Stellar classification systems provide a way for astronomers to categorize stars based on their mass, and other properties, which helps us better understand the nature and evolution of stars.
Let’s explore more about the different types of stars. Note: this is a detailed read, so grab a coffee, and get comfortable before you dive in!
Types of Stars on the Hertzsprung-Russell Diagram
The Hertzsprung-Russell (HR) diagram represents one of the most important trends found in Stellar Astrophysics. It was developed independently by astronomers Ejnar Hertzsprung and Henry Norris Russell in the early 20th century.
By plotting the observed stellar absolute magnitudes against the respective color indexes, Hertzsprung was able to show that the relationship between these two quantities is not random, on the contrary, stars appear to fall into distinct groups.
Related: What are birthday stars and how to find yours
The same correlation is also evident when plotting stellar spectral classes against absolute magnitudes, as reported by Russell.
Today, the HR Diagram is a powerful tool for understanding stellar evolution. It allows astronomers to classify stars based on their temperature and luminosity and to identify different stages of stellar evolution.
For example, high-mass stars are hotter and more luminous than low-mass stars, and they evolve more quickly.
As a result, they occupy a different region of the HR Diagram, and their evolutionary paths can be easily traced.
Luminosity versus Surface Temperature
The position of a star in the HR diagram provides information about its present stage and its mass.
The types of Stars on the Hertzsprung-Russell diagram are:
- Main sequence stars: These are the most common type of stars in the universe. They range in size from small, cool red dwarfs to large, hot blue giants, and their characteristics depend on their mass. It has a distinctive S-shape, with most stars clustering around the main sequence, which represents stars in their stable hydrogen-burning phase.
- Red giants: These are stars that have used up most of their hydrogen fuel and have expanded to several times their original size. They are cooler than main sequence stars, but much brighter.
- White dwarfs: These are the remnants of low- to medium-mass stars that have exhausted their nuclear fuel. They are very dense, about the size of Earth, and incredibly hot.
- Supergiants: These are very large and luminous stars, many times larger than the sun. They are rare and short-lived, and when they explode as supernovae, they can release more energy than our entire galaxy.
Stars spend most of their lives in the Main Sequence, followed by the Red Giant phase, and White Dwarf phase.
Stellar structure models and evolution codes are developed with the intent of reproducing the observable relationships of the HR diagram.
Size (Mass) Classification
The exact path a star takes through its lifecycle depends primarily on its mass, which determines its internal temperature, pressure, and nuclear fusion reactions.
Here is a breakdown of the lifetimes of stars based on their mass:
Very low-mass stars (less than 0.3 times the mass of the sun) can have lifetimes of trillions of years, as they burn their fuel very slowly. An example of a very low-mass star is AB Doradus C. It has a mass estimated to be about 60 times that of Jupiter, and it will never leave the main sequence since it burns so little hydrogen.
Low-mass stars (between 0.3 and 1.5 times the mass of the sun) have lifetimes of tens to hundreds of billions of years.
They are also known as red dwarfs and are the most common type of star in the universe.
After forming from a molecular cloud, a low-mass star enters the main sequence phase, during which it generates energy through hydrogen fusion in its core.
Related: What is An A-Type Star?
As the star’s hydrogen fuel is consumed, its core contracts and heats up, causing the outer layers to expand and cool, making the star appear redder and cooler.
Eventually, the core runs out of hydrogen fuel and the star enters a new phase of its life, during which it burns helium in its core and hydrogen in a shell around the core.
This causes the star to expand into a red giant or supergiant before eventually shedding its outer layers and forming a planetary nebula around a small, hot core called a white dwarf.
Intermediate-mass stars (between 1.5 and 8 times the mass of the sun) have lifetimes of millions to billions of years. These are less common than low-mass stars.
These follow a complex life cycle that begins with their formation from a molecular cloud and continues through the main sequence, helium-burning, asymptotic giant branch, and white dwarf phases.
During the main sequence phase, the star generates energy through hydrogen fusion in its core, while during the helium-burning phase, the core contracts and heats up, causing helium fusion to occur. The asymptotic giant branch phase sees the star expanding and contracting and losing mass through stellar wind.
Finally, the star fades into a white dwarf, which slowly cools and fades over billions of years. Intermediate-mass are able to generate enough heat and pressure in their cores to fuse helium into carbon, unlike low-mass stars that do not generate enough heat and pressure for helium fusion.
However, intermediate-mass stars are not able to generate enough heat and pressure to fuse heavier elements like oxygen and silicon, as high-mass stars do.
Examples of intermediate-mass stars include Aldebaran, Capella, and Betelgeuse, which are all visible to the naked eye.
High-mass stars (greater than 8 times the mass of the sun) have much shorter lifetimes, typically only a few million years. They are relatively rare and are typically much more luminous than low-mass and intermediate-mass stars.
Like low-mass stars, they begin their lives on the main sequence, but they burn through their hydrogen fuel much more quickly, causing them to evolve rapidly.
They enter a series of new phases, during which they burn helium, carbon, and other elements in their cores, causing them to expand and contract multiple times and form various structures such as red supergiants and Wolf-Rayet stars.
