PG. 127 Measuring the Stars

Groups of stars Long ago, people grouped bright stars and named them after animals, mythological characters or every day objects. Called constellations There are 88 constellations Constellations that appear to move around the north pole are called circumpolar (Big Dipper) Classified as summer, fall, winter, and spring constellations

Star clusters Groups of stars that are gravitationally bound to one another are called clusters Open cluster- stars that are not densely packed Globular cluster-stars that are densely packed into a spherical shape

Binaries Two stars that are gravitationally bound that orbit a common center of mass are binaries More than half the stars are binary Appear to be single stars

Stellar positions and distances Two units to measure long distances  Light Year (ly) =9.461 x km)  Parsec (pc)= 3.26 ly Scientists use parallax to calculate the distance to a star The closer the star, the larger the shift

Properties of Stars Diameter, mass, brightness, energy output, surface temperature, and composition. Can be smaller than the Sun or bigger (our Sun is average) Brightness is measured using magnitude Apparent magnitude relies on what we see Absolute magnitude takes into account how far a star is from Earth (can only be calculated if you know the distance of the star) Luminosity measures the energy output and you must know the apparent magnitude

Spectra of stars The rainbow you see when you shine white light through a prism is called a spectrum. 3 types: continuous, emission, and absorption Stars have an absorption spectrum

Classification of stars Stars are assigned spectral types (O, B, A, F, G, K, and M) Each spectral type is then subdivided into more specific divisions using numbers 1-9 O stars are the hottest and M the coolest Hotter stars have simple spectra while cooler stars have more lines in their spectra Our Sun- G2 73% Hydrogen and 25% helium and 2% other elements

Wavelength shifts Shifts in spectral lines happen when there is motion between the source of light and the observer If the star is moving toward the observer, the spectral lines are shifted toward shorter wavelengths (called blueshifted) Star is moving away, wavelengths become longer (redshifted)

H-R Diagrams Properties of mass, luminosity, temperature, and diameter can be related and shown on a graph called the Hertzsprung-Russell diagram (H-R diagram) Absolute magnitude is plotted on the vertical axis and temperature or spectral type is plotted on the horizontal axis 90 % of stars fall on a strip of the H-R diagram called the main sequence (runs from upper left corner to lower right) Upper left = hot, luminous stars Lower right= cool, dim stars

Classification on the H-R Upper-right= cool but luminous; very large; red giants (100x larger than the Sun) Lower-left= hot but dim; very small; white dwarfs

Use the spectra to answer the question. Each element absorbs light at a particular wavelength. This absorption of light produces dark lines in spectra. Scientists analyze spectra to identify the composition of a star. According to the spectra shown, which elements does the unknown star contain? 1.A. calcium and sodium 2.B. calcium and iron 3.C. iron and magnesium 4.D. sodium, iron, and magnesium