The diagram illustrates the standard stellar classification system—O, B, A, F, G, K, and M—ordered from the most massive and hottest to the least massive and coolest. At the top of the hierarchy are the O and B type stars, which are colossal, luminous blue giants. An O-type star, for instance, boasts a staggering mass 50 times that of our Sun and scorches with surface temperatures around 40,000 Kelvin (40 kK). Because these giants crush and burn through their nuclear fuel at a ferocious rate, they have incredibly brief lifespans; an O-type star lives for only about 10 million years (10 My), while a B-type survives for roughly 100 million years. Their accompanying spectra, spanning from 365 nm to 900 nm, are intensely bright in the shorter, high-energy blue and ultraviolet wavelengths.

Moving down the sequence, we encounter the intermediate-mass stars: types A, F, and G. These stars represent a middle ground in terms of size, temperature, and longevity. An A-type star is about twice the mass of the Sun and burns at 10,000 Kelvin, living for around 1 billion years (1 Gy). As we reach the G-type stars, we find the classification that includes our own Sun. A typical G-type star has a baseline mass of exactly 1 solar mass, a surface temperature of approximately 5,000 Kelvin, and a steady lifespan of about 10 billion years. The spectra for these intermediate stars, particularly A and F types, begin to show distinct dark absorption lines, which become increasingly complex in G-type stars as cooler atmospheres allow a wider variety of atomic transitions to be visible.

At the bottom of the diagram are the K and M type stars, often referred to as orange and red dwarfs, respectively. These are the smallest, coolest, and most abundant stars in the universe. An M-type star possesses a mere fraction of the Sun’s mass—around 0.2 solar masses—and has a comparatively cool surface temperature of 3,000 Kelvin. However, their slow, extremely efficient rate of nuclear fusion grants them staggering longevity. A K-type star can live for 50 billion years, and an M-type red dwarf can burn steadily for 100 billion years (100 Gy), far exceeding the current age of the universe. Their spectra are noticeably darker in the blue regions and brilliantly bright in the red and infrared, characterized by broad absorption bands created by surviving molecules in their relatively cool atmospheres.