Research Interest

"The reward of the young scientist is the emotional thrill of being the first person in the history of the world to see something or to understand something." - Cecilia Payne-Gaposchkin

My research group tackles the most challenging aspects of galactic evolution by dissecting the Milky Way in light of large data sets. My work draws heavily on a combination of theoretical modeling, statistical inferences, and machine learning which leads to innovative angles that shed light on the most fundamental questions of star formation, galactic evolution, interstellar medium, and cosmology.


I primarily work on the Milky Way, capitalizing on a wide range of on-going large-scale surveys and most key future surveys in the next decade, including spectroscopy (SDSS-V, DESI, GALAH, APOGEE, LAMOST, JWST), astrometry (Gaia), photometry (DES, LSST, Euclid, WFIRST) and asteroseismology (TESS, PLATO). I am an "end-to-end" large survey-oriented scientist — I develop novel methods to maximally harness information in the data, build theoretical models, and confront them with observation via statistical inference.



Formation of the Milky Way


Stellar astrophysics


Statistical inferences


Large survey data mining


Finite representation of data


Deep learning

I study the Milky Way

"We are part of this universe; we are in this universe, but perhaps more important than both of those facts, is that the universe is in us." - Neil deGrasse Tyson

The question — deciphering the Milky Way

Understanding physical processes responsible for the formation and evolution of galaxies is a fundamental but unsolved problem in astrophysics. Galaxy evolution over cosmic time is complex, and the influences of multiple processes can be hard to disentangle. Our own galaxy, the Milky Way, offers a unique chance to study individual structural components of a galactic system (e.g., disk, bulge, and halo) because it is the only galaxy for which we can study many individual stars in great detail. Since most stars are long-lived, using the stars as "fossil records" (what is known as Galactic archaeology) offers unparalleled insight into the assembly of galaxies.

The opportunity — enabled by the big data

The observational landscape of Galactic archaeology is rapidly changing; ongoing spectroscopic surveys, such as APOGEE and GALAH are collecting high-resolution spectra for 105—106 stars in the Milky Way, three orders of magnitude more stars than before, and are measuring 20—40 elemental abundances. The next generation of surveys such as SDSS-V and DESI will further increase the sample size by another order of magnitude. Importantly, the spectroscopic data is complemented by other crucial surveys; the astrometric data from Gaia has now provided kinematic phase space information for 1.3 billion stars, and the asteroseismic data from PLATO and TESS will reveal precise evolutionary states and stellar ages for about 104 stars.

My recent research focus

  • Measuring the chemical and dynamical evolution of the Milky Way via statistical inferences;

  • Describing complex systems through the lens of machine learning — in particular, designing robust summary statistics to compare simulations with observations;

  • Understanding how stars formed and how stellar ejecta mixed in the interstellar medium;

  • Applying spectral analysis and machine learning techniques that I developed to a broad range of questions which includes binaries, exoplanets, black holes and cosmology (weak lensing).

Upcoming Colloquia

02/2020 @ Cambridge Univesity


02/2020 @ Oxford University


02/2020 @ University College London





Public Outreach

"All men have stars, but they are not the same things for different people. For some, who are travelers, the stars are guides. For others they are no more than little lights in the sky. For others, who are scholars, they are problems." - The Little Prince

TED: How do we study the stars (full link)


How to measure distances [ 1.5 M views ]




Interactive Modules: Interstellar Absorption and the Lyman Alpha Forest (full screen)



"The struggle itself towards the heights is enough to fill a man's heart. One must imagine Sisyphus happy." - Albert Camus

Download Resume

Professional Appointment


Assistant prof. (with tenure) in astrophysics and computer science

Jointly affiliated with Research School of Astronomy and Astrophysics and

Research School of Computer Science


NASA Hubble Fellow

Funded by NASA for a fully independent postdoctoral research for 3 years.


Carnegie-Princeton-IAS Fellow

A 6-year joint postdoctoral fellowship from Princeton University, the Institute for Advanced Study in Princeton (where Albert Einstein worked!) and the Carnegie Observatories (where Erwin Hubble worked!).



