Stars and Galaxies

1. Stars and Galaxies

2. Chapter 25-1 • Objectives: • Describe constellations. • Discuss circumpolar constellations. • Discuss apparent and absolute magnitude.

3. Constellations • Ancient Greeks and Romans observed patterns of stars in the sky. • They imagined that the constellations represented mythological creatures. • In reality the stars in a constellation may not be near each other.

4. Circumpolar Constellations • As the Earth moves, some constellations, such as Ursa Minor and Ursa Major, circle around Polaris. • They appear to move because the Earth is in motion. They are visible all year.

5. Apparent and Absolute Magnitude • The absolute magnitude of a star is the amount of light it gives off. • The apparent magnitude is the amount of light that is received on earth. • A star that is dim can appear bright if it is close to Earth.

6. Brightness of Stars The brightness of a star depends on • Size • Surface temperature • Distance from the Earth (absolute and apparent magnitude)

7. Measuring Star Distance • One method of measuring the distance to stars is called parallax. • Parallax refers to the apparent change in the position of a star in the sky. • Measurement is based on the triangle formed by the Sun, Earth and star. The greater the parallax angle, the closer the star is to Earth. • This method cannot be used for a star more than 100 light-years away.

8. Light Years • A light-year is the distance that light travels in one year. Light travels at 300,000 km/s or 9.5 trillion km in one year. • The nearest star to Earth, other than the sun, is Proxima Centuri, 4.3 light years away.

9. Properties of Stars • The color of a star indicates its temperature. • Astronomers study the composition of stars by observing their spectra as it is split by a prism. • The dark lines in a spectra tell the scientist which elements the star contains.

10. Chapter 25-2 • Objectives: • Describe the layers of the Sun. • Discuss the features of the Sun.

11. The Sun • The sun is 150 million kilometers from the Earth. • The sun is 4.6 billion years old. • The volume of the is 1 million times greater than Earth. • The sun’s density is only ¼ of the Earth because the sun is made only of gases.

12. Layers of the Sun • Corona (hottest part of outer layers – 1,700,000 degrees C) • Chromosphere- (atmosphere 27,800 ºC) • Photosphere (surface of the sun – 6000 ºC) at the top of the convective zone • The radiative zone is between the core and the convective zone • Core- interior of sun- 15,000,000ºC

13. Sunspots • Sunspots – cooler than the rest of the sun’s surface they appear as dark spots. Their movement shows that the sun rotates but that the sun rotates more quickly at the equator than the poles. This tells us it does not rotate as a solid object.

14. Prominences • Prominences – loops of gas originating in the chromosphere

15. Solar Flare • Solar flares – bright bursts of light on the sun’s surface

16. CME • CME –Coronal mass ejections are a continuous stream of high-energy particles released into space in all directions from the sun. They can interfere with radio signals and telephone communications.

17. Chapter 25-3 • Objectives: • Determine how stars are classified. • Compare the Sun to other types of stars on the H-R diagram • Describe how stars evolve.

18. Size of Stars • Neutron star- A dying high mass star that is16 km in diameter. • White dwarf star- This is a dying low to medium mass star which is earth sized, but can be as small as Asia. • Medium sized star- This low to medium mass star at birth is 1/10 to 10 times the size of the Sun. • Giant star-This high mass star at birth is 10-100 times our Sun. • Super giant star- This very high mass star at birth is100-1000 times the size of our Sun.

19. Composition of Stars • Hydrogen and helium (96-99%) • Oxygen • Neon • Carbon • Nitrogen • Calcium • Very high mass stars can fuse iron.

20. Surface Temperature of Stars • The surface temperature of a star can be determined by the color. • Blue-white (hottest) – 25,000º C • White - 10,000º C • Yellow (Sun) - 6,000º C • Red-orange- 5,000º C • Red (coolest) - 3,000º C

21. The Hertzsprung-Russell • As the absolute magnitude increases, the temperature also increases.

22. Why do stars shine? • Within the core of a star, gravitational forces cause nuclear fusion. • Four hydrogen atoms are fused to produce one helium atom. The remaining matter is given off in the form of heat and light energy.

23. The Evolution of Stars • New stars are born from the gases in a nebula. • When hydrogen in the cloud reaches a certain temperature (15,000,000 degrees C), nuclear fusion begins. A protostar, or new star, is formed. • The main factor that shapes the evolution of a star is the mass it began with. • A more massive stars have a shorter life.

24. Evolution of a Star

25. Medium-Sized Stars • For the first few billion years, a star shines as nuclear fusion occurs in the core. • When most of the hydrogen is gone, the helium core shrinks and heats up again. • As the outer shell expands, it cools and its color reddens and become a red-giant. • When all of the helium atoms are fused into carbon, the star begins to die.

