Binary Stars. Binary Stars. Methods of Observation. Need to look for binary signatures. Only direct method of measuring star masses!!!

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1 Binary Stars Two stars gravitationally bound after formation (~ 55% stars in MW) Each star orbits the center of mass (COM) ( balance point ) Stars of equal mass: COM equidistant from each star Binary Stars Stars of unequal mass: COM closer to more massive star Massive star small orbit; Low mass star larger orbit Only direct method of measuring star masses!!! Methods of Observation Binary Stars are classified according to how they are observed two stars close in sky may not be close in space Mizar Alcor Big Dipper Optical Double: two stars that appear close from our perspective but are not gravitationally bound Need to look for binary signatures. 1

2 Methods of Observation Visual Binary System Stars directly observed to orbit each other over an extended time Simulated orbit of Sirius A & B Visual Binaries Center on one star & watch the other move around it Orbital properties will provide masses Methods of Observation If a binary system is too far away, then we ll see the system as one source of light. Need to interpret light in order to determine if a binary star is present. 2

3 Doppler Effect A perceived change in wavelength (frequency) of a wave that is produced by a source moving toward or away from an observer Doppler Equation: λ λ As source moves toward an observer, the waves are compressed As source moves away from observer, the waves are stretched The faster the source, the more compression/stretching Sound: frequency (pitch) of a siren gets higher as ambulance moves toward an observer (vice versa) Doppler Effect Light wavelength of light shrinks as star moves toward Earth speed of a star is << c, λ changes very little (same color) use a star s spectrum as a guide Lab Spectrum Hα Star moving away from Earth Star moving toward from Earth Spectrum is redshifted Spectrum is blueshifted Methods of Observation Spectroscopic Binary Single point source produces two different spectra Each spectrum will exhibit a periodic redshift and blueshift 3

4 Methods of Observation Eclipsing Binary As each star passes in front of the other, the total brightness changes. Methods of Observation The properties of a binary system can be determined by its light curve Effects on Stellar Evolution As one star matures more quickly and becomes a (Super) Giant matter can transfer to its companion to form an accretion disk 4

5 Novae Matter accretes onto white dwarf H fusion will ensue Explosion of surface material Nova Cygni 1992 Supernovae (Type I) If material accretes too quickly then fusion reactions can blow up the white dwarf entirely Light curve indicates type of Supernova Black Holes Accretion disk heats around a black hole to produce X-rays Cygnus X-1 Steps for finding a black hole: (1) find a point source of X-rays (2) look for a star orbiting an unseen companion (3) determine mass of the unseen companion If M > 3 M Suns, then the object is a black hole 5

6 Star Clusters Large groups of gravitationally bound stars Diameter # of stars Location Types of stars Shape Example Open 75 L.Y Disk Young, metal-rich Asymmetric Pleiades Globular L.Y Halo Old, metal-poor Spherical Omega Cen Pleiades Omega Centauri 6

7 Variable stars used to obtain distance Star Clusters Assumption: all stars of a given cluster formed from same Nebula All stars of a single cluster are (1) the same distance from Earth & (2) the same age Create an HR Diagram using all stars of a single cluster HR Diagram will change in appearance over time because the stars evolve Determining Ages Mass of a star determines its spectral type, formation time, & Main Sequence lifetime Spectral Type O & B A F G K & M Formation Time (years) 100,000 1 million 7.5 million 50 million 300 million MS Lifespan (years) 5 million 125 million 1 billion 10 billion 50 billion 7

8 Determining Ages All stars of a cluster begin to contract at the same time After 3 Million years: - All high mass stars (O, B, A) are in MS stage - Lower mass stars not formed Determining Ages After 30 Million years - Moderate mass stars (F & G) have entered MS stage of evolution - Lowest mass stars not formed - Highest mass stars have evolved to later stages Determining Ages Star found on the Main Sequence Turnoff will give the cluster s age 8

9 Determining Distances Spectroscopic Parallax - Determine the Luminosity of MS Turnoff - Measure the Brightness of MS Turnoff 9