Lecture 23: Doppler Effect - Binary Stars - Neutron Stars and Black Holes

author: Walter H. G. Lewin, Center for Future Civic Media, Massachusetts Institute of Technology, MIT
recorded by: Massachusetts Institute of Technology, MIT
published: Oct. 10, 2008,   recorded: November 1999,   views: 4323
released under terms of: Creative Commons Attribution Non-Commercial Share Alike (CC-BY-NC-SA)

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1. Doppler Shift with Sound Waves - Circular Orbits:

The received frequency changes if the source of sound moves towards or away from an observer. This is the Doppler Effect, demonstrated by Professor Lewin with a tuning fork. The fractional change in frequency reveals the velocity component along your line of sight to the moving sound source. If the source of sound is in circular motion, and if the observer is somewhere in the orbital plane, you can determine the orbital radius and the speed of the source in its orbit. This is demonstrated with a rotating wind organ.

2. Doppler Shift of Electromagnetic Radiation:

Electromagnetic radiation travels at the speed of light, c, in vacuum. If a source of light has a velocity component towards you, the frequencies that you will observe will be higher than those of the emitted radiation, and the received wavelengths will be shorter (blue-shift) than the emitted wavelengths. If the source is receding from you the received wavelength is longer (red-shift). The spectroscopic Doppler shift is used by astronomers to measure the radial velocity of emitters and absorbers of light.

3. Star Mass Determinations from Doppler Shift:

A binary star system consists of a pair of stars orbiting about their center of mass. By measuring the Doppler shifts of both stars as a function of time, you can determine the orbital period, the radial velocity of each star and, if the observer is located in the orbital plane, the orbital radii can be found for both stars. The orbital radii and Kepler's third Law determine the total mass of the system, enabling the determination of each star's mass separately.

4. X-ray Binary Systems:

In an X-ray binary system, there is a neutron star (or black hole) pulling matter off its donor companion. Matter spirals toward the neutron star, and potential energy is converted to kinetic energy. This, coupled with the high mass transfer rate between the pair, generates tremendous power and astronomical temperatures (it radiates mainly X-rays). The accreting ionized matter gets funnelled onto hot spots by the neutron star's magnetic field, which spins with the neutron star (making it an X-ray pulsar). Doppler shifts in the pulsar period and X-ray eclipses can provide orbital parameters (and masses) for the stellar system.

5. Black Holes:

A black hole is a massive object with no size, but with a characteristic surface called the event horizon from within which nothing can escape the black hole. In black-hole binary systems the accreting matter radiates X-rays as it approaches the event horizon of the black hole, but the black hole has no surface, therefore does not exhibit a pulsar-like behavior. You can measure the optical Doppler shift of the donor star, and from its spectrum estimate the donor's mass. This then leads to the mass of the accretor. If this mass is in excess of about 3 solar masses, it is believed to be a black hole. Cygnus X-1 was the first such discovery (in 1972). Its black hole is about 10 solar masses.

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Reviews and comments:

Comment1 Mr. Jamahl Peavey, January 6, 2009 at 3:57 a.m.:

The binary stars travel in opposite directions. Your diagram shows them moving in the same direction.


Comment2 marjorie alexander, March 29, 2009 at 8:56 a.m.:

Thank you so much for your teaching and to you and MIT for making this freely available for all who yearn to understand and grow!

With the CERN experiments coming closer to a reality, I want to learn as much as I can to fully understand this exciting history. I plan to study away with these online classes.

Bless your heart for your passion to spread knowledge!


Comment3 Deadulas Delta, January 31, 2012 at 3:18 a.m.:

Thank you so much for this this helped me so much for this clip on a project (middle schooler) this helped me much on the project with the Doppler affect. thanks for helping me spread the knowledge.


Comment4 Davor form VideoLectures, December 18, 2017 at 10:30 a.m.:

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