photometry and spectroscopy

Photometry and Spectroscopy

Astronomy 315Professor Lee

CarknerLecture 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 () Centauri, Beta () Centauri, Gamma () Centauri, etc.

Next Time Read Chapter 17.1-17.6