The question of whether our Sun is an average star has a complex answer that depends entirely on the criteria used for comparison. If “average” refers to its physical characteristics, such as size, mass, and temperature, the Sun is a very typical star among its kind. However, if “average” is defined by how common a star type is within the Milky Way galaxy, the Sun is statistically quite rare. Stellar astronomers classify stars based on their physical properties and population frequency, leading to two distinct perspectives on the Sun’s place in the universe.
Defining the Sun’s Stellar Classification
Astronomers use the Morgan-Keenan (MK) classification system to formally define a star’s identity, which is based on its temperature and luminosity. The Sun is classified as a G2V star, a designation that precisely describes its current state and composition. The letter “G” indicates its spectral type, which corresponds to a surface temperature of approximately 5,772 Kelvin, giving it a yellowish color.
The number “2” further refines this spectral class, placing it toward the hotter end of the G-type range. The Roman numeral “V” is the luminosity class, which signifies that the Sun is a dwarf star currently residing on the main sequence. This means the Sun is in the most stable and longest phase of its life, steadily fusing hydrogen into helium in its core.
The Sun has been in this stable state for about 4.6 billion years and is expected to continue for another 5 billion years. This main sequence phase is characterized by a state of hydrostatic equilibrium, where the outward pressure from nuclear fusion perfectly balances the inward pull of gravity.
The Sun’s classification as a main sequence star means it is a typical star in terms of its evolutionary stage, as the majority of stars spend most of their existence in this phase. The Sun’s relatively moderate temperature and stability are necessary for supporting a long-lived, habitable zone around it.
The Sun Compared to the Most Common Stars
When comparing the Sun to the stars in the Milky Way, it is revealed to be statistically uncommon. The galactic population is overwhelmingly dominated by stars that are far smaller, cooler, and dimmer than our Sun. The most frequent stellar type is the M-type star, commonly called a Red Dwarf.
Red Dwarfs make up an estimated 70% to 85% of all stars in the galaxy. M-type stars have a mass significantly less than the Sun, and their surface temperatures are typically below 4,000 Kelvin. They are so faint that they are often difficult to observe across large distances, despite their immense population.
In contrast, G-type stars like the Sun constitute only about 6% to 7.5% of the total stellar population. This means that for every G-type star, there are roughly ten Red Dwarfs in the galaxy. The Sun is also estimated to be brighter and more massive than about 85% of the stars within the Milky Way.
The statistical average star in the Milky Way is small, dim, and red, belonging to the M-class. This disparity highlights the difference between being physically mid-range and statistically common.
The Sun’s Middle-of-the-Road Position on the H-R Diagram
The Sun’s physical properties are best understood by its placement on the Hertzsprung-Russell (H-R) Diagram, a graph that plots a star’s luminosity against its surface temperature. The H-R Diagram is a map of stellar evolution, where most stars fall along a diagonal band called the main sequence. The Sun resides almost exactly in the middle of this prominent main sequence band.
This central position on the diagram confirms the Sun’s balanced physical characteristics relative to other main sequence stars. Its luminosity is defined as one solar luminosity (1 \(L_{\odot}\)), and its mass is one solar mass (1 \(M_{\odot}\)), making it the benchmark for all other stellar measurements. The Sun is neither an extremely hot, massive, and luminous blue-white O- or B-type star, nor is it an ultra-cool and dim Red Dwarf.
The massive O and B stars, which occupy the upper-left of the main sequence, are hundreds of times more luminous than the Sun and burn through their fuel in only a few million years. Conversely, the M-type Red Dwarfs in the lower-right are so faint that their luminosities are often less than one-hundredth of the Sun’s. The Sun’s moderate mass allows it to enjoy a long, stable lifetime of approximately 10 billion years, placing it squarely in the middle range of stellar longevity.
The Sun’s temperature of about 5,772 Kelvin also falls between the extreme heat of the blue giants and the low temperatures of the red dwarfs. Therefore, the Sun is physically average in terms of scaling, sitting at a midpoint on the continuous spectrum of mass, temperature, and luminosity found among stars that are fusing hydrogen.