STARS

The Different Types of Stars

We have all heard about the stars and witnessed their beauty in the night sky since our childhood. But have you ever heard about the stellar classification of stars?

Let us understand what exactly we mean by the stellar classification of stars. And In astronomy, stellar classification is the classification of stars based on their spectral characteristics, i.e., the relation between wavelength and some other variables. Stellar classification, a scheme for assigning leads to types according to their temperatures as estimated from their spectra. For most of its active life, a star shines due to the thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space.

A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. According to the latest data, the observable universe contains 1×1024 stars. The nearest one to our Planet is our sun.

Scientists have studied stars for quite a time now and divided them based on their age and temperature. We will discuss one such classification today, which is the stellar classification of stars. Stars are classified by their spectra (the elements that they absorb) and their temperature. There are seven main types of stars. To decrease the temperature, O, B, A, F, G, K, and M.

MORGAN KEENAN spectral classification

We all see the stars twinkling but have you ever wondered how do stars twinkle? 

The passing of light causes the twinkling of stars through different layers of an explosive atmosphere. But in reality, the stars don't twinkle. They only appear because of the interaction between our eyes, stars, and earth's atmosphere. The formation of a star is not a quick process. It takes around a million years to form a star from an accumulation of gas and dust, collapsing due to gravity in its nebulae.

Our universe has nebulas, in which new stars are being formed all the time. Though we haven't observed much of these stars present in our infinite universe, those observed give a perfect idea of their composition and properties. Now let us go back to the base of the topic, stellar classification. It is a scheme assigning stars their type based on their temperatures as calculated from their spectra. The variety of temperature is done based on wien's law regarding black body radiation.

The universally accepted system of stellar classification is a combination of two schemes: the Harvard system, which is based on the star's surface temperature, and the MK, i.e., the Morgan Keenan system based on the luminosity of the star. The MK, also called the Yerkes system, is the American astronomer w.w. Morgan, p.C. Keenan, and a few others. It is based on two sets of parameters-a refined version of the Harvard o-m scale and a luminosity scale of grades i-for supergiants, ii-bright giants, iii-normal giants, iv-sub-giants, and v-main sequence, or dwarf, stars; further specifications may be used, such as a grade in for bright supergiants and grades vi and vii for sub-dwarfs and white dwarfs, respectively. In the Harvard system, the stars are classified as the hottest to coolest using letters such as O, B, A, F, G, K, M, where O is the hottest(25,000 k), and blue in colour, and M is the coolest (3,500 k) star which is red in color. The letters are further subdivided from hot to cool. An adapted and famous mnemonic for memorizing this stellar classification sequence is "oh be a fine girl/guy, kiss me." The hotter stars are sometimes referred to as early and the cooler as late. With the discovery of brown dwarfs, objects that form like stars but do not shine through thermonuclear fusion, the stellar classification system has been expanded to include spectral types L, T, and Y. Now, let us know about the classification in detail.

O Type Stars

Starting with the letter O: O, type stars are the hottest, luminous and rarest of all main stars. They have very complicated surroundings, which makes the measurement of their spectra difficult. Most of the massive stars lie within this particular type. Because of their massive and giant size, they have very hot cores and burn through their hydrogen fuel very quickly, so they tend to burn brighter and are the first stars to leave the main sequence. Their spectra have dominant absorption lines and sometimes emission for HE ii lines, neutral helium lines, and prominent ionized. Further subdivision includes O3, O7, O9, etc. Spectral type O7 is the point at which the two intensities are equal, and type O3 is when the line vanishes.

B Type Of Stars

The letter B: B-type stars are also very luminous and blue in colour. Their spectra have neutral helium lines and are very energetic. Their life span is relatively short, around 40 million years. Many stars that we can see from our naked eyes are of this type only. B-type stars are originated from an OB association that is related to giant molecular clouds. Now, the question here is, what is an OBassociation? An OB association is a loose grouping of several thousand stars, a small fraction of spectral types O and B. B-type stars also produce a 'be' type spectrum and are also detected in o and a shell star. Be stars are a heterogeneous set of stars with B spectral types and emission lines. According to the research, it was found that they can have a disk that creates and can create emission lines. They are considered to be main-sequence stars, but several sub-giants and giant stars are also included.

