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 108m

[ calculated using stefan’s law]

Physical properties of star

2. MASS:

Range- 0.1 M to 50 M

M – solar mass =1.99 x 1039

[ 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 – 6th

• Brightest star observed in night sky – 1st

1st magnitude star is100 times brighter than 6th

magnitude

L2/L1= 2.512 m2 –m

Stars having negative magnitude are brighter than 0 magnitude stars

Magnitude of sun = -26.8

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 M3.9

As mass increases luminosity increases

a graph of log M v/s log L is a straight line

Spectral Classes

Class colour temperature

O Blue white 30,000

B Light Blue 20,000K

A White 10,000K

F Light yellow 7,000K

G Yellow 6,000K

K Orange 4,000K

M Red 3,000K

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--- 200 billion years

3 times the mass--- 500 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