Lecture 3 – The Sky Constellations, Star Names, and Magnitudes

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The Constellations Origins Origins Star patterns named by ancient peoples after gods, goddesses, animals, monsters, and mythic heroes. Star patterns named by ancient peoples after gods, goddesses, animals, monsters, and mythic heroes. The constellations used by Western Culture today originated in Mesopotamia around 3,000 B.C. The constellations used by Western Culture today originated in Mesopotamia around 3,000 B.C.

The Constellations Only certain stars were part of the pattern and belonged to the constellation. Faint stars were not part of any constellations. Frequently constellations were named for their brightest star. Of the constellations defined by the Babylonians, Egyptians, and Greeks, 48 are still used today.

The Constellations The ancient civilizations we got the 48 classical constellations from couldn’t see the entire southern sky. During the Age of Sail, European explorers added 40 modern constellations in the “uncharted” areas of the sky.

The Constellations The 48 classical and the 40 modern constellations make up the 88 official constellations used by astronomers today. The 48 classical and the 40 modern constellations make up the 88 official constellations used by astronomers today. The term constellation also now has a new meaning: The term constellation also now has a new meaning: No longer refers to the pattern of stars itself. No longer refers to the pattern of stars itself. Now refers to a well defined region of the sky that contains the traditional star pattern. Now refers to a well defined region of the sky that contains the traditional star pattern. Everything inside that region of the sky is now part of the constellation, like a “celestial state”. Everything inside that region of the sky is now part of the constellation, like a “celestial state”.

The Constellations

What Is An Asterism? In addition to the 88 official constellations there are several unofficial but popular star patterns. In addition to the 88 official constellations there are several unofficial but popular star patterns. Called asterisms. Called asterisms. Examples: Examples: The Big Dipper The Big Dipper The Little Dipper The Little Dipper The Northern Cross The Northern Cross The Great Square The Great Square

Daily Grade 3 – Question 1 1.Which statement below most accurately describes modern constellations? A.They are 88 well defined regions in the sky. B.They are 88 connect-the-dot mythological sky figures. C.They are 13 connect-the-dot mythological sky figures along the ecliptic. D.They are 13 well defined sky regions along the ecliptic.

3D Constellations The stars of a constellation only appear to be close to one another The stars of a constellation only appear to be close to one another Usually, this is only a projection effect. Usually, this is only a projection effect. The stars of a constellation may be located at very different distances from us. The stars of a constellation may be located at very different distances from us.

The Celestial Sphere Many ancient cultures, including the Greeks and Romans, assumed that the Earth was stationary in the center of the Universe. Many ancient cultures, including the Greeks and Romans, assumed that the Earth was stationary in the center of the Universe. Given this assumption, they concluded that the Sun, Moon, planets, and stars were attached to gigantic crystalline spheres that surrounded the Earth. Given this assumption, they concluded that the Sun, Moon, planets, and stars were attached to gigantic crystalline spheres that surrounded the Earth. Today we call this imaginary globe the Celestial Sphere. Today we call this imaginary globe the Celestial Sphere.

Celestial Sphere Concepts THE CELESTIAL SPHERE REPRESENTS THE APPARENT VIEW OF THE UNIVERSE AS SEEN FROM THE EARTH. A huge globe of stars surrounding the Earth. A huge globe of stars surrounding the Earth. An ancient concept of the Cosmos. An ancient concept of the Cosmos. IT IS A USEFUL CONCEPT TODAY BECAUSE: it is understandable in terms of spherical geometry. it is understandable in terms of spherical geometry. it can be used to extend map concepts for the Earth to the sky. it can be used to extend map concepts for the Earth to the sky. it is useful in navigation. it is useful in navigation. RELATIONSHIP TO THE VISIBLE SKY The visible sky is 1/2 the celestial sphere. The visible sky is 1/2 the celestial sphere. Half of the celestial sphere is above the horizon, while the other half is below the horizon. Half of the celestial sphere is above the horizon, while the other half is below the horizon.

