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 spectrum It 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 emission The red glow is from H  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 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