# Luminosity to magnitude

1. Nov 17, 2011

### the riddick25

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 cant answer the question.

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

If anyone can help it would be greatly appreciated

Thanks :D

2. Nov 17, 2011

### Uniquebum

Depends whether you want apparent or absolute magnitude.
For apparent the formula would be
$m_1 - m_2 = -2.5 \log{\frac{L_1}{L_2}}$

3. Nov 17, 2011

### the riddick25

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. Nov 17, 2011

### Uniquebum

$m_1 = m_{sun} - 2.5\log{\frac{L}{L_{sun}}}$
where $m_{sun} = –26.74$

So for the value you gave you'd get
$m_1 = -26.74 - 2.5*4.4799$
which gives a pretty high magnitude as the lower the magnitude, the brighter the object.