Apparent magnitude

apparent visual magnitudemagnitudevisual magnitudemagnitudesapparent brightnessmagvisible magnitudestellar magnitudebrightnessm
The apparent magnitude () of an astronomical object is a number that is a measure of its brightness as seen by an observer on Earth.wikipedia
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Sirius

SothisDog StarSirius B
The brighter an object appears, the lower its magnitude value (i.e. inverse relation), with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46.
With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star.

List of brightest stars

brightest starsbrightest starone of the brightest stars
The brightest stars in the night sky were said to be of first magnitude ( = 1), whereas the faintest were of sixth magnitude ( = 6), which is the limit of human visual perception (without the aid of a telescope).
This is a list of the brightest stars down to magnitude +2.50, as determined by their maximum, total, or combined visual magnitudes as viewed from Earth.

Magnitude (astronomy)

magnitudemagnitudesmag
The brighter an object appears, the lower its magnitude value (i.e. inverse relation), with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46.
Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude.

First magnitude star

brightestfirst magnitudefirst magnitude stars
The brightest stars in the night sky were said to be of first magnitude ( = 1), whereas the faintest were of sixth magnitude ( = 6), which is the limit of human visual perception (without the aid of a telescope).
First magnitude stars are the brightest stars in the night sky, with apparent magnitudes lower than +1.50.

N. R. Pogson

Norman PogsonNorman R. PogsonPogson
In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star that is 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today.
He introduced a mathematical scale of stellar magnitudes with the ratio of two successive magnitudes being the fifth root of one hundred (~2.512) and referred to as Pogson's ratio.

Sun

solarSolThe Sun
What is the ratio in brightness between the Sun and the full Moon?
The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74.

Photometry (astronomy)

photometricphotometryphotometrically
The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry. For example, photometry on closely separated double stars may only be able to produce a measurement of their combined light output.
Also, accurate photometry is usually more difficult when the apparent brightness of the object is fainter.

Vega

2828Botercadentconstellation of Vega
Astronomers later discovered that Polaris is slightly variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
This apparent magnitude is a numerical value that decreases in value with increasing brightness of the star.

Logarithmic scale

logarithmiclogarithmic unitlog
The magnitude scale is logarithmic.
stellar magnitude scale for brightness of stars

Betelgeuse

α OriBetelg'''euseBetelgeuse mass loss
On early 20th century and older orthochromatic (blue-sensitive) photographic film, the relative brightnesses of the blue supergiant Rigel and the red supergiant Betelgeuse irregular variable star (at maximum) are reversed compared to what human eyes perceive, because this archaic film is more sensitive to blue light than it is to red light.
It is distinctly reddish, and is a semiregular variable star whose apparent magnitude varies between 0.0 and 1.3, the widest range of any first-magnitude star.

Almagest

cataloghis book on astronomyMagna Syntaxis
This rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is generally believed to have originated with Hipparchus.
Books VII and VIII cover the motions of the fixed stars, including precession of the equinoxes. They also contain a star catalogue of 1022 stars, described by their positions in the constellations, together with ecliptic longitude and latitude. Ptolemy states that the longitudes (which increase due to precession) are for the beginning of the reign of Antoninus Pius (138 AD), whereas the latitudes do not change with time. (But see below, under The star catalog.) The constellations north of the zodiac and the northern zodiac constellations (Aries through Virgo) are in the table at the end of Book VII, while the rest are in the table at the beginning of Book VIII. The brightest stars were marked first magnitude (m = 1), while the faintest visible to the naked eye were sixth magnitude (m = 6). Each numerical magnitude was considered twice the brightness of the following one, which is a logarithmic scale. (The ratio was subjective as no photodetectors existed.) This system is believed to have originated with Hipparchus. The stellar positions too are of Hipparchan origin, despite Ptolemy's claim to the contrary.

Brightness

brightintensitybrilliant
The apparent magnitude () of an astronomical object is a number that is a measure of its brightness as seen by an observer on Earth.
With regard to stars, brightness is quantified as apparent magnitude and absolute magnitude.

Photographic magnitude

Magnitudes obtained from this method are known as photographic magnitudes, and are now considered obsolete.
Before the advent of photometers which accurately measure the brightness of astronomical objects, the apparent magnitude of an object was obtained by taking a picture of it with a camera.

Phase curve (astronomy)

phase curvephase curvesphase function
For planets and other Solar System bodies the apparent magnitude is derived from its phase curve and the distances to the Sun and observer.
The brightness usually refers the object's absolute magnitude, which, in turn, is its apparent magnitude at a distance of one astronomical unit from the Earth and Sun.

Venus

Morning Starevening starCytherocentric
It is the second-brightest natural object in the night sky after the Moon, reaching an apparent magnitude of −4.6 – bright enough to cast shadows at night and, rarely, visible to the naked eye in broad daylight.

Double star

visual companiondouble starsoptical double
For example, photometry on closely separated double stars may only be able to produce a measurement of their combined light output.
Since that time, the search has been carried out thoroughly and the entire sky has been examined for double stars down to a limiting apparent magnitude of about 9.0.

R136a1

In 1960, a group of astronomers working at the Radcliffe Observatory in Pretoria made systematic measurements of the brightness and spectra of bright stars in the Large Magellanic Cloud.

Jupiter

Jovianplanet JupiterGiove
When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, bright enough for its reflected light to cast shadows, and making it on average the third-brightest natural object in the night sky after the Moon and Venus.

Mars

Martianplanet MarsRed Planet
Its apparent magnitude reaches −2.94, which is surpassed only by Jupiter, Venus, the Moon, and the Sun.

SN 1006

supernova of 1006supernova that occurred in 1006 CEthe 1006 supernova
SN 1006 was a supernova that is likely the brightest observed stellar event in recorded history, reaching an estimated −7.5 visual magnitude, and exceeding roughly sixteen times the brightness of Venus.

Weber–Fechner law

Weber's lawis non-linearFechner scale
In Pogson's time this was thought to be true (see Weber–Fechner law), but it is now believed that the response is a power law (see Stevens' power law).
The eye senses brightness approximately logarithmically over a moderate range (but more like a power law over a wider range), and stellar magnitude is measured on a logarithmic scale.

GRB 080319B

The burst set a new record for the farthest object that was observable with the naked eye: it had a peak visual apparent magnitude of 5.3 and remained visible to human eyes for approximately 30 seconds.

Eta Carinae

η Carinaeη CarEta Carinae A
Previously a 4th-magnitude star, it brightened in 1837 to become brighter than Rigel marking the start of the Great Eruption. Eta Carinae became the second-brightest star in the sky between 11 and 14 March 1843 before fading well below naked eye visibility after 1856.

Canopus

α Carinaea first magnitude starCanopean
Canopus' visual magnitude is −0.74, and it has an absolute magnitude of −5.71.

Generalized continued fraction

continued fractionscontinued fractionfundamental recurrence formulas
. This figure, the fifth root of 100, became known as Pogson's Ratio.
Pogson's ratio (100 1/5 or 5√100 ≈ 2.511886...), with x = 5, y = 75 and 2z - y = 6325: