Regulus is it the 20 brightest star or not?

Answer 1

Regulus is not in the 20th brightest stars as seen from Earth. It is the 22nd brightest star, 23nd if you include the sun.

In terms of absolute magnitude it is way down the list, there are many hundrededs of catalogued stars more luminous.

List of brightest stars and (apparent magnitude.)

1. Sun (-26.74)
2. Sirius, Alpha Canus Magoris (-1.46)
3. Canopus, Alpha Carinae (-0.72)
4. Arcturus, Alpha Bootes (-0.04 variable)
5. Alpha Centauri A (-0.01)
6. Vega, Alpha Lyrae (0.03)
7. Rigel, Beta Orionis (0.18)
8. Procyon, Alpha Canus Minoris (0.34)
9. Achernar, Alpha Eridani (0.50)
10. Betelgeuse, Alpha Orionis (0.58 variable)
11. Hadar, Beta Centauri (0.60)
12. Capella A, Alpha Aurigae A (0.71)
13. Altair, Alpha Aquilae (0.77)
14. Aldebaran, Alpha Tauri (0.85 variable)
15. Capella B, Alpha Aurigae B (0.96)
16. Spica, Alpha Virginis (1.04)
17. Antares, Alpha Scorpii (1.09)
18. Pollox, Beta Geminorum (1.15)
19. Fomalhault, Alpha Piscis Austrini (1.16)
20. Deneb, Alpha Cigni (1.25)
21. Mimosa, Beta Crucis (1.30)
22. Alpha Centauri B (1.33)
23. Regulus, Alpha Leonis (1.35)
24. Acrux A, Alpha Crucis A (1.40)
25. Adara, Epslion Canis Majoris (1.51)
26. Gacrux, Gamma Crucis (1.63)