Astronomy 101. 1. Where we are in the Universe. 2. Motions on the sky. 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. The parallax angle p. Small-angle formula:.
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1. Where we are in the Universe
2. Motions on the sky
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
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
Galactos = milk in Greek
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!
~ 100 billion galaxies in the observable Universe
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!
Wide-angle optical view of the GC region
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
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
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)
Size ~ 1 AU (12 Schwarzschild Radii)
Density ~ 7x1021 Msun/pc3
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.
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)
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
Density ~ 0.05 atoms/cm3
Temperature ~ 105 K
Remnant of supernova
= 3 pc
= 1 AU
= 10 kpc
= 100 Mpc
Looking through space = travel in time!
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.
In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures.
Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.
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.
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!
Born: about 85 in EgyptDied: about 165 in Alexandria, Egypt
Shares with Euclid's "Elements" the glory of being the scientific text longest in use.
Venice: Petrus Liechtenstein, 1515.
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.
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.
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.
90o - L
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
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.''.
If you are in the northern hemisphere, celestial objects north of the celestial equator are above the horizon for more than 12 hours because you see more than half of their total 24-hour path around you. Celestial objects on the celestial equator are up 12 hours and those south of the celestial equator are above the horizon for less than 12 hours because you see less than half of their total 24-hour path around you. The opposite is true if you are in the southern hemisphere.
Notice that stars closer to the NCP are above the horizon longer than those farther away from the NCP. Those stars within an angular distance from the NCP equal to the observer's latitude are above the horizon for 24 hours---they are circumpolar stars. Also, those stars close enough to the SCP (within a distance = observer's latitude) will never rise above the horizon. They are also called circumpolar stars.
At left, gravity is pulling on a slanted top. => Wobbling around the vertical.
The Sun’s gravity is doing the same to Earth.
The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.
As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years.
It will be closest to Polaris ~ A.D. 2100.
There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.)
~ 12,000 years from now, it will be close to Vega in the constellation Lyra.
Earth’s rotation is causing the day/night cycle.
Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations.
The Sun’s apparent path on the sky is called the Ecliptic.
Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere.
Sun travels 360o/365.25 days ~ 1o/day
Earth’s axis of rotation is inclined vs. the normal to its orbital plane by 23.5°, which causes the seasons.
We experience Summer in the Northern Hemisphere when the Earth is on that part of its orbit where the N. Hemisphere is oriented more toward the Sun and therefore:
Note: the Earth is actually closest to the Sun in January 4!
Perihelion: 147.09 × 106 km; Aphelion: 152.10 × 106 km
The ecliptic and celestial equator intersect at two points: the vernal (spring) equinox and autumnal (fall) equinox. The Sun crosses the celestial equator moving northward at the vernal equinox around March 21 and crosses the celestial equator moving southward at the autumnal equinox around September 22.
When the Sun is on the celestial equator at the equinoxes, everybody on the Earth experiences 12 hours of daylight and 12 hours of night for those two days (hence, the name ``equinox'' for ``equal night'').
The day of the vernal equinox marks the beginning of the three-month season of spring on our calendar and the day of the autumn equinox marks the beginning of the season of autumn (fall) on our calendar. On those two days of the year, the Sun will rise in the exact east direction, follow an arc right along the celestial equator and set in the exact west direction.
Since the ecliptic is tilted 23.5 degrees with respect to the celestial equator, the Sun's maximum angular distance from the celestial equator is 23.5 degrees. This happens at the solstices. For observers in the northern hemisphere, the farthest northern point above the celestial equator is the summer solstice, and the farthest southern point is the winter solstice. The word ``solstice'' means ``sun standing still'' because the Sun stops moving northward or southward at those points on the ecliptic.
The Sun reaches winter solstice around December 21 and you see the least part of its diurnal path all year---this is the day of the least amount of daylight and marks the beginning of the season of winter for the northern hemisphere. On that day the Sun rises at its furthest south position in the southeast, follows its lowest arc south of the celestial equator, and sets at its furthest south position in the southwest.
The Sun reaches the summer solstice around June 21 and you see the greatest part of its diurnal path above the horizon all year---this is the day of the most amount of daylight and marks the beginning of the season of summer for the northern hemisphere. On that day the Sun rises at its furthest north position in the northeast, follows its highest arc north of the celestial equator, and sets at its furthest north position in the northwest.
Polar Circle: when L > 66.5o
Puzzle: Ice Ages!
32% of land covered with ice
Sea level 120 m lower than now
Milutin Milankovitch 1920
Our Earth makes a complicated motion through space , like a crazy spaceship
As a result, the flux of solar radiation received by the Earth oscillates with different periodicities and amplitudes
This triggers changes in climate
f1 = sin[2 t + 1]
f2 = 0.7 sin[3.1 t + 2.4]
f3 = 1.3 sin[4.5 t + 0.3]
Adding oscillations with different phases and incommensurate frequencies
Adding Milankovitch cycles of solar irradiation for 65 degree North latitude
Note the last peak 9,000 years ago when the last large ice sheet melted
Other factors? Volcanic winters, impacts, …
71,000 yr ago: eruption of Mount Toba (Sumatra)
2,800 km3 of material thrown in the atmosphere
Instant ice age?
Meteorite impacts; Mass extinctions
Diameters from 50-70 m to 200 km
49,000 yr old
Iron meteorite of size 50 m, mass 300,000 ton
Impact velocity 11 km/sec
¾ of all living species have been killed
4.5 billion years ago
October 9, 1992
12 kg stony meteorite hit the Earth