Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step. Numbers come from NASA and OpenStax; the pictures are not to scale.
Tap the buttons and slide the sliders.
Switch between ordinary light and Webb’s infrared view. What was hiding in the dark pillars?
Dust absorbs visible light but infrared passes through it. Toggle the view to reveal the young stars inside the Pillars of Creation.
Tap the two buttons. Where are the baby stars hiding?
A sketch of the Pillars, not the real photo.
Slide the temperature. When does gravity win, and when does the jiggling gas spread out?
Low temperature means low gas pressure, so gravity gathers the gas into clumps. Heat the cloud and thermal motion disperses them.
Slide to make the cloud cold or warm. When do the bits clump together?
A toy model: the pull and the jiggle are simplified.
Squeeze the clump. What happens to its heat and spin? When do the disk and jets appear?
As the core contracts it heats up and spins faster; the spin flattens infalling gas into a disk, and jets appear along the poles.
Slide to squeeze the ball. Does it get hotter?
Not to scale: real collapse shrinks a core far more.
Pick a clump, then press Squeeze. Does its core reach 10 million °C, or does it end as a brown dwarf?
Fusion needs a core of about 10 million °C. Below ~1/13 of the Sun’s mass (0.075 Suns) the core never gets there: a brown dwarf.
Pick a clump and squeeze it. Does it light up?
Temperatures are rounded; real cores differ from star to star.
Pick a star’s mass. How long does it live, what colour is it, and how does it end?
Main-sequence lifetimes from OpenStax Table 22.1, interpolated on a log scale. Above ~8 Suns the star ends as a supernova.
Lifetimes from OpenStax; the in-between values are estimates.
Slide through the Sun’s life, from its birth to a white dwarf. Where are we now?
About 4.6 billion years in, halfway through ~10 billion years on the main sequence; then a red giant and a white dwarf.
Slide to see the Sun as a baby, now, and very old.
Sizes are not to scale.
Every star started as a giant cloud of gas and dust in space. The cloud is very, very cold.
Gravity is the pull that makes things fall. In the cold cloud, gravity pulls bits of gas together into balls.
Gravity squeezes one ball tighter and tighter. The falling gas crashes together and gets hot. The ball spins faster, like a spinning skater. Now it glows: a baby star!
In the middle of the baby star, tiny bits squeeze together and join up. That makes lots of energy. The energy pushes out, the star stops shrinking, and it shines. A real star is born!
If the ball is too small, it never gets hot enough. It stays a "failed star", called a brown dwarf.
Our Sun was born like this, a long, long time ago. Leftover dust and gas made the planets, and our Earth!
Stars form inside giant, cold clouds of gas and dust called molecular clouds. Gravity pulls a dense clump of the cloud together. As the clump collapses it heats up and becomes a glowing protostar. When its core reaches about 10 million °C, hydrogen nuclei start fusing into helium. That nuclear fusion releases enough energy to push back against gravity, and the collapse stops. At that moment a true star is born.17
Stars are still being born right now. Our Milky Way holds more than 100 billion stars,1 and new ones keep switching on in places like the Orion Nebula and the Pillars of Creation.34
The raw material for stars is spread thinly between the stars as gas and dust. In some places it gathers into huge molecular clouds. They are mostly hydrogen, can stretch hundreds of light-years across, and hold from about 1,000 to 10 million times the mass of the Sun.1 Their insides are astonishingly cold: only about 10–20 kelvin, which is 10–20 degrees above absolute zero.3
Because young stars appear in batches inside these clouds, astronomers call them stellar nurseries.1 A few famous ones:
Why can't we just photograph stars being born? Dust blocks visible light. Infrared light gets through, so telescopes like Webb and Herschel use infrared to look inside.34
Every star's life is a tug of war between two forces: gravity pulling inward and gas pressure pushing outward.38 Star birth is the story of gravity winning for a while, until fusion forces a truce.
