THE UNIVERSE10th Grade – Physics

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INTRODUCTIONWhat is the universe?

The universe is commonly defined as the totality of everything that exists including planets, stars, galaxies, the contents of intergalactic space, and all matter and energy Definitions and usage vary and similar terms include the cosmos, the world and nature

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GALAXIESGalaxy Origins Most astronomers suggest that galaxies

formed shortly after a cosmic "big bang" that began the universe some 10 billion to 20 billion years ago.

In the milliseconds following this explosion, clouds of gases began to coalesce, collapse, and compress under gravity to form the building blocks of galaxies.

Galaxies are sprawling space systems composed of dust, gas, and countless stars

Most of the galaxies in the Universe are probably tiny dwarf galaxies. For example, in our Local Group of galaxies there are only 3 large spiral galaxies: the Milky Way, Andromeda, and the Triangulum Galaxy. The rest are dwarf and irregular galaxies

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Types of Galaxies

Spiral galaxies, such as the Milky Way, consist of a flat disk with a bulging center and surrounding spiral arms. The galaxy's disk includes stars, planets, dust, and gas—all of which rotate around the galactic center in a regular manner.

Elliptical galaxies are shaped as their name suggests. They are generally round but stretch longer along one axis than along the other. They may be nearly circular or so elongated that they take on a cigar like appearance

Galaxies that are not spiral or elliptical are called irregular galaxies. Irregular galaxies appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by.

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THE SOLAR SYSTEM• Our solar system consists of an average

star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune,.

• It includes: the satellites of the planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium.

• The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system.

• Note: Pluto no longer belongs to the category of planet because it does not confirm to the recognized ellitical path around the Sun. Out of the 3 moos Pluto has, Charon is almost equal in its size.

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• The Sun is the most prominent feature in our solar system. It is the largest object and contains approximately 98% of the total solar system mass.

• One hundred and nine Earths would be required to fit across the Sun's disk, and its interior could hold over 1.3 million Earths.

• The Sun's outer visible layer is called the photosphere and has a temperature of 6,000°C (11,000°F). This layer has a mottled appearance due to the turbulent eruptions of energy at the surface.

• The Sun is the closest star to Earth and is the center of our solar system. A giant, spinning ball of very hot gas, the Sun is fueled by nuclear fusion reactions.

THE SUN

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• The light from the Sun heats our planet and makes life possible. The Sun is also an active star that displays sunspots, solar flares, erupting prominences, and coronal mass ejections.

• The distance of the Sun from the earth is nearly 1.5 x 108 km

• Light takes about 8 minutes to travel from the sun to the earth, hence sun is about 8 minutes light away from earth

• The mass of the sun can be calculated by Kepler’s law, the mass of the sun is 2 x 1030 kg. the mass of the sun is represented by (M-Sun)

Prominences and Solar Flares• When you look at the sun through a

telescope (with special filters so you eyes don't get damaged!), at the sides of the photosphere there appear to be large eruptions of gases like it was from a volcano. Each of these is called a prominence.

THE SUN

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• The amount of energy radiated per second by the sun in all direction is called solar luminosity, it is equal to 3.9 x 1026 Watt

• The solar telescope in India is situated in Kodaikanal and Udaipur

• The sun rotates from west to east with a period about 26 days

• Spectroscopes have shown the existence of dark lines called Fraunhofer lines in the solar spectrum, these lines show the existence of elements like sodium, calcium in addition to hydrogen in the sun.

THE SUN

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THE SUN Granulation pattern:The "surface" of the Sun (the photosphere) is covered with a "granulation pattern" caused by the convective flow of heat rising to the photosphere from the Sun's interior.

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Sunspots : are the dark irregular patches present on the sun, they don't look that big when you see them on the Sun The energy from the interior is prevented from reaching the photosphere in the regions of the sun spots. This is due to magnetic activities below the photosphere. The charges particles ejected at great speeds from the sun, ionize the air molecules which results in spectacular colour display. These are seen from polar regions and are called aurora or polar lights.

THE STRUCTURE OF THE SUN• The Core: or the center of the Sun, is the region

where the energy of the Sun is produced The energy travels first through the radiative zone, The Sun's core has a tremendously high temperature and pressure.

• The Radiative Zone (or radiation zone): The next layer out from the core is this zone which emits radiation. This radiation diffuses outwards..

• The Convective Zone: In this next layer, photons continue to make their way outwards via convection. The energy is transferred more rapidly. This time it is the motion of the gases in the Sun that transfers the energy outwards. The gas at this layer mixes and bubbles

• The Photosphere: This is the lower atmosphere of the Sun and the part that we see (since it emits light at visible wavelengths). This layer is about 300 miles (500km) thick. The temperature is about 5,500 °C.

