model exists to explain Gould’s Belt (_Palouš and Ehlerová_ , 2017). Here, we
summarize recent literature which uses Gaia data to argue in favor of the
Gould’s Belt, including how analysis of the 3D motion of young stars towards
Gould’s Belt clouds provides kinematic evidence of the Belt’s expansion.
However, we also summarize significant opposition to the existence of the
Gould’s Belt, as revealed by unbiased surveys of stars and dust in the solar
neighborhood.

_Dzib et al._ (2018) compiled Gaia data on young stellar objects for a subset
of star-forming regions associated with the Gould’s Belt. Using the 3D spatial
distribution of these YSOs, Dzib et al. determined the shape of the Gould’s
Belt, parameterizing it as an ellipsoid with a size of $\approx$ 350 pc in $x$
and $y$ and $\approx$ 70 pc in $z$, centered about 100 pc from the Sun.
Combining the 3D spatial positions of the YSOs with Gaia proper motions and
ancillary radial velocity measurements from the literature, Dzib et al. argue
that the Gould Belt is expanding at a rate of $\rm 2.5\;km\;s^{-1}$. _Bobylev
and Bajkova_ (2020) analyzed the spatial and kinematic properties of a large
sample of young T Tauri type stars in a 500 pc radius from the Sun and derived
updated physical parameters for the Gould’s Belt. Using a sample of 100,000
red clump giants, _Gontcharov and Mosenkov_ (2020) analyzed the 3D spatial
dust distribution in the Gould’s Belt, finding it is also well-described by an
ellipse but about double the size as constrained by the YSOs in Dzib et al.
_Palouš et al._ (2019) perform a study using a Gaia-detected sample of OB
stars, finding that OB associations are expanding locally, but with no
evidence of a common origin.

While several studies in the Gaia era find evidence of the Gould’s Belt in
both gas and young stars, most of these are targeted studies focused on
regions commonly associated with the Belt. However, Gaia has also enabled
dozens of new, unbiased maps that adopt a more broad-brush approach to
Galactic structure, targeting all gas and stars within the nearest few
kiloparsecs. None of these maps support the existence of the Gould’s Belt. In
fact, already during the Hipparcos era, pre-Gaia, both _Elias et al._ (2009)
and _Bouy and Alves_ (2015) raised doubts on the existence of the Gould’s
Belt. Using Gaia data, _Zari et al._ (2018) construct a uniform three-
dimensional density map of young stars out to 500 pc. Zari et al. “find that
the 3D density maps show no evidence for the existence of the ring-like
structure, which is usually referred to as the Gould Belt.” There is no “link”
between opposite sides of the Belt, either in gas or young stars. The lack of
evidence for the Gould’s Belt in unbiased studies of young stars is also
supported by the catalog of _Prisinzano et al._ (2022), who use DBSCAN in
combination with Gaia EDR3 parallaxes, proper motions, and broadband
photometry to identify a uniform sample of young stars $\rm<10\;Myr$ old
within 1.5 kpc of the Sun.

###  5.3 Unexpected Large-Scale Features in the Gaia Era

Rather than affirming traditional models of the local solar neighborhood, new
3D dust maps, new catalogs of molecular cloud distances, and new maps of the
3D spatial distribution of OB stars and young stellar clusters provide
evidence for structures much larger than Gould’s Belt and challenge existing
models for spiral structure at high resolution. These features, shown in
Figure 12, include the 2.7 kpc “Radcliffe” Wave containing most of the Gould
Belt clouds, a 2 kpc-long “Split” extending out of the Sco-Cen association,
prominent spurs between the Sagittarius/Local arms and Perseus/Local arms
defined by the distribution of OB stars, and a 1-kpc-long high-pitch-angle
structure in the Sagittarius arm defined by the 3D spatial distribution of
young stellar clusters.

