Transcript Slide 1

What Are Constellations?
The first thing you need to know is that constellations are not real!
The constellations are
totally imaginary things
that poets, farmers and
astronomers have made
up over the past 6,000
years (and probably even
more!). The real purpose
for the constellations is
to help us tell which
stars are which, nothing
more
Constellations help by breaking up
the sky into more manageable bits
They are used as
mnemonics, or memory
aids
On a really dark night, you can see about 1000 to 1500 stars.
Stars are identified by their Brightness or Magnitude 0 magnitude
Being the brightest then becoming fainter as you count 1, 2, 3…
The magnitude scale was invented by an ancient Greek astronomer
named Hipparchus in about 150 B.C
a.
b.
c.
d.
e.
There are about 21 zero and first magnitude stars.
There are about 50 2nd magnitude stars including Polaris.
The 3rd magnitude stars total about 150
There are some 600 4th magnitude stars
5th magnitude stars are about the faintest you can see on a good
night. There are about 1500 of these stars, but less than 100
of them appear on the charts.
f. Some 6th magnitude stars can be seen by the “keen of sight” in
constellations such as the Dolphin, Cup, and the Fishes.
g. For anything fainter binoculars or a telescope is needed.
As it turns out, the eye senses brightness
logarithmically, so each increase in 5
magnitudes corresponds to a decrease in
brightness by a factor 100. The absolute
magnitude is the magnitude the stars
would have if viewed from a distance of
10 parsecs or some 32.6 light years.
Obviously, Deneb is intrinsically very
bright to make this list from its greater
distance. Rigel, of nearly the same
absolute magnitude, but closer, stands
even higher in the list. Note that most of
these distances are really nearby, on a
cosmic scale, and that they are generally
uncertain by at least 20%. All stars are
variable to some extent; those which are
visibly variable are marked with a "v".
What are apparent and absolute magnitudes?
a. Apparent is how bright the appear to us in the sky
b. Absolute magnitudes are how bright a star would appear from
some standard distance, arbitrarily set as 10 parsecs or about
32.6 light years.
Betelgeuse (alpha
Orionis) is the secondbrightest star in the
constellation Orion and
one of the brightest
stars in the sky.
The Brightest Stars, as Seen from the Earth
Common
Name
Scientific
Name
Sun
Distance (light
years)
Apparent
Magnitude
Absolute
Magnitude
Spectral
Type
-
-26.72
4.8
G2V
Sirius
Alpha CMa
8.6
-1.46
1.4
A1Vm
Canopus
Alpha Car
74
-0.72
-2.5
A9II
Rigil
Kentaurus
Alpha Cen
4.3
-0.27
4.4
G2V +
K1V
Arcturus
Alpha Boo
34
-0.04
0.2
K1.5IIIp
Vega
Alpha Lyr
25
0.03
0.6
A0Va
Capella
Alpha Aur
41
0.08
0.4
G6III +
G2III
Rigel
Beta Ori
~1400
0.12
-8.1
B81ae
Procyon
Alpha CMi
11.4
0.38
2.6
F5IV-V
Achernar
Alpha Eri
69
0.46
-1.3
B3Vnp
Common
Name
Scientific
Name
Distance (light
years)
Apparent
Magnitude
Absolute
Magnitude
Spectral
Type
Betelgeuse
Alpha Ori
~1400
0.50 (var.)
-7.2
M2Iab
Hadar
Beta Cen
320
0.61 (var.)
-4.4
B1III
Acrux
Alpha Cru
510
0.76
-4.6
B0.5Iv +
B1Vn
Altair
Alpha Aql
16
0.77
2.3
A7Vn
Aldebaran
Alpha Tau
60
0.85 (var.)
-0.3
K5III
Antares
Alpha Sco
~520
0.96 (var.)
-5.2
M1.5Iab
Spica
Alpha Vir
220
0.98 (var.)
-3.2
B1V
Pollux
Beta Gem
40
1.14
0.7
K0IIIb
Fomalhaut
Alpha PsA
22
1.16
2.0
A3Va
Becrux
Beta Cru
460
1.25 (var.)
-4.7
B0.5III
Common
Name
Scientific
Name
Distance (light
years)
Apparent
Magnitude
Absolute
Magnitude
Spectral
Type
Deneb
Alpha Cyg
1500
1.25
-7.2
A2Ia
Regulus
Alpha Leo
69
1.35
-0.3
B7Vn
Adhara
Epsilon CMa
570
1.50
-4.8
B2II
Castor
Alpha Gem
49
1.57
0.5
A1V +
A2V
Gacrux
Gamma Cru
120
1.63 (var.)
-1.2
M3.5III
Shaula
Lambda Sco
330
1.63 (var.)
-3.5
B1.5IV
There are 88 recognized constellations, with their names
tracing as far back as Mesopotamia, 5000 years ago.
Currently, 14 men and women, 9 birds, two insects, 19
land animals, 10 water creatures, two centaurs, one
head of hair, a serpent, a dragon, a flying horse, a
river and 29 inanimate objects are represented in the
night sky (the total comes to more than 88 because
some constellations include more than one creature.
