What is Luminosity: Definition and 147 Discussions

Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object.In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical object.In SI units, luminosity is measured in joules per second, or watts. In astronomy, values for luminosity are often given in the terms of the luminosity of the Sun, L⊙. Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (Mbol) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band.
In contrast, the term brightness in astronomy is generally used to refer to an object's apparent brightness: that is, how bright an object appears to an observer. Apparent brightness depends on both the luminosity of the object and the distance between the object and observer, and also on any absorption of light along the path from object to observer. Apparent magnitude is a logarithmic measure of apparent brightness. The distance determined by luminosity measures can be somewhat ambiguous, and is thus sometimes called the luminosity distance.

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  1. R

    Collider Luminosity Calculation

    Homework Statement A collider has an average electron current of 10 milliAmps and an average positron current of 5 milliAmps.If the ring radius is 100 m and there is one electron bunch and one positron bunch with uniform density, radius 1 µ and length 2 mm, what is the average luminosity in...
  2. 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...
  3. B

    Calculating Luminosity from Brightness: Where Did I Go Wrong?

    Hello, I've been trying to solve for the luminosity of Gliese 581 using its brightness. I've been using the equation below: F = (L/(4*pi*r^2)) I have the flux as the absolute brightness of Gliese 581, which is 11.6. The distance, r, to the star is 1.89210568 × 10^17 meters. However, when I...
  4. btouellette

    Integrating monochromatic luminosity to get total luminosity

    Homework Statement Integrate the monochromatic luminosity (of a blackbody model star) over all wavelengths to obtain an expression for the total luminosity. This is 3.14(a) from An Introduction to Modern Astrophysics 2e by Carroll and Ostlie. Homework Equations ## L_\lambda d\lambda =...
  5. mokeejoe5

    Calculating Sun's Luminosity: Step-by-Step Guide

    I'm trying to work out the luminosity of the sun. 1630 Watts/m2 apparent magnitude reduces with distance squared so 1/(1.496*1011)2 = 4.468*10-23m2 has the same brightness 1630 Watts from earth Surface area of the sun divided by 4.468*10-23m2 = # of 1630 Watt sections *1630 = 2.21*1044...
  6. C

    Day length and orbital period + apparent luminosity

    Homework Statement I am actually an MD student, but I have been working on writing a novel. I wanted to create exoplanets that were suitable for human habitation, and I had a guy help me over the summer come up with plausible numbers for a variety of variables. Where I am having trouble now is...
  7. C

    How do they define luminosity function in astrophysics?

    hi, please explain me how to define a luminosity function? what is meant by luminosity interval? and the last one , what is meant by Present Day Mass Function (PDMF)? reagrds
  8. Ken G

    Understanding the Luminosity of Radiative Stars

    In the past I have tried to start threads that provide surprising insights into the processes that set the luminosity of main-sequence stars that have radiative diffusion as their primary energy transport mechanism (as opposed to fully convective stars). However I failed to include professional...
  9. S

    Luminosity and Surface Area

    Homework Statement A typical adult burns about 2500 Calories in one day. 1)How much energy does the average human emit every second 2)What is another term to use for expressing a “Joule per second”? 3)From your answers to (1) and (2), you just determined the “luminosity” of the average human...
  10. B

    Determining distance using luminosity

    I have a question where I am asked to determine the apparent distance in light years using brightness and luminosity: With a very large telescope on a very dark night we can detect a star like our Sun that appears just 10^−20× as bright as our Sun does during the day. Use this information to...
  11. Safinaz

    What are the integrated luminosity values for CMS at Cern?

    Hi all, I'd like to know what is the integrated luminosity of CMS exp. Cern ? S.
  12. 2

    Estimate the number of stars with present-day luminosity function

    Folks, Please don't flame me for this as it is probably already answered somewhere. How does one estimate the number of stars in a 500pc^3 which have a range of 10^-2 to 0.1 Lsun? I am not a maths head, so simple answer that would have the formula would be great. Thanks
  13. C

    Can Luminosity Give You a Star's Age?

    You find two yellow stars at the same distance (but not in a binary system!). They have the same mass, but one is much more luminous than the other. Which one is older?
  14. B

    Black hole Accretion Luminosity

    Homework Statement Suppose that a black hole of mass M accretes mass at a rate ##\dot{M}##. Further suppose that accretion of mass Δm leads to the radiation of energy ##\Delta E= \eta Δmc^2##, for some effeciency of energy conversion, ##\eta##. What is the luminosity of emitted radiation in...
  15. P

    Finding a constant of proportionality from a mass luminosity relation

    Homework Statement For main sequence stars, the mass–luminosity relation can be approximated by L\proptoM^{3.5} f) If luminosity and mass are both measured in solar units, what is the constant of proportionality? {2} I know that the luminosity value of the sun is 4x10^{26}W and M =...
  16. M

    Luminosity Distance: Explaining the Length Differences

    Explain the difference between distances. Red lines indicating light travel path. Green lines indicating distance. Why there is length difference between green lines? In this diagram object moves at 0.8c away from the observer in t FOR. How far away observer in t' FOR sees an object...
  17. F

