Types of stars in the universe

                             

                                      Types of Stars 

     

   

• Protostar
• T Tauri Stars 
• Main Sequence Stars
• Dwarf Stars ( Yellow , Orange , Red , White )
• Blue Stars 
• Giant Stars ( Red , Blue )
• Supergiant  Stars ( Red , Blue )
•   Neutron Stars 
• Magnetar Stars 

     Protostar

A protostar  is a very young star that is still gathering mass from its parent molecular cloud .

The protostellar phase is the earliest one in the process of stellar evolution . For a low mass star  (ie that of the Sun or lower ) it lasts about 500,000 years . The phase begins when a molecular cloud fragment first collapses under the force of self gravity and an  Opaque , pressure supported core forms inside the collapsing fragment. It ends when the infalling gas is depleted , leaving a pre main sequence star , which contracts to later become a main sequence star at the onset of hydrogen fusion producing helium .

  

 T- Tauri 

T Tauri stars (TTS) are a class of variable stars that are less than about ten million years old . This class is named after the prototype , T Tauri , a young star in the Taurus star-forming region. They are found near molecular cloud and identified by their optical variability and strong chromospheric lines . T Tauri stars are pre main sequence stars in the process of contracting to the main sequence along the Hayashi track , a luminosity-temperature  relationship obeyed by infant stars of less than 3 solar masses in the pre main sequence phase of stellar evolution. These stars central temperature are too low for hydrogen fusion. They typically rotate with a period between one and twelve days , compared to a month for the Sun, and a very active and variable . There is evidence of large areas of starspot coverage , and they have intense and variable X-ray and radio emission (approximately 1000 times that of the Sun ).

  Main Sequence Star 

The main sequence is a continuous and distinctive band of stars that appear on plots of stellar color versus brightness. After condensation and ignition of a star, it generates thermal energy in its dense core region through nuclear fusion of hydrogen into helium. During this stage of the star's lifetime , it is located on the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and age .  The cores of the main sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward pressure of gravitational collapse from the overlying layers .The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. 

Dwarf Stars 

A dwarf star is a star of relatively small size and low luminosity . These stars lie on the main sequence the phase of life in which a star steadily converts the hydrogen fuel in its core to helium. They are either much brighter than Sun , or much fainter . They don't consume their hydrogen quickly as our sun does , so even though they're less massive and thus have less hydrogen , they still live for a much longer time than our Sun will .

 

              Types of  Dwarf Star 

• Yellow Dwarf
• Orange Dwarf 
• Red Dwarf 
• White Dwarf 

     

     Yellow Dwarf 

The is the term used to describe a medium sized star. These stars are also known as G dwarf stars and G type main sequence stars . Noticeable characteristic  of these stars are their size. 

Yellow dwarf stars are between 0.84  and 1.15 times the mass of our sun .Our  sun is a yellow dwarf. The term yellow dwarf is a misnomer because G type stars actually range from white , for more luminous type like Sun , to one very slightly yellow for the less massive and luminous G-type main sequence stars. These types of stars outshines 90 percent of stars in our milky way galaxy .These stars are almost perfect hydrostatic equilibrium , but not quite as the core heats up , it expands slightly ,which has the effect cooling the core down.

  Orange Dwarf 

A K type main sequence star also referred to as K dwarf or orange dwarf is a main sequence (hydrogen-burning) star of spectral type K and luminosityV. These stars are intermediate in size between red M-type sequence stars and yellow G type main sequence stars. They have masses between 0.5 and 0.8 times the mass of the sun and surface temperature between 3,900 and 5200 K . These K-type stars emit Ultra radiation ( which can damage DNA and thus hamper the emergence of nucleic acid based life.) than G type stars like Sun . Orange dwarfs , on the other hand , with masses between 50% and 80% that of the sun, have only a little bit more flare activity than Sun like stars. 

Red Dwarf 

Red dwarf stars make up the largest population of stars in the galaxy , but they are too dim to be seen with naked eye from earth . Limited radiance help to extend their lifetimes , that's far greater than sun . The closest star to the Sun Proxima Centauri, is a Red Dwarf . Red dwarf include the smallest of the stars, weighing between 7.5% and 50% the mass the sun. The reduced size means that they burn at a lower temperature , reaching only 6,380 degrees Fahrenheit (3,500 degree Celsius ).  These contain metals , which are heavier than hydrogen and helium . They have relatively low pressure , a low fusion rate and hence, a low temperature . The energy generated is the product of nuclear fusion of hydrogen into helium by way of the proton-proton (PP) chain mechanism.

  

 White Dwarf

                                               

 A white dwarf , also called a degenerate dwarf , is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense , its mass is comparable to that of Sun ,while its volume is comparable to that of Earth. A white dwarf faint luminosity comes from the emission of stored thermal energy , no fusion take place in a white dwarf. The nearest known white dwarf is Sirius B , at 8.6 light years , the smaller component of Sirius binary star. These stars are thought to be the final evolutionary state of stars whose mass is not enough to become a neutron star, which is about 10 solar masses.

