sensitive to yellow-green light with a wavelength of about 555 nm.
The difference (U-B or B-V) between the magnitudes of the same object in different bands shows its color and is called the color index. The higher the color index, the redder the object.
There are other photometric systems, each of which has different bands and, accordingly, different quantities can be measured. For example, in the old photographic system, the following quantities were used:
- photovisual magnitude (m pv)- a measure of blackening the image of an object on a photographic plate with an orange light filter;
- photographic magnitude (m pg)- measured on a conventional photographic plate, which is sensitive to the blue and ultraviolet ranges of the spectrum.
Apparent stellar magnitudes of some objects
| An object | m |
|---|---|
| The sun | -26,73 |
| Full moon | -12,92 |
| Iridium flare (maximum) | -9,50 |
| Venus (maximum) | -4,89 |
| Venus (minimum) | -3,50 |
| Jupiter (maximum) | -2,94 |
| Mars (maximum) | -2,91 |
| Mercury (maximum) | -2,45 |
| Jupiter (minimum) | -1,61 |
| Sirius (the brightest star in the sky) | -1,47 |
| Canopus (2nd brightest star in the sky) | -0,72 |
| Saturn (maximum) | -0,49 |
| Alpha Centauri cumulative brightness A, B | -0,27 |
| Arcturus (3rd brightest star in the sky) | 0,05 |
| Alpha Centauri A (4th brightest star in the sky) | -0,01 |
| Vega (5th brightest star in the sky) | 0,03 |
| Saturn (minimum) | 1,47 |
| Mars (minimum) | 1,84 |
| SN 1987A - supernova 1987 in the Large Magellanic Cloud | 3,03 |
| Andromeda's nebula | 3,44 |
| Faint stars that are visible in metropolitan areas | 3 … + 4 |
| Ganymede is a moon of Jupiter, the largest moon in the solar system (maximum) | 4,38 |
| 4 Vesta (bright asteroid), at maximum | 5,14 |
| Uranus (maximum) | 5,32 |
| Triangulum Galaxy (M33), visible to the naked eye in clear skies | 5,72 |
| Mercury (minimum) | 5,75 |
| Uranus (minimum) | 5,95 |
| Naymanishi stars visible to the naked eye in the countryside | 6,50 |
| Ceres (maximum) | 6,73 |
| NGC 3031 (M81), visible to the naked eye under perfect skies | 6,90 |
| Nightmanish stars visible to the naked eye in a perfect sky (Mauna Kea Observatory, Atacama Desert) | 7,72 |
| Neptune (maximum) | 7,78 |
| Neptune (minimum) | 8,01 |
| Titan is a moon of Saturn, the 2nd largest moon in the solar system (maximum) | 8,10 |
| Proxima Centauri | 11,10 |
| The brightest quasar | 12,60 |
| Pluto (maximum) | 13,65 |
| Makemake in opposition | 16,80 |
| Haumea in opposition | 17,27 |
| Eris in opposition | 18,70 |
| Faint stars seen in a 24" CCD image with a 30 minute exposure | 22 |
| The smallest object available on the 8-meter ground-based telescope | 27 |
| The smallest object available on the Hubble Space Telescope | 31,5 |
| The smallest object that will be available on the 42-meter ground-based telescope | 36 |
| The smallest object that will be available on the OWL orbiting telescope (launch is scheduled for 2020) | 38 |
Let's continue our algebraic excursion to the heavenly bodies. In the scale that is used to assess the brightness of stars, in addition to fixed stars, other luminaries - planets, the Sun, the Moon - can find a place for themselves. We will talk separately about the brightness of the planets; here we indicate the stellar magnitude of the Sun and Moon. The magnitude of the Sun is expressed as a number minus 26.8, and the full moon - minus 12.6. Why both numbers are negative, the reader must think, is understandable after all that has been said before. But, perhaps, he will be perplexed by the insufficiently large difference between the magnitude of the Sun and the Moon: the first is "only twice as large as the second."
Let's not forget, however, that the designation of magnitude is, in essence, a certain logarithm (based on 2.5). And just as it is impossible, when comparing numbers, to divide their logarithms one by another, so it makes no sense, when comparing stellar magnitudes, to divide one number by another. What is the result of a correct comparison, shows the following calculation.
If the magnitude of the Sun minus 26.8", this means that the Sun is brighter than a star of the first magnitude
2.5 27.8 times.
The moon is brighter than a star of the first magnitude
2.5 13.6 times.
This means that the brightness of the sun is greater than the brightness of the full moon at
Calculating this value (using tables of logarithms), we get 447,000. Here, therefore, is the correct ratio of the brightness of the Sun and the Moon: a daytime star in clear weather illuminates the Earth 447,000 times stronger than the full Moon on a cloudless night.