Eventually, they reach a point where they can no longer support their own mass, and they explode in a spectacular supernova, leaving behind either a neutron star or a black hole. Some examples of high-mass stars are Eta Carinae, and Cygnus OB2 #1.
Ultimately, the fate of a star depends on its mass.
Low and intermediate-mass stars will eventually shed their outer layers and become white dwarfs, while high-mass stars will explode in supernovae and can leave behind neutron stars or black holes.
Spectral (Color) Classification
Spectral classification is a system used by astronomers to classify stars based on their spectra, which is the light that they emit and absorb. The Morgan-Keenan (M-K) classification system and the Harvard spectral classification system are two related systems used to classify stars based on their spectral characteristics.
The Harvard spectral classification system, developed in the late 1800s by Edward Pickering and Williamina Fleming, was the first system to classify stars based on their spectral features.
It originally classified stars into four main types: A, B, C, and D, based on the strength of hydrogen and helium lines in their spectra.
The system was later expanded to include additional subclasses, such as the luminosity classes I-V.
The M-K classification system, developed in the 1940s by William Wilson Morgan, Philip C. Keenan, and Edith Kellman, builds on the Harvard system by refining the spectral types and introducing a numerical classification system for the subclasses.
The M-K system divides stars into seven main spectral types, labeled O, B, A, F, G, K, and M, with each type further subdivided into 10 subclasses numbered 0-9.
| Spectral Type | Surface Temperature | Color | Examples |
| O | > 25,000K | Violet | 10 Lacertra |
| B | 10,000-25,000K | Blue | Rigel |
| A | 7,500-10,000K | Blue | Sirius |
| F | 6,000-7,500K | Blue-White | Procyon |
| G | 5,000-6,000K | White-Yellow | Sun |
| K | 3,500-5,000K | Orange-Red | Arcturus |
| M | < 3,500K | Red | Betelgeuse |
The O stars are the hottest and brightest, with surface temperatures of up to 50,000 Kelvin, while the M stars are the coolest and dimmest, with surface temperatures of around 3,000 Kelvin. The other spectral types fall in between these extremes.
Today, the M-K classification system is the most widely used system for classifying stars and is commonly used in modern astronomical research.
However, the Harvard system remains an important historical landmark in the development of stellar classification. Both systems use the strength of certain spectral lines, such as those of hydrogen and helium, to determine the spectral type and luminosity class of a star.
Presently, the M-K classification system also includes additional spectral types such as L, T, and Y, which are used to classify objects that are cooler and less massive than M stars. L dwarfs have spectral features that are dominated by metal hydrides, while T dwarfs have features dominated by methane.
Y dwarfs, the coolest objects in the extension, have yet to be spectroscopically confirmed, but are predicted to have spectra dominated by ammonia and water.
This extended classification system is important for studying and characterizing objects such as brown dwarfs, which have temperatures and masses intermediate between those of planets and stars.
Spectral classification is an important tool for studying stars because it provides information about their temperatures, luminosities, and chemical compositions. By analyzing the spectra of stars, astronomers can determine their physical properties, such as their masses, radii, and ages.
Star Population Classification
Stars can also be classified according to their stellar populations, which refers to the generation in which they formed.
There are three main types of stellar populations:
Population I: These stars are relatively young, with ages of less than 10 billion years. They are typically found in the disk of the galaxy, where ongoing star formation occurs. Population I stars are rich in heavy elements, such as carbon, nitrogen, and oxygen, which are produced by previous generations of stars.
They are also generally located in the spiral arms of galaxies.
Population II: These stars are older, with ages of between 10 and 13 billion years. They are found in the galactic halo and the bulge of the galaxy. Population II stars have much lower metallicities than Population I stars, meaning that they contain fewer heavy elements. They are also more common in globular clusters than in the disk of the galaxy.
Population III: These hypothetical stars are thought to have formed in the early universe, shortly after the Big Bang. They are believed to have been massive and short-lived, with masses of up to several hundred times that of the Sun.
Population III stars are predicted to have had very low metallicities, as heavy elements had not yet been produced in significant quantities. However, no Population III stars have been observed to date.
The classification of stars according to their stellar populations is important for understanding the history and evolution of galaxies. By studying the distribution and properties of different types of stars in galaxies, astronomers can learn about the formation and evolution of galaxies over cosmic time.
Oscillation Pattern Classification
Stars can be classified according to their oscillation pattern, which is the way they vibrate and emit waves of energy through their interiors. This field of study is known as asteroseismology, and it provides valuable information about a star’s internal structure and composition.
There are different types of stellar oscillators, each with its own unique characteristics and properties. Some of the main types of stellar oscillators are:
Cepheid variables: These are luminous, pulsating stars that have a well-defined period-luminosity relation, which makes them useful as distance indicators. Cepheid variables are typically yellow or red supergiants, and their pulsations are caused by changes in their ionization levels.
RR Lyrae variables: These are similar to Cepheid variables but are smaller and less luminous. RR Lyrae stars are typically found in old stellar populations such as globular clusters, and their pulsations are caused by changes in the helium ionization zone.
