Chapter 21
Stars and galaxies
Chapter 21
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Stars and their Characteristics
Kinds of Stars
Formation of Stars
Galaxies and the Universe
Constellations
• A group of stars that appear to form a pattern
in the sky.
• 88 constellations can be seen from Northern
and Southern Hemispheres.
• The Big Dipper and Little Dipper are famous
constellations
• finding Polaris
Seasonal Changes in Constellations
• Constellations that appear to move around
Polaris are called circumpolar constellations.
• These constellations never set below the
horizon and can be seen all year long.
• The number of stars that are seen circumpolar
depends on the observer’s latitude.
• Number of circumpolar constellations increase
as we move towards north pole.
The Apparent Motion of Circumpolar
Constellations
• Earth turns from west to east.
• As a result whole sky appears to turn from
east to west.
• The part of sky above the Earth’s Axis does not
rise or set.
• When Earth rotates on its axis the, Polaris
seems stationary.
• Stars around Polaris appear to move around in
a counterclockwise directions.
Seasonal Changes in Circumpolar
Constellations
• Follow the position of Ursa Major around
Polaris for all seasons
• This reveal that the constellation is moving
around the star(Polaris).
• Orion is visible in autumn and winter only.
• The change from Orion in the winter to Lyra in
the summer occurs as the Earth points to that
part of the sky at night during that season.
Circumpolar Constellations
• Here is a look at circumpolar constellations.
• Star movement in 24 hours
Distance to Stars
• Closet star is the Sun
• Distance from Earth to Sun is called an Astronomical
Unit.
• This is equal to 150 million KM or 93 million miles.
• We use Light year to measure distances for objects like
stars that are too far away to be measured in KM.
• Light year=distance light travels in one year=9.6 trillion
KM=about 6 trillion miles
• One parsec= 3.26 light years from the Sun
Physical Properties of Stars
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Size
Temperature
Mass
Density
Size, Density, and Mass
• Stars may be as small as Earth ,or 2000 times
the diameter of the Sun
• The smallest star is .01 of the Sun, largest star
may be 50 times the mass of the Sun
• Betelgeuse is one ten-millionth of the Sun in
Density, Sirius is so dense that one teaspoon
of this star will weigh more than a ton.
• Density is million times the density of the Sun
Color of the Stars
• Star color depends on the temperature of the
star
Color of the Stars
• Cool stars are red
• Hotter stars emit more blue than red.
• Yellow stars are hotter than the orange stars
Elements in the Stars
• Stars are mainly made from hydrogen and
helium
• One or two percent of the mass may be
heavier elements
Star Brightness
Apparent Magnitude
Luminosity
Absolute Magnitude
Apparent Magnitude
• The apparent magnitude is how bright the star
appears to an observer on earth.
• In the star- magnitude system higher number
means star is not as bright. For example
magnitude 1 is 2.5 times brighter than magnitude
2. Magnitude 1 is (2.5X2.5X2.5) 15.625 times
brighter than magnitude three star.
• Stars which are brighter than magnitude 1 are
expressed as 0 or negative numbers.
Luminosity
• Apparent brightness does not provide
information on temperature or the size of the
star
• The actual or true brightness of a star is its
luminosity.
• Luminosity depends on the temperature and size
of the stars.
• If sizes are equal hot star will be more luminous
than the cold star.
• If two stars had the same temperature larger star
will be more luminous.
Absolute Magnitude
• Astronomers use the term absolute
magnitude to express the luminosity of the
stars.
• Absolute magnitude is the apparent
magnitude of a star if it is placed at a distance
of 32.6 light years from the sun.
• Sun is an average star with a magnitude of 4.8,
Rigel in Orion has magnitude of – 6.4.
Parsec
• 1 parsec is 3.26 light years.
• 32.6 light years = 10 parsecs
Kinds of Stars
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Color of the Star depends on its temperature
Luminosity depends on temperature and size
Sirius and Vega are hot blue-white stars.
Aldebaran and Arcturus are more luminous because
they are huge. These are called red giants.
Stars that are larger and more luminous are called
super giants.
Less luminous stars are dwarf stars.
These are faint and dense stars.
Typically the size of the Earth but 100,000 times denser
than the Earth
Hertzsprung- Russell Diagram
Pulsars
• Some objects that give off powerful bursts of
radio waves
• One such object is in Crab Nebula
• Astronomers call these objects pulsars.
• These objects are rotating very rapidly and as
a pulsar rotates it gives off radio waves
Variable Stars
• Stars that vary in brightness over regular periods
or cycles that take from days to years.
• Stars that change in brightness as they expand
and contract are pulsating stars.
• An important example of pulsating stars are
cephid variable.
• Another non-pulsating may change in brightness
because it is not one star.
• It is system of two eclipsing stars called eclipsing
binary.
Life Cycle of a Star
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Life and Death of Star
Part 2
Part 3
Part 4
Part 5
Star Life