Measuring the Stars

1. Measuring the Stars Pg. 41

2. 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

3. 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

4. BinariesBinaries 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

5. Stellar positions and distances • Two units to measure long distances • Light Year (ly) =9.461 x 1012 km) • Parsec (pc)= 3.26 ly • Scientists use parallax to calculate the distance to a star • The closer the star, the larger the shift

6. Properties of Stars Diameter, mass, brightness, energy output, surface temperature, and composition. Can be smaller than the Sun or bigger 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

7. 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

8. 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 73% Hydrogen and 25% helium and 2% other elements

9. 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)

10. 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 stip 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

12. 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

13. Most of the energy that reaches Earth’s surface comes from the Sun. Which process best describes how that energy is produced? A. Uranium fuses to plutonium in the core of the Sun. B. Greenhouse gases accumulate in the Sun’s atmosphere. C. Hydrogen fuses to helium in the core of the Sun. D. Sunspots erupt on the Sun.

14. The nebular hypothesis describes the probable origin of A.the Milky Way galaxy. B.the expanding universe. C.the Sun and planets. D.the aurora borealis.

15. Use the Hertzsprung-Russell diagram to answer the question.An astronomer finds a star with a very cool temperature and a luminosity 200 times that of the Sun (LSun). This star is best classified as a • A. red giant. • B. red supergiant. • C. main sequence star. • D. white dwarf.

16. 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? • A. calcium and sodium • B. calcium and iron • C. iron and magnesium • D. sodium, iron, and magnesium

17. Chemical elements can exist in several states of matter and under a wide range of temperature and pressure conditions. Hydrogen in the core of the Sun undergoes fusion to produce helium and energy. What is the physical state of the hydrogen in the core of the Sun? A. hydrogen gas molecules at high temperature and pressure B. single atoms of hydrogen at relatively moderate temperature but under enough pressure to condense to a liquid C. high-temperature plasma under extreme pressure D. long-chain hydrogen polymers at high temperature but relatively low pressure

18. A spectrograph is used to measure wavelengths of light and to analyze spectra. When the dark lines in a spectrum shift toward the blue end of the spectrum, an object is moving toward Earth. When the dark lines in a spectrum shift toward the red end of the spectrum, an object is moving away from Earth. Scientists have found that light from nearly all galaxies shows a red shift. What is the best conclusion from this finding? A.Earth is at the center of the universe. B.Galaxies can move in only one direction. C.The universe is moving through space. D.The universe is expanding.