Delta Scuti stars: These are intermediate-mass stars that pulsate in multiple modes, with periods ranging from a few minutes to a few hours. Delta Scuti stars are typically main sequence or slightly evolved stars, and their pulsations are caused by changes in their outer layers.
White dwarf pulsators: These are compact, degenerate stars that pulsate due to changes in their outer layers or in their magnetic fields. White dwarf pulsators include the ZZ Ceti stars, which have hydrogen-dominated atmospheres, and the DAV stars, which have helium-dominated atmospheres.
Solar-like oscillators: These are pulsating stars that have masses similar to that of the Sun and exhibit oscillations that are similar to those observed on the Sun. Solar-like oscillators pulsate with periods of a few minutes to a few hours and have amplitudes of a few parts per million.
They are mainly observed in main sequence stars and subgiants. Some red giants exhibit oscillations that are similar to those of the Sun, but are much more pronounced due to their larger size and lower surface gravity.
These oscillations are known as radial oscillations, and they cause the star to pulsate in size and brightness.
Gamma Doradus variables: These are pulsating stars that have masses between 1.5 and 3 times that of the Sun. They pulsate with periods of a few hours to a few days and have amplitudes of a few tenths of a magnitude.
Gamma Doradus variables are mainly observed in young open clusters and star-forming regions.
Rapidly oscillating Ap (roAp) stars: These are pulsating stars that have strong magnetic fields and high surface temperatures. They pulsate with periods of a few minutes to a few hours and have amplitudes of a few hundredths of a magnitude. RoAp stars are mainly observed in the upper main sequence and have masses between 1.5 and 2 times that of the Sun.
Variable subdwarf B (sdB) stars: These are compact stars that are believed to be the remnants of red giants that have lost their outer envelopes. They are characterized by high surface temperatures and low luminosities, and exhibit rapid, irregular variations in brightness due to pulsations in their outer layers.
Slowly-pulsating B (SPB) stars: these are a class of massive main-sequence stars that exhibit periodic variations in brightness with periods ranging from a few hours to several days. These pulsations are thought to be caused by pressure waves in the stellar interior that is partially reflected at the surface.
SPB stars have a relatively low amplitude of pulsation, which makes them difficult to detect and study.
Blue stragglers: These are stars that appear to be younger and more massive than the other stars in their cluster. They pulsate with periods of a few hours to a few days and have amplitudes of a few tenths of a magnitude. Blue stragglers are mainly observed in old open clusters and globular clusters.
By studying the different types of pulsating stars, astronomers can gain a better understanding of the internal structure and properties of stars, including their ages, masses, radii, and chemical compositions.
This information is crucial for testing and refining theories of stellar evolution and for understanding the role of stars in the formation and evolution of galaxies.
Star Systems Classification
The classification of star systems is important for understanding the properties and behavior of stars and their environments. By studying different types of star systems, astronomers can learn about the formation and evolution of stars, the dynamics of stellar systems, and the formation and properties of planets.
Single stars: These are stars that exist on their own, without any companions. Single stars are the most common type of star in the universe. Single stars that have planets orbiting around them. These planets are known as exoplanets, and they are detected through a variety of methods, including the transit method and the radial velocity method.
The discovery of exoplanets has revolutionized our understanding of planetary systems and has opened up new avenues of research in astrobiology and planetary science.
Binary stars: These are star systems that consist of two stars that are gravitationally bound to each other. There are two main types of binary stars: visual binaries, which can be resolved into two separate stars through telescopes, and spectroscopic binaries, which appear as a single star.
However, spectroscopic binaries show periodic Doppler shifts in their spectra due to the gravitational influence of their companion.
There are binary star systems that also have planets orbiting one or both of the stars. These systems are less common than single-star systems with planets, but they are still important for understanding the formation and evolution of planetary systems.
Multiple stars: These are star systems that consist of three or more stars that are gravitationally bound to each other. There are different types of multiple stars, including triple, quadruple, and higher-order systems.
Star clusters: These are groups of stars that are bound together by their mutual gravity. There are two main types of star clusters: open clusters, which contain young stars and are found in the disk of the galaxy, and globular clusters, which contain old stars and are found in the galactic halo.
FAQ
Q: What are Protostars?
A: The process of star formation begins when a region of a molecular cloud becomes dense enough that gravity begins to dominate over other forces, such as gas pressure and magnetic fields. As the gas in the cloud collapses under its own gravity, it becomes increasingly dense and hot, forming a protostar at the center.
The protostar continues to accrete gas and dust from the surrounding cloud, gradually increasing in mass and temperature.
At some point, the temperature and pressure in the core of the protostar become high enough to initiate nuclear fusion, the process by which hydrogen atoms combine to form helium, releasing enormous amounts of energy in the process. Once fusion begins, the protostar becomes a true star and enters the main sequence phase of its life.
Q: What are Neutron Stars?
Neutron stars are extremely dense and compact remnants of a supernova explosion. They are only about 12 miles (20 km) in diameter, but have a mass greater than the sun. Black holes are also the result of a supernova explosion of a very massive star. They have a gravitational field so strong that nothing, not even light, can escape from them.