Figure 13: 3D distribution of local clouds. The position of the Sun is marked
with $\odot$. The size of the symbols is proportional to the column density.
The red points describe a spatially and kinematically coherent structure, the
Radcliffe Wave. Remarkable, and currently unexplained, the structure appears
to be undulating on the Galactic plane. The greyscale map in the left panel
show an integrated dust map. For an interactive version of this figure,
including additional layers not shown here (for example, a model of the Gould
Belt and log-spiral arm fits), see here. Reproduced from _Alves et al._
(2020), Nature, 578, 237.

####  5.3.1 The Radcliffe Wave and the Split

There is mounting evidence that the star-forming regions traditionally
associated with Gould’s Belt clouds are only a small part of two linear kpc-
long gas structures: the Radcliffe Wave (_Alves et al._ , 2020) and the Split
(_Lallement et al._ , 2019). The Sun is currently located in between these
kpc-long structures. In Figure 12, we show the Radcliffe Wave in red and the
Split in blue. The Radcliffe Wave and the Split were initially detected with
3D dust mapping, using either the global distribution of molecular clouds with
accurate dust-based distances (Figure 2) or in gross topological maps of the
interstellar medium (Figure 1).

The larger of the two local structures, the Radcliffe Wave, has an aspect
ratio of about 1:20, extends for about 3 kpc and contains about three million
solar masses of gas (see Figure 13). Inside it, star-forming regions are
connected by lower-density gas. The Radcliffe Wave contains four of the major
Gould’s Belt clouds (Perseus, Taurus, Orion, and Cepheus) as well as several
other famous star-forming regions, including CMa OB1, Mon R2, Cygnus X, and
the North America Nebula. The Radcliffe Wave appears to be undulating and is
well described by a damped sinusoidal wave on the plane of the Milky Way with
an average period of about 2 kiloparsecs and a maximum amplitude of about 160
parsecs. The Radcliffe Wave is likely the densest gas distribution of the
Local Arm (_Reid et al._ , 2019) (cyan trace in Figure 12), containing the
bulk of its gas in the nearest 2 kpc, placing the Sun at about 100 pc from the
inner side of a major arm of the Milky Way. However, it has a higher pitch
angle ($\approx 30^{\circ}$) than traditionally associated with the Local Arm,
estimated to be $11^{\circ}$ in _Reid et al._ (2019).

An obvious question now is, what is the relationship between the young
populations used to define the spiral arms in §5.1 (_Castro-Ginard et al._ ,
2021; _Poggio et al._ , 2021; _Zari et al._ , 2021) and kpc-long gas
structures like the Radcliffe Wave emerging from Gaia 3D dust maps (_Alves et
al._ , 2020; _Green et al._ , 2019; _Lallement et al._ , 2019; _Zucker et al._
, 2020)? The answer, as we will see, is not obvious. In one study, _Quillen et
al._ (2020) estimates the birth location of young $(\rm<70\;Myr)$ stellar
clusters by performing backward orbit integrations in a gravitational
potential, finding that the variation in the birth heights of these stellar
associations suggest that they were born in a corrugated disk of molecular
clouds, similar to those inferred from current 3D dust maps. In another study,
_Swiggum et al._ (2022), compared the distribution of massive stars from the
literature (_Castro-Ginard et al._ , 2021; _Pantaleoni González et al._ ,
2021; _Poggio et al._ , 2021; _Zari et al._ , 2021) for the Orion (Local) arm,
with the location of the Radcliffe Wave, and found that the dust and massive
stars do not overlap exactly. In Figure 14, we present the distribution of
young stars from Zari et al. where we overlay the Radcliffe Wave. In this
figure, the location of the Radcliffe Wave is roughly parallel but shifted to
the inner Galaxy compared to the massive stars and clusters in the Orion
(Local) arm. This is similar to the morphology of spiral arms in nearby
galaxies and suggests that the Radcliffe Wave could be the gas component of
the Orion (Local) arm. If this is the case, the spatial distribution is
tantalizingly close to the classical scenario for arm formation via a spiral
shock (_Elmegreen et al._ , 2018), but this result is in tension with _Castro-
Ginard et al._ (2021), which disfavors classical density waves as the drivers
of spiral structure. In the spiral shock scenario, the enhancements seen in
Poggio and Zari et al. would correspond to the slightly older populations
(ages $<$ 50 Myr) in front of the currently star-forming gas of the Radcliffe
Wave. In this scenario, one would expect an age (color) gradient across the
arm, but this was not observed (nor ruled out).

