What is a Star?
A Star is the luminous astronomical object which is spheroid of plasma. Many stars are visible by naked eyes from earth at night. The star shines due to thermonuclear fusion of hydrogen into helium present in its core which produces heat and light. The stars are looked so small but actually they are very big in size but due to very large distance from earth they look small.
The journey of a star starts from gravitational collapsing of gaseous nebula with hydrogen-helium in its core. When its core becomes sufficient dense than start conversion of hydrogen into helium and relies the light and heat. When total amount of hydrogen in its core converted into helium the star becomes Red Giant. The core of star becomes dense and star may convert into a stellar remnant, a white dwarf, a neutron star and if core is more massive than it convert into black hole.
The stars are classified in various groups according to mass and temperature. They are classify in seven groups, they are O,B,A,F,G,K and M. “M” group stars are very small and cool having temperature less than 3000 degree Celsius. “O” group stars are biggest and hottest star having temperature more than 30000 degree Celsius. The Sun is visible as yellow in colour, its show that sun is “G” type star having temperature is about 5500 degree Celsius.
Nebulae (Interstellar Cloud) with Protostar
The stars are formed from cloud of gas and dust which is called “nebulae” or “interstellar cloud”. The interstellar clouds are the accumulation of plasma, gas and dust. The velocity of the interstellar cloud is higher than the rotation of the milkyway. These clouds have the velocity greater than 90 km/s.
The interstellar cloud will remain in hydrostatic equilibrium until kinetic energy of gas pressure is in balance of potential energy of internal gravitational force. The cloud becomes too massive so gas pressure in cloud become insufficient to support it and so cloud will collapse by gravitational attraction. As the cloud collapse, the centre of the cloud heat up which is called “Protostar”. As the cloud collapse, it breaks up into smaller and smaller pieces. Each of these parts radiates the energy gain by gravitational potential energy which is called born of star. The protostar is the first stage in the process of star formation. It is the very young star which is collecting the mass from its parent molecular cloud.
Average Star & Massive Star
The Average stars are the stars with mass of 0.5 to 8 times that of the earth’s sun. A star having the mass greater than eight solar mass during its life cycle is called Massive star. Both average and massive stars are born from cloud made of dust, gas called nebulae.
The Blue star is classified as O-types star is very hot. They have about 30000 K temperature which is found in active star forming region of galaxy like arms of galaxy due to its blue light surrounding dust and gas cloud visible blue. Their mass is about 2.5 to 90 times that of sun and last about 40 million years. They have strong Helium II absorption line in their spectra. Beside the B-type star, hydrogen-helium lines are weaker in their spectra. Due to more massive and hotter, blue stars have short life and end its life in supernova and finally it transform into either black hole or neutron star.
Examples: AE Aurige, Delta Circini, theta 1 Orionis
The Yellow dwarf is G- types of star having temperature about 5200 K to 6000K. It has luminosity of 0.5 to 0.6 that of the sun and mass is 0.8 to 1.4 that of the sun having life about 4 to 17 billion years. Our sun is also G-type star but it is fact white. In the core of all G-types stars, hydrogen converts into helium and it will convert in red giant when all hydrogen finished in core. G-type star is range from white to slightly yellow in colour. The sun is in fact white but it is seen yellow, orange and red in colour from earth due to scattering effect of earth’s atmosphere.
Examples: Alpha Centaury A, Tau ceti.
The Orange dwarf star is K-type star having temperature about 3900K to 5200K. It has luminosities of 0.08 to 0.6 that of sun and mass is about 0.45 to 0.8 that of sun. The average life of this type of star is about 15 to 30 billion years. K-type stars are as common as g-type of star. They emit less UV radiation than g-type of star.
Examples: Alpha Centaury B
Red dwarf is the coolest and smallest star on main sequence. The red dwarf star is generally lies between K-type and m-type star having temperature about 4000K. It has luminosity of 0.001 to 0.08 that of sun and mass is about 0.08 to 0.45 that of sun. They last long about many trillion years.
Due to very low luminosity, these stars are not observed easily. This star emits very low light sometimes 1/10000 that of the sun. In the massive red dwarf having more than 0.35 solar masses, conversion of hydrogen into helium occurs in core as well as outer surface so nuclear fusion in this type of star is very slow and prolonged.
Examples: Proxima Centaury
GIANT AND SUPER GIANT
All hydrogen from the core of star is exhausted and started the burning of helium. So the core collapse and become hotter and outer layer of star start expand outwards and star becomes either Giant Star or Super giant Star. The star which has low or medium mass becomes Giant Star. If the star which has high mass or more than 10 times mass than sun are converted into Super giant Star.
