B. Sc-III Semester - V Physics- P-XII

1. B. Sc-IIISemester - VPhysics- P-XII Chapter Milky Way Galaxy & Solar System By Dr. V. S. Jamadade

2. Milky Way Galaxy & Solar System • Points to be covered :- • Introduction • Milky Way Galaxy & Origin of Solar System • Condensation theory- arguments for & against the theory • Early History of planets • Planetary properties of Mars • Evidence of Geological activities, Prospects for life on Mars • Surface of the Sun, Sunspots,& Sunspot Cycle

3. Introduction • Universe Contains About 100 Billion Galaxies • Each Galaxy- 100 Billion Stars. • Stars – Own Planetary system • For Stability - • Satellites Rotate about Planets • Planets Rotate about Sun • Sun Rotates about Galaxy • Galaxy Rotates about Its Centre

4. Seen at Moonless Night Milky Way Galaxy – Luminous Band stretching from Horizon to Horizon. 1st Observed- Galileo – 1610- by 1 inch Telescope. & Showed – made up of Stars- Close – Diffused band. In 1750- T. Wright – M. W.G. – look like a Disc- Few Stars along Thickness.- This idea Confirmed by W. Herchael. Visible Milky way is Edge on View of disc type Galaxy. Milky Way Galaxy

8. Milky Way Contains – 100 billion Stars held Together by gravitational Force Arranged in Flat Disc About 100 thousand l.y.- Diameter 5000 l. y. Thickness. Stars Concentrated at Centre=> Galactic Nucleus.- Stars bulges out of the Disc. Galactic Nucleus Obscured by Clouds of Dust Present in Space between Stars. Galactic Centre is Along Sagittarius. .

9. The Galactic Centre

10. Spiral Arms- 1500l.y. Wide 3000 l.y. Apart. Arms Trailing S.G. consists - Most Bright stars Space filled with ‘H’ & Dust. Sun – 30000ly from Centre Front View

11. from above ("face-on") see disk and bulge Perseus arm Orion arm Sun Cygnus arm Carina arm from the side ("edge-on")

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13. Entire Spiral Rotates About Axis passing through centre & Perpendicular to Disc. Regions Near Centre rotate faster Solar System- Completes 1 round – 250 million years. Majority Stars- individuals Galactic Clusters =>Few Groups =1000 members Globular Clusters => million Stars

14. The Inter Stellar Medium • About 90% Interstellar medium = Hydrogen (Primordial) => To Form New Stars + Other heavier Elements. • 40 different types of molecules Present in Space between Stars. • Density of matter in Space between Stars is LOW. • Interstellar molecules- Nh3, C2H6O, CO, HCN, Interstellar dust.

15. Origin of Solar System • Sun & Solar System – formed about 4.6 billion years ago. When concentration of heavy elements was 1%. • Essential Features : - • i] Planets Division – • a) Terrestrial Planets : - Mercury, Venus, Earth ,Mars • Small masses But Higher Densities • b) Giant Planets :- Jupiter, Saturn, Uranus, Neptune. • Large masses but Low Densities. • ii] Planets Revolve around Sun- clockwise • iii] Orbital Planes – Close together

16. Planetary System

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18. Different theories of the formation of Solar System • i] Collision Theory • ii] Nebular Theory • iii] Condensation Theory

19. Condensation Theory • The most Modern theory. • It provides a working hypothesis that how planets formed form a large nebula of gas and dust • According to this gas cloud started to contract under its own gravity and it leads to increase in density of gas • Therefore during occasional collision the atoms and molecules stuck together to form heavier particles which carried along gas swirling around the centre of cloud • But when particle becomes to large they fall towards central plain

20. Thus density of matter increases towards central plane of thin disk of matter • The central cloud continues to contract under gravity when its temperature increase and reached around 10 million degree Kelvin. • Here thermonuclear reaction are ignited in the central core when star or sun is born and this took place 4.6 billion year ago. • During this period aggregation of small bodied called planetismals continued to form planets. • Thus No definite proof is provided to justify Condensation theory

21. Arguments for & Against theory • A] Favorable reasons to Support the theory :- • i. Acc. To Steller Evol. Sun & Planets - Formed by Condensation of interstellar gas & dust . • ii. During Cond. 2 or more massive objects – Formed. • iii. During Cond. gas & dust in nebula- swirl around the centre of System, hence heavier particles moving towards centre by gravity. Thus conc of matter in cloud nearer the centre & hence terrestrial planets are more dense • iv. Uniform direction of revolution of planets is consequence of direction of revolution of nebula. • v. Acc.to law of conservation of ang. Mom. , the planets formed at larger distance from centre will be rotating faster about their axes. • vi. Clockwise rotation and Revolution of Planets is again due to rotation of nebula.