The origin of the undulation in the Radcliffe Wave is also unclear but could
arise internally in the disk (e.g., because of the Kelvin-Helmholtz
instability (_Fleck_ , 2020) or externally due to a collision (e.g. _Tepper-
García et al._ , 2022). To elucidate its origin, the Radcliffe Wave has been
the focus of several studies seeking to characterize its kinematics. In the
first of such studies, _Donada and Figueras_ (2021) presented a study of 13
clusters that are probable members of the Radcliffe Wave and found that the
vertical motion of 11 of these “is not contradictory with the behaviour
expected from a simple model of harmonic motion in the vertical direction.”
_Li and Chen_ (2022) used the proper motions of YSOs associated with Radcliffe
Wave to estimate that the Radcliffe Wave is “possibly oscillating around the
Galactic disk mid-plane with an amplitude of 130$\pm$20 pc”. _Bobylev et al._
(2022) confirmed the existence of the Radcliffe Wave via the position of young
stars. They also recognized the Radcliffe Wave in the positions and in the
vertical velocities of masers and radio stars belonging to the Local Arm but
argue that the Radcliffe Wave is less of a wave but “more like a local high-
amplitude burst, rapidly fading away.”

Figure 14: A Galactic XY (bird’s-eye) view of various stellar/dust
distributions within a few kiloparsecs of the Sun (centered yellow dot). The
over-density map of the OBA-type stars from _Zari et al._ (2021) (black-blue-
white) is shown in the background of each panel. The emergent spiral features
in this map, from decreasing to increasing values in X, are the Perseus,
Orion, and Sagittarius-Carina arms. Star-forming clouds from _Zucker et al._
(2020) are displayed as red points in the first two panels, with the Radcliffe
Wave appearing as a narrow alignment of clouds from ($x$, $y$) $\approx$ (-1,
-1) kpc to (X, Y) $\approx$ (0.5, 2) kpc. The cyan points in the first panel
are the open clusters from _Cantat-Gaudin and Anders_ (2020) with ages less
than 30 Myr, sized relative to their membership count. The second panel shows
the fit (solid white line) to the Orion arm from masers (_Reid et al._ ,
2019), three circles of constant, Galactocentric radius (white dashed lines),
and two arrows pointing in the directions of the Galactic center/Galactic
rotation. Overlaid in the right-most panel is the dust map of _Lallement et
al._ (2019) with a heat map color-scale showing increasing dust density.
Credit: Swiggum et al., A&A, 664, L13, 2022, reproduced with permission ©ESO.

_Thulasidharan et al._ (2022) investigate whether the Radcliffe Wave may be
correlated with a kinematic oscillation perpendicular to the Galactic disk.
Leveraging a combination of open clusters, massive stars, and red giant stars
in and beyond the Radcliffe Wave, _Thulasidharan et al._ (2022) find evidence
of an oscillation in the vertical velocity of a few $\rm km\;s^{-1}$ in the
younger populations, suggesting that the amplitude of oscillation is tied to
the age of the stellar population. Running simulations of a Sgr-like dwarf
galaxy impacting the Milky Way, _Thulasidharan et al._ (2022) find that the
resulting density wave aligns with the Radcliffe Wave but that the kinematic
wave is misaligned, underlining the need for further studies on the effects of
a perturber on the Galactic disk. Following up on the study of _Thulasidharan
et al._ (2022), _Tu et al._ (2022) investigate the vertical angle of pre-main
sequence stars in Gaia DR2 along the wave — corresponding to their orbital
phase perpendicular to the Galactic plane — finding that the vertical angle
varies significantly from trough to peak, in a pattern potentially in
agreement with a wavelike-oscillation. In an analysis of clustered star
formation in the solar neighborhood, _Alfaro et al._ (2022) finds that warp,
corrugations, and high local deviations in both $z$-height and vertical
velocity all likely play an interconnected role in shaping the phase-space
structure of young open clusters near the Sun, and better understanding the
interplay of these phenomena will also likely give rise to better
understanding of the formation of the Radcliffe Wave.

