Chapter 21

Stars and galaxies

Chapter 21

• 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

• 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 MagnitudeLuminosity

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

• 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

• Life and Death of Star• Part 2• Part 3• Part 4• Part 5• Star Life