quantity

abundance

The relative amount of a molecule compared to a reference species in a sample.
11 related articles · 2 formulas

Description

Abundance, in a scientific context, refers to the relative amount of a specific molecule or element compared to a reference species in a given sample. It is a measure of 'how much is there' relative to something else, rather than an absolute count. This concept is fundamental to chemistry, astrophysics, and biology, because it allows scientists to characterize the composition of a sample — be it a star, a rock, or a microbial community — and to infer processes that led to that composition.

History

The concept of abundance has roots in early chemistry, but it became a quantitative tool in the 19th century with the development of spectroscopy, which allowed astronomers to measure the abundance of elements in the Sun and stars, as hinted by the title 'Chemical map of a star swarm'.

How it works

Abundance is typically expressed as a ratio, such as parts per million (ppm) or as a fraction relative to a standard. In spectroscopy, for example, the intensity of a spectral line is proportional to the abundance of the emitting species. In genomics, the abundance of a gene sequence in a metagenomic sample reveals which microbes are dominant. The key is that abundance is a relative measure, so it is robust to variations in sample size or total signal, but it requires a well-defined reference.

In practice

Abundance measurements are used in medical diagnostics (e.g., the ratio of specific proteins in blood), in environmental monitoring (e.g., pollutant concentrations), and in astrobiology to search for biosignatures — where the relative abundance of certain molecules could indicate life.

Fun fact

The title 'Why a species thrives where it shouldn't' refers to the 'abundance-occupancy' relationship in ecology, where locally abundant species tend to be more widespread — a pattern that also holds for molecules in a chemical mixture.

Concept graph →

Formulas

$$\mathrm{C}/\mathrm{O} \approx 1$$
Carbon to oxygen abundance ratio is approximately unity.
What is in the formula
CvariableCarbon abundance — abundance
OvariableOxygen abundance — abundance
$$A(\mathrm{X}) = \log_{10}(N_\mathrm{X}/N_\mathrm{H}) + 12$$
Abundance of element X relative to hydrogen on a logarithmic scale.
What is in the formula
A(X)variableLogarithmic abundance of element X — abundance
N_XvariableNumber density of element X — number density per cubic metre
N_HvariableNumber density of hydrogen — number density per cubic metre
log10operatorBase-10 logarithm — logarithm base 10

Related articles · 11

Stellar2026-08-263 minarXiv:2608.26097
Chemical Passport for a Star with Seven Planets
Forty light-years away lives a star that went unnoticed for a long time: TRAPPIST-1 emits two thousand times less light than the Sun. But it turned out to have seven Earth-sized planets, three of which orbit in the zone where water could remain liquid. Yet little was known about the star itself, about its chemical kitchen.
Stellar Palimpsests: How We Read the History of Swallowed Planets
Every star is a living archive, and its spectrum a multilayered manuscript. Over billions of years, the history of the galaxy is written into it: the galactic chemical evolution of the Milky Way leaves a smooth trace in elemental abundances.
Stellar2026-07-023 minarXiv:2607.01699
Galactic Recipe: How the Sun Stopped Being Special
For years, the Sun teased astrophysicists. In its atmosphere, refractory elements, like spices in a basic recipe, were slightly less abundant than in nearly identical twin stars. Suspicion arose: could our star be modest, skipping the planetary "dessert" that others devour with gusto?
Instrumentation2026-06-241 minarXiv:2606.26296express
The Indigenous Perspective on the Drake Equation
The Drake Equation is an attempt to guess the number of intelligent civilizations in our Galaxy. It’s like counting finished cakes in an unknown bakery: you know the ovens (stars), the baking sheets (planets), and the baking time. But were eggs added to the batter? Western science, relying on a single earthly recipe, considers them a rare ingredient.
q-bio.QM2026-06-151 minarXiv:2606.16726express
How Many Mice Does It Take to Understand the Microbiome?
When you're planning a renovation, you don't buy paint by eye: you measure the wall area. In science, deciding how many samples to collect is a similar guess, but the stakes are higher. Skimp on samples, and your study becomes a lottery: you might miss a real effect, or you might mistake noise for a discovery. Overshoot, and you're wasting tubes, months, and, in animal experiments, lives.
q-bio.GN2026-06-121 minarXiv:2606.14823express
Genes Point Drugs in the Right Direction
Imagine a flashlight that only lights up when you're near a road sign: that's roughly how drug discovery works when scientists follow genetic clues. A team of researchers examined over 26,000 target-disease pairs: combinations of a protein that a drug acts on and the disease it treats.
Galaxies2026-03-271 minarXiv:2603.26873express
Patchy Blast: How a Regular Supernova Created Anomalous Stars
Ancient stars, nearly devoid of heavy elements, are like fossilized remnants of the first cosmic flashes. For a long time, the odd ratio of carbon, hydrogen, and helium in three such stars was thought to require a hypernova — an ultra-bright supernova with hundreds of times more energy than a regular one.
Galaxies2026-03-012 minarXiv:2603.01041express
A Chemical Map of a Starry Swarm
At the heart of Omega Centauri, the most massive globular cluster in our Galaxy, thousands of stars are packed so tightly that their light merges into a single glowing blob when seen from Earth. Separating this swarm into individual stars and determining the composition of each is a task akin to sorting through an overfilled anthill.
Ideas Nobody Was Looking For
Science is like a city where residents pave the streets themselves. Where paths are already worn, new houses go up, while untouched vacant lots go unnoticed — not because building there is impossible, but because no one ever looks.
Galaxies2026-01-131 minarXiv:2601.08939express
Exoplanets2025-11-011 minarXiv:2511.00525express
How to Find Life: Look for Molecular Diversity
The chemical composition of a sample is like a soundscape. A living forest sounds polyphonic: chirping, rustles, the hum of insects. A barren desert is just the whistle of the wind.