A Type Of Stars

The letter A: A-type stars are young, around a few hundred million years. They have masses ranging from 1.4 to 2.1 times the sun's mass and surface temperatures between 7112 k to 11500 k. They appear bluish-white in color, very bright, and can be seen by our naked eyes. A-type stars don't have a convective zone or convective region, i.e., A region where a layer of the star is unstable to convection, and as a result, they lack strong stellar winds and are not able to bear magnetic dynamo. Research proved that gigantic planets formed around them are hard to detect using a certain method called doppler spectroscopy. It is an indirect method for finding ex-o-planet, i.e., Planet outside of the solar system and brown dwarfs from their radial velocity measurements.

F Type Of Stars

Type F: F-type stars are moderately hot and yellowish-white in colour, usually weighing between 1.0 and 1.4 times the sun's mass. Their temperature lies between 6,000k to 7,600 k. It is also believed that life can also exist on this particular type. They are known for emitting a large amount of light in the form of radiation, such as UV radiation, which harms our planet earth. They are likely to represent the brightest and hottest main-sequence stars that could allow life. The best example off-type stars are Polaris, i.e., The current northern pole star. A type F7 and is 430 light-years away.

G Type Of Stars

The letter G: G type stars, also called yellow dwarf, are yellow in colour, and their weight is between 0.8 to 1.25 times the mass of the sun. They have currently believed to 100 light-years away. Their surface temperatures are between 5,300k and 6,000k. Class G contains the yellow evolutionary void, i.e., It is an area in the heart sprung-Russell diagram where atmospheres of blue-ward evolving super- and hyper-giants are moderately unstable. Thus, this type is unstable for supergiants to be. Compared to hotter and brighter type stars, O, B, A, F type stars. G-type stars radiate more light towards the spectrum's infrared end. The most famous G type, G2 type, a star in the sun. Each second, the sun fuses approximately 600 million tons of hydrogen into helium in a process known as the proton-proton chain (4 hydrogens from 1 helium), converting about 4 million tons of matter to energy. Other known stars under this type are ALPHA CENTAURI A, TAU CETI, AND 51 PEGASI.

K Type Of Stars

Next is type K: K-type stars appear orange and are cooler than the sun. Their range of temperature is between 3,500k to 6,000k. They are of intermediate size between M type and G type. The keen interest of astronomers is to research this type to find life on this star because of their stability, small mass, greater heat zones, and they emit fewer UV rays. Also, the velocity of the solar wind is not too much. Thus, they are the most approving stars to focus on the research for ex-o-planets and extraterrestrial life. The nearest K-type stars known are EPSILON ERIDANI which is 10.5 light-years away, HD 192310 is 29 light-years away, GLIESE 86 is 35 light-years away, and 54 PISCIUM is 36 light-years away.

M Type Of Stars

And the last we have is type M: M type stars, also known as red dwarfs, are the smallest and coolest red colour star with such low luminosity that none can be seen by our naked eyes. Despite being called as smallest, they too include some giant, supergiant stars and hypergiant stars. The range of their surface temperature is from 2,400k to 3,700k. Red dwarf stars have masses from about 0.08 to 0.6 times that of the sun. Lighter stars are more abundant in space than heavier stars. That is why they are the most common and tend to live longer than usual. There are other extended spectral types have also been included. Several new spectral types have been taken into use from newly discovered types of stars.

Extended Spectral Types 

Of Stars

Protostar

A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down 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 down. All of the energy released by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet.

T Tauri Star

A T Tauri star is the stage in a star’s formation and evolution right before it becomes a main-sequence star. This phase occurs at the end of the protostar phase when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble main-sequence stars; they’re about the same temperature but brighter because they’re larger. T Tauri stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years.

Main Sequence Stars

Main Sequence stars are young stars. They are powered by the fusion of hydrogen (H) into helium (He) in their cores, a process that requires temperatures of more than 10 million Kelvin. Around 90 percent of the stars in the Universe are main-sequence stars, including our sun. The main sequence stars typically range from between one-tenth to 200 times the Sun’s mass. A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward.

White Dwarf Star

When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star. The outward light pressure from the fusion reaction stops and the star collapses inward under its gravity. A white dwarf shines because it was a hot star once, but no fusion reactions are happening anymore. A white dwarf will just cool down until it becomes the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have cooled down that far yet.

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