# Why is the Sun's Luminosity not equal to the Zero Point Luminosity?

I was doing some research and found that the zero-point luminosity was $$L_\odot = 3.75\times 10^{28}$$, while the sun's reported luminosity was $$L_{Sun} = 3.828\times 10^{26}$$, according to Wikipedia. Why isn't it the same value? Doesn't the symbol $$\odot$$ represent the value of our sun?

Furthermore, when Googling the Luminosity of the sun, it gives $$1L_\odot$$, which suggests that $$L_{Sun} = L_\odot$$ when it is evidently not. What's going on here? I'm very confused.

For background, I have little to no background knowledge of astrophysics, and I'm trying to learn about it.

Thanks.

• Could you link/clarify what the 'zero point luminosity' is? I'm not familiar with the concept myself. Also units should always be added for any physical quantities. – AtmosphericPrisonEscape Aug 16 '19 at 11:26
• @Anurag Do you mean the zero point of the absolute magnitude scale? – SpaceCore Aug 16 '19 at 12:59
• $L_{\odot}$ is the bolometric luminosity of the Sun. I don't know where you get $3.75 \times 10^{28}$ W from? – ProfRob Aug 16 '19 at 14:11
• What units are these two values in? Are they both in Watts? If not, they could be equivalent, but using different unit systems. – HDE 226868 Aug 16 '19 at 15:34

## 1 Answer

The zero point luminosity L0 is the luminosity of a star whose absolute bolometric magnitude Mbol is 0. IAU recommends the value L0 = 3.0128 × 1028 W. If the Sun's Mbol is +4.74, then L0 / Lsun = 104.74 / 2.5 = 78.7, which is consistent with Lsun = 3.828 × 1026 W.

Your understanding that Lsun ≡ L is correct. Maybe there was an accidental substitution of L for L0 at some point. Curiously, the number 3.75 × 1028 appears in the Wikipedia:Sun sidebar as the Sun's luminous flux in lumens, a quantity more applicable to a light bulb than to a star.