The Galactic Center (Chiefland Astrofest 2026)

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ALMA · radio light from cold dust

JWST · infrared light from stars

The Milky Way, as your eye sees it The same sky, star by star: Gaia The constellations Toward the Galactic Center: Sagittarius Back to the whole sky

The Teapot Tonight at 7 pm, looking southwest In infrared light In millimeter and radio light

Tonight: low in the southwest, right after dark

ESO/S. BrunierGaia star colors via ESAGaia star colors via ESA · constellation lines: d3-celestial (O. Frohn)Simulated sky: Gaia DR3 + Hipparcos (ESA)SPHEREx via NASA/JPL-CaltechPlanck + HI4PI via lambda.gsfc.nasa.govGaia star colors via ESA

The whole sky: the Milky Way as your eye sees it from a dark site (ESO photo). The bright band is our own Galaxy, seen edge-on from inside; the Galactic Center is in the middle, behind the dark dust lanes.
Build 1: the same sky, mapped star by star by the Gaia satellite. Build 2: the constellations.
Build 3: Sagittarius. The spout of the Teapot points toward the Galactic Center.
Tonight the Teapot is low in the southwest after dark (~25° up at 6 pm, ~18° at 7 pm) and sets ~8:50 pm; the Galactic Center itself sets ~8 pm.
Build 5: zoom in on the Teapot; its eight stars circled.
Build 6: back out to tonight's view from here, 7 pm, looking southwest under a dark (Bortle 3) sky, simulated from the Gaia and Hipparcos star catalogs. The Galactic Center is ~10° up, just off the spout.
Build 7: the same view in infrared (SPHEREx), which sees through much of the dust.
Build 8: the same view in millimeter and radio light (Planck + HI4PI): the gas and dust between the stars.
Build 9: back out to the whole sky (Gaia). Next: what other kinds of light show there.

2MASS via IPACSPHEREx via NASA/JPL-Caltech

Planck + HI4PI via lambda.gsfc.nasa.gov

Atomic hydrogen gas

HI4PI

Dust

Planck

Molecular gas

Planck

Gamma rays: the highest-energy light (Fermi)

An unexplained glow right at the center (2009). Dark matter? Pulsars? More later…

NASA/DOE/Fermi LAT Collaboration · inset: T. Linden (U. Chicago), NASA

Gamma rays again: the “Fermi bubbles”

Two giant gamma-ray bubbles above and below the center (2010), each ~8,000 parsecs tall.

NASA/DOE/Fermi LAT/D. Finkbeiner+ · Su+ 2010

X-rays: gas at millions of degrees (ROSAT, 1990s)

A faint loop arcs high above the center…

ROSAT all-sky survey (MPE)

eROSITA (2020): two giant X-ray bubbles

Where do the bubbles start?

Zoom in on the center

The X-ray hot gas goes all the way to the center, tracing where the stars are

Each reaches ~14,000 parsecs from the plane. Energy: ~100,000 supernovae.

X-ray “chimneys” rise from the central ~15 parsecs. The black hole, or many exploding stars? Still debated.

Predehl+ 2020 (eROSITA, MPE/IKI)

Ponti+ 2019 (XMM-Newton, Chandra) · Predehl+ 2020

Predehl+ 2020 (eROSITA, MPE/IKI)

Anastasopoulou+ 2023 (XMM-Newton)

The eROSITA telescope mapped the X-ray sky much more deeply and found two enormous bubbles, above and below the center.
They reach ~14 kpc (14,000 parsecs) from the plane and hold ~10⁵⁶ erg (Predehl+ 2020): roughly 100,000 supernovae' worth.
Build: the color version.
Build: the base of the bubbles: X-ray “chimneys” above and below Sgr A* (Ponti+ 2019, 2021). The launching region is the inner ~15 pc. The driver could be the black hole or supernovae; we don't know. The outflow carries ~0.6–0.8 solar masses of gas per year (Bordoloi+ 2017; Di Teodoro+ 2018, 2020).
Build: box on the central ~10° × 5°. Build: zoom to the XMM-Newton 6.7 keV (Fe XXV) map of that box, same sky coordinates.
The X-ray excess requires ~1.9× solar metallicity (Anastasopoulou+ 2023). Heavy elements are made in stars, so this gas has been through many generations of stars.
Next: ALMA’s view of the center.

The hot gas at the center is rich in heavy elements

ALMA + Green Bank: the central ring of gas (ACES + MUSTANG)

Cold dust glowing at 3 mm across the whole Central Molecular Zone, ~200 parsecs end to end.

