Chapter 2 User’s Guide to the Sky: Patterns and Cycles 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 light-years 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.

Constellations All around the world, ancient cultures celebrated heroes, gods, and mythical beasts by naming groups of stars called constellations.

Constellations 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 use.

Constellations 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.

Constellations 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.

Constellations In recent centuries, astronomers have added 40 modern constellations to fill gaps.

Constellations 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 direction. Any star within the region belongs to only that one constellation.

Asterisms 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)

Asterisms Another example known to all people of the southern hemisphere: the Southern Cross.

Asterisms Another example known to all people of the southern hemisphere: the Southern Cross.

Asterisms Another asterism is the Great Square of Pegasus that includes three stars from Pegasus and Alpheratz, now considered to be part of Andromeda only.

Constellations 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 Earth.

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 centuries. 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 brightness. Thus, the brightest star is usually designated alpha (α), the second brightest beta (β), etc.

The Names of the Stars For many constellations, the letters follow the order of brightness. However, some constellations are exceptions.

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 Majoris. This name identifies the star and the constellation and gives a clue to the relative brightness of the star. Compare this with the ancient individual name for that star, Sirius, which tells you nothing about its location or brightness.

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 stars. 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 Egyptian-Greek 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: 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 numbers. 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 infrared).

The Brightness of the Stars Limitation 4: an apparent magnitude informs you only how bright the star is as seen from Earth. 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.

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 distances. However, it is still convenient to think of the sky as a great sphere enclosing Earth with stars all at one distance.

The Celestial Sphere The celestial sphere is an example of a scientific model, a common feature of scientific thought.

As you study the sky, you will notice three important points. 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 rotation. This produces day and night – the Sun during 1 day follows almost exactly the motion of stars

The N celestial hemisphere rotates around Polaris

Two, what you can see of the sky depends on where you are on Earth. 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’ Angles One degree of arc = 60 arcminutes 1o = 60’ One arcminute = 60 arcseconds 1’ = 60’’ Therefore, 1 degree = 3600 arcsec.