The Lives of the Stars

Part Of: Demystifying Physics sequence
Followup To: Deep Time
Content Summary: 1100 words, 11 min reading time.

Why does the Earth orbit a slow-burning hydrogen bomb? And why is the night sky illuminated with trillions of such explosions?

Let’s find out.


Stars emit light. The most important characteristics of light are brightness and color.  A Hertzsprung-Russell (HR) Diagram puts brightness on the x-axis, and color on the y-axis.  In this way, a star can be represented by a single point.

What happens if you plot the location of all visible stars onto the same HR diagram? The result is rather striking:

Stellar Evolution- Main Sequence Stars

Most stars seem to fit inside a continuous swathe known as the Main Sequence. Why?

As we will see, there are five stages in the stellar lifecycle:

  1. Formation: Clouds congeal into protostars.
  2. Dwarf Phase: Hydrogen begins to fuse.
  3. Giant Phase: Hydrogen runs out, switch to helium fusion and beyond.
  4. Fuel Crisis: nuclear fusion runs out of raw materials.
  5. Termination: whatever remains of the star slowly becomes cold.

Stars spent 90% of their lives as dwarves. The radiation of dwarves vary continuously based on solar mass. This explains the Main Sequence.

As we will see, the five life-stages of the stars differ based on how big they are:

Stellar Evolution- Lifecycle Flowchart (1)

The Dynamics of Stars

Stars are born when hydrogen clouds begin to collapse in on themselves.

If gravity was the only force in play, all stars would quickly become black holes. But compressed gases develop high outward pressure. So there are a tension:

Stellar Evolution- Force Interactions (1)

Phase 1: Protostars

Star formation begins when a molecular cloud begins to collapse into a dense core. As this core accretes mass, gravity’s pull intensifies. Soon, the site of collapse becomes a protostar.

Protostars are not yet hot enough to induce fusion. But the compression of gravity still makes these objects extremely hot. Once their radiation blows away surrounding clouds, the appear on what is called the stellar birthline of the HR diagram.

  • Small protostars are called T Tauri stars.
  • Large protostars are known as Herbig Ae/Be stars

As protostars mature, they become more hot and dense. On the HR diagram, their signatures will move from the stellar birthline towards the main sequence.

Stellar Evolution- Phase 1

HR movement is not movement in space. “Travel” away from the stellar birthline means that protostars are growing hotter (left) but less bright (down).

Phase 2: Dwarves (Main Sequence)

The interior of the Sun is not homogenous. The closer to the center, the more extreme the climate. If the protostar is large enough, the core of the star will become so intense, that it will trigger nuclear fusion, releasing enormous amounts of energy. If space was not a vacuum, and the sound of the Sun could travel to Earth, its volume would equal that of a motorcycle.

Nuclear fusion { hydrogen → helium } depletes hydrogen, and creates helium. The helium core of the star expands, as hydrogen is depleted.

  • Small dwarf stars are called yellow dwarves.
  • Large dwarf stars are called blue dwarves.

Over 90% of a star’s lifetime is spent in this dwarf phase. 

Stellar Evolution- Phase 2

Stars are the crucible of matter. What does this mean?

The Primordial Era produced only hydrogen clouds, intermixed with helium. How is it possible for our bodies to be 65% oxygen? Hydrogen does not spontaneously become oxygen, after all.

With few exceptions, all naturally occurring substances were forged in the heart of stars. Nucleosynthesis describes how the raw material of the Big Bang was forged into the chemically diverse world of modernity. In the dwarf phase, we only see the construction of helium. We will soon see nuclear fusion carried much further.

Stellar Evolution- Nucleosynthesis (1)

We are literally made of starstuff.

Phase 3: Giants

Eventually, stars run out of hydrogen fuel. At this point, helium atoms are so hot that they start to fuse: { helium → carbon }. This new kind of fusion changes the thermal output of the star, which leaves the main sequence.

Helium-burning stars expand dramatically. For this reason, we call main-sequence stars dwarves, and post main-sequence stars giants.  

Again we see a difference along solar mass:

  • Small stars in their giant phase are too small to combust carbon. These are red giants
  • Large giants can combust further elements. These are red supergiants

Stellar Evolution- Phase 3

The interior of red supergiants, therefore, is shaped a bit like an onion, with each deeper layer “raising” the atomic number of its exterior. But why does this chain stop at iron?

Iron has the lowest mass per quark: fusing iron consumes energy, instead of creating it.  

Stellar Evolution- Fusion Limits (1)

Small Stars: Fuel Crisis & Termination

Red giants eventually radiate away most of their mass. 😦 Thus, gravity slowly loses its hold on the star, and the outer shells are propelled outward by thermal pressure. This ejection of inert hydrogen is known as a planetary nebula.

The abandoned cores of a shell comprise white dwarves. Since these objects have no source of fuel, they slowly cool, resulting in down-right movement on a HR diagram:

Stellar Evolution- Phase 5- Small Star

Large Stars: Fuel Crisis & Termination

In contrast to smaller stars, supergiants die in fantastic ways. Their iron core is unstable, and will eventually explode in a supernova. Supernovae are no trifling matter. Their outputs are often brighter than their host galaxies (trillions of stars). If a nearby star in the Milky Way were to go supernova, it would obliterate the human race.


If the core survives the explosion, it becomes a neutron star. Neutron stars are rather dense. One teaspoon of its material would weigh more than ten Pyramids of Giza. Neutron stars are highly magnetic, and rotate quite swiftly: this is the root cause of quasars.

But sometimes the core will be even more dense. In this scenario, it will fully collapse into itself; ripping the fabric of spacetime to become a black hole. Black holes are a bit like predators; they “hunt” and “eat” other stars. At their center, most galaxies possess a supermassive black hole (SMBH). Our galaxy’s SMBH (Sagittarius A*) is fortunately 26,000 light-years away from Earth.

Stellar Evolution- Phase 5- Large Star (2)

The Story of Our Sun

As we have seen, the universe is 13.8 billion years old, and stars began to form 13.4 billion years ago. We can categorize stars by birthday into three stellar populations.  

The earliest stars were chemically simple because the universe contained nothing but hydrogen and helium. But as nucleosynthesis progressed, the universe began to accumulate more complex atoms. The second generation of stars had small amounts of metal. The last generation had substantial metal content.

Did you know that planets outnumber stars? There are about 200 billion stars in the Milky Way, and about 220 billion planets.

Planets are recent inventions. They are created only if a nebula sufficiently high metallic content. In that case, a protoplanetary disc will orbit the protostar, which will ultimately condense into extrastellar satellites.

As a recently-created star, born only 4.6 billion years ago, the Sun’s birthing nebula was sufficiently metallic to create such a disc. This disc eventually consolidated into our eight planets. The Sun is now in its second phase of life, a yellow dwarf. 5.5 billion years from now, it will – like so many of its brothers and sisters – start burning helium as a red giant.

Stellar Evolution- Our Sun (3)


Stellar Evolution- Lifecycle Flowchart (1)

 Until next time.


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