Anastasopoulou+ 2023 (XMM-Newton)

ACES: the ALMA CMZ Exploration Survey (Longmore, Ginsburg+) · MUSTANG-2 (Green Bank Telescope)

The Galactic Center has the most stuff, in the Galaxy and on the sky

MeerKAT radio: Heywood+ 2022 (SARAO) · combined with Spitzer by Judy Schmidt

Massive stars: bright, hot, short-lived
small stars: dim, cool, live practically forever

Ekström+ 2012 models, vendian.org colors, www.tinyurl.com/HR-figure

One example of each type of star, on the sky

Gaia star colors via ESA · constellation lines: d3-celestial (O. Frohn)

Optical: DSS2

How a star like the Sun forms (a cartoon)

original cartoon by Cormac Purcell, modified by AG

Missing: neighbors and environment

The cartoon is not universal
Most stars form in crowds, near massive stars.

Guarcello+ 2025: JWST EWOCS Wd1

Madau & Dickinson 2014

Star formation peaked ~10 billion years ago

Back then, stars formed in denser, more turbulent clouds

Kruijssen & Longmore 2013

The Galactic Center is our nearest look at those conditions

The solar neighborhood is not crowded

young star clusters: under 15 · under 60 million years

massive O stars

massive stars forming now

dust: Edenhofer+ 2024 · clusters: Hunt & Reffert 2023 (Gaia DR3), Sco-Cen: Ratzenböck+ 2023 · viz: C. Swiggum (live 3D)

Seen from above: dust and young star clusters near the Sun

young star clusters: under 15 · under 60 million years

massive O stars

dust: Edenhofer+ 2024 · clusters: Hunt & Reffert 2023 (Gaia DR3), Sco-Cen: Ratzenböck+ 2023 · viz: C. Swiggum (live 3D)

Sgr C

Mystery: giant radio filaments

~1,000 thin, straight threads, some ~30 parsecs long
Radio glow from electrons spiraling around magnetic fields
Found in 1984. What makes them is still unknown.

MeerKAT: Heywood+ 2022, Yusef-Zadeh+ 2022 · image: Judy Schmidt

Mystery: star-forming bubbles shaped by magnetic fields?

Sgr C: NASA, ESA, CSA, STScI, S. Crowe (UVA), J. Bally (CU), R. Fedriani (IAA-CSIC)

Mystery: where are the neutron stars?

← the magnetar,
next to Sgr A*

radio pulsars (5)

30 parsecs

10⁸–10⁹neutron stars predicted in the Milky Way: one for each massive star that has died

thousandsfound, almost all as pulsars

~6known within ~30 parsecs of the center. The closest, a magnetar found in 2013, is 0.1 parsecs from Sgr A*.

Theory expects hundreds or more near the black hole. Hidden behind blurring gas, or really missing?

Known pulsars: ATNF Pulsar Catalogue (Manchester+ 2005), Johnston+ 2006, Deneva+ 2009 · magnetar: Mori+ 2013, Eatough+ 2013 · MeerKAT: Heywood+ 2022 · image: Judy Schmidt

Every massive star that dies leaves a neutron star or black hole. Over the Galaxy's life that adds up to ~10⁸–10⁹ neutron stars (e.g., Sartore+ 2010).
We've found only a few thousand, mostly as radio pulsars.
Near the center: five radio pulsars within ~15′ of Sgr A* (Johnston+ 2006; Deneva+ 2009), plus the magnetar SGR/PSR J1745−2900, 2.4″ from Sgr A* (found by Swift in 2013).
Predictions: ~100–1000 pulsars orbiting close to Sgr A* (Pfahl & Loeb 2004). The usual excuse is that turbulent ionized gas smears out the radio pulses, but the magnetar's pulses are less smeared than expected (Spitler+ 2014), so the “missing pulsar problem” is still with us (Dexter & O'Leary 2014).
Thousands of faint old pulsars might also explain the gamma-ray glow: next slide.

Mystery: is the gamma-ray glow dark matter?

Dark-matter particles that collide and annihilate would make gamma rays, brightest where dark matter is densest: the center.

Since 2009: an extra glow at about the right brightness and shape.

T. Linden (U. Chicago), NASA · Goodenough & Hooper 2009; Daylan+ 2016

If dark matter is a particle that can annihilate, the center is the best place to look: two particles must meet, so the signal grows as density squared.
Since 2009 Fermi data show an excess of ~1–3 GeV gamma rays around the center that fits a ~40 GeV particle with the “natural” annihilation rate (Daylan+ 2016).
But the excess only exists after subtracting a model of the whole gamma-ray sky, and it changes with that model.
Millisecond pulsars, old recycled neutron stars, have a similar spectrum; thousands too faint to see one by one would add up to a glow. Recent work finds the glow follows the bulge stars more than a round dark-matter halo (Macias+ 2018; Bartels+ 2018), but this is still argued.
The dark-matter density profile in the inner Galaxy is unknown, and dwarf galaxies have not shown a matching signal.

