Observing the Universe
Reading: Chapter 2.1, 2.2, 2.6, 2.8
What the universe looks like from Earth
When you look up at the stars, you look out through a layer of air only about 100 kilometers deep. With the naked eye, we can see more than 2,000 stars as well as the Milky Way.
1 The Milky Way
A band of light making a circle around the celestial sphere.
What is it? Our view into the plane of our galaxy.
Milky Way over Ontario (K. A Hepburn)
The Milky Way
2 Constellations • Ancient civilizations around the world named groups of stars called constellations. • Names were based on ancient heroes, gods, animals, shapes and mythology was associated with them. • Many star patterns recognized today originated 5,000 years with the Babylonians, Egyptians and later the Greeks.
Constellations
• In 1928, the International Astronomical Union (IAU) established 88 official constellations with clearly defined permanent boundaries that together cover the entire sky.
• Just like a map of the earth is divided into countries, a map of the sky is divided into constellations.
• 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.
3 Constellations • The sky also contains some star groupings called asterisms. Examples: – Big Dipper in the constellation Ursa Major (the Great Bear) – Great Square of Pegasus that includes three stars from Pegasus and Alpheratz, now considered to be part of Andromeda only.
The brightest stars in a constellation…
A. All belong to the same star cluster. B. All lie at about the same distance from Earth. C. May actually be quite far away from each other.
4 The brightest stars in a constellation…
A. All belong to the same star cluster. B. All lie at about the same distance from Earth. C. May actually be quite far away from each other.
Constellations
• Most constellations are made up of stars that are not physically close to one another.
• Some stars may be moving through space in different directions.
• The only thing they have in common is that they lie in approximately the same direction from Earth.
5 Star Names Most individual star names derive from ancient Arabic, Greek or Latin, altered over centuries. Examples: Sirius: Greek meaning glowing Aldeberan: Arabic meaning follower (of the Pleiades or Seven Sisters) Vega: Arabic meaning landing (constellation was viewed as a landing vulture) Spica: Latin from spica virginis meaning the ear of wheat of Virgo
Star Names • 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), and so on. – There are some exceptions
6 The Brightness of Stars • Astronomers measure the brightness of stars using the magnitude scale. • 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.
The Brightness of Stars • The Greek astronomer Hipparchus (190–120 BC) is believed to have compiled the first star catalog and may have used the magnitude system in that catalog. • Around AD 140, the Egyptian-Greek astronomer Claudius Ptolemy used the magnitude system in his own catalog. • Successive generations of astronomers have continued to use the system.
7 The Brightness of Stars • Star brightnesses are called apparent visual magnitudes
(mV) They describe how the stars look to human eyes from Earth. • Today scientists no longer rely just on their eyes. • Modern instruments measure the flux: the total light energy hitting one square metre per second. • Flux can then be used to calculate apparent visual magnitude
The Brightness of Stars • Some stars are so bright they have negative magnitudes • Faint stars detected by telescopes have magnitudes larger than 6 • Apparent visual magnitude is based only on visible light. • Apparent visual magnitude does not say anything about the stars actual energy output since their distance is not included.
8 The Celestial Sphere
When we look at the sky, we cannot tell how far different stars are from us
Ancient astronomers thought that the sky was a giant celestial sphere on to which all the stars were attached.
Although we now know better, the celestial sphere is still useful for describing the location (but not distance) of stars in the sky. (We will see later in the course how to measure distances to stars).
The Celestial Sphere
Stars at different distances all appear to lie on the celestial sphere.
Ecliptic is Sun s apparent path through the celestial sphere. Why is it tilted from the celestial equator ?
Note: The celestial sphere is a a useful tool for describing the sky - it does not really exist.
9 Angular Size and Angular Distance Since we cannot accurately judge how far objects in the sky are, we CANNOT tell their true size.
However, we CAN talk about the angular size of an object or the angular distance between two objects.
Angular size or distance is measured in degrees.
Try to estimate the angular size of an object for yourself…
• Full circle = 360º • 1º = 60! (arcminutes) • 1! = 60" (arcseconds)
The angular size of your finger at arm s length is about 1°. How many arcseconds is this?
A. 60 arcseconds B. 600 arcseconds C. 60 × 60 = 3,600 arcseconds
10 The angular size of your finger at arm s length is about 1°. How many arcseconds is this?
A. 60 arcseconds B. 600 arcseconds C. 60 × 60 = 3,600 arcseconds
Angular Size An object s angular size appears smaller if it is farther away. This relationship is very useful for calculating actual distances or sizes
Angular size
Physical size
Distance
360 degrees angular size = physical size × 2π × distance
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