Astronomy Exam 3

Density between stars

less dense than the best vacuum we can make on earth—very few atoms.

Interstellar Dust

dust particles are roughly the size of the wavelength of visible light, so it is mostly blocked, but infared radiation (just larger) go right through—results in a dark cloud with “reddish” stars at edges

Preferential scattering

happens at the edges of a dust cloud, or at the horizon at sunset: most blue light is scattered, and red light (at a longer wavelength than dust particles) sometimes makes it through

Interstellar gas

90% hydrogen, 9% helium

Types of nebulae

Emission nebulae= glows reddish due to the radiation from a hot star within the cloud

Dark nebulae= dust cloud

Reflection nebulae=light from surrounding stars bounces off cloud particles to create blue color due to preferential scattering

Dark nebulae

Very cold

Absorb visible light

Emit radio waves

Have strong CO emission lines

Sculpting of dust lanes

radiation blows back less dense dust and leaves just the unique shape of the high-density dust within it

Importance of atomic hydrogen in interstellar gas

when the hydrogen transitions from the electron and proton being in “parallel spin” to “anti-parallel” spin, which reduces it’s energy—causing a lower energy wavelength longer than the typical size of interstellar dust particles.

So, this hydrogen radiation reaches earth unaffected by stellar debris

Molecular Clouds

regions of interstellar gas between 10-20 K, where density is a bit higher due to most gas particles are molecules—only longer wavelength radio waves can escape

Molecular Clouds

H2 (molecular hydrogen)

CO (carbon monoxide)

H2O (water

H2CO (formaldehyde)

The last 3 emit radio waves as they are created by chemical processes within the clouds—called “Tracer molecules”

Ratio of Hydrogen to tracer molecules in molecular clouds

1 billion: 1

Molecular cloud complexes

huge groups of molecular clouds, 10s of parsecs across

Triggers star formation

Some kind of external event causes a molecular cloud to lose it’s hydrostatic equilibrium and gravity dominates over heat, causing it to contract and collapse on itself. 

Seven stages of stellar evolution

1)Interstellar Cloud—begins to collapse and fragment

2) & 3) Contracting Cloud fragment—temp and density increases rapidly

4) Protostar—shrinks as density and temp increase to 1,000,000 K, ignites the proton-proton chain

5) Protostellar Evolution—outward directed pressure grows, heat escapes and contraction slows, luminosity decreases, gas is ionized and temp continues to increase. Strong winds result in bipolar flow and ejects matter perpendicularly

6) Newborn Star—10 million years after stage 4, radius decreases and temp increases. Proton-proton chain begins producing helium, luminosity less than the sun, and radiation is absorbed by dust and reemitted as infrared

7) Main Sequence Star—hydrostatic equilibrium reached, energy emitted is stable

Brown Dwarfs

“Failed stars” whose original fragments were too small, so hydrostatic equilibrium was achieved before temp became high enough to start nuclear fusion—very cool and smaller

Star clusters

a group of stars that all formed from the same parent cloud

Open cluster

type of star cluster that is loose and irregular (often found on Milky Way disc)

Associations

star clusters that are smaller than open clusters, but more spread out and expanding

Globular clusters

spherical star clusters with millions of stars, not found in Milky Way, lack upper-main sequence stars and are the oldest ones