Towards the Galactic center, the remaining Gould Belt clouds in the solar
vicinity (Ophiuchus, Lupus, Chamaeleon, Corona Australis, and the Pipe Nebula)
belong to the newly discovered “Split”, which _Lallement et al._ (2019) argue
could be a spur-like feature between the Sagittarius and Local arms with a
pitch angle of $\approx 45^{\circ}$ The Split begins in the famous Sco-Cen
association and extends almost 2 kpc in length, with a similar width as the
Radcliffe Wave. Its kinematics have yet to be systematically investigated in
the Gaia era, but morphologically it shares many properties with the newly-
discovered Sagittarius spur (_Kuhn et al._ , 2021), as discussed in §5.3.3.

While much work remains, what seems clear is that emerging kpc-scale linear
features like the Radcliffe Wave and the Split detected in 3D dust maps are
the ideal laboratory to test different spiral arm formation theories. The
expected increase of astrometric precision in the subsequent Gaia data
releases and better radial velocities from ground-based surveys promises to
settle this 70-year-old question regarding the nature of spiral structure.

####  5.3.2 The Blue Streams

_Bouy and Alves_ (2015) constructed a 3D map of the spatial density of OB
stars within 500 pc from the Sun using the Hipparcos catalog and found three
large-scale stream-like structures they called Blue Stream, for being composed
of young stars (shown in blue in Figure 12d). The spatial coherence of these
blue streams and the monotonic age sequence over hundreds of parsecs suggest
they are made of young stars, similar to the young streams that are
conspicuous in nearby spiral galaxies. The three streams are 1) the Scorpius
to Canis Majoris stream (Sco-CMa), covering 350 pc and 65 Myr of star
formation history; 2) the Vela stream, encompassing at least 150 pc and 25 Myr
of star formation history; and 3) the Orion stream, including the well-known
Orion OB1abcd associations. Their map reveals a remarkable and previously
unknown nearby OB association, between the Orion stream and the Taurus
molecular clouds (that they name Taurion), which they argued could be
responsible for the observed structure and star formation activity in the
Taurus complex. Post-Gaia, it became clear that the Orion Blue Stream is part
of the Radcliffe Wave. The Sco-CMa Blue Stream, minus the CMa region, which is
part of the Radcliffe Wave, is likely associated with the Split. The Sco-CMa
Blue Stream age gradient would then suggest an ordered transformation of the
Split gas into stars, although more work is needed to test this hypothesis.

####  5.3.3 The Sagittarius Spur

The Sagittarius spur (_Kuhn et al._ , 2021) is a 1-kpc-long+ band of star-
forming regions, shown in yellow in Figure 12. The Spur was originally
detected using the Spitzer/IRAC Candidate YSO (SPICY) catalog (_Kuhn et al._ ,
2021), which leveraged mid-infrared photometry to classify over 100,000 young
stellar objects toward the inner Galactic plane. These YSOs were later grouped
using a clustering algorithm on the plane-of-the-sky, before being cross-
matched with Gaia to get distance and proper motion information for each
group. The Sagittarius spur contains 25 such YSO groups in the SPICY catalog,
which together form a linear structure with a pitch angle of 56∘,
significantly different from the traditional view of the Sagittarius spiral
arm defined by masers, which _Reid et al._ (2019) argue has a pitch angle of
17∘. The Sagittarius Spur also contains about half of the masers used to
define the traditional global log-spiral fit to the near Sagittarius arm in
Reid et al (magenta trace in Figure 12) within the nearest 2 kpc. Like the
Radcliffe Wave and the Split, the Sagittarius spur has a high aspect ratio, of
$\approx$ 7:1 and links many of the most famous star-forming regions toward
the inner Galaxy, including M8, M16, M17, and M20.