Mystery: antimatter at the center

When an electron meets its antimatter twin, a positron, both vanish into gamma rays of exactly 511 keV.

511 keV sky model, INTEGRAL/SPI (Siegert+ 2016)

Possible sources

Radioactive atoms from exploding stars
Black holes & neutron stars feeding on companions
Past outbursts of Sgr A*
Dark matter

Siegert+ 2016 (INTEGRAL/SPI)

The 511 keV line is the signature of positrons annihilating with electrons. Balloon experiments saw it from the Galactic Center in the 1970s; ESA's INTEGRAL mapped it.
INTEGRAL/SPI: 58σ detection; bright bulge, faint thick disk, and an unresolved central source at Sgr A* (Siegert+ 2016). The Galaxy needs a few × 10⁴³ positrons per second.
Candidates: radioactive decay from supernovae and other stellar explosions (e.g., ⁴⁴Ti from faint thermonuclear supernovae in old stars, Crocker+ 2017), X-ray binaries and microquasars, past Sgr A* activity, pulsars, light dark matter.
Why it's hard: positrons can travel far before slowing down and annihilating, the bulge-heavy shape doesn't match where young stars are, and the instruments have ~2.7° resolution. COSI (NASA, 0.2–5 MeV) should map it better.

Three major surveys change our perspective

ALMA · ACES

Steve Longmore · John Bally · Cara Battersby · Adam Ginsburg · Jonathan Henshaw · Paul Ho · Izaskun Jiménez‑Serra · Betsy Mills · Daniel Walker · Ashley Barnes · Mattia Sormani · Robin Tress · Jennifer Wallace · Laura Colzi · Maya Petkova · Danya Alboslani · Emad Alkhuja · Jairo Armijos‑Abendaño · Lucia Armillotta · N. Bijas · Nazar Budaiev · Rojita Buddhacharya · Alyssa Bulatek · Michael Burton · Laura Busch · Natalie Butterfield · Paola Caselli · Mélanie Chevance · Claire Cook · Samuel Crowe · Ana Karla Díaz‑Rodríguez · Simon Dicker · Katarzyna Dutkowska · Adam Fairley · Christoph Federrath · Rubén Fedriani · Zi‑Xuan Feng · Karl Fiteni · Gary Fuller · Pablo García · Philipp Girichidis · Simon Glover · Javier Goicoechea · Mark Gorski · Savannah Gramze · Qi‑Lao Gu · Perry Hatchfield · Christian Henkel · Rebecca Houghton · Pei‑Ying Hsieh · Yue Hu · Katharina Immer · Namitha Issac · Desmond Jeff · Janik Karoly · Jens Kauffmann · Ralf Klessen · Diederik Kruijssen · Mark Krumholz · Cornelia Lang · Alex Lazarian · Rachel Lee · Emily Levesque · Fu‑Heng Liang · Dani Lipman · Xunchuan Liu · Xing Lu · Qiu‑yi Luo · Alessandro Lupi · Sera Markoff · Sergio Martín · Brian Mason · Farideh Mazoochi · Laura McCafferty · Emily Moravec · Mark Morris · Francisco Nogueras‑Lara · Marie Nonhebel · Tomoharu Oka · John Orlowski‑Scherer · Jürgen Ott · Marco Padovani · Xing Pan · Dylan Paré · Thushara Pillai · Jaime Pineda · Marc Pound · Denise Riquelme‑Vásquez · Víctor Rivilla · Charles Romero · Galaxy Salo · Álvaro Sánchez‑Monge · Miriam Santa‑Maria · Anika Schmiedeke · Rainer Schödel · Matthias Schultheis · Howard Smith · Yoshiaki Sofue · Jack Sullivan · Enrico Di Teodoro · Leonardo Testi · Volker Tolls · Miguel Requena Torres · Grant Tremblay · Arianna Vasini · Gijs Vermariën · Alexey Vikhlinin · Serena Viti · Q. Daniel Wang · Gwenllian Williams · Fengwei Xu · Qizhou Zhang · Suinan Zhang

8 papers so far (2025–26)

The whole central ring at 3 mm wavelength: dust and dozens of molecules.
sites.google.com/view/aces-cmz

