Stars & Galaxies Chapter 28

1. Stars & GalaxiesChapter 28 Most of what we know about the universe comes from analyzing the light that reaches us from distant stars & galaxies.

2. A Closer Look at Light Ch 28 Sec 1 • We learn about the stars & the universe by studying the electromagnetic radiation (EM) they give off. Crab Nebula

3. A Closer Look at Light • What is light? • A form of electromagnetic (EM) radiation • Energy that travels in waves • Ex. radio, x-rays, visible • Moves at ~300,000 km per second (the speed of light)

4. A Closer Look at Light • Length of wave determines characteristics of each form of EM • Longest wavelengths = radio waves • Can be longer than a soccer field • Shortest wavelengths = gamma rays

5. A Closer Look at Light • Can be arranged in a continuum with the longest wavelength at one end & the shortest at the other • This is known as the electromagnetic spectrum • Stars (including the sun) emit a wide range of wavelengths

7. Looking at your reference table… • Wavelength decreases to as move to the left (Gamma= shortest) & increases as move to the right (Radio= longest) • Visible light (color spectrum) is a tiny part (near the middle) of the entire EM spectrum • Ultraviolet = sunburn Infrared waves = heat

8. A Closer Look at Light • Can travel through empty space • Don’t need a medium (like air or water) to travel • EM waves emitted (given off) by an object can give information about: • the chemical elements present in the object • the object’s motion Observing an Exploded Star at Different Wavelengths Visualization

9. The Spectroscope • Visible light  made up of light of various colors (different wavelengths) • Seen in a rainbow or as light passes through a triangular prism • Light waves are refracted (bent), forming a band of colors called the visible spectrum • Longer wavelengths are refracted less than shorter wavelengths • Red = longest • Violet = shortest • Astronomers use spectra of distant stars to learn more about them • To separate starlight into its colors  use a spectroscope • Uses a prism to split light entering a telescope into a spectrum

10. Types of Visible Spectra • Spectroscopes break light into 3 types of spectra which astronomers can analyze & compare to help figure out what elements make up the atmospheres of stars & planets: • Continuous • Emission • Absorption

11. Types of Visible Spectra Animation for 3 Types of Spectra • Continuous • Unbroken • source emitting all visible wavelengths) • Glowing solids (ex. filament) • Glowing liquids (ex. molten iron) • Hot, compressed gases inside stars • Emission • Unevenly spaced lines of different colors & brightnesses • Source is emitting light of only certain wavelengths • Glowing thin gases • Every element has unique emission spectrum • Able to identify elements in object by analyzing spectra • Absorption • Continuous spectrum crossed by dark lines formed when light from a glowing object passes through a cooler gas, which absorbs some of the wavelengths • Same as wavelengths that the gas would emit • By comparing emission & absorption spectra, can determine what elements are present in cooler gas that is absorbing some of the light

12. Types of Visible Spectra • A star’s absorption spectrum tells us the composition of its outer layer • Ex. the sun’s spectrum • Hot, compressed gases of interior radiate a continuous spectrum • As these EM waves pass through the sun’s cooler, outer layers (photosphere & chromosphere) some of the waves are absorbed • As a result, sun’s spectrum crossed by many dark lines ~By matching these lines with the emission spectra of gaseous elements heated in a lab, scientists have identified more than 67 elements in sun’s outer layer.

14. Types of Visible Spectra • Absorption spectra can also be used to determine composition of planet’s atmosphere. • A planet shines by reflecting sunlight. • If spectrum contains dark lines that are not found in the sun’s spectrum, then these lines must be caused by substances in the planet’s atmosphere.

15. The Doppler Effect • Using spectral analysis, it can be determined if a star is moving in relation to Earth b/c of a phenomenon known as the Doppler Effect • Think of what appears to happen (to the sound of the siren) as an ambulance approaches you and then moves away from you

16. What is “Blue Shift” ? • When an object is coming towards you, waves coming from it are compressed • (wavelengths shorten & shift towards blue end of spectrum) Higher pitch

17. What is “Red Shift” ? • If an object is traveling away from you, waves are stretched out • (wavelengths get longer & shift towards red end of spectrum) Lower pitch

18. Stars • Stars moving away from us show a “Red Shift” • Stars moving towards us show a “Blue Shift” bluered Red shifted spectra Normal spectra Blue shifted spectra Colors shift, but pattern (spacing) is the same

