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Home , Celestial equator, Johann Bayer, Twinkling

ASTR 101

The and the Sky

August 29, 2018

• Sky and the atmosphere • Twinkling of • The • • brightness and the magnitude system • Naming of stars Scattering of light and color of the sky

Sun light

more less scattering scattering air particle

• During the we cannot see stars due to the glare in the atmosphere (from sunlight). • When sunlight travels through the Earth’s atmosphere some of the light is scattered (deflected) off air molecules. • White light is a combination of different colors, blue light is scattered far more than red light. Blue Sky during the day

Sun

Earth atmosphere

• During the day we see this scattered sunlight in the atmosphere. • Since most of it is blue, a lot of blue light is seen in the atmosphere in all directions, which makes the sky look blue. • Light from stars are fainter than scattered sunlight in the atmosphere, so we cannot see them (unless a star happen to be very bright) In the sky is always dark

• Away from the atmosphere (in outer space, on …) sky is always dark. • It is possible to see stars and the sun at the same time. The Sun looks red at Sunrise/Sunset

Sun

atmosphere Earth

• At sunrise and sunset, Sun is close to the . Sunlight has to travel a longer distance in the atmosphere to reach us. • By the time sunlight reaches ground most of the blue light has scattered off. Only red light remains. • So the Sun (moon…) looks red at sunrise or sunset. • If the sky is cloudy, smoggy or there is a lot of dust particles in air they may reflect the sunlight and that part of the sky also looks red. Twinkling of Stars (stellar scintillation)

• When starlight travels through the atmosphere light refracts slightly (changes direction) as it passes through air layers of different densities. • When the atmosphere is turbulent this refraction is not uniform or steady, it changes from moment to moment, changing the direction of starlight all the time. • Since angular size of stars are extremely small (less than 0.01”) even a tiny change in the direction is noticeable, resulting “twinkling” of stars. • When a star is low in the sky, light has to travel a longer distance through the atmosphere. – Stars closer to the horizon appear to twinkle more than stars that are overhead Twinkling

View of a star through a telescope in poor seeing conditions

• Twinkling is undesirable for astronomy. For astronomical observations, a steady atmosphere with good “seeing” is needed.

• Major Astronomical observatories are located in higher elevations (mountain tops). Twinkling

• Planets have larger angular sizes. • So a very small changes in the position of a planet in the sky is not so noticeable, they twinkle less. – This is one way to distinguish a planet from a star.

• But in poor seeing conditions planets also twinkle, especially when they are low in the sky. The Earth and the Sky

• From a location on ground, the Earth looks flat – because we see only a tiny part of the vast spherical surface of the Earth • At a given time and location we see half of the sky, the sky directly above that location. • View of the other half is blocked by earth. Earth spins on its axis (west to east) and goes around the sun Celestial objects (planets, stars, galaxies…) are located at different distances Just as someone in a boat sailing forward sees unmoving [objects] going backward , on the earth, sees the unmoving stars going uniformly westward… -Aryabhata, ca 500CE https://en.wikipedia.org/wiki/Aryabhata

• On earth we do not feel its motion or . • Instead it appears that the whole universe (sky), sun, moon, stars… goes around the Earth east to west. • All celestial objects appears to be located at the same distance, like located inside a giant sphere centered at Earth. Just as someone in a boat sailing forward sees unmoving [objects] going backward , on the earth, sees the unmoving stars going uniformly westward… -Aryabhata, ca 500CE https://en.wikipedia.org/wiki/Aryabhata

• But on earth we do not feel its motion or rotation. • Instead it appears that the whole universe, sun, moon, stars… goes around the Earth east to west. • All celestial objects appear to be located at the same distance, like they are on a giant sphere centered at Earth. The Celestial Sphere

www.phy.olemiss.edu/~perera/animation_cele/celes_sp.gif

• This apparent sphere around us with all celestial objects on it, is called the Celestial Sphere. • We know this is an illusion: – different celestial objects are at different distances from us. – it is the earth that rotates, not the sky • Nevertheless considering sky as a sphere centered at earth is a useful concept for observational purposes. The Earth and the Sky

N W E

S nadir

• At a given location and time only half of the celestial sphere (sky) is visible, view of the other half is blocked by the earth. • On a typical moonless clear , about 3000 stars are visible from a dark location. – Zenith: the point directly above in the sky at a given time and location. – Nadir: Point in the sky directly below. It cannot be seen because it is on the other side of the earth. – Celestial : The imaginary circle passing through the North and South points on the horizon and through the zenith. – Celestial object has the highest elevation (altitude) when it cross the meridian. Daily Motion of the Sky

transit

N rising E

setting celestial meridian W S

• All celestial objects appears to rise in the East, moves across the sky and sets in the West. • They have the highest elevation above horizon (altitude) when crossing the meridian (also called transit) Celestial Poles and

North

Celestial equator

South celestial pole

• The Earth’s rotation axis extended to the sky (to the celestial sphere) meets the celestial sphere at two locations. – They are called celestial poles. • Celestial equator is the great circle on the celestial sphere in between celestial poles. – the imaginary circle in the sky directly above the Earth's equator. 16 +

North Celestial pole – the North Star Long exposure image of the north pole region Star trails over the Kennon Observatory • Earth’s axis always points to the same point in the sky regardless the rotation or the orbital movement. • locations of celestial poles in the sky are fixed • the whole sky appears to rotate about celestial poles. • Since celestial poles don’t move, so do any objects located at the poles. • There happen to be a bright star very close to the north pole: The north star ( Polaris). • It appears to stay fixed in the sky while other stars move around it. Stars near the north celestial pole

• There isn’t any bright star close to the South celestial pole. • So there is no “south star”.