Zenith = Point on the celestial sphere directly overhead Nadir = Point on the c.s. directly underneath (not visible!) Celestial equator = projection of Earth’s equator onto the c. s. North celestial pole = projection of Earth’s north pole onto the c. s. The Celestial Sphere

Angular Measure In The Sky 360 o = A COMPLETE CIRCLE 1 o = 60’ (minutes) 1’ = 60” (seconds)

Latitude Affects Visible Sky What we see in the sky depends on our latitude. What we see in the sky depends on our latitude. At the north pole, the north celestial pole is at the zenith. At the north pole, the north celestial pole is at the zenith. At the equator, the north celestial pole is at the horizon. At the equator, the north celestial pole is at the horizon. The altitude of the north celestial pole = latitude. The altitude of the north celestial pole = latitude.

The Sky From Abilene – 32.4 o N North Celestial Pole is 32.4 o above the northern horizon. North Celestial Pole is 32.4 o above the northern horizon. Celestial Equator is 57.6 o above the southern horizon. Celestial Equator is 57.6 o above the southern horizon. Circumpolar stars lie within 32.4 o of the North Celestial Pole. Circumpolar stars lie within 32.4 o of the North Celestial Pole. Rising & Setting stars are visible from 32.4 o away from the South Celestial Pole. Rising & Setting stars are visible from 32.4 o away from the South Celestial Pole. Stars within 32.4 o of the South Celestial Pole are never visible from Abilene. Stars within 32.4 o of the South Celestial Pole are never visible from Abilene.

Looking North The stars appear to circle around the North Celestial Pole (near Polaris) once in 24 hours. Stars within the angle of the observer's latitude away from the North Celestial Pole never rise or set. They are always above the horizon (circumpolar stars).

Circumpolar Constellations Never Rise Or Set Are Always Above The Horizon Circumpolar Region Depends On Latitude

Daily Grade 3 – Question 2 2.At what location on Earth is an observer that has the South Celestial Pole directly overhead? A.At Earth's equator (0 degrees latitude). B.At Earth's North Pole (90 degrees North latitude). C.At Earth's South Pole (90 degrees South latitude). D.At 45 degrees North latitude.

Naming The Stars The brightest stars have had proper names for thousands of years. The brightest stars have had proper names for thousands of years. Typically from Arabic Typically from Arabic Islamic astronomers produced many detailed star charts during the middle ages. Islamic astronomers produced many detailed star charts during the middle ages. Also many stars have names that are from the Greek Also many stars have names that are from the Greek Greek astronomers also produced many star charts. Greek astronomers also produced many star charts.

Naming the Stars Proper names are often a literal description of the star’s location in the constellation: Proper names are often a literal description of the star’s location in the constellation: Betelgeuse – “Shoulder of the giant” Betelgeuse – “Shoulder of the giant” Rigel – “Foot” Rigel – “Foot” Deneb – “Tail” Deneb – “Tail” Procyon – “Before the dog” Procyon – “Before the dog” Algol – “Eye of the ghoul” Algol – “Eye of the ghoul” Star proper names also sometimes describe the star. Star proper names also sometimes describe the star. Sirius – “Scorching” Sirius – “Scorching” Antares – “Rival of Mars” Antares – “Rival of Mars” Kochab – “Star” Kochab – “Star”

Scam Alert! NO organization has been given official legal power to name the stars. NO organization has been given official legal power to name the stars. Astronomers consider the names designated by the International Astronomical Union official, but the IAU has no true legal authority to name stars (or demote planets, for that matter). Astronomers consider the names designated by the International Astronomical Union official, but the IAU has no true legal authority to name stars (or demote planets, for that matter). Companies that sell star names as gifts are a SCAM! Companies that sell star names as gifts are a SCAM!