Due to slow fusion period, the star can contract itself and convert into blue super giant. They are called blue because temperature of star spread over small surface area and due to that it is hot and blue.
The Blue Giant is generally lies in spectra O-type, B-type and A-type occasionally. They have temperature about 10000K to 33000K having luminosity 10000 that of the sun and mass about 2 to 15 that of the sun. The life of it may be 10 to 100 million years.
There is no perfect definition of blue giant but the stars which composed luminosity class III & II are defined as blue giant which has also temperature above 10000K.
Examples: Meissa, Xi Persei.
BLUE SUPER GIANT
The Blue Super giant is spectra OB -type. They have temperature about 10000K to 50000K having luminosity 10000 to 1000000 that of the sun and mass about 10 to 100 that of the sun. The life of blue super giant is short about 10 million years.
Blue super giant have luminosity class I and spectral class B9 or earlier. Blue super giants are bigger than the sun and smaller than the red super giant. Due to high massive, the burning rate of hydrogen of this star is very high so they lives for few million years.
Examples: UW Canis Majoris, Rigel,Tau Canis Majoris.
Red Giant star is smaller and with low mass than red super giant. They are M-type and K- type with mass about 0.3 to 10 that of the sun. They have luminosity about 100-1000 and temperature is about 3300K to 5300K. The average life of it may be 0.1 to 2 billion years.
Most of the red giant stars are on red giant branch in which hydrogen is still converting into helium in shell surrounding inert helium core. In other type of red clump star, helium is being fused into carbon in their core.
Examples: Aldebaran, Arcturus
RED SUPER GIANT
The Red Super giant star has super giant luminosity. They are spectral K-type and M-type. They have luminosity around 1000 to 800000 that of the sun and temperature about 3500K to 4500K. They compose the mass about 10-40 that of the sun having life 3 to 100 million years.
They are the stars which have given off all hydrogen from its core so their outer layer is huge expanded. They are the biggest stars in the universe but they are not most massive and luminous. Some of the red super giants are so massive that they fuse very heavy elements like iron.
Examples: Antares, Betelgeuse, Mu Cephei.
The dead stars are those stars which have no longer fusion processes doing in their core.
The name White Dwarf is first announced in 1922 by Willem Luyten. White dwarfs are spectral D- type stars having luminosity 0.0001 to 100 that of the sun. About 97 % of the stars in milkyway galaxy are white dwarf. They have temperature about 8000K to 40000K and mass about 0.1 to 1.4 that of the sun. They live about 100000 years to 10 million years. The white dwarf is that stage of the star which has no enough mass to be Neutron star. There is no material for fusion reaction in white dwarf so no any source of energy which generated by fusion against gravitational collapse but it due to electron degeneracy pressure which make the star very dense.
Example: Sirius B, Procyon B
The Neutron Stars are the smallest and densest star in the universe. They have radius about 10km and having mass 1.4 to 3.2 that of the sun. This star is D-type star have luminosity is very low due to its small size. This star is very hot having temperature about 600000K and life is about 100000 to 10million years.
Neutron stars are formed from supernova explosion of massive star. Once the star formed it will no longer active to generate heat and cool over the time. The neutron stars are composed mainly of neutrons.
Example: PSR J0108-1431, PSR B1509-58.
The Black Dwarf is the form of white dwarf which cools sufficiently so that it unable to emit the heat and light. There is no any black dwarf is present in universe now because the time for converting the white dwarf into black dwarf is too longer than the life of universe. If the black dwarfs are present in universe, it is not possible to detect because of very little radiation.
The small star converts into white dwarf or neutron star after finishing its nuclear fusion in core. But Massive star which has masses more than three times converted into black hole after supernova explosion with gravity so strong even light cannot escapes from it. After forming the black hole, it is continuously growing by absorbing the mass from surrounding. By continuously absorbing stars and other black holes, it will become super massive black hole which is present in the center of galaxy.
The Failed star is formed just like normal star from collapsed of gravitational cloud of hydrogen. They are generally known as brown dwarf. There is no sufficient mass in core to fuse hydrogen so they do not shine.
Brown dwarf is spectral M,L,T,Y-type star. They have luminosity is about 0.00001 that of the sun and mass 13 to 80 times of Jupiter. The temperature of brown dwarf is about 300K to 2800K which lives for many trillion years. This star is not enough massive to conduct fusion of hydrogen into helium. The brown dwarf having mass more than 13 times of Jupiter fuses deuterium and if mass more than 65 times Jupiter fuses lithium. They are shown as magenta or sometimes red/orange in colour.
Example: Gliese 229 B, Luhman 16.
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Authored by BK Suthar.
This site is about various topics of astronomical and geographical things which present in universe and also surrounding us. It will give you information about various planets, incidents occurs in universe and also about nature.
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