JWST survey

Rainer Schödel · Steve Longmore · Adam Ginsburg · Konstantina Anastasopoulou · Jay Anderson · John Bally · Ashley Barnes · Cara Battersby · Alessandra Mastrobuono Battisti · Giuseppe Bono · Aaron Bryant · Santi Cassisi · Mélanie Chevance · Anna Ciurlo · Angela Cotera · Samuel Crowe · Tuan Do · Rubén Fedriani · Anja Feldmeier‑Krause · Sebastiano von Fellenberg · Lucia Bravo Ferres · Dimitri Gadotti · Carme Gallart · Teresa Gallego‑Calvente · Eulalia Gallego‑Cano · Pablo García · Angela Gardini · Abhimat Gautam · Andrea Ghez · Stefan Gillessen · Miguel Cano González · Alessia Gualandris · Robert Gutermuth · Daryl Haggard · Matthew Hankins · Jonathan Henshaw · Matthew Hosek · Rebecca Houghton · Yue Hu · Tracy Huard · Ralf Klessen · Diederik Kruijssen · Ryan Lau · Alex Lazarian · Rebecca Levy · Mattia Libralato · Anan Lu · Jessica Lu · Xing Lu · Nora Luetzgendorf · John Magorrian · Shifra Mandel · Roeland van der Marel · Macarena García Marín · Sera Markoff · Alvaro Martinez · Maria Melamed · Betsy Mills · Kaya Mori · Mark Morris · Elena Murchikova · Paco Najarro · David Nataf · Nadine Neumayer · Shogo Nishiyama · Francisco Nogueras‑Lara · Dylan Paré · Florian Peissker · Maya Petkova · Thushara Pillai · Gabriele Ponti · Rodrigo Contreras Ramos · R. Michael Rich · Carlos Román‑Zúñiga · Michael Rugel · Nils Ryde · Nadeen Sabha · Joel Sánchez‑Bermúdez · Álvaro Sánchez‑Monge · Matthias Schultheis · Stephen Serjeant · Lijing Shao · Janet Simpson · Mattia Sormani · Christopher Sorrell · Jonathan Tan · Enrico Di Teodoro · Brian Thorsbro · Pablo Torne · Robin Tress · Elena Valenti · Sill Verberne · Pierre Vermot · Eduardo Vitral · Gunther Witzel · Siyao Xu · Farhad Yusef‑Zadeh · Michal Zajacek · Qizhou Zhang · Manuela Zoccali

half done (Sep 2026); the rest in 2027

Infrared images of millions of stars: ~4 million so far.
sites.google.com/view/jwst-gc

Roman surveys

et al.

coming

Repeated images of the inner Galaxy: star motions, variable stars, and planets.

The Galactic Center: open questions (a partial list)

	question	open since
	What makes the giant radio filaments?	1984
Q2	How does gas flow into the center?	1991
Q4	What shape is the central ring of gas?	1995
Q3	(How) does gas reach the black hole, Sgr A*?	1996
	Was the black hole brighter in the recent past?	1996
	What are the fast-moving compact clouds?	1998
Q7	Is the mix of star masses different here?	1999
Q6	Where do the center’s young stars go?	2002
	How did the central disk of stars form?	2002
	Why are there so many cosmic rays?	2005
Q8	Is the chemistry different?	2009
	How does dust dim and redden the light?	2009
	Is the extra gamma-ray glow dark matter, or pulsars?	2009
	Where are the massive stars, and how many are there?	2010
	Where do the lone massive stars come from?	2010
Q5	Why does the center form so few stars?	2013
	Does star formation come in bursts?	2015
	What accelerates the most energetic cosmic rays?	2016
Q1	What planets form there?	2016

Q1	What planets form there?	2016	←
Q2	How does gas flow into the center?	1991
Q3	(How) does gas reach the black hole, Sgr A*?	1996
Q4	What shape is the central ring of gas?	1995
Q5	Why does the center form so few stars?	2013
Q6	Where do the center’s young stars go?	2002
Q7	Is the mix of star masses different here?	1999
Q8	Is the chemistry different?	2009

Almost every planet we know is within ~1,000 parsecs of the Sun

NASA Exoplanet Archive, Sept 2026

Roman will find tens of thousands of planets ~8,000 parsecs away

(not counting any found in the Galactic Center field)

Planets within ~1,000 parsecs: 82% of those known today → ~10% after Roman

NASA Exoplanet Archive, Sept 2026 · microlensing: Penny+ 2019, ROTAC 2025, Terry+ 2026 · transits: Wilson+ 2023

What happens to
planet-forming disks
in these extreme
places?