26. White Dwarfs • When a red-giant begins to die, gravity causes the star’s matter to collapse inward and become a white dwarf which is very dense. • The amount of time it takes the white dwarf to die depends on the mass of the star when it first formed. The smaller the starting mass, the longer a star will live.

27. Massive Stars • The evolution of high and very high mass stars are the same as low and medium mass stars until they become red giants. Low and medium mass stars will become white dwarfs. Higher mass stars will begin to produce oxygen, nitrogen and iron. • The iron atoms will not fuse and begin to absorb energy. The star will explode in a supernova which can light the sky for weeks.

28. Evolution • Low to Medium Mass -nebula- main sequence- red giant- white dwarf- brown dwarf • High Mass -nebula- main sequence- red giant- supernova-neutron star • Very High Mass- nebula- main sequence- red giant- supernova- black hole and new nebula

29. Supernova • During a supernova explosion, the heat reaches such high temperatures that iron atoms can fuse to form new elements. • These elements explode into space and form a new nebula. • Chinese astronomers recorded a supernova in 1054 that lit the night sky for 23 days and could be seen for 600 nights. (Crab Nebula)

30. High Mass and Very High Mass Stars • After the supernova explosion, a high mass star will become a neutron star. • After a supernova explosion, in a very high mass star the core that remains will be so massive, that without the energy created by nuclear fusion to support it, the core is swallowed by its own gravity. • The gravity of the core is so strong that light cannot escape – a black hole.

31. Detection of Black Holes • Most black holes have companion stars. • When the gases from the companion hole are pulled into the hole and heated, they give off a burst of X-rays.

32. Chapter 25-4 • Objectives: • Describe the Sun’s position in the Milky Way Galaxy • Explain that the same natural laws apply to our solar system also apply to other galaxies.

33. Nebulae • A nebula is a massive cloud of dust and gas. • Nebulae are the birthplace of new stars. • Stars are held together by gravity in galaxies. There are three main types of galaxies, spiral, elliptical and irregular.

34. Spiral Galaxies • Galaxies contain various star groups. • Most galaxies are spiral galaxies. Spiral galaxies are shaped like pinwheels.

35. Elliptical Galaxies • Galaxies that vary in shape from nearly spherical to flat disks are called elliptical galaxies. They contain very little dust and gas. They are usually older.

36. Irregular Galaxies • Irregular galaxies have no definite shape.

37. The Milky Way Galaxy • The Milky Way Galaxy is a spiral galaxy. • Most of the older stars in the Milky Way are found near the nucleus of the galaxy. • The Milky Way is estimated to be 100,000 light-years in diameter and about 15,000 light-years thick. • The Sun is located in one of the pinwheel arms. • The stars rotate counter clockwise around the center. This takes 200 million years.

38. Local Group Galaxy Cluster • Astronomers have concluded that the Universe is expanding since all the galaxies, with the exception of other galaxies in the Local Group Galaxy Cluster, show a red shift. Within the Local Group, some show a red shift and others a blue shift, since some are moving toward the Milky Way and others away.

39. The Big-Bang Theory • Astronomers believe that the expanding universe is the result of an enormous explosion known as the big bang. • The explosion occurred 15-20 billion years ago. • As the matter moved away from the explosion, gravity caused clusters to form. • These clusters became the galaxies of the universe. • Support comes from Red Shift and background radiation.

40. Spectroscope • Astronomers use telescopes and spectroscopes to study the stars. A spectroscope can break up the light from a distant star into colors. • After the light is focused into a lens, a prism separates light into its different colors, called a spectrum. • By using a spectroscope, scientists can determine if an object is moving toward the Earth or away, how fast the star is rotating and what elements make up the star.

41. The Red Shift • When a star is approaching the Earth, the light waves will be compressed. • The wavelengths are shorter and are characteristic of blue and violet light. • If the star is moving away from Earth, the waves will be expanded. Longer wavelengths are characteristic of red light. • The spectrum of a star moving away from Earth is shifted toward the red end. (the Red Shift)

42. Doppler Effect • The Red Shift Theory is based on the doppler effect in which waves light and sound, appear to compress as they approach the viewer. • They appear to lengthen as they move away from the viewer.

43. The Steady State Theory • This theory states that the universe is and has always been the same.

44. The Oscillation Theory • In this universe, the gravitational attraction between the galaxies will cause the movement away from each other to slow down. Then gravity will begin to pull the galaxies back toward the center of the universe until another big bang occurs. This will continue to happen over and over.