You all must have seen the stars in the sky every night, some bigger, some a little smaller, some twinkling, some are not. Some yellow in color, some in white, and some in red. And you must have tried to count them many times too, but you never succeed because the number is too big to be counted on fingers. According to European Space Agency (ESA), there are approximately 1,000 billion billion stars (10^24) in the universe. The number will undoubtedly increase as humans become accustomed to technology and explore deep space. And the fact that our universe is continuously expanding, speaks to this point in a more convincing way.
So have you ever wondered, how scientist deals with all these stars in this amazing universe, which are infinite in number? Is there any method to classify these stars? The answer is Yes !!
And what would be your reaction if you get to know that the enormously large number of stars are all classified into just 7 different types. Yes, you heard right. The Morgan Keenan Classification is the latest classification which is a combination of the old Harvard System and the Yerkes System.
Let's dig into them in detail.
Harvard Classification of Stars
First, the Harvard Star Classification System is a one-dimensional system that classifies stars into seven main categories according to their spectra. This classification is based on the temperature of the star's surface. These seven categories are denoted by the seven alphabets which in order of hotter to colder are O, B, A, F, G, K, M. Thus, type O stars are the hottest stars with a surface temperature of about 50,000 K and M. A typical star is the coldest star with a surface temperature of only 2500K. The color of the star depends on the surface temperature, as shown.
The easiest way to find the order is to associate a sentence word with each alphabet:
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The temperature range and spectral characteristics for each are given in the figure above.
These 7 categories are the ones originally developed in the classification scheme. With the discovery of brown dwarfs, objects that form like stars but do not glow by fusion, the star classification system has been expanded to include spectral types L, T, and Y. These new spectral types L, T, and Y were created to classify the infrared spectra of cold stars. This includes both red dwarfs and brown dwarfs, which are very low in the visible spectrum.
The 7 Main Spectral Types of Stars:
- O (Blue) (10 Lacerta)
- B (Blue) (Rigel)
- A (Blue) (Sirius)
- F (Blue/White) (Procyon)
- G (White/Yellow) (Sun)
- K (Orange/Red) (Arcturus)
- M (Red) (Betelgeuse)
Type O includes bluish-white stars with surface temperatures typically between 25,000 and 50,000 K (although several O-type stars with much higher temperatures have been described); Lines of ionized helium appear in the spectrum. Class B stars are typically between 10,000K and 25,000K and are also bluish-white in color but exhibit helium-neutral lines. The surface temperatures of A-type stars range from 7,400 K to about 10,000 K; hydrogen lines are prominent and these stars are white. F-type stars are yellowish-white, reach from 6000 to 7400 K, and exhibit many metallic-induced spectral lines. The Sun is a G-class star; they are yellow, with a surface temperature of 5,000 to 6,000 K. K-type stars are yellow to orange, at about 3,500 to 5,000 K, and M stars are red, at about 3,000 K, with titanium oxide prominent in their spectra. The Brown L dwarf has a temperature of about 1,500 to 2,500 K and has spectral lines caused by alkali metals such as rubidium and sodium and metallic compounds such as iron hydrides. T brown dwarfs have significant methane absorption in their spectra and temperatures between about 800 and 1,500 K. Y-type brown dwarfs are cooler than 800 K and have spectral lines of ammonia and ammonia in water.
In this stellar classification, within the same class, there are 10 additional divisions. So each star has a number starting at 09, with a lower number representing a hotter star. Therefore, a K0 star is hotter than a K7 star. The usual color description is traditional in astronomy and represents the color associated with the average color of an A-class star, considered white. Also, clear color descriptions are what an observer would see if they tried to depict stars under a dark sky without the aid of the eye or with binoculars. However, most of the stars in the sky, except the brightest, are white or bluish-white to the naked eye because they are too dark for color vision to work. Red supergiant stars are cooler and redder than dwarfs of the same spectral class, and stars with distinctive spectral features such as carbon stars can be much redder than any black body.
