# Apparent magnitude

The apparent magnitude (m) of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value (i.e. inverse relation). The Sun, at apparent magnitude of −26.7, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere. Furthermore, the magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5100, or about 2.512.

The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry. Apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is usually measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V ("visual") filter band would be denoted either as mV or often simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object.

## History

Visible to
typical
human
eye[1]
Apparent
magnitude
Bright-
ness
relative
to Vega
Number of stars
brighter than
apparent magnitude[2]
in the night sky
Yes −1.0250%1 (Sirius)
0.0100%4
1.040%15
2.016%48
3.06.3%171
4.02.5%513
5.01.0%1602
6.00.4%4800
6.50.25%9100[3]
No 7.00.16%14000
8.00.063%42000
9.00.025%121000
10.00.010%340000

The scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the night sky were said to be of first magnitude (m = 1), whereas the faintest were of sixth magnitude (m = 6), which is the limit of human visual perception (without the aid of a telescope). Each grade of magnitude was considered twice the brightness of the following grade (a logarithmic scale), although that ratio was subjective as no photodetectors existed. 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.

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. This implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio.[4] The zero point of Pogson's scale was originally defined by assigning Polaris a magnitude of exactly 2. 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.

Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution (SED) closely approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess presumably due to a circumstellar disk consisting of dust at warm temperatures (but much cooler than the star's surface). At shorter (e.g. visible) wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance (usually expressed in janskys) for the zero magnitude point, as a function of wavelength, can be computed.[5] Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.

With the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30 (for detectable measurements). The brightness of Vega is exceeded by four stars in the night sky at visible wavelengths (and more at infrared wavelengths) as well as the bright planets Venus, Mars, and Jupiter, and these must be described by negative magnitudes. For example, Sirius, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other very bright astronomical objects can be found in the table below.

Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The most widely used is the AB magnitude system,[6] in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be approximately equal in the V filter band.

## Calculations

As the amount of light actually received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. The dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by

which is more commonly expressed in terms of common (base-10) logarithms as

where Fx is the observed flux density using spectral filter x, and Fx,0 is the reference flux (zero-point) for that photometric filter. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5100 2.512 (Pogson's ratio). Inverting the above formula, a magnitude difference m1m2 = Δm implies a brightness factor of

### Example: Sun and Moon

What is the ratio in brightness between the Sun and the full Moon?

The apparent magnitude of the Sun is −26.74 (brighter), and the mean apparent magnitude of the full moon is −12.74 (dimmer).

Difference in magnitude:

Brightness factor:

The Sun appears about 400000 times brighter than the full moon.

Sometimes one might wish to add brightnesses. For example, photometry on closely separated double stars may only be able to produce a measurement of their combined light output. How would we reckon the combined magnitude of that double star knowing only the magnitudes of the individual components? This can be done by adding the brightnesses (in linear units) corresponding to each magnitude.[7]

Solving for yields

where mf is the resulting magnitude after adding the brightnesses referred to by m1 and m2.

### Absolute magnitude

Since flux decreases with distance according to the inverse-square law, a particular apparent magnitude could equally well refer to a star at one distance, or a star four times brighter at twice that distance, and so on. When one is not interested in the brightness as viewed from Earth, but the intrinsic brightness of an astronomical object, then one refers not to the apparent magnitude but the absolute magnitude. The absolute magnitude M, of a star or astronomical object is defined as the apparent magnitude it would have as seen from a distance of 10 parsecs (about 32.6 light-years). The absolute magnitude of the Sun is 4.83 in the V band (yellow) and 5.48 in the B band (blue).[8] In the case of a planet or asteroid, the absolute magnitude H rather means the apparent magnitude it would have if it were 1 astronomical unit from both the observer and the Sun, and fully illuminated (a configuration that is only theoretically achievable, with the observer situated on the surface of the Sun).

## Standard reference values

Standard apparent magnitudes and fluxes for typical bands[9]
Band λ
(μm)
Δλ/λ
(FWHM)
Flux at m = 0, Fx,0
Jy 10−20 erg/(s·cm2·Hz)
U0.360.1518101.81
B0.440.2242604.26
V0.550.1636403.64
R0.640.2330803.08
I0.790.1925502.55
J1.260.1616001.60
H1.600.2310801.08
K2.220.236700.67
L3.50
g0.520.1437303.73
r0.670.1444904.49
i0.790.1647604.76
z0.910.1348104.81

It is important to note that the scale is logarithmic: the relative brightness of two objects is determined by the difference of their magnitudes. For example, a difference of 3.2 means that one object is about 19 times as bright as the other, because Pogson's Ratio raised to the power 3.2 is approximately 19.05.

