Learning from Light

There are three different types of spectra… Continuous spectrum (“Thermal radiation”) Emission spectrum Absorption spectrum hot cool

“Thermal radiation” Everything gives off light – electromagnetic radiation – due to its thermal energy. Visible light cannot pass through the bag … … but infrared light can! (The bag is opaque to visible light, but transparent to infrared light.) At temperatures we are used to – room temperature up to hundreds of degrees – that radiation is below the visible part of the spectrum  Infrared (IR)

The lowest energy part of the visible spectrum is RED But as an object heats up, the energy of the light it gives off goes up – eventually into the visible part of the spectrum The lowest energy part of the visible spectrum is RED As it gets hotter still, it turns yellow, then white. WHY?

This is energy per unit surface area. As an object gets hotter, it gives off more of the higher energy light More of the blue An even mix More of the red The hottest objects look BLUE! This is energy per unit surface area.

Notice that the wavelength of the PEAK depends on temperature

Notice that the wavelength of the PEAK depends on temperature This can be expressed mathematically as Wien’s Law Or, solving for T:

Notice that the wavelength of the PEAK depends on temperature This can be expressed mathematically as Wien’s Law Or, solving for T: What is the peak wavelength in the thermal spectrum of a star whose surface temperature is 15,000 K?

Notice that the wavelength of the PEAK depends on temperature This can be expressed mathematically as Wien’s Law Or, solving for T: What is the peak wavelength in the thermal spectrum of a star whose surface temperature is 15,000 K?

Notice that the wavelength of the PEAK depends on temperature This can be expressed mathematically as Wien’s Law Or, solving for T: The peak wavelength in the thermal spectrum of a star you’re studying is 350 nm. What is the surface temperature of the star?

Notice that the wavelength of the PEAK depends on temperature This can be expressed mathematically as Wien’s Law Or, solving for T: The peak wavelength in the thermal spectrum of a star you’re studying is 350 nm. What is the surface temperature of the star?

Sound waves from a stationary train sound the same in all directions

Sound waves from a stationary train sound the same in all directions The Doppler Effect Sound waves from a stationary train sound the same in all directions Sound waves from a moving train sound different depending on your location

It’s the same with light… The Doppler Effect It’s the same with light… All lines shifted to longer wavelengths All lines shifted to shorter wavelengths

Source at rest w.r.t. laboratory The Doppler Effect Source at rest w.r.t. laboratory Source moving away slowly Source moving away faster Source moving toward us slowly Source moving toward us faster

The Doppler Effect

The Doppler Effect A yellow line in the helium spectrum has a wavelength of 587.5 nm when measured in the lab. The same helium line has a wavelength of 585.0 nm in the spectrum of a star. Is the star moving towards us or away from us? At what speed?

The Doppler Effect A purple line in the spectrum of mercury has a wavelength of 435.8 nm when measured in the lab. The same line in the spectrum of a nebula has a wavelength of 438.2 nm. What is the nebula’s radial velocity with respect to the Earth?

The Doppler Effect allows us to measure an object’s radial motion only “proper motion”

The Doppler Effect also allows us to measure a star’s rotation! Rotating slowly Rotating quickly One side of the star is moving toward us, the other is moving away from us. Each line is shifted both up AND down – they spread out.

Astro-Cash Cab! Jack Crystal Emily Isaiah Amanuel

1) A particular line in the spectrum of a galaxy has a wavelength of 378.0 nm. The same line, when measured in a lab on Earth, has a wavelength of 381.5 nm. The galaxy must be moving… … towards us. … away from us. … can’t tell from this information.

2) Which statement is always true? a) A hotter object gives off more long wavelength photons than a cooler object. b) A hotter object gives off more short wavelength photons than a cooler object. c) The thermal photons given off by a hotter object have a higher peak wavelength than those given off by a cooler object. d) The thermal photons given off by a hotter object have a higher peak energy than those given off by a cooler object.

3) Which color star is hottest? Red Blue Yellow White

The Doppler Effect can tell us how quickly a star is rotating. 4) True or False ? The Doppler Effect can tell us how quickly a star is rotating.