Astronomy 101

1. Astronomy 101 1. Where we are in the Universe 2. Motions on the sky

2. Milky Way Galaxy 25,000 light years, Or ~ 8 kpc 200 billion stars Galactic year = 225 million yr Our sun is 4.6 billion yr old 1 pc = 3.26 ly

3. The parallax angle p Small-angle formula: Define 1 parsec as a distance to a star whose parallax is 1 arcsec d (in parsecs) = 1/p 1 pc = 206265 AU = 3.26 ly

4. “Milky Way” – a milky patch of stars that rings the Earth Galactos = milk in Greek

5. Galileo found that the Milky Way is made up of stars

6. Galaxy M31 in Andromeda – similar to the Milky Way Galaxy 1 Mpc from us

7. Hubble Deep Field 10 day exposure photo! Over 1500 galaxies in a spot 1/30 the diameter of the Moon Farthest and oldest objects are 13 billion light years away! Hubble Space telescope ~ 100 billion galaxies in the observable Universe

8. 500 Mpc scale

9. What’s in the Center?

10. The Galactic Center Our view (in visible light) towards the galactic center (GC) is heavily obscured by gas and dust Extinction by 30 magnitudes  Only 1 out of 1012 optical photons makes its way from the GC towards Earth! Galactic center Wide-angle optical view of the GC region

11. If one looks at this region with big telescopes and near-infrared cameras one can see lots of stars. If one takes pictures every year it seems that some stars are moving very fast (up to 1500 kilometers per second). The fastest stars are in the very center - the position marked by the radio nucleus Sagittarius A* (cross). Distance between stars is less that 0.01 pc

12. A Black Hole at the Center of Our Galaxy? By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to ~ 2.6 million solar masses

13. Radio observations with Very Long Baseline Interferometry (VLBI) that are thousands of times more precise than optical observations (good enough to easily pin-point a source the size of a pea in New York when sitting in Paris)

16. Recent VLBI observations (latest issue of Nature) Size ~ 1 AU (12 Schwarzschild Radii) Density ~ 7x1021 Msun/pc3

17. Will we see a black-hole shadow soon??

18. 1 Astronomical Unit = 1.51011 m

19. The Kuiper Belt – home for short-period comets?? Starting in 1992, astronomers have become aware of a vast population of small bodies orbiting the sun beyond Neptune. There are at least 70,000 "trans-Neptunians" with diameters larger than 100 km in the radial zone extending outwards from the orbit of Neptune (at 30 AU) to 50 AU.

20. 1-day motion of Varuna

23. Voyagers 1 and 2 Launched in 1977 Voyager 1 is now 95 AU from the Sun! (13 light-hours, or 14 billion km) The most distant human-made object in the Universe Speed 17.2 km/sec (3.6 AU per year)

24. Proxima Centauri (Alpha Centauri C) Closest star (4.2 light-years from the Sun) It would take ~ 80,000 years for Voyager 1 to reach a neighboring star Plutonium battery will be dead by 2020 Mission may be shut down by 11/2005 Golden record

25. Local Bubble Density ~ 0.05 atoms/cm3 Temperature ~ 105 K Remnant of supernova explosion?

27. 1017 m = 3 pc distance between stars 107 m planets 109 m Sun 1011 m = 1 AU Solar System 1021 m = 10 kpc galaxy 1025 m = 100 Mpc Largest structure 1026 m = Gpc Hubble radius Distance scale Looking through space = travel in time!

28. Classification of objects on the sky • Description of motions of these objects • Understanding 1 and 2

29. The constellations are an ancient heritage handed down for thousands of years as celebrations of great heroes and mythical creatures. Here Sagittarius and Scorpius hang above the southern horizon.

30. Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures.

31. Constellations (2) Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

32. International Astronomical Union (IAU) http://www.iau.org/IAU/Activities/nomenclature/const.html Names and Standard Abbreviations of Constellations The following list of constellation names and abbreviations is in accordance with the resolutions of the International Astronomical Union (Trans. IAU, 1, 158; 4, 221; 9, 66 and 77). The boundaries of the constellations are listed by E. Delporte, on behalf of the IAU, in, Delimitation scientifique des constellations (tables et cartes), Cambridge University Press, 1930; they lie along the meridians of right ascension and paralleIs of declination for the mean equator and equinox of 1875.0. 88 constellations

33. Asterisms

34. Small dipper

35. Summer triangle

36. Hipparchus of Rhodes Born: 190 BC in Nicaea (now Iznik), Bithynia (now Turkey)Died: 120 BC in probably Rhodes, Greece Catalogue of 850 stars Discovered precession of the Earth’s orbit Determined the distance to the moon Compiled trigonometric tables For thousands of years, discoveries in math and science were driven by astronomical observations!