Yerkes Classification of Stars
Simply assigning an alphabet to each star based on its surface temperature is not enough to classify stars. Stars come in all sizes and at different stages of evolution. There are main-sequence stars that still burn hydrogen to helium in their nuclei (the Sun) and there are white dwarfs that have ended their lives. So we need another parameter to distinguish them. This parameter is the brightness.
Luminance, in astrophysics, is the total energy output per second. Dense stars with higher surface gravity exhibit larger spectral line pressure expansions. The gravitational force, and therefore the pressure, on the surface of a giant star, is much lower than that of a dwarf star because the giant star's radius is much larger than that of a massive dwarf star. similar amount. The difference in the spectrum can therefore be interpreted as a lighting effect, and a luminance class can only be assigned when examining the spectrum. The Luminosity class and its description are as follows:
Morgan-Keenan (MK) Classification of Stars
Finally, when the Harvard system and the Yerkes brightness classes are combined, we get the current Morgan Keenan (MK) star classification system. It is based on two sets of parameters: a fine-tuned version of the Harvard OM scale and the luminosity scales of classes I (for supergiant), II (bright giant), III (normal giant), IV (subgiant), and V (main sequence, or dwarf); other specifications may be used, such as class Ia for bright supergiant and classes VI and VII for dwarfs and white dwarfs, respectively. Thus, each star is assigned a spectral class according to its surface temperature and a luminosity class corresponding to its surface gravity (luminosity). Therefore, our Sun is a G2V star. Its surface temperature is about 5,900 K (G-type) and it fuses hydrogen to helium in its nucleus, creating a main-sequence star (V); while Barnard's Star, an approximately 3,100 K red dwarf, is rated M5 V; and the supergiant Rigel is rated B8 Ia. The MK system works by plotting all the stars in the Universe on a single diagram, the Hertzsprung Russell diagram, which is currently one of the most powerful tools in stellar physics.
Some types of Stars
When looking at the night sky on a clear day, it might seem that most stars are cold blue stars that would be of type B or A. However, the red dwarfs in the main sequence are the most common type of star in our universe.
Our Sun is the main sequence, a G-type star, but most of the stars in the Universe are much cooler and have low mass. In fact, most of the red dwarfs in the main sequence are too faint to be seen with the naked eye from Earth. Red dwarfs burn slowly, which means they can live very long compared to other types of stars. The closest star to Earth (Proxima Centauri) is a red dwarf. Red dwarfs include the smallest star in the Universe, weighing between 7.5% and 50% of the Sun's mass. Although red dwarfs are the most common stars in the universe, there are a total of 7 main types of stars.
Here is some information about each known type of star in our universe.
Protostar
Protostar is what you have before a star forms. Protostars are a collection of gases that have collapsed from a giant molecular cloud. The protostar phase of stellar evolution lasts about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to collapse. All the energy released by the protostar comes only from gravity-induced heating - the nuclear fusion reactions have not yet begun.
T Tauri Star
A T Tauri star is a stage in a star's formation and evolution just before it becomes a main-sequence star. This phase occurs towards the end of the protostar when the gravitational pressure holding the star together is the source of all of its energy. T Tauri stars do not have enough pressure and temperature in their cores to induce nuclear fusion, but they look like main sequence stars; they have the same temperature but are brighter because they are bigger. T Tauri stars may have large sunspot coverage and intense X-ray flares and intense stellar winds. The stars will stay at the T Tauri stage for about 100 million years.
Main Sequence Stars
The main sequence stars are young stars. They are powered by the fusion of hydrogen (H) with helium (He) in their nuclei, a process that requires temperatures above 10 million Kelvin. About 90% of the stars in the Universe are main-sequence stars, including our sun. Main sequence stars typically vary from 1/10 to 200 times the mass of the Sun. A main-sequence star is in hydrostatic equilibrium. Gravity pulls the star in, and the slight pressure of all the fusion reactions in the star pushes it out. The internal and external forces balance each other, and the star maintains a spherical shape. Main sequence stars will have a size that depends on their mass, which determines how much gravity pulls them in.
