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. L

    Sun - Increasing in Luminosity

    I am currently researching how the sun's steady increase in luminosity is affecting the four inner planets. It's a fascinating subject. I haven't done any of the calculations yet (mostly because I am not that advanced in math yet), but I was watching a show called, "The Universe." On this...
  2. R

    Calculating Luminosity Distances: Converting Angular Distances to Parsecs

    Hey I'm getting very muddled with my units, and would really appreciate some clarity :-) I have angular distances between galaxies at some redshift, in arcseconds I want to calculate the distance in parsecs, taking into account the luminosity distance. In the equation; r =...
  3. Ken G

    How to change a star's luminosity

    I have a question to ponder. Let's say you had unlimited access to resources, and wanted to reduce the luminosity of a main-sequence star by a factor of 2. To make it cleaner, let's take a somewhat more massive star than the Sun, so it has a shallower convection zone and the luminosity is...
  4. J

    Astronomy, Luminosity of 100 solar mass stars

    This is just a small part of one of my problems. I need to find the luminosity of a stellar population containing 100 solar mass stars and one 20 solar mass star. I think that the 20 solar mass star will dominate, but I am not sure. Here is the formula to use: L/Lsun = (M/Msun)^3.5 I...
  5. Ken G

    Bad astronomy in massive star luminosity

    I've often wondered why there is so much misinformation around the topic of the reasons that very massive main sequence stars are so luminous. It even shows up in books, but if we restrict to seemingly reliable internet sites, I'll bet that in 10 google hits of sites that offer an explanation...
  6. J

    What is the meaning of luminosity per unit frequency?

    I am just trying to understand exactly what luminosity per unit frequency is. I know that luminosity (L = 4PI(r2)(flux)) is amount of energy per unit time, but not sure what it means when it is also per unit frequency. The question has a cosmic source of 100 solar luminosity, all emitted...
  7. Q

    Total luminosity of a galactic disk

    This is basically problem 2.8 from Sparke & Gallagher "Galaxies in the universe". There's only one area I'm having trouble with. I've solved the surface density for stars in the galactic disk (# stars per unit area) as \Sigma(R)=2\exp(\frac{-R}{h_R})h_R Now, with L_0 being the...
  8. K

    Calculating Luminosity of a Star at Different Stages

    The question is A main sequence star is barely visible at a distance of 20 pc with a certain telescope. The star subsequently ascends the red giant branch, during which time its surface temperature drops by a factor of 3 and its radius increases by a factor of 50. a) Determine the how...
  9. M

    Poynting vector and luminosity

    Hello Say I have a source of radiation placed at my origin and I know its power is spread uniformly about 2*pi at a certain frequency, I believe this is also called luminosity. (or a 2D equivalent) This means if I put a shell around the source and collect this power, then as long as the shell...
  10. H

    Estimating the Sun's Present-Day Luminosity

    I want to make an order-of-magnitude estimate of the Sun's present-day luminosity. For this, I want to use just the following two pieces of information in a simple idealised model of (the mechanism of energy production in a star): 1) the mass of a helium nucleus is about 1 percent less...
  11. S

    How Do Brightness and Luminosity Differ in Energy Terms?

    I cannot get a feel of what is luminosity physically... Can anyone explain me the difference between energy transfer in terms of brightness (which is measured in Watts) and luminosity (which requires a new definition of units, candela)??
  12. G

    Eddington luminosity for star made out of protons

    Imagine that a star could be composed entirely of protons so that Thomson scattering by protons dominated the stellar opacity. How luminous, in L_{\odot}, would the star have to be to blow itself apart by radiation pressure? I know that the luminosity has to be greater than the eddington...
  13. S

    Visual Luminosity and flux of sun.

    Homework Statement Some values for the Sun are, bolometric luminosity Lbol = 3.83 x 10^26 W, bolometric correction = –0.07, and radius = 6.96 x 105 km. Calculate the visual luminosity, LV, of the Sun and the flux, FV, radiated into space by the solar surface. Homework Equations M1-M2=-2.5...
  14. Barnak

    Mass of star as function of Luminosity, Temperature and size

    Hello all, First, since I'm just a physics teacher, and not an astrophysicist, my questions may sound "obscure" or badly formulated. Especially since I'm not an English native speaker. Sorry about that :shy: I need a mathematical relation which could give the mass of a "theoretical" star as...
  15. Z

    Which star has the greater luminosity

    Homework Statement Question 1 - If two stars A of luminosity 9000 and B of luminosity 90 (both relative to the Sun) appear equally bright from the Earth, how much further away is A from B? Question 2 - Star A is twice as bright as Star B, but B is twice as far away as A. Which star has...
  16. I

    Schechter luminosity function (integration)

    Suppose the luminosity function of galaxies is given by the approximation: dn = \varphi L dL = \varphi* (L / L*)^-\alpha dL/L* when L < L* and dn = \varphi L dL = 0 when L > L* where \alpha = 1.25, L* = 2.0E10 solar luminosities and \varphi* = (3Mpc)^-3 Integrate the above eqn to find the...
  17. N

    Solar Luminosity: Calculating Sphere of Light Illuminating Earth's Orbit

    Homework Statement The formula for the surface area of a sphere is 4(3.14) r2, where r is the mean distance from the sun to the earth. Determine the sphere of light (square meters) illuminated by the sun the size of the Earth's orbit.Homework Equations astronomical Unit-149,597,829 k The...
  18. P

    What exactly does radio luminosity mean?

