3 - Stellar Spectra

Why a slit?

No slit

Slit

Sky Backgrounds and Telescope Nods

star

slit

Visual & Photographic & 2D Electronic Detectors

1D or “Compressed” 2D Electronic Detectors

Increasing T CatalogNames

...also collisions with electrons...

Balmer Series & Balmer Jump

IR

UV

Visual!

Old Photographic NEGATIVES that Defined Spectral Classification

BLUE part of the visual spectrum....

Pressure Broadening & Pressure Ionization

(Energy Level Perturbation & Changing Recombination Rate)

Modern Digital Spectra (from Silva & Cornell 1992, ApJS, 81, 865)

Near-IR Spectra (Rayner et al. 2009, PASP, 185, 289)

Near-IR Detail

Originally, classification was based on spectra at visible wavelengths, since that’s all that was available at the time!

Log L

T

Luminosity Classes

V = Main Sequence

Spectral Resolution

Measuring Stars at Different λs – “Alphabet Soup Photometry” - UBVRIJHKLMNQ

“Standard” Johnson System (and newer Bessell Cousins-Kron filters)

Filter λeff(μm) Δλ(μm)U 0.36 0.07B 0.44 0.10V 0.55 0.09RCK 0.64 0.16R 0.70 0.22ICK 0.80 0.15I 0.90 0.24J 1.25 0.23H 1.65 0.29K 2.2 0.42L 3.5 0.57M 4.6 0.34N 10 6Q 19 5edge originally set by detector – now by filter

J H K

L M

Photometry of 4 dusty stars

Photometry & Stellar Magnitudes

where const(λ) is set by the photometric system

Relative brightnesses of 2 stars at a given λ:

The relative brightness of a star at 2 different λs:

For a collection of stars at the same distance from us, and T versus L diagram translates into a Color-Magnitude diagram. The magnitude can be either M or m without destroying this correspondence.

Beyond OBAFGKM

S Stars - dominated by ZrO

C/O~1, sometimes Tc present!

(99Tc has half-life of 2.1x105 yrs)

Carbon Stars C/O >1

R - C/O > 1 bands of C2, C3, CH, CN, etc.

N - C/O > 1 and s-process elements like Ba & Sr

CH - Old stars with stronger CH

Brown Dwarfs

Brown Dwarf Spectral Classes

M - Above 2000 K, TiO and VO dominate

T=2000-2200 K, TiO condenses into solids

CaTiO3 in M, Ti3O5 & Ti2O3 in cooler objects

L (“new”!) - TiO and VO gone

T (“new”!) - CH4 appears in “H” and “K” photometric bands (actually in “L” at M5, in “K” at L8, and in “H” at T0 - “The Goldilocks Problem”)

Y (discovered in 2012) – NH3 (H2O??)

L & T (brown dwarfs)

NOTE: Very Non-Planckian

HD189733b Hubble Data

Swain et al. 2008

Exoplanet “Spectra”