Proplyds: disks boiled away by nearby massive stars

Disks trimmed by surrounding gas
Wijnen+ 2017; Otter+ 2021

Disks with tails in the Arches cluster
Hosek+ in prep

Madau & Dickinson 2014

EWOCS: JWST in Westerlund 1 & 2

Monsch et al.,
incl. Lovell, Galarza

Guarcello+ 2024, 2025

Wd1

Wd2

Planet-forming disks exist in the center

Sgr C
disk: 2,000× the Earth–Sun distance
star: 32 Suns
spirals: a passing star?

Hundreds detected, a few resolved

We can study planet formation in this environment.

Early days: so far we can only see the biggest disks.

Soon: how many young stars have disks?

Lu+ 2022 (top figure) · Weng+ 2026 · Zhang+ 2025 · Budaiev+ 2024 · Daley+ in prep

Q1	What planets form there?	2016	future!
Q2	How does gas flow into the center?	1991	progress
Q3	(How) does gas reach the black hole, Sgr A*?	1996	future
Q4	What shape is the central ring of gas?	1995	←
Q5	Why does the center form so few stars?	2013
Q6	Where do the center’s young stars go?	2002
Q7	Is the mix of star masses different here?	1999
Q8	Is the chemistry different?	2009

Distance: astronomy's most fundamental question

Overhead tonight: the Summer Triangle

Vega and Deneb look similar, but Deneb is 50 to 100 times farther away

→ Deneb gives off more than 1,000× as much light as Vega

Measuring distance: parallax

Blink one eye, then the other: your thumb jumps against the background

Earth's orbit gives us two “eyes” 300 million km apart

Gaia (2013–2025): distances to 1.5 billion stars

best: a dime seen from 160,000 km (4 laps around the earth) away

But dust dims the Galactic Center's visible light a trillion times

Gaia star colors via ESA

What you can see toward Sagittarius, by distance

The best distance to the center: a star orbiting the black hole

The star S2 orbits the center every 16 years

→ a black hole of 4 million Suns
Nobel Prize 2020: Reinhard Genzel & Andrea Ghez

Orbit size in angle + orbit size in kilometers (from its speed) → 8,200 parsecs, within a few percent

We measure velocity through Doppler shift and sky motion by watching the orbit, giving distance.

orbit: GRAVITY Collaboration 2020

Stars around the black hole, 1995–2026

Observed (green label): Keck/NIRC2 (Keck Observatory Archive), VLT/NACO (ESO archive), JWST/NIRCam (MAST) images, aligned for this talk; GRAVITY images from Abd El Dayem et al. 2026. In between: the nearest real image with stars moved along published orbits and motions (Gillessen et al. 2017; GRAVITY Collaboration 2021; Abd El Dayem et al. 2026), blurred as that instrument measured. Before 2002: model at the 1998 Keck speckle resolution.

S2 and S301, 1995–2030

Positions from published orbits (Gillessen et al. 2017; GRAVITY Collaboration 2021; Abd El Dayem et al. 2026), drawn at GRAVITY resolution. Bright bar: years with observations; dim bar: orbit extrapolated backward or forward in time.

The best distance to the center comes from a star, S2, that orbits the central black hole every 16 years.
Its orbit tells us the black hole's mass: 4 million Suns (Nobel Prize 2020, Reinhard Genzel and Andrea Ghez).
We measure the orbit's size as an angle on the sky, and its size in kilometers from the star's speed (Doppler shift). Together: ~8,200 parsecs, within a few percent.
Build: thirty years of resolution. Snapshots labeled "Observed" are real images (Keck, VLT, and JWST archival data, then GRAVITY images from the S301 discovery paper). In between, each frame is the nearest real image with the cataloged stars moved along their published orbits and motions, so it keeps that instrument's own blur (circle in the corner) and depth; everything before 2002 is a model using the resolution measured in 1998 Keck speckle data. The progression is not strictly sharper each step: Keck speckle in 1998 already had a ~55 milli-arcsecond core but was shallow, VLT adaptive optics ~75, Keck laser-guide-star adaptive optics ~50–65, and JWST (6.5 m mirror) ~75; the leap is GRAVITY, where stars are about 3 milli-arcseconds across, roughly 20 times sharper than a single 8–10 m telescope (zoom, rewinding to 2021). The last frame draws the orbits of S2 (16 years; closest approach in 2018) and S301 (8.7 years).
Next build: the same zoom on just those two stars, 1995–2030, two years per second. Bright part of each bar on the timeline = years with observations; dim = orbit extrapolated back (S301 before 2021) or forward (everything after 2025). White diamonds mark closest approach: S2 in 2002 and 2018 (next 2034), S301 every 8.7 years (1997, 2005, 2014, 2023, next 2032).