Blue Stars
Blue stars are generally hot O-type stars, commonly found in regions of active star formation, particularly in the arms of spiral galaxies, where their light illuminates dust clouds and gases in the surrounding areas, often making these areas appear blue. Blue stars are also commonly found in complex many-star systems, where their evolution is much more difficult to predict due to interstellar mass transfers, as well as the possibility of other stars. each other in the system will end their lives as supernovas at different times. Blue stars are mainly characterized by strong absorption lines of helium II in their spectra, and the lines of neutral hydrogen and helium in their spectra are considerably weaker than those of stars in type B.
Because blue stars are very hot and massive, they have relatively short lifetimes and end in violent supernova events, which eventually lead to the creation of black holes or neutron stars.
Red Dwarfs
Red Dwarfs are the most common stars in the Universe. They are main-sequence stars, but they are so low mass that they are much cooler than stars like our Sun. This colder state makes them appear pale. They have another advantage. Red dwarfs can maintain a hydrogen-fuel mixture in their nuclei, and therefore can hold their fuel much longer than other stars. Astronomers estimate that some red dwarfs will burn out over a period of up to 10 trillion years. The smallest red dwarfs have a mass of 0.075 times the mass of the Sun, and they can be as much as half the mass of the Sun.
Yellow Dwarfs
The yellow Dwarf is a G-type main-spectrum star and weighs between 0.7 and 1 times the mass of the Sun. About 10% of the stars in the Milky Way are dwarf yellow. They have a surface temperature of about 6000°C and glow bright yellow, almost white. Our Sun is an example of a G-type star, but it is actually white because all the colors it emits are mixed. However, although all visible light from the Sun is mixed to produce white, its visible light emission peaks in the green part of the spectrum, the green component is absorbed and/or scattered by other frequencies both in the Sun's spectrum. itself and in the Earth's atmosphere. Typical G-type stars have masses between 0.84 and 1.15 Suns and temperatures between 5,300 K and 6,000 K.
Like the Sun, all G-type stars convert hydrogen to helium in their cores and will evolve into red giants when their hydrogen fuel supplies run out.
Orange Dwarfs
Orange Dwarfs are K-type main sequence stars that, in size, lie between the red M-type main-sequence stars and the yellow G-type main-sequence stars. K-type stars are particularly interesting in the search for extraterrestrial life, because they emit significantly less UV radiation (which damages or destroys DNA) than G-type stars, and they remained stable in the main chain until about 30 billion. years, compared with about 10 billion years for the Sun. In addition, K-type stars are about four times more common than G-type stars, which makes finding alien planets much easier.
Supergiant Stars
The most massive stars in the Universe are supergiant stars. Giants and supergiants are formed when a star lacks hydrogen and begins to burn helium. As the star's core collapses and becomes hotter, the resulting heat causes the star's outer layers to expand outward. Low- and intermediate-mass stars later evolve into red giants. However, stars with masses 10 times the size of the Sun become red supergiants during their helium-burning phase. Supergiants consume a lot of hydrogen and will consume all the fuel in their hearts within a few million years. An example of a red supergiant is Herschel's Garnet star in Cepheus. The garnet star, Mu Cephei, appears garnet red and lies at the edge of the nebula IC 1396. Mu Cephei is visually 100,000 times brighter than our Sun, with a magnitude of -7.6.
Supergiant stars live fast and die young, exploding like supernovas; completely disintegrate in the process.
Blue Giants
Stars of luminosity III and II (giant and bright giant) are called blue giants. This term applies to many types of stars in different stages of development. These are evolved stars that have left the main sequence but have little in common. Thus, a blue giant simply refers to stars in a particular region of the HR diagram rather than a particular type of star. An example of a blue/white giant star is Alcyone in the constellation Taurus. Blue giants are much rarer than red giants, because they only develop from larger and less common stars, and because they have short lifespans. Some stars are mistakenly labeled as blue giants because they are large and hot.