Star Groupings and Asterisms
Some of the more
familiar
"constellations" are
technically not
constellations at all.
For example, the
grouping of stars
known as the Big
Dipper is probably
familiar to most, but
it is not actually a
constellation. The Big
Dipper is part of a
larger grouping of
stars called the Big
Bear (Ursa Major)
that is a constellation
Constellations Are Not Physical Groupings
The apparent groupings of stars into constellations that we
see on the celestial sphere are not physical groupings. In
most cases the stars in constellations and asterisms are
each very different distances from us, and only appear to
be grouped because they lie in approximately the same
direction.
The Constellations of the Zodiac
The zodiac is an imaginary band 18 degrees wide and centered on the
ecliptic. The constellations that fall in the zodiac are called the 12
constellations of the zodiac.
The constellations of the zodiac are still of importance because
the planets, as well as the Sun and Moon, are all near or on the
ecliptic at any given time; thus, they are always found within one
of the zodiac constellations.
The 12 Constellations of the Zodiac
Aquarius, the water bearer
Aries, the ram
Cancer, the crab
Capricorn, the goat
Gemini, the twins
Leo, the lion
Libra, the scales
Pisces, the fish
Sagittarius, the archer
Scorpius, the scorpion
Taurus, the bull
Virgo, the virgin
When you look in a sky atlas, you might see diagrams like this:
This type of schematic draws the stars as different
sizes to represent different brightness
You might also notice that every star
on the chart is labeled
In addition, other things
besides stars are also
labeled on the chart
Common Names
Most of the brighter stars in the sky
have common names that are of
historical and mythological significance.
For example, the bright red star in the
shoulder region of the constellation
Orion (the Hunter) is called Betelgeuse,
which comes from Arabic and means
(roughly) "the armpit of the mighty one"
(see adjacent figure). The brightest
star in Orion is a blue-white star called
Rigel that is situated at the opposite
corner of the constellation from
Betelgeuse (adjacent figure).
The Bayer Naming System
One more systematic method is the
Bayer system, which names the
brighter stars by assigning a
constellation (using the Latin
possessive of the name) and a greek
letter (Alpha, Beta, Gamma, Delta,
Epsilon, . . .) in an approximate
order of decreasing brightness for
stars in the constellation. The
adjacent figure illustrates for
Orion. Betelgeuse is also called
Alpha-Orionis and Rigel is called
Beta Orionis in the Bayer system.
The ordering of stars by brightness in the classical Bayer
system is only approximate. For example, Rigel (Beta Orionis) is
actually slightly brighter than Betelgeuse (Alpha Orionis), and
Kappa Orionis is considerably brighter than the position of
Kappa in the Greek alphabet would suggest.
The brightest star
in the nighttime sky
is Sirius, which is in
the constellation
Canis Major and is
termed Alpha Canis
Majoris in the Bayer
naming system.
The Flamsteed Naming System
The Flamsteed naming system can in
principle be used to name any
number of stars. In this system one
uses the same Latin possessive of
the constellation name as in the
Bayer system, but the stars are
distinguished, not by their
brightness, but by their nearness to
the western edge of the
constellation by assigning an arabic
numeral. Thus, the closest star to
the western edge of the
constellation Cygnus is called 1-Cygni
in the Flamsteed system and 61Cygni denotes the star that is the
61st closest to the western edge.
Star Maps
To use star
maps
effectively, you
need to know
your latitude
and longitude on
the surface of
the Earth, and
the offset of
your timezone
from the
Greenwich
meridian.
The celestial sphere is an imaginary sphere of infinite
radius centred on the Earth, on which all celestial bodies
are assumed to be projected. This Earth-centred
Universe is, of course, not an accurate model of the real
Universe, so why introduce it?
First, it forms a convenient
pictorial representation of
the different directions of
astronomical objects, and
second, calculations involving
these directions can be
performed using some
formulae of spherical
trigonometry.
The celestial sphere is assumed to be fixed, so as the Earth rotates
the celestial sphere appears to rotate in the opposite direction once
per day. This apparent rotation of the celestial sphere presents us
with an obvious means of defining a coordinate system for the
surface of the celestial sphere - the extensions of the north pole
(NP) and south pole (SP) of the Earth intersect with the north
celestial pole (NCP) and the south celestial pole (SCP), respectively,
and the projection of the Earth's equator on the celestial sphere
defines the celestial equator (CE). The celestial sphere can then be
divided up into a grid in a similar manner to the way in which the
Earth is divided up into a grid of latitude and longitude.
Celestial Coordinate
System
Declination
The celestial equivalent
of latitude is called
declination and is
measured in degrees
North (positive
numbers) or South
(negative numbers) of
the Celestial Equator.
Right Ascension
The celestial equivalent of
longitude is called right
ascension. Right ascension
can be measured in
degrees, but for
historical reasons it is
more common to measure
it in time (hours, minutes,
seconds): the sky turns
360 degrees in 24 hours
and therefore it must
turn 15 degrees every
hour; thus, 1 hour of right
ascension is equivalent to
15 degrees of (apparent)
sky rotation.
