Download calculated using stefan`s law

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