  Stellar atmospheric parameters (effective temperature, luminosity
classifications, and metallicity) estimates for some 24 million stars
(including over 19 million dwarfs and 5 million giants) are determined from the
stellar colors of SMSS DR2 and Gaia EDR3, based on training datasets with
available spectroscopic measurements from previous high/medium/low-resolution
spectroscopic surveys. The number of stars with photometric-metallicity
estimates is 4--5 times larger than that collected by the current largest
spectroscopic survey to date -- LAMOST -- over the course of the past decade.
External checks indicate that the precision of the photometric-metallicity
estimates are quite high, comparable to or slightly better than that derived
from spectroscopy, with typical values around 0.05--0.10 dex for [Fe/H] $>
-1.0$, 0.10--0.20 dex for $-2.0 <$ [Fe/H]$ \le -1.0$ and 0.20--0.25dex for
[Fe/H] $\le -2.0$, and include estimates for stars as metal-poor as [Fe/H]
$\sim -3.5$, substantially lower than previous photometric techniques.
Photometric-metallicity estimates are obtained for an unprecedented number of
metal-poor stars, including a total of over three million metal-poor (MP;
[Fe/H] $\le -1.0$) stars, over half a million very metal-poor (VMP; [Fe/H] $\le
-2.0)$ stars, and over 25,000 extremely metal-poor (EMP; [Fe/H] $\le -3.0$)
stars. From either parallax measurements from Gaia EDR3 or
metallicity-dependent color-absolute magnitude fiducials, distances are
determined for over 20 million stars in our sample. For the over 18 million
sample stars with accurate absolute magnitude estimates from Gaia parallaxes,
stellar ages are estimated by comparing with theoretical isochrones.
Astrometric information is provided for the stars in our catalog, along with
radial velocities for ~10% of our sample stars, taken from completed or ongoing
large-scale spectroscopic surveys.
