Research · Glossary

Glossary

Plain-language explanations of the terms on my research pages, for students and for readers from other fields. Each assumes only introductory physics and calculus.

Jump to: Images and telescopes · Cosmology · Stars · Abbreviations and academic terms

Images and telescopes

Arcsecond: A unit of angle on the sky: 1/3600 of a degree. The full Moon is about 1,800 arcseconds across; Roman's pixels are 0.11 arcseconds across. Used on: Image Reconstruction.
Astrometry: Measuring the precise positions of objects on the sky. Combining images needs accurate astrometry, so that each pixel is placed at the right spot. Used on: Image Reconstruction.
Coadd, coaddition: An image made by combining several exposures of the same part of the sky, which reduces noise and, with dithering, recovers detail lost to undersampling. Used on: Image Reconstruction, Image Analysis.
Correlated noise: Noise that is shared by neighboring pixels instead of varying independently from pixel to pixel; for example, stripes that make a whole row slightly brighter or darker. Unlike white noise, it is not beaten down efficiently by combining neighboring pixels, and if uncorrected it can mimic real patterns. Used on: Image Analysis.
Diffraction limit: The sharpest image a telescope can make, set by the wave nature of light and the size of its mirror: even a perfect telescope spreads a point of light into a small pattern. Space telescopes, free of the blurring by Earth's atmosphere, can reach it. Used on: Image Reconstruction.
Dithering: Shifting the telescope slightly between exposures, often by fractions of a pixel, so that different exposures sample the sky at different positions. Combined, they recover detail that a single undersampled exposure misses. Used on: Image Reconstruction.
Drizzle: The traditional method for combining dithered images from space telescopes such as Hubble: each input pixel is shrunk and "dropped" onto a finer output grid. It is simple and robust, but it does not control the output PSF precisely. Used on: Image Reconstruction.
Flux calibration: Converting the numbers a detector records into physical brightness (flux: the energy arriving per second per unit area), so that measurements from different exposures and instruments agree. Used on: Image Reconstruction.
IMCOM: Short for IMage COMbination: an image-combination method that chooses the weights of input pixels so that the output image has a PSF as close as possible to a target the user specifies, while keeping the noise under control. PyIMCOM is its modern implementation for Roman, and Effortless is a faster successor. Used on: Image Reconstruction.
Inverse problem, linear inverse problem: Inferring the causes from their observed effects, such as the true sky from blurred, sampled and noisy images. It is linear when the observations are a weighted sum of the unknowns plus noise, as in image reconstruction.
Noise: Random fluctuations in a measurement that are not part of the signal, for example from the random arrival of photons or from the detector's electronics. Used on: Image Analysis.
Nyquist–Shannon sampling theorem: A result from signal processing. Any signal, including an image of a star, can be written as a sum of waves of different sizes (a Fourier series). If the smallest wave in it has a certain size, the signal can be recovered exactly from samples taken more than twice per wavelength of that smallest wave. Sample more coarsely, and fine detail is lost or disguised as coarser patterns, which is called aliasing. Used on: Image Reconstruction.
Photometry: Measuring how bright an object is, often through several color filters. Brightness in different filters gives information about color, and so about temperature or redshift. Used on: Cosmological Inference, Image Analysis.
Pixel: One element of a detector or digital image. A pixel value is best thought of as a sample of the sky's brightness at one point, after blurring by the telescope and by the way the pixel collects light across its area, not as a little square of sky. Used on: Image Reconstruction, Image Analysis.
Point spread function (PSF): The image that a telescope and camera make of a single point of light, such as a distant star: a small blur. Every image is the true sky blurred by the PSF, so knowing the PSF precisely is essential for measuring shapes. Used on: Image Reconstruction, Image Analysis.
PSF leakage: How far a reconstructed image's PSF falls short of its target: the part of the result that is a bias, depending on the sky itself, rather than noise.
Sampling: Recording a continuous quantity at discrete points; a camera samples the sky's brightness at each pixel. See also Nyquist–Shannon sampling theorem. Used on: Image Reconstruction, Image Analysis.
Systematics, systematic errors: Errors that push results in a consistent direction, instead of scattering them randomly. More data do not average them away, so they must be understood and corrected. Uncorrected noise patterns, such as stripes, can become systematics when they mimic the signal. Used on: Image Analysis, Asteroseismology, Image Reconstruction.
Undersampled, well-sampled: An image is undersampled when its pixels are too coarse to capture its finest details (fewer than two samples across them), and well-sampled when they are fine enough. Roman's single exposures are undersampled; reconstructed images are well-sampled. Used on: Image Reconstruction.
White noise: Noise that varies independently from one pixel (or moment) to the next, with equal strength on all scales, by analogy with white light, which mixes all colors equally. Averaging N pixels of white noise reduces it by a factor of √N. Used on: Image Analysis.

