What are the most common kinds of stars are low luminosity stars?

The night sky, as we see it, is a dazzling but incomplete portrait of the cosmos. The stars that blaze brightest for us—the brilliant blue giants and the vivid red supergiants—are the celebrities of the stellar population, but they are far from being the norm. When astronomers move past what is merely bright and focus instead on sheer numbers, the story shifts dramatically toward the dim, the small, and the incredibly long-lived. The most common kinds of stars in the galaxy are those possessing low luminosity compared to our own Sun. Understanding this massive population is central to understanding galactic evolution, as these faint residents dominate the count, even if they contribute little to the overall visual light we perceive from afar.

# The Overwhelming Majority

To quantify this reality, we must look at the Morgan–Keenan (MK) classification system, which sorts main-sequence stars primarily by temperature using the letters O, B, A, F, G, K, and M, from hottest to coolest. When observing the local Milky Way, the numbers paint an undeniable picture: one class swamps all others.

The M-type stars, commonly known as Red Dwarfs, account for approximately 70% to 76% of all main-sequence stars observed in the solar neighborhood. This means roughly three out of every four stars currently fusing hydrogen in their core belongs to this cool, small category. Even if we look at slightly warmer K-type stars, which are classified as Orange Dwarfs, they add another significant chunk, making up about 12% to 15% of the main-sequence population.

When we combine these two coolest main-sequence spectral classes, the M and K dwarfs, we account for well over 80% of all the stars actively undergoing fusion in our local galactic disk. For contrast, our Sun, a G-type star, represents only about 7.6% of the count, while the hotter O and B types combined make up less than 0.5%.

This numerical dominance creates an interesting paradox for the casual sky-watcher. The giant, luminous stars are precisely what we see, yet they are the rarest; they live fast and die young, burning through their fuel in mere millions of years. Conversely, the faint M-dwarfs burn so frugally that they can sustain core hydrogen fusion for trillions of years, potentially longer than the current age of the universe itself. It is a profound realization that nearly every star we could potentially see without optical aid is not representative of the typical stellar object; we are predisposed to noticing the brief, bright flash over the endless, steady glow.

# Red Dwarfs Dominance

The Red Dwarf is the archetypal low-luminosity star, representing the ultimate end state for the vast majority of stellar formation—at least in terms of number counts. These are the smallest of the main-sequence stars, possessing masses ranging from about 7.5% to 50% the mass of our Sun, or Sol.

Their low mass dictates their low luminosity and cool surface temperatures, falling within the 2,300 K to 3,900 K range in the Harvard classification. Because they are so cool and dim, virtually no red dwarf is visible to the naked eye, despite several residing relatively close to us, such as our nearest stellar neighbor, Proxima Centauri, which is an M5.5Ve type. To observe these dim stellar bodies, astronomers require telescopes or sensitive instruments.

The key to their longevity lies in their internal structure and fuel efficiency. Larger stars, like the massive O-types, operate like a paper fire, consuming their hydrogen rapidly, sometimes in less than a million years. Red dwarfs, however, manage their hydrogen supply with remarkable thrift. They generate energy primarily through convection, which constantly churns the star’s material. This churning mechanism continuously brings fresh hydrogen from the outer layers down into the core where fusion occurs. This process means the star can exhaust all its hydrogen fuel over its life, rather than just the core supply, which extends their main-sequence lifetime to estimates reaching 14 trillion years. In older parts of the galaxy where star formation has ceased, the stellar population is almost entirely composed of these long-lived red dwarfs.

It is also worth noting that stellar remnants, like White Dwarfs, though not burning hydrogen, are often quite faint due to their small size, making them another form of low-luminosity object to consider, though they are not part of the main-sequence count.

Do high mass stars form faster than low mass stars?

# Cooler Companions

Just above the Red Dwarfs in temperature, but still distinctly cool and low-luminosity compared to the Sun, are the K-type stars, frequently referred to as Orange Dwarfs. These main-sequence stars form the next most populous group, making up about 12% to 15% of the total number of main-sequence stars locally.

K-type stars have masses ranging from about 0.45 to 0.8 solar masses, and their surface temperatures are in the 3,900 K to 5,300 K range. They are visibly orange and possess extremely weak hydrogen lines in their spectra; by the late K spectral types, molecular bands like titanium oxide begin to appear.

If we use the Sun (G2V) as our benchmark for a stable, long-lived star with a projected main-sequence life of about 15 billion years, the K-type dwarfs are even better in terms of sheer endurance. Mainstream theories suggest these stars can remain stable on the main sequence for up to 30 billion years. This longevity, combined with their cooler temperatures, places them in an intriguing position regarding the potential for life elsewhere.

The theoretical advantage of K-dwarfs over G-dwarfs is twofold: their habitable zone—the region where liquid water could exist—is closer to the star, and they emit significantly less DNA-damaging ultraviolet (UV) radiation than hotter G-type stars like our Sun.

Imagine a star system orbiting a K-dwarf about 10 billion years ago. If an Earth-like planet were positioned correctly, it would be receiving less intense, less harmful radiation than Earth does today, extending the window for complex life to evolve by perhaps another 10 to 20 billion years compared to a system around a Sun-like star. However, to be in that habitable zone, the planet must orbit much closer to its star than Earth orbits the Sun. This proximity introduces a secondary, critical problem for these systems: tidal locking. A closely orbiting body is far more likely to become tidally locked, where one side perpetually faces the star, leading to extreme temperature differences between the day and night sides, which complicates the emergence of life as we know it. Thus, while K-dwarfs offer an expanded timeline, they demand a more precarious orbital placement than the Sun’s G-type neighborhood.

