How do I convert luminosity to magnitude?

In summary, the conversation discusses finding the rate of change of a star's magnitude over time. The speaker has two columns of data, one for time since the star left the main sequence and the other for Log (L/L_sun). They are unsure how to convert the luminosity values into magnitudes and are seeking help. The formula for apparent magnitude is mentioned and a specific example is given. The speaker clarifies that they are looking for apparent magnitude and provides the formula for it. The conversation ends with a calculation for the given value.
  • #1
the riddick25
29
0
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 the question.

I thought it might be m=2.5 log L but I don't know if this is correct or not.

If anyone can help it would be greatly appreciated

Thanks :D
 
Physics news on Phys.org
  • #2
Depends whether you want apparent or absolute magnitude.
For apparent the formula would be
[itex]m_1 - m_2 = -2.5 \log{\frac{L_1}{L_2}}[/itex]
 
  • #3
But I'm not given actual luminosities, I've been given Log (L/L_sun)
so for example one of my values is 4.4799, how would i turn this into a magnitude?

And yes, I was looking for an apparent magnitude, forgot to mention it in my first post
 
  • #4
[itex]m_1 = m_{sun} - 2.5\log{\frac{L}{L_{sun}}}[/itex]
where [itex]m_{sun} = –26.74[/itex]

So for the value you gave you'd get
[itex]m_1 = -26.74 - 2.5*4.4799[/itex]
which gives a pretty high magnitude as the lower the magnitude, the brighter the object.
 
  • #5


I can provide you with the formula to convert luminosity to magnitude. The correct equation is m = -2.5 log(L/L_sun) + C, where m is the magnitude, L is the luminosity, and C is a constant value. This formula is known as the distance modulus and is used to calculate the apparent magnitude of a star.

To use this formula, you will need to know the luminosity of the star in relation to the Sun (L/L_sun). This can be found by taking the logarithm of the luminosity value and dividing it by the logarithm of the Sun's luminosity. Once you have this value, you can plug it into the formula to calculate the star's magnitude.

It is important to note that this formula assumes the star is at a distance of 10 parsecs from Earth. If you have data for the star's distance, you will need to use the distance modulus formula m = -2.5 log(L/L_sun) + 5 log(d/10), where d is the distance in parsecs.

I hope this information helps you in your research on the changing magnitude of stars over time. If you have any further questions, please don't hesitate to reach out for assistance. Good luck with your study!
 

1. What is luminosity and how does it relate to magnitude?

Luminosity refers to the total amount of energy emitted by a star or other astronomical object. Magnitude, on the other hand, is a measure of the brightness of an object as seen from Earth. The higher the luminosity, the brighter the object will appear in magnitude.

2. How is luminosity measured?

Luminosity is typically measured in terms of the amount of energy emitted per second, known as watts. For stars, luminosity can also be measured in terms of solar luminosity, which is the amount of energy emitted compared to the sun.

3. Can luminosity change over time?

Yes, luminosity can change over time due to various factors such as changes in the size or temperature of the object, or fluctuations in nuclear reactions within the object.

4. What is the difference between absolute and apparent magnitude?

Absolute magnitude is a measure of how bright an object would appear if it were located at a standard distance from Earth. Apparent magnitude, on the other hand, is a measure of how bright an object appears from Earth. Absolute magnitude takes into account the intrinsic brightness of the object, while apparent magnitude is affected by the distance between the object and Earth.

5. How is the relationship between luminosity and magnitude used in astronomy?

The relationship between luminosity and magnitude is used to classify and compare objects in the night sky. By knowing the luminosity and distance of an object, astronomers can calculate its apparent magnitude and determine its position and characteristics in the universe.

Similar threads

  • Advanced Physics Homework Help
Replies
1
Views
1K
  • Advanced Physics Homework Help
Replies
1
Views
5K
  • Advanced Physics Homework Help
Replies
1
Views
1K
  • Advanced Physics Homework Help
Replies
2
Views
2K
  • Advanced Physics Homework Help
Replies
1
Views
2K
  • Astronomy and Astrophysics
Replies
15
Views
6K
  • Advanced Physics Homework Help
Replies
4
Views
3K
  • Advanced Physics Homework Help
Replies
5
Views
4K
  • Introductory Physics Homework Help
Replies
1
Views
2K
  • Advanced Physics Homework Help
Replies
6
Views
2K
Back
Top