What is Apparent magnitude: Definition and 26 Discussions

Apparent magnitude (m) is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's light caused by interstellar dust along the line of sight to the observer.
The word magnitude in astronomy, unless stated otherwise, usually refers to a celestial object's apparent magnitude. The magnitude scale dates back to the ancient astronomer Ptolemy, whose star catalog listed stars from 1st magnitude (brightest) to 6th magnitude (dimmest). The modern scale was mathematically defined in a way to closely match this historical system.
The scale is reverse logarithmic: the brighter an object is, the lower its magnitude number. A difference of 1.0 in magnitude corresponds to a brightness ratio of 5√100, or about 2.512. For example, a star of magnitude 2.0 is 2.512 times brighter than a star of magnitude 3.0, 6.31 times brighter than a star of magnitude 4.0, and 100 times brighter than one of magnitude 7.0.
The brightest astronomical objects have negative apparent magnitudes: for example, Venus at −4.2 or Sirius at −1.46. The faintest stars visible with the naked eye on the darkest night have apparent magnitudes of about +6.5, though this varies depending on a person's eyesight and with altitude and atmospheric conditions. The apparent magnitudes of known objects range from the Sun at −26.7 to objects in deep Hubble Space Telescope images of magnitude +31.5.The measurement of apparent magnitude is called photometry. Photometric measurements are made in the ultraviolet, visible, or infrared wavelength bands using standard passband filters belonging to photometric systems such as the UBV system or the Strömgren uvbyβ system.
Absolute magnitude is a measure of the intrinsic luminosity of a celestial object rather than its apparent brightness and is expressed on the same reverse logarithmic scale. Absolute magnitude is defined as the apparent magnitude that a star or object would have if it were observed from a distance of 10 parsecs (3.1×1014 kilometres). When referring to just "magnitude", apparent magnitude rather than absolute magnitude is normally intended.

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  1. M

    B Apparent magnitude / absolute magnitude / luminosity

    Hello, When we talk about the luminosity, apparent magnitude or absolute magnitude of a star, are we talking about all wavelengths of EM radiation or just a certain range? I've read that luminosity is basically the power output of all types of radiation, but the apparent magnitude scale is...
  2. R

    B Apparent Magnitude of Moon: Exploring its Impact at 1000x Closer Proximity

    Hello everyone! What would've been the apparent magnitude of the moon if it were 1000 times closer? Can we even talk about an apparent magnitude in the case of a large solid angle?
  3. 2

    Stefan-Boltzmann law, luminosity, brightness and magnitude?

    From what I understand, in the equation P=\sigma AT^4, P is the power output of the star which is the energy radiated per second in EM radiation of all frequencies, and I think luminosity is also defined as the energy radiated per second in EM radiation of all frequencies. Therefore luminosity...
  4. thegirl

    Apparent Magnitude and Intensity Formula

    Hey, Could anyone explain why Ia/Ib=2.512^(Mb-Ma), Where "Ia" and "Ib" are intensities of star a and star b and "Mb" and "Ma" are the apparent magnitudes of star A and star B? I thought the formula would be Ia/Ib=(2.512)^(Ma-Mb) because Ia=2.512^Ma and Ib=2.512^Mb. Thank You
  5. L

    Binary Stars and Apparent Magnitudes

    Homework Statement A binary star system at a distance of 85pc consists of two stars of equal luminosity that are so close together that they are observed in a telescope as a single image. If the apparent magnitude of the two stars combined is 10.7 what would be the apparent magnitude of just...
  6. S

    Flux --> Apparent magnitude ( only 1 source )

    Hi guys, I've seen a lot of textbooks converting between 2 apparent magnitudes to the ratio of 2 fluxes. But I just want to know how to convert 1 ( ONE, UNO , 1, NOT 2 ) flux to 1 ( ONE, UNO, 1, NOT 2 ) 1 Apparent Magnitude. I think I saw my professor wrote this down, but am not sure . Is this...
  7. D

    Apparent Magnitude: Understanding Star X's Brightness

    What does this statement mean: "Apparent magnitude of a star, X, is m." ? m_2 - m_1 = -2.5 log (B_2/B_1) Apparent magnitudes are defined relatively, right? We can talk about differences in apparent magnitudes. If i know the ratio of the brightnesses, i can find out the difference in...
  8. L

    Calculating Apparent Magnitude of The Moon

    Hello everyone, I've been wondering if I can calculate the apparent magnitude of The Moon since I know the apparent magnitude of The Sun(say -27mag) and reflection coefficient of Moon's surface(say 0.12). We know the equation \Delta m = -2.5 log_{10}(I/I_0) Where I is the flux of the...
  9. P

    How to find the distance to a cluster using the apparent magnitude

    Homework Statement The main sequence can be approximated by a relation of the form L\proptoT^5 . In Cluster A, a main sequence star called Star α has the same B – V colour as the Sun, and a V-band apparent magnitude of 11.3. The effective temperature of Star α is 5800K. In Cluster B, a main...
  10. R

    Apparent magnitude of the Moon from Mercury?

