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Chapter 28 section 2. 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.
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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?
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 -26.5 • The moon is -12.5 • 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 3.258 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.
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.