22. B. Objections to the Theory • i. Sun Spins about its Axis at much slower rate than Planets. But Acc. To conservation of ang. Mom. It is expected to rotate very fast. But it is not so. • ii. Terrestrial planets – very small in size & mass. => its Gravity is insufficient to condense . • iii. The mass of Planets should form a large fraction of mass of Original cloud. But the mass of all planets is about 1% of mass of Sun. • These Difficulties are not Resolved Easily. However evidences From Carbon dating , that earth formed by condension along the Sun, about 4.6 billion years ago.

23. Early History of Planets • Acc to C.T. Solar system, Sun,, Planets -> formed due to Condensation of gaseous matter in original cloud. • original cloud =H + He+ 1% heavier elements. • H & He+> Major Constituents of Sun & planets. • Sun & Giant Planets- composed of H & He -> gravity not Sufficient to raise temp. 10 million degrees-> to set thermonuclear reactions=> remain as Planets • Thus C.T. Accounts the formation & composition of Giant Planets.

24. But Terrestrial Planets – different – composition is heavier Rocky Elements. • A small fraction of gaseous material trapped in Rock Cavities- later- released- during volcanic irruptions. To form oceans & atmosphere on planets. • Complete process of formation of terrestrial planets slowed over 10 million years. • Initially the Condensation of Rocky particles was very Slow.- When they grew up to attract other particles by own gravity, material was added quickly to planets . Thus present Planetary system is formed.

25. Planetary Properties of Mars

26. Mars .

27. Planetary Properties of Mars • Mars is the fourth Terrestrial planet from the center of the solar system. Also called Exterior planet. • Called the "red planet" for its distinct orange-red color, Mars has been the object of intense interest for over a century. • Atmosphere- not Very dense. • Two Satellites – Phobos & Diemos • % of water vapour is very small.

28. Mars has numerous earthlike features. There are large, extinct volcanoes dotting its surface, eroded channels where water once flowed freely, and ice caps covering its poles that look very much like Earth's polar regions. • But , the thin Martian atmosphere is made mainly of carbon dioxide. Although Mars may now be a cold, dead world, the variety of features on its surface suggests a complex and fascinating past. • Perhaps we will learn more about this fascinating planet when the new NASA Odyssey mission • NASA  Rover missions Curiosity is launched recently.

29. The temperature on Mars can be very, very cold. On its warmest day, Mars can still be a very cold place. At the top and bottom of the planet are poles just like on Earth. During the Martian winter, ice caps can be seen at the poles. • Space probes have landed on Mars. These probes were sent on a fact-finding mission by the United States. They performed experiments on the Martian dirt and atmosphere. The dirt was found to contain clay which was rich in iron. The iron is what gives Mars its red color. • Mars has many craters which were formed by meteorites or asteroids hitting it. Mars also has some of the tallest volcanoes and some of the deepest valleys in our solar system. Mars has two moons, Phobos and Deimos which have unusual shapes. Scientists think these potato-shaped moons were once asteroids captured by Mars' gravitational pull .

30. Evidence for Geological Activities • The Radio Active Heating in interior- causes increase in Temp.=> Rocks melt => heat released in the form of volcanoes. • Due to Small Gravity – Water vapor Escaped from Atmosphere. • Volcanic Activities confirmed by Photographs of Surface. • Photographs show • conically shaped Huge Volcanic mountains with a crater at summit. • & Number of meteorite craters on flancks (sides) of Volcanoes. • More than dozens mountains- discovered. • Estimated that, Martian volcanoes died out at 500 million years ago. • As Lava runs down volcanic mountains washes away the small meteorite craters on the sides of mountains. • Volcanic activity- associated with movements of large Slabs of Rock called plates.

31. When Plates move apart & during head –on collision of plates- cracks developed. • Valley of Mariner ( Long & Straight crack) is found. • At some point of Geological Evolution of mars- the deep forces must have pulled the crust apart along a line to form Valley of Mariner. • Repeated land Slides at the rims of cracks=> Valley is Widened. • No major Signs of plate movements on mars .