    Like the title says, I'm wondering what exactly is the definition for radio luminosity. A search on Google doesn't turn up with anything. I think it means the intensity of radio waves emitted, but I'm not sure. I'd rather you explain the term without using the word luminous or any variation of...
  19. V

    Solving Brightness Equation with Procyon: 7L☉ and 11.41 light years

    In a question is says that Brightness = Luminosity/4πr₂ (for a star). Procyon = 7L☉ and 11.41 light years as distance. the equation in B= 7L☉/ 4π(11.41)₂ the answer turned out to be 0.004 L☉ watts per square light year (its a textbook question). What i don't understand is what is 7L☉...
  20. Q

    Exploring Sun's Luminosity Evolution

    Hi All As most of us know, in a qualitative sense, the Sun will eventually become a Red Giant before its core explodes (the Helium Flash) and it's heaved onto the Helium Main Sequence, to shine merrily for about 100 million years. What I want to know, in a quantitative sense, is just how its...
  21. W

    How to calc absolute magnitudes by comparing to the sun's luminosity

    hi guys, (i) By comparing with the luminosity of the Sun, calculate the absolute magnitude of a 60W light bulb. [The absolute magnitude of the Sun is +4:75; you may ignore colour differences between the bulb and the Sun.] (ii) The Hubble Space Telescope (HST) can detect objects as faint...
  22. V

    Help to determine the luminosity of a star

    I need to work out the luminosity of 3 stars 85 PEG A / B / C using the following data and equation, Absolute Mag of stars A/B/C/SUN MA = 5.28 MB = 8.53 MC = 15 MS = 4.84 Apparent Mag of stars A/B/C mA = 5.75 mB = 9.00 mC = 15.47 LS = Luminosity of Sun – 1 ? Equation MA-MS =...
  23. I

    Calculating luminosity density of the universe

    Homework Statement l = Phi*L*Gamma(a+2) b) The Sloan Digital Sky Survey (SDSS) has recently measured the following Schechter parameters in the r passband: a = -1.16 +/- 0.03, M*= -20.80 +/- 0.03, Phi* = (1.50 +/-0.13). Given that the sun has absolute magnitude M = 4.62 in the SDSS r...
  24. I

    Calculating the luminosity density of the universe

    l = Phi*L*Gamma(a+2) b) The Sloan Digital Sky Survey (SDSS) has recently measured the following Schechter parameters in the r passband: a = -1.16 +/- 0.03, M*= -20.80 +/- 0.03, Phi* = (1.50 +/-0.13). Given that the sun has absolute magnitude M = 4.62 in the SDSS r band, calculate the...
  25. T

    How Does the Schechter Luminosity Function Determine Galaxy Luminosity Density?

    Homework Statement The galaxy luminosity function  \phi(L) tells us the number density of galaxies as a function of luminosity L. The luminosity density of the universe l is thus given by the expression l = \int^{\infty}_{0}L\phi(L)dl a) For a Schechter luminosity function...
  26. R

    Stellar Formation, Life, Death - Temp, Radius, Luminosity

    How does the temperature of a star relate to the radius of a star? How does the mass of a star affect the temperature of the star's core? How does the luminosity of a star relate to how long a star will live? How do the spectral lines of a star determine if the star is a first generation...
  27. A

    Why do the Sun's/star's luminosity increase during the main sequence?

    Hi, I'm wondering why the Sun's luminosity has increased over time. I have learned that every star in the main sequence also tend to increase their luminosity but I haven't figured out why. Luminosity is related to star's radius and temperature so does these parameters rise during the...
  28. T

    Optics (Parallax and Luminosity)

    Does anyone know the optical algorithms for calculating (or at least estimating) parallax and luminosity? So far I have only found something for parallax, and not quite sure it is correct. For luminosity I haven't really found anything useable. This is all in regards to rifle scopes and similar...
  29. E

    Discovering the Solar Corona Luminosity Curve: Explained and Explored

    I've been trying to find out what's the curve for the solar corona luminosity vs distance like. I know that the corona can be practically observed up to 4 solar radii. I also found out that the curve is more or less exponential but I need the actuall equation of the curve. Can anyone help me?
  30. P

    A question about star's luminosity, temperature and mass.

