Astronomy · Unit 1
The Life of Stars
Every star is a tug-of-war it will eventually lose.
Gravity pulls a star inward. Heat and pressure from its core push outward. For billions of years the two stay balanced, and the star shines. When the fuel runs out, gravity wins, and what happens next depends almost entirely on one number: the star's mass.
On this page you'll learn what stars are made of, how they shine, how we read their color and distance, and how they die. Then test yourself with the quiz.
What is a star?
A star is a giant ball of hot gas held together by its own gravity. It is mostly hydrogen, with some helium and small amounts of other elements. The closest one is our Sun.
Stars are common. The Milky Way alone contains more than 100 billion of them, and astronomers estimate the whole universe could hold up to one septillion (a 1 followed by 24 zeros).
Stars are born inside molecular clouds: cold clouds of gas and dust that can be hundreds of light-years across. Dense clumps in a cloud collapse under gravity and heat up. Each hot clump becomes a protostar, a baby star. Because one cloud forms many stars at once, these clouds are called stellar nurseries.
Fusion: how stars shine
A protostar first glows from the heat of its own collapse. But as it squeezes tighter, its core gets hotter and denser. Once the core reaches about 15 million kelvin, hydrogen nuclei are forced together to make helium. This is nuclear fusion, and it releases enormous energy.
Stars take two main routes to the same result. Sun-like stars mostly use the proton–proton chain, which joins protons step by step into helium. Stars heavier than about 2 Sun-masses mostly use the CNO cycle, where carbon, nitrogen and oxygen nuclei act as helpers that are recycled at the end.
That energy creates outward pressure that balances gravity's inward pull. Astronomers call this balance hydrostatic equilibrium. A star fusing hydrogen into helium in its core this way is a main sequence star. It is the longest stage of a star's life.
Here's the twist: massive stars have more fuel but live shorter lives. To hold up their huge weight they must fuse fuel far faster. Some massive stars last only a few million years. Some small, dim stars will shine for trillions of years, longer than the universe has existed so far.
Color and temperature
A star's color is a thermometer. Bluer stars have hotter surfaces. Redder stars have cooler surfaces. That's the reverse of the "red hot, blue cold" labels on a faucet, so it's worth remembering.
Our Sun is in the middle. Its visible surface, the photosphere, is about 5,500 °C (10,000 °F), and it looks yellow-white. Proxima Centauri, the nearest star after the Sun, is a cool, faint red dwarf.
| Color | Surface | Where on the main sequence |
|---|---|---|
| Blue / blue-white | Hottest | Massive stars, upper left |
| Yellow (like the Sun) | Medium | Intermediate-mass stars, middle |
| Red | Coolest | Low-mass stars, lower right |
Astronomers sort main sequence stars into classes by color and temperature, from hottest to coolest: O (blue), B (blue-white), A (white), F (yellow-white), G (yellow, like the Sun), K (orange) and M (red).
Magnitude and brightness
Astronomers rate brightness with the magnitude scale, an idea that goes back to the Greek astronomer Hipparchus, who sorted naked-eye stars by brightness. It runs backward: a smaller number means a brighter star. A first-magnitude star is brighter than a second-magnitude star.
The scale is logarithmic: a magnitude 1 star is 100 times brighter than a magnitude 6 star, and each single step is a factor of about 2.512. Very bright objects get negative magnitudes. In a dark place, the unaided eye can see stars down to about magnitude 6; ESA's Gaia maps stars as faint as magnitude 20.7.
How bright a star looks is its apparent magnitude. That mixes up two things: how much light the star truly gives off and how far away it is. To compare stars fairly, astronomers use absolute magnitude: how bright a star would look from a standard distance of 10 parsecs (about 32.6 light-years). To get it, you need the distance.
Light-years and parallax
A light-year is a distance, not a time. It is how far light travels in one year: about 9.46 trillion km (5.88 trillion miles). Light from the Sun reaches us in about 8 minutes. Light from Proxima Centauri takes about 4.25 years.
How can we measure that? Hold up a thumb at arm's length and blink one eye, then the other. Your thumb seems to jump against the background. That shift is parallax. Move your thumb farther away and the shift gets smaller.
Astronomers do the same with Earth's orbit. They observe a star, wait six months until Earth is on the other side of the Sun, and look again. A nearby star shifts slightly against far more distant stars. The shift is tiny and measured in arcseconds (1/3600 of a degree). Half the total shift is the parallax angle p, and the distance in parsecs is simply:
d (parsecs) = 1 / p (arcseconds), and 1 parsec ≈ 3.26 light-years.
Example from ESA: Sirius, the brightest star in the night sky, has a parallax of 0.379 arcseconds. So d = 1 / 0.379 ≈ 2.64 parsecs ≈ 8.6 light-years.
From the ground, parallaxes smaller than about 0.01 arcsecond are very hard to measure. Space telescopes do better. ESA's Hipparcos measured about 120,000 stars, and ESA's Gaia is measuring parallaxes for more than a billion.
Try it: parallax to distance
Slide to choose a parallax angle. A bigger shift means a closer star. Press a preset to load a real star.
Distance ≈ 2.64 parsecs ≈ 8.6 light-years
The HR diagram
In the early 1900s, the astronomers Ejnar Hertzsprung and Henry Norris Russell had the same idea: plot stars by color and true brightness and look for patterns. The result, the Hertzsprung-Russell (HR) diagram, is still one of astronomy's most useful tools.
