Lecture 24 - The Interstellar Medium (3/18/96)

Seeds: Chapters 8, 9

1. A Binary Star Primer
   • Estimates are that as much as 50% of the stars are actually in binary systems, that is, two stars orbiting each other like the Earth and its Moon.
   • This high rate of double stars is telling us something about how stars were formed.
   • For us, binary stars give us a tool to measure the masses of the stars using Kepler's 3rd Law (as modified by Newton).
   • We have: (M_1 + M_2)/Msun = a(AU)^3 / P(yrs)^2
   • Note that this relation was calibrated using the Sun-Earth orbit, but gravitation is universal, so we can use it anywhere!
   • How can we locate binary star system? There are three kinds of binary star systems:
     1. Visual Binary - you can see both stars orbiting the center of mass. These must be nearby, since 1 AU is 1 arcsec at 1 pc distance, so at 100 pc 1 AU is 0.01 arcsec! You get the period P and apparent semi-major axis of the orbit A.
     2. Spectroscopic Binary - even though the two stars appear too close together on the sky to resolve, you can see the orbital motions through the Doppler effect shifting emission or absorption lines in the stellar spectrum back and forth as the star moves toward then away from us. This is sensitive only to the component of the orbital velocity along the line of sight - if the orbital plane is perpendicular to our line of sight, no red/blue shifting of lines is seen. You get the period P, and the projected velocity v from which you can derive the projected axis A.
     3. Eclipsing Binary - a spectroscopic binary system where the orbit is almost exactly aligned with the line-of-sight and thus one or both stars cross in front of the other causing eclipses. You can see the brightness dim and thus determine the period (and the radius of the stars from the timing). From spectroscopy, velocity can be determined.
   • Please read the part of Chapter 8 (section 8-4) for more on binary stars!
2. Gravitational Collapse
   • Gravity is always attracting, there is no "anti-gravity" repulsion.
   • What do you think happens if you start with a bunch of matter (of any type) distributed far across space, and let gravity do its work?
   • Unless you make the matter distribution exactly uniform and infinite then it will always collapse eventually, unless something stops it.
   • What can stop gravitational collapse?
   • The matter can hit something first, namely the rest of the matter that is falling in also. These collisions cause heat and pressure which resists the collapse. This is called gas pressure.
   • If the collapsing cloud of matter becomes hot enough, and starts emitting photons like stars do, then the light pressure can stop the collapse. This is called radiation pressure.
   • Some other sort of pressure, like that from magnetic fields, can stop the collapse. This is called magnetic pressure.
   • We know our solar system is stable and not collapsing, but the planets are orbiting. They possess momentum, specifically, angular momentum that keeps it spinning instead of collapsing.
   • It turns out that stars (and gas giant planets like Jupiter and Saturn) use gas and radiation pressure to support themselves against gravity. This is what defines the size of a star.
   • It also turns out that in the early stages of star formation, when large gas clouds have to collapse by many orders of magnitude in size to form stars, that magnetic pressure (and to some extent gas pressure) and angular momentum must be overcome.
   • We will discuss the star formation process in more detail in the next lecture.
   • For now, where is the stuff from which stars are made?
3. A View of the Interstellar Medium
   • Early in the history of telescopic astronomy, it was noticed that in addition to lots and lots of stars, there appeared to be fuzzy patches, or clouds, in the celestial sky.
   • These were called nebulae, or nebula in the singular, which is Latin for "cloud".
   • Catalogs for nebulae were made so that people would not confuse them with fuzzy comets, which they were more interested in.
   • What are the nebulae? What different kinds do we see? This is the usual first step in observational science - classify the objects that we find!
   • Diffuse Nebulae - amorphous bright fuzzy blobs, sometimes with one or more bright stars in the center. These turn out to be gas clouds ionized and lit up by very bright young stars.
   • Planetary Nebulae - shells or rings of bright nebulosity surrounding a more or less hollow center. These turn out to be shells of matter thrown off of a dying star.
   • Supernova Remnant - veil like wisps, filamentary rings, or a filled shell with lots of tendrils (often similar in appearance to diffuse nebulae). These turn out to be the result of titanic stellar explosions (supernovae) marking the death of a massive star.
   • Dark Nebulae - dark clouds that block out the stars behind them, and redden those few stars that manage to be seen through them. Often look like "holes" or "gaps" in the Milky Way. These are relatively dense clouds of gas containing dust grains that absorb visible light. Some times dark nebulae are seen in conjunction with bright diffuse nebulae, which seem to live on the edges of dark clouds.
   • Diffuse nebulae can shine by their own light if they are hot, then being called emission nebulae. Lines of hydrogen, carbon, oxygen, calcium, and other atoms can be seen their spectrum. Most bright diffuse nebulae are emission nebulae.
   • Some diffuse nebulae shine by reflected starlight, and are thus called reflection nebulae. Faint blue wisps around the stars of the Pleiades, a star cluster in the constellation Taurus visible to the naked eye are a refection nebulae caused by the bright stars.
   • The nebulae that we are interested in for star formation are the dark nebulae, which contain large amounts of cold gas primed for gravitational collapse and the birthing of stars!
4. What is the Interstellar Medium Made Of?
   • We call all this gas and nebulosity the interstellar medium, the stuff "between the stars", like interstate means between states. We abbreviate interstellar medium as ISM.
   • From spectra of the nebulae, we know they are made of gas, just like the photospheres of stars like our Sun.
   • The gas in the emission nebulae often have compositions very similar to the Sun's: mostly hydrogen and helium, with traces of other elements like carbon, nitrogen, oxygen, and iron. In fact, it is as if they came from stars (...hmmmmm...).
   • There is also dust in the ISM, small grains about 1 micron (10^-6 m) in size. About 1% of the ISM is in dust grains.
   • Scattering of light off of dust grains is what reddens the light of stars seen through dust clouds. The short wavelength blue photons are more easily scattered than the longer wavelength red photons. This same effect reddens the Sun at sunset - and after dust storms or volcano eruptions or big fires sunsets are spectacularly red.
   • The dust is made of solids, like ice, carbon compounds, silicates, and iron.
   • Dust is formed in low temperature regions, below 100K or so, since high temperatures will cause collisions and sputtering of the grains thus destroying them.
   • The surfaces of dust grains can act as a matrix to hold molecules close together and allow chemistry to occur. Some very large molecules have been detected in space, with more than ten atoms. These tend to be chains of hydrocarbons and other organic molecules, and there are claims of seeing some proteins, like those that make up DNA, in some molecular clouds! These complex molecules are believed to be made on the surfaces of grains. In the past decade, the field of cosmochemistry has grown to study the formation of these molecules.
   • The densities in the molecular clouds are as high as 10^5 atoms/cm^3. This is still very tenouous, these dark clouds are still better vacuums than anything we can make here on Earth!
   • Molecular clouds range in masses from 100 up to 10^8 solar masses! You can make lots of stars out of that much stuff.

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