# Stellar Remnants: White Dwarfs

When a star like our Sun exhausts its core hydrogen fuel, it becomes a red giant, then sheds its outer layers, leaving behind the super-dense remnant known as a White Dwarf. While these objects are very low in luminosity relative to their progenitor stars when they were on the main sequence, they are not formed from the lowest-mass stars like the Red Dwarfs. Instead, they are the fate of stars up to about eight times the Sun’s mass.

White dwarfs are typically Earth-sized but possess enormous mass, meaning a teaspoon of their material would weigh hundreds of thousands of times more than a pickup truck. They no longer generate heat through nuclear fusion; their faint light is merely the leftover thermal energy radiating away over eons. Over the span of 10 billion years, they will gradually cool down. Their luminosity class is distinct, often designated with the letter D (for Degenerate) or Roman numeral VII in the MK system, separating them from the live, hydrogen-fusing stars classified as luminosity class V (dwarfs).

Their atmospheres are categorized based on their remaining composition. For instance, DA white dwarfs have a hydrogen-rich atmosphere, identifiable by strong hydrogen spectral lines, while DB dwarfs have helium-rich atmospheres. Despite being too dim to see with the unaided eye, some can be found in binary systems with a bright companion, such as Procyon B.

Which stars are low-mass stars?

# Substellar Faintness: Brown Dwarfs

Perhaps the lowest luminosity objects that still hold a tenuous connection to stellar classification are the Brown Dwarfs, often nicknamed "failed stars". These objects sit right on the boundary between the largest planets and the smallest stars. Generally, they possess masses between 13 and 80 times the mass of Jupiter, but critically, they never accumulate enough mass or core temperature to ignite sustained hydrogen fusion like a true main-sequence star.

Brown dwarfs evolve by cooling down continuously, progressing through a sequence of spectral types that are too cool to fit neatly into the standard OBAFGKM scheme. They transition through the L, T, and Y spectral classes, which are characterized by infrared observations because they emit almost no visible light.

Class L dwarfs are cooler than M-stars, and their spectra show the presence of metal hydrides. Class T dwarfs are even cooler, with methane becoming prominent in their spectra, peaking in the infrared, and having surface temperatures between about 550 K and 1,300 K. The Y dwarfs are the coldest category, with the coolest known examples hovering around 250 K—the temperature of a cold winter day. These objects straddle the boundary with giant planets, as the deuterium-fusion limit (the defining characteristic of a brown dwarf) is around 13 Jupiter masses.

The theoretical formation of stars also explains the low-luminosity prevalence. Star formation is governed by a power law where smaller outcomes are statistically far more likely than larger ones. If we liken the process to cutting biscuits from a sheet of pastry, the large, intact sections yield a few big stars, but the remaining scraps and shreds can generate a vast number of tiny, low-mass objects. Furthermore, massive stars, which produce the brilliant light we notice, die quickly, removing themselves from the population count, whereas the Red Dwarfs are virtually immortal on cosmic timescales. Because massive stars die and less massive stars persist for eons, any region of space undisturbed by recent star formation becomes overwhelmingly populated by these low-luminosity veterans.

# Luminosity and Spectral Context

To truly appreciate why these stars are low luminosity, it helps to frame them within the full stellar spectrum. The MK system uses luminosity classes to further distinguish stars within the same temperature/spectral class.

• Class V (Dwarfs): This is the main-sequence class, encompassing the Red Dwarfs (M V), Orange Dwarfs (K V), and Yellow Dwarfs (G V) like our Sun. These are the stars fusing hydrogen in their cores and represent the longest phase of a star’s life.
• Giants and Supergiants (Classes I, II, III, IV): These are evolved stars that have exhausted core hydrogen. They are dramatically larger and thus far more luminous than their dwarf counterparts, even if they share a similar surface temperature (e.g., a K-type giant versus a K-type dwarf).

The term "luminosity" here directly relates to the star's intrinsic energy output. The Sun is our reference point, classified as G2V. A K-dwarf (K V) has a bolometric luminosity of 0.08 L☉ or less. In stark contrast, the massive O-type main-sequence stars emit 30,000 L☉ or more, and even B-type stars begin at 25 L☉. The difference between the least luminous M-dwarfs (less than 0.08 L☉) and the hottest O-dwarfs is astronomical, solidifying the Red Dwarf’s position as the epitome of low-luminosity existence.

The low luminosity of these stars is inextricably linked to their spectral characteristics. Cooler temperatures mean fewer atoms are ionized, resulting in different absorption lines. Red Dwarfs (M-class) show molecular bands, particularly TiO, dominating their visible spectrum by type M5, with very weak or absent hydrogen absorption lines. K-type stars show increasing neutral metal lines, with molecular bands appearing late in the class. This spectral fingerprint, combined with their measured distances (often via parallax for closer ones) and apparent brightness, is how astronomers calculate their true, low intrinsic luminosity.

In summary, the universe is populated overwhelmingly by the slow-burning embers of stellar formation. While the bright, massive stars provide the spectacular fireworks that catch our immediate attention, the bulk of the galaxy’s observable mass belongs to the faint, enduring Red Dwarfs (M-type V), supported closely by the slightly brighter, longer-lived Orange Dwarfs (K-type V). Add in the cooling stellar corpses of White Dwarfs and the sub-stellar Brown Dwarfs, and the low-luminosity population is clearly the dominant, though visually modest, component of our cosmic neighborhood.