# 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. ### I Calculating Proportional Luminosity of Fictitious Star

I am attempting to calculate the proportional luminosity of a fictitious star using surface area and temperature in kelvin. To what level of accuracy can I expect from the formula L ∝ AT^4? Where L = Luminosity, A = surface area and T = Temperature L ∝ AT^4 L ∝ 4π · (r)^2 · (T)^4 If I replace...
2. ### I What is the luminosity of Earendel?

I just read an article about it, and an internet search & Wikipedia did not have this information. A rough interpretation of the HR diagram for a mass of 75 suns in Main Sequence) says it is about 106 suns. https://en.wikipedia.org/wiki/WHL0137-LS
3. ### I Lux-meter, candelas or lux, measuring nits in practice

My lux-meter can be used in two modes, candela and lux. How do I use it to accurately measure a display's luminance in nits? The screen isn't Lambertian, I have to assume this as an unknown. I'm interested in the luminance as seen by the eye looking at the screen perpendicular to it. Now, first...
4. ### I Red-Shifting vs. Luminosity Question

Main Question or Discussion Point Is red-shifted light brighter than what its black-body emission would be at the same temperature? As a figurative example, a star, neutron star, or other astronomical body has a temperature of 6,000 K, and is so close to its Schwarzschild radius that its light...
5. ### Calculating distance between lux meter and light source

I am currently performing some experiments to calculate the distance between a light source and a lux meter where the light source points directly towards the lux meter. Is it possible to calculate the exact distance between the light source and the lux meter given that we have a lux value...
6. ### B How to add up luminosity

Say you had a green and red LED, both with a luminosity of 2 W would the combined luminosity be 4 W or still 2 W?
7. ### Mass, luminosity and magnitude of a galaxy

So i first calculated the angular area of the galaxy. a=22.6'' and a/b=0.85 => b = 26.588''( btw I do not know why a/b =0.85 since b is the semi minor axis). Then the area of the elipse is a*b*π =1887.745''. Then using the first equation we get an apparent magnitude of m=16.511. Using the second...
8. ### I Lab frame and accelerator frame

In the file I present below,it shows a way to calculate luminosity in boost frame when crossing angle exists.I can't understand why luminosity calculation is done in boost frame instead of lab frame where calculations are much easier.I also have difficulty understanding lab frame,accelerator...
9. ### B Intensity of Spherical Shell of Stars

Given that L is the luminosity of a single star and there are n stars evenly distributed throughout this thin spherical shell of radius r with thickness dr, what is the total intensity from this shell of stars? My calculations were as follows: Intensity is the power per unit area per steradian...
10. ### I What is Earth's future warming due to the Sun's increasing luminosity?

Obviously, at some point, the Sun will be a red giant, and it's going to be very, very hot on Earth. I'd like to know what the expected rise in temperature would be due solely to the Sun, while holding the composition of the Earth's atmosphere constant - or at least constant except for the gas...
11. ### What is Hubble's Luminosity Law?

Problem Statement: Please help me understand the variables for the equation, 5 log(R) = -m + k. Relevant Equations: 5 log(R) = -m + k Here is the link to where I found the equation. I know it's on wikipedia but I checked the Hubble's paper and it seems to be credible. I'm trying to make...
12. ### Stargazing Accelerating universe - luminosity data

I am looking for the measurement data for distant objects (e.g. type Ia supernovae) The red shift and the luminosity flux data. What time interval is used in the luminosity flux measurement? What formula is used for the luminosity distance calculation?
13. ### Cosmology: luminosity and magnitude

Homework Statement Homework Equations [/B]The Attempt at a Solution (a) (b) Could you tell me if what I am doing is correct? In part (b) they want M as a function of L so I assumed we have to get rid of the l. Do I have to use l(m=0) = 1.52 x 10 ^(-8) ?[/B]
14. ### I Luminosity Distance: Finding the Connection

If photons are emmitted from a galaxy A at te to galaxy B (receives at present time a=1,t0) ,x being the comoving distance ,and current distance be dL L(t0)=a(te)2L(te) Flux(Φ0)=a(te)2L(te)/4πdL2 In comoving frame ∅=L(te)/4πx2 Is there any way to relate Φ and ∅ to find the luminousity distance dL??
15. ### I Correlation between a Star's Luminosity and Distance?