Cosmology

Cosmological parameters: The handful of numbers in the standard model of cosmology, such as the amount of matter and how clumpy it is. Surveys try to measure them precisely, and to test whether the model itself holds. Used on: Cosmological Inference.
Cosmological simulation: A computer calculation that follows matter (and sometimes gas and stars) under gravity from the early Universe to today. N-body simulations represent matter as many particles. Used on: Cosmological Simulations.
Dark matter: Matter that does not emit or absorb light but has gravity. It makes up most of the matter in the Universe, and weak lensing measures it directly through its gravity. Used on: Cosmological Inference.
Figure of merit: A single number that summarizes how precisely an experiment pins down a set of parameters; larger means smaller error bars. Used on: Cosmological Inference.
Fisher forecast: A fast way to estimate how precisely an experiment will measure some parameters. It looks at how quickly the predicted measurements change when each parameter changes: the faster they change, the better the data can pin that parameter down. It assumes the probability of each parameter value falls off like a bell curve (a Gaussian) around the best value. Named after the statistician Ronald Fisher. Used on: Cosmological Inference.
Forecast: An estimate, made before the data exist, of how precisely a survey will measure something, used to plan the survey and its analysis. Used on: Cosmological Inference.
Galaxy cluster: A group of hundreds to thousands of galaxies bound by gravity, filled with gas at tens of millions of degrees that shines in X-rays. Clusters are the most massive gravitationally bound objects. A cluster is called relaxed when it has settled down since its last merger. Used on: Cosmological Simulations.
Markov chain Monte Carlo (MCMC): A family of algorithms that explore the possible values of many parameters at once by taking a guided random walk ("Monte Carlo" for the randomness; "Markov chain" because each step depends only on the current position). The walk is built to spend more time where the parameters fit the data better, so a histogram of where it went shows the range of good values: the error bars. It is robust but computationally expensive. Used on: Cosmological Inference.
Matter power spectrum: How much the density of matter fluctuates on each length scale. It is the same idea as the oscillation power spectrum of a star, but over distances in space instead of frequencies in time. Many cosmological statistics, including those from weak lensing, are predicted from it. Used on: Cosmological Inference.
Prior: What is assumed about a parameter before looking at the data, for example a range from other experiments. "Tight priors" mean strong outside knowledge. Used on: Cosmological Inference.
Redshift: The stretching of light to longer, redder wavelengths as the Universe expands while the light travels. More distant galaxies have larger redshifts, so redshift serves as a measure of distance and of cosmic time. Used on: Cosmological Inference.
Redshift bins: Groups of galaxies with similar redshifts: slices at different distances, and so at different times in cosmic history. Used on: Cosmological Inference.
Shear: The stretching of an image in one direction and squeezing in the perpendicular one, which turns a circle into a slight ellipse. Weak lensing shears the images of distant galaxies by about 1%; cosmic shear is the pattern of this effect across the sky. Used on: Cosmological Inference, Image Reconstruction.
Standard model of cosmology (ΛCDM): The current standard picture: the Universe has expanded from a hot, dense early state (the Big Bang); it contains ordinary matter, cold (slow-moving) dark matter and dark energy (Λ, the cosmological constant); and structure grew by gravity from tiny early ripples. A handful of cosmological parameters describe it. Used on: Cosmological Inference.
Symplectic method: A numerical method for following motion under forces such as gravity, built so that each time step respects the geometric (symplectic) structure of motion under such forces. One consequence is that "phase-space volume" (a combined measure of the spread in positions and velocities) is conserved. As a result, errors in conserved quantities such as energy stay small over very long runs instead of growing steadily. ContEvol, my family of numerical methods, is nearly symplectic. Used on: Cosmological Simulations.
Two-point correlation function: A statistic that asks how a quantity at one point is related, on average, to the same or another quantity at a point a given distance away. For galaxies: are pairs found more often than by chance, or are their shapes aligned? Three of them are used together for weak lensing: cosmic shear, galaxy–galaxy lensing and galaxy clustering. Combined, they form a "3×2pt" analysis. Used on: Cosmological Inference.
Weak gravitational lensing: The slight bending of light from distant galaxies by the gravity of matter in front of them, which stretches their images by about 1%. Averaged over millions of galaxies, it maps all the matter, including dark matter, and so tests cosmological models. Used on: Image Reconstruction, Image Analysis, Cosmological Inference.
Zel'dovich approximation: A simple model of how structure forms: each bit of matter keeps moving in a straight line in the direction set by the early ripples, so that matter piles up where paths cross. It captures how the cosmic web begins. Used on: Cosmological Simulations.
σ₈ (sigma-8): A measure of how clumpy matter is today: the typical relative fluctuation in the density of matter (as predicted by the simplest theory, which treats the fluctuations as small), averaged over spheres about 37 million light-years in radius. Cosmologists write this radius as 8 h⁻¹ megaparsecs, where a megaparsec is about 3.3 million light-years and h ≈ 0.7 describes the expansion rate. Used on: Cosmological Inference.
Ω_m (omega-m): The fraction of the energy density of the Universe today that is in matter (ordinary and dark), about 0.3. Nearly all the rest is dark energy, which makes the expansion of the Universe speed up. Used on: Cosmological Inference.