A common misconception is that the logarithmic nature of the scale is because the human eye itself has a logarithmic response. 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).[10]

Magnitude is complicated by the fact that light is not monochromatic. The sensitivity of a light detector varies according to the wavelength of the light, and the way it varies depends on the type of light detector. For this reason, it is necessary to specify how the magnitude is measured for the value to be meaningful. For this purpose the UBV system is widely used, in which the magnitude is measured in three different wavelength bands: U (centred at about 350 nm, in the near ultraviolet), B (about 435 nm, in the blue region) and V (about 555 nm, in the middle of the human visual range in daylight). The V band was chosen for spectral purposes and gives magnitudes closely corresponding to those seen by the light-adapted human eye, and when an apparent magnitude is given without any further qualification, it is usually the V magnitude that is meant, more or less the same as visual magnitude.

Because cooler stars, such as red giants and red dwarfs, emit little energy in the blue and UV regions of the spectrum their power is often under-represented by the UBV scale. Indeed, some L and T class stars have an estimated magnitude of well over 100, because they emit extremely little visible light, but are strongest in infrared.

Measures of magnitude need cautious treatment and it is extremely important to measure like with like. 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. Magnitudes obtained from this method are known as photographic magnitudes, and are now considered obsolete.

For objects within the Milky Way with a given absolute magnitude, 5 is added to the apparent magnitude for every tenfold increase in the distance to the object. For objects at very great distances (far beyond the Milky Way), this relationship must be adjusted for redshifts and for non-Euclidean distance measures due to general relativity.[11][12]

For planets and other Solar System bodies the apparent magnitude is derived from its phase curve and the distances to the Sun and observer.

