Doppler Effect

Doppler Effect: sound waves

As the train approaches, the soundwaves get crunched together. Thewavelength gets shorter. (higher pitch)

As the train recedes, the soundwaves get stretched apart. Thewavelength gets longer. (lower pitch)

Stationary train

Moving traindemo

Doppler Effect: Light Waves

•If a source of light is approaching, the waves of light will be crunched, and smaller. •Smaller wavelength (λ) = blue.•This is called blueshift.

•If a source of light is receding away, the waves will be stretched and the light will become redder. •Longer wavelength (λ) = red

•This is called redshift.

Approaching

Receding

Doppler Shift: First Measure “rest wavelength” λo

If a star is moving away from us based, light waves from it will be stretched out to longer wavelengths (a “redshifted”)

(If moving toward then “blueshifted”)

Not Moving

Doppler Effect Calculation

The light of a moving source is blueshifted

or redshifted by

Δλ/λ0 = vr/cλ0 = rest wavelength emitted by the source

Δλ = wavelength change due to Doppler effect

vr = radial velocity (speed)

The faster something moves, the bigger the change in wavelength.

c = speed of light

Example:A certain spectral line (Hα) has a rest wavelength of 656 nm

Suppose we observe a star’s spectrum with the Hα line at λ = 658 nm.

Question: How Fast is this Star moving?

Is it moving toward us or away?

Example:

λ = 658 nm (observed wavelength) The change in wavelength is: Δλ = λ− λ0 = 2 nm.

vr/c = Δλ/λ0

We find Δλ/λ0 = 2nm/656nm = 0.003 = 3*10-3

vr/c = Δλ/λ0 = 0.003 vr = 0.003 * c vr = 0.003 *(300,000 km/s) = 900 km/s.

The star is receding from us at 900 km/s.

λ0 = 656 nm (rest wavelength)

Doppler Effect: Applications

• The Doppler Effect can be used to measure how fast something is moving

• Police: Speeding Tickets• Weather: Doppler Radar• Astronomy: Motions of stars, including planet detection.

Chapter 4 Summary

! Wien’s Law: Measure Temperature ! Bohr model ! Atoms & Light ! How a spectrum is formed. ! Doppler Effect: Measure motion

Earth

The Sun – Our Star

The Sun: Our Nearest Star

! The Sun is a star ! Because it is so close, its very bright ! The Sun is an “ordinary” star.

! Some stars are larger, some smaller ! Some stars are brighter, some dimmer ! Some stars are older, some younger

! So, we can apply what we learn about the Sun to other stars.

Observations Reveal The Sun’s Properties

! The Sun is 150 million km (93,000,000 miles) away=1AU ! Distance found during the Transit of Venus, 1761

! The Sun 109 times larger than Earth ! ...from its angular size

! It is 333,000 times more massive than Earth ! Newton’s Law of Gravity lets us measure mass

! Its surface temperature is 5,800 Kelvin ! From Wien’s Law & star’s spectrum & λmax

! It is composed of Hydrogen & Helium ! Determined from spectral lines.

Composition of The Sun! The Sun is a plasma -- an extremely hot gas ! Electrons in its atoms have been stripped away, or ionized! The Sun contains: Hydrogen (~70%) Helium (~30%)

To study the Sun, we consider its interior and its atmosphere

Atom Ion

The Outer Parts of the Sun

! The photosphere is the layer we see ! The chromosphere is just above the

photosphere ! The corona is the outermost part of the Sun.

The outer layers of the Sun are called the Sun’s Atmosphere.

It is usually divided into the 3 layers.

Convection & GranulationGas in the Sun is undergoing convection.

Cells of hot gas are bubbling up in the Convection Zone

The gas then cools and sinks down.

Each “bubble” is about 700 km across.

The top of the convection zone is called the photosphere of the Sun.

The Photosphere

• Light from the interior leaves the Sun at the Photosphere

• Temperature ≈ 5800 K

• Is the apparent surface layer of the Sun

Image ©JulioC

• The Chromosphere is a thin layer above the Photosphere

Chromosphere seen Total Solar Eclipse Aug. 21, 2017

The Chromosphere has Prominences, which erupt of the Sun’s Surface

The Corona

! Outside the chromosphere is the Sun’s Corona ! Its temperature is even hotter than the

photosphere: 1 million Kelvins.

! But, the density of the corona is very low.

! Because it is so hot, it emits X rays ! It has streamers that follow magnetic field lines

(c) James Van Zandt, 2017

Sun’s corona seen on August 21, 2017

The Sun’s Corona looks different every eclipse....

1916 2005 2017NASA

Solar Wind

! The corona expands outwards, becoming the solar wind

! A Solar Flare can bombard the Earth with high-energy particles.

“Coronal Mass Ejection” moves out at 4 million M.P.H.!

Sun

Eruption on the Sun seen in Ultraviolet light.

Fortunately, Earth’s magnetic field deflects the particles.

They strike Earth at the North & South Poles.

Anim.

Such a “solar storm” could endanger astronauts.

! When they strike the Earth’s atmosphere, they produce the Aurora Borealis, the Northern Lights.

Image from: http://astroguyz.com/

Oct. 2016 Aurora over Iceland

check: spaceweather.com for reports on solar flares & aurorae