We know our neighborhood in 3D, but not the Galactic Center

Molinari+ 2011: the 100-pc ring, face-on

Molinari+ 2011: a twisted ring, seen from above

Sofue+ 2025: Galactic Center arms from ACES

Sofue+ 2025: spiral arms in the center, from ALMA

Lipman+ 2026: nested orbits

Tress+ 2026: a simulation seen from above, ~1,000 parsecs across, over 125 million years

Progress: a 3D map of the central ring, 2025–26

Lipman+ 2026, 3D-CMZ V Fig. 6

~3/4 of known clouds are in front of the black hole
Walker+ 2025; Lipman+ 2026; Nogueras-Lara+ 2026; Mills+ 2026
For most clouds, the best we can say is "in front"
Most of the gas isn't in clouds, so we can't place it yet

Roman will track the motions of tens of millions of stars: a new way to map the gas in 3D

□ Roman Galactic Center survey □ Roman bulge survey

Mapping the gas in 3D with moving stars

Stars circle the center: on the near side they drift one way across the sky, on the far side the other way

A cloud of gas dims and reddens the stars behind it

Which stars are dimmed tells us where the cloud sits, to within ~10 parsecs

Orbits: Nieuwmunster+ 2024; method: Vasini+ 2026

Q1	What planets form there?	2016	future!
Q2	How does gas flow into the center?	1991	←
Q3	(How) does gas reach the black hole, Sgr A*?	1996
Q4	What shape is the central ring of gas?	1995
Q5	Why does the center form so few stars?	2013
Q6	Where do the center’s young stars go?	2002
Q7	Is the mix of star masses different here?	1999
Q8	Is the chemistry different?	2009

The bar feeds the center

bar model: the bar's near- and far-side dust lanes feeding the CMZ, overshooting gas, and G1.3, BC2 and G5

The bar feeds the central ring

bar model: the bar's near- and far-side dust lanes feeding the CMZ, overshooting gas, and G1.3, BC2 and G5, on a barred galaxy, with the l = 0 and +-5 deg sightlines

Gas falls along the bar's dust lanes: about one Sun's worth per year

About a third lands on the ring on its first pass; the rest overshoots Hatchfield+ 2021

...and hits the far side dust lane

cartoon: S. Gramze (Gramze+ 2023)

simulation: D. Lipman

Q1	What planets form there?	2016	future!
Q2	How does gas flow into the center?	1991	progress
Q3	(How) does gas reach the black hole, Sgr A*?	1996	←
Q4	What shape is the central ring of gas?	1995
Q5	Why does the center form so few stars?	2013
Q6	Where do the center’s young stars go?	2002
Q7	Is the mix of star masses different here?	1999
Q8	Is the chemistry different?	2009

The central black hole barely eats

Sgr A*: 4 million Suns

Gas flowing into the center along the bar: ~1 Sun's worth per year

What the black hole swallows: less than a millionth of that

Where does the rest go?

EHT Collaboration / ESO, CC BY 4.0

Is gas flowing in toward the black hole?

Tress+ 2026 (Rhea-RT)

Some fast-moving gas features are shocks: gas crashing into gas — Lipman+ 2026

Lipman+ 2026 Fig. 8: high-velocity-width features in ACES

Lipman+ 2026 ACES VIII, Fig. 8

Is this inflow?

Mills+ 2026 sky and top-down views of the central 10 pc, with LF 2 overlaid

LF 2: Battersby+ 2026 · figure: Mills+ 2026

Hints of gas flowing inward — Battersby+ 2026

Future: Roman 3D maps; detailed simulations

The Galactic Center: open questions (a partial list)

	question	open since
	What makes the giant radio filaments?	1984
Q2	How does gas flow into the center?	1991
Q4	What shape is the central ring of gas?	1995
Q3	(How) does gas reach the black hole, Sgr A*?	1996
	Was the black hole brighter in the recent past?	1996
	What are the fast-moving compact clouds?	1998
Q7	Is the mix of star masses different here?	1999
Q6	Where do the center’s young stars go?	2002
	How did the central disk of stars form?	2002
	Why are there so many cosmic rays?	2005
Q8	Is the chemistry different?	2009
	How does dust dim and redden the light?	2009
	Is the extra gamma-ray glow dark matter, or pulsars?	2009
	Where are the massive stars, and how many are there?	2010
	Where do the lone massive stars come from?	2010
Q5	Why does the center form so few stars?	2013
	Does star formation come in bursts?	2015
	What accelerates the most energetic cosmic rays?	2016
Q1	What planets form there?	2016