## Table of notable celestial objects

Apparent visual magnitudes of known celestial objects
App.
mag.
(V)
Celestial object ... ... as seen from
−67.57gamma-ray burst GRB 080319Bas seen from 1 AU.
−40.07star Zeta1 Scorpiias seen from 1 AU.
−38.00star Rigelas seen from 1 AU. It would be seen as a large very bright bluish disk of 35° apparent diameter.
−30.30star Siriusas seen from 1 AU.
−29.30star Sunas seen from Mercury at perihelion
−27.40star Sunas seen from Venus at perihelion
−26.74star Sunas seen from Earth (about 400,000 times brighter than mean full moon)[13]
−25.60star Sunas seen from Mars at aphelion
−25.00Minimum brightness that causes the typical eye slight pain to look at
−23.00star Sunas seen from Jupiter at aphelion
−21.70star Sunas seen from Saturn at aphelion
−20.20star Sunas seen from Uranus at aphelion
−19.30star Sunas seen from Neptune
−18.20star Sunas seen from Pluto at aphelion
−16.70star Sunas seen from Eris at aphelion
−14.20An illumination level of 1 lux[14][15]
−12.90full moonmaximum brightness of perigee+perihelion full moon (mean distance value is −12.74,[16] though both values are about 0.18 magnitude brighter when including the opposition effect)
−11.20star Sunas seen from Sedna at aphelion
−10.00Comet Ikeya–Seki (1965)which was the brightest Kreutz Sungrazer of modern times[17]
−9.50Iridium (satellite) flaremaximum brightness
−7.50supernova of 1006the brightest stellar event in recorded history (7200 light-years away)[18]
−6.50The total integrated magnitude of the night sky as seen from Earth
−6.00Crab Supernova of 1054(6500 light-years away)[19]
−5.90International Space Stationwhen the ISS is at its perigee and fully lit by the Sun[20]
−4.89planet Venusmaximum brightness [21] when illuminated as a crescent
−4Faintest objects observable during the day with naked eye when Sun is high
−3.99star Epsilon Canis Majorismaximum brightness of 4.7 million years ago, the historical brightest star of the last and next five million years
−3.82planet Venusminimum brightness when it is on the far side of the Sun
−2.94planet Jupitermaximum brightness [22]
−2.91planet Marsmaximum brightness[23]
−2.5Faintest objects visible during the day with naked eye when Sun is less than 10° above the horizon
−2.50new moonminimum brightness
−2.45planet Mercurymaximum brightness at superior conjunction (unlike Venus, Mercury is at its brightest when on the far side of the Sun, the reason being their different phase curves)
−1.61planet Jupiterminimum brightness
−1.47star SiriusBrightest star (except for the Sun) at visible wavelengths[24]
−0.83star Eta Carinaeapparent brightness as a supernova impostor in April 1843
−0.72star Canopus2nd brightest star[25]
−0.49planet Saturnmaximum brightness at opposition and perihelion when the rings are full open (2003)
−0.3Halley's cometExpected apparent magnitude at 2061 passage
−0.27star system Alpha Centauri ABThe total magnitude (3rd brightest star to the naked eye)
−0.04star Arcturus4th brightest star to the naked eye [26]
−0.01star Alpha Centauri A4th brightest individual star visible telescopically in the sky
+0.03star Vegawhich was originally chosen as a definition of the zero point[27]
+0.50star Sunas seen from Alpha Centauri
+1.47planet Saturnminimum brightness
+1.84planet Marsminimum brightness
+3.03supernova SN 1987Ain the Large Magellanic Cloud (160,000 light-years away)
3...4Faintest stars visible in an urban neighborhood with naked eye
3.44Andromeda GalaxyM31[28]
4.38moon Ganymedemaximum brightness[29] (moon of Jupiter and the largest moon in the Solar System)
4.50open cluster M41an open cluster that may have been seen by Aristotle[30]
5.20asteroid Vestamaximum brightness
5.32planet Uranusmaximum brightness[31]
5.72spiral galaxy M33which is used as a test for naked eye seeing under dark skies[32][33]
5.73planet Mercuryminimum brightness
5.8gamma-ray burst GRB 080319BPeak visual magnitude (the "Clarke Event") seen on Earth on March 19, 2008 from a distance of 7.5 billion light-years.
5.95planet Uranusminimum brightness
6.49asteroid Pallasmaximum brightness
6.5Approximate limit of stars observed by a mean naked eye observer under very good conditions. There are about 9,500 stars visible to mag 6.5.[1]
6.64dwarf planet Ceresmaximum brightness
6.75asteroid Irismaximum brightness
6.90spiral galaxy M81This is an extreme naked-eyetarget that pushes human eyesight and the Bortle scale to the limit[34]
7...8Extreme naked-eye limit, Class 1 on Bortle scale, the darkest skies available on Earth[35]
7.78planet Neptunemaximum brightness[36]
8.02planet Neptuneminimum brightness
8.10moon Titanmaximum brightness of largest moon of Saturn[37][38] mean opposition magnitude 8.4[39]
8.94asteroid 10 Hygieamaximum brightness[40]
9.50Faintest objects visible using common 7×50 binoculars under typical conditions[41]
10.20moon Iapetusmaximum brightness[38], brightest when west of Saturn and takes 40 days to switch sides
12.91quasar 3C 273brightest (luminosity distance of 2.4 billion light-years)
13.42moon TritonMaximum brightness[39]
13.65dwarf planet Plutomaximum brightness[42], 725 times fainter than magnitude 6.5 naked eye skies
15.4centaur Chironmaximum brightness[43]
15.55moon Charonmaximum brightness (the large moon of Pluto)
16.8dwarf planet MakemakeCurrent opposition brightness[44]
17.27dwarf planet HaumeaCurrent opposition brightness of[45]
18.7dwarf planet ErisCurrent opposition brightness
20.7moon Callirrhoe(small ~8 km satellite of Jupiter)[39]
22Faintest objects observable in visible light with a 600 mm (24″) Ritchey-Chrétien telescope with 30 minutes of stacked images (6 subframes at 5 minutes each) using a CCD detector[46]
22.91moon Hydramaximum brightness of Pluto's moon
23.38moon Nixmaximum brightness of Pluto's moon
25.0moon Fenrir(small ~4 km satellite of Saturn)[47]
27.6planet Jupiterif it were located 5,000 AU (750 billion km) from the Sun[48]
27.7Faintest objects observable with an 8-meter class ground-based telescope such as the Subaru Telescope in a 10-hour image[49]
28.2Halley's Cometin 2003 when it was 28 AU from the Sun[50]
28.4asteroid 2003 BH91observed magnitude of ~15-kilometer Kuiper belt object Seen by the Hubble Space Telescope (HST) in 2003, dimmest known directly-observed asteroid.
31.5Faintest objects observable in visible light with Hubble Space Telescope[51]
34Faintest objects observable in visible light with James Webb Space Telescope[52]
35unnamed asteroidexpected magnitude of dimmest known asteroid, a 950-meter Kuiper belt object discovered by the HST passing in front of a star in 2009.[53]
35star LBV 1806-20a luminous blue variable star, expected magnitude at visible wavelengths due to interstellar extinction

