ASTROPHYSICS

Physical properties of star 1.SIZE spherical depends on mass, temperature, gravity & age Range- 0.2R to 220 R, R- solar radius = 6.96 x 10 8 m [ calculated using stefan’s law]

Physical properties of star 2. MASS: Range- 0.1 M to 50 M M – solar mass =1.99 x [ Keppler’s iii law]

Physical properties of star 3. BRIGHTNESS / LUMINOSITY : magnitude – measure of brightness when observed from earth Faintest star observed in night sky – 6 th Brightest star observed in night sky – 1 st 1 st magnitude star is100 times brighter than 6 th magnitude

L 2 /L 1 = m2 –m Stars having negative magnitude are brighter than 0 magnitude stars Magnitude of sun = Absolute brightness of a star are defined by by placing all stars at a distance of 10 parsec 1 parsec = 3.260ly

Physical properties of star 4. TEMPERATURE : Surface temperature varies 3,000K to 30,000 K Temperature is measured using spectral type

MASS –LUMINOSITY RELATION L α M 3.9 As mass increases luminosity increases a graph of log M v/s log L is a straight line

Spectral Classes Classcolourtemperature OBlue white30,000 BLight Blue20,000K AWhite10,000K FLight yellow7,000K GYellow6,000K KOrange4,000K MRed3,000K

Absolute Luminosity of star V/S temperature graph of a star is called HR diagram

Features Most of the stars are concentrated in narrow band- called main sequence stars. As one moves from O to M type stars mass, temperature and luminosity of the stars decrease in main sequence. Stars spend most of their life span in regions e.g main sequence, giants, white dwarfs. Super giants are thinly populated occupy the top. White dwarfs lay left of the main sequence.

TIME OF STAY IN THE MAIN SEQUENCE More the mass, less is the time of stay Ex: sun life – about 10 billion years ½ the mass billion years 3 times the mass Million years

Internal Temperature and Pressure of a Star Pressure and the temperature are maximum at the stellar core and decrease towards the surface of the star.

Photon diffusion Time Time taken by a photon to defuse from the center of the star to its surface. - In a star energy generated at the core. - Energy spread in the form of photons - While moving towards the surface it faces a large number of frequent collision - Energy and direction of travel of the photon changes. In case of sun T=30,000 years

Stellar Evolution

1. Proto star Large cloud of interstellar dust and gases mostly hydrogen compressed due to gravitational force. High pressure and temperature are produced. Nuclear fusion of hydrogen starts. Gravitational contraction is balanced by outward pressure.

Helium Star As hydrogen fuel is exhausted, energy generation decreases - star begins to contract. Temperature increases and star becomes very hot. Helium atoms begins to fuse to form carbon.

Red giant Fusion of helium continues at the centre. Heat generated at the core expands the outer layer enormously ( 10 to 20 times the size of the sun ) This cools the outer layer. So star appears reddish and dim.

White dwarf After spending millions of years in red giant stage carbon fuses. Radiation oozes the outer layer of the star. As a result generation of energy further decreases, star collapses further. Star sinks and acquire high density and high temperature. This is the end stage of the star.l

Chandrashekhar limit If mass of the star at birth < 1.4Mo star ends with white dwarf stage. If mass of the star at the time of its birth > 1.4 Mo the core of the star collapses further, temperature and pressure increases enormously results in explosion called supernova.

Debris of supernova has high temperature and pressure that electrons and protons fused into neutron called neutron star. If the mass of the neutron star > 5Mo it further collapses under its own gravity. Any radiation entering this mass can not come out. Which sucks everything like a hole- black hole.

Stellar Evolution