Spectra and Stars

Taking a Spectrum

The Diffraction Grating

An example spectrum

BLUE RED

Am

ount

of l

ight

Things you can learn from a spectrum

Blackbody RadiationIdeal producers of energy are called blackbodies.

The energy they emit is blackbody radiationThe amount of light emitted at each wavelength

depends only on the blackbody’s temperatureHotter blackbodies have brighter (more intense)

and bluer (shorter wavelength) spectra

Red Hot or Blue/White Hot?

A Blackbody SpectrumAs temperature increases, a blackbody spectrum

peaks at shorter wavelengthsThe Sun’s spectrum looks similar to a

5800 K blackbody

Things you can learn from a spectrum - Temperature

BLUE RED INFRA-RED

BLUE RED INFRA-RED

How Do Spectral Lines Form?Spectral lines correspond to electron transitions

between energy levels in an atomAbsorption: light energy absorbed, electron

jumpsto a higher energy level (ionization is the electron receiving enough energy to jump out of the atom)

Emission: electron falls down to a lower energy level; releases energy as light

Things you can learn from a spectrum - Composition

BLUE RED INFRA-RED

• Temperature (how hot something is)measured from the continuum (is the amount of light larger in the blue or in the red? Red=cool, blue=hot!!!)

• Composition (what something is made of)measured from the position of patterns of lines

• these lines can be from a source of hot gas (emission lines, which contain more light than the continuum)

• or from cool gas in front of a hot source (absorption lines, which contain less light than the continuum)

• Velocity (how fast a source is moving along the line-of-sight)measured from whether the position of patterns of lines are all shifted to the red (the object is moving away from you) or all shifted to the blue (the object is moving towards you)

What (main) things can be learned from spectra?

Some spectra and images of stars

Order your spectra

(and put one RA/Dec into the SDSS Navigate Tool to recover one of the images and spectra

yourself)

BLUE RED

BLUE REDBLUE RED

BLUE RED

BLUE REDBLUE RED

BLUE REDBLUE RED

BLUE RED

BLUE RED

BLUE RED

The Main Sequence of Stars

Stars do not have random temperatures and luminosity

91% of all stars are on a hot-is-bright, cool-is-faint line called the Main Sequence

But, there are also very bright cool stars and very dim hot stars

Hertzsprung-Russell Diagram

Mags?

Main Sequence Stars Are Stable

Main Sequence stars are in hydrostatic equilibrium

The battle between Gravity and Pressure

Pressure from fusion pushes out and gravity pulls in – an equilibrium

This is why a Main Sequence star isn’t shrinking even though it’s a big ball of gas like Jupiter or Saturn

Bright, cool stars must be large - Red GiantsDim, hot stars must be small - Blue/White Dwarfs

How do we know that some stars are bigger than other stars?

Spectra and Stars