Phys 1080 Ch 9

Front 1

interstellar medium

Back 1

the gas and dust between the stars, is mostly concentrated near the plane of our Milky Way Galaxy and has an average density of about one atom per cubic centimeter

Front 2

interstellar extinction

Back 2

or dimming, makes the distant stars look fainter than they should

Front 3

interstellar reddening

Back 3

makes distant stars appear too red because dust particles in the interstellar medium scatter blue light more easily than red light. Dependence of this extinction on wavelength shows that scattering dust particles are very small. Dust made of carbon, silicates, iron, ice, and other common atoms and molecules

Front 4

interstellar absorption lines

Back 4

made by interstellar gas that is cold and has very low density. Much narrower than the spectral lines produced in stars. Usually obvious in stellar spectra because they represent ions that cannot exist in the atmospheres of the stars.

Front 5

interstellar emission lines

Back 5

produced at many wave lengths by low-density gas of the interstellar medium

Front 6

21-cm radio line

Back 6

an emission line produced when the electron in hydrogen atoms change the direction of their spin and emit radio-wavelength photons. This radiation allows radio astronomers to map the distribution of neutral hydrogen gas in the interstellar medium

Front 7

Composition of interstellar medium

Back 7

about 70% hydrogen gas

28% helium

2% is atoms heavier than helium

this is approx the same composition as the sun and other stars

Front 8

interstellar dust

Back 8

makes up roughly 1% of the mass of the interstellar medium. The remaining 99% of the mass is gas

Front 9

molecular clouds

Back 9

large, dense clouds of gas and dust; because they are so dense molecules can form inside them

Front 10

giant molecular clouds

Back 10

largest o molecular clouds and the sites of star formation. Radio, infrared, and Xray telescopes have detected emission from over 150 molecules in the interstellar medium

Front 11

nebula

Back 11

cloud of gas in space and an HII region also known as an emission nebula

Front 12

HII region/ emission nebula

Back 12

produced when ultraviolet radiation from hot stars ionizes nearby gas, making it glow. The red, blue, and violet Balmer lines blend together to produce the characteristic pink-red color of ionized hydrogen

Front 13

reflection nebula

Back 13

produced by gas and dust illuminated by a star that is hot enough to ionize the gas. Rather, the dust scatters the starlight to produce a reflection of the stellar absorption spectrum

Front 14

Why does the daytime sky look blue

Back 14

because shorter wavelength photons scatter more easily than longer wavelength photons reflection nebula look blue

Front 15

dark nebula

Back 15

cloud of gas and dust that is noticeable because it blocks the light of distant stars. Irregular shapes reveal the turbulence in interstellar medium

Front 16

evidence that stars have formed recently

Back 16

existence of massive hot stars that cannot live very long, such as prominent stars of Orion

Front 17

contraction of molecular clouds

Back 17

gravity makes them contract but is resisted by thermal every in the gas, and also by magnetic fields, rotation, and turbulence

Front 18

shocks

Back 18

passing shock waves by which some clouds are compressed and star formation is triggered. Birth of massive stars can produce shock waves that trigger further star formation

Front 19

bok globules

Back 19

small dark nebulae, some of which may be contracting to form stars

Front 20

free-fall collapse

Back 20

when cold gas of interstellar space heats up as it contracts because atoms fall inward and pick up speed. When atoms collide, gravitational energy is converted into thermal energy

Front 21

protostars

Back 21

collapsing clouds dense enough to be opaque slow their contraction

Front 22

cocoons

Back 22

protostars form deep inside gas and dust cocoons and are not directly visible at visual wavelengths until the cocoons dissipate

Front 23

Orion Nebula

Back 23

visible, small part of a much larger dusty molecular cloud. Ionization by ultraviolet photons from the hottest star is ionizing the gas, lighting up the nebula and making it glow brightly

Front 24

infared observations

Back 24

reveal clear evidence of active star formation deeper in the molecular cloud just to the northwest of the Trapezium

Front 25

birth line

Back 25

line in the H-R diagram where protostars become visible

Front 26

T Tauri stars

Back 26

solar-mass objects that have just emerged from their cocoons and are located in the H-R diagram between the birth line and the main sequence

Front 27

Young Stellar Objects

Back 27

stars that are nearing the main sequence stage

Front 28

protostellar disks

Back 28

surrounds many protostars

Front 29

bipolar flows

Back 29

jets of gas emitted along the axis of a spinning protostellar disk

Front 30

Herbig-Haro objects

Back 30

nebulae formed where the jets push into the surrounding gas

Front 31

Associations: T associations/ OB associations

Back 31

groups of stars born together but not bound to each other by their mutual gravity. The presence of these associations in the area is evidence of recent star formation

Front 32

principle of hydrostatic equilibrium

Back 32

says the weight pressing down on a layer of gas in a star must be balanced by the pressure in the gas for the star to be stable. That shows that the inner layers of stars must be hotter because they must support more weight

Front 33

law of energy transport

Back 33

states that energy must flow from hot regions such as the core of the sun to relatively cool regions such as the surface of the sun (conduction, radiation, or convection)

Front 34

opacity of a gas

Back 34

its resistance to the flow of radiation. Regions where opacity of gas does not permit radiation to carry away enough energy the gas can churn in convection

Front 35

why is convection important in stellar evolution

Back 35

it can mix material between inner and outer parts of stars (conduction is less efficient that radiation or convection except in few rare types of stars with very high densities)

Front 36

how do stars make energy

Back 36

the same way the sun does, using the proton-proton chain, operates only at temperatures above 4million K needed to overcome the Coulomb barrier of electrical repulsion between positively charged atomic nuclei

Front 37

CNO cycle

Back 37

more efficient than proton-proton chain but requires a higher temperature because of the larger repulsion by carbon and other nuclei

(both processes combine 4 hydrogen nuclei to make 1 helium nucleus plus energy)

Front 38

pressure-temperature thermostat

Back 38

relationship between pressure and temperature, ensures that the star generates just enough energy to support itself

Front 39

stellar models

Back 39

astronomers can study the interiors of stars and the way they change overtime by calculating detailed stellar models. (based on 4 previous laws)

Front 40

why can the mass-luminosity relation among main sequence stars not be understood from stellar models

Back 40

more massive stars have more weight to support so that makes them more luminous

Front 41

why does the main sequence have a lower end; what objects are seen there

Back 41

because stars less massive than .08 solar mass cannot get hot enough to begin hydrogen fusion;

brown dwarfs

Front 42

zero-age main sequence (ZAMS)

Back 42

where a star is located on the H-R diagram when it first begins fusing hydrogen into helium. As hydrogen is converted to helium the total number of nuclei in the stars core declines, slowly contracting the core, the outer layers expand, moves up and to the right across the band of the main sequence

Front 43

how long can a star remain on the main sequence

Back 43

depends on its mass; more massive the faster it uses hydrogen fuel.

25-solar-mass will die in 7 million years, sun expected to last for 10 billion