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The Chromosphere: This reddish layer is an area of rising temperatures. The temperature ranges from 6,000 °C (at lower altitudes) to 50,000 °C (at higher altitudes). The Chromosphere is visible during solar eclipses (when the moon blocks the Photosphere).

The Corona: This is the outer layer of the Sun's atmosphere. The corona extends for millions of miles and the temperatures are tremendous, reaching one million °C.

Holes in the corona occur where the Sun's magnetic field loops out into space. These coronal holes may be the source of the solar wind, a stream of energetic particles that permeate the Solar System.

THE STRUCTURE OF THE SUN

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THE SUN – Solar Activity

The Sun is not a quiet place, but one that exhibits sudden releases of energy. One of the most frequently observed events are solar flares: sudden, localized, transient increases in brightness that occur in active regions near sunspots

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• A star is a massive, luminous sphere of plasma held together by gravity

• The night sky is sprinkled with an enormous number of stars that appear like bright points.

• Stars are cosmic energy engines that produce heat, light, ultraviolet rays, x-rays, and other forms of radiation. They are composed largely of gas and plasma, a superheated state of matter composed of subatomic particles.

• Though the most familiar star, our own sun, stands alone, about three of every four stars exist as part of a binary system containing two mutually orbiting stars

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THE STAR

• Hot stars are white or blue, whereas cooler stars appear to have orange or red hues.

• Stars may occur in many sizes, which are classified in a range from dwarfs to super giants. Super giants may have radii a thousand times larger than that of our own sun.

• Hydrogen is the primary building block of stars. The gas circles through space in cosmic dust clouds called nebulae.

• Building pressures cause rising temperatures inside such a nascent star, and nuclear fusion begins when a developing young star's core temperature climbs to about 27 million degrees Fahrenheit (15 million degrees Celsius).

• Stellar distances : The stars are so far away from us that they appear to be fixed. • Stellar distance being very large, a unit called ‘parsec’ is used in addition to the

unit light year (ly), to measure stellar distances. 1 parsec (pc) = 3.26ly = 3 x 1013 km.

• Brightness and luminosity : Some stars are bright and some are dim; some stars look white while some may look blue and some red. The difference in colour is due to different temperatures.

• Some stars have always stood out from the rest. Their brightness is a factor of how much energy they put out, which is called their luminosity, and also how far away from Earth they are.

• The brightness of stars is represented by a system called magnitude scale• Stellar Spectra : The spectrum of a star like that of the sun, is a continuous

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THE STAR : Appearance

THE STAR• Sizes of stars : they appear as pin points even through the most

powerful telescopes. However the radius of a star can be deduced from its luminosity and temperature. Majority of stars have radii in the range of a tenth to twenty times solar radius.

• Masses of stars : When two stars form a gravitationally bound system, and go round in circles around their common centre of mass, their individual masses can be determined by observing their motion. Such a star system is called a binary star.

• Sirius is a binary star system; its components have masses 1.4 MO and 2.4 MO. It is found in general that the luminosity of stars increases with mass. Stars about forty times heavier than the sun are roughly million times luminous than the sun.

• Stellar Interiors : Mean densities of the stars can be calculated, knowing their sizes and masses. The densities vary from 5 x 104 kg m-3 in the case of the coolest star to around 10 kg m-3 for the hot stars. The central temperature ranges from 10 to 30 million degrees as we go from the coolest to the hot stars.

• Strange stars : Among stars, there are strange types of stars known as binary stars, red giants, white dwarfs, black dwarfs, neutron stars, black holes and events called novae and supernovae. In addition, there are quasars which are galaxies thousands times brighter than ordinary galaxies, and pulsars which are stars that emit radiation in pulses

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• We can see, on a clear night, a hazy luminous band stretched across the sky among the stars. This is known as the Milky way. (Akasha ganga).

• It was Galileo, who first observed (about 1610) that the Milky way is comprised of countless number of individual stars.

• Careful examination has shown that the Milky Way viewed from an edge, looks like a flat disc with a central bulge and the thickness tapers off towards the edges.

• Milky Way is a conglomeration of stars (Such a huge group of stars is called a galaxy).

• The diameter of the Milky Way is about one lakh light years, and the central thickness is about 6000 ly. Our sun is located more than halfway (about 28,000 ly) from the centre. As we belong to this Galaxy, it is called Our galaxy.

• Our Galaxy is a spiral galaxy. Milky way is rotating about its centre. Our sun along with the planetary system is revolving around the centre of our galaxy with a speed of 250 km/s and takes about 250 million years, to go round once. Our galaxy contains about 1011 stars and the total mass of all stars in our Galaxy is about 3 x 1041 kg.

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MILKY WAY

COSMOLOGY• Cosmology is the study of the universe. It deals especially with

the search for suitable theories to understand the observed universe, its origin and its future.

• Birth and death of stars (Stellar evolution): The process from the birth to the death of a star is called stellar evolution.

