Download Introduction to Astronomy (high school)

1. Where we are in the Universe
2. Motions on the sky
Milky Way Galaxy
200 billion stars
Galactic year = 225 million yr
Our sun is 4.6 billion yr old
25,000 light years,
Or ~ 8 kpc
1 pc = 3.26 ly
The parallax angle p
Small-angle formula:
p(in arcsec) 1 AU

206265
d
206265  1 AU
d
p(in arcsec)
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
“Milky Way” – a milky patch of stars that
rings the Earth
Galactos = milk in Greek
Galileo found that the Milky Way is made up of stars
Galaxy M31 in Andromeda – similar to the Milky Way Galaxy
1 Mpc from us
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
500 Mpc scale
What’s in the Center?
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
If one looks at this region with big telescopes and nearinfrared 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
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
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)
Recent VLBI observations (latest issue of Nature)
Size ~ 1 AU (12 Schwarzschild Radii)
Density ~ 7x1021 Msun/pc3
Will we see a black-hole shadow soon??
1 Astronomical Unit = 1.51011 m
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.
1-day motion of Varuna
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)
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
Local Bubble
Density ~ 0.05 atoms/cm3
Temperature ~ 105 K
Remnant of supernova
explosion?
Distance scale
1017 m
107 m 109 m 1011 m
1021 m
= 3 pc
planets Sun = 1 AU
= 10 kpc
Solar System distance galaxy
between
stars
Looking through space = travel in time!
1025 m
= 100 Mpc
Largest
structure
1026 m
= Gpc
Hubble
radius
1.
2.
3.
Classification of objects on the sky
Description of motions of these objects
Understanding 1 and 2
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.
Constellations
In ancient times, constellations only referred to
the brightest stars that appeared to form
groups, representing mythological figures.
Constellations (2)
Today, constellations are well-defined regions
on the sky, irrespective of the presence or
absence of bright stars in those regions.
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.
Nominative
Genitive
Nominative
Genitive
Andromeda
And
Andromedae
Lacerta
Lac
Lacertae
Antlia
Ant
Antliae
Leo
Leo
Leonis
Apus
Aps
Apodis
Leo Minor
LMi
Leonis Minoris
Aquarius
Aqr
Aquarii
Lepus
Lep
Leporis
Aquila
Aql
Aquilae
Libra
Lib
Librae
88 constellations
Asterisms
Small dipper
Summer triangle
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!
Claudius Ptolemy
Born: about 85 in Egypt
Died: 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.
• 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.
Star naming business: stay away from charlatans!
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.
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.
We can define a useful coordinate system
for locating objects on the celestial sphere
by projecting onto the sky the latitudelongitude 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.
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 L
degrees above 90o - L
the horizon.
• Celestial
equator
L
culminates
90º – L above
the horizon.
Equatorial coordinates
Right ascension (similar to longitude)
Counted from Vernal Equinox
Measured in hours, minutes, seconds
Full circle is 24 hours
Declination (similar to latitude)
Counted from celestial equator
Measured in degrees etc.
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.
Star trails
Precession (1)
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.
Precession (2)
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.
The Sun and Its Motions
Earth’s rotation is causing the day/night cycle.
The "Road of the Sun" on the Celestial Sphere
1.
2.
Diurnal motion from east to west due to the earth’s
spinning around its axis, with ~ 24 h period
Drift eastward with respect to the stars ~ 1 degree per
day with the period of ~ 365.25 days.
This causes the difference of 4 min per day between
the Solar and Sidereal day.
The Ecliptic
Due to Earth’s revolution around the sun, the sun
appears to move through the zodiacal
constellations.
Sun travels
360o/365.25 days
~ 1o/day
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.
The Seasons
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:
1. the Sun rises higher in the sky and is above the horizon longer,
2. The rays of the Sun strike the ground more directly.
Likewise, in the N. Hemisphere Winter the hemisphere is oriented
away from the Sun, the Sun only rises low in the sky, is above the
horizon for a shorter period, and the rays of the Sun strike the
ground more obliquely.
Seasons
1.
Seasons are NOT caused by varying distances from the Earth to the Sun
2. The primary cause of seasons is the 23.5 degree tilt of the
Earth's rotation axis with respect to the plane of the ecliptic.
Note: the Earth is actually closest to the Sun in January 4!
Perihelion: 147.09 × 106 km; Aphelion: 152.10 × 106 km
Sun’s altitude at local noon at equinox: 90o - L
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.
Solstices
p. 22
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.
Sun’s altitude at noon: 90o – L + 23.5o
Sun’s altitude at noon: 90o – L - 23.5o
Polar Circle: when L > 66.5o
There are no seasons on the equator
(except for the changes related to
weather)
 In reality the seasons “lag”: for example,
maximum summer temperatures occur ~ 1
month later than the summer solstice.
Blame oceans that act as storages of heat!

Puzzle: Ice Ages!
Myr ago
• Occur with a period of ~ 250 million yr
• Cycles of glaciation within the ice age occur with a period of 40,000 yr
• Most recent ice age began ~ 3 million yr ago and is still going on!
Last Glacial Maximum: 18,000 yr ago
32% of land covered with ice
Sea level 120 m lower than now
Ice Age: Cause

Theory: climate changes due
to tiny variations in the Earth’s
orbital parameters



Precession of the rotation axis
(26,000 yr cycle)
Eccentricity (varies from 0.00 to
0.06 with 100,000 and 400,000
yr cycles)
Axis tilt (varies from 24.5o to
21.5o with 41,000 yr cycle
Milutin Milankovitch 1920
26,000 yr cycle
• Varies from 0.00 to 0.06 (currently 0.017)
• Periodicity 100,000 and 400,000 yr
• Eccentricity cycle modulates the amplitude of the precession cycle
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
1
f1
f1+f2
0.5
2
2
4
6
8
10
1
-0.5
-1
2
1
f2
4
6
8
10
8
10
-1
0.5
-2
2
4
6
8
10
f1+f2+f3
-0.5
f3
-1
2
1
1
0.5
2
-1
2
-0.5
-1
4
6
8
10
-2
4
6
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
(Berger 1991)
Very good agreement!
Are these effects enough to explain
the Ice Ages???
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
150 known impact sites on Earth
Diameters from 50-70 m to 200 km
Barringer crater, Arizona
49,000 yr old
Iron meteorite of size 50 m, mass 300,000 ton
Impact velocity 11 km/sec
65 million years ago a huge meteorite of 10 km size hit the Earth
• World-wide fires
• 1-km-hign tsunamis
• Acid rains and atmospheric pollution
• Darkness and severe winter for many decades
¾ of all living species
have been killed
Our Moon could have been formed in a giant collision
4.5 billion years ago
The Peekskill meteorite
October 9, 1992
12 kg stony meteorite hit the Earth