Right ascension (RA; symbol α: Greek letter alpha) is the
astronomical term for one of the two coordinates of a point on the
celestial sphere when using the equatorial coordinate system. The
other coordinate is the declination. RA is comparable to longitude,
measured from a zero point known as the vernal equinox point. RA
is measured in hours, minutes, and seconds. Being closely tied with
sidereal time, it is both a unit of time and of angle. An hour of
right ascension is equal to 15 degrees of arc, a minute of right
ascension equal to 15 minutes of arc, and a second of right
ascension equal to 15 seconds of arc.
The apparent path of the Sun in the sky is known as the ecliptic
and is actually the intersection of the plane of the Earth's orbit
with the celestial sphere. Because the rotation axis of the Earth
(which defines the celestial sphere) is tilted at an angle (23.5°)
with respect to the plane of the Earth's orbit, the ecliptic is
inclined at an angle to the celestial equator.
The ecliptic and the equator intercept at two points, associated with
the zodiacal constellations of Aries and Libra.
spring equinox (or vernal equinox), on March 21
autumnal equinox, on September 21
The maximum altitude of the Sun in the sky, as viewed from the northern
hemisphere, gradually increases from the spring equinox until it reaches a
maximum on June 21 - the summer solstice (when the Sun appears to
`stand still' in the sky before starting to move back towards the celestial
equator)
The Sun reaches its minimum altitude in the sky when viewed from
the northern hemisphere on December 21 - the winter solstice which marks the beginning of northern hemisphere winter.
The Earth rotates from west to east and hence the
stars appear to revolve from east to west about the
celestial poles on circular paths parallel to the
celestial equator once per day. Some stars never set
and remain visible at night all year. These are called
circumpolar stars
upper culmination.
lower culmination
Which stars are
circumpolar depends
on the latitude of the
observer
REVIEW
Coordinates on the Celestial Sphere
The right ascension
(R.A.) and declination
(dec) of an object on the
celestial sphere specify
its position uniquely, just
as the latitude and
longitude of an object on
the Earth's surface
define a unique location.
Thus, for example, the
star Sirius has celestial
coordinates 6 hr 45 min
R.A. and -16 degrees 43
minutes declination, as
illustrated in the
following figure.
The Constellations of the Southern Hemisphere (some are
seasonally visible in the Northern Hemisphere):
Apus, the bird of paradise
Ara, the altar
Carina, the ship's keel
Centauras, the centaur
Chamaeleon, the chameleon
Circinus, the compass
Crux, the southern cross
Dorado, the swordfish
Eridanus, the river
Grus, the crane
Hydrus, the water snake
Indus, the Indian
Lepus, the rabbit
Mensa, the table
Musca, the fly
Norma, the surveyor's level
Octans, the octant
Pavo, the peacock
Phoenix, the phoenix
Pictor, the easel
Reticulum, the net
Triangulum Australe, the southern triangle
Tucana, the toucan
Vela, the ship's sails
Volans, the flying fish
The Constellations of the Northern Hemisphere (some are
seasonally visible in the Southern Hemisphere):
Andromeda, the princess
Antlia, the pump
Aquila, the eagle
Auriga, the chariot driver
Bootes, the herdsman
Caelum, the chisel
Camelopardalis, the giraffe
Canes Venatici, the hunting dogs
Canis Major, the big dog
Canis Minor, the little dog
Cassiopeia, the queen
Cepheus, the king
Cetus, the whale
Columba, the dove
Coma Berenices, Berenice's hair
Corona Australis, the southern crown
Corona Borealis, the northern crown
Corvus, the crow
Crater, the cup
Cygnus, the swan
Delphinus, the dolphin
Draco, the dragon
Equuleus, the little horse
Fornax, the furnace
Hercules, the hero
Horologium, the clock
Hydra, the water snake
Lacerta, the lizard
Leo Minor, the little lion
Lupus, the wolf
Lynx, the lynx
Lyra, the harp
Microscopium, the microscope
Monoceros, the unicorn
Ophiuchus, the sepent holder
Orion, the hunter
Pegasus, the flying horse
Perseus, the Medusa killer
Pisces Austrinus, the southern fish
Puppis, the ship's stern
Pyxis, the ship's compass
Sagitta, the arrow
Sculptor, the sculptor
Scutum, the shield
Serpens, the snake
Sextans, the sextant
Telescopium, the telescope
Triangulum, the triangle
Ursa Major, the big bear
Ursa Minor, the little bear
Vulpecula, the little fox
Constellations Sorted by Month
January
Caelum
Dorado
Mensa
Orion
Reticulum
Taurus
Bibliography
http://www.enchantedlearning.com/subjects/astronomy/
http://csep10.phys.utk.edu/astr161/lect/time/naming.html
http://www.astro.wisc.edu/~dolan/constellations/constellations.html
http://www.astro.columbia.edu/~archung/labs/fall2001/lec02_fall
01.html