THE UNIVERSE AND THE SOLAR SYSTEM

Universe and formation of universe (Big Bang Theory) Stellar Evolution Galaxies

The Universe

Universe all existing matter (tiniest subatomic particles to galactic super-clusters) and space

The universe is incomprehensively large in size and dimension. 100 billion galaxies each comprising an average of 100 billion of stars. universe began about 13.8 billion years ago in what has become known as the Big Bang.

Big Bang Theory and Expanding Universe

The Big Bang theory is the prevailing cosmological model for the birth of the universe. It states that at some moment all of space was contained in a single point from which the

Universe has been expanding ever since. 13.8 billion years ago , which is thus considered the age of the universe After the initial expansion, the Universe cooled sufficiently to allow the formation of subatomic

particles, and later simple atoms. primordial elements later coalesced through gravity to form stars and galaxies. The majority of atoms produced by the Big Bang were hydrogen, along with helium Universe would reach a maximum size and then begin to collapse. It would become denser and

hotter again, ending with a state similar to that in which it started—a Big Crunch

THE LIFE CYCLE OF A STAR

Outlined below are the many steps involved in a stars evolution, from its formation in a nebula, to its death as a white dwarf or neutron star.

NEBULA

cloud of gas (hydrogen) and dust in space. birthplaces of stars . Planetary Nebula are the outer layers of a star that are lost when the star changes from a red

giant to a white dwarf.

STAR

luminous globe of gas producing its own heat and light by nuclear reactions (nuclear fusion). consist mostly of hydrogen and helium gas. The faintest stars are the red dwarfs, less than one-thousandth the brightness of the Sun. Towards the end of its life, a star like the Sun swells up into a red giant, before losing its outer

layers as a Planetary Nebula and finally shrinking to become a white dwarf.

RED GIANT

large bright star with a cool surface. final stages of the evolution of a star like the Sun, as it runs out of hydrogen fuel at its centre. They are very bright because they are so large, although their surface temperature is lower than

that of the Sun Very large stars (red giants) are often called Super Giants. These stars have diameters up to 1000

times that of the Sun and have luminosities often 1,000,000 times greater than the Sun. A red giant is a luminous giant star in a late phase of stellar evolution. The appearance of the red giant is from yellow-orange to red

WHITE DWARF

shrunken remains of normal stars whose nuclear energy supplies have been used up. This is very small, hot star, the last stage in the life cycle of a star like the Sun. Extremely dense. White dwarfs have a mass similar to that of the Sun, but only 1% of the Sun's

diameter; approximately the diameter of the Earth. White dwarf consist of degenerate matter with a very high density due to gravitational effects, i.e.

one spoonful has a mass of several tonnes. White dwarfs cool and fade over several billion years. Fusion in a star's core produces heat and outward pressure, but this pressure is kept in balance by

the inward push of gravity generated by a star's mass.

SUPERNOVA

A supernova is a stellar explosion that briefly outshines an entire galaxy, radiating as much energy as the Sun or any ordinary star is expected to emit over its entire life

span, before fading from view over several weeks or months. A great proportion of primary cosmic rays comes from supernovae Supernovae can be triggered in one of two ways

1. by the sudden re-ignition of nuclear fusion in a degenerate star; or 2. by the gravitational collapse of the core of a massive star.

In the first case, a degenerate white dwarf may accumulate sufficient material from a companion, either through accretion or via a merger, to raise its core temperature, ignite carbon fusion, and trigger runaway nuclear fusion, completely disrupting the star.

In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy that can create a supernova explosion.

Nova

nuclear explosion on a white dwarf, which causes a sudden brightening of the star. Novae are thought to occur on the surface of a white dwarf in a binary system.

NEUTRON STARS

These stars are composed mainly of neutrons and are produced when a supernova explodes, forcing the protons and electrons to combine to produce a neutron star.

Neutron stars are very dense. Typical stars having a mass of three times the Sun but a diameter of only 20 km. If its mass is any greater, its gravity will be so strong that it will shrink further to become a black

hole.

Great densities are only possible when electrons are displaced from their regular shells and pushed closer to the nucleus, allowing atoms to take up less space. The matter in this state is called 'degenerate matter'.

BLACK HOLES

Black holes are believed to form from massive stars at the end of their life times. The gravitational pull in a black hole is so great that nothing can escape from it, not even light. The density of matter in a black hole cannot be measured. Black holes distort the space around

them, and can often suck neighbouring matter into them including stars.

Cosmic microwave background

thermal radiation left over from the "Big Bang" of cosmology. In older literature, the CMB is also variously known as cosmic microwave background radiation

(CMBR) or "relic radiation." The CMB is a cosmic background radiation that is fundamental to observational cosmology because

it is the oldest light in the Universe, dating to the epoch of recombination. With a traditional optical telescope, the space between stars and galaxies (the background) is

completely dark. However, a sufficiently sensitive radio telescope shows a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum.

The CMB is well explained as radiation left over from an early stage in the development of the Universe, and its discovery is considered a landmark test of the Big Bang model of the Universe.

Galaxies

Galaxy a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction.

A galaxy is a huge mass of stars, nebulae, and inter-stellar material. The smallest galaxies contain about 100,000 stars, while the largest contains up to 3000 billion stars.

Galaxies are the major building blocks of the universe. A galaxy is a giant family of billions of stars, and is held together by its own gravitational field. From the billions of galaxies, two basic types have been identified:

1. Regular galaxies, and 2. Irregular galaxies.

The regular galaxies (Fig. 1.1 ) may be disc-shaped, elliptical and they generally have new stars. The stars of the irregular galaxies are very old (Fig. 1.2).

1. Regular Galaxies

The regular galaxies may be of either spiral or elliptical in shape.

a) Spiral Galaxies

The Milky Way is an example of large spiral galaxies—disc-shaped with greater concentration of stars near their centres (Fig. 1.2). The spiral galaxies have curved spiral arms. About 25% of all galaxies have curved spiral arms.

Spiral galaxies are well supplied with the interstellar gas in which new bright, young stars form.

The Milky Way and other spiral galaxies consist of populations of old stars in the centre, and the youngest stars located in the arms.

b) Elliptical Galaxies

The most abundant are probably the elliptical galaxies. These are generally smaller than the spiral galaxies. They are generally symmetrical or spheroidal very old and no new star forming The biggest and the brightest galaxies

About two-thirds of all the galaxies are elliptical.

c) Irregular Galaxies

one-tenth of all galaxies The stars of the irregular galaxies are generally very old.

Our Galaxy (Milky Way)

Milky Way galaxy of which our solar system is a part Orion Arm The Solar System is located in the Orion Arm, 26,000 light years from the center of

the Milky Way Our galaxy has the shape of a flat disc with a central bulge. Its diameter is about a hundred thousand light years. In the nucleus the thickness reaches ten thousand light years, whereas in the disc it is five

hundred to two thousand light years thick.

It consists of stars, gas and dust; superimposed on the general distribution of the star in the pair of spiral arms.

We do not know exactly how far the Sun is from the centre, but it is conventionally taken to be 33,000 light years away. Thus our solar system is relatively far from the galactic centre.

Our galaxy is vast, about 100,000 light years across. In comparison the solar system seems small of about 12 light years across

The entire galaxy is rotating in the space, although the inner stars travel faster than those further out. The Sun which is about one-third out from the centre, completes one lap of galaxy in about every 220 million years (Fig.1.3).