32. Prospects of Life on Mars • The only member , where life can be Expected. • Since the conditions are quite Favorable for Evolution of life. • Mars – characterized by Climatic intervals- winter, summer moisture. A Substantial amount of water – accumulated during volcanic Activities. • The source of water – disappeared when volcanic subsided. • Due to low gravity- individual water molecules leaked to space & eventually a trace of moisture is left in atmosphere with many dry streams & rivers. • The free flow of building blocks of life (C, N, O) resulted in Frequent collisions & large molecules can be formed. ( protein, DNA, RNA).

33. At early time, there was plenty of water with denser atmosphere & favorable climate. Life may have evolved during this period. • But when volcanoes ceased , present drier climate transit. • During gradual changes the Martian life adopted to present conditions. • The discovery of Martian life would lead to Probability of life evolving on earth like. • The Experience to detect Martian life Suggest that life or some process imitating life, exist on Mars today.

34. The Sun • .

35. The SUN • The Sun is a low mass star on the outer reaches of the Milky Way galaxy. • The Sun is some 30,000 light years from the center of the Milky Way and lies on one of the spiral arms. At this distance, it takes about 250 million years to complete an orbit around the galaxy. • One light year is the distance traveled by light in one year. One light year is about 10,000 billion kilometers.

36. The sun is the star at the center of the Solar System. • The Sun's color is white although from the surface of the Earth it may appear yellow because of the atmosphere. •  The Sun is so big that you could fit more than one million earths inside of it!. It measures more that a million kilometers across. • The Sun is about 75% hydrogen and 25% helium. This changes slowly over time as the sun converts hydrogen to helium in its core. The Sun´s energy is produced by nuclear fusion.

37. Earth-Sun Distance: 150 million km (93 million miles) • Elemental Composition of Sun: 74% Hydrogen, 25% Helium, 1% other (by mass). • Solar Luminosity: ~ 4 x 10³³ erg/s. [As bright as 4 trillion 100-watt light bulbs] • Age of Sun: 4.5 billion years.

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41. PARTS OF OUR SUN • Chromosphere : is a narrow layer above the photosphere that raises in temperature. • Core : where energy is created by nuclear reactions. • Corona: the outermost layer of the solar atmosphere. The corona consist in a gas with a high temperature. • Sunspots: appear dark because it is cooler than the surrounding areas. • Photosphere: the visible surface of the sun. • Prominences: a solar prominence is an arc of gas that erupts from the surface of the sun.

42. 1. Core2. Radiative zone3. Convective zone4. Photosphere5. Chromosphere6. Corona7. Sunspot8. Granules9. Prominence

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46. . Visible Sun- Bright, Dazzling disc (400km thickness) called Photosphere Emitts continuous Spectrum Average Temp.6000degree k Temp- increases towards centre to 20 million degrees. Thermonuclear reactions Form the Source of Energy. Produced Energy reaches the Surface by Convection. Surface of the SUN

47. Surface Exhibits granular structure • Granules are irregular shaped bright regions with darker boundary. • Temp of darker region is 200k less than bright grain • Average Size of Granule about 400- 500 km • Darker boundaries are cooled gases sinking back into the photosphere. • Thus, observation of Granules is evidence for existence of convection currents below the photosphere. • Individual granules have different shape & Size and last for few minutes.

48. Sun Surface Shows Number of Dark Region => Sunspots. Predicted by Greek Observers in 4th Century Sun Spots

49. 1. Shows Activity of Disturbed Sun Surface. 2. Consists of umbra (Dark Central area) & penumbra (less dark fringe like structure). 3. Different Shape & Size. Small spots (pores) = 2500km in diameter Large spots1,30000km in diameter. 4. Different life times ( minutes - months) 5. Temp. is about 4000k Prominent Features: -

50. 6. Sun-Spot Cycle One of the most dramatic properties of the Sun's activity is the existence of a solar cycle, which is seen in the pattern of sunspots. • sunspots cycle :- the number of sunspots on the Sun is not constant but varies significantly with a peak number approximately every 11 years. • The Sun, thus, has a cycle of behavior which repeats every 11 years. • Average time from min. to max = 5 years • Average time from max to min. = 6 years • Also Exhibits drift motion of 2km/s towards west • It is not known exactly causes of this repeatable behavior but processes in the deep interior of the Sun, which generate magnetic field, are thought to be responsible. • Over the last 30 years scientists have developed theories to explain this behavior and improved observations from telescopes, providing needed information to know the answer to this.