Blue Supergiants
Blue Supergiants are known as OB supergiants, and usually have a luminosity classification of I and spectral classification of B9 or earlier. Blue supergiant stars are generally larger than the Sun, but smaller than red supergiant stars, and range in mass from 10 to 100 times the mass of the Sun. Normally, type O and type B main sequence stars do not leave the main sequence for several million years, because they burn off their hydrogen supply very quickly due to their high mass.
These stars begin their expansion in the blue supergiant phase shortly after heavy elements appear on their surfaces, but in some cases, some stars evolve directly into stars known as Wolf-Rayet stars, skipping the "normal" blue supergiant stage.
Red Giants
When a star consumes all of the hydrogens in its nucleus, fusion stops and the star no longer generates external pressure to counteract the internal pressure that brings it closer. A hydrogen shell around the nucleus ignites, continuing the life of the star, but its size increases dramatically. In these stars, hydrogen is always molten to helium, but in a shell surrounding an inert helium nucleus. The old star has become a red giant and maybe 100 times more massive than it was in its main sequence. When this hydrogen fuel is depleted, other helium shells and even heavier elements can be consumed in fusion reactions.
The red giant phase of a star's life cycle will last only a few hundred million years before it runs out of fuel and becomes a white dwarf.
Red Supergiants
Red Supergiant Stars are stars that have exhausted their supply of hydrogen in their nuclei and as a result, their outer layers expand greatly as they evolve out of the main sequence. Stars of this type are among the most massive known in terms of mass, although they are not usually among the most massive or brightest.
Antares, in the constellation Scorpio, is an example of a red supergiant star late in its life.
White Dwarfs
When a star runs out of hydrogen fuel in its nucleus and does not have enough mass to force the elements to undergo fusion, it becomes a white dwarf. The mild external pressure of the fusion reaction stops and the star collapses inward under its own gravity. A white dwarf glows because it was once a hot star, but no longer has fusion reactions.
A white dwarf will cool down until it becomes the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarf has cooled down to this point.
Neutron Stars
Neutron stars are the collapsed nuclei of massive stars (between 10 and 29 solar masses) that are compressed outside the white dwarf stage in a supernova explosion. The remaining nucleus becomes a neutron star. A neutron star is an unusual type of star made up entirely of neutrons; Particles that are slightly more massive than protons, but carry no charge. Neutron stars are raised relative to their own mass by a process known as "neutron degeneracy pressure". The strong gravity of a neutron star crushes protons and electrons to form neutrons. If the stars were even more massive, they would become black holes instead of neutron stars after the supernova activates.
Black Holes
While smaller stars can become a neutron star or white dwarf once their fuel begins to run out, larger stars with three times the mass of our sun may end their lives in a supernova explosion. The dead remnant that has no external pressure to resist gravity will then continue to collapse into a gravitational singularity and eventually become a black hole, with an object's gravity so strong that Not even light can escape. There are a variety of different black holes. Stellar-mass black holes are the result of a star around 10 times heavier than the Sun ending its life in a supernova explosion, while supermassive black holes found at the center of galaxies maybe millions or even billions of times more massive than a typical stellar-mass black hole.
Known examples of black holes include Cygnus X1 and Sagittarius A.
Brown Dwarfs
Brown dwarfs are also known as failure stars. This is as a result of their training. Brown dwarfs are shaped like stars. However, unlike stars, brown dwarfs do not have enough mass to burn and melt the hydrogen in their nuclei. Therefore, they do not shine and can be small.
As a general rule, brown dwarfs fall within the mass range of 13 to 80 Jupiter masses, with sub-brown dwarfs falling below this range.
The spectral classification of stars, along with the Hertzsprung Russell diagram, is one of the most fundamental concepts in stellar astrophysics. The entire history of the stars revolves around these two concepts. This article is important to learn more about stellar astrophysics. Today, in the name of astrophysics, most people just talk about black holes, wormholes, white holes, and other popular science concepts. But in fact, Astrophysics is much more than that. It is a very wide branch of Physics.
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