19. The Doppler Effect Visualization of Red & Blue Shift of Star's Spectrum Red & Blue Shift Animation • So, if a star is: • Moving away • Red shift • (spectral lines shift towards red) • Moving closer • Blue shift • (spectral lines shift towards blue) • Dividing a spectral line’s shift in wavelength by the wavelength determined in the lab gives astronomers the ratio of the star’s velocity to the speed of light • Ex. red shift of 0.001 • Star is moving away from Earth at about one-thousandth the speed of light (or 300 km/sec)

22. Stars & Their Characteristics Ch 28 Sec 2 • Early observations • Astronomy is one of the oldest human pursuits • Some observations made by ancient astronomers are still used today

23. Stars & Their Characteristics • Constellations • Ancient ppl saw stars in much the same way we do • Gave names to describe groups of stars (constellations) • Often based on: • Mythic heroes (Hercules) • Animals (Leo the lion, Taurus the bull) • Monsters (Draco the Dragon, Centaurus the centaur) • familiar objects (Crater the goblet, Lyra the lrye/harp) • Some names given as early as 2450 B. C. E. • Many ancient names still used today • Other names given in last few centuries • Telescopium the telescope & Microscopium the microscope

24. Stars & Their Characteristics • Constellations (cont’d) • 88 can be seen from N & S Hemispheres • Not natural groupings like solar systems & galaxies. They are human inventions. • Stars in most appear to be together only as they are viewed from Earth • Actually at widely varying distances from Earth & are moving in relation to one another • Since so far away, takes 1000s of years before their motions alter the pattern of the constellation

25. Stars & Their Characteristics • Constellations (cont’d) • Big Dipper • Best known asterism (smaller group of stars within a constellation) • Part of constellation known as Ursa Major (the Great Bear or the Big Bear) • Can be used to find other constellations • Imagine a line drawn through the two stars farthest from the dipper’s handle (“pointer stars”) • This line will lead to the last star in the asterism known as the Little Dipper (which is part of the constellation Ursa Minor or the Little Bear) • This star is Polaris or The North Star

28. Stars & Their Characteristics • Constellations (cont’d) • Big Dipper • Can be used to find other constellations • “Draw” the line from the “pointer stars” to Polaris and then continue about the same distance past Polaris (and the Little Dipper) • You will see a large, lop-sided “W”, which is the constellation Cassiopeia

30. Stars & Their Characteristics • Constellations (cont’d) • The apparent & regular motion of the constellations/stars, sun, & moon across our sky are caused by Earth’s motions • Rotation (spin on axis) & revolution (orbit) • Earth rotates from west to east • So, whole sky appears to move from east to west • Constellations/stars, sun & moon appear to rise in the east & set in the west

31. Stars & Their Characteristics • Sections of sky directly above poles seem stationary (“stand still”) as Earth rotates on its axis • Polaris (The North Star) seems fixed in the sky while the other stars near by move counterclockwise around it ~These are “circumpolar” stars ~Ursa Major, Ursa Minor, & Cassiopeia are circumpolar constellations that can be seen all year b/c they don’t set below the horizon Circumpolar Star Animation

32. Star Trails Animation

33. Stars & Their Characteristics • Constellations (cont’d) • Position in sky changes with the seasons due to Earth’s change in position along orbit • Ex. Big Dipper • Fall  seen near northern horizon • Spring  high overhead • Ex. Cassiopeia • Fall  nearly straight overhead • Spring  just above northern horizon • Some constellations can be seen only in certain seasons also due to Earth’s position along orbit • Ex. Lyra in summer, Orion in winter

34. Stars & Their Characteristics

35. Stars & Their Characteristics • Apparent Magnitude • Some stars seem bright, others faint, & others even more faint • Around 120 B. C. E. Hipparchus devised a system of classifying stars by how bright they looked • Rated the brightness of 850 stars on a scale of 1-6 • Each value on the scale is the apparent magnitude • 1 = brightest • 6 = faintest • Ptolemy expanded the scale to include more than 1000 stars

36. Stars & Their Characteristics • Apparent Magnitude of a star is a measure of how bright a star appears to be to an observer on Earth • Lower the magnitude number, the brighter the star is • Some of the brighter stars are classified as first-magnitude stars • Some stars are even brighter than first-magnitude stars & have magnitudes less than one • A few of the brightest stars have negative magnitudes • Ex. Sirius  apparent magnitude = -1.45 • Ex. Sun  apparent magnitude = -26.7 (b/c so close) • Faintest stars that can be seen with unaided eye are called sixth-magnitude stars

