Stars & Galaxies

1. Chapter 28 Stars & Galaxies

2. 28.2 Stars & their Characteristics

3. Constellations • Groups of stars • Many were named during ancient times • Hercules • Leo – the lion • Taurus – the bull • Draco – the dragon • Lyra – lyre or harp • Some modern constellations include: telescopium and microscopium

4. Constellations • 88 constellations can be seen from Earth’s northern and southern hemisphere • Human made groupings , not natural groupings • Most stars appear to be grouped together when only view from Earth, when in actuality they widely vary in distances from Earth and are moving

5. Big Dipper • Best-known asterism – small star grouping • Part of a the larger constellation • Ursa Major – the Great Bear • Points towards the handle of the Little Dipper in the constellation Ursa Minor – Little Bear

6. Polaris • The North Star • The last star in the handle of the Little Dipper

7. Cassiopeia • Opposite of the Little Dipper • Looks like a large lopsided W

8. Star Movement • Since Earth rotates on an axis the whole sky appears to turn from east to west • That is why the sun, moon, and stars are said to rise in the east and set in the west

9. Circumpolar Stars • Sections of sky directly above Earth’s pole appear to be stationary • When viewed from some northern latitudes, these constellations never set below the horizon and can be seen all year long • Examples: Ursa Major, Ursa Minor, and Cassiopeia

10. Seasonal Changes • A constellation’s position in the sky changes with the seasons • Caused by Earth’s changes in position as it orbits the sun • Some constellations can be seen only in certain seasons • Northern Hemisphere • Summertime: Lyra • Winter: Orion the Hunter

11. Apparent Magnitude • How bright a star appears to be to an observer on Earth • The lower the star’s magnitude number, the brighter the star is • Each magnification differs from the next by a factor of 2.5 • A first-magnitude star is 100 times brighter than a six-magnitude star

12. Distances to Stars Astronomical Units • Equals 150 million kilometers • The average distance between Earth and the Sun • Nearest star – Proxima Centauri is about 40 trillion kilometers from the sun or more than 260,000 AUs • Jupiter is 5.2 AUs from the Sun

13. Distances to Stars Light-Year • The distance that a ray of light travels in one year • Equals 9.5 trillion kilometers • Proxima Centauri is about 4.2 light-years away from Earth

14. Distances to the Stars Parallax & Parsec • A change in an object’s direction due to a change in the observer’s position • Astronomers can calculate the distance to a star by knowing the angle between two observed positions and the distance between the observation points • Parsec: short for “parallax second” equals 3.258 light-years or 3.086 x 1013 kilometers

15. Elements in Stars • Spheres of super- hot gases • Hydrogen (69%) and Helium (29 %) with small amounts (2%) of oxygen, carbon, nitrogen, and sodium • No two stars contain the same elements in the same proportions • Spectral analysis looks at wavelength of light which depend on composition and temperature • Each star has a spectrum that is unique as your fingerprints

16. Mass of Stars • Cannot be observed directly and therefore must be calculated based on other observations • Greater mass will have a stronger gravitational effect on the bodies around it

17. Solar Mass • Stellar masses are expressed as multiples of the mass of the sun, which is called one solar mass • Some stars are five, ten, or more times more massive than the sun • Others are less massive, perhaps with only 1/5 or 1/10 the sun’s mass

18. Size of Stars • Stars vary in size more than they do in mass • Smallest stars are smaller than Earth • The largest star known has a diameter more than 2,000 times that of the sun

19. Temperature & Color of Stars • The range of colors a star emits depends on its surface temperature • Stars appear to be mostly whitish, yet they are tinged with faint colors

20. Temperature & Color of Stars • Astronomers group stars by temperature and color into spectral classes • Called the Harvard Spectral Classification Scheme, devised by Annie Jump Cannon in the 1920’s • Red = cool stars • Yellow/white = medium stars • Blue= hot stars (The sun appears very yellow to us because Earth’s atmosphere scatters blue light)

21. Luminosity • The actual brightness of a star • Depends only on its size and temperature • Larger = more luminous • Hotter = more luminous

22. Would the sun be more luminous if it were the same size but hotter? Explain • What if it had the same surface temperature but was more massive? Explain