4 hydrogen nuclei → 1 helium nucleus + energy
That energy is huge. Each second, the Sun fuses about 600 million tons of hydrogen into helium, and about 4 million tons of that mass turns into pure energy.3 The Sun's core sits at about 15 million °C.2
It depends on the star's mass. Stars much heavier than the Sun reach the main sequence in a few thousand to a million years. The Sun needed millions of years, and the smallest stars need tens of millions.5 The first free-fall collapse of a core is fast by comparison, just thousands of years, which is one reason it's so rarely caught in the act.3
A clump between roughly 13 Jupiters and about 0.075 times the Sun's mass (roughly 1/13) never gets hot enough in the middle for steady hydrogen fusion.5 It becomes a brown dwarf, a "failed star" of roughly 13 to 80 times Jupiter's mass that gives off almost no visible light.9 At the other end, stars top out at somewhere between 100 and 200 Suns, because a very heavy newborn star shines so fiercely that it blows away any extra gas.5
Once a star is born, its mass sets the rest of its story: how hot and what colour it is, how long it lives, and how it dies.18 You might guess that bigger stars last longer because they have more fuel. It's the other way round. A heavier star has more weight pressing on its core, so the core has to be hotter to hold it up, and hotter cores burn fuel much, much faster.10
| Star (mass vs the Sun) | Surface temperature | Colour | Time on the main sequence |
|---|---|---|---|
| 40 × Sun | ~54,000 K | blue | ~1 million years |
| 16 × Sun | ~29,000 K | blue-white | ~10 million years |
| 3.3 × Sun | ~9,600 K | white | ~500 million years |
| 1 × Sun | ~6,000 K | white (looks yellow from Earth) | ~10 billion years |
| 0.8 × Sun | ~5,200 K | orange | ~14 billion years |
| 0.4 × Sun | ~3,800 K | orange-red | ~200 billion years |
Temperatures and lifetimes from OpenStax Table 22.1; colours from OpenStax 17.2.1011 The universe is about 13.8 billion years old,16 so the longest-lived red dwarfs have not had time to burn out.
Hot stars glow blue and cool stars glow red: a star's colour tells you its surface temperature.11 Small, cool red dwarfs are by far the most common stars, about three quarters of the stars in the Milky Way, and some may shine for trillions of years.9
Our Sun formed about 4.6 billion years ago from a spinning cloud of gas and dust called the solar nebula. As it collapsed, it spun faster and flattened into a disk. Most of the material fell to the centre and became the Sun, which now holds 99.8% of the solar system's mass. Much of what was left in the disk built the planets, including Earth.2 That's no accident: disks are a normal by-product of star birth, which is why so many stars have planets.3
The Sun is about halfway through its roughly 10-billion-year life. In about 5 billion years it will become a red giant, and later a white dwarf.214
Star birth and star death are linked. Material thrown out by dying stars and supernovae mixes into future molecular clouds and ends up in the next generation of stars.1 The blast of a supernova, or the strong winds of massive stars, can even squeeze a nearby cloud and trigger new star birth.37 Many of the elements needed for life were made in supernovae,12 so most of the atoms in you, other than hydrogen, were cooked up in stars that died before the Sun was born.
1) A cold molecular cloud of gas and dust forms dense clumps. 2) Gravity makes a clump collapse. 3) The falling gas heats up, spins faster and flattens into a disk, forming a protostar. 4) When the core reaches about 10 million °C, hydrogen fuses into helium. 5) The energy from fusion balances gravity, and a stable star is born.
Stars come from giant, cold clouds of gas (mostly hydrogen) and dust called molecular clouds, or stellar nurseries. Famous examples are the Orion Nebula and the Pillars of Creation in the Eagle Nebula.
It depends on mass. Very massive stars reach the stable, hydrogen-fusing stage in a few thousand to a million years. A Sun-like star takes millions of years, and the smallest stars take tens of millions of years.
A star's mass decides its lifespan. A star 40 times the Sun's mass lasts about a million years, the Sun about 10 billion years, and small red dwarfs hundreds of billions of years or more, longer than the universe has existed so far.
Not for a very long time. Stars are forming right now in our galaxy, for example in Orion and the Eagle Nebula. Star formation only uses a small part of a cloud's gas, and exploding stars return material to space for future stars, but the supply of gas will slowly run down over many billions of years.
A protostar is a baby star: a hot, glowing ball of gas that is still shrinking and pulling in material. It shines from the heat of its collapse, not from fusion. It becomes a true star when hydrogen fusion starts in its core.
A brown dwarf is a 'failed star'. It forms like a star but has less than about 1/13 of the Sun's mass (roughly 13 to 80 times Jupiter's mass), so its core never gets hot enough for steady hydrogen fusion.