I made three charts from several different sources plotting the luminosity and distance from Earth of 26 to 300 Stars. In each chart, there appears to be a correlation between a star's luminosity (in solar units) and its distance from Earth (or solar system, whichever you prefer) in light years...
16. ### I Resources to find Data/Models for Nonradial Pulsations of Va

Hello! I'm currently working on a project in which I need to make an animated visualization demonstrating the physical changes and luminosity changes of a star with non radial stellar pulsations. This visualization has to be based on real or theoretical data or a real or theoretical model but I...
17. ### The Mystery of Human Luminosity: A Comparison to Daily Energy Intake

Homework Statement If we assume that a human radiates like a black body, calculate the luminosity in watts, then compare it to the daily energy a human gets from eating. (The area of a human A=1.3 m2) The Attempt at a Solution The temperature of a human is 310K so L=AσT4 =...
18. ### I Astrophysics simple question about luminosity

Why in Astrophysics it is used νLν (where ν is the frequency and Lν is the luminosity at that frequency) in the emission spectrum of an accretion disk?
19. ### I Curve fitting the luminosity distance and redshift data

Can anyone recommend papers that directly curve-fit redshift as a function of luminosity distance for type Ia supernova and gamma ray bursts? I am looking for papers that do not curve-fit the data via an assumed model, even one as simple as Friedmann–Lemaître–Robertson–Walker (FLRW) metric. I...
20. ### I What does L star mean in the Schechter function?

I'm trying to understand the Schechter function, I read on Wikipedia that the L star term in the function is the 'characteristic galaxy luminosity where the power-law form of the function cuts off'. What does this mean exactly?
21. ### Calculating the Feeding Rate of Supermassive Black Holes to Power Quasars

Homework Statement Quasars arise when supermassive black holes swallow nearby gas, or even entire stars. Their masses are typically 10&7 solar masses, and their luminosities are typically around 10&39 W. Hide the children! Roughly how many stars must one of these monsters eat per year, in order...
22. ### I Relationship between star radius and luminosity

In a PDF presentation on star formation that I'm currently reading, I ran into the following statement: "If we observe an increase in a star's temperature but without any changes in its luminosity, it means the star is shrinking (its radius is decreasing)" I'm having trouble understanding...
23. ### 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...
24. ### I Lumens and the Luminosity Function

Hello, I have a small confusion regarding the formula that calculates the luminous flux of a light source, which is given by: (See this Wiki) The ##\bar y (λ)## is the so-called luminosity function that corrects for the sensitivity of the human eye for a particular chosen wavelength. The...
25. ### I Average Luminosity of Moving Body: A Relativistic Analysis

Lets suppose a radiating body moving relative to a frame. At a certain time because of relativistic beaming one observer in that frame would observe a different luminousity of the body. But observes placed at a different angle in that frame relative to the body would also observe different...
26. A

### How do I calculate the luminosity of a Random Beanbag Machine?

Homework Statement A big drunken alumni tailgate game has an area of 11 ft2. Engineering alumni have constructed a Random Beanbag Machine that uniformly drops beanbags on the surface so that in each second 660 beanbags drop randomly all over the surface. The beanbag luminosity of this machine...
27. ### I Calculating planet's radius using a star's luminosity

Hello, I’ve been watching a lecture about how in astronomy one would be able to calculate the radius of 2 stars by measuring the velocity of the orbit and then measure the time how long the luminosity of a star dims when one star is behind the other. After thinking this a bit through, I came...
28. ### A Understanding the Impact of Triggers on Integrated Luminosity in Detectors

I was wondering whether the integrated luminosity depends on the triggers that were running. Thing is that for several time periods, some triggers in the detector may not be running. Wouldn't that affect the luminosity of the data?
29. ### How does the luminosity equation work?

I've read that L = 4 x Pi x b x d2 Where L = Luminosity (W) d = distance (m) b = brightness (W m-2) But in a real life applied situation, how does it work, how does one measure the distance and its brightness? And what does the 4Pi do?
30. ### Backing-out luminosity togo from lumens to radiant flux in W

Hi. Has anyone already done the work to back out the standard luminosity function so I can go from lumens at a particular wavelength to radiant flux in Watts? I have visible LED spec sheets with optical characteristics in lumens and I want to calculate the outputs in W. My NIR LED has specs in...
31. ### B Luminosity and neutrino questions

IK Pegasi B is considered the white dwarf most likely to supernova in our galactic neighborhood, although some professors (eg: Neil F. Cumins of Maine) thinks it might nova instead. (I agree with Professor Cumins) Questions: At about 150 light years distant would this faint star become...
32. ### Luminosity of the Earth covered in bulbs

1. The problem statement, all variables and given/known ddata a.) Assume that a 100W light bulb covers 30cm^2 of area, and covers the surface of the Earth (land and water) with such 100W bulbs. How would the total power output compare with the luminosity of the Sun? Note: L_sun = 10^26W and...
33. ### I Changing luminosity in days?

