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THE SKY

(Part 1)

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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.

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It’s all about light

THE SKYWhen you look up to the stars, you are looking out through a layer of air a little more than a hundred kilometers deep. Beyond that, space is nearly empty and the stars are light years apart.

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On a dark night far from city lights, you can see a few thousand stars. The ancients organized what they saw by naming stars and constellation.

Ancient cultures celebrated gods and heroes by giving names to groups of stars, Constellations.

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Constellations

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

• 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).

Different cultures named groups of stars differently. Orion was known as Al Jabar to ancient Syrians and the White Tiger to the Chinese.

The Pawnee Indians knew the constellation Scorpius as two groupings; the long tail was a snake, and the two bright stars at end were swimming ducks.

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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.

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Constellations

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

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Star Chart

• http://www.telescope.com/assets/pdf/starcharts/2015-2-starchart_bw.pdf

• HOMEWORK: LOCATE ORION IN THE SKY!– Due by Monday

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Asterisms

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

Pegasus.

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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.

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Names of Stars

• The “ancients” gave names to the brightest stars.– Instead of Latin (translated from Greek), star

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

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

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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.

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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.

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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.

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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.

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Brightness of Stars

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

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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.

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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.

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Luminosity and Temperature

• Luminosity – Refers to the total amount of energy that a star emits in terms of the sun’s luminosity (our sun is the reference point), which is 1.0.

• Temperature refers to the temperature of its surface.

• The temperature and the luminosity of a star are used to group stars according to type.

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Sequence of Stars

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Sequence of Stars (not all types included)

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Main Sequence Stars Measurements (our sun as reference)

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Neutron Stars

• Neutron star – contains about 1 solar mass compressed into a radius of about 10 km. – It is left behind by a supernova – the death of a

large, highly luminous star. – Called a neutron star because of its density which

can only be stable as a “fluid of neutrons.” – Magnetic field a trillion times stronger than

Earth’s – Spins multiple times per second.

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White Dwarfs

• A white dwarf has about the same mass but is about the size of Earth.

• White dwarfs are thought to be the final evolutionary state of all stars whose mass is not high enough to become a neutron star (including our Sun).– Includes 97% of stars in Milky Way

• Considered a stellar “remnant.”

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Death of Our Sun

• The sun is becoming more luminous as it fuses hydrogen into helium (the early sun was less luminous than today’s). – It will continue to get more luminous, which will

eventually vaporize Earth’s oceans. – It has 5 billion years left before it exhausts its hydrogen.

– Once out of hydrogen it will fuse helium.

• Eventually as it runs out of fuel, it will become a red giant before collapsing into a white dwarf.

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