• Huge gaseous clouds mostly hydrogen exist in space. It is believed that the birth of a star begins when such a gaseous cloud contracts due to gravity. There will be increase in density which leads to increase in pressure. Gradually the cloud settles with a spherical mass at the centre of the cloud.

• The central portion has a mass of about 99% of the mass of the cloud. The sphere formed at the centre is called a protostar.

• This represents the cocoon stage of a star. A protostar has a hydrogen core formed at its central portion.

• When the temperature is high enough, fusion of hydrogen begins. The energy released during nuclear fusion tries to expand the matter.

• The energy released during nuclear fusion tries to expand the matter. Eventually the outward pressure due to the radiation generated balances the inward gravitational pull. This gives the star a stable state. A protostar thus reaches steady state.

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Birth and Death of Stars• The star is now in the youth stage and it is said to be in the main sequence.• The youth stage of a star may last for several billions of years depending on its mass. Less massive

stars stay longer in the main sequence. • The sun has remained in the youth stage for about five billion years. It will continue in the same

stage for another five billion years. For massive stars.

Evolution of Sun - like star• As hydrogen continues to fuse forming helium a helium core forms at the centre. This is

surrounded by hydrogen in the outer shell and hydrogen fuses at a rapid rate. The resulting radiation causes the star’s envelope to expand and cool. The colour of the star changes to red and the star is now called red giant.

• The sun in its red giant stage, will swallow up mercury and venus and the earth will be burnt in the heat.

• As a star’s envelope expands, its core contracts and heats up. When the temperature reaches 108 K, helium is converted into carbon in the core.

• Once the fusion of helium in the core is complete the core can not contract further.

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Evolution of Sun - like star

• Outer envelop of red giant gets detached and thrown into space. It forms a cloud of hydrogen gas called planatary nebula.

• As the temperature increases, the core develops pressure which prevents further collapse; the star becomes what is called White dwarf.

• White dwarfs shine due to their high temperature. When further nuclear reactions stop, a white dwarf gradually cools losing its internal energy resulting in decrease of temperature. It becomes dimmer and dimmer until it emits no light. It has then become a black dwarf a dark cold chunk of mass. This corresponds to the death of a sun like star.

• Stars which are 5 times or still more massive than the sun, evolve differently

• The carbon nuclei produced during the fusion of helium form a carbon core.

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Evolution of massive stars

• This will get ignited to produce a core of oxygen. The chain continues leading to the production of heavier elements.

• Thus cores of oxygen, magnesium, silicon, etc are formed and the cores lie one inside the other like the layers of an onion. Iron is formed at the inner most core. At this stage, the star explodes and the event is called supernova .

• The supernova of 1054 is today recognisable as a nebula, located in the constellation of Taurus. It is called crab nebula.

• The supernova of 1572 (called Tycho’s star) and 1604 (called Kepler’s star) have been studied using radiotelescope.

• A sphere of neutrons remains at the centre and this is called neutron star.

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Evolution of massive stars• In the case of stars with masses about 30 times that of

sun, the remnant at the end of supernova, has huge amount of matter compressed into a very small region. This is a region of intense gravitational field and is called a black hole.

• The sun would become a black hole, if compressed to a radius of 3 km. The only recognizable property of a black hole is its mass

• Thus there can be three end states in stellar evolution namely - white dwarf, neutron star and black hole. This is decided by the stellar mass. Their presence can be inferred indirectly.

• The star clusters are of two types - open and globular clusters based on their appearance.

• In open type, stars appear to be loosely bound where as in globular type, stars appear to be tightly bound.

• The open clusters have many blue stars which are young. The older globular clusters have mostly red stars.

• The study of star clusters can verify theories of stellar evolution.

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ORIGIN OF THE UNIVERSE

• Study of galaxies shows that the galaxies in space are uniformly distributed.• The galaxies are the building blocks of our vast universe. Since the time of Aristotle, it was believed

that the universe was static and unchanging. • This was shown to be false in the 1920s leading to the realization of a dynamic evolving universe.• The spectral lines of all galaxies show red shift. Spectral lines of a given galaxy show equal red

shifts. However it varies from galaxy to galaxy. • The red shifts of galaxies show that they are moving away from us with enormous speeds.• This demonstrates that the universe is expanding.

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ORIGIN OF THE UNIVERSE

• The velocity of recession of a celestial body is proportional to its distance from us. This is known as Hubble’s law.

• The age of the universe is estimated to be 10 to 20 billion years.• At the beginning of the universe, all matter comprising galaxies and star• and radiation were compressed into a fiery region, with extremely high• temperature and enormous density. This is known as the “primordial fireball”• The fireball exploded with a bang. There was an awesome explosion which• launched the expansion of the universe. This is called the “Big Bang”. As matter• and radiation cooled, stars and galaxies formed. Planets came into existence.

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