    Luminosity & flux equations confusion

    Hi folks First off I have only just figured out that v is used as bloody frequency in loads of astrophysics equations. Not fun. I've got monochromatic flux as: Fv = dE / dt·dA·dv now I'm happy with this. But now the lecture notes I'm looking at describes monochromatic luminosity as...
  18. wolf1728

    Mass Luminosity relation doesn't hold true when applied to actual data

    I see there have been many postings about this topic in this forum. The formula for this relation is Luminosity = Mass^3.5 Taking logs of both sides we get log (lum) = 3.5 * log (mass) and using a little algebra we find that the exponent (3.5) should equal log (lum) ÷ log (mass) I have...
  19. S

    Question about eddington luminosity

    I have a question regarding the Eddington limit. My question is, an object with any mass has luminosity greater than the Eddington limit. What will happen to a small particle in the nearby region of the object? I believe the small particle will be pushed away by radiation force because it...
  20. S

    HR diagram: magnitude - luminosity relation

    Hey PF, I'm just being stupid today, because I can't figure out the way HR diagram is constructed, hope you guys can help. So suppose that you complete an observation in several different filters, and you need to work out the luminosity and temperature for each object in your field of view...
  21. S

    Luminosity, Intensity and flux

    what is the difference between luminosity, Intensity and flux in particle physics? Some time people use them in similar meanings.
  22. A

    Find gravitational acceleration through luminosity

    Homework Statement Use equation 1.3 to estimate the solar radius Ro from its luminosity and effective temperature. Show that the gravitational acceleration g at the surface is about 30 times larger than that on Earth.Homework Equations (1.3) L=4∏R2σSBT4The Attempt at a Solution Well we know the...
  23. K

    Einstein Luminosity and Speed of Light

    Homework Statement Find the Einstein luminosity (LE) in terms of just c and G (the speed of light and the gravitational constant), i.e. determine a power (in watts) from just these two terms using dimensional analysis. What is this value? Once determined, you should be able to show that an...
  24. Z

    Calculating Luminosity (Easy Atronomy Question)

    Capella is the same temperature as our sun, but about ten times the suns diamter; it must be about_____ times as luminous as our star. Could someone please tell me how to solve this type of problem?? I am aware of the formula R=sqrt(L)/Temp squared. But I'm not sure how to apply that...
  25. G

    How to Calculate Luminosity of an RR Lyrae Star?

    Homework Statement Given pulsation period in days, (surface temperature) and mass of an RR Lyra variable star. How do you find luminosity? Homework Equations The relevant equations seem to be for finding the luminosity of Cepheids... there are no equations for Lyrae... Other than, absolute...
  26. D

    Help with relative star flux and luminosity

    Homework Statement A star as an apparent visual magnitude of 14 and an absolute visual magnitude of 14.7. I have worked out that its distance is 7 parsecs. The sun has an absolute visual magnitude of 4.8 and an effective temperature of 5800k. If the star has the same effective temperature of...
  27. W

    Surface brightness to luminosity

    Can anyone offer insight into the derivation of the surface brightness to solar luminosity conversion S(mag/arcsec^2) = Mabs + 21.572 - 2.5log S (Solar luminosities/pc^2) More or less my concern is the constant term, is it empirically observed value or is there a derivation for it? thanks
  28. A

    Flux Density (Jy) to Luminosity when wavelength is involved

    Hi, I think my problem may be a little trivial however I have been stuck on it for quite some time. I have plots of flux density (Jy) versus wavelength in order to look at a particular forbidden line. I want to find the luminosity of the line, however as I am dealing with Jy [W/(m^2 Hz)] I do...
  29. C

    Luminosity Distance, Determination From Type Ia SN

    The light curve of a type Ia supernova in a cosmologically remote galaxy requires two particular corrections. The time axis must be corrected for time dilation, and the apparent-magnitude axis must be corrected for redshift dimming. It is my understanding that the "distance" determined...
  30. J

    Calculating Cross Sections from Events + Luminosity

    Hello, How can I convert the number of events + the luminosity in a cross section? I think this must be trivial... thanks..
  31. S

    Half of moon fluctuates in luminosity and hue randomly, why?

    I looked up at the moon with several friends and family members and the moon would gradually get brighter for 30 seconds. It would stay bright for a couple minutes and then dim down also large dark spots randomly appeared on the visible part of the moon. I did some research and read that this...
  32. E

    Red Giant Luminosity: Exploring the Causes

    Well, I've seen in almost all sites and books that Red Giants are more luminous than the star itself... Particularly, let us take our sun as an example... First of all, we know that due to the He- Fusion and the enormous pressure acting on the H- on the surface, it IGNITES or fuses faster...
  33. J

    Exploring the Proportionality of Luminosity & Mass in Stars

    Homework Statement Homework Equations L = M^3.5 The Attempt at a Solution Am I right in saying that this is the correct proportionality for luminosity and mass in a star? If it is, i am confused. In an example I have it states that when mass is doubled the luminosity...
  34. D

