* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Motions of the Stars
Survey
Document related concepts
International Ultraviolet Explorer wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Astronomical unit wikipedia , lookup
Copernican heliocentrism wikipedia , lookup
Corvus (constellation) wikipedia , lookup
Geocentric model wikipedia , lookup
Newton's laws of motion wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Observational astronomy wikipedia , lookup
Dialogue Concerning the Two Chief World Systems wikipedia , lookup
Transcript
The Motions of the Stars Doppler Effect Shift in the observed wavelength when the source is moving relative to the observer. Examples: Sound Waves (Siren or Train Horn) Light Waves Amount of the shift and its sign depends on relative speed of the source & observer direction (towards or away) Stationary Source Moving Source High Pitch (short waves) Same Pitch Same Pitch Doppler Effect in Sound High Pitch (short waves) Doppler effect in light Low Pitch (long waves) Low Pitch (long waves) The Doppler Effect in Light Amount of Shift depends upon the emitted wavelength (lem) and the relative speed v: If the motion is away from observer –Wavelength gets longer = REDSHIFT If the motion is towards the observer –Wavelength gets shorter = BLUESHIFT Doppler Effect in Practice Used by astronomers to measure the speeds of objects towards or away from the Earth. Other Uses: Traffic Radar Guns: • Bounce microwaves or laser light of known wavelength off of cars, measure reflected wavelength: Doppler shift gives the car’s speed. Doppler Weather Radar: • Bounce microwaves off of clouds, measure speed and direction of motion. Strength of the reflected signal gives the amount of rain or snow. The “Fixed” Stars Way to Measure Speeds Observe the wavelength (lobs) of a source with a known emitted wavelength (lem) The difference is directly proportional to the speed of the source, v: l ( observed) l ( emitted ) V l ( emitted ) c Q: The observed change in wavelength due to Doppler effect occurs: A) Only when the temperature of an object changes. B) Whenever the light source is moving with respect to the observer (independent of the direction) C) Only when the source moves across the line of sight D) Only when the source has radial velocity towards or away from the observer. Radial Velocities To the naked eye, the stars appear “fixed” to the sky. No Shift In reality, the stars are in constant motion. Great distances make the amount of motion small on a human lifetime. Blueshift Sun Redshift Proper motion = how fast a star is moving relative to background objects (in arcseconds per year). Radial Velocity Motion along the line of sight. 360 degrees in a circle, 60 arcminutes in a degree, 60 arcseconds in an arcminute. (An arcsecond is a tiny angle.) Measure using the Doppler Shift of the star’s spectrum: • Moving towards Earth: Blueshift • Moving away from Earth: Redshift • Moving across line of sight: No Shift Proper Motions Barnard’s star has the highest proper motion of any star: 10.3 arcseconds per year (1 degree per 350 years). 1902 Angular motion across the sky of nearby stars with respect to distant stars. • Typical proper motion: ~0.1 arcsec/year. 1952 Reflects true motion of the stars through space (not parallax) 2002 Discovery of Proper Motions Proper motions are cumulative Edmund Halley (1718): Effects build up over time… Noticed that the positions of 3 bright stars, Sirius, Aldebaran, & Arcturus had moved by about 20 arcminutes from the positions given by Hipparchus of Rhodes in 300BC. • The longer you wait, the greater the angular motion is. Measuring proper motions: • Compare photos of the sky taken at two separate times • Measure how much stars have moved. Example Case Study: The Big Dipper Star with a proper motion of 0.1 arcsec/year: • After 1 year: it moves 0.1 arcsec • After 10 years, it moves 100.1=1 arcsec • After 100 years, it moves 1000.1=10 arcsec It takes many millennia for the constellations to noticeably change shape. Today 50,000 BC More distant stars tend to have smaller Proper Motions for same transverse velocity: Today Can usually measure accurate proper motions out to distances of ~1000 parsecs. 50,000 BC 50,000 AD Sun Space Motions Want to know the true motion of a star through space. Observed motions: • proper motions: angular motion • Transverse or tangential velocity: need to know the Distance and Proper Motion • radial velocity: motion along line-of-sight True space motion • Combine Transverse & Radial Motions Sun Why measure space motions? Most useful when measured for many stars. Use statistics of the motions to find: • Motion of the Sun through nearby space (towards the constellation of Hercules) • Local rotation of the Milky Way galaxy Important tool for studying the structure of the Milky Way galaxy. To measure a star’s tangential velocity, we need to measure its: A) B) C) D) Proper motion and parallax Proper motion and radial velocity Doppler shift and spectral type Parallax and radial velocity Parallax or Proper Motion (1) If observed at the same time of the year, every year, parallax is zero but proper motion would be visible. (2) Parallax is only along the direction of earth’s motion. Star’s movement in other directions would imply proper motion. (3) Parallax has to go back and forth--- but proper motion is cumulative, i.e. the effect builds up with time. A spectrum of a star gives no information on the star’s A) B) C) D) E) Radial velocity Tangential velocity Composition Luminosity Temperature