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Binary Stars Book page 643

©cgrahamphysics.com 2016

Doppler Effect


Red and blue shift in stars • Light from moving objects will appear to have

different wavelengths depending on the relative motion of the source and the observer


EM Spectrum


• Light moving away from an observer has a longer wavelength than it had when emitted. Longer wavelength means red color.

• Light approaching an observer has a shorter wavelength and appears to be blue shifted.

• Red shift – star moving away, longerλ

• Blue shift – star moving closer, shorter λ

Object at rest

Object moving towards observer

Object moving away from observer ©cgrahamphysics.com 2016

Binary Stars • Two stars that orbit about a common center

• If they can be resolved with a telescope they are called visual binary.

• The period of rotation depends on the sum of the masses of the two stars and the separation between them.

• The orbit depends on the mass of each star.

• Knowing the sum of the masses each individual mass can be found.


Spectroscopic binaries

• The binary nature of the system can be deduced from its spectrum.

• The spectrum of a star is observed over a period of days


• If stars A and B do not belong to same spectral class, spectrum 1 and 3 will not be identical.

• If One star too dim to detect its spectrum, a single spectrum will shift forth and back


Eclipsing binaries


Eclipsing binaries

• The binary nature can be deduced from a periodic change in brightness


• With respect to Earth the stars will block light out from the other as they orbit each other.

• The deeper or primary eclipse occurs when the hotter star passes behind the cooler star


• If two stars of equal brightness overlap, the total brightness drops by 50%

Variation in brightness gives information about

• The ratio of surface temperature

• The relative size

• The size of the orbit



Example

• a) What type of object is this?

• b) Account for the shape of the observed light curve.

• c) What does the information on the time axes suggest about the physical properties of the object?

An astronomer observes an object, producing the following relative light curve.

Eclipsing binary system

Stars are side by side

Bigger star Hotter star passes behind cooler smaller star

Cooler smaller star passes behind hotter bigger star

The period is ~ 60h they must be close together <1Au. they must orbit each other at velocities


Example • Two stars, A and B form

a spectroscopic binary as shown in the diagrams below.

• a) Which star is more massive?

• b) Explain why the spectral lines periodically split when move together in a system such as this.

• Doppler shift star moving tw us blue shifted star moving away red shifted

• Moving across line of sight no shift two line are superimposed

• c) Why do the stars have different ranges in their radial velocities?

• 𝑀𝐴 < 𝑀𝐵 wider orbit, must travel further in orbital period radial velocity must be greater

B


Review

• Visual binary

• Eclipsing binary

• Spectroscopic binary


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Binary Stars - cgrahamphysics.files.wordpress.com€¦ · Binary Stars •Two stars that orbit about a common center •If they can be resolved with a telescope they are called visual