Ornamented
Universe
TANUKA CHATTOPADHYAY
F
or centuries or more theUniverse has fascinated mankind with its mysteries. How big is it? How is it formed? Whether it is endless and everlasting? With groundbreaking new discoveries it is wondrous and deadly adventurous through space and time. Fifty years have passed since man first ventured into the outer space but Universe is only now unveiling its greatest secrets and garlanding features. Scientists are discovering new phenomena we didn’t even know existed a year ago. The study of the Universe is the totality of its existence with planets, stars,
galaxies, intergalactic space,
magnetic field, matter and energy. The most convincing theory about the formation of the Universe is that it was born as a result of a big explosion called “Big Bang” from an extremely hot, dense point
when it was just 10-34 second old.
From then onward it suffered an incredible burst of expansion, known as “inflation” whereas after inflation it expanded at a slower rate. Now, due to expansion, the Universe got cooled and matter formed thereafter. Regarding
“cosmic dark age”. The transition from this phase to the next active phase is known as “epoch of reionization”. During the latter phase, clumps of gas collapsed to form “first stars and galaxies”. Although the expansion of the Universe gradually slowed down, five to six billion years after the Big Bang a mysterious force called “dark energy” began speeding up the Universe again (accelerated Universe), which is still prevailing. Nine billion years after Big Bang our solar system was born. The present age of the Universe is roughly 13.7 billion years.
When we look at the clear summer night sky we see a white patch of light extending across the vault of the sky. This is what we call “Milky Way”. Milky Way is the name of our Galaxy which we belong to. Naturally
the question arises “what do we mean by a galaxy”? Galaxy is a vast ensemble of stars and gaseous malls pervaded by magnetic fields, cosmic rays together with unseen matter. There are many galaxies like our own but their morphological
structures vary. Depending
upon their morphological
structures, E. P. Hubble in 1920 classified them into three main categories (i) Elliptical, (ii) Spiral /Barred Spiral and (iii) irregulars
(Fig. 1).
Elliptical galaxy, as is evident from its name, has an ellipsoidal structure. Spiral galaxy consists of a very dense “nucleus” surrounded by a less dense “bulge” and the “nucleus” and the “bulge” are embedded in a disc like structure in which spiral arms generate. In our Galaxy the “Sun” is situated in one such arm “Orion”. The whole structure is surrounded by a speherical structure called galactic “halo”. In “halo” primarily old objects, called “Population II” objects, are found. Along the spiral arms recently formed stars and gas prevail. These are called “Population I” objects (Fig. 2).
Fig.1 Hubble’s tuning fork diagram of galaxy classification.
Fig.2 face on view of a spiral galaxy.
the evolutionary pattern of the Universe theoreticians enunciated that one second after Big Bang the Universe was filled with elementary particles like neutrons, protons, electrons, antielectrons, photons and neutrinos. For the first 380000 years the Universe was so hot that the heat smashed the atoms to break them into a
dense plasma which scattered like fog and the Universe remained opaque to light and radiation. After 380000 years matter was cooled enough to form stable atoms during the era of “recombination”
and the Universe became
transparent consisting of mostly electrically neutral gas. After this era the Universe remained quiet and dark as no stars or any other bright objects had formed. Four hundred million years after the Big Bang the
Universe emerged from that If the spiral galaxy contains a “bar” in the nuclear region it
is called a “barred spiral”. Our Galaxy is a spiral galaxy. On the other hand galaxies having no particular shapes are called “irregular galaxies” e.g. Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). The mass of our Galaxy has been estimated by various authors to
be of the order of 1011 M⊙ where
M⊙ is the mass of Sun.
When electromagnetic radiation emerges from an object which is moving away from us, the wave length of the emerging radiation increases a little bit depending upon the speed of the object when it is observed by an observer. This phenomenon is known as the “Doppler Shift” and the ratio of increase in the wavelength to its original length is known as redshift. It is denoted by the symbol “z”. In 1920, Hubble observed that all galaxies are redshifted and the above conjecture for the first time gave an idea of an expanding Universe from an observational point of view. Also at very low redshift (z < < 1) if V is the velocity of the galaxy along line of sight and D is
its distance from us then he found
V = H x D, where H is known as
“Hubble’s constant” at the present epoch. The above relation is known as “Hubble’s law”.