Bayer Letter Names Johann Bayer Johann Bayer Bavararian lawyer Bavararian lawyer Published a sky atlas: Uranometria. Published a sky atlas: Uranometria. Assigned lower case greek letters to brighter stars in each constellation. Assigned lower case greek letters to brighter stars in each constellation. Usually assigned in rough order of brightness. Usually assigned in rough order of brightness. Alpha = brightest star. Alpha = brightest star. Beta = second brightest Beta = second brightest Gamma = third brightest Gamma = third brightest And so on… And so on… Astronomers continue to use these “Bayer letters” Astronomers continue to use these “Bayer letters”

Bayer Letter Names A star’s Bayer Letter Name is: A star’s Bayer Letter Name is: It’s greek letter first… It’s greek letter first… …followed by the possessive form of the constellation name. …followed by the possessive form of the constellation name. Example: The star Rigel Kentarus is the brightest star in the constellation Centarus: Example: The star Rigel Kentarus is the brightest star in the constellation Centarus: Alpha Centauri Alpha Centauri

Daily Grade 3 – Question 3 3.What is the most likely Greek letter name of the second brightest star in the constellation Lyra? A.Alpha Lyrae. B.Beta Lyrae. C.Gamma Lyrae. D.Delta Lyrae.

The Magnitude Scale Invented by the Greek Astronomer Hipparchus in the 2 nd century B.C. Invented by the Greek Astronomer Hipparchus in the 2 nd century B.C. Called the brightest stars in the night sky “stars of the first magnitude.” Called the brightest stars in the night sky “stars of the first magnitude.” Second brightest were “stars of the second magnitude.” Second brightest were “stars of the second magnitude.” All the way down to “stars of the sixth magnitude.” All the way down to “stars of the sixth magnitude.”

The Magnitude Scale Very useful system, but not perfect Very useful system, but not perfect Technically, it refers to Apparent Visual Magnitude Technically, it refers to Apparent Visual Magnitude Brightness in visible wavelengths as seen from Earth Brightness in visible wavelengths as seen from Earth Does NOT tell how bright the star actually is (absolute magnitude) because it does not take into account how far away the star is from us. Does NOT tell how bright the star actually is (absolute magnitude) because it does not take into account how far away the star is from us. Still useful because it is related to the brightness that you “see” (even through a telescope). Still useful because it is related to the brightness that you “see” (even through a telescope).

The Magnitude Scale Used for centuries, and then refined by modern astronomers. Used for centuries, and then refined by modern astronomers. 1 st magnitude stars are 100 times brighter than 6 th magnitude stars. 1 st magnitude stars are 100 times brighter than 6 th magnitude stars. So 5 “steps” in the magnitude scale corresponds to a 100× difference in brightness. So 5 “steps” in the magnitude scale corresponds to a 100× difference in brightness. What brightness difference corresponds to 1 step in magnitude? What brightness difference corresponds to 1 step in magnitude? Need to know what number you have to multiply by itself five times to get 100. Need to know what number you have to multiply by itself five times to get 100. That number is …. (usually rounded to when used in calculations). That number is …. (usually rounded to when used in calculations). So 1 step in magnitude corresponds to a 2.512× difference in brightness. So 1 step in magnitude corresponds to a 2.512× difference in brightness.

The Magnitude Scale If Star A is has a magnitude this much less than Star B… …then Star A is this many times brighter than Star B × × × × 5100×

Sirius (brightest star in the sky): m v = Full moon: m v = Sun: m v = The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): The Magnitude Scale

Here’s the actual equation: Here’s the actual equation: I A /I B = (2.512) (m B -m A ) This number right here is “how bright star A is compared to star B” If it is >1, then star A is brighter than star B. If it is <1, then star A is fainter than star B. This thing in parentheses here is an exponent. So say m B – m A is a number we’ll call X. You would read this equation as “2.512 raised to the X power.”

Daily Grade 3 – Question 4 4.The apparent visual magnitude of star A is 2 and the apparent visual magnitude of star B is 1. Based on this information which statement below must be true? A.Star A emits more light than star B. B.Star B emits more light than star A. C.Star A is closer than star B. D.Light output and distance cannot be determined from a star's apparent visual magnitude alone.

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