Our Milky Way should host an ancient, metal-poor, and centrally concentrated stellar population, which reflects the star formation and enrichment in the few most massive progenitors that coalesced at high redshift to form the proto-Galaxy. While metal-poor stars are known to reside in the inner few kiloparsecs of our Galaxy, current data do not yet provide a comprehensive picture of such a metal-poor “heart” of the Milky Way. We use information from Gaia Data Release 3, especially the XP spectra, to construct a sample of 2 million bright (G BP < 15.5 mag) giant stars within 30° of the Galactic center (GC) with robust [M/H] estimates, δ[M/H] ≲ 0.1. For ∼1.25 million stars we calculate orbits from Gaia Radial Velocity Spectrometer velocities and astrometry. This sample reveals an extensive, ancient, and metal-poor population that includes ∼18,000 stars with −2.7 < [M/H] < −1.5, representing a stellar mass of ≳5 × 107 M ⊙. The spatial distribution of these [M/H] < −1.5 stars has a Gaussian extent of only σ R GC ∼ 2.7 kpc around the GC, with most orbits confined to the inner Galaxy. At high orbital eccentricities, there is clear evidence for accreted halo stars in their pericentral orbit phase. Most stars show [α/Fe] enhancement and [Al/Fe]–[Mn/Fe] abundances expected for an origin in the more massive portions of the proto-Galaxy. Stars with [M/H] < −2 show no net rotation, whereas those with [M/H] ∼ −1 are rotation dominated. These central, metal-poor stars most likely predate the oldest disk population (τ age ≈ 12.5 Gyr), which implies that they formed at z ≳ 5, forging the proto-Milky Way.

We use chemistry ([α/Fe] and [Fe/H]), main sequence turnoff ages, and kinematics determined from H3 Survey spectroscopy and Gaia astrometry to identify the birth of the Galactic disk. We separate in-situ and accreted stars on the basis of angular momenta and eccentricities. The sequence of high−α in-situ stars persists down to at least [Fe/H] ≈ −2.5 and shows unexpected non-monotonic behavior: with increasing metallicity the population first declines in [α/Fe], then increases over the range −1.3 [Fe/H] −0.7, and then declines again at higher metallicities. The number of stars in the in-situ population rapidly increases above [Fe/H] ≈ −1. The average kinematics of these stars are hot and independent of metallicity at [Fe/H] −1 and then become increasingly cold and disk-like at higher metallicities. The ages of the in-situ, high−α stars are uniformly very old (≈ 13 Gyr) at [Fe/H] −1.3, and span a wider range (8 − 12 Gyr) at higher metallicities. Interpreting the chemistry with a simple chemical evolution model suggests that the non-monotonic behavior is due to a significant increase in star formation efficiency, which began ≈ 13 Gyr ago. These results support a picture in which the first ≈ 1 Gyr of the Galaxy was characterized by a "simmering phase" in which the star formation efficiency was low and the kinematics had substantial disorder with some net rotation. The disk then underwent a dramatic transformation to a "boiling phase", in which the star formation efficiency increased substantially, the kinematics became disk-like, and the number of stars formed increased tenfold. We interpret this transformation as the birth of the Galactic disk at z ≈ 4. The physical origin of this transformation is unclear and does not seem to be reproduced in current galaxy formation models.

