<p>Two stars in a binary system are at the same distance from Earth.</p>
<p>If they have similar spectral types then the difference in their magnitudes tells us the ratio of their luminosities.</p>
<p><span class="math-container">$$\Delta m = m_1 - m_2 = -2.5\log_{10}\left(\frac{L_1}{L_2}\right) \ . $$</span></p>

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2021-11-09T13:30:17.443Z
2021-11-09T20:13:10.357Z
2021-11-09T20:13:10.357Z
ProfRob
<p>Does a circumstance exist for subtracting one apparent magnitude from another to reach a solution to  a problem? Is it ever suitable to do so?</p>

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2021-11-09T11:43:26.557Z
If you assume two asteroids have the same albedo, then if you know the distances to each from the Earth and from the Sun you can get the ratio of their diameters from the difference in their absolute magnitude. It's a stretch though.
uhoh
<apparent-magnitude>
Is subtracting one apparent magnitude from another ever practical?
Constantthin
# Is subtracting one apparent magnitude from another ever practical?

(Asked by: username_0 on 2021-11-09)

Does a circumstance exist for subtracting one apparent magnitude from another to reach a solution to a problem? Is it ever suitable to do so?

**Tags:** apparent-magnitude
> **Comments:**
> **username_1**: If you assume two asteroids have the same albedo, then if you know the distances to each from the Earth and from the Sun you can get the ratio of their diameters from the difference in their absolute magnitude. It's a stretch though.  

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## 1 answers

### Answer by username_2 on 2021-11-09. Score: 6

Two stars in a binary system are at the same distance from Earth.

If they have similar spectral types then the difference in their magnitudes tells us the ratio of their luminosities.

$$\Delta m = m\_1 - m\_2 = -2.5\log\_{10}\left(\frac{L\_1}{L\_2}\right) \ . $$


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