Objectives Students will be able to determine how distances between stars are measured. Students will be able to distinguish between brightness and luminosity. Students will be able to identify properties used to classify stars.
Word of the Day Star: A self-luminous celestial body consisting of a mass of gas held together by its own gravity.
29.2 Measuring the Stars • Page 837 • Main Idea: Stellar classification is based on measurement of light spectra, temperature, and composition.
29.2 Measuring the Stars • Page 837 • Constellations: Groups of stars. • There are 88 of them. • Circumpolar Constellations: Visible in Northern Hemisphere all year. • Summer, Fall, Winter, Spring: Only visible during certain seasons. • Constellations are used by people to know when to prepare for planting, harvest, and ritual celebrations. Circumpolar Star Trails
Seasonal Constellations Hercules: Visible in NH only during summer Orion: Visible in NH only during winter
29.2 Measuring the Stars • Page 838 • Star Clusters: Groups of stars that are gravitationally bound to each other. • 2 kinds: • Open Cluster: Not densely packed. • Globular Cluster: Densely packed into spherical shape.
Open Star Cluster Globular Star Cluster
29.2 Measuring the Stars • Page 838 • Binary Stars: Two stars that are gravitationally bound together and that orbit a common center mass. • Hard to tell two stars apart. • One star often is much brighter than the other.
Classwork • Constellation in a cup
29.2 Measuring the Stars • Page 840 • Stellar Position and Distances: • 2 Units used to measure long distances: • Light Year (ly): The distance that light travels in 1 year = 9.461 x 10 km. • Parsec (pc): 3.26 ly. 12
29.2 Measuring the Stars • Page 840 • Parallax: Apparent shift in position of an object caused by the motion of the observer. • Used to calculate distance. • Earth’s orbit around the Sun means that an observer’s position in space is always changing. • Distance to a star can be estimated using the star’s parallax (angle of change.) • Closer the star, the larger the shift. • Estimates are accurate up to 500 pc away.
29.2 Measuring the Stars • Page 841 • 4 Basic Properties of Stars: • Mass • Diameter • Luminosity • Temperature – Estimated by finding spectral type of star. Controls nuclear reaction rate and governs luminosity and magnitude.
29.2 Measuring the Stars • Page 842 • Magnitude: • Apparent Magnitude: How bright a star appears. • Brightest stars = +1 • Next brightest = +2 • A difference of 5 magnitudes = 100 x more bright. • Ex) +1 is 100 x more bright than +6.
29.2 Measuring the Stars • Page 842 • Magnitude: • Absolute Magnitude: Accounts for distance (apparent does not.) • How bright a star would appear if it were 10 pc away.
29.2 Measuring the Stars • Page 842 • Luminosity: Energy output from surface of star per second – a star’s power. • Requires that one know a star’s apparent magnitude and distance away. • Measure in watts – energy emitted per second. • Sun’s luminosity = 3.85 x 10 w. 26
29.2 Measuring the Stars • Page 483 • Classification of Stars: • Word of the Day: • Spectral Line: An isolated bright or dark line in a spectrum produced by emission or absorption of light of a single wave length.
29.2 Measuring the Stars • Page 483 • Spectral Lines: Provide information about a star’s temperature and composition.
29.2 Measuring the Stars • Page 483 • Temperature: Corresponds to a star’s spectral lines. • Types: O Hot 50,000 K • B • A • F • G • K • M Cool 2,000 K • Each type is subdivided into more specific divisions by numbers 0 – 9.
Spectral Lines of Stars Hotter Stars Cooler Stars
29.2 Measuring the Stars • Page 485 • Composition: All stars have similar compositions. • Hot Stars: Less lines in spectra (fewer kinds of gasses.) • Cool Stars: More lines in spectra (more kinds of gasses.) • Typical Composition of Stars: • 73% hydrogen (H) • 25% helium (He) • 2% other elements.
Composition of Stars “Heavier Elements” = Anything heavier than Helium
29.2 Measuring the Stars • Page 485 • Hertzprung – Russell Diagram (H-R Diagram): • Shows the lifetimes of stars. • A graph on which absolute magnitude is plotted on vertical axis and temperature or spectral type is plotted on horizontal axis. • Each class of star has a specific mass, luminosity, magnitude, temperature and diameter.
29.2 Measuring the Stars • Page 485 • H-R Diagram: Plots evolution of stars from one class to another. • Main Sequence: Area of the H-R Diagram in which most stars are located. • Runs diagonally from upper left corner (Hot Bright Stars) to lower right corner (Cool Dim Stars.)
29.2 Measuring the Stars • Page 486 • Main Sequence: Contains 90% of stars including Sun (which is at the center because it has an average temperature and luminosity.) • Stars here fuse hydrogen. • As hydrogen runs out stars fuse helium and begin to evolve off of the main sequence.
29.2 Measuring the Stars • Page 486 • Mass determines a star’s lifetime. • More Mass = More Pressure = Shorter Lifetime • Higher mass stars burn hydrogen faster. • Lower Mass = Less Pressure = Longer Lifetime • Lower mass stars burn hydrogen slowly.
29.2 Measuring the Stars • Page 486 • Red Giants and White Dwarfs: • Red Giants: Top right of H-R diagram = Cool, Bright Stars. • Because these stars are cool, but still bright, they must have large surface areas. • Mass is more than 100 x greater than the Sun.
29.2 Measuring the Stars • Page 486 • Red Giants and White Dwarfs: • White Dwarfs: Lower left of H-R diagram = Hot, Dim Stars. • Appear dim, though they are very hot, must have small surface areas. • Size of Earth, but has a mass same as the Sun.