While originally detected using YSO groups, the Sagittarius spur is also
clearly seen in 3D dust, with the groups coinciding with a prominent dust lane
showing the same high-pitch angle. While nominally 1 kpc in length (starting
in the first quadrant and ending at the Galactic center) it is possible that
the Sagittarius Spur extends even further into the fourth quadrant, comprising
what _Chen et al._ (2020c) calls the “Lower Sagittarius-Carina Spur” as
detected in their 3D molecular cloud catalog, shown in Figure 2.

Associating the proper motions of the spur’s young stellar objects with its
radial velocities obtained from CO emission of their natal clouds, _Kuhn et
al._ (2021) find the structure to be remarkably kinematically coherent, moving
slightly faster than Galactic rotation and drifting marginally radially
inward, with a bulk motion toward the negative $z$ direction. _Kuhn et al._
(2021) calculate that the rotational shear experienced by the spur is,
$\Delta\Omega$ is $\rm 4.6\;km\;s^{-1}\;kpc^{-1}$, implying a relatively short
predicted timescale of $\approx 90$ Myr for the structure to alter its pitch
angle by a factor of two (_Elmegreen_ , 1980).

####  5.3.4 The Cepheus Spur

Finally, an additional spur in the Gaia era has been identified using new
catalogs of OB stars. _Pantaleoni González et al._ (2021) cross-match the
“Alma Luminous Star” catalog (_Cameron Reed_ , 2003) with Gaia DR2 astrometry
to obtain a probable sample of massive stars and analyze their spatial and
kinematic distribution in the solar neighborhood. Pantaleoni González et al.
identify a prominent overdensity of OB stars (first proposed by _Morgan et
al._ , 1953) extending 2 kpc in length and bridging the Local Arm and Perseus
arm toward the Milky Way’s outer galaxy, which they call the Cepheus spur
(shown in purple in Figure 12). The Cepheus spur contains the prominent OB
associations Cep OB4, Cam OB1, Aur OB1, and Gem OB1. Analysis of the vertical
distribution of its massive stars indicates that the Cepheus spur has an
anomalous average z-height, extending 50 - 100 pc above the Galactic plane.
Analysis of its proper motions indicates that the Cepheus spur is
kinematically distinct, with a common peculiar motion with respect to
neighboring distributions of OB stars. Pantaleoni González et al. suggest that
the Cepheus spur is likely related to the Radcliffe Wave, as the spur touches
the Wave at one end, coinciding with a region of enhanced star formation at
the crest of the wave.

Complementary maps of the distribution of OB stars derived from Gaia
astrometry and photometry show many of the same defining features as the
Pantaleoni González et al. map (see e.g. _Xu et al._ , 2018, 2021; _Chen et
al._ , 2019a) used to discover the Cepheus spur. For example, all maps show
kiloparsec-scale overdensities associated with the Local and Sagittarius arms
— particularly aligned with these arms’ prominent sub-features like the
Radcliffe Wave and the Sagittarius Spur.

###  5.4 A New Framework for Connecting Kiloparsec to Parsec Scales

Together, the discovery of these kiloparsec-scale structures — containing
long-studied clouds previously thought to be in relative isolation — provides
a new framework for testing the influence of galactic dynamics in cloud
formation and evolution.

