THE SKY

CHAPTER 2 (Part 1)

Objectives

• To be able to interpret and apply the term “brightness” to stars.

• To be able to describe how the sky moves with reference to the Earth’s rotation.

• To be able to predict the seasons and describe what causes them.

• To be able to synthesize information on astronomical cycles to predict Earth’s climate.

Constellations

• Are simply groups of stars that have been given a name and specific boundries.

• Stem from “ancient (on a human timeline)” cultures.

• They were a way of celebrating heroes and gods. • And they were also practical – utilized for

navigation (ex. Southern Cross) and agriculture (creation of almanacs).

Constellations

• Ursa Major was a constellation which was shared by various cultures in Europe, Asia, and North America. – The conjecture is that the “concept of the celestial

bear” might have crossed the Bearing Land Bridge during the last ice age.

• At the current time, there are 88 official, by International Astronomical Union, constellations with defined boundaries.

Constellations

• Due to its defined boundaries, each constellation represents an area of the sky and includes all of the stars within that region.

Asterisms

• Less formally defined groupings of stars.– Examples. The Big Dipper and the Great Square of

Pegasus.

Constellations and Asterisms

• While the stars appear close together in the sky, in reality they are not physically associated with one another.

• They may even be moving through space in different directions.

• Just lie in the same approximate direction from Earth.

Names of Stars• The “ancients” gave names to the brightest

stars.– Instead of Latin (translated from Greek), star

names often come from ancient Arabic.– During the European Dark Ages, the seat of

knowledge moved east to Arabia and Persia. – Names such as Sirius, Aldebaran, and Betelguese.

Identifying Stars by Brightness• Names are a clumsy way to ID stars – there are

too many to remember.• So, assign letters to the bright stars in a

constellation in order of “brightness.”• The Greek alphabet is used to rank the stars of

a constellation in terms of brightness.– Brightest is Alpha (sometimes the Greek letter is

used), second brightest is Beta, etc.– There are exceptions to this rule.

Identifying Stars by Brightness• Example: Canis Major – it is identified as alpha Canis

Majoris.

• This IDs the star and constellation and its relative brightness.

• Formerly called Sirius, which tells nothing of location or brightness.

Book’s Favorite Stars

• The book lists 25 “favorite stars” in Figure 2-5 from the book.

Southern View, in north it is found in far south in spring

Brightness of Stars

• Describing brightness: astronomers use “magnitude scale.”– First appeared in writings of Ptolemy (AD 140),

though may have been pilfered from Hipparchus (BC 190-120).

– Ptolemy used magnitude system in his star catalog, setting tone for other astronomers who have used the same system.

Brightness of Stars

• Divided into six classes – estimating brightness.

• Brightest stars were designated “first-magnitude” stars; fainter stars second-magnitude; and so-on-and-so-forth.

• Sixth-magnitude – the faintest to the human eye.

Brightness of Stars

• Modernity has led us to very precise measures of brightness.

Brightness of Stars – Apparent Visual Magnitudes

• A few stars are so bright, the modern magnitude scale extends into negative numbers.

• Sirius, the brightest star in the sky, actually has a magnitude that is -1.47.

• The faintest stars we can see with the naked eye are about sixth magnitude.

• Telescopes allow us to see fainter stars.

Apparent Visual Magnitudes (APV)

• Describes how the stars look to human eyes observing from earth (indicated as mv).

• Visible light only, does not take into account infrared or UV.

• The subscript V stands for visible. • APV does not take into account distance to the

stars, telling us only how bright the star looks seen from Earth.