Like almost anything in the universe, stars born, live their life and then die. They are also one of the most diverse things we know today. The most commonly used criteria of classifying stars is based on their spectral characteristics, in which stars are basically sorted out from the hottest O type to the coolest M type (I am been extremely modest here).
Anyway, there are few other means to classify stars, in which they can be physically described. Below are 13 types of stars based on stages of their evolution and kinematics.
Types of Stars Based on Stages of their Evolution
A Protostar outburst in the Orion Star-forming complex. The image was taken by Spitzer Space Telescope. NASA
The process of star formation begins with the collapse and fragmentation of molecular clouds. These fragments, also known as dense cores, gain mass through accretion (by accumulating gas from the surrounding cloud) while the gravitational contraction kicks in.
The dense cores eventually transform into a rotating sphere of extremely hot gas due to increased pressure and temperature. The resulted star-like object is called protostar.
By the end of the protostellar phase, which may last over 500,000 years, the star has already acquired almost all its mass but has not yet started nuclear fusion at the core. Moreover, protostars are observable only at infrared and microwave wavelengths.
2. T Tauri Star or Herbig Ae/Be star
On an evolutionary scale, between the protostellar phase and the main sequence stage, there is an intermediary phase in which the young stars continue the accretion process and reach a certain temperature limit.
While stars in this phase are generally labeled as pre-main sequence stars, they can be classified either as a T Tauri star, with a mass less than 2 solar masses, or Herbig Ae/Be star, if it has a mass somewhere between 2 to 8 solar masses.
Since pre-main sequence stars don’t have enough temperature and pressure to initiate hydrogen fusion, they are powered by gravitational contraction rather than a nuclear reaction. Most pre-main sequence stars feature planet-forming or protoplanetary disks at their early stages.
3. Main Sequence Star
Main sequence stars in each spectral class (Morgan-Keenan spectral classification)
A pre-main sequence star eventually acquires the core temperatures (approx 10 million kelvin) needed to initiate a nuclear chain reaction. Once started, it leads the star towards a hydrostatic equilibrium in which the energy released from the nuclear fusion ceases its gravitational collapse. Both these forces balance each other out.
A majority of stars in the Milky Way, including our Sun, are main sequence stars. They can vary in size and brightness. The lower observable mass limit for a main sequence star at which it can sustain nuclear fusion is about 0.08 solar masses. Such low-mass main sequence stars are called red dwarfs.
4. Red Dwarfs
Spectral Types: M and sometimes K-types
Red dwarfs are the most common type stars in the Milky Way galaxy and quite possibly in the universe. Most stars in the immediate vicinity of the Sun are red dwarfs including the Proxima Centauri located at 4.2 light years away.
They’re essentially low mass (less than 0.8 M☉) stars on the main sequence with low fusion rate and low temperature. Due to their prolonged hydrogen fusion, red dwarfs are expected to live between 1 trillion to 10 trillion years depending on the mass.
It’s evident that red dwarfs have not had enough time to evolve from the main sequence phase since their estimated lifespans are much longer than the current age of the universe.
However, according to the current stellar models, red dwarfs need a minimum mass of 0.25 solar masses to evolve into a red giant. Anything less than that is more likely to end up as white dwarfs.
5. Red Giant Stars
An artist’s concept of the Sun as a red giant (current Sun in the inset for reference) | Image Courtesy: Wikimedia
Spectral Types: Class M, K and most Carbon Stars
Towards the end of their life, main sequence stars (with masses between 0.3 M☉ to 8 M☉) enter one last phase which is generally characterized by a dramatic increase in both their size and luminosity. These stars are called red giants. The Sun is destined to become a red giant.
After millions of years of steady nuclear fusion, main sequence stars eventually run out of hydrogen in their core, 5-6 billion years in Sun’s case. Without fusion and a counteracting force, the stellar core begins to collapse under the influence of gravity.
But before it could collapse much further, the earlier dormant hydrogen shell around the star’s core kicks in and starts supporting the nuclear fusion. Once the hydrogen is completely depleted, it starts fusing helium causing the star to swell up at an extremely rapid pace.
For a Sun-like star, this phase usually lasts up to a few hundred million years.
6. White Dwarfs
Sirius B, the smaller star in the lower left, is part of the Sirius binary star system Image Courtesy: NASA
White dwarfs are considered as the last stage (not theoretically though) in the evolutionary journey of the low to medium mass (0.5 and 8 M☉) main sequence stars. A typical white dwarf’s mass is similar to that of the Sun, while its volume is likely to correspond that of the Earth’s.
After the helium-fusing phase, if a red giant doesn’t have sufficient mass to produce core temperatures needed to fuse carbon, an inert gas of carbon and oxygen piles at the core while it starts collapsing. When the red giant explodes and forms a planetary nebula, it leaves behind a dense, carbon-rich white dwarf.
7. Supergiant Star
Size comparison of planets and stars | Image Courtesy: Dave Jarvis
Spectral Type: A to M types
Supergiants, as the name implies, are the largest stars in the universe. These gigantic stellar beasts have masses at least ten times than that of the Sun. Betelgeuse, the nearest supergiant (724 light-years approx.) for example, has a mass between 11 M☉to 12 M☉.
Stars with initial mass above 10 solar masses are much quicker to initiate helium fusion process than the red giants after hydrogen exhaustion. The process causes their atmosphere to inflate drastically as they leave the main sequence phase.