Stars

[C/N]: The ratio of carbon to nitrogen at a star's surface, compared with the Sun's, on a logarithmic scale: [C/N] = −0.2 means the ratio is 10^−0.2 ≈ 0.63 times the Sun's. The first dredge-up lowers it, so it measures how deep mixing reached. Used on: Stellar Modeling.
Convection: Energy transport by the motion of gas: hot gas rises and cool gas sinks, as in a pot of water heating on a stove. The outer layers of red giants are convective. Used on: Stellar Modeling.
dex: A difference of 1 in a base-10 logarithm, that is, a factor of 10. So 0.15 dex is a factor of about 1.4. Used on: Stellar Modeling.
First dredge-up: The stage, early on the red giant branch, when the star's convective outer layer reaches deep enough to bring material processed by nuclear burning up to the surface, changing its measured composition. Used on: Stellar Modeling.
Fourier transform: A mathematical tool that breaks a signal varying in time into the frequencies it contains. Used on: Asteroseismology.
Gaia: A European Space Agency mission (2013–2025) that measured positions, distances and motions of more than a billion stars. A distance turns a star's apparent brightness into its true luminosity, and luminosity and temperature give its radius, independently of asteroseismology. Used on: Asteroseismology.
Galactic archaeology: Reconstructing the history of the Milky Way from the ages, compositions and motions of its stars. Used on: Asteroseismology.
Hertzsprung–Russell diagram: A plot of stars' luminosity against their surface temperature, with hotter stars on the left. Stars at different stages of life fall in different regions, so a star's evolution traces a path across it. Used on: Stellar Modeling.
Kepler: A NASA space telescope (2009–2018) that, in its main mission (2009–2013), measured the brightness of about 150,000 stars almost continuously for four years, mainly to find planets. Its light curves also revealed the oscillations of thousands of red giants. Used on: Asteroseismology, Stellar Modeling.
Light curve: A record of an object's brightness over time. Used on: Asteroseismology.
Main sequence: The long, stable stage of a star's life, when it burns hydrogen in its core; about 90% of a star's life is spent there. The Sun is on the main sequence. Used on: Stellar Modeling.
Metallicity: The fraction of a star's material made of elements heavier than helium (all "metals" to an astronomer), usually given relative to the Sun on a logarithmic scale. Used on: Stellar Modeling.
Power spectrum (of oscillations): How much a signal varies at each frequency, found with a Fourier transform; "power" because it measures the square of the variation's amplitude, as power in a wave does. For a red giant's light curve, it shows a comb of oscillation peaks. Used on: Asteroseismology.
Red giant branch bump: A pile-up of red giants at one brightness. It is caused by a brief dimming when the hydrogen-burning shell reaches the sudden change in composition left where the convective layer once reached deepest; each star passes that brightness three times. Its position tests how deep convection mixes. Used on: Stellar Modeling.
Red giant, red giant branch: A star that has run out of hydrogen in its core and swollen to many times its former size (up to a hundred times or more), with a cool, red surface. The red giant branch is the path such stars follow in the Hertzsprung–Russell diagram as they grow brighter. Used on: Stellar Modeling, Asteroseismology.
Scaling relations: Simple formulas that give a star's mass and radius from ν_max, Δν and its surface temperature, by comparison with the Sun. They work well but need calibration and corrections. Used on: Asteroseismology.
Stellar model: A computer solution of the equations of a star's structure (gravity, pressure, energy transport and nuclear burning) followed over time. Codes such as MESA and YREC compute them. Used on: Stellar Modeling.
Surface gravity (log g): The gravitational acceleration at a star's surface, g = GM/R². Astronomers quote log g, the base-10 logarithm of g in cm/s². The Sun's is 4.44 (about 27,000 cm/s², 28 times Earth's); most observed red giants' are about 1 to 3.5. Used on: Stellar Modeling, Asteroseismology.
α-rich population: Stars with more "α elements" (such as oxygen, magnesium and silicon, which stars build by adding helium nuclei, or α particles) relative to iron than the Sun has. They formed early in the Milky Way's history, so they are mostly old. Used on: Asteroseismology, Stellar Modeling.
Δν (large frequency separation): The regular spacing between the peaks in a star's oscillation power spectrum ("large" to tell it from smaller spacings between other kinds of peaks). It is set by the time sound takes to cross the star, and grows with the star's mean density (as its square root). Used on: Asteroseismology.
ν_max (nu-max): The frequency at which a star's oscillations are strongest. It depends mainly on the star's surface gravity, and also on its temperature. Used on: Asteroseismology.

