Lecture 2 Flashcards
when were the atoms in your body first formed?
in the big bang when the universe began.
why were constellations to historical civilizations?
ancient cultures around the world celebrated heros, gods and mythical beast by giving their names to a group of stars called constellation. constellations simply celebrate the most important mythical figures in each culture
where did the oldest constellations in western cultures originate?
they originated in Assyria over 3000 years ago, others were added later by the Babylonian and greek astronomers.
of the ancient constellations in the west how many are still in use?
48
what were constellations originally?
they were only loosely defined groupings of stars. many of the feinter stars were not included in any constellation and the stars of the southern sky not visible to ancient astronomers of northern latitudes were not grouped into constellations.
what were Constellation boundaries defined for?
they were only approximates so that a star like Alpheratz could be thought of as part of Pegasus or part of Andromeda
what did astronomers do to make up for the gaps and ambiguities in the constellations?
they added 40 modern constellations, and in 1982 the international astronomical union established 88 official constellations with clearly defined boundaries.
in addition to the 88 official constellations we have?
the sky contains a number of less formally defined groupings called asterisms. The Big Dipper for example is a well known asterism that is part of the constellation Ursa Major(the great bear)
what stars did astronomers particularly give names too?
the brightest individual stars
what language are the constellation names in now?
although the constellation names came from greek translated to latin - the language of science until the 19th centaury - most star names come from ancient Arabic, much altered by the passing centauries.
what was the more useful method of assigning names to stars that made them easier to remember and keep track of?
assigning letters to the bright stars in the constellation to approximate order of brightness. astronomers use the greek alphabet for this. the brightest - alpha, second - beta and so on.
how would you identify a star by its greek letter designation.
you would give it a greek letter followed by the possessive form of the constellation name. eg the brightest star in the constellation Canis Major is Alpha Canis Majoris, which can also be written Canis Majoris. this both identifies the star and the constellation and gives a clue to the relative brightness of the star. compared to the ancient name of the star Sirus which does not give any info about its properties.
how do astronomers describe the brightness of stars?
using the magnitude scale. a system that first appeared in the writing of claudius ptolemaeus in about the year 140.
how was the magnitude scale divided?
the stars were divided into 6 classes. The brightest were called first magnitude stars and those fainter second-magnitude. the scale continued down to sixth-magnitude starts, the feintest visible to the human eye. the larger the magnitude number the fainter the star. measured in Apparent magnitude(Mv)
can the brightness magnitude of a star be negative?
some stars are so bright that the modern magnitude scale must be extended to negative numbers eg Sirus = -1.46. the same must be done with faint starts, yes the feintest that can be see by the human eye is magnitude 6. but with a telescope u can see those much fainter. these numbers are called apparent visual magnitudes (mV) and they describe how the stars look to human eyes observing from earth.
to be accurate and not subjective by determining star brightness with the naked eye what must be done?
to be accurate you must refer to the flux - the amount of energy that hits one square meter in one second. This makes a precise definition of brightness.
what did ancient astronomers believe the sky to be?
a great sphere surrounding earth with the stars stuck on the inside like thumbsticks in a ceiling.
the celestial sphere
a way of thinking of the sky as a great starry sphere enclosing the earth
why does the sky appear to rotate westward around the earth each day?
it is a consequence of the eastward rotation of the earth. That rotation produces day and night.
how do astronomers measure distance across the sky?
they measure angular distance across the sky as angles and express them as degrees , arc minutes and arc seconds. The same units are used to measure the angular diameter of an object.
is the celestial sphere an example of a scientific model?
yessir
are stars there during the day time?
yes they are just invisible during the day because the sky is lit up by the sun.
in addition to causing the obvious daily motion of the sky what is earths rotation connect to?
a very slow celestial motion that can be detected over centuries. more than 2000 yrs ago Hipparchus compared the positions of some stars with their positions recorded nearly 2 centauries previously and realized that the celestial poles and the equator were slowly moving across the sky. later astronomers understood that this motion is caused by the top like motion of the earth known as precession.
precession
the toplike motion of the earth