    Supernovae Type Ia: Redshift, Luminosity & Speed

    Homework Statement Supernovae type Ia has a red shift z=1.1. Observed luminosity of the star has changed in the interval of 60 days. What time interval would an observer measure in the near vicinity of the star? What is the speed that the star is moving away from us? What is the distance of the...
  35. mesa

    How is galaxy mass calculated using luminosity

    This seems like it would be fairly complex as any material that is not in the suns would absorb photons and convert part of that energy over to kenetic energy giving a false value for actual luminosity from the stars themselves. Galaxies appear to be very different from one another, is it...
  36. M

    Luminosity vs Power: Stars & W

    If one star is say 10 times more luminous than another star would that mean it would have 10 times more power as in W? And would that relationship continue with increased luminousity as in a star 1000 times more luminous would have 1000 times more power?
  37. T

    Understanding Stellar Lifespans: Comparing Luminosity & Mass

    Hi, all how would you go about finding out how much longer a star would live compared to another if you knew the one star was x times more luminous and y times more massive?
  38. H

    Calculate Redshift & Luminosity Distance for Hubble's Law

    Homework Statement Hubble's law may be written as H0dL = z +(1/2)(1 - q0)z2 Working to O(z) only, what is the redshift of a galaxy at a luminosity distance of 100 Mpc from Earth? What is the role of the constant q0? Given that observations show that q0 = -0.77, calculate the luminosity...
  39. O

    Type 1a Supernova - linking Nickel decay to luminosity

    Homework Statement I have a question in my coursework where I don't know where to start. I've got Nickel decaying post the explosion, I've got a time t, I've got an energy release per nucleus of Nickel, I've got a decay constant, I've got a light curve decay half life, and I've got a...
  40. C

    Mass of galaxy from luminosity

    Homework Statement I'm trying to understand how the mass of a galaxy at redshift 1 can be estimated by knowing its luminosity in the K-band. I've been told you are able to use the IMF but I'm not sure of the details. I really don't see how you could use the salpeter, or the luminosity...
  41. W

    Calculating Photon Energy given Luminosity

    Homework Statement NOTE: This is part two of a problem. The first part used the equation for the energy density of photons in a blackbody to find the photon energy, but that doesn't seem to apply here. Imagine that you are looking at a 100 W lightbulb from 1 meter away. At any instant in...
  42. M

    Surface gravity as a function of luminosity?

    I've been using these forums for awhile but this is my first post. So thanks in advance for taking the time to consider this problem. It is much appreciated. The question is: How does surface gravity vary as a function of luminosity along the main sequence? By surface gravity, I'm sure...
  43. W

    The Blushing Face of Saturn: Radiance, Luminosity & Habitable Zone

    Okay, reeling from a defeat in the stellar ring with a self gravitating mass of gas that imploded at a massive temperature when all I wanted it to do was hang around nice and calmly while I put gravitating bodies in it...now I move on to a slightly more conventional model. I do plan to do most...
  44. N

    Calculating luminosity given accretion power

    Homework Statement I have a question regarding calculating luminosity given accretion power. I have accretion power given to me as Δm/Δt= 1.5 solar masses/yr, in order to solve for luminosity are we supposed to convert the accretion power into solar masses/seconds or is it just some kind of...
  45. T

    How do I convert luminosity to magnitude?

    I am trying to find out how fast a star's magnitude changes over time, and have two columns of data. The first is the time since the stay left the main sequence, and the second is Log (L/L_sun). I am unsure as to how to chance the luminosity values into magnitudes, which means I can't answer...
  46. O

    Converting Lumens to Joules for 1cm Plastic/Glass Sphere

    (Note that this is not a homework question, but rather something I'm pursing in my leisure) I'd to do a quick calculation to see what sort of power I would need to raise the temperature of a small sphere (1cm diameter, made of plastic or glass) with an absorption coefficient of about .5. I...
  47. Z

    Light's Reach: How Far Does 1 Candela's Luminosity Go?

    Like say I have one candela of luminosity. How far away do I have to go until the photons of this light are completely undetectable?
  48. K

    Luminosity in LHC: Engineer's Guide to Understanding

    I'm an engineer. What's luminosity? I don't understand the explanation of Wikipedia. Why are physics glad if the luminosity grows up in LHC, respect of last year?
  49. A

    Eddignton Luminosity or maybe not

    Hi there Problem is: Assuming spherical Symmetry, estimate the rate of mass loss from the sun, if at the Earth, the measured velocity of the solar wind is 400km/s and the proton density of the wind is roughly 7 particles/cm^3. Give your answer in solar Masses per year. My attempted...
  50. A

    Eddignton Luminosity or maybe not

    Hi there Problem is: Assuming spherical Symmetry, estimate the rate of mass loss from the sun, if at the Earth, the measured velocity of the solar wind is 400km/s and the proton density of the wind is roughly 7 particles/cm^3. Give your answer in solar Masses per year. My attempted...
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