Abbreviations and academic terms

ADS: The NASA Astrophysics Data System, the standard search engine for astronomy papers. On this site, "ADS" links open a paper's record there.
APOGEE: The Apache Point Observatory Galactic Evolution Experiment, a survey of the Sloan Digital Sky Survey that took infrared spectra of hundreds of thousands of stars, measuring their temperatures and chemical compositions.
APOKASC-3: A catalog that combines APOGEE spectroscopy with Kepler asteroseismology for more than 15,000 evolved stars, mostly red giants. "3" is its third release.
arXiv: A free online archive where physicists and astronomers post papers ("preprints"), usually before or while they are reviewed by a journal. Astronomy papers are in the section astro-ph.
Carpentry: In the Schmidt AI in Science fellowship, a small project that uses AI. MapHub is one.
Data challenge: A team exercise in which everyone analyzes the same simulated data, to test the analysis pipeline before real data arrive.
DOI: Digital Object Identifier: a permanent link to a published paper, which keeps working even if the journal's website changes.
Postdoctoral fellowship, project postdoc: A postdoc (postdoctoral researcher) holds a temporary research position after the PhD, usually for two to three years. A fellowship is a postdoc position won in an open competition, which usually comes with its own funding and the freedom to choose one's research; a project postdoc is hired to work on a specific funded project. Here, a fellowship means a postdoctoral position, not a scholarship for students.
Hack session, hack week: A short event, from a few hours to a week, in which people work together on code or data, often helping newcomers get started.
HLWAS, HLIS: The High-Latitude Wide-Area Survey, one of the Roman Space Telescope's core surveys, which will image and take spectra of a large area of sky away from the Milky Way's disk. Its imaging part is the High Latitude Imaging Survey (HLIS), the name used in team names such as "Roman HLIS Cosmology PIT."
MIDAS: The Michigan Institute for Data and AI in Society, the University of Michigan's institute for data science and AI, which hosts the Schmidt AI in Science postdoctoral fellows there.
ORCID: A free, permanent identifier for a researcher (mine is 0000-0002-1699-6944), which tells apart people with the same name.
PIT, Project Infrastructure Team: One of the teams NASA selected to build the software and analysis tools for the Roman Space Telescope's surveys before launch.
Wide-Field Science Team: One of the teams NASA selected to prepare particular science with the Roman Space Telescope's wide-field surveys.

Missing a term, or is an explanation unclear? Let me know.