Download A Brief History of Planetary Science

Photometry and Spectroscopy
Astronomy 315
Professor Lee Carkner
Lecture 7
Quiz #1
Next Monday (March 26)
Covers lectures 1-9
About 16 multiple choice (~50% weight)
About 4 short answer/problems (~50%
weight)
Equations and constants provided
But unlabeled
You must bring pencil and calculator!
No cell phones/PDAs
Observing list 1 due this Friday
Studying for Quiz #1
Study
lectures
exercises
homework
readings
Can you:
Identify the key concepts of the class?
Write a paragraph explaining key concepts?
Solve math problems from exercises and book
without help?
Study guide posted on web page
Studying Stars

Stars are too small to see structure

Spectra are studied through
spectroscopy
Spectroscopy
If we take a spectrum of a star, what does it
look like?

What do the lines and their strength tell us?
Strength of line depends on:

The temperature being such that the transitions
can occur
Spectral Signatures
An atoms electron’s can be in a number of
states from 1 (the ground state) to removed
from the atom completely

At higher temperature they are in higher states

Ionized atoms are represented with roman
numerals (e.g. Ca II, calcium with 2 electrons
missing)
Hydrogen Transitions
Spectral Lines in Stars
Most stars have very similar compositions

The spectrum we take only covers a certain
energy range

Three reasons:

Temperature so high that electrons only produce
higher energy transitions

The Balmer Series
All stars are made primarily of hydrogen,
but many stars have weak H lines

In what stars do we see Balmer lines?
Not in cool stars (electrons all in ground state)

Only in medium hot stars are the Balmer lines
strong
Spectral Types

The spectral types are (from high to
low temperature):

Each spectral type is divided into 10
sub classes 0 - 9 (from high to low T)

Temperature Dependence of
Stellar Spectral Lines
Stellar Spectra -- Image
Spectral Typing

Spectral type gives us temperature
O and B stars T ~
A and F stars T ~
G, K and M stars T ~
Spectral typing is accurate to about 2-3
subcategories or a few hundred degrees
The Spectral Types
Stars were first classified by strength of the H
Balmer line

Eventually it was determined that this
sequence did not reveal anything of
astrophysical significance

Photometry
We want to get an accurate quantitative
measure of brightness
Our system is composed of two things:


Magnitude

The magnitude scales is logarithmic and is
related to the flux by:
m2 – m1 = 2.5 log10 (f1/f2)
where the flux is defined as the amount of energy
received from the star per unit area per unit time
(watts/m2/s)
Notes on Magnitude
Magnitude scale runs backwards

Scale is semi logarithmic
A star that is n less magnitudes has 2.5n times the
flux

5 magnitude difference is factor of 100 difference
in flux

Magnitude is sometimes indicated with an
“m”
Magnitudes of Selected
Objects
Sun:
Moon:
Venus:
Sirius:
Faintest star you can see:
Faintest star with small telescope:
Large telescope and CCD camera:
Hubble Space Telescope:
Filters

Use a set of standard filters, such as the
UBVRI scale

e.g. V = 500-600 nm, B = 400-480 nm
We report the magnitudes with the letter of
the filter

Standard UBVRI Passbands
Color Index

The color index gives an estimate of the
temperature
Example B-V:

Negative B-V means smaller B magnitude, which means
more blue light, indicating a hot star

Star Names
Only the brightest stars in the sky have proper
names

e.g., Rigil Kentaurus from Rijl al-Qanturis
meaning “Foot of the Centaur”
Bright stars also have a Bayer designation

Alpha (a) Centauri, Beta (b) Centauri, Gamma
(g) Centauri, etc.

Next Time
Read Chapter 17.1-17.6