The Birth of Stars -part I Chapter Twenty. Announcements I need from you a LIST on questions every end of the class near the door so I can KNOW what you.

The Birth of Stars -part I

Chapter Twenty

Announcements

• I need from you a LIST on questions every end of the class near the door so I can KNOW what you didn’t get and REVIEW it next class!

• Review Class for Exam 1 will be in definitively on the 21st during class

• Question: How do I determine the mass of stars?

…in 1977 Voyager 1 & 2

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Mass-Luminosity Relation For radius: look at H-R diagramREVIEW of some Concepts of the Last Class

Why Mass Control the Properties of a Main-Sequence Star?

• Main sequence stars are same as Sun->thermonuclear reactions

• Greater the Mass, greater the pressure and temperature at the Core->FASTER thermonuclear reaction->more LUMINOUS

• Models of Main-Sequence Stars: for the stars to maintain equilibrium, more massive->larger radius, higher surface T

(look at the HR diagram)

Spectroscopy makes it possible to study binarysystems in which the two stars are close together

• Some binaries can be detected and analyzed, even though the system may be so distant or the two stars so close together that the two star images cannot be resolved

• A spectrum binary appears to be a single star but has a spectrum with the absorption lines for two distinctly different spectral types

• A spectroscopic binary has spectral lines that shift back and forth in wavelength

• This is caused by the Doppler effect, as the orbits of the stars carry them first toward then away from the Earth

Doppler Shift can only Occur for RADIAL velocities Perpendicular motion do not affect the wavelength of the star

From the Doppler Shift -> Radial Velocity

Orbital Period

RADIAL VELOCITY CURVE

Double-Line Spectroscopic binaries (those binaries where we can detect the spectrallines of both stars. Most are single-line.

Light curves of eclipsing binaries provide detailedinformation about the two stars

• An eclipsing binary is a system whose orbits are viewed nearly edge-on from the Earth, so that one star periodically eclipses the other

• Detailed information about the stars in an eclipsing binary can be obtained from a study of the binary’s radial velocity curve and its light curve

Light Curves of Eclipsing Binaries

From the shape of the light curves of Eclipsing Binaries we can see if the eclipse was total or not.

Light curves can yield information about sizes and theirOrbits. + Spectroscopic binaries - > we can determine very accurately mass and Radius.

…END OF REVIEW

Stellar Evolution: Understanding how stars evolve requires both

observation and ideas from physics

• Because stars shine by thermonuclear reactions, they have a finite life span

• The theory of stellar evolution describes how stars form and change during that life span

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Emission nebulae are found near O and B stars (emitting copious amountof UV radiation). When atoms in the nearby interstellar gas absorbthese energetic UV photos they emit in the visible by a process callRecombination.

Emission Nebulae are direct evidence of gas atoms in the Int. Medium

Interstellar Medium: space between stars is not empty!

Further evidence of interstellar gas comes from the spectraof binary star systems: spectra lines shift back and forth but certain Lines remain at fixed wavelengths: stationary absorption lines.

Interstellar gas and dust pervade the Galaxy

• Interstellar gas and dust, which make up the interstellar medium, are concentrated in the disk of the Galaxy

• Clouds within the interstellar medium are called nebulae

• Dark nebulae are so dense that they are opaque

• They appear as dark blots against a background of distant stars

• Emission nebulae, or H II regions, are glowing, ionized clouds of gas

• Emission nebulae are powered by ultraviolet light that they absorb from nearby hot stars

• Reflection nebulae are produced when starlight is reflected from dust grains in the interstellar medium, producing a characteristic bluish glow

The HII regions emit visible Light on the 656nm Transition from n=3 to n=2,on the red portion of thespectrum->this is whythey have the distinctive reddish color. (think about Fluorescent light fixtures…)

Dark nebulae are so opaque(large bits of matter-dustGrains) relatively very denseT ~ 10 - 100 K

Ffine grains in a lowerConcentration than that Found in dark nebulaeThe grain size ~ 500nm andThey scatter more Efficiently short wavelenght(blue) so they look bluish…

Don’t confuse with Doppler effect. Interstellar reddening makesObjects appear red not by shifting wavelengths, but by filtering out short wavelengths.

Protostars form in cold, dark nebulae

• Star formation begins in dense, cold nebulae, where gravitational attraction causes a clump of material to condense into a protostar

• As a protostar grows by the gravitational accretion of gases, Kelvin-Helmholtz contraction causes it to heat and begin glowing

How do Stars Form?

Gravity must DOMINATE internal pressure

VISIBLE

IR

The only parts of the Interstellar medium with high enough densityand low enough temperature for starsto form are dark nebulae

Density of a Bok globules are on The order of 100 to 10,000 particles/cm3 per comparison with the interstellar medium that is 0.1-20 particles/cm3

Within these clouds, the densest portions can contract and form clumpscalled protostars.

Protostars evolve into main-sequence stars

• A protostar’s relatively low temperature and high luminosity place it in the upper right region on an H-R diagram (heating<->L)

• Further evolution of a protostar causes it to move toward the main sequence on the H-R diagram

• When its core temperatures become high enough to ignite steady hydrogen burning, it becomes a main sequence star

EVOLUTIONARY TRACKS

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The more massive the protostar, the more rapidly it evolves

During the birth process, stars both gainand lose mass

• In the final stages of pre–main-sequence contraction, when thermonuclear reactions are about to begin in its core, a protostar may eject large amounts of gas into space

• Low-mass stars that vigorously eject gas are called T Tauri stars