The Lower Main SequenceThe Lower Main Sequence• UV Ceti Stars

– M dwarf flare stars– About half of M dwarfs are flare stars (and a few K dwarfs,

too) – A flare star brightens by a few tenths up to a magnitude in

V (more in the UV) in a few seconds, returning to its normal luminosity within a few hours

– Flare temperatures may be a million degrees or more– Some are spotted (BY Dra variables)– Emission line spectra, chromospheres and coronae; x-ray

sources– Younger=more active– Activity related to magnetic fields (dynamos)– But, even stars later than M3 (fully convective) are active –

where does the magnetic field come from in a fully convective star?

– These fully convective stars have higher rotation rates (no magnetic braking?)

Solar Type StarsSolar Type Stars

• Pulsators– The delta Scuti stars– SX Phe stars

• Binaries– FK Comae Berenices Stars– RS CVn stars– W UMa stars– Blue Stragglers

Chemically Peculiar Stars of Chemically Peculiar Stars of the Upper Main Sequencethe Upper Main Sequence

• Ap stars– SrCrEu stars– Silicon Stars– Magnetic fields– Oblique

rotators– Slow rotators

• Am-Fm stars– Ca, Sc deficient– Fe group,

heavies enhanced

– diffusion• HgMn stars• The Boo stars• Binaries?

The Upper Main SequenceThe Upper Main Sequence• 100 (or so) solar masses, T~20,000 – 50,000 K• Luminosities of 106 LSun• Generally cluster in groups (Trapezium, galactic

center, eta Carinae, LMC’s R136 cluster)

(Some of) The Brightest Stars in the GalaxyStar mV MV Mbol Sp. T. Dist.Pistol Star … … -11.8 7 kpcHD 93129A 7.0 -7.0 -12 O3If 3.4 kpcEta Carina 6.2 -10 -11.9 B0 0 2.5 kpc

Cyg OB2#12

11.5 -10 -10.9 B5 Ia+e 1.7 kpc

Zeta-1 Sco 4.7 -8.7 -10.8 B1.5 Ia+ 1.9 kpc

Types of Massive StarsTypes of Massive Stars• Luminous Blue Variables (LBVs)

– Large variations in brightness (9-10 magnitudes)– Mass loss rates ~10-3 Msun per year, transient rates of 10-1

Msun per year– Episodes of extreme mass loss with century-length periods

of “quiescence”– Stars’ brightness relatively constant but circumstellar

material absorbs and blocks starlight– UV absorbed and reradiated in the optical may make the

star look brighter– Or dimmer if light reradiated in the IR– Hubble-Sandage variables are also LBVs, more frequent

events– Possibly double stars?– Radiation pressure driven mass loss?– Near Eddington Limit?

Wolf-Rayet StarsWolf-Rayet Stars• Luminous, hot supergiants• Spectra with emission lines• Little or no hydrogen• 105-106 Lsun

• Maybe 1000 in the Milky Way• Losing mass at high rates, 10-4 to 10-5 Msun per year• T from 50,000 to 100,000 K

•WN stars (nitrogen rich)•Some hydrogen (1/3 to 1/10 HE)•No carbon or oxygen

WC stars (carbon rich)NO hydrogenC/He = 100 x solar or moreAlso high oxygen

•Outer hydrogen envelopes stripped by mass loss•WN stars show results of the CNO cycle•WC stars show results of helium burning•Do WN stars turn into WC stars?

Red GiantsRed Giants• Miras (long period variables)

– Periods of a few x 100 to 1000 days– Amplitudes of several magnitudes in V (less in K near flux

maximum)– Periods variable– “diameter” depends greatly on wavelength– Optical max precedes IR max by up to 2 months– Fundamental or first overtone oscillators– Stars not round – image of Mira– Pulsations produce shock waves, heating photosphere,

emission lines– Mass loss rates ~ 10-7 Msun per year, 10-20 km/sec– Dust, gas cocoons (IRC +10 216) some 10,000 AU in

diameter• Semi-regular and irregular variables (SRa, SRb, SRc)

– Smaller amplitudes– Less regular periods, or no periods

Amplitude of Mira Light Amplitude of Mira Light CurveCurve

More Red GiantsMore Red Giants• Normal red giants are oxygen rich – TiO dominates the spectrum• When carbon dominates, we get carbon stars (old R and N

spectral types)• Instead of TiO: CN, CH, C2, CO, CO2• Also s-process elements enhanced (technicium)• Double-shell AGB stars

Weirder Red GiantsWeirder Red Giants• S, SC, CS stars

– C/O near unity – drives molecular equilibrium to weird oxides

• Ba II stars– G, K giants– Carbon rich– S-process elements enhanced– No technicium– All binaries!

• R stars are warm carbon stars – origin still a mystery– Carbon rich K giants– No s-process enhancements– NOT binaries– Not luminous for AGB double-shell burning

• RV Tauri Stars

Mass Transfer BinariesMass Transfer Binaries

The more massive star in a binary evolves to the AGB, becomes a peculiar red giant, and dumps its envelope onto the lower mass companion

• Ba II stars (strong, mild, dwarf)• CH stars (Pop II giant and subgiant)• Dwarf carbon stars• Nitrogen-rich halo dwarfs• Li-depleted Pop II turn-off stars

After the AGBAfter the AGB• Superwind at the end of the AGB phase strips most of the

remaining hydrogen envelope• Degenerate carbon-oxygen core, He- and H-burning shells, thin H

layer, shrouded in dust from superwind (proto-planetary nebula)• Mass loss rate decreases but wind speed increases• Hydrogen layer thins further from mass loss and He burning shell• Star evolves at constant luminosity (~104LSun), shrinking and

heating up, until nuclear burning ceases• Masses between 0.55 and 1+ solar masses (more massive are

brighter)• Outflowing winds seen in “P Cygni” profiles• Hydrogen abundance low, carbon abundance high (WC stars)• If the stars reach T>25,000 before the gas/dust shell from the

superwind dissipates, it will light up a planetary nebulae• Temperatures from 25,000 K on up (to 300,000 K or even higher)• Zanstra temperature - Measure brightness of star compared to

brightness of nebula in optical hydrogen emission lines to estimate the uv/optical flux ratio to get temperature