We have mentioned above that galaxies contain gas and stars besides other constituents. Stars, as we know, are bright sources of energy and at the center of a star nuclear reactions occur. This produces huge amounts of energy. The radiation pressure generated from this huge energy halts the gravitational collapse and the star remains in the stage of what we call “hydrostatic equilibrium”. If we plot the luminosity of the star which is the total amount of energy emitted from its surface in unit time (or magnitude which is the scale of luminosity) versus temperature (or colour) for all stars in our Galaxy, about ninety per cent stars are found in such a stage. We say that the stars belong to the “Main Sequence”. The main sequence of stars extends from top left to the bottom right
(Fig. 3) in the diagram. This
diagram is known as Hertsprung
– Russell diagram (H-R
diagram). This diagram has been constructed by E. Hertsprung and H. N. Russell independently in 1913. The significance of this diagram is that if we know the magnitude and colour of a star and plot it in the diagram,
it at once describes the
evolutionary status of the star. In the nuclear reactions inside a star, as mentioned above, hydrogen is converted to helium, but when helium gets exhausted in the core, the core contracts
The study of the Universe is the totality of its
existence with planets, stars, galaxies, intergalactic
space, magnetic field, matter and energy.
and the reaction passes over to a less dense region, called shell, surrounding the core.
Here radiation pressure
overweighs the gravitational force and the shell expands. As the shell expands its surface temperature falls so it emits light of comparatively longer wavelength (red) and the star appears larger, bigger and brighter and it occupies top right position of the H-R diagram. Due to contraction of the core and expansion of the envelope the envelope blows away and the core density is so high that the degenerate pressure of the electrons balances gravitational collapse of the core and it occupies the bottom left position in the H –R diagram. We call it a “white dwarf”. It is the excellence of Sir
Subrahmanyan Chandrasekhar
who invented the maximum mass of such white dwarfs to be 1.45
M⊙ and got the Nobel prize in
the year 1983 for his great work.
This maximum mass is named as “Chandrasekhar Mass” in his honour. The surface temperature of a white dwarf is initially hot and it radiates thermal energy and
and the surrounding medium gets enriched by these heavy elements where next generations of stars form. As a result of supernova explosion the outer envelope of the star is completely blown out and the core contracts to become a “neutron star” or “black hole” depending upon the mass. Now the stars in a galaxy form either as a single entity, or in binary or in triplet or in clusters. When a star forms in an isolated way it is called a “field star” whereas the stars in binary system are called binary stars. Similarly when a system of stars are gravitationally bound together having a common motion with respect to the surrounding it is called a “star cluster”. There are two types of star clusters, (i) galactic or open clusters and (ii) globular clusters. Galactic clusters are small in size, flattened and
Fig. 3 H – R diagram of stars.
metal deficient. They are mostly found in the galactic halo. These observations led astronomers to believe that they were formed at the early phase of the galaxy when sufficient amount of heavy elements had not been formed. Thus globular clusters contain Population II objects .
In the Universe we observe another type of spectacular objects called “Quasar”. It is a star like object with the luminosity comparable to that of a galaxy having large ultraviolet flux of radiation accompanied by generally broad emission lines and absorption lines found at large redshift. Moreover there are “Active
galactic nuclei” (AGN), observed in the Universe. These are the nuclei of some galaxies which are undergoing violent activities and are emitting infrared, radio, ultraviolet and X ray radiations of the electromagnetic spectrum. AGN is modeled as supermassive black hole existing at the centre of the galaxy and materials which accrete onto the black hole and release large amounts of energy in the high energy zone. Again there are “pulsars” which are highly
magnetized rotating neutron
stars emitting electromagnetic radiation similar to that of a light house. Sometimes intense flashes of gamma rays are observed, which last for tens of seconds.
These are the most spectacular astronomical events to be explored yet. These are termed as “Gamma ray bursts” (GRBs).
Thus Universe contains a vast multitude of astronomical objects whose complete physical structure are yet not known or unexplored. Hence human activities to master the cosmic space will greatly enrich future progress of human society. Perhaps one day one of our near inheritants will enjoy