chapter 15: star formation and the interstellar medium
TRANSCRIPT
Chapter 15: Star Formation and the Interstellar Medium
The “vacuum of space” isn’t a perfect vacuum
There are places out there where there is actually a lot of stuff, though it is still very
tenuous
Most of the interstellar medium is hydrogen gas but
about 1% is dust
The dust tends to block our
view
It’s not that there aren’t stars in that direction, it’s that the dust block the light from the stars
The gas and dust scatters the blue wavelengths better than
others
The visible light scatters because it’s wavelength is close to the size of the dust grains. We call this effect Interstellar Reddening
Like everything, the dust glows by
its blackbody spectrumIt is very cold so it glows in infrared and microwave wavelengths
As blackbodies,
the dust grains absorb
visible and UV light and
emit in IR
Most of the dust is cool so it glows in the far IR
Recent far IR image taken by the ESA Planck satellite telescope
Some of the interstellar gas is extremely hot
ROSAT X-ray image of the Milky Way
Some of the gas is just warm so it gives off atomic
emissionThe red glow is from Ha emission of hydrogen gas. The dark areas are where there is lots of dust. The blue is light scattered off dust near bright young stars
If hydrogen is really cold it will only emit at the 21cm
wavelength (radio wavelength)
21 cm emission is due to hydrogen spin flip
The largest collections of gas and dust are called Giant Molecular Clouds (GMC’s)
The Orion Giant Molecular Cloud is over 1000 lightyears across
In Spiral Galaxies the GMC’s are concentrated in the arms
Inside the GMC’s, in the coldest, darkest places, stars
form
The temperature has to be cold enough for gravity to overcome the natural gas pressure
Something triggers the pockets to form and begin
collapsing and star formation begins
The pockets that form stars will be those with the highest density
New Star Formation Shock can also trigger star
formation
The collapse
from nebula to protostar is driven by gravity but moderated by angular momentum
The central region flattens out in a pizza dough effect
The process not only
leads to a star but a
solar system as
well
Protostars are shrouded in a dense cocoon
You can’t see them in visible but IR and X-rays penetrate through the dust and gas surrounding them
As it collapses,
the protostar heats up
When the core temperature reaches 10,000,000°, fusion
begins
The protostar will continue to shrink and heat up as it tries to find its equilibrium temperature
More massive stars collapse differently than low mass
stars
Mass loss in young stars
The formation and ignition process tends to be very violent. Young stars tend to blow off material during this stage of their life.
Massive stars blow off mass
at prodigious
rates
The most massive stars can lose around 10-4 solar masses every year with major eruptions where they may lose mass much faster over a few years
Mass Loss in low
mass stars
The swirling disk of gas and dust and the spinning protostar can lead to the formation of jets that shoot out the polar axis if the star: bipolar outflows
Herbig-Haro Objects
H-H Objects are young stars that produce jets. When the jets collide with the surrounding
interstellar gas they light it up.
Formation of Bipolar Outflows (fancy name for jets)
The jets form as a result of strong magnetic fields being concentrated and twisted by the collapse from a nebula
The twisted magnetic fields also lead to intense activity on the surface of the star.
How long it takes to get born depends on the mass of the star
Evidence of our theory of star formation is out there
Watch Orion Fly Through Video at http://www.youtube.com/watch?v=UCp-XKeSvSY