WJEC AS Physics Unit 1.6 - Using Radiation to Investigate Stars

Stellar spectra of stars

• Analysis of a star’s stellar spectrum, can identify the EM radiation emitted from the star

• Consists of a continuous emission spectrum, from the dense gas of the surface of the star, and a line absorption spectrum arising from the passage of the emitted electromagnetic radiation through the tenuous atmosphere of the star

Continuous emission spectrum

• Spectrum of the wavelengths of light being emitted from a gas cloud (wavelengths it had previously absorbed when light passed through it)

• From the dense gas of the surface of the star

• Emitted light from photons being emitted

• Coloured lines on black background

• Lines of light produced by the atoms emitting photons of light

• Used to identify unknown elements via diffraction grating

Line absorption spectrum

• A continuous spectrum of light crossed by black absorption lines

• Absorption lines where photons have been absorbed by the gas cloud

• Transmitted light

• Black lines correspond to particular photons

• Gas absorbed certain wavelengths of like

• Atoms absorb photons of light

Black bodies

A black body is a body/surface which absorbs all the electromagnetic radiation that falls upon it. No body is a better emitter of radiation at any wavelength that than a black body at the same temperature. Stars are very good approximations to back bodies

Best emitter of radiation at any wavelength

A black body, no body is a better emitter of radiation at any wavelength than a black body at the same temperature

Good approximations to black bodies

Stars, although not black, they are almost perfect emitters of electromagnetic radiation

How intensity of radiation emitted from black bodies varies

With wavelength

Absolute or Kelvin temperature

The temperature, T in Kelvin (K) is relayed to the temperature, θ, in Celsius (°C) by;

T/K=θ/°C + 273.15

At 0K (-273.15°C), the energy of particles in a body is the lowest I can possibly be

θ = T-273

T=θ+273

Relationship between peak wavelength and the absolute temperature

Inversely proportional

Relationship between temperature and intensity

Directly proportional

Higher temperature=higher the peak intensity, lower the peak intensity wavelength, more curve + more blue light

Black Body spectrum

• y-axis=spectral intensity (au)

• x-axis=wavelength

Wien’s displacement law

Stefan’s law

Inverse square law

Multiwavelength astronomy