    Blue Stars 

Blue stars are stars that have at least 3 times the mass of the Sun and up . Whether a star has 10 times the mass of the Sun or 150 solar masses, its going to appear blue to our eyes .Blue stars burn through their fuel at a tremendous rate. They have temperature in excess of 30,000 kelvin (K).These type stars have strong absorption lines of ionized helium , strong lines of other ionized elements , and hydrogen and neutral helium lines weaker than spectral type B. These are located in regions of active star formation , such as the spiral arms of a spiral galaxy or a pair of galaxies undergoing collision and merger ( such as the Antennae Galaxies ). These emit ultra violet light, and so appear in the visible spectrum as bluish-white. 

Giant Star 

A giant star is a star with substantially larger radius and luminosity than a main sequence (dwarf ) star of the same surface temperature . They have a radii up to few hundred times the Sun and luminosities between 10 and a few hundred thousand times that of the Sun .  A star becomes a giant after all the hydrogen available for fusion  at its core has depleted and, as a result , leaves the main sequence .

Types of Giant Star

•  Red Giant
• Blue Giant

Red Giant 

A red giant is a star that has exhausted the supply of hydrogen in its core and has begun thermonuclear fusion of hydrogen in a shell surrounding the core . They have radii tens to hundreds of times larger than that of the Sun. However , their outer envelope is lower in temperature , giving them a reddish-orange hue. Despite the lower energy density of their envelope , red giants are many times more Luminous than the Sun because of their great size. They have surface temperature of 3,000-4,000K . Red giants are evolved from main-sequence stars with masses in the range from about 0.3M to around 8M. The stellar limb of a red giant is not sharply defined, contrary their depiction in many illustrations. Rather , due to the very low mass density of the envelope , such stars lack a well defined photosphere , and the body of the star gradually transitions into a corona. 

          Blue Giant 

The coolest and least luminous stars referred to as blue giants are on the horizontal branch, intermediate mass stars that have passed through a red giant phase and are now burning helium in the cores. Depending on mass and chemical composition these stars gradually move blue wards until they exhaust the helium in their cores and then they return red wards to asymptotic giant branch (AGB). They have temperatures from around 10,000K upwards. These stars are only 5-10 times the radius of the Sun. 

   Super Giant 

Super Giants have masses from 8 to 12 times the Sun upwards , and luminosities from about 1,000 to over a million times the Sun. They vary greatly in radius , usually from 30 to 500 , or even in excess of 1,000 solar radii . They are massive enough to begin helium-core burning gently before the core becomes degenerate, without a flash and without strong dredge-ups that lower-mass stars experience.  There are supergiant stars at all of the main spectral classes and across the whole range of temperatures from mid-M class stars at around 3,400 K to the hottest O class stars over 40,000 K. 

  Red Super Giant

Red super giants are cool and large. They have spectral types of K and M , hence surface temperature below 4,100 K. They are typically several hundred to over a thousand times the radius of  the Sun . Surface abundances of red super giants are dominated by hydrogen even though hydrogen at the core has been completely consumed. These are observed to rotate slowly or very slowly. The red super giant contain  a relatively small number of very large convection cells compared to stars like the Sun . This causes variations in surface brightness that can lead to visible brightness variations  as the star rotates. These pre -red supergiant main sequence  stars exhaust the hydrogen in their cores after 5-20 million years. 

   

    Blue super giant 

   

Super giants are evolved high-mass stars, larger and more luminous than main- sequence stars . O class and early B class with initial masses around 10-300M evolve away the main sequence in just few million years as their hydrogen is consumed and heavy elements start to appear near the surface of the star. These stars usually become blue super giants. Because of their extreme masses they have relatively short lifespans and are mainly observed in young cosmic structure such as open clusters, the arms of spiral galaxies and in irregular galaxies. They have surface temperature of 10,000-50,000 K and luminosities from about 10,000 to a million times that of the Sun.

  Neutron Star

       A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses , possibly more  if the star was especially metal-rich. Neutron stars are the smallest and densest stellar objects ,excluding black holes and hypothetical white holes, quark stars, and strange stars . Neutron stars have a radius on the order of 10 kilometers(6.2 mi) and a mass of about 1.4 solar masses. They result from the supernova explosion of a massive star, combined with gravitationally collapse , that compresses the core past white dwarf star density to that of atomic nuclei.

 

  Magnetar

 A magnetar is a type of neutron star believed to have an extremely powerful magnetic field . The magnetic-field decay power the emission of high-energy electromagnetic radiation particular X rays and gamma rays. During the following decade , the magnetar hypothesis became widely accepted as likely explanation for soft gamma repeaters (SGRs) and anamous X-ray pulsars(AXPs). Like other neutron stars, magnetars are around 20 kilometers(12mi) in diameter and have a mass about 1.4 solar masses . They are formed by the collapse of a star with a mass 15 times that of the Sun. When  in a supernova , a star collapses to a neutron star, and its magnetic field increases dramatically in strength through conservation of magnetic flux. Halving a linear dimension increases the magnetic filed fourfold.