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 Can you: Study guide posted on web page 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 Spectra are studied through spectroscopy 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