Stars: structure and life cycle

When does a celestial body become a star?

A celestial body is only called a star when it can sustain hydrogen fusion in its core.

The minimum threshold is approximately:

• 0.08 solar masses
• Equivalent to about 80 times the mass of Jupiter

Objects lighter than this threshold are called brown dwarfs, which cannot sustain long-term hydrogen thermonuclear fusion.

Why do stars shine?

Under immense gravity, a star's core reaches temperatures of millions of degrees.

Here, hydrogen nuclei fuse into helium through thermonuclear reactions, releasing energy according to the formula:

E = mc²

This energy creates outward pressure, balancing the inward pull of gravity.

Composition of a star

Most stars consist of:

• Approximately 71% hydrogen.
• Approximately 27% helium.
• Approximately 2% heavier elements.

Surface temperature determines the color of a star:

• Hottest: blue.
• Medium (like the Sun): yellow-white.
• Cooler: orange and red.

The Sun is a G-type star, with a surface temperature of about 5,800 K.

Life cycle of a star

Main sequence phase

This is the longest phase in a star's life.

The star remains stable thanks to the balance between:

• Gravity.
• Pressure from thermonuclear reactions.

The Sun is currently in this phase.

When hydrogen runs low

The core contracts and heats up.

The outer layers expand to form a red giant.

Fate depends on mass

Sun-like stars

After the red giant phase:

• Shed the outer shell.
• Form a planetary nebula.
• Leave behind a core known as a white dwarf.

Lifespan on the main sequence is about 10 billion years.

Stars about 8 times heavier than the Sun

Continue to burn heavier elements such as:

• Carbon.
• Oxygen.
• Silicon.

Eventually, an iron core forms.

At that point, thermonuclear reactions no longer produce energy and the core collapses, triggering a supernova explosion.

Three types of "stellar remnants"

After death, the remaining core can become:

White dwarf

• Core mass below approximately 1.4 solar masses.
• Size equivalent to Earth.

Neutron star

• Higher mass.
• Diameter of only about 20 km.
• A teaspoon of its matter can weigh billions of tons.

Black hole

• If the remaining core is too heavy.
• No force is strong enough to resist gravitational collapse.

The most interesting thing

The heavy elements in our bodies such as:

• Carbon.
• Oxygen.
• Calcium.
• Iron.

were all created inside stars and dispersed into space when they died.

⭐ It can be said literally that humans are made of "stardust." Every atom heavier than hydrogen in our bodies was once born in the cores of stars that existed before the Sun was formed.

Read more

• Galaxies: definition and classification
• 20 brightest stars in the night sky
• Black holes: where the geometry of space collapses