Read it like this. Bluer, hotter stars go on the left; redder, cooler stars on the right. Brighter stars go at the top; fainter ones at the bottom. Notice that you need each star's distance to place it, because the vertical axis is true brightness (absolute magnitude), not how bright it looks.
Most stars fall on a diagonal stripe, the main sequence. These are stars fusing hydrogen into helium. Massive blue stars sit at the upper left, Sun-like yellow stars in the middle, and small red stars at the lower right. Above the stripe are giants: cool but bright, so they must be huge. Below and to the left are white dwarfs: hot but faint, so they must be small.
Life cycles: low-mass vs. massive stars
Every star starts the same way, in a molecular cloud. The ending depends on mass. When a star's core runs out of hydrogen, fusion's outward push weakens and the core contracts. Squeezing heats the core, and the star's outer layers swell, often to a few hundred times their old size. The star has become a red giant.
Low- and medium-mass stars (like the Sun)
NASA's Imagine the Universe puts the dividing line at about 7 times the Sun's mass. Below it, the red giant's core gets hot enough to fuse helium into carbon. When the helium runs out, the star sheds its outer layers into a glowing shell called a planetary nebula (nothing to do with planets; it's an old name). The core left behind is a white dwarf: a roughly Earth-sized stellar cinder that slowly cools over billions of years. This is the Sun's fate, several billion years from now.
Massive stars
Heavier stars swell into red supergiants and keep going. Their cores fuse carbon into heavier elements like oxygen, neon and magnesium, and the largest continue until silicon fuses into iron. Each new fuel buys less time. The final silicon stage lasts only days.
Iron is a dead end: fusing it uses energy instead of releasing it. The iron core collapses, rebounds and sends a shock wave out through the star. The result is a supernova, one of the most powerful explosions in the universe. About 75% of the star's mass is blasted into space.
What's left depends on the core. A core of about 1.4 to 5 solar masses becomes a neutron star, an incredibly dense remnant. A larger core collapses into a black hole. Roughly, stars that start with about 7 to 20 Sun-masses make neutron stars; stars above about 20 make black holes.
Quiz: The Life of Stars
Six questions. Pick an answer for each, then press Check my answers. Your score stays on your device.
Talk about it
- When you look at a star 8.6 light-years away, you see light that left it 8.6 years ago. What does that mean for what "now" means in astronomy?
- Massive stars live short lives but make the heavy elements in your body. Is a short, bright life or a long, dim one more important to the universe?
- Almost everything we know about stars comes from their light. What kinds of questions can light never answer?
Sources
- NASA Science, "Star Basics" (composition, number of stars, molecular clouds, fusion, main sequence, lifetimes, red giants, planetary nebulae, white dwarfs, iron core collapse, supernovae, neutron stars and black holes). https://science.nasa.gov/universe/stars/
- NASA Goddard, Imagine the Universe!, "Stars" (hydrostatic equilibrium, 15 million K for fusion, 10 billion years on the main sequence, the 7 solar mass and 20 solar mass dividing lines, 1.4–5 solar mass neutron star cores, 75% of mass ejected). https://imagine.gsfc.nasa.gov/science/objects/stars1.html
- NASA Science, "Sun: Facts" (core about 15 million °C, photosphere about 5,500 °C). https://science.nasa.gov/sun/facts/
- NASA Science, "What is a light-year?" (9.46 trillion km; Proxima Centauri 4.25 light-years; 8 light-minutes to the Sun). https://science.nasa.gov/exoplanets/what-is-a-light-year/
- NASA Science, "Stars in an Exoplanet World" (O, B, A, F, G, K, M classes and their colors). https://science.nasa.gov/exoplanets/stars/
- NASA Technical Reports Server, M. Adams and Q. Qiang, "The Sun: A Star to Study in Our Backyard" (2021) (proton–proton chain in solar-type stars; CNO cycle above about 2 solar masses). https://ntrs.nasa.gov/api/citations/20210009695/downloads/m_adams_qiang_2021_v2.pdf
- ESA Science & Technology, "The Hertzsprung-Russell diagram" (color and brightness axes, main sequence placement by mass, the need for distances). https://sci.esa.int/web/gaia/-/60240-the-hertzsprung-russell-diagram
- ESA Science & Technology, "Measuring stellar distances by parallax" (six-month baseline; Gaia measuring over a billion stars). https://sci.esa.int/web/gaia/-/53278-measuring-stellar-distances-by-parallax
- ESA Science & Technology, "Stellar Distances" (Hipparchus and the magnitude scale, 5 magnitudes = 100 times, d = 1/p, arcseconds, 3.26 light-years per parsec, Sirius example, 0.01″ ground limit, Hipparcos 120,000 stars). https://sci.esa.int/web/education/-/35616-stellar-distances
- ESA Science & Technology, "About Star Mapper" (Hipparcos) (apparent vs. absolute magnitude, 10 parsecs = 32.6 light-years, naked-eye limit about magnitude 6, Gaia to magnitude 20.7). https://sci.esa.int/web/hipparcos/-/58154-star-mapper
- ESA, Gaia DR3 story, "How far away are the stars?" (thumb-and-eyes parallax analogy). https://www.cosmos.esa.int/web/gaia/dr3-how-far-away-are-the-stars
Credits: all illustrations on this page, including the share image, are original SVG drawings made for Jugami. No photos or third-party images are used.