Lecture 2, ASTA01
Chapter 2
User’s Guide to the Sky:
Patterns and Cycles
• The night sky is the rest of the universe as
seen from our planet.
• When you look up at the stars, you look out
through a layer of air only about 100
kilometres deep (10 km most of it!)
• Beyond that, space is nearly empty – with the
planets of our solar system several AU away and
the far more distant stars scattered many lightyears apart.
• Keep in mind that you live on a planet, a
moving platform.
• Earth rotates on its axis once a day.
• So, from our viewpoint, sky objects appear to
rotate around us each day.
• For example, the Sun rises in the east and sets in the
west, and so do the stars.
• The Sun, the Moon, planets, stars, and galaxies all
have an apparent daily motion that is not real but is
caused by a real motion of Earth.
The Stars
• On a dark night, far from city lights, you
can see a few thousand stars.
• Your observations can be summarized by
naming individual stars and groups of stars
and by specifying their
relative brightness.
• All around the world, ancient cultures
celebrated heroes, gods, and mythical
beasts by naming groups of stars called
• 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
• In those former times, a constellation was
simply a loose grouping of bright stars.
• Many of the fainter stars were not included in
any constellation.
• Constellation boundaries, when they were defined at all,
were only approximate.
• A star like Alpheratz could be thought of as part of
Pegasus and also part of Andromeda.
• In recent centuries, astronomers have added
40 modern constellations to fill gaps.
• In 1928, 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
• Any star within the region belongs to only
that one constellation.
• 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 you
probably recognize that is part of the constellation
Ursa Major (the Great Bear)
• Another example known to all people of the
southern hemisphere: the Southern Cross.
• Another example known to all people of the
southern hemisphere: the Southern Cross.
• Another asterism is the Great Square of
Pegasus that includes three stars from
Pegasus and Alpheratz, now considered
to be part of Andromeda only.
• 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.
• 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
The Names of the Stars
The Names of the Stars
• The names of the constellations are in
Latin or Greek, the languages of science
in Medieval and Renaissance Europe.
The Names of the Stars
• Most individual star names derive from
ancient Arabic, much altered over
• The name of Betelgeuse, the bright red star in
Orion, comes from the Arabic phrase ‘yad
aljawza,’ meaning ‘armpit of Jawza (Orion).’
• Aldebaran, the bright red eye of Taurus the
bull, comes from the Arabic ‘aldabar an,’
meaning ‘the follower.’
The Names of the Stars
• Another way to identify stars is to assign Greek letters to the
bright stars in a constellation in the approximate order of
• Thus, the
brightest star is
alpha (α), the
second brightest
beta (β), etc.
The Names of the Stars
• For many constellations, the
letters follow the order of
• However, some
constellations are
The Names of the Stars
• 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
• This name identifies the star and the constellation
and gives a clue to the relative brightness of the
• Compare this with the ancient individual name for that
star, Sirius, which tells you nothing about its location or
The Brightness of the Stars
• Astronomers measure the brightness of stars using the
magnitude scale.
The Brightness of the Stars
• The ancient astronomers divided the stars
into six brightness groups.
• The brightest were called first-magnitude
• The scale continued downward to sixth-magnitude
stars – the faintest visible to the human eye.
Copyright © 2013 by Nelson Education Ltd.
The Brightness of the Stars
• Thus, the larger the magnitude number,
the fainter the star.
• This makes sense if you think of the bright
stars as first-class stars and the faintest stars
visible as sixth-class stars.
The Brightness of the Stars
The Greek astronomer Hipparchus (190–
120 BC) is believed to have compiled the
first star catalogue.
• There is evidence he used the magnitude
system in that catalogue.
About 300 years later (around 140 CE), the EgyptianGreek astronomer Claudius Ptolemy definitely used
the magnitude system in his own catalogue.
The Brightness of the Stars
• 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.
The Brightness of the Stars
• Brightness is quite subjective.
• It 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 we call it ‘intensity’.
The Brightness of the Stars
• With modern scientific instruments, you
can measure the intensity of starlight with
high precision and then use a simple
mathematical relationship that relates light
intensity to apparent visual magnitude.
• Mathematically: m 1  m 2   2 .5 log I1 / I 2
• Thus, e.g., if two stars differ in their intensity I
one hundred times, then they differ in
magnitude by 5 magn. units, denoted as 5m
That’s because log (100) = 2 (decimal logarithm of 100)
The Brightness of the Stars
In order to form the ratio of fluxes (intensities),
we need to have an object with which to
compare other objects. Such a standard
object with assigned 0th magnitude was the
star Vega (alpha Cygni, i.e. the brightest star
in constellation Cygnus).
A star 100 times dimmer than Vega is 5th
magnitude (5m), a star that is 10000 times
dimmer is 10th magnitude (10m), and so on.
The Brightness of the Stars
• Thus, precise modern measurements of the
brightness of stars are still connected to
observations of apparent visual magnitude that go
back to the time of Hipparchus.
• Limitations of the apparent visual magnitude
system have motivated astronomers to
supplement it in various ways.
The Brightness of the Stars
• Limitation 1: some stars are so bright that
the scale must extend into negative
• Sirius, the brightest star in the sky, has a
magnitude of –1.47.
The Brightness of the Stars
• 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.
The Brightness of the 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.
• Other magnitudes systems have been invented to
express the brightness of invisible light arriving at Earth
from the stars. For instance, IR means infrared
(wavelengths longer than visible red) and UV means
ultraviolet (wavelengths shorter than visible violet.
• Separate colors are often denoted: U B V R I
(ultraviolet, blue, visible yellow-green, red, near
The Brightness of the Stars
• Limitation 4: an apparent magnitude informs
you only how bright the star is as seen from
• It doesn’t reveal anything about a star’s true power
output – because the star’s distance is not known!
• There is an “absolute magnitude scale” where we
assign magnitudes that the object would have if
placed at a certain distance known as 10 parsecs
from us (parsec is yet another measure of length in
astronomy). That scale informs about the true
power emitted, rather than flux received by us. 35
The Sky and Its Motions
• The sky above you seems to be a great
blue dome in the daytime and a sparkling
ceiling at night.
• Learning to understand the sky requires that
you first recall the perspectives of people who
observed the sky thousands of years ago.
The Celestial Sphere
• Ancient astronomers believed the sky was
a great sphere surrounding Earth, with the
stars stuck on the
inside – like thumbtacks
in a ceiling.
The Celestial Sphere
• Modern astronomers know that the stars
are scattered through space at different
• However, it is still
convenient to think of
the sky as a great
sphere enclosing Earth
with stars all at one
The Celestial Sphere
• The celestial sphere is an example of a
scientific model, a common feature of
scientific thought.
The Celestial Sphere
• As you study the sky, you will notice three
important points.
The Celestial Sphere
• One, sky objects appear to rotate
westward around Earth each day, but that
is a consequence of Earth’s eastward
• This produces day and night – the Sun during
1 day follows almost exactly the motion of
The N celestial hemisphere rotates around Polaris
The Celestial Sphere
• Two, what you can see of the sky depends on
where you are on Earth.
• For example, Australians see many constellations
and asterisms invisible from North America (like
Southern Cross), but they never see the Big Dipper.
The Celestial Sphere
• Three, astronomers measure distances across the sky as
angles. These are expressed in units of degrees and
subdivisions of degrees called arcminutes and arcseconds.
• One degree of arc = 60 arcminutes
• 1o = 60’
• One arcminute = 60 arcseconds
• 1’ = 60’’
• Therefore, 1 degree = 3600 arcsec.

The Scale of the Cosmos