Balls of hot plasma generating energy through nuclear fusion, illuminating the cosmos, classified by temperature and undergoing a life journey determined by their initial mass.

Stars

What is a Star?

A star is essentially a ball of hot plasma held together by its own gravity. Like giant nuclear furnaces, they generate energy through nuclear fusion in their cores, illuminating the cosmos. Our most familiar star, the Sun, is an unremarkable yet vitally important star to us.

How to Classify Stars? - Stellar Spectra

Just as biologists classify living organisms, astronomers have established a classification system for stars, known as the Morgan–Keenan (MK) spectral classification. This system primarily categorizes stars based on their surface temperature—that is, their color.

*Caption: The Hertzsprung-Russell diagram, one of the most important charts in astronomy, showing the relationship between stars' temperatures, luminosities, and evolutionary stages.

From the hottest blue to the coolest red, the main types are O, B, A, F, G, K, and M. A famous mnemonic for remembering this is: "Oh, Be A Fine Guy/Girl, Kiss Me!"

Temperature Scale:**

(Hottest) O — B — A — F — G — K — M (Coolest) <————————————————————————————————————— 50,000K 20,000K 10,000K 7,000K 6,000K 4,500K 3,000K

*Quick Reference Table:

Spectral Type	Surface Temp. (approx.)	Color	Typical Mass (Sun=1)	Example
O	> 25,000 K	Blue	> 15 M☉	Alnitak (Zeta Orionis)
B	11,000-25,000 K	Blue-White	2 - 15 M☉	**Rigel**¹
A	7,500-11,000 K	White	1.5 - 2 M☉	Sirius
F	6,000-7,500 K	Yellow-White	1.05 - 1.5 M☉	Polaris
G	5,000-6,000 K	Yellow	0.8 - 1.05 M☉	The Sun
K	3,500-5,000 K	Orange	0.45 - 0.8 M☉	Arcturus
M	< 3,500 K	Red	0.08 - 0.45 M☉	Proxima Centauri

¹ A Blue Supergiant, like Rigel, is an extremely luminous, hot, and massive star of spectral type O or B, but its life is also very short.

The Life Journey of a Star

The life of a star is like a grand drama, with its plot largely determined by its "weight" at birth—its initial mass.

• The Fate of Low-mass Stars (like the Sun):
  1. In a molecular cloud, a clump of matter collapses under gravity into a protostar.
  2. When the core temperature is high enough, nuclear fusion ignites, and it becomes a stable main-sequence star, where it spends the longest part of its life.
  3. After exhausting its fuel, it expands into a huge red giant.
  4. The outer layers are cast off, forming a beautiful planetary nebula.
  5. The remaining core collapses into a dense white dwarf, which slowly cools in the cosmos.
• The End of High-mass Stars (over 8 solar masses):
  1. They go through similar birth and main-sequence stages but in a much shorter and more violent manner.
  2. They swell into even larger red supergiants.
  3. Eventually, they end their lives in a spectacular explosion called a Type II Supernova.
  4. The remnant, depending on its mass, will collapse into either a neutron star or an infinitely dense black hole.

Recommended Reading

• Popular Science:
  • (Book) Stars: A Very Short Introduction by P. P. Eggleton.
  • (Book) Astrophysics for People in a Hurry by Neil deGrasse Tyson.
• Textbooks:
  • (Book) An Introduction to Modern Astrophysics by Bradley W. Carroll and Dale A. Ostlie.
• Further Reading:
  • (Book) Stellar Structure and Evolution by Rudolf Kippenhahn, Alfred Weigert, and Achim Weiss.