ASTA01H3 Lecture Notes - Lecture 2: International Astronomical Union, Star Star, Sun-2
Chapter 2 — User’s Guide to the Sky: Patterns and
Cycles
Lecture 2
-Earth rotates on its axis once a day
Constellations
-all around the world, ancient cultures celebrated heroes, gods and mythical beasts by
naming groups of stars called constellations
-the constellations named within Western culture originated in Mesopotamia, Babylon,
Egypt and Greece beginning as many as 5000 years ago
•of these ancient constellations, 48 are still in use
•a constellation was simply a loose grouping of stars, many of the fainter stars were
not included in any constellation
-constellation boundaries, when they were defined at all, were only approximate
-in recent centuries, astronomers have added 40 modern constellations to fill gaps
-in 1982, the International Astronomical Union (IAU) established 88 official
constellations with clearly defined permanent boundaries that, together, cover the
entire sky
-a constellation now represents not a group of stars but a section of the sky - a
viewing direction
-any star within the region belongs to only that one constellation
Asterisms
-in addition to the 88 official constellations, the sky contains a number of less formally
defined groupings known as asterisms
-for example, the Big Dipper is an asterism that is part of the constellation Ursa
Major (the Great Bear)
-although constellations and asterisms are named as if they were real groupings, most
are made up of stars that are not physically associated with one another
!1
find more resources at oneclass.com
find more resources at oneclass.com
-some stars may be many times farther away than others in the same constellation
and moving through space in different directions
-the only thing they have in common is that they lie in approximately the same
direction from Earth
The Names of the Stars
-the names of the constellations are in Latin or Greek
-most individual star names derive from ancient Arabic, much altered over centuries
-another way to identify stars is to assign Greek letters to the bright stars in a
constellation in the approximate order of brightness
-thus, the brightest star is usually designated alpha (⍺), the second brightest beta
(β), etc
-for many constellations, the letters follow the order of brightness
-however, some constellations are exceptions
-a Greek-letter star name also includes the possessive form of the constellation name
-for example, the brightest star in the constellation Canis Major is alpha Canis
Majoris
-this name identifies the star and the constellation and gives a clue to the relative
brightness of the star
The Brightness of the Stars
-astronomers measure the brightness of stars using the magnitude scale
-the ancient astronomers divided the stars into six brightness groups
-the brightest were called first-magnitude stars
-the scale continued downward to sixth-magnitude stars — the faintest visible to the
human eye
-thus, the larger the magnitude number, the fainter the star
-star brightnesses expressed in this system are known as apparent visual
magnitudes (mv); these describe how the stars look to human eyes observing from
Earth
!2
find more resources at oneclass.com
find more resources at oneclass.com
-brightness depends on both the physiology of human eyes and the psychology of
perception
-to be scientifically accurate, you should refer to flux
-flux is the light energy from a star that hits one square metre of perpendicular area
in one second. Sometimes called ‘intensity’
-to measure the intensity of starlight: m1 — m2 = -2.5 log I1/I2
-thus, if two stars differ in their intensity, I, one hundred times, then they differ in
magnitude by 5 magn. units, denoted as 5m
-log (100)=2(decimal logarithm of 100)
-in order to form the ratio of fluxes (intensities), we need to have an object with
which to compare other objects
-limitations of the apparent visual magnitude system have motivated astronomers to
supplement it in various ways
-Limitation 1: some stars are so bright that the scale must extend into negative
numbers
-Limitation 2: with a telescope, you can find stars much fainter than the limit for your
unaided eyes
-Thus, the magnitude system has also been extended to include numbers larger
than sixth magnitude to include fainter stars
-Limitation 3: although some stars emit large amounts of infrared or ultraviolet light,
those types of radiation are invisible to human eyes
-the subscript ‘V’ in mv is a reminder that you are counting only light that is visible
-Limitation 4: an apparent magnitude informs you only how bright the star is as seen
from Earth
-it doesn’t reveal anything about a star’s true power output — the star’s distance
is not known
The Celestial Sphere
-modern astronomers know that the stars are scattered through space at different
distances
!3
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Chapter 2 user"s guide to the sky: patterns and. Earth rotates on its axis once a day. All around the world, ancient cultures celebrated heroes, gods and mythical beasts by naming groups of stars called constellations. The constellations named within western culture originated in mesopotamia, babylon, Constellation boundaries, when they were de ned at all, were only approximate. In recent centuries, astronomers have added 40 modern constellations to ll gaps. In 1982, the international astronomical union (iau) established 88 of cial constellations with clearly de ned permanent boundaries that, together, cover the entire sky. A constellation now represents not a group of stars but a section of the sky - a viewing direction. Any star within the region belongs to only that one constellation. In addition to the 88 of cial constellations, the sky contains a number of less formally de ned groupings known as asterisms.