Considering that the number warmth , allocated by the Moon, is proportional to the amount of light scattered by it - and this is probably close to the truth - it must be admitted that the Moon sends us heat 447,000 times less than the Sun. It is known that each square centimeter at the boundary of the earth's atmosphere receives from the Sun about 2 small calories of heat per minute. This means that the Moon sends to 1 cm 2 of the Earth every minute no more than 225,000th part of a small calorie (that is, it can heat 1 g of water in 1 minute by 225,000th part of a degree). This shows how unsubstantiated all attempts to attribute any influence to the moonlight on the earth's weather.
The common belief that clouds often melt under the action of the rays of the full moon is a gross misconception, explained by the fact that the disappearance of clouds at night (due to other reasons) becomes conspicuous only in moonlight.
Let us now leave the Moon and calculate how many times the Sun is brighter than the most brilliant star in the entire sky - Sirius. Arguing in the same way as before, we obtain the ratio of their brightness:
i.e., the Sun is 10 billion times brighter than Sirius.
The following calculation is also very interesting: how many times is the illumination given by the full moon brighter than the total illumination of the entire starry sky, i.e., all the stars visible to the naked eye in one celestial hemisphere? We have already calculated that stars from the first to the sixth magnitude inclusive shine together like a hundred stars of the first magnitude. The problem, therefore, is reduced to calculating how many times the moon is brighter than a hundred stars of the first magnitude.
This ratio is equal
So, on a clear moonless night, we receive from the starry sky only 2700th of the light that the full moon sends, and 2700 x 447,000, that is, 1200 million times less than the sun gives on a cloudless day.
magnitude
Dimensionless physical quantity characterizing , created by a celestial object near the observer. Subjectively, its meaning is perceived as (y) or (y). In this case, the brightness of one source is indicated by comparing it with the brightness of another, taken as a standard. Such standards are usually specially selected non-variable stars. The magnitude was first introduced as an indicator of the apparent brightness of optical stars, but later extended to other radiation ranges:,. The magnitude scale is logarithmic, as is the decibel scale. In the magnitude scale, a difference of 5 units corresponds to a 100-fold difference in the fluxes of light from the measured and reference sources. Thus, a difference of 1 magnitude corresponds to a ratio of light fluxes of 100 1/5 = 2.512 times. Designate the magnitude of the Latin letter "m"(from Latin magnitudo, value) as a superscript in italics to the right of the number. The direction of the magnitude scale is reversed, i.e. the larger the value, the weaker the brilliance of the object. For example, a star of 2nd magnitude (2 m) is 2.512 times brighter than a 3rd magnitude star (3 m) and 2.512 x 2.512 = 6.310 times brighter than a 4th magnitude star (4 m).Apparent magnitude (m; often referred to simply as "magnitude") indicates the radiation flux near the observer, i.e. the observed brightness of a celestial source, which depends not only on the actual radiation power of the object, but also on the distance to it. The scale of apparent magnitudes originates from the stellar catalog of Hipparchus (before 161 ca. 126 BC), in which all the stars visible to the eye were first divided into 6 classes according to brightness. The stars of the Bucket of the Great Bear have a shine of about 2 m, Vega has about 0 m. For particularly bright luminaries, the magnitude value is negative: for Sirius, about -1.5 m(i.e. the flux of light from it is 4 times greater than from Vega), and the brightness of Venus at some moments almost reaches -5 m(i.e. the light flux is almost 100 times greater than from Vega). We emphasize that the apparent stellar magnitude can be measured both with the naked eye and with the help of a telescope; both in the visual range of the spectrum, and in others (photographic, UV, IR). In this case, "apparent" (English apparent) means "observed", "apparent" and is not specifically related to the human eye (see:).
Absolute magnitude(M) indicates what apparent stellar magnitude the luminary would have if the distance to it were 10 and there would be no . Thus, the absolute stellar magnitude, in contrast to the visible one, allows one to compare the true luminosities of celestial objects (in a given range of the spectrum).
As for the spectral ranges, there are many systems of magnitudes that differ in the choice of a specific measurement range. When observed with the eye (with the naked eye or through a telescope), it is measured visual magnitude(m v). From the image of a star on a conventional photographic plate, obtained without additional light filters, the photographic magnitude(mP). Since photographic emulsion is sensitive to blue light and insensitive to red light, blue stars appear brighter (than it appears to the eye) on the photographic plate. However, with the help of a photographic plate, using orthochromatic and yellow, one obtains the so-called photovisual magnitude scale(m P v), which almost coincides with the visual one. By comparing the brightness of a source measured in different ranges of the spectrum, one can find out its color, estimate the surface temperature (if it is a star) or (if it is a planet), determine the degree of interstellar absorption of light, and other important characteristics. Therefore, standard ones have been developed, mainly determined by the selection of light filters. The most popular tricolor: ultraviolet (Ultraviolet), blue (Blue) and yellow (Visual). At the same time, the yellow range is very close to the photovisual one (B m P v), and blue to photographic (B m P).
