Starry Monday at Otterbein

1. Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- November 7, 2005 Dr. Uwe Trittmann

2. Today’s Topics • Classification of Stars • The Night Sky in November

3. Feedback! • Please write down suggestions/your interests on the note pads provided • If you would like to hear from us, please leave your email / address • To learn more about astronomy and physics at Otterbein, please visit • http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.) • http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)

4. Classification of Stars • We can classify stars by many categories • Name • Position • Constellation • Distance • Color • Temperature • Size • Brightness • Spectra • Features: double stars, variable stars, …

5. How Stars Got Their Names • Some have names that go back to ancient times (e.g. Castor and Pollux, Greek mythology) • Some were named by Arab astronomers (e.g. Aldebaran, Algol, etc.) • Since the 17th century we use a scheme that lists stars by constellation • in order of their apparent brightness • labeled alphabetically in Greek alphabet • Alpha Centauri is the brightest star in constellation Centaurus • Some dim stars have names according to their place in a catalogue (e.g. Ross 154)

6. Positions of Stars The Celestial Sphere • An imaginary sphere surrounding the earth, on which we picture the stars attached • Axis through earth’s north and south pole goes through celestial north and south pole • Earth’s equator Celestial equator

7. Celestial Coordinates Earth:latitude, longitude Sky: • declination (dec) [from equator,+/-90°] • right ascension (RA) [from vernal equinox, 0-24h; 6h=90°] Examples: • Westerville, OH 40.1°N, 88°W • Betelgeuse (α Orionis) dec = 7° 24’RA = 5h 52m

8. But: What’s up for you… Observer Coordinates • Horizon – the plane you stand on • Zenith– the point right above you • Meridian – the line from North to Zenith to south

9. …depends where you are! • Your local sky – your view depends on your location on earth

10. Constellations of Stars • About 5000 stars visible with naked eye • About 3500 of them from the northern hemisphere • Stars that appear to be close are grouped together into constellations since antiquity • Officially 88 constellations (with strict boundaries for classification of objects) • Names range from mythological (Perseus, Cassiopeia) to technical (Air Pump, Compass)

11. Constellations of Stars (cont’d) Orion as seen at night Orion as imagined by men

12. Constellations (cont’d) Orion “from the side” Stars in a constellation are not connected in any real way; they aren’t even close together!

13. Distances to the Stars • Parallax can be used out to about 100 light years • The parsec: • Distance in parsecs = 1/parallax (in arc seconds) • Thus a star with a measured parallax of 1” is 1 parsec away • 1 pc is about 3.3 light years • The nearest star (Proxima Centauri) is about 1.3 pc or 4.3 lyr away • Solar system is less than 1/1000 lyr

14. Our Stellar Neighborhood

15. Scale Model • If the Sun = a golf ball, then • Earth = a grain of sand • The Earth orbits the Sun at a distance of one meter • Proxima Centauri lies 270 kilometers (170 miles) away • Barnard’s Star lies 370 kilometers (230 miles) away • Less than 100 stars lie within 1000 kilometers (600 miles) • The Universe is almost empty! • Hipparcos satellite measured distances to nearly 1 million stars in the range of 100 pc • almost all of the stars in our Galaxy are more distant

16. Brightness • A measure of the apparent brightness • Logarithmic scale • Notation: 1m.4 (smaller brighter) • Originally six groupings • 1st magnitude the brightest • 6th magnitude the dimmest • The modern scale is more complex • The absolute magnitude is the apparent magnitude a star would have at a distance of 10 pc: 2M.8

17. Electromagnetic Spectrum

18. Three Things Light Tells Us • Temperature • from black body spectrum • Chemical composition • from spectral lines • Radial velocity • from Doppler shift

19. Peak frequency Black Body Spectrum (gives away the temperature) • All objects - even you - emit radiation of all frequencies, but with different intensities

20. Measuring Temperatures • Find maximal intensity  Temperature (Wien’s law) Identify spectral lines of ionized elements  Temperature

21. Wien’s Law • The peak of the intensity curve will move with temperature, this is Wien’s law: λT = const. = 0.0029 m · K So: the higher the temperature T, the smaller the wavelength λ, i.e. the higher the energy of the electromagnetic wave

22. Luminosity and Brightness • Luminosity L is the total power (energy per unit time) radiated by the star • Apparent brightness B is how bright it appears from Earth • Determined by the amount of light per unit area reaching Earth • B  L / d2 • Just by looking, we cannot tell if a star is close and dim or far away and bright

23. Measuring the Sizes of Stars • Direct measurement is possible for a few dozen relatively close, large stars • Angular size of the disk and known distance can be used to deduce diameter

24. Sizes of Stars • Dwarfs • Comparable in size, or smaller than, the Sun • Giants • Up to 100 times the size of the Sun • Supergiants • Up to 1000 times the size of the Sun • Note: Temperature changes!

