What is the critical lower mass limit ($M_{\odot}$) for sustained core hydrogen fusion defining a true red dwarf star?

$0.075 M_{\odot}$

The determination of whether a celestial body qualifies as a true star hinges on its ability to sustain stable, long-term hydrogen fusion in its core. For red dwarfs, which represent the smallest true stars, the lower boundary of mass is precisely defined at $0.075$ times the mass of the Sun ($M_{\odot}$). Objects falling below this critical threshold lack the necessary gravitational compression to achieve and maintain the sustained core temperatures required for this reaction, leading instead to classification as brown dwarfs. The $0.5 M_{\odot}$ figure represents the upper boundary of the red dwarf mass spectrum. The concept relies on reaching an internal pressure high enough to generate temperatures near 10 million Kelvin.