Harvard University -- MA, PhD Astrophysics

Advisor: Prof. Charlie Conroy


AURA Future Leader - 2019

Selected by The Association of Universities for Research in Astronomy (AURA) as a young representative

NASA Hubble Fellowship - 2018-21

Prized postdoctoral fellowship from NASA

Carnegie-Princeton Fellowship - 2017-23

A joint prized postdoctoral fellowship from the Carnegie Observatories and Princeton University

Institute for Advanced Study School of Natural Sciences Fellowship - 2017-21

Awarded full funding for an independent research at the Institute for Advanced Study, Princeton

CCAPP Price Prize - 2016

Two finishing Ph.D.s awarded internationally

Selected to attend the Lindau Meeting of Nobel Laureates, Lindau, Germany

Prestigious meeting for young scientists below the age of 35

NASA Earth and Space Science Fellowship - 2015-16

Funded by NASA for a Ph.D. study

Research Milestones

"One of the principal objects of theoretical research is to find the point of view from which the subject appears in the greatest simplicity." - Josiah Willard Gibbs


Journal publications


First/Supervising Author


Advisees' publications






Seminars/conference presentations


"The limits of my language mean the limits of my world." - Ludwig Wittgenstein

A pristine sample of 4.2 million red clump stars
M. Lucey, YST+, MNRAS, in prep.

Radial migration in action space in the Galactic disk
N. Frankel, H.-W. Rix, YST+, ApJ, in prep.

Forecasting chemical abundance precision for extragalactic stellar archaeology
N. Sanford, D. Weisz, YST+, ApJS, in prep.

The GALAH survey: Temporal enrichment of the Galactic disk
2020, MNRAS, 491, 2043

Abundance estimates for 16 elements in 6 million stars from LAMOST DR5 low-resolution spectra
2019, ApJ, 245, 34

The inside-out growth of the Galactic disk
2019, ApJ, 884, 99

Stars that move together were born together
2019, ApJL, 884, L42

A dynamical model for clustered star formation in the Galactic disk
2019, ApJ, 884, 173

Non-LTE chemical abundances in Galactic open and globular clusters
2019, A&A, 728, 54.

The Payne: self-consistent ab initio fitting of stellar spectra
2019, ApJ, 879, 69.

The vertical motion history of disk stars throughout the Galaxy
2019, ApJ, 878, 21.

Measuring radial orbit migration in the Galactic disk
2018, ApJ, 865, 96.

Measuring Oxygen abundances from stellar spectra without Oxygen lines
2018, ApJ, 860, 159.

A large and pristine sample of standard candles across the Milky Way
2018, ApJL, 858, L7.

Stellar ages and masses in the solar neighbourhood: Bayesian analysis using spectroscopy and Gaia DR1 parallaxes
2018, MNRAS, 477, 2606.

Discovery and characterization of 3000+ main-sequence binaries from APOGEE spectra
2018, MNRAS, 476, 528.

Metallicity fluctuation statistics in the interstellar medium and young stars. I. Variance and correlation
2018, MNRAS, 475, 2236.

Signatures of unresolved binaries in stellar spectra: implications for spectral fitting
2018, MNRAS, 473, 5043.

Photospheric diagnostics of core helium burning in giant stars
2018, ApJ, 853, 20

Measuring 14 elemental abundances with R=1,800 Lamost spectra
2017, ApJL 849, L9

Prospect for measuring abundances of >20 elements with low-resolution stellar spectra
2017, ApJ, 843, 32

Constructing polynomial spectral models for stars
2016, ApJL, 826, L25

Accelerated fitting of stellar spectra
2016, ApJ, 826, 83

APOGEE chemical tagging constraint on the maximum star cluster mass in the α-enhanced Galactic disk
2016, ApJ, 816, 10

Prospects for chemically tagging stars in the Galaxy
2015, ApJ, 807, 104

Constraining the Galactic potential via action-based distribution functions for mono-abundance stellar populations
2013, MNRAS, 434, 652

High resolution elemental abundance analysis of the open cluster IC 4756
2012, MNRAS, 427, 882

Galaxy Zoo: Dust lane early-type galaxies are tracers of recent, gas-rich minor mergers
2012, MNRAS, 423, 59

Galaxy Zoo: Dust and molecular gas in early-type galaxies with prominent dust lanes
2012, MNRAS, 423, 49

Principal component analysis on chemical abundance spaces
2012, MNRAS, 421, 1231


"We cannot work without hoping that others will advance further than we have. In principle, this progress goes on ad infinitum. " - Max Weber

Intermediate Algebra

Princeton Prison Teaching Innitiative
Teaching assistant

Stellar astrophysics

Harvard University
Teaching assistant

Classical Mechanics

National University of Singapore
Teaching assistant


National University of Singapore
Teaching assistant


National University of Singapore

Linear Algebra

National University of Singapore

Abstract algebra

National University of Singapore

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