Stars near the Magellanic south celestial pole clouds zenith 90⁰ Sky Coordinates

altitude

N

W 270⁰ E 90⁰

S 180⁰

• Simplest way to give the location of a celestial object in the sky is to give its apparent position in the sky – Altitude: vertical angular elevation above the horizon – Azimuth: direction from North to the object: angle measured along the horizon towards the east along the horizon. • But the apparent position of a celestial object changes with time and location. – Not the best way to identify an object. Logitude and

• Position of a place on the Earth can be given by its longitude and latitude. – Latitude: angular distance from the equator. – Longitude: angular separation of the meridian through the location and the reference meridian trough Greenwich. • A similar mechanism can be used to give the position of a celestial object on the celestial sphere (sky). N and celestial north pole

Vernal (first point of ) declination

celestial equator right ascension

reference median

• Position of an object on the celestial sphere can be specified similarly. – When refer to locations in the sky they are called declination and right ascension. • Declination: angular distance to an object from the celestial equator. • Right ascension: angle between the celestial meridian through an object and a reference median. Constellations

• View of the southern sky this evening from Oxford MS. – Stars, Saturn, • Ancient observers imagined groups of brighter stars as patterns in the sky, called constellations. – Representing mythological characters, heroes, animals, royalty whatever important or fascinated them. • Every culture saw patterns in the sky and attributed legends and beliefs according to their culture. • Northern constellations in use now date back to Babylonian times, 4-5 millennia ago. Later Greeks adopted them and named according to their mythology. Western (Greek) Ancient Egyptian Ancient Chinese Constellations in the same part of the sky according to Greeks ,Egyptians and Chinese

• From the planetarium software : Stellarium – Available from (free): www.stellarium.org • Also demo version of Skygazer planetarium software is available form http://www.carinasoft.com/downloads.html#VdemoAnchor Examples of few figures

• All above are constellations – the narrow band in the sky where sun, moon and planets are visible. • Their origin dates backs to ancient Sumerians and Babylonian times Modern Constellations

constellation boundaries

• 16th-19th century astronomers added many new constellations to fill in the gaps between classical constellations and cover uncharted regions of the southern sky. • In 1922, the International Astronomical Union (IAU) adopted 88 constellations as internationally accepted constellations. – That included the 48 classical constellations, 32 southern constellations and few other constellations formed since the 16th century. – 88 constellations divide the sky in to 88 regions according to boundaries defined by the IAU. distances to stars in the Big Dipper.

• Constellations are not physical objects: – Group of stars just happened to be in the same direction in the sky. – No physical relationship between stars in a constellation • distances between stars could be hundreds of light years Antique star maps/

Albrecht Dürer, 1515, Germany Alexander Jamieson, early 19th Century • Ancient star maps were vividly decorated with constellation figures – Stars were identified from their locations in the constellations. • Today constellations are used as mnemonics, an easy way to remember and identify different regions of the sky. They play less important role in professional astronomy. • Instead star catalogs and databases play a major role. • One of the largest catalog, Hubble guide star catalog has over 945 million stars down to magnitude 21. Asterisms

Sickle (in )

Tea pot (in Sagittarius)

Orion’s belt

W in Cassiopeia

A distinctive pattern of stars, but often not an official constellation. examples: Big dipper, Tea pot , belt of , Pleiades, W in Cassiopeia, Delphinus, southern cross Star brightness: magnitude system • Stars have different apparent brightness. • In astronomy apparent brightness of a star (or other celestial object) is given by a number called the magnitude. • It is a backward scale, smaller the magnitude number brighter the star and vice versa. • Faintest stars can be seen with the naked eye are about magnitude 6

700

600

500

• It is a nonlinear scale: 400 Star brightness Star • For each magnitude difference, actual 300 brightness changes by a factor of about 2.5 200

100

0 0 1 2 3 4 5 6 7 Star magnitude magnitude 2 2.5 x brighter magnitude 3 2.5 x2.5 = 6.25 x brighter 2.5 x brighter magnitude 4