Some of the above magnitudes are only approximate. Telescope sensitivity also depends on observing time, optical bandpass, and interfering light from scattering and airglow.

## References

1. "Vmag<6.5". SIMBAD Astronomical Database. Retrieved 2010-06-25.
2. "Magnitude". National Solar Observatory—Sacramento Peak. Archived from the original on 2008-02-06. Retrieved 2006-08-23.
3. Bright Star Catalogue
4. See .
5. Oke, J. B.; Gunn, J. E. (March 15, 1983). "Secondary standard stars for absolute spectrophotometry". The Astrophysical Journal. 266: 713–717. Bibcode:1983ApJ...266..713O. doi:10.1086/160817.
6. "Magnitude Arithmetic". Weekly Topic. Caglow. Retrieved 30 January 2012.
7. Evans, Aaron. "Some Useful Astronomical Definitions" (PDF). Stony Brook Astronomy Program. Retrieved 2009-07-12.
8. Huchra, John. "Astronomical Magnitude Systems". Harvard-Smithsonian Center for Astrophysics. Retrieved 2017-07-18.
9. Schulman, E.; Cox, C. V. (1997). "Misconceptions About Astronomical Magnitudes". American Journal of Physics. 65 (10): 1003. Bibcode:1997AmJPh..65.1003S. doi:10.1119/1.18714.
10. Umeh, Obinna; Clarkson, Chris; Maartens, Roy (2014). "Nonlinear relativistic corrections to cosmological distances, redshift and gravitational lensing magnification: II. Derivation". Classical and Quantum Gravity. 31 (20): 205001. arXiv:. Bibcode:2014CQGra..31t5001U. doi:10.1088/0264-9381/31/20/205001.
11. Hogg, David W.; Baldry, Ivan K.; Blanton, Michael R.; Eisenstein, Daniel J. (2002). "The K correction". arXiv:.
12. Williams, David R. (2004-09-01). "Sun Fact Sheet". NASA (National Space Science Data Center). Archived from the original on 15 July 2010. Retrieved 2010-07-03.
13. Dufay, Jean (2012-10-17). Introduction to Astrophysics: The Stars. p. 3. ISBN 9780486607719.
14. McLean, Ian S. (2008). Electronic Imaging in Astronomy: Detectors and Instrumentation. Springer. p. 529. ISBN 978-3-540-76582-0.
15. Williams, David R. (2010-02-02). "Moon Fact Sheet". NASA (National Space Science Data Center). Archived from the original on 23 March 2010. Retrieved 2010-04-09.
16. "Brightest comets seen since 1935". International Comet Quarterly. Retrieved 18 December 2011.
17. Winkler, P. Frank; Gupta, Gaurav; Long, Knox S. (2003). "The SN 1006 Remnant: Optical Proper Motions, Deep Imaging, Distance, and Brightness at Maximum". The Astrophysical Journal. 585 (1): 324–335. arXiv:. Bibcode:2003ApJ...585..324W. doi:10.1086/345985.
18. "ISS Information - Heavens-above.com". Heavens-above. Retrieved 2007-12-22.
19. "HORIZONS Web-Interface for Venus (Major Body=299)" (2006 February 27 (GEOPHYSICAL DATA)). JPL Horizons On-Line Ephemeris System. Retrieved 2010-11-28. (Using JPL Horizons you can see that on 2013-Dec-08 Venus will have an apmag of −4.89)
20. Williams, David R. (2007-11-02). "Jupiter Fact Sheet". National Space Science Data Center. NASA. Retrieved 2010-06-25.
21. Williams, David R. (2007-11-29). "Mars Fact Sheet". National Space Science Data Center. NASA. Archived from the original on 12 June 2010. Retrieved 2010-06-25.
22. "Sirius". SIMBAD Astronomical Database. Retrieved 2010-06-26.
23. "Canopus". SIMBAD Astronomical Database. Retrieved 2010-06-26.
24. "Arcturus". SIMBAD Astronomical Database. Retrieved 2010-06-26.
25. "Vega". SIMBAD Astronomical Database. Retrieved 2010-04-14.