    I know there there has to be a simple answer to this, but I can't understand where it's at. Here is the question. Find the apparent magnitude of the Moon [Earth's] as seen from Mercury. Assume Mercury is 0.52 AU from the Moon and that Mercury sees the Moon fully [it's a full moon]. Okay...
  11. S

    How can I calculate the visual magnitude of a star with only its flux data?

    How to calculate the visual magnitude of a star having only the flux of such star? The equation involves 2 magnitudes and 2 fluxes, corresponding to 2 objects a and b: ma-mb= -2.5log10\left( \frac{F_a}{F_b}\right) but I just have Fa and I have to obtain ma. Is it valid to take...
  12. I

    Apparent magnitude of a light bulb

    Homework Statement What is the apparent magnitude of a 100W light bulb at a distance of 3 m? [Hint: Compare with the Sun to eliminate the unknown constant in the expression relating flux density to apparent magnitude.] . The solar luminosity is L \approx 4 \times 10^{26} W. . The Earth-Sun...
  13. M

    Absolute and apparent magnitude

    is that both of absolute and apparent magnitude counts only visible light? since i found some books said absolute magnitude counts luminosity instead of intensity, where luminosity includes also the non-visible frequencies. thank you!
  14. Z

    Apparent magnitude of a binary star system

    Homework Statement A binary star system consists of a hot main sequence star (m=0.5m) and a red giant (m=1.2m), the parallax of the system is p=0.0012". Calculate the apparent magnitude of the system. Homework Equations I have b1/b2 = 2.512^(m2-m1) and p=1/r The Attempt at a...
  15. N

    How to Calculate Combined Apparent Magnitude for Binary Star Systems

    Hi guys, first post (hopefully not only). If 2 stars are in an apparent binary system, how do you calculate the "combined apparent magnitude" of the system if you know both their apparent magnitudes and their distance from you. I can work out the apparent magnitude (even though that's not...
  16. P

    What is the apparent magnitude of the binary system?

    Homework Statement Two stars are in a circular visual binary system. The orbital period of the binary is 30 years. The distance to the binary is 20 parsecs. The angular radius of the orbit of each star is 1". What are the masses of the two stars? each star works out to be 4.4 solar...
  17. T

    Finding the apparent magnitude of the sun, and finding Distances from it.

    Homework Statement During its lifetime a massive star converts about one solar mass of hydrogen into iron in its core, releasing about 0.7 percent of the rest-mass energy in the form of radiation. When it explodes as a supernovae, the outer envelope of the star (with a mass of 10Msun, say)...
  18. D

    Calculating absolute magnitude and apparent magnitude

    Ummm... never mind, we found it... Homework Statement Given Proxima Centauri with parallax angle of 0.769" and apparent bolometric magnitude of 11.1... what is its absolute magnitude?Homework Equations I get that I should use m-M = 5 log10(d/10 pc) and I understand that d = 1/p". What I...
  19. G

    Astrophysics - Apparent Magnitude of stars in a close binary system

    Homework Statement 2 solar type stars are in a close binary system. Each indivual star has an apparent brightness of m = 10 mag. Determine the apparent magnitudes of the combined system of the 2 stars assuming they cannot be resolved as individual objects. Determine the minimum brightness...
  20. G

    Apparent Magnitude of Binary Star System

    I am having trouble understanding this problem. It has several parts describing a binary star system. The last part ask for the apparent magnitude of the combined light from both stars, if they were viewed as a single point. I guess my first thought was to add them, but I'm worried because...
  21. R

    Apparent Magnitude of planets as seen from earth

    I've seen that Mars' apparent magnitude varies between -2.9 and 1.8 and Mercury's from -2.0 and 5.5 (wikipedia said so at least). I'm trying to figure out which planet would have the greatest variation in brightness, as seen from Earth. The numbers at wikipedia points in the direction of...
  22. S

    Apparent magnitude of binary star?

    [SOLVED] Apparent magnitude of binary star? Homework Statement A binary star has a total apparent magnitude of 15.00. One component star is twice as bright as the other. a) Show that the apparent magnitude of the brighter star is 15.44. b) The fainter star has an apparent magnitude of...
  23. B

    Calculate combined apparent magnitude of two stars

    Homework Statement What is the combined apparent magnitude o a binary system consisting of two stars of apparent magnitudes 3.0 and 4.0 Homework Equations m-n=2.5 log(f(m)/f(n)) The Attempt at a Solution I know m= 3 and n=4 ,or vice versa. I'm not sure what this problem means by...
  24. M

    Apparent Magnitude: Meaning & 3 Star System

    What is the meaning of: "the apparent magnitude of a star is obtain with a system of 3 stars"? Thanks
  25. S

    Apparent magnitude of two stars

    Homework Statement Two stars have apparent magnitudes of V = 5.1 and V = 4.6 but are too close together to be resolved with the naked eye and appear to be a single object. What is its apparent magnitude? Homework Equations I don't know what the relevant equations are if there are any...
  26. T

    Apparent Magnitude of Sun at 8000 Parsecs from Galactic Center

    The extinction to our galactic center is about Av=20mag. What would be the apparent magnitude of our sun at the 8000parsec distance of the galactic center? From what I know, I think I need to use http://en.wikipedia.org/wiki/Absolute_magnitude#Computation the luminosity-distance formula. So...