31 • 5 magnitudes difference corresponds to 100 times in actual brightness • When the object is too bright (like Venus, Sun, Moon) its magnitude could be negative. • Examples: – Faintest object (galaxy) in Hubble extreme deep field: magnitude 31 – Faintest object observable with a 6 inch aperture telescope: 13 – Faintest object observable with a typical binocular magnitude: 9 – Faintest stars visible to naked eye: magnitude 6 – North star 2 – Mars these days 1.1, – faintest stars visible near the physics department: magnitude < 1 – Betelgeuse: 0.4 – Vega: magnitude 0 – reference star brighter – (brightest star in the sky): -1.3 – Venus these days: –4.7, – Full Moon -13 – Sun -27

32 Hipparchus’ magnitude system

• The reason for this seemingly awkward system is in its’ history. • Greek astronomer Hipparchus prepared a detailed star catalog in 129 BCE. – It was the most accurate star catalog of the antiquity. – He wanted to record the brightness of a star as well as its position in the sky

• He called the brightest stars in the sky 1st class stars • the next brightest stars 2nd class stars, and so on • until the 6th class, which were the faintest stars he could see.

• Hipparchus’ star classification became popular with later astronomers and they kept using his system of 1st to 6th (magnitude) for almost two millennia. • With a telescope stars fainter than 6th magnitude are visible – so after the invention of the telescope in 17th century, there was a need to extend the magnitude scale. • 19th century astronomers refined and quantified the magnitude system to the modern form, while keeping most of the features of Hipparchus original ‘backward’ system. 33 • Response of the human eye is not linear, but logarithmic

actual brightness

• Thus the star brightness classification scale which was originally based on observations by eye has this nonlinear logarithmic nature Naming of Stars

A page from the star catalog in Ptolemy’s Almagest (2nd century CE)

• Most bright stars have proper names, like Polaris, Sirius, Rigel or Antares. – Most star names are of Arabic origin few are Greek • During the middle ages Arabic astronomers adopted Greek constellations. – Greek astronomers identified stars by their location in constellations. – Arabic astronomers followed the same in Arabic. 35 Naming of Stars

When the Arabic texts were translated to Latin in late Medieval period, those descriptions of star positions in Arabic were directly passed down to the west:

– Often in garbled Arabic with translation errors and misplaced meanings.

• and became the star names in use today.

• : armpit (of Orion in Arabic) • Rigel: left foot (of Orion) • Deneb: tail of Swan • Denebola: Lions tail

A page from Johann Bayer’s star , (1603 CA). 36 Algol Polaris

Arcturus Deneb Vega

Altair

Antares Fomalhaut

• Few bright stars in the evening sky these days Polaris

Capella Regulus Pollux Castor

Procyon Betelgeuse

Rigel Sirius

• Few bright stars in the morning sky these days Naming of Stars

• In 1603, the German astronomer Johann Bayer produced a star atlas, Uranometria. – There he assigned each star in a constellation a Greek letter, approximately in the order of their apparent brightness. • Bayer naming scheme is still in use for most of the bright stars – ex: : star a in the constellation Centaurus • If the constellation has more than 24 stars, Greek letters followed by upper and lower case Latin letters. – a, b, g... w, A, b, c, … z • In addition index numbers from many other catalogs are used - HD, SAO… same star: Betelgeuse, α Orionis, 58 Orionis, HD39801, SAO 113271, HIP27989 39 Motion of stars and shape of constellations over time.

A star moving 100 km/s travels 3.2x109km in a year

100 ly=9.5x1014 km

• Stars appear to have fixed positions in the sky while whole sky rotates with them. • Constellation patterns have not changed over millennia

• Hence the name “fixed stars” given by ancient astronomers 0.7” • Stars do move in space with considerable speeds (e.g. the Sun moves 120 km/s in the Milky way) • But distances to them are so large, change in their angular positions too small for us to see. • A star moving 100 km/s travels 3.2 billion km in a year • If it is 100 light years (950 trillion km) away its position in the sky changes only by 0.7 arc-second. 100,000 years ago

now

Shape of the Big Dipper over time in 100,000 years Animation: www.phy.olemiss.edu/~perera/animations/um.gif Change of the shape of Big Dipper due to motion of stars.

• But over long time spans (many thousands of years), there could be noticeable changes. Review Questions • What would be the color of the sky few thousand kilometers above the Earth. • It is often said that stars twinkle but planets won’t. Do you agree? • On hot summer days often you see the view closer to the surface of a paved road blurry and wavy. Why? • Twinkling is not desirable for astronomical observations since it blurs astronomical images. What could astronomers do to overcome that? • Why giving the location of altitude and azimuth is not the best way to specify the location of an object in the sky. • What is the declination of a celestial object if it is located – (a) on the celestial equator – (b) on the North celestial pole – (c) on the South celestial pole – (d) if sometimes it is directly overhead (at the Zenith) from Oxford MS. • Why does the North Star appear to be fixed in the sky while all other object move? • Why haven’t constellation patters changed since they have been established 3-4 thousand years ago. • How many constellation are there according to modern convention? • Name the three brightest celestial objects in the ? • Why isn’t the star magnitude scale linear? • What magnitude are the faintest visible stars to naked eye? • Why are most star names Arabic while constellation names are Latin (or Greek) origin?

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