Types of Stars Spectral Classifications: Based on the luminosity and effective temperature , the stars are categorized depending upon their positions in the HR diagram. Hertzsprung -Russell Diagram (H-R Diagram) : 1. The H-R Diagram is a graphical tool that astronomers use to classify stars according to their luminosity (i.e. brightness), spectral type, color, temperature and evolutionary stage. 2. HR diagram is a plot of luminosity of stars versus its effective temperature. 3. Most of the stars occupy the region in the diagram along the line called the main sequence. During that stage stars are fusing hydrogen in their cores. Various Types of Stars Brown Dwarf: White Dwarf: Brown dwarfs are sub-stellar objects After a star like the sun exhausts its nuclear that are not massive enough to sustain fuel, it loses its outer layer as a "planetary nuclear fusion processes. nebula" and leaves behind the remnant "white Since, comparatively they are very cold dwarf" core. objects, it is difficult to detect them. Stars with initial masses Now there are ongoing efforts to study M < 8Msun will end as white dwarfs. them in infrared wavelengths. A typical white dwarf is about the size of the This picture shows a brown dwarf around Earth. a star HD3651 located 36Ly away in It is very dense and hot. A spoonful of white constellation of Pisces. dwarf material on Earth would weigh as much as First directly detected Brown Dwarf HD 3651B. few tons. Image by: ESO The image is of Helix nebula towards constellation of Aquarius hosts a White Dwarf Helix Nebula 6500Ly away. Green arrow points the star at the centre. White Dwarf Red Dwarf: Red dwarfs are stars that are Neutron Star small, cooler and dimmer than Neutron stars are formed when a Magnetar: our Sun. They are much less massive star dies in a "supernova Magnetars are a class of neutron massive than the Sun. The very explosion". During this dramatic stars, the surface magnetic field first Red dwarfs formed in our event, the core of the star suddenly of which ranges around 1015 -1016 Galaxy should be still in their collapses under its own weight and Gauss. This magnetic field is main sequence stage and they the outer parts are violently ejected approximately thousand trillion retain their initial composition. into surrounding space. times stronger than the Earth's The image along side shows the A typical neutron star is the size of a magnetic field. famous double star Alcor-Mizar small city. from constellation of Ursa A spoonful of material form such star The picture on the left is an Major. Alcor has a companion Constellation of Ursa Major, Alcor-Mizar on Earth would weigh as much as all artistic view of a magnetar. red dwarf star Alcor-B marked the cars on Earth put together. with green arrow. and Red Dwarf star Alcor B. SUN Pulsar: Our Sun is an ordinary, main Pulsars are spinning neutron stars sequence star which is fusing that have jets of particles, hydrogen into helium at its core. beamed along magnetic poles, moving It is around 4.5 billion almost at the speed of light. years old, and is around halfway This beam of light from the jets sweep around as the pulsar rotates, through its lifetime. just as the spotlight in a lighthouse It is expected to evolve into a does, hence pulses of radiation are red giant and subsequently into seen as the beam sweeps over the white dwarf. Earth. X-ray image of SUN, by-TRACE Team, NASA An artist image of a Pulsar. Red giants: Black Hole: 1. When stars like our Sun reach the 1. Black holes are the Artist view of Black Hole end of their life by exhausting the evolutionary endpoints of very supply of hydrogen at their cores, massive stars that undergo a they enter one last phase of supernova explosion leaving ballooning up to many times of the behind a burnt out stellar original size. Such objects are called remnant. red giant stars 2. With no outward forces to 2. Such stars have radii ten to oppose gravitational forces, the hundreds of times larger than that remnant collapses in on itself. of the Sun. The object left behind has such 3. Our own Sun, for example will a strong gravity that its escape grow so large that it will engulf the velocity is greater than the speed Infrared image of red giant star U Hydrae of light. orbits of Mercury, Venus and even located 500 light years away from us. Earth. 3. It means that not even light can escape from black hole. .