37. Claudius Ptolemy Born: about 85 in EgyptDied: about 165 in Alexandria, Egypt Almagest • A treatise in 13 books • Mathematical theory of the motions • of the Sun, moon, and planets • Catalogue of 1022 stars and 48 constellations • Introduced minutes and seconds • Geocentric system Shares with Euclid's "Elements" the glory of being the scientific text longest in use.

38. Original book title is Syntaxis • Translated to Arabic as Almagest (al majisti) and then to Latin • That is why stars have Arabic names Venice: Petrus Liechtenstein, 1515.

39. Star naming business: stay away from charlatans!

40. OFFICIAL STAR-NAMING PROCEDURES Bright stars from first to third magnitude have proper names that have been in use for hundreds of years. Most of these names are Arabic. Examples are Betelgeuse, the bright orange star in the constellation Orion, and Dubhe, the second-magnitude star at the edge of the Big Dipper's cup (Ursa Major). A few proper star names are not Arabic. One is Polaris, the second-magnitude star at the end of the handle of the Little Dipper (Ursa Minor). Polaris also carries the popular name, the North Star. A second system for naming bright stars was introduced in 1603 by J. Bayer of Bavaria. In his constellation atlas, Bayer assigned successive letters of the Greek alphabet to the brighter stars of each constellation. Each Bayer designation is the Greek letter with the genitive form of the constellation name. Thus Polaris is Alpha Ursae Minoris. Occasionally, Bayer switched brightness order for serial order in assigning Greek letters. An example of this is Dubhe as Alpha Ursae Majoris, with each star along the Big Dipper from the cup to handle having the next Greek letter. Faint stars are designated in different ways in catalogs prepared and used by astronomers. One is the Bonner Durchmusterung, compiled at Bonn Observatory starting in 1837. A third of a million stars are listed by "BD numbers." The Smithsonian Astrophysical Observatory (SAO) Catalogue, the Yale Star Catalog, and The Henry Draper Catalog published by Harvard College Observatory are all widely used by astronomers. The Supernova of 1987 (Supernova 1987a), one of the major astronomical events of this century, was identified with the star named SK -69 202 in the very specialized catalog, the Deep Objective Prism Survey of the Large Magellanic Cloud, published by the Warner and Swasey Observatory. These procedures and catalogs accepted by the International Astronomical Union are the only means by which stars receive long-lasting names.

43. The celestial sphere The entire sky appears to turn around imaginary points in the northern and southern sky once in 24 hours. This is termed the daily or diurnal motion of the celestial sphere, and is in reality a consequence of the daily rotation of the earth on its axis. The diurnal motion affects all objects in the sky and does not change their relative positions: the diurnal motion causes the sky to rotate as a whole once every 24 hours. Superposed on the overall diurnal motion of the sky is "intrinsic" motion that causes certain objects on the celestial sphere to change their positions with respect to the other objects on the celestial sphere. These are the "wanderers" of the ancient astronomers: the planets, the Sun, and the Moon.

44. We can define a useful coordinate system for locating objects on the celestial sphere by projecting onto the sky the latitude-longitude coordinate system that we use on the surface of the earth. The stars rotate around the North and South Celestial Poles. These are the points in the sky directly above the geographic north and south pole, respectively. The Earth's axis of rotation intersects the celestial sphere at the celestial poles. Fortunately, for those in the northern hemisphere, there is a fairly bright star real close to the North Celestial Pole (Polaris or the North star). Another important reference marker is the celestial equator: an imaginary circle around the sky directly above the Earth's equator. It is always 90 degrees from the poles. All the stars rotate in a path that is parallel to the celestial equator. The celestial equator intercepts the horizon at the points directly east and west anywhere on the Earth.

45. The Celestial Sphere (2) • From geographic latitude L (northern hemisphere), you see the celestial north pole L degrees above the horizon; • From geographic latitude –L (southern hemisphere), you see the celestial south pole Ldegrees above the horizon. 90o - L • Celestial equator culminates 90º – L above the horizon. L

46. Equatorial coordinates Declination (similar to latitude) Right ascension (similar to longitude) Counted from celestial equator Measured in degrees etc. Counted from Vernal Equinox Measured in hours, minutes, seconds Full circle is 24 hours

49. The arc that goes through the north point on the horizon, zenith, and south point on the horizon is called the meridian. The positions of the zenith and meridian with respect to the stars will change as the celestial sphere rotates and if the observer changes locations on the Earth, but those reference marks do not change with respect to the observer's horizon. Any celestial object crossing the meridian is at its highest altitude (distance from the horizon) during that night (or day). During daylight, the meridian separates the morning and afternoon positions of the Sun. In the morning the Sun is ``ante meridiem'' (Latin for ``before meridian'') or east of the meridian, abbreviated ``a.m.''. At local noon the Sun is right on the meridian. At local noon the Sun is due south for northern hemisphere observers and due north for southern hemisphere observers. In the afternoon the Sun is ``post meridiem'' (Latin for ``after meridian'') or west of the meridian, abbreviated ``p.m.''.