The video: When Astrophysicists detected Quasars, they thought that they could not be more than a few light-days across as they were changing their brightness in just a few days. If we assume that they are light-years long, then the change in brightness from the other side would be observed...
34. ### Apparent Flux and number of stars

The stars in our Galaxy have luminosities ranging from $L_{\text{min}}$ to $L_{\text{max}}$. Suppose that the number of stars per unit volume with luminosities in the range of $L$, $L+dL$ is $n(L)dL$. The total number of stars per unit volume if clearly $$n = \int_{L_{min}}^{L_{max}} n(L)dL.$$...

The video:
36. ### A satellite, the sun and the satellites heat protector

Homework Statement A spacecraft is protected from the Sun’s radiation by a planar baffle whose size is much greater than that of the spacecraft itself. The baffle is aligned perpendicular to the direction of the Sun. Show that the equilibrium temperature of the baffle is...
37. ### I Star Radius & Mass from Spectral Class, B-V, Luminosity

I have a data set of 120k star systems that I'd like to import into a project, and, while it has a lot of useful infomation, I'd like to display these stars in a visual fashion. This means that I need to figure out the radius, when zoomed into the star system, and its mass, to simulate objects...
38. ### B Effect of Density and Geometry on Gravity and Luminosity

Imagine a planet similar to Earth, but exposed to a completely different star. The star has the same mass and emits the same amount of photons as the sun, but it is a huge, extremely slender torus made of 1 mm diameter neon tubing. The planet is on the axis of the torus and at the same distance...
39. ### Two stars with the same mass equals the same luminosity?

As the Stephan-Boltzmann's Law says, the minimal change in a star's mass would have a massive effect on its luminosity, but can necessarily two stars with the same mass have the same luminosity?
40. ### Luminosity on Henyey track

Why does star luminosity change while on Henyey track? A lecture: https://astro.uni-bonn.de/~astolte/StarFormation/Lecture2012_PMS.pdf Page 31 claims that luminosity changes as 4/5 power of temperature. I´m not even asking for derivation as to why the power is specifically 4/5, which presumably...
41. ### Using Eddington Luminosity to calculate mass

Homework Statement The nuclear reactions in a star’s core are very sensitive to pressure and temperature, so high mass stars have much higher luminosities. The luminosities of massive stars have been observed to obey the following scaling relationship with mass (M): Lstar = (34.2)*M^2.4...
42. ### Link redshift with luminosity distance?

Homework Statement Plot luminosity distance and redshift z Homework Equations $$d_L(z)=(1+z)r(z)$$ where d_L(z) is luminosity distance and r(z) is the comoving distance. and we have $$r(z)= \frac{H_0^{-1}}{\sqrt\Omega_K}*sinn[\sqrt{\Omega_K}\int_0^z\frac{dz'}{\sqrt{\Omega_M(1+z')^3}}]$$ where...
43. ### How to calculate delivered luminosity?

I found CMS total delivered luminosity from Cern Statistics website (22.98 inverse fb). But i want to calculate myself and using values from website's supertable page. (https://acc-stats.web.cern.ch/acc-stats/#lhc/overview-panel) But my result is 8.16E+42 Where is my mistake?
44. ### Calculating Luminosity and Temperature of the Sun from Pluto

Hey all, I think that I'm doing this problem correctly but I'm getting an answer that's a couple thousand Kelvins off. Sorry if I should have posted this in the "advanced" section. 1. Homework Statement Part of a flat region of Pluto has the Sun directly overhead. Its surface temperature is 58...
45. ### Luminosity & Stellar Lifetimes

Homework Statement Quasars have a luminosity on the order of 10^12 times more than our Sun. Our Sun is expected to last 5 billion years. Using this number estimate (in seconds) how long our Sun would last if it started using energy at the rate of a Quasar. Homework Equations None The Attempt...
46. ### How to convert Ohms to luminosity?

So, I recently joined a stargazing club, and for our summer activity, we're supposed to use a telescope, a photoresistor, and a multimeter to measure the luminosity of the moon. I know that luminosity can be calculated with the equation, L = σ AT 4, and that brightness can be measured with the...
47. ### Globular cluster luminosity function

So I was interested in how astronomers measure the distances to other stars, galaxies, etc and I found this pdf about the subject http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1992PASP..104..599J&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf [Broken] It seems to...
48. ### Absolute magnitude and luminosity

Do two stars of the same absolute bolometric magnitude have the same absolute visual magnitude? If this is true; does this mean that two stars with the same power output have the same have the same bolometric magnitude and therefore same visual magnitude?
49. ### 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...
50. ### Wrong Explanations for the Luminosity of Main-Sequence Stars

There is a rather surprising thing afoot in many introductory astronomy texts, and many online web course notes, relating to a commonly seen but completely wrong explanation for why high-mass main-sequence stars have a much higher luminosity than low-mass main-sequence stars. You would really...