For example, new simulations featuring resolved cloud populations (_Duarte-
Cabral and Dobbs_ , 2016; _Kim et al._ , 2017, 2020; _Seifried et al._ , 2017;
_Smith et al._ , 2019; _Walch et al._ , 2015; _Jeffreson et al._ , 2020)
provide the key context for understanding relationships between kiloparsec-
scale structures and the clouds and young stars they host, offering the
potential to explain how spiral shocks and differential rotation affect the
large-scale accumulation of gas in spiral galaxies. Many of these simulations
offer the same dynamic range now available in the solar neighborhood. For
example, the Latte suite of FIRE-2 simulations of Milky Way-like galaxies can
track massive molecular clouds with $\approx 1$ pc spatial resolution and
$\approx\rm 1\;Myr$ temporal resolution, and has been shown to reproduce the
observed distribution of molecular cloud masses seen in the Milky Way
(_Benincasa et al._ , 2020). Detailed work has also been done to map the
ensemble of giant molecular clouds formed consistently in the FIRE-2
simulations to their stellar clusters using new cluster formation models,
rendering detailed constraints on the cluster mass, age, and metallicity as a
function of both space and time (_Grudić et al._ , 2022). The FIRE
cosmological simulations also offer synthetic Gaia surveys for select Milky
Way-like galaxies, facilitating further comparison between simulations and the
observations presented in this review (_Sanderson et al._ , 2020).

Beyond simulations, JWST is also opening up a new era of resolved cloud
populations, offering the same high-resolution “face-on” view now attainable
in the solar neighborhood. In particular, the PHANGS-JWST treasury program
(_Lee et al._ , 2021, 2022) will target nineteen nearby ($\rm d\lesssim
20\;Mpc$) galaxies with MIRI and NIRCam imaging with a resolution ranging from
$\rm 5-50\;pc$. In combination with UV-optical data on these galaxies’ young
stellar clusters from Hubble (PHANGS-HST; _Lee et al._ , 2022), spectroscopy
of their HII regions from MUSE (PHANGS-MUSE; _Santoro et al._ , 2022), and CO
observations of their molecular clouds with ALMA (PHANGS-ALMA; _Leroy et al._
, 2021), the PHANGS-JWST treasury program will enable a complete inventory of
star formation activity across the electromagnetic spectrum in these galaxies.

Given the diversity of tracers now capable of probing the 3D structure of our
solar neighborhood, including 3D dust, young stellar objects, OB stars, and
open clusters, future efforts to contextualize one tracer in the context of
complementary tracers should allow better comparisons to both these
extragalactic observations of face-on spiral galaxies and synthetic galaxies
from numerical simulations.

##  6 CONCLUSIONS

The deluge of data from the Gaia satellite has already revolutionized our
understanding of the distribution of the gas and young stars in the Milky Way
and has revealed a new Galactic solar neighborhood with unmatched detail. The
previous model for the distribution of gas and young stars in the local
kiloparsec (the Gould’s Belt) is now in crisis, and this structure appears to
be made up of parts of previously unrecognized kiloparsec-long gas structures,
the Radcliffe Wave and the Split. At the same time, Gaia challenges the
current description of the structure, pitch angles, location, and mass of the
Galactic spiral arms. These unexpected Galactic-scale features can also be
resolved down into their individual star-forming complexes in 3D on parsec
scales for the first time, opening up a new window for understanding the
multi-scale physics of star formation.

The main conclusions of this chapter are:

  * •

We can now begin to reconstruct the 3D spatial distribution of dense gas and
young stars at $\approx$ 1 pc spatial resolution using a combination of 3D
dust mapping and astrometry of young stars obtained from Gaia and VLBI.
Detailed studies of individual star-forming regions show that most clouds are
highly structured, with filamentary and/or sheet-like morphologies. Many
clouds show evidence of being shaped by recent stellar and supernova feedback
events, and either contain sub-cavities or are draped on the edge of large-
scale superbubbles.

