Starry Monday at Otterbein

Astronomy Lecture Series-every first Monday of the month-

April 4, 2005

Dr. Uwe Trittmann

Welcome to

Today’s Topics

• Spectra – Fingerprints of the Elements

• The Night Sky in March

Feedback!

• Please write down suggestions/your interests on the note pads provided

• If you would like to hear from us, please leave your email / address

• To learn more about astronomy and physics at Otterbein, please visit– http://www.otterbein.edu/dept/PHYS/weitkamp.asp

(Obs.)

– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)

Light and Spectra

• Color of light determined by its wavelength

• White (visible) light is a mixture of all colors

• Can separate individual colors with a prism

Light is an electromagnetic Wave

• Medium = electric and magnetic field• Speed = 3 105 km/sec

Electromagnetic Spectrum

Visible Light

400–440 nm Violet440–480 nm Blue480–530 nm Green530–590 nm Yellow590–630 nm Orange630–700 nm Red

Three Things Light Tells Us

• Temperature – from black body spectrum

• Chemical composition– from spectral lines

• Radial velocity– from Doppler shift

Black Body Spectrum

(gives away the temperature)

• All objects - even you - emit radiation of all frequencies, but with different intensities

Peak frequency

Cool, invisible galactic gas(60 K, mostly low radio

frequency)

Dim, young star(600K, mostly infrared)

The Sun’s surface(6000K, mostly visible)

Hot stars in Omega Centauri(60,000K, mostly ultraviolet)

The hotter the object, the higher the peak frequency!

Wien’s Law

• The peak of the intensity curve will move with temperature, this is Wien’s law:

Temperature / frequency = constant

So: the higher the temperature T, the smaller the frequency f, i.e. the higher the energy of the electromagnetic wave

Measuring Temperatures

• Find maximal intensity

Temperature (Wien’s law)

Identify spectral lines of ionized elements Temperature

Spectral Lines – Fingerprints of the Elements

• Can use this to identify elements on distant objects!

• Different elements yield different emission spectra

Origin of Spectral

Lines

• Atoms: electrons orbiting nuclei

• Chemistry deals only with electron orbits (electron exchange glues atoms together to from molecules)

• Nuclear power comes from the nucleus

• Nuclei are very small– If electrons would orbit the

statehouse on I-270, the nucleus would be a soccer ball in Gov. Bob Taft’s office

– Nuclei: made out of protons (el. positive) and neutrons (neutral)

• The energy of the electron depends on orbit• When an electron jumps from one orbital to

another, it emits (emission line) or absorbs (absorption line) a photon of a certain energy

• The frequency of emitted or absorbed photon is related to its energy

E = h f

(h is called Planck’s constant, f is frequency)

Origin of Spectral Lines: Emission

Heated Gas emits light at specific frequencies “the positive fingerprints of the elements”

Origin of Spectral Lines: Absorption

Cool gas absorbs light at specific frequencies

“the negative fingerprints of the elements”

Spectral Lines

1. Light of a low density hot gas consists of a series of discrete bright emission lines: the positive “fingerprints” of its chemical elements!

2. A cool, thin gas absorbs certain wavelengths from a continuous spectrum dark absorption ( “Fraunhofer”) lines in continuous spectrum: negative “fingerprints” of its chemical elements, precisely at the same wavelengths as emission lines.

Doppler Shift

Application: Separate close Binary Stars• Too distant to resolve the individual stars

• Can be viewed indirectly by observing the back-and-forth Doppler shifts of their spectral lines

Application:Classification of the Stars

Class Temperature Color Examples

O 30,000 K blue

B 20,000 K bluish Rigel

A 10,000 K white Vega, Sirius

F 8,000 K white Canopus

G 6,000 K yellow Sun, Centauri

K 4,000 K orange Arcturus

M 3,000 K red Betelgeuse

Mnemotechnique: Oh, Be A Fine Girl/Guy, Kiss Me

The Hertzprung-

Russell Diagram• A plot of absolute

luminosity (vertical scale) against spectral type or temperature (horizontal scale)

• Most stars (90%) lie in a band known as the Main Sequence

Hertzsprung-Russell diagrams … of the closest stars …of the brightest stars

Stellar Lifetimes• From the luminosity, we can

determine the rate of energy release, and thus rate of fuel consumption

• Given the mass (amount of fuel to burn) we can obtain the lifetime

• Large hot blue stars: ~ 20 million years

• The Sun: 10 billion years• Small cool red dwarfs: trillions

of years

The hotter, the shorter the life!

The Night Sky in March

• The sun is getting higher -> shorter nights!

• Spring constellations (Cancer,Leo,Coma,Virgo,…) contain few bright stars but many galaxies

• Jupiter is in opposition this month (i.e. at its brightest)

Moon Phases

• Today (Waning crescent, 20%)

• 4 / 8 (New Moon)

• 4 / 16 (First Quarter Moon)

• 4 / 24 (Full Moon)

• 5 / 1 (Last Quarter Moon)

Today at

Noon

• Sun at meridian, i.e. exactly south

10 PM

Typical observing hour, early March

• no Moon

• Jupiter

• Saturn at meridian

South-East

Perseus andAuriga

with Plejades and the Double Cluster

Zenith

• Big Dipper points to the north pole

South-West

• The Winter Constellations– Orion

– Taurus

– Canis Major

– Gemini

– Canis Minor

South

Spring Constellations

- Cancer

- Leo

- Hydra

Deep Sky Objects:

- Beehive Cluster (M44)

Mark your Calendars!

• Next Starry Monday at Otterbein: May 2, 2005, 7 pm (this is a

Monday )

• Web pages:– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)

Mark your Calendars II

• Physics Coffee is every Wednesday, 3:30 pm

• Open to the public, everyone welcome!

• Location: across the hall, Science 256

• Free coffee, cookies, etc.

• Details about Otterbein’s Rocket Contest there!