Q1	What planets form there?	2016	future!
Q2	How does gas flow into the center?	1991	progress
Q3	(How) does gas reach the black hole, Sgr A*?	1996	future
Q4	What shape is the central ring of gas?	1995	progress
Q5	Why does the center form so few stars?	2013	←
Q6	Where do the center’s young stars go?	2002
Q7	Is the mix of star masses different here?	1999
Q8	Is the chemistry different?	2009

The central ring of gas

Gas: 50 million Suns' worth · new stars: ~1 Sun's worth per decade

Image: NRAO, Ginsburg

Ginsburg+ 2018a

>1/3 of its stars are born in clusters that stay together (3–8× the local rate)

About half of its star formation happens in one cloud: Sgr B2

Barnes+ 2017, Hatchfield+ 2024, Ginsburg+ 2018b, Budaiev+ 2024

We're going to zoom in ~300× to count young stars.

Sgr B2 with JWST/ NIRCam MIRI on MeerKAT radio: glowing gas around massive stars

Budaiev+ 2026

Clusters forming now

1 pc

MIRI sees massive young stars still buried in dust

1 pc

Budaiev+ 2026

JWST sees an outflow:
the first infrared light from deep inside Sgr B2 North

a cluster of ~3,000 Suns, still gaining ~1 Sun's worth of gas every 10 years

Budaiev+ 2026

The puffy red stuff, only at 25μm, is still a mystery

JWST: slightly older young stars, spread out

ALMA: stars forming right now, still buried

◯ JWST: young stars with warm dust (Budaiev+ 2026)

■ ALMA: forming stars (Ginsburg+ 2018, Budaiev+ 2024, Daley+ in prep)

━ JWST: where young stars are most crowded

Star formation in Sgr B2 is lopsided: JWST and ALMA see different sides

The transition is sharp

Budaiev+ 2026

Star formation is ongoing
Lopsided because the cloud is being squeezed or hit
Still open: why does a collision make stars?

Gas and young stars can go separate ways

Tress+ 2026 Fig. 22: schematic of a star-forming region in the CMZ

(a) A cluster forms, perhaps where the bar's gas hits the ring

(b) The new stars blow a hole in the cloud, but don't destroy it

(c) The gas is pushed around and stays on circular orbits; the stars drift off

(d) stars may drift back and affect a different cloud

Tress+ 2026 (Rhea-RT), Fig. 22

Sgr B2 is forming stars — is the rest of the center?

Henshaw+ 2023; Longmore+ 2013

JWST looked at a few of the center's clouds before 2026

Sgr B2: >500 young stars forming (ALMA)
~100 bubbles of glowing gas around new massive stars

Ginsburg+ 2018, Jeff+ 2024, Budaiev+ 2024, 2025, 2026, Daley+ in prep

Cloud E/F: 100,000 Suns' worth of gas, maybe 1 young star

Houghton, Esteve, Gutermuth, Longmore, Huard, Kauffmann subm.

Clouds C & D: 100,000 Suns' worth of gas, one small cluster

Gramze+ in prep

The Brick: 100,000 Suns' worth of gas, about 10 young stars

Walker+ 2021

Stars are forming in the center, but we've only checked a small part of it

The JWST Galactic Center Treasury Survey (GO 10678) — NIRCam: half observed

starformation.astro.ufl.edu/avm_images/jwst_gc_aladin.html
starformation.astro.ufl.edu/jwst-gc

When did the stars form?
Where are the dust and gas?
Finding young stars

■ observed Sep 2026 (68) ■ planned Aug–Sep 2027 (64) ■ skipped, to be rescheduled (7) status 2026-10-15

…and the same survey with MIRI

starformation.astro.ufl.edu/avm_images/jwst_gc_aladin.html
starformation.astro.ufl.edu/jwst-gc

Finding young stars
How new stars heat their surroundings

■ observed Sep 2026 (68) ■ planned Aug–Sep 2027 (64) ■ skipped, to be rescheduled (7) status 2026-10-15

Progress:

starformation.astro.ufl.edu/avm_images/jwst_gc_aladin.html
starformation.astro.ufl.edu/jwst-gc

Half observed, Sep 2026
two infrared colors
the rest: Aug–Sep 2027

Early highlights

~4 million stars so far
positions precise to a dime seen from 3,200 km
star motions, by comparing with earlier JWST images

□ observed mosaic of 2026-10-15

JWST Galactic Center survey

stars removed: now you see the gas and dust

JWST + ALMA (red: cold dust)

JWST MIRI: longer-wavelength infrared (Budaiev+ in prep)

Q1	What planets form there?	2016	future!
Q2	How does gas flow into the center?	1991	progress
Q3	(How) does gas reach the black hole, Sgr A*?	1996	future
Q4	What shape is the central ring of gas?	1995	progress
Q5	Why does the center form so few stars?	2013	progress
Q6	Where do the center’s young stars go?	2002	←
Q7	Is the mix of star masses different here?	1999	←
Q8	Is the chemistry different?	2009

Where do the center’s star clusters go?