25. Star Systems: Binary Stars • Some stars form binary systems – stars that orbit one another • visual binaries • spectroscopic binaries • eclipsing binaries • Beware of optical doubles • stars that happen to lie along the same line of sight from Earth • We can’t determine the mass of an isolated star, but of a binary star

26. Visual Binaries • Members are well separated, distinguishable

27. Spectroscopic Binaries • Too distant to resolve the individual stars • Can be viewed indirectly by observing the back-and-forth Doppler shifts of their spectral lines

28. Eclipsing Binaries (Rare!) • The orbital plane of the pair almost edge-on to our line of sight • We observe periodic changes in the starlight as one member of the binary passes in front of the other

29. Spectral Classification of the Stars Class Temperature Color Examples O 30,000 K blue B 20,000 K bluish Rigel A 10,000 K white Vega, Sirius F 8,000 K white Canopus G 6,000 K yellowSun,  Centauri K 4,000 K orange Arcturus M 3,000 K red Betelgeuse Mnemotechnique: Oh, Be AFine Girl/Guy, Kiss Me

30. Spectral Lines – Fingerprints of the Elements • Can use spectra to identify elements on distant objects! • Different elements yield different emission spectra

31. Origin of Spectral Lines • Atoms:electrons orbiting nuclei • Chemistry deals only with electron orbits (electron exchange glues atoms together to from molecules) • Nuclear power comes from the nucleus • Nuclei are very small • If electrons would orbit the statehouse on I-270, the nucleus would be a soccer ball in Gov. Bob Taft’s office • Nuclei: made out of protons (el. positive) and neutrons (neutral)

32. The energy of the electron depends on orbit • When an electron jumps from one orbital to another, it emits (emission line) or absorbs (absorption line) a photon of a certain energy • The frequency of emitted or absorbed photon is related to its energy E = h f (h is called Planck’s constant, f is frequency)

33. Hertzsprung-Russell-Diagram • Hertzsprung-Russell diagram is luminosity vs. spectral type (or temperature) • To obtain a HR diagram: • get the luminosity. This is your y-coordinate. • Then take the spectral type as your x-coordinate. This may look strange, e.g. K5III for Aldebaran. Ignore the roman numbers ( III means a giant star, V means dwarf star, etc). First letter is the spectral type: K (one of OBAFGKM), the arab number (5) is like a second digit to the spectral type, so K0 is very close to G, K9 is very close to M.

34. Constructing a HR-Diagram • Example: Aldebaran, spectral typeK5III, luminosity = 160 times that of the Sun L 1000 Aldebaran 160 100 10 1 Sun (G2V) O B A F GK M Type … 01234567890123456789 012345…

35. The Hertzprung-Russell Diagram • A plot of absolute luminosity (vertical scale) against spectral type or temperature (horizontal scale) • Most stars (90%) lie in a band known as the Main Sequence

36. Hertzsprung-Russell diagrams … of the closest stars …of the brightest stars

37. Mass and the Main Sequence • The position of a star in the main sequence is determined by its mass All we need to know to predict luminosity and temperature! • Both radius and luminosity increase with mass

38. Stellar Lifetimes • From the luminosity, we can determine the rate of energy release, and thus rate of fuel consumption • Given the mass (amount of fuel to burn) we can obtain the lifetime • Large hot blue stars: ~ 20 million years • The Sun: 10 billion years • Small cool red dwarfs: trillions of years The hotter, the shorter the life!

39. Preview: Stellar Lifecycle • Next Starry Monday: • How stars are born and die • What makes stars “shine” • Planetary nebulae are dead stars! • …and much more

40. The Night Sky in November • Back to standard time -> earlier observing! • Autumn constellations are up: Cassiopeia, Pegasus, Perseus, Andromeda, Pisces  lots of open star clusters! • Mars at opposition • Saturn is visible later at night

41. Moon Phases • Today (New Moon, 36%) • 11 / 8 (First Quarter Moon) • 11 / 15 (Full Moon) • 11 / 23 (Last Quarter Moon) • 12/ 1 (New Moon)

42. Today at Noon • Sun at meridian, i.e. exactly south

43. 10 PM Typical observing hour, early November • Mars • Uranus at meridian • Neptune Moon

44. South-East Plejades Mars at its brightest in Aries

45. West The summer triangle is still hanging on …

46. Due North • Big Dipper points to the north pole

47. High up – the Autumn Constellations • W of Cassiopeia • Big Square of Pegasus • Andromeda Galaxy

48. Andromeda Galaxy • “PR” Foto • Actual look

49. South-East High in the sky: Perseus and Auriga with Plejades and the Double Cluster

50. South-West • Planets • Uranus • Neptune • Zodiac: • Capricorn • Aquarius