  * •

We are starting to be able to measure the 3D motion of cloud complexes using
embedded young stars as proxies for the motion of their parental gas.
Variations in the 3D structure of star-forming regions map to variations in
their dynamics. Several regions show evidence of sequential star formation via
gradients in their ages, kinematics, and spatial distribution, suggesting that
feedback may shape the 3D dynamics of the local gas and that large-scale
triggering may give rise to many of the local young stellar populations.

  * •

3D dust mapping of the interstellar medium out to 2 kpc and beyond reveals
that molecular clouds are not isolated objects. Instead, clouds are connected
and embedded in narrow kpc-long gas structures with widths of the order of
$\approx$ 100 pc. These long and narrow structures resemble the “dust lanes”
seen in nearby galaxies.

  * •

There are two main families of molecular clouds and star-forming regions in
the solar vicinity, corresponding to two of these narrow kpc-long gas
structures 1) the “Radcliffe Wave” family of clouds containing Canis Major,
Monoceros R2, Orion, Taurus, Perseus, Cepheus, North America, and the Cygnus
regions, and 2) the “Split” family of clouds, including the Sco-Cen related
clouds (Ophiuchus, Lupus, Corona Australis, Chameleon), Serpens, and Aquila.
These 3D dust features — and similar features detected in the distribution of
OB stars and young stellar clusters — possess higher pitch angles than
predicted by classical spiral structure theory.

  * •

Regarding the fundamental question of how clusters and associations dissolve
to form the Galactic field, Gaia has opened the critical $\rm 10-100\;Myr$
window on the study of stellar populations, allowing the identification of
young stellar populations without infrared excesses near star-forming
complexes and young stellar populations far from interstellar gas.

  * •

Many open clusters, long thought to be roughly spherical structures, appear to
be cigar-shaped cluster coronae, with most of the stars residing beyond the
cluster core.

Ultimately, Gaia has enabled for the first time a coherent 3D spatial model of
young stars, gas, and dust at a large dynamic range. This 3D spatial
distribution can be combined with 3D stellar motions to probe the dynamical
state and star formation history of the solar neighborhood in 6D. Yet this
field is still in its infancy, with more data and insights still to come. The
next Gaia data release — Gaia DR4 — is expected after late 2025 and will be
based on 66 months of data collection. Gaia DR4 will contain full astrometric,
photometric, and radial velocity catalogs, all variable and non-single-star
solutions, as well as epoch and transit data for all sources (including all
available $\rm BP-RP$ and RVS spectra) (_Gaia Collaboration et al._ , 2022b).
Parallaxes will improve by a factor of $1.4\times$ and proper motions by a
factor of $2.8\times$ compared to Gaia DR3 (_Brown_ , 2019). The final Gaia
data release, Gaia DR5, should include $>10$ years of observations and release
after 2030 (_Gaia Collaboration et al._ , 2022b). Compared to Gaia DR4,
parallaxes, photometry, and radial velocities should improve by $1.4\times$
and proper motions by a factor of $2.8\times$ in Gaia DR5 (_Brown_ , 2019).
Accurate tangential velocities should also be available for a $>22\times$
larger volume. This combination of future Gaia data releases with the wealth
of photometric and spectroscopic surveys on the horizon — including SDSS-V and
LSST (_Kollmeier et al._ , 2017; _Ivezić et al._ , 2019) — will only continue
to enhance our models of the 6D distribution of gas and young stars, paving
the way for a unified understanding of how star formation is mediated by
physical processes occurring across vastly different scales in both time and
space.

Acknowledgments We thank Eric Mamajek and a second anonymous referee for
excellent comments that substantially improved this chapter. Catherine Zucker
acknowledges that support for this work was provided by NASA through the NASA
Hubble Fellowship grant #HST-HF2-51498.001 awarded by the Space Telescope
Science Institute (STScI), which is operated by the Association of
Universities for Research in Astronomy, Inc., for NASA, under contract
NAS5-26555. Phillip Galli acknowledges financial support from São Paulo
Research Foundation (FAPESP) under grants 2020/12518-8 and 2021/11778-9.

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