Luminosity: Meaning, Factors, Units & Examples

Ans.The differences between Luminosity and Luminous Flux are:

Ans. We calculate the apparent brightness (m) of a star by just observing how bright it is shining when compared to other visible stars. 

• The absolute magnitude (M) of a star is the apparent magnitude that stars would have if they were 10 parsecs (32.6 light years) away. 
• Deneb, a far-away star, which is approximately 1600 light years away, could appear bright like Sirius, the brightest star in the night sky, which is just 8.6 light years away. 
• Deneb has an apparent magnitude (m) of 1.25 magnitude, its absolute magnitude (M) is around -7. Sirius has an apparent magnitude of -1.46 magnitude, its absolute magnitude (M) is 1.42 magnitudes. 
• If you know a star’s parallax you know its distance. Once the star's distance and apparent magnitude are determined, the calculation of its absolute magnitude (M) can be performed. 
• Formula used to determine M is: M = m + 5 - 5 (log d), where d is the distance in parsecs (1 parsec = 3.26 light years). 

Ans. We can find out the Distance using the formula:
10 ((m − M + 5)/5) = d
⇒ 10 ((m − M + 5)/5) = d 

⇒ 10((11 − 3 + 5)/5) = d 

⇒ 10((11−3+5)/5)=d 

⇒ d = 398.11 parsecs

Next let us find out the Absolute Magnitude:
⇒ m − M = dm − M = d
⇒ 11 − M = 811 − M = 8
⇒ M = 3M = 3 

In order to find Luminosity, we assume n to represent the 5th root of 100, because of the fact that magnitude scales are based on a 5 point difference which represents a 100-fold increase. 

⇒ l = (n) (4.83−M) l= (n) (4.83 − M)
⇒ I = (n) (4.83 − 3) l= (n) (4.83−3)
⇒ l = 5.3951
Therefore, luminosity equals 5.4 approximately. 

Ans. When the total visible light flux becomes equal to the reflected sunlight due to the sky, environment or shading/light redirecting surfaces, they have a smaller total energy per luminous flux which then results in heat gain than the direct sunlight which is Possibly caused by a larger ability to produce luminosity that is more visible radiation compared to ultraviolet and infrared light which are cooler light. 

Ans. Astronomers express the luminosity of an object in terms of solar luminosities. The most luminous stars present in the universe emit several million solar luminosities, with the most luminous supernovae shining with 1017 solar luminosities and the dim brown dwarfs have luminosities which is a few millionths when compared to that of the Sun. 

Ans. First, measure the bolometric flux across all wavelengths and if we are unable to do this then we need to measure the flux in as many wavebands as possible or we can also obtain a flux-calibrated spectrum over as wide a wavelength as possible. 

There is one other way to measure a star’s luminosity is to calculate its apparent brightness about how it appears to the eye and compare that to its distance. We can convert a flux when in specified wavebands to a bolometric flux with the help of bolometric corrections or you can extend the application and then integrate it from a fit to the observed part of the spectrum for the bolometric flux to work out. 

We can then multiply the bolometric flux that we observed from the star by 4πD², where D is the distance to the star. We can then correct it for absorption and scattering by extending material if it is necessary. 

Ans. The measure of the luminosity of a star means how bright the star would be if it is viewed from a distance of 10 parsecs or 32.58 light years is called absolute magnitude. Generally, 10 parsecs is taken as the standard distance by the Astronomers and it is referred to the intrinsic brightness of the star as its absolute visual magnitude. 

The apparent magnitude of the star appears to be if it were 10 parsecs or 32.58 light years away. Absolute magnitude holds a relationship to the intrinsic luminosity of the star. If we put this in simple terms, it is expressed as the apparent magnitude at a distance of 10 parsecs from the star and its symbol is “Mv”. 

Ans. The measure of how bright the star appears when viewed from Earth is Apparent magnitude. In this case, Magnitude and apparent magnitude are the same thing which is namely how bright a celestial object appears to us on Earth ranked on the historic logarithmic magnitude system. The apparent magnitude usually depends on three things such as how big the celestial body is, how far away it is from Earth and finally how much light it produces per diameter of the star. It is related to the observed energy flux from the star. The symbol for absolute magnitude is “mv”. 

Ans. By taking the incoming light, astronomers study the different wavelengths of light from celestial objects using a spectrometer or spectroscope to break or partition the light into its component wavelengths. This method used here is called spectroscopy. 

Ans. Variable stars are whose brightness as seen from Earth can change over the short term or over the long term which may be caused by a change in emitted light or by something partly blocking the light. 

They are of two types: One is intrinsic variable stars which means that their luminosity changes due to expansion, contraction, eruption or pulsation. The second is extrinsic variable stars, which means that a star or planet passes in front of the star and blocks the light or it changes due to stellar rotation.