Constellations  A man made pattern of stars.  The constellations can change depending on the culture and what is important to that culture.  There are 88 constellations.  The IAU or International Astronomical Union are the only people who can name stars.

Cassiopeia  What do you see here?

Cassiopeia

Circumpolar Constellations  A circumpolar constellation is on that can be seen year round.  The stars never set below the horizon.  They are different for the northern and southern hemisphere.

Circumpolar Constellations  Can you think of any that can always be seen in the sky?  What star can always be seen in the Northern Hemisphere?  Some circumpolar constellations in the northern hemisphere are  Big Dipper  Draco  Cassiopeia  Ursa Minor (Polaris)

Movement of the Constellations  The constellations move because the Earth is moving.  The constellations change as the seasons change.  Some constellations can only be seen during certain seasons.

Apparent Magnitude (m)  A measure of how bright a star appears to be to an observer on earth.

The Magnitude Scale  Each magnitude is 2.5 times the magnitude of the next one.  The more negative the number the brighter the star.  For example our sun is a  The moon is  The dimmest that we can see is a +30.  Why is the magnitude scale backwards?

Absolute Magnitude (M)  How bright an object would look if distance was not a factor.

Luminosity  The true energy output (the wattage).  Depends on both the temperature and the surface area (radius)

Distance to Stars  Astronomical Units (AU): 150 million kilometers (the distance from Earth to the sun)  Light-year: the distance that a ray of light travels in one year. (9.5 x 10^12 km)  Parsec: one parsec is equal to light years or 3.068x 10^ 13 km.

Elements in stars  Stars are made of mostly hydrogen and helium.  Only 1 or 2 percent of a star’s mass consists of heavier elements such as oxygen, carbon, nitrogen, and sodium.  Astronomers use spectral analysis to examine what a star is composed of.  Every stars composition is different.

Mass of stars  Star masses are described as multiple masses of the sun.  Our sun is known as one solar mass.  Some stars can be 20 solar masses while others are less than one.

Size of stars  The smallest stars are smaller than the earth, and can be as big as 2000 times the mass of the sun!

Temperature of Stars  The range of colors that a star emits depends on the surface temperature.  This is the same as when you heat a metal, it goes from red to yellow to white as it gets hotter.  Very hot objects give off a blue light.  The sun has a surface temperature of 5500 degrees Celsius is white and yellow.  The sun looks very yellow to us because our atmosphere scatters the blue light, which makes the sun look more yellow.

Stellar Color and Temperature

Variable Stars  Any star that changes in brightness.

Cepheid Variables  They are yellow supergiant whose cycle of brightness range from 1 day to 50 days.  Most have a cycle of 5 days.  Astronomers use these stars to find the distance from Earth to the star by comparing their absolute and apparent magnitudes.

Why are some stars variable stars?  One reason is that there are two stars, not one.  This is called a binary system.  The stars revolve around each other and the brightness changes.