After helium, they start fusing much heavier elements until their core explodes into a type II supernova. Supergiant stars die much younger after evolving at a fast rate.
8. Neutron Stars or Stellar Black Hole
An isolated neutron star (illustration), one without associated supernova or companion star Image Courtesy: NASA
A neutron star is one of the two possible evolutionary end-points of a high mass star (above 8 solar masses). As the name suggests, these stars are composed entirely of neutrons and perhaps the smallest and the densest stars in the observable universe. Neutron stars have radii between 12 to 13.5 km.
When a massive star (supergiant) finally runs out of fuel, its core starts contracting under gravity. This contraction squeezes protons and electrons together to produce neutrons which build up in very tight spaces and it goes on until their density matches that of an atomic nucleus.
The star eventually explodes into a supernova leaving behind a dense neutron star. Similar to the Chandrasekhar limit for white dwarfs, if the remnant star has a mass greater than 2.2 -3.0 solar masses (the Tolman–Oppenheimer–Volkoff limit), it will collapse further to become a black hole.
9. Variable Stars
Stars whose apparent magnitude, or brightness when seen from the Earth, fluctuates over time are called variable stars. These stars can be classified into two types; intrinsic variables, in which fluctuations are caused by changes in the star itself (drastic swelling and shrinking or mass ejections) and extrinsic variables, whose apparent changes are caused by external factors.
Stellar variability, apart from changes in brightness, can occur due to specific changes in the spectrum. According to the most recent edition of the General Catalogue of Variable Stars, there are nearly fifty thousand variable stars in the Milky Way with seven thousand suspected ones under consideration.
Few known examples of variable stars are – R Scuti, P Cygni and Algol or Beta Persei, possibly the first ever variable star ever discovered.
Types of Stars Based on Kinematics
Stars can also be classified based on their kinematics. As you may know, the Sun along with all the stars and cosmic dust in the Milky Way revolves around the galactic core with some uniformity. But, within the local stream of stars, there are few that moves independently or erratically due to one reason or the other.
Over the years, researchers have been able to identify a number of stars that are racing away from their stellar associations at an unusual velocity. These stars are broadly classified as high-velocity stars which can be further subdivided into runaway stars, hypervelocity stars, and halo stars.
10. Runaway Stars
This is an image of the Tarantula Nebula taken from the Hubble Space Telescope. The enlarged inset shows, what appears to be a runaway star, kicked out of the nebula by its much heavier peers. The dashed arrow points toward the presumed direction of that star. Image Courtesy: NASA.
The Sun revolves around the galactic core at a speed of about 22 km/s relative to the average velocity of other stars in its neighborhood. A runaway star, based on its current definition, however, is a star that moves at a velocity of up to 100 km/s (relative to Sun’s neighborhood).
According to the most recent theory derived from years of observations, there are two possible ways for a star to gain enough velocity to become a runaway star:
The first scenario involves gravitational interaction between two binary star system, each containing two stars. Here more than one stars can be ejected. The second possible cause of a runaway star is a supernova explosion in a multiple star system.
One of the best examples of runaway star(s) is AE Aurigae, 53 Arietis, and Mu Columbae, all of which appears to have originated from Bernard’s Loop in the Orion Nebula. They are speeding away from each other at a velocity of 100 km/s.
11. Hypervelocity Stars
An artist’s concept of US 708 ( star on the left), one of the fastest moving stars in the galaxy | Image Courtesy: NASA
Hypervelocity stars or HVS are extremely fast-moving stars with velocities much higher than those of runaway stars. The average velocity of stars in the Milky Way varies between 100 km/s to 200 km/s, whereas a hypervelocity star can have velocities more than 1000 km/s and are likely to exceed the escape velocity of the galaxy.
These stars are found in abundance near the center of galaxy compared to the other parts. One of the fastest, if not the fastest star in the galaxy, US 708 is moving at an exceptional velocity of 1,200 km/s, but it doesn’t appear to have originated from near the galactic core.
Many of the known HVS’ are main-sequence stars with masses identical to that of the Sun. Few neutron stars and dwarf stars are also theorized to be moving at hypervelocity speeds.
12. Intergalactic Stars
A possible mechanism for no longer gravitationally bound star HE 0437-5439 Image Courtesy: NASA/ESA
Stars which are not associated or, more precisely, gravitationally bound to any galaxy are called intergalactic stars. Like other stars, intergalactic ones are also believed to have originated inside the galaxies but later ejected due to some unclear reason.
Their existence was first reported in 1997 by the Hubble Space Telescope after surveying the Virgo Cluster. A few years later, another large group of intergalactic stars was discovered this time in the Fornax cluster of galaxies.
In 2015, a group of astronomers from the Vanderbilt university detected more than 675 rogue stars just outside the Milky Way, which they argued were hypervelocity stars ejected from our galaxy.
13. Halo Stars
Halo stars are fascinating in many ways. They were formed in the early universe before the supernova explosions that disseminated large amounts of heavy elements into the interstellar space.
Located in the galactic halo, spherical portion of the galaxy (most notable in spiral galaxies), these stars have highly optical orbits around the center of the galaxy.
Due to this, halo stars we observe today have low metal content than relatively younger star such as our Sun. While their exact age is extremely difficult to determine but if the current assumptions are correct some of them are possibly as old as the age of the universe.
Kapteyn’s Star, located roughly about 12.8 light years away, is the nearest halo star from the Sun. It’s also a high-velocity star and has a retrograde orbit around the galactic core.