23. Absolute Magnitude • A measure of how bright the star would be if all stars were at the same distance

24. What is the difference between apparent magnitude and absolute magnitude?

25. Variable Stars • Some stars show a regular variation of brightness over cycles that last from days to years • Pulsating Stars: change in brightness as they expand and contract • Cepheid Variables: yellow supergiants whose cycles of brightness range from about 1 day to 50 days

26. 28.3 – Life Cycles of Stars

27. Life Cycle of Stars • Stars are born from great clouds of gas and dust. • They mature, grow old, and die • When they die, they may produce new clouds of dust, which may led to the formation of a new star and planets • The more massive a star is, the shorter its life will be.

28. The Hertzsprung-Russell Diagram • The stars in the universe are at different stages in their life cycles. • Some are young and hot and others are older and colder

29. The Hertzsprung-Russell Diagram • The diagram plots luminosity of stars against their surface temperatures

30. Main Sequence • About 90% of stars are in the main sequence • They are all actively fusing hydrogen into helium

31. Giant Stars • Have diameters from 10 to 100 times greater than that of the sun • Have greater luminosity than main sequence stars

32. Supergiants • Stars with diameters more than 100 times that of the sun • Even more luminous than giant stars • Because of their great size, red supergiant are very luminous despite being relatively cool stars

33. White Dwarfs • Stars that are near the end of their lives • Were once red giant stars • Red giant stars lost their outer atmosphere, and now are only the glowing stellar core

34. What does the Hertzsprung-Russell Diagram depict?

35. Birth of a Star • Begins as a Nebula – a cloud of gas and dust • 99% is hydrogen gas • An outside force like a shock wave causes the nebula to begin condensing • As regions become denser, their temperature increases

36. Birth of a Star • Parts of the nebula will begin to glow – called protostars • As contraction continues, protostar becomes hotter and brighter • Eventually a fusion reaction begins, and a star is born

37. Death of a Star like the Sun • Main sequence stars with a mass similar to the sun’s remain about the same size for billions of years • Hydrogen in the core continues to fuse into helium • Energy produced by the fusion reaction balances the force of gravity that is pulling the star’s matter inward

38. What forces balance each other in a main-sequence star?

39. Death of a Star like the Sun • Eventually the hydrogen is used up and gravity takes over • The hydrogen core shrinks and its contraction produces additional heat, which triggers hydrogen fusion outside of the core • The entire star begins to expand

40. Death of a Star like the Sun • Temperature rises to the point at which helium can fuse into heavier elements of carbon and oxygen • Temperature never rises enough for these heavier elements to fuse • Gasses at the stars surface begin to blow away, resulting in a glowing halo of gasses called a planetary nebula

41. Death of a Star like the Sun • Eventually the planetary nebula fades as its gasses dissipate into space • All that is left is the hot carbon-oxygen core called a white dwarf

42. Death of a Massive Star • Stars that are eight or more times as massive as the sun face a different fate • Fusion process will continue until iron nuclei are formed • Star swells to more than 100 times the diameter of the sun, becoming a supergiant

43. Death of a Massive Star • The formation of iron nuclei does not release energy, it absorbs energy • The iron core quickly and suddenly collapses • Produces a shock wave that blasts the stars outer layers into space

44. Death of a Massive Star • Creates a burst of light millions of time brighter than the original star’s brightness called a supernova • A single supernova can outshine all of the other stars in its galaxy for a time

45. Remnants of a Massive Star • After a massive star goes supernova it leaves behind its core • The remaining core may become a neutron star or a black hole

46. Neutron Star • Superdense remains that are created when electrons are pushed into the nucleus • Spins and gives off burst of radio waves • A rapidly spinning neutron star is called a pulsar because it pulses energy

47. Black hole • The remnant of a star at least 15 times as massive as the sun. • The gravitational force of a black hole is so strong that light cannot escape

48. Compare and contrast the death of a star like the sun and the death of a massive star

49. 28.4 Galaxies and the Universe

50. 28.4 Galaxies and the Universe • The universe is everything that exists • The Observable Universe refers to everything that we can observe • Limited in extent by a combination of the age of the universe and the speed of light