The astrophysical origins of r-process elements remain elusive. Neutron star mergers (NSMs) and special classes of core-collapse supernovae (rCCSNe) are leading candidates. Due to these channels’ distinct characteristic timescales (rCCSNe: prompt, NSMs: delayed), measuring r-process enrichment in galaxies of similar mass but differing star formation durations might prove informative. Two recently discovered disrupted dwarfs in the Milky Way’s stellar halo, Kraken and Gaia-Sausage Enceladus (GSE), afford precisely this opportunity: Both have M ⋆ ≈ 108 M ⊙ but differing star formation durations of ≈2 Gyr and ≈3.6 Gyr. Here we present R ≈ 50,000 Magellan/MIKE spectroscopy for 31 stars from these systems, detecting the r-process element Eu in all stars. Stars from both systems have similar [Mg/H] ≈ −1, but Kraken has a median [Eu/Mg] ≈ −0.1 while GSE has an elevated [Eu/Mg] ≈ 0.2. With simple models, we argue NSM enrichment must be delayed by 500–1000 Myr to produce this difference. rCCSNe must also contribute, especially at early epochs, otherwise stars formed during the delay period would be Eu free. In this picture, rCCSNe account for ≈50% of the Eu in Kraken, ≈25% in GSE, and ≈15% in dwarfs with extended star formation durations like Sagittarius. The inferred delay time for NSM enrichment is 10×–100× longer than merger delay times from stellar population synthesis—this is not necessarily surprising because the enrichment delay includes time taken for NSM ejecta to be incorporated into subsequent generations of stars. For example, this may be due to natal kicks that result in r-enriched material deposited far from star-forming gas, which then takes ≈108–109 yr to cool in these galaxies.

The mass–radius relation of white dwarfs is largely determined by the equation of state of degenerate electrons, which causes the stellar radius to decrease as mass increases. Here we observationally measure this relation using the gravitational redshift effect, a prediction of general relativity that depends on the ratio between stellar mass and radius. Using observations of over 3000 white dwarfs from the Sloan Digital Sky Survey and the Gaia space observatory, we derive apparent radial velocities from absorption lines, stellar radii from photometry and parallaxes, and surface gravities by fitting atmospheric models to spectra. By averaging the apparent radial velocities of white dwarfs with similar radii and, independently, surface gravities, we cancel out random Doppler shifts and measure the underlying gravitational redshift. Using these results, we empirically measure the white dwarf mass–radius relation across a wide range of stellar masses. Our results are consistent with leading theoretical models, and our methods could be used with future observations to empirically constrain white dwarf core composition and evolution.

The Milky Way's satellite galaxies ("surviving dwarfs") have been studied for decades as unique probes of chemical evolution in the low-mass regime. Here we extend such studies to the "disrupted dwarfs", whose debris constitutes the stellar halo. We present abundances ([Fe/H], [α/Fe]) and stellar masses for nine disrupted dwarfs with M ≈ 10 6 − 10 9 M from the H3 Survey (Sagittarius, Gaia-Sausage-Enceladus, Helmi Streams, Sequoia, Wukong/LMS-1, Cetus, Thamnos, I'itoi, Orphan/Chenab). The surviving and disrupted dwarfs are chemically distinct: at fixed mass, the disrupted dwarfs are systematically metal-poor and α-enhanced. The disrupted dwarfs define a massmetallicity relation (MZR) with a similar slope as the z = 0 MZR followed by the surviving dwarfs, but offset to lower metallicities by ∆[Fe/H]≈0.3−0.4 dex. Dwarfs with larger offsets from the z = 0 MZR are more α-enhanced with [α/Fe] = 0.43 +0.09 −0.09 × ∆[Fe/H] + 0.08 +0.03 −0.03 . In simulations as well as observations, galaxies with higher ∆[Fe/H] formed at higher redshifts -exploiting this, we infer the disrupted dwarfs have typical star-formation truncation redshifts of z trunc ∼1 − 2. We compare the chemically inferred z trunc with dynamically inferred accretion redshifts and find almost all dwarfs are quenched only after accretion. The differences between disrupted and surviving dwarfs are likely because the disrupted dwarfs assembled their mass rapidly, at higher redshifts, and within denser dark matter halos that formed closer to the Galaxy. Our results place novel archaeological constraints on low-mass galaxies inaccessible to direct high-z studies: (i) the redshift evolution of the MZR along parallel tracks but offset to lower metallicities extends to M ≈ 10 6 − 10 9 M ; (ii) galaxies at z ≈ 2 − 3 are α-enhanced with [α/Fe]≈0.4.