Arches

Quintuplet

Expected to dissolve within a few tens of millions of years, and blend in even sooner — Portegies Zwart+ 2002

Future: look for trails of stars behind the Arches and Quintuplet with JWST + Roman

Is the mix of star masses different here?

Hosek+ 2019 Fig. 17: IMF slope vs mass range; IMF cartoons

Extra heavy stars
(above 1.8 Suns)
(Hosek 2019)

Hubble: ≥ 1.8 Suns

JWST: ≥ 0.4 Suns

(GO 2045 & 10678)

Hosek+ 2019, Fig. 17 · JWST goal: GO 2045 (PI Hosek), NIRCam on the Arches & Quintuplet

Q1	What planets form there?	2016	future!
Q2	How does gas flow into the center?	1991	progress
Q3	(How) does gas reach the black hole, Sgr A*?	1996	future
Q4	What shape is the central ring of gas?	1995	progress
Q5	Why does the center form so few stars?	2013	progress
Q6	Where do the center’s young stars go?	2002	future
Q7	Is the mix of star masses different here?	1999	future
Q8	Is the chemistry different?	2009	←

Ice hints that the center has more heavy elements

cloud c

the Brick

Gramze+ 2026, OJAP

JWST's colors show ice

Next: measuring the ice with JWST spectra

JWST spectra, observed Sep 2026

PI Matt Ashby (SAO) · Cargile, Ginsburg, Hora, Lisse, Melnick, Paladini, Tolls, Werner, Young, Zucker, Kim

Heavy elements matter for forming stars; chemistry matters for planets

Rivilla+ 2026, Fig. 1

Gas pours in along the bar

Stars are forming, but too few

Icy, with rich chemistry

3D maps coming!

	question	now	future
Q1	What planets form there?	disks seen; no planets yet	Roman planet hunt, ALMA disks
Q2	How does gas flow into the center?	measured: ~1 Sun per year	ALMA maps + simulations
Q3	(How) does gas reach the black hole?	hints of inflow	Roman + simulations
Q4	What shape is the central ring?	~3/4 of known clouds in front	Roman 3D maps
Q5	Why so few stars?	6–11× too few	JWST young-star census
Q6	Where do the young stars go?	no tails found yet	JWST star motions
Q7	Is the mix of star masses different?	Arches: extra heavy stars	deeper JWST, more clusters
Q8	Is the chemistry different?	ices: more heavy elements?	JWST spectra

Answered: Why is the gas so warm? (Ginsburg+ 2016)
Partly: how dust dims the light (Bravo Ferres+ 2025)
Still open: radio filaments · the black hole’s past · fast compact clouds · cosmic rays · lone massive stars · bursts of star formation

Thank you! Questions?

More questions, more answers

Massive stars: bright, hot, short-lived small stars: dim, cool, live practically forever

Almost all of the light in star-forming galaxies comes from massive stars

Stars form from gas (seen here via its dust)

Most of what we know about how stars form comes from small, nearby clouds

How a star like the Sun forms (a cartoon)

The cartoon is not universal Most stars form in crowds, near massive stars.

The solar neighborhood is not crowded

Seen from above: dust and young star clusters near the Sun

Mystery: where are the neutron stars?

Mystery: is the gamma-ray glow dark matter?

Mystery: antimatter at the center

Three major surveys change our perspective

The Galactic Center: open questions (a partial list)

Almost every planet we know is within ~1,000 parsecs of the Sun

Roman will find tens of thousands of planets ~8,000 parsecs away

Planet-forming disks exist in the center

Distance: astronomy's most fundamental question

Measuring distance: parallax

What you can see toward Sagittarius, by distance

The best distance to the center: a star orbiting the black hole

We know our neighborhood in 3D, but not the Galactic Center

Progress: a 3D map of the central ring, 2025–26

The bar feeds the center

The bar feeds the central ring

The central black hole barely eats

Is gas flowing in toward the black hole?

The Galactic Center: open questions (a partial list)

The central ring of gas

Gas and young stars can go separate ways

Where do the center’s star clusters go?

Is the mix of star masses different here?

Next: measuring the ice with JWST spectra

Heavy elements matter for forming stars; chemistry matters for planets

Massive stars: bright, hot, short-lived
small stars: dim, cool, live practically forever

The cartoon is not universal
Most stars form in crowds, near massive stars.