We have all heard about the stars and witnessed there beauty in the night sky since our childhood. But Have you ever heard about Stellar Classification of Stars? If your answer is no, then you have landed at the right place because this video will touch the bottom of the topic. Before going too deep first, let us understand what exactly we mean by Stellar Classification of Stars! In astronomy, Stellar Classification is the classification of stars based on their spectral characteristics i.e. relation between wavelength and some other variables. Stellar classification, scheme for assigning stars to types according to their temperatures as estimated from their spectra. -------------------------------------------------------------------------------------------
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For most of its active life, a star shines due to 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 quiet a time now and divided them on them on the basis of 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. In order of decreasing temperature, O, B, A, F, G, K, and M. We all see the stars twinkling but have you ever wondered how do stars twinkle? To answer this, let us go back to our school days where we were taught refraction of lights. Twinkling of stars is caused by the passing of light through different layers of a tempestuous atmosphere. But in real the stars don't twinkle, they only appear as such 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 of years to form a star from an accumulation of gas and dust which collapses 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 but those which are observed give a very good 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 on the basis of their temperatures as calculated from their spectra. The classification of temperature is done on the basis of 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. Morgan Keenan system, which is based on the stars luminosity. The MK also called Yerkes system, is the work of the American astronomers W.W. Morgan, P.C. Keenan, and 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 super-giants, 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 Ia for bright super-giants and grades VI and VII for sub-dwarfs and white dwarfs, respectively. In the Harvard system the stars are classified on 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 colour. The letters are further subdivided from hot to cool. An adapted and famous mnemonic for memorising this sequence of stellar classification 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 system of stellar classification has been expanded to include spectral types L, T, and Y.
Now let's us know about the classification in details.
Credits: Nasa
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