Stellar Classification: From O to M Type — The Secrets of the Universe’s Hottest and Coolest Stars
Astronomers have classified stars into seven major types based on their size, brightness, temperature, and color. These spectral classes are known as O (blue), B (blue-white), A (white), F (white-yellow), G (yellow), K (orange), and M (red).
Within our own Milky Way Galaxy, scientists estimate there are between 100 to 400 billion stars scattered across the spiral arms and galactic center.
According to astronomical research, M-type stars are the coolest and smallest stars in the known universe. Their surface temperature can drop to around 3,700 Kelvin. Despite their relatively small size and low temperature, these stars are remarkable for their extraordinarily long lifespans, which can last from hundreds of billions to even trillions of years.
A well-known example of an M-type star is Proxima Centauri, located approximately 4.26 light-years away from Earth. It is considered the closest neighboring star system to our Solar System. Although 4.26 light-years might sound near, it would still take a human-made spacecraft roughly 70,000 to 100,000 years to reach it with current technology.
In contrast, our Sun belongs to the G-type (yellow) category — a medium-sized, medium-lifespan star. The Sun’s surface temperature ranges between 5,200 to 6,000 Kelvin, and its estimated lifespan is about 10 billion years. This means the Sun is now about halfway through its life cycle. Interestingly, while the Sun’s surface appears relatively “cool,” its outer corona can reach a staggering 1–2 million Kelvin in temperature.
On the other extreme, O-type stars represent the hottest, brightest, and most massive stars in the universe. These extremely rare giants have surface temperatures exceeding 30,000 Kelvin, shine hundreds of thousands of times brighter than the Sun, and are often found in young star clusters or active galaxies.
However, their immense size and energy output come at a cost — O-type stars live very short lives, lasting only a few million years. They emit enormous amounts of ultraviolet radiation and eventually end their lives in cataclysmic supernova explosions, often leaving behind neutron stars or black holes. Through this process, they play a crucial role in the creation of heavy elements and help trigger new star formation in surrounding gas clouds, turning regions of space into vast stellar nurseries.
In essence, the classification of stars does not merely reveal the structure and diversity of the cosmos — it also offers profound insight into the mysteries of time, matter, and energy. Especially the short-lived but powerful O-type supergiants, whose violent deaths often mark the beginning of new cosmic creation, reminding us that in the grand theater of the universe, destruction often precedes rebirth.
