contains 10% of all the known mass in our galaxy

                -composed of atoms, molecules, and tiny pieces of dust

-90% of particles in interstellar medium are hydrogen atoms and molecules

-9% are helium

-1% all other naturally forming elements

-Variety of different molecules are found in interstellar medium, identified by their spectral emissions

Dust in space has a variety of structures

-carbon-based (organic) particles typically .005 micrometers across

                -similar to molecules found in engine exhaust and burnt meat

                -called polycyclic aromatic hydrocarbons (PAHs)

                -PAHs composed only of carbon, and hydrogen

Larger space dust composed of carbon or silicon cores surrounded with mantle of ice or other

                -about .30 micrometers across

Gas and dust in interstellar medium sometimes glow due to scattering light from stars in vicinity

bluish haze, caused by fine grains of interstellar dust that scatter blue light from surrounding stars

you can see a star or other object through the interstellar medium, it appears redder than it actually is

When you see a star or other object through the interstellar medium, it appears redder than it actually is

-occurs because short wavelength starlight scattered more than long wavelengths

                -different then the Doppler shift because that causes all wavelengths to lengthen equally, reddening causes the intensities of wavelengths to change

                -locate these areas by looking in the radio or infrared spectrum, because these are scattered relatively little by the gas and dust

Stars are formed from gas and dust that is cool, so that it can collapse together

Stars originally condense out of a cold, interstellar cloud

-interstellar hydrogen used in this process in the molecular form

                -hard to find, so astronomers look for carbon monoxide

-dark patches in the sky

example: Horsehead nebula

Remains of a star

-have a distinctly arched appearance

                -when star blows up, if the scattering remnants run into a giant molecular cloud, it can cause the cloud to contract, stimulating star birth

Collision between two interstellar clouds can also create regions dense and cool to collapse and form star

Radiation from O and B stars will ionize the gas surrounding them, expanding and compressing nearby interstellar medium enough to make a star

A small, roundish, dark nebula in which stars are forming

When a giant molecular cloud contracts and cools enough, gravitational attraction causes small regions of gas and dust in it to collapse.

As this region becomes denser, it blocks light from behind and inside it, becoming dark bok globules

                -in order to collapse, dark cores must be cool

The condition under which gravitational forces overcome thermal forces to cause part of an interstellar cloud to collapse and form stars and planets

When dense cores are cool enough so that the gravitational attraction of the matter in the region overwhelms the pressure there and pulls the gas together to form new stars 

Giant molecular clouds have several thousand dense cores—which means it will have thousands of stars forming an open cluster

Dense core actually collapses from inside out. Inner region falls in rapidly, outer region comes in more leisurely (process of increasing mass in central region)

The earliest stage of a star's life before fusion commences and when gas is rapidly falling onto it

Proto-star forms…

Inner part of fragment becomes Proto-star continues to grow as mass falls inward

Density and temperature increase

center ~1,000,000 K

gravity > outward pressure of the heat

Pre-main sequence star on the HR-diagram

Newly formed object at center of core during accretion

Even though fusion has not begun, protostar still releases energy

Protostars are larger than main-sequence stars

                -emit large quantities of radiation and gas

The stage of star formation just before the main sequence; it involves slow contraction of the young star

Radiation and particles flowing off of the protostar prevent the remaining gas and dust on the outside from reaching the protostar---stops mass accretion

pre sequence star contracts slowly and when temperature is high enough hydrogen fusion begins

this process releases enormous amounts of energy

                -contractions are halted from this energy

                -outer shell of gas and dust finally dissipates and star becomes visible

More massive pre sequence star begin hydrogen fusion sooner/faster

  -instead become planetlike orbs of hydrogen and helium

Forms because extremely hot stars (such as O and B) emit vast quantities of ultraviolet radiation which ionizes any surrounding hydrogen gas, creating a H II region

Why are H II regions visible?

Some hydrogen atoms are free being knocked apart by the ultraviolet photons, and some free protons and electrons manage to get back together

When these hydrogen atoms assemble, the electrons return to their ground state creating enough energy to make the nebulae glow

    -usually a reddish hue

OB associations also affect giant molecular clouds

                -ultraviolet radiation eats hole in cloud, and where this

outflow is supersonic it creates a shock wave

                -shock wave forms along edge of H II region, compressing

the hydrogen gas as it passes and stimulating new star birth

Main sequence stars are those that are in hydrostatic equilibrium with nuclear reactions fusing hydrogen into helium in their cores at a nearly constant rate

Stars do not move along the main sequence

set of locations on H-R diagram where pre-main-sequence stars first become stable objects

More massive a star is, the faster it evolves and the less time it spends on the main sequence

                -occurs because gravity presses down with greater pressure on massive star’s core than little star forcing the more massive star to consume hydrogen in just a few million years

-Stars spend most of their life on main sequence-which is why  most stars are on main sequence

Called red dwarfs

-different than higher mass main sequence stars because they create helium in their cores which moves up and out, while hydrogen is in the outer lays and moves downward (inward)

                -eventually convert whole mass into helium

                -once this happens, fusion will cease because it will not have enough pressure in its core to heat and fuse helium into anything else

                -simply move down and right t the main sequence

Under influence of gravity, gas is compressed and heated enough to begin fusing into helium

When stars leave hydrostatic equilibrium, they expand

                -because more energy is being generated in shell than before

                -extra energy absorbed by gasses in outer layer, and they

heat and expand, causing the whole star to swell

surface gases however cool

Giants so enormous they constantly leak gasses into space resulting sometimes in extreme mass loss

Surface of giant is cooler, but it is more luminous than main sequence stars—because it has more surface area

When central temperature reaches 100 million K fusion begins at core of star

                -First time it has a central energy source since it left main sequence

                -turns helium into carbon

In lower mass stars, helium fusion begins suddenly

Stars larger than 2 M have a safety valve that keeps them from exploding or collapsing

However, stars smaller than this do not

                -outer layers do not provide enough inward force to keep the cores as a normal gas

A large spherical cluster of gravitationally bound stars usually found in the outlying regions of a galaxy

Globular clusters are called “metal-poor” for their lack of heavy elements

                -called Population II stars

The location of the brightest main sequence stars on the H-R diagram of a globular cluster

-stars are just beginning to exhaust their hydrogen in their cores

                -used to determine age of cluster

Pauli exclusion Principle

This principle says that two identical particles cannot exist in the same place at the same time

                -as electrons are pushed closer and closer together they must vibrate faster so as not to become “identical” (same place with same speed) as other electrons

This state for electrons is called degenerate, and when in this state a greater outward pressure is provided than from the normal pressure before degenerate

                -keeps star from collapsing further

                -this pressure does not change with temperature

This leads to the helium flash

Post-helium-flash stars often found in old star clusters, called globular clusters

                -form at same time

                -but are gravitationally bound to each other, unlike open clusters

the region on the H-R diagram between the main sequence and the giant branch

                -unstable and pulsate

                -stars called variable stars because their luminosities change or pulsate

Protostar begins fusion...

Star moves onto Main Sequence

Groups of stars form and condense in part of the cloud

Lifetimes of Stars (in regards to mass)

High Mass stars...

Low mass stars...

High mass stars…

Low mass stars…