A Spectrum: what it is and what it can tell us

Mary Lou West

AAI, July 26, 2013

Our Theme

An alternation of experiment and theory (new equipment and new ideas) propels us to understand the world better and better.

My Goal

To show you some neat stuff,

and to have you begin to understand it.

Are you ready?

Dispersion of white light by a glass prism

Isaac Newton reversed the process in 1666.

Astronomy became Astrophysics

• We learned about the stars’ make-up and slow motions, opening up binary star orbits, exoplanets, orbits in galaxies, and cosmology.

• We knew about the types of visible light, then we learned about other types of light.

Infrared Light

• William Herschel, February 1800

• Looking for good filters for the sun, to observe sunspots

• Thermometer read hot

below “red”

Sunlight as white light, thin dark lines found

In 1814 Joseph Fraunhofer invented the spectroscope to characterize types of glass more accurately.

In the 1820’s he saw the thin dark lines in the solar spectrum.

Wilhelm Wien

• Explained how the color of a hot body depends on its temperature in 1893

Colors of Stars

Stefan-Boltzmann Law

• 1879 Jozef Stefan and Ludwig Boltzmann

• total energy/surface area of light emitted by a very hot solid object depends on the temperature of the object to the 4th power

• Wow!

Spectrum of a hot gas

Hydrogen Helium Neon

Gas spectral lines

• In the 1850’s Gustav Kirchoff and Robert Bunsen explained the dark Fraunhofer lines in the sun’s spectrum as absorption by the chemical elements, each one with a different “fingerprint”

Spectrum portions

Gamma Peg

The Sun

Spectra of Sun, Gam Peg

Gamma Peg

Gamma Peg Sivo 8

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Gamma Peg Sivo 4

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Sivo 2

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Gamma Peg Sivo 6

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Calibration, hydrogen tube

Spectral Classification

• Annie Jump Cannon, one of Harvard’s “Computers”

• Classified thousands of stars

• System started ABC..O

• But ended up as OBAFGKM

Chemical Composition

• Cecelia Payne modeled the stars

• Found that they were mostly (70%) hydrogen

• Very different from the Earth!

• They are also 25% helium

• There was only 5% for all the remaining chemical elements

Emission Lines

• There are hot dense gas clouds (M42)

• There are dense winds around some stars (Wolf-Rayet stars)

• There are planetary nebulae

• There are supernova

remnants

Doppler Shift

• Velocity:

• blue-shifted means approaching Earth

• Red-shifted means receding from Earth

• The amount of color shift indicated the speed

• Some speeds are 20% the speed of light, or more!

Hubble’s Law of Cosmological Redshift

Types of spectroscopes Sivo Star Analyzer 100

• Shelyak

Star Analyzer 100• The star image is the zero-order, so

makes calibration easy

• Unfortunately, the resolution is poor

RSpec software

Projects to Try In the Solar System

•Velocities of the rings of Saturn, calculate the mass of Saturn itself

•Comets, as tails grow

•Asteroids, carbonaceous are farther out

Atmospheres of jovian planets, especially ammonia at 643 nm in

Jupiter but not Saturn.

Projects with Stars

• Examples of spectral types of stars

• Wolf-Rayet stars, emission lines of dense hot gas thrown out. Clumpy? Periodic?

• Colliding wind binaries, especially at periastron

• Stars eclipsed by disks, clouds,…

• Young T Tauri stars, formation of planets

• Be stars with emission lines

More projects with stars

• Stars with variable lines, (Alioth)

• Modeling with synthetic spectra

• Stars with unusual elements, technicium, europium

• Stars doing thermal pulses

• Planetary nebulae

• Novae

• Supernovae

Extragalactic Projects

• Different types of galaxies

• Velocity dispersion by line broadening

• Rotation curves of galaxies

• Quasars, red shift (3C 273, 16% c)

Inspiration at the

Meadowlands, yesterday

Inspiration from articles

• Field, Tom, Spectroscopy for Everyone, Sky and Telescope, Aug 2011, p 68

• Hattenbach, Jan, Deciphering Starlight, Sky and Telescope, Sep 2013, p 30

• Kannappan, Sheila, Fabricant, Daniel, Getting the Most from a CCD Spectrograph, Sky and Telescope, July 2000, p 125