    My question is : if two stars have the same luminosity and temperature, do they have to be at the same mass and size?
  31. B

    Calculating the Sun's Luminosity: The Equation Revealed

    [SOLVED] luminosity of the sun What equation do you use to figure out the luminosity of the Sun if you are given the amount of energy the Earth receives from it?
  32. B

    Dimensional analysis on Luminosity

    I am having trouble with one of the problems on my hw. The question is: Because light carries momentum, it creates pressure when it shines on something. This has led people to propse using "solar sails" instead of conventional rocket engines on interplanetary or interstellar space craft. A...
  33. D

    Spectral and Luminosity Classification

    I am looking for a table of spectral and luminosity classification that shows the breakdown of spectral classes, mass, luminosity, radius, and includes evolved stars. Has anyone seen something like this. I did find this: http://nrumiano.free.fr/Estars/classes.html It is helpful, but doesn't...
  34. K

    Flux, Luminosity, Distance, Size

    1) You observed two galaxy clusters. You have determined the distance to Cluster A (by Cephied variables) to be 50 Mpc. You find the galaxies in Cluster B to be on average 16 times fainter than those in Cluster A. How far away is Cluster B? How does the (apparent) average size of galaxies in...
  35. V

    Computing luminosity from surface brightness

    Homework Statement I'm trying to find the central luminosity per square parsec of a galaxy with central surface brightness I(0) = 15 \; mag \; arcsec^{-1}. I need the answer to be in multiples of the solar bolometric luminosity per square parsec. Homework Equations m_1 - m_2 =...
  36. L

    Estimating Radius for Half of Total Luminosity: Luminosity Integral Help

    If the total luminosity is given by L_T = \int_0^{\infty} e^{-(r/a)^{1/4}} r^2 dr estimate the radius [tex]r(a)[/itex] corresponding to half of the total luminosity. This would be to integrate from zero to r and get a function r(a,L) but this is impossible so anyone got an idea on how...
  37. L

    Does the Hubble Profile of Surface Brightness Lead to Infinite Total Luminosity?

    Homework Statement Show that the Hubble profile of surface brightness I(r) = I_0 \left(1+\frac{r}{R}\right)^{-2} leads to an infinite total luminosity. r is the distance from the center and R is a constant. 2. The attempt at a solution For large r this is related as I(r) \propto...
  38. B

    How many M type stars equal the luminosity of one O type star?

    Hello. what equation do i use to figure out how many .75*Msolar main sequence stars would it take to equal the luminosity of one 3*Msolar O tpe star? I know O type are much brighter but how many times brighter? thank you
  39. A

    Luminosity Conversion Question

    Could someone help me out with a conversion question. How do you convert 290mcd to candelas/meter square? Or what additional information do I need to make the converstion? Thanks!
  40. P

    Plasma Luminosity: Estimating Rocket Exhaust Heat Loss

    I'm interested in the general topic of the luminosity of plasmas. I would expect that this topic would come up most often in astrophysics, so that's where I'm posting the question as to get the best shot at getting an answer. I'm interested in applying the information in a rather unusual...
  41. Chronos

    An HST Program for the Luminosity Calibration of Type Ia Supernovae

    Here is an interesting [and important] paper, IMO: The Hubble Constant: A Summary of the HST Program for the Luminosity Calibration of Type Ia Supernovae by Means of Cepheids Authors: A. Sandage (1), G.A. Tammann (2), A. Saha (3), B. Reindl (2), F.D. Macchetto (4), N. Panagia (4)...
  42. Z

    Calculating solar mass using luminosity

    Okie dokie, I searched the web and found L=M ^ 3.5 (luminosity equals mass to the power of 3.5). But, I'm pretty clueless on anything beyond Algebra I. How do I solve for M if I know L? [If the answer can be expressed in Visual Basic syntax, all the better.] I also read several caveats...
  43. J

    How does the diameter of Polaris compare to the diameter of the sun?

    Hey.. need some help in solving this problem: Q) The luminosity of Polaris is 10,000 times the luminosity of the sun. The surface temperature of Polaris is about 5800 kelvins. Using k=33,640,000 find how the diameter of polaris compares with the diameter of the sun. ( Equation: D=...
  44. turbo

    Average luminosity of night sky vs frequency

    Average luminosity of night sky vs frequency... What is the average luminosity of the night sky in all observable wavelengths, from the shortest down to the longest? Are there estimations/approximations anywhere of average luminosity plotted against wavelength? Are there links to papers that...
  45. N

    Luminosity determining E Field

    I'm trying to determine how the luminosity affects the amplitude of the resulting E field at a given distance. I know that the irradiance and amplitude are linked by the fact that the irradiance is proportional to the square of the amplitude of the E field but I don't have that. I have...
  46. M

    How Do Cepheid Variable Stars Illuminate Distance Calculations in Astronomy?

    Here are a few more questions that I really need help on, thanks! 3. A Cepheid variable star is observed in the nearby galaxy M31. Its period of variation is measured to be 40 days. Using the period - luminosity relation given in the table below, what is the luminosity of the Cepheid...
  47. S

    Luminosity, Flux, Time: What Do They Mean?

    L=A[sig]T4 f=L/A=[sig]T4 Where does time come into these equations? If one telescope of a known diameter can reach a certain magnitude, it is easy to find what magnitude a different telescope of known magnitude can reach. But if a telescope integrating over 10s can reach a certain...
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