The Electromagnetic The Electromagnetic SpectrumSpectrum

All black bodies when heated start to glow,

emitting radiation, some we can see, some

invisible.

The The VisibleVisible Spectrum Spectrum

• White light (from a light bulb or from the sun) is actually a mixture of many different wavelengths.

• When separated, a rainbow or a continuous visible spectrum is revealed.

• However, there are many other wavelengths that human sight cannot detect.

This is the electromagnetic spectrum.

Short wave, high energy

Long wave, low energy

Wavelength• Electromagnetic energy

is wave energy, similar to waves in water.

• The distance between two crests is called the wavelength ().

• The number of waves per second is the frequency (f).

• All wavelengths of light travel at 186,000 mi/sec (known as c).

Gases that glow produce emission spectra.

This is what you see when you point a spectroscope toward a neon light.

Energy Levels

• Electrons when energized jump to a higher level.• When they drop down, they emit a specific wavelength of

light.• In a spectroscope, this is seen as a line in an emission

spectrum.• Hydrogen has the simplest emission spectrum, since it only

has one electron.

Emission spectra are like fingerprints…each element is

different!

Helium tube Argon tube

A hot radiating body surrounded by cool gas

produces an absorption spectra.

HOT

COOL

This is the situation for the Sun and most stars.

The Hydrogen absorption spectrum for the Sun.

It’s a negative image of the emission spectra.

SUMMARY:SUMMARY: Three Spectra• Stars produce continuous spectra, with no gaps

between the colors.• Thin gases emit emission spectra.• When light passes through cool gas, colors are

removed, forming an absorption spectrum.

The Effect of Temperature

• Temperature can be measured in Fahrenheit, Celsius, or Kelvin scales.

• In the Celsius scale, 0 °C is freezing and 100 °C is boiling.

• The Kelvin scale is measured from absolute zero, the coldest possible temperature (– 273 °C).

• The higher the temperature the more energy is emitted from a glowing object.

Spectral Graph for the Sun

Most of the sun’s energy is in the visible part of the spectrum as well as the infrared.

This indicates a temperature of 5800 °K.

Other Stars, Different Temperatures

• The hotter the star, the more energy it emits into space.

• The peak of its spectrum shifts toward the violet and even the ultraviolet.

• Cool stars have peaks in the red.

• Planets (like Earth) peak in the IR part of the spectrum.

Wien’s LawWien’s Law• Max Plank determined

that black bodies have a wavelength of maximum energy.

• It drops off in both directions from that point.

• Wien’s Law states the higher the temperature, the shorter that maximum wavelength will be and the more intense its light will be.

Using Wien’s Law…• If we know the

temperature of a star, we can determine the wavelength of its maximum intensity.

• Use the formula on the right.

• Temperature must be in °K and the wavelength will be in nanometers (one billionth of a meter).

max = 2,900,000/12000 =

242 nm (blue-whiteblue-white star)

Picturing a Nanometer…

• The average width of a human hair is 1 mm or 100,000 nanometers.

• A chlorine atom is around 0.2 nm across, so a nanometer is equal to 5 Cl atoms.

0.2nm

Finding Temperature…Finding Temperature…

• This red supergiant star has a max of 950 nm. What is its surface temperature?

• T= 2,900,000 ÷ 950 = 3053 °K

• This version of Wien’s Law is much more practical, since we can directly measure wavelength in a spectroscope.

The peak of the sun’s radiation is in the middle of the visible spectrum, so…

The Sun is yellow. Its peak radiation is about 500nm, so its temperature is 5800 °K.

Introducing the Doppler Effect

• The train has a higher pitch whistle when approaching you.

• The train has a lower pitch when moving away from you.

• This Doppler Effect is caused by compression or stretching of sound waves.

• The same phenomenon occurs with light, only the object must be moving very fast to detect it.

BlueBlue Shifts, RedRed Shifts

• Light waves moving away from an observer are stretched.

• They shift toward the red end of the spectrum.

• Those waves moving toward an observer are compressed.

• They shift toward the blue end of the spectrum.

• They larger the shift, the faster that object is moving.

Detecting Rotation…

• If a galaxy is rotating, then one end should show a blueblue shift and the other a redred shift.

• This principle applies to stars with planets. They show a spectral wobble.

Magnitude

• Stars differ by brightness, which is measured by magnitude.

• The lower the magnitude, the brighter the star.

• The Sun has a magnitude of -27, by far the brightest object in the sky.

• Without telescopes you cannot objects with magnitudes over 6.

If a photograph is taken of the sky, the stars appear as dots. The larger the dot, the brighter the star (the lower the magnitude).

When you decrease the magnitude by 1, the brightness is 2.5 stronger.

1

2

Star Names• Bright stars have traditional

names, many from Arabic: Betelgeuse, Aldebaran, Sirius, Arcturus, etc.

• Today, astronomers name stars using Greek letters () followed by the constellation name with a Latinized ending.

• Therefore, Arcturus is also known as Alpha ( Bootis.

EXAMPLE:

The brightest star in the constellation Ursa Minor is Polaris. In modern astronomy it is also called Alpha ( Ursa Minoris.The second brightest star Kocab (on the “bowl” itself) is known as Ursa MinorisIf the Greek alphabet is used up, then Roman letters are used.

Polaris

Quick Quiz!• Short wave radiation is produced by objects

with…?• What is an emission spectrum?• What is an absorption spectrum?• Why do we know that Hydrogen and Helium are

found on most stars?• How do we know that the Sun’s surface

temperature is 5800 °K?• What is the Doppler Effect? What does it tell us?• How would we name the fourth brightest star in the

constellation Orion?