27. Yeomans; Chamberlin. "Horizon Online Ephemeris System for Ganymede (Major Body 503)". California Institute of Technology, Jet Propulsion Laboratory. Retrieved 2010-04-14. (4.38 on 1951-Oct-03)
28. "M41 possibly recorded by Aristotle". SEDS (Students for the Exploration and Development of Space). 2006-07-28. Retrieved 2009-11-29.
29. Williams, David R. (2005-01-31). "Uranus Fact Sheet". National Space Science Data Center. NASA. Archived from the original on 29 June 2010. Retrieved 2010-06-25.
31. Lodriguss, Jerry (1993). "M33 (Triangulum Galaxy)". Retrieved 2009-11-27. (Shows bolometric magnitude not visual magnitude.)
32. "Messier 81". SEDS (Students for the Exploration and Development of Space). 2007-09-02. Retrieved 2009-11-28.
33. John E. Bortle (February 2001). "The Bortle Dark-Sky Scale". Sky & Telescope. Retrieved 2009-11-18.
34. Williams, David R. (2007-11-29). "Neptune Fact Sheet". National Space Science Data Center. NASA. Archived from the original on 1 July 2010. Retrieved 2010-06-25.
35. Yeomans; Chamberlin. "Horizon Online Ephemeris System for Titan (Major Body 606)". California Institute of Technology, Jet Propulsion Laboratory. Retrieved 2010-06-28. (8.10 on 2003-Dec-30) Archived November 13, 2012, at the Wayback Machine.
36. "Classic Satellites of the Solar System". Observatorio ARVAL. Archived from the original on 31 July 2010. Retrieved 2010-06-25.
37. "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2009-04-03. Archived from the original on 23 July 2009. Retrieved 2009-07-25.
38. "AstDys (10) Hygiea Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2010-06-26.
39. Zarenski, Ed (2004). "Limiting Magnitude in Binoculars" (PDF). Cloudy Nights. Retrieved 2011-05-06.
40. Williams, David R. (2006-09-07). "Pluto Fact Sheet". National Space Science Data Center. NASA. Archived from the original on 1 July 2010. Retrieved 2010-06-26.
41. "AstDys (2060) Chiron Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2010-06-26.
42. "AstDys (136472) Makemake Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2010-06-26.
43. "AstDys (136108) Haumea Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2010-06-26.
44. Steve Cullen (sgcullen) (2009-10-05). "17 New Asteroids Found by LightBuckets". LightBuckets. Archived from the original on 2010-01-31. Retrieved 2009-11-15.
45. Sheppard, Scott S. "Saturn's Known Satellites". Carnegie Institution (Department of Terrestrial Magnetism). Retrieved 2010-06-28.
46. Magnitude difference is 2.512 log10[(5000/5)2 × (4999/4)2] ˜ 30.6, so Jupiter is 30.6 magnitudes fainter at 5000 AU
47. What is the faintest object imaged by ground-based telescopes?, by: The Editors of Sky Telescope, July 24, 2006
48. "New Image of Comet Halley in the Cold". ESO. 2003-09-01. Archived from the original on 1 March 2009. Retrieved 2009-02-22.
49. Illingworth, G. D.; Magee, D.; Oesch, P. A.; Bouwens, R. J.; Labbé, I.; Stiavelli, M.; van Dokkum, P. G.; Franx, M.; Trenti, M.; Carollo, C. M.; Gonzalez, V. (21 October 2013). "The HST eXtreme Deep Field XDF: Combining all ACS and WFC3/IR Data on the HUDF Region into the Deepest Field Ever". The Astrophysical Journal Supplement Series. 209 (1): 6. arXiv:. Bibcode:2013ApJS..209....6I. doi:10.1088/0067-0049/209/1/6.
50. http://www.jaymaron.com/telescopes/telescopes.html (retrieved Sep 14 2017)
51. "NASA - Hubble Finds Smallest Kuiper Belt Object Ever Seen". www.nasa.gov. NASA. Retrieved 16 March 2018.