EXTERNAL GALAXIES

INTRODUCTION

• Galaxies were first classified in 1920s by hubble into different types according to their shape and structural features.
• His classification schemes consisted of 3 regular classes elliptical spiral and barred spirals.
• The irregular galaxies formed the fourth class of objects in his system of classification.
• The eleptical galaxies were further sub classified by hubble according to the degree of ellipticity.

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DIFFERENT TYPES OF GALAXIES.

CONTD…

• Our galaxy and m31 which is believed to be much like ours are example of spiral galaxies.
• A typical spiral galaxy comprises of a nucleus ,a disc, a corona and spiral arms.
• Large number of spiral galaxies have bars passing through their nuclei. if spiral arms originates from bars it is called barred spirals.

SPIRAL GALAXY M101- NASA

DISTRIBUTION OF GALAXIES

• IN A SURVEY OF 1920, WITH 100 INCH TELESCOPE ON THE MOUNT WILSON ,HUBBLE FOUND THAT THE NUMBER OF GALAXIES REMAINED MORE OR LESS CONSTANT OVER THE LARGE GALACTIC POLAR CAPS BUT ON APPROACHING THE MILKY WAY THE NUMBER BECAME FEWER.
• THE GALACTIC PLANE REGION WAS CALLED ZONE OF AVOIDANCE.
• ACCORDING TO HUBBLE OSERVATION OF GALAXIES THE DEPENDENCE OF THE NUMBER OF GALAXIES PER UNIT AREA BRIGHTER THAN THE GIVEN LIMITING MAGNITUDE ON GALACTIC LATITUDE IS GIVEN BY

LOG N = CONSTANT -0.15 COSEC 90°

LUMINOSITY DISTRIBUTION OF GALAXIES

SPECTRA OF GALAXIES

• The spectroscopic study of galaxies and assignment of spectral class for each of these involves some inherent difficulties.
• Their spectral study is difficult because their surface brightness is very low and the spectra are small and of low dispersion.
• Individual stars cant be studied .only the integrated spectrum with its nature varying according to wavelength is obtained.
• The large red shift of the entire spectrum of a distant galaxy presents problem for the correct assessment of the features like spectral energy distribution.
• Most of the galaxies exhibit composite nature of the spectrum as would be expected if it produced by a mixture of stars belonging to all sorts of spectral class and luminosity.

LOCAL GROUP OF GALAXIES

• THE DISCOVERY OF NEARBY GALAXIES SUCH AS M31 M32 M33 THE LARGE AND SMALL MAGELLANIC CLOUDS WITHIN A SMALL RANGE DISTANCE OF LESS THAN 1MPC LED TO THE CONCEPT THAT OUR GALAXIES BELONG TO LOCAL CLUMPING.THIS HAS BEEN CALLED LOCAL GROUP OF GALAXIES.
• AT PRESENT MORE THAN FORTY GALAXIES ARE KNOWN IN LOCAL GROUP BUT THIS NUMBER IS INCREASING SYSTEMATICALLY.
• OF THE GALAXIES BELONGING TO LOCAL GROUP THREE ARE SPIRALS FOUR ARE IRREGULAR AND OTHER ARE ELLIPTICAL AND DWARF ELLEPTICAL.

DISTANCE OF GALAXIES

• THE GALAXIES ARE DISTANT STELLAR SYSTEM AND THE CALCULATION OF ACCURATE DISTANCE TO THEM IS AN ASTRONOMICAL PROBLEM.
• THE ASSUMPTION IS MADE THIN THE EXTERNAL GALAXIES HAVE THE SAME MAGNITUDE AS THOSE BELONGING TO OUR OWN GALAXY.
• THE NEXT METHOD WOULD BE IDENTIFYING THE EXTERNAL SYSTEM SIMILAR TO THE KNOWN OBJECT IN OUR GALAXY HAVING THE SAME ABSOLUTE MAGNITUDE.
• THE DISTANCE OF THE SYSTEM CAN BE CALCULATED AS (m-M) =5log R/10,WHERE R IS IN PARSEC ,(m-M) IS THE DISTANCE MODULUS.

NUCLEI OF GALAXIES

• THE CENTRAL REGION OF THE GALAXY IS ITS DENSE PART AND THE CORE OF THIS REGION OF MAXIMUM DENSITY IS TERMED AS THE NUCLEUS OF GALAXY.
• OVERALL EVOLUTION OF GALAXIES INCLUDIND THE FORMATION OF NEW GENERATION OF STARS AND MORPHOLOGICAL DEVELOPMENT IS GUIDED CHIEFLY BY THE ACTIVITIES UNDERGONE INTO THEIR NUCLEI.
• AMBERTSUMYAN HAS CLASSIFIED THE NUCLEI OF GALAXIES INTO FOUR DIFFERENT TYPES ACCORDING TO THEIR ACTIVITY.

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NUCLEUS OF ANDROMEDA GALAXY

DWARF GALAXIES

• THE GALAXIES WITH LOW LUMINOSITY,LOW METALLICITY HAVING SMALLER SIZE ARE TERMED DWARF GALAXIES.
• THE DISTINGUISHABLE CRITERIA BETWEEN GIANT AND DWARF ARE GENERALLY ABSOLUTE V MAGNITUDE OR MASS.
• LUMINUOUS CRITERION IS MAINLY USED FOR DIFFERENTIATING DWARF FROM GIANT GALAXIES.

DWARF GALAXY - NGC 7320

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