Did We Overestimate the Number of Stars in the Universe?
Why Read This
What Makes This Article Worth Your Time
Summary
What This Article Is About
Astrophysicist Ethan Siegel dismantles the intuitive but deeply flawed method of estimating the Universe’s stellar population by multiplying Milky Way stars by the number of galaxies—an approach that overestimates the true figure by a factor of hundreds. The error stems from three false assumptions: that the Milky Way is a typical galaxy, that its stars represent the cosmic average, and that distant galaxies observed billions of years in the past have as many stars as galaxies do today.
Siegel explains the correct method: integrating the star-formation rate throughout cosmic history, accounting for the early Universe’s massive but short-lived Population III stars, and subtracting the fraction of stars that have already died. The result is approximately 2.14 sextillion (2.14 × 1021) stars currently existing in the observable Universe—a finite, well-constrained number that is still hundreds of times smaller than the naive multiplication estimate, and far smaller still than the 80 quintillion stars we can actually observe from our vantage point in space.
Key Points
Main Takeaways
The Milky Way Is Atypical
Our galaxy is larger and more massive than the vast majority of galaxies; using it as a proxy for the “average” galaxy inflates any stellar estimate by hundreds of times.
Early Stars Were Giants
The Universe’s first stars formed without heavy elements, grew to 10–25+ solar masses, burned a thousand times brighter than our Sun, and died within a few million years.
Star Formation Has Peaked and Declined
The cosmic star-formation rate peaked more than 10 billion years ago; today it runs at just 3% of that maximum, and roughly 50% of all stars had already formed by the time our Universe was 4.9 billion years old.
~2.14 Sextillion Stars Exist Today
Integrating the star-formation rate across cosmic history and subtracting the ~3% of stars that have since died yields approximately 2.14 × 1021 stars currently alive in the observable Universe.
We Can Only See 4% of Them
Due to the finite speed of light, we observe distant galaxies as they appeared in the past—with fewer stars—meaning only about 80 quintillion stars (4% of the total) are actually visible to us today.
Heavy Elements Enable Modern Stars
Carbon, oxygen, and other heavy elements are essential for cooling collapsing gas clouds into stars. Their absence in the early Universe forced the first stars to grow enormous—and die fast.
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Article Analysis
Breaking Down the Elements
Main Idea
The Naive Estimate Is Wrong by Orders of Magnitude
The common intuition—multiply Milky Way stars by the number of galaxies—rests on three false assumptions that together inflate the true stellar count by hundreds of times. Siegel’s central contribution is showing that the correct approach requires integrating cosmic star-formation history, understanding that most of the Universe’s stars formed in small, low-mass galaxies unlike our own, and recognising that our telescopes see galaxies as they were, not as they are.
Purpose
To Correct a Widely Shared Misconception
Siegel writes explicitly to correct an error made even by prestigious institutions such as the European Space Agency. His purpose is both educational and corrective: to replace an intuitive but wrong shortcut with a rigorous, evidence-based method. The article is aimed at a scientifically curious general audience—one that can follow quantitative reasoning without needing graduate-level mathematics.
Structure
Problem Statement → Why It Fails → Historical Deep-Dive → Correct Calculation → Two Answers
The article opens by stating the naive method and immediately flagging its inadequacy, then spends several sections tracing star-formation from the Big Bang through cosmic history—explaining Population III stars, the initial mass function, and the landmark 2014 Madau & Dickinson review paper. It concludes by distinguishing two different valid questions (stars that exist vs. stars we can observe), arriving at different but equally well-grounded numerical answers for each.
Tone
Precise, Enthusiastic & Instructive
Siegel writes with the infectious enthusiasm of a scientist who finds wonder in exact numbers. He uses specific figures throughout—sextillions, parsecs, percentages—and never shies from quantitative precision, yet wraps each calculation in plain-language explanation. Occasional rhetorical gestures (“Don’t fret”; a Blade Runner quote) keep the tone approachable without sacrificing rigour.
Key Terms
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Tough Words
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The process by which atomic nuclei are created from lighter particles; in cosmology, Big Bang nucleosynthesis produced the Universe’s first hydrogen, helium, and trace lithium.
“…something that occurs during the first few minutes of the Big Bang in a process known as Big Bang nucleosynthesis.”
To extend known data or a trend beyond the range of direct observation in order to estimate values or conditions that cannot be measured directly.
“If we look out across cosmic time and extrapolate what must be out there, based on both what we can see and what we know about the Universe…”
Consisting of two atoms bonded together; diatomic hydrogen (H₂) was the early Universe’s only significant mechanism for cooling collapsing gas clouds—a highly inefficient one compared to heavier elements.
“…is arguably through the occasional diatomic hydrogen molecule (H₂), which is still tremendously inefficient…”
Emitting or reflecting a great deal of light; in astrophysics, luminosity measures the total energy output of a star per unit time, scaling steeply with mass—roughly as the cube of the star’s mass.
“…mass continues to fall onto them, meaning they become heavier, more massive, more luminous, and increasingly shorter-lived.”
The diffuse gas and plasma that fills the vast space between galaxies; its state of ionisation is used as an indirect clock for when the first stars began radiating enough energy to heat the Universe.
“…the neutral atoms in the Universe’s intergalactic medium would have become reionized far earlier than we observe…”
In a way that is temptingly close to fulfilling a desire or answering a question, but not yet complete; used here to describe JWST’s near—but not yet achieved—detection of the Universe’s earliest stars.
“…the James Webb Space Telescope is taking us tantalizingly close in just its first ~14 months of science operations…”
Reading Comprehension
Test Your Understanding
5 questions covering different RC question types
1According to the article, approximately 50% of all stars in the observable Universe had formed by the time the Universe was about 7 billion years old.
2Why were the Universe’s first stars so much more massive than stars that form today?
3Which sentence best explains why multiplying Milky Way stars by the number of galaxies produces such a large overestimate?
4Evaluate whether each of the following statements accurately reflects the article’s content.
The number of stars we can currently observe from Earth (80 quintillion) is only about 4% of the total stars estimated to exist within the observable Universe today.
Approximately 10% of all stars that have ever formed in the Universe have since died, leaving roughly 90% still alive today.
The landmark 2014 review paper by Madau and Dickinson allowed astronomers to trace the star-formation history of the Universe back to when it was only about 5% of its current age.
Select True or False for all three statements, then click “Check Answers”
<div class="aa-quiz__feedback" data-explanation="Statement 1 is True: the article explicitly states we can see about 8 × 1019 stars, which is approximately 4% of the 2.14 sextillion that exist today. Statement 2 is False: Siegel states only about 3%—not 10%—of stars have died by now, leaving approximately 97% still alive. Statement 3 is True: the article states the 2014 paper covered the star-formation rate back to a time when the Universe was only ~650 million years old, or ~5% of its current age of 13.8 billion years.”>5Based on Siegel’s discussion of stellar mass and lifespan, what can we infer about why the current Universe is dominated by low-mass M-class stars rather than the massive O- and B-class stars of the early Universe?
