Glowing pink and blue nebula in deep space

Module 17 · Stellar Nurseries

Nebulae &
star formation.

Stars are not eternal. They condense out of cold clouds of gas, burn for millions or billions of years, then return their matter to the interstellar medium — often as the same nebulae the next generation will collapse out of.

01 · Field guide

Five kinds of cloud.

"Nebula" just means cloud. But astronomers sort them by how they glow, what they're made of, and what phase of stellar life they belong to.

Cosmic Dust

24 Light-Years
Diameter of the Orion Nebula, the closest region of massive star formation to Earth.
10,000 °C
Average temperature of an emission nebula, glowing brightly from ionized hydrogen gas.
5 Billion
Years until our Sun exhausts its fuel and blossoms into a glowing planetary nebula.

Emission nebula

Glowing hydrogen ignited by newborn stars

Clouds of hydrogen gas ionized by ultraviolet light from very hot young stars. Electrons recombine and emit a characteristic red glow. These are the classic stellar nurseries.

Temperature: ~10,000 K
Appearance: Deep red / pink (H-alpha)
Example: Orion Nebula (M42), Eagle Nebula (M16)

02 · Live · NASA Image Library

Real photos of emission nebulae.

Fetched live from NASA's public image archive for the search "Orion Nebula".

03 · From cloud to sun

How a star is
assembled.

Every star begins the same way: a tiny local overdensity in an enormous cold cloud loses its battle with gravity. What happens next takes millions of years and rewrites the neighborhood. As the gas collapses inward, it rapidly heats up, forming a dense, swirling accretion disk. Eventually, the core pressure and temperature cross the threshold for nuclear fusion, and a new sun violently ignites, blowing away the remaining stellar dust with powerful solar winds.

Protostar forming inside a glowing cloud with bipolar jets

Stage metrics · order-of-magnitude estimates

Log-scaled bars · 1 M⊙ track

01
Molecular Cloud
Millions of years
A cold, dense region of a giant molecular cloud (mostly H₂) starts to contract under its own gravity, often triggered by a nearby supernova shockwave.
Stage metrics
Est · NASA GSFC / ESA Herschel
Temperature
10 K
Density
10² cm⁻³
Radius
31.6 ly
Duration
10 Myr
02
Prestellar Core
~100,000 years
A dense clump collapses. As it falls inward, gravitational energy is converted to heat. The core spins faster and flattens into a disk.
Stage metrics
Est · Herschel Gould Belt Survey
Temperature
20 K
Density
10⁵ cm⁻³
Radius
20,000 AU
Duration
100 kyr
03
Protostar
~1 million years
A hot central object forms, still gaining mass from its accretion disk. Powerful bipolar jets punch out along the rotation axis, blowing away leftover gas.
Stage metrics
Est · Spitzer / JWST c2d survey
Temperature
3.0k K
Density
10¹⁰ cm⁻³
Radius
5 AU
Duration
1 Myr
04
T Tauri Star
~10 million years
The star is now visible, but not yet fusing hydrogen. It contracts slowly, surrounded by a protoplanetary disk where planets are already assembling.
Stage metrics
Est · NASA ADS / Hubble
Temperature
4.0k K
Density
10¹⁵ cm⁻³
Radius
4.30 R⊙
Duration
10 Myr
05
Main Sequence
Billions of years
Core temperature crosses ~10 million K. Hydrogen fusion ignites. The star settles into a long, stable adulthood — like our Sun, currently 4.6 billion years in.
Stage metrics
Est · NASA Sun Fact Sheet
Temperature
5.8k K
Density
10²⁴ cm⁻³
Radius
1.01 R⊙
Duration
10 Gyr

Estimate · characteristic values for a Sun-mass star. Real ranges span orders of magnitude. Sources: NASA GSFC, JPL Sun Fact Sheet, ESA Herschel, Spitzer c2d.

04 · The recycling

Every atom in your body heavier than helium was forged inside a star.

Hydrogen · Helium

Made in the first three minutes after the Big Bang. Still ~98% of ordinary matter in the universe.

During the era of Big Bang Nucleosynthesis, which lasted from roughly 10 seconds to 20 minutes after the Big Bang, the universe was a super-hot, super-dense plasma. Protons and neutrons fused together to form the first atomic nuclei. By the time the universe cooled enough for fusion to stop, the elemental composition was locked in at roughly 75% Hydrogen, 25% Helium, and trace amounts of Lithium. For the next 380,000 years, the universe remained too hot for electrons to bind to these nuclei, existing as an opaque plasma until recombination allowed the first neutral atoms to form. These primordial gases are the ultimate source material for every star in the cosmos.

Carbon → Iron

Fused in the cores of stars over billions of years, then dispersed by stellar winds and planetary nebulae.

Once a star exhausts the hydrogen in its core, gravity compresses it, raising temperatures to over 100 million Kelvin—hot enough to ignite helium fusion via the triple-alpha process, creating Carbon and Oxygen. In massive stars (over 8 times the mass of our Sun), this process continues in concentric layers, resembling an onion. The core progressively fuses heavier and heavier elements: Carbon fuses into Neon, Neon into Oxygen, Oxygen into Silicon. The final stage is Silicon burning, which produces Iron and Nickel in a matter of days. Iron is the ultimate dead-end for stellar fusion because fusing it consumes more energy than it releases, leading directly to core collapse.

Gold · Uranium

Forged in the extreme conditions of supernovae and neutron star collisions. Scattered across space by the blast.

Elements heavier than Iron cannot be formed through standard stellar fusion. Instead, they require the r-process (rapid neutron capture process), which occurs in the universe's most extreme and violent environments. When a massive star runs out of fuel and undergoes a core-collapse supernova, the resulting shockwave produces a flood of free neutrons that slam into iron nuclei, rapidly building heavier elements like Gold, Platinum, and Uranium before they have time to decay. Similarly, when two incredibly dense neutron stars collide (a kilonova), the resulting explosion ejects massive amounts of these heavy elements into the interstellar medium, seeding future solar systems.