37. Stars & Their Characteristics • In the modern magnitude system, each magnitude differs by a factor of about 2.5 • So, a first-magnitude star is about 2.5x brighter than a second-magnitude star and a second-magnitude star is about 2.5x brighter than a third-magnitude star • A first-magnitude star, by definition, is 100x brighter than a sixth-magnitude star • With the use of telescopes, astronomers can see stars that are far dimmer than those Hipparchus observed • Well beyond twentieth-magnitude

38. Stars & Their Characteristics • Distance to Stars • The problem in expressing distances in space is that the units we use for most measurements (cm, m, km) are too small for measuring such large distances • Numbers become so large that they are difficult to work with

39. Stars & Their Characteristics • Distance to Stars (cont’d) • Astronomers have devised special units for measuring large distances in space… the astronomical unit (AU) • Avg. distance between Earth & sun is ~150 million km (93 million miles) • This distance is known as an astronomical unit (AU) • Next closest star (Proxima Centauri) is ~40 trillion km • This is more than 260,000 AU!!! • Astronomers use AU to express distances w/in solar system • Ex. Jupiter is 5.2 AUs from the sun

40. Stars & Their Characteristics • Distance to Stars (cont’d) • Neither km nor AU are satisfactory for expressing the great distances in space (beyond the solar system) • Instead use 2 other units: light-years & parsecs • A light-year measures the distance that a ray of light travels in 1 year • light travels ~300,000 km/sec… So, in one year, light travels ~9.5 trillion (9.5 x 1012) km • Proxima Centauri is ~4.2 light-years away from Earth Light-year Animation

41. Stars & Their Characteristics Parallax Animation • Distance to Stars (cont’d) • One way of measuring the distance to the nearest star is based on parallax • Parallax is a change in an object’s direction due to a change in the observer’s position • Hold your thumb out and observe its apparent position while switching which eye you have open • B/c Earth orbits the sun, parallax is experienced when observing stars • At different times of year (different position in orbit) a nearby star does not seem to be in exactly the same position against the backdrop of distant stars • Astronomers can calculate the distance to a star by knowing the angle btw the 2 observed positions & the distance btw the observation points ~ use a special unit called a “parsec” (“parallax second”)  1 parsec = 3.258 light-years (3.086 x 1013 km)

43. Elements in Stars • Star = sphere of super-hot gases • Mostly hydrogen & helium • 1-2% of mass may consist of heavier elements (ex. oxygen, carbon, nitrogen, sodium) • At its surface, the sun ~ 69% hydrogen, 29% helium, 2% heavier elements • No 2 stars contain exactly the same elements in the same proportion • Spectral analysis used to determine composition • Wavelengths of light that a star radiates depend on 2 qualities that are different for every star: • Composition • Temperature • Each star has unique spectrum

44. Elements in Stars • Stars get their energy fromNuclear Fusion. • Hydrogen atoms serve as fuel & release energy as theyfusetogether to make Helium atoms. Hydrogen/Helium Fusion Animation Energy

45. Mass, Size, & Temperature of Stars • Mass • Can’t observe mass directly • Can only calculate what its mass might be on the basis of other observations • Inertial properties of the body • Gravitational influence on other bodies • Great mass  strong gravitational effect • Lesser mass  weaker gravitational effect • Expressed as multiples of the mass of the sun, which is called 1 solar mass • Some stars = 5x, 10x, or more massive than the sun • Others are less massive (ex. 1/5 or 1/10 mass of sun)

46. Mass, Size, & Temperature of Stars • Size • Varies more than mass • Smallest stars are smaller than Earth • Largest known star  diameter more than 2000x that of the sun • Density • Differs even more • Betelgeuse ~ one ten-millionth as dense as the sun • One star near Sirius is so dense that one teaspoonful of it would weigh more than a ton on Earth

48. Mass, Size, & Temperature of Stars • Temperature & Color of Stars • Astronomers group stars by temp. & color into spectral classes • Harvard Spectral Classification Scheme • Devised by Annie Jump Cannon in 1920s • Expanded since then to include other stars • Blue Stars = HOT !!! • Sirius • White • Yellow • Sun = 5500°C • Red Stars = COOL !!! • Ex. Betelgeuse 3000°C

49. Investigation: What Does the Spectrum of a Star Tell Us about its Temperature?

50. Long Infrared Red Orange Yellow Green Blue Violet Ultraviolet Low Energy radiated by cooler stars Visible Light High Energy radiated by hotter stars Short