Download April 2005

Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthApril 4, 2005
Dr. Uwe Trittmann
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 Violet
440–480 nm Blue
480–530 nm Green
530–590 nm Yellow
590–630 nm Orange
630–700 nm Red
Three Things Light Tells Us
• Temperature
– from black body spectrum
• Chemical composition
– from spectral lines
• Radial velocity
– from Doppler shift
Peak frequency
Black Body
Spectrum
(gives away the
temperature)
• All objects - even you emit radiation of all
frequencies, but with
different intensities
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=hf
(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
O
B
A
F
G
K
M
Temperature
30,000 K
20,000 K
10,000 K
8,000 K
6,000 K
4,000 K
3,000 K
Color
blue
bluish
white
white
yellow
orange
red
Examples
Rigel
Vega, Sirius
Canopus
Sun,  Centauri
Arcturus
Betelgeuse
Mnemotechnique: Oh, Be A Fine Girl/Guy, Kiss Me
The
HertzprungRussell 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
SouthEast
Perseus and
Auriga
with Plejades
and the
Double
Cluster
Zenith
Big Dipper
points to the
north pole
SouthWest
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!