The Interstellar Medium

Red, White, and Blue : Nebulae

Components of the ISM Gas (hydrogen and helium) –Clouds Molecular Clouds T~20K, n>1000/cc Cold HI (neutral H)T~100K, n~20/cc Warm HIT~5000K, n~0.1-1/cc –Diffuse gas HII regions (ionized H)T~10000K, n~ /cc Hot intercloud mediumT~1 million K, n~0.001/cc (most of the volume) Dust (silicates, graphites, ices) –Dark Clouds –Cirrus

The Orion Region – Visible and Infrared

Molecular Clouds Optical Infrared

HII regions Ionizing radiation from hot young stars makes hydrogen clouds glow red (other elements: other colors)

Scattering (and the blue sky)

Reflection Nebulae Blue light is scattered by dust more efficiently than red light, so dust seen in scattered light looks bluish.

Dark Clouds Associated with dense gas is about 1% (by mass) of “rocky/icy” grains that could eventually make terrestrial planets.

Visible and Infrared Extinction The dark dust clouds are very opaque in the visible, but we can see through them better and better, the longer the wavelength of light that is used. Looking through the galactic plane has the same effect; to see to the heart of the Galaxy you must use infrared or radio (or X-rays!).

Emission, Extinction, Scattering, and Reddening Ionizing radiation “HII region” Reflection nebula extinction & reddening Balmer emission Emission nebula

Kirchoff’s Laws 1)An opaque object emits a continuous (blackbody) spectrum. 2)An thin gas cloud produces an emission line spectrum. 3)A thin gas cloud in front of a blackbody source usually produces an absorption line spectrum.

Astro Quiz Suppose the thin cloud of gas had the same temperature as the hot solid object. The spectrum would look like: 1)A continuous spectrum 2)An absorption spectrum 3)An emission spectrum

Emission and Absorption Spectra More accurately, a gas cloud is only opaque within spectral lines, while a star is opaque at all wavelengths. The brightness of each depends on the usual T 4 relation. If, as is usually the case, the cloud is colder than the star (or the star’s atmosphere is colder than its surface), then an absorption line spectrum is produced. If one looks only at the cloud, the background (empty space) is even colder, so you always get an emission line spectrum. If you look at a cloud through a hotter cloud of gas, you will get an emission line spectrum which includes a continuum.

The Milky Way

Discovery of the Galaxy Democritus (400 BC) Milky Way is unresolved stars? Galileo (1610) that’s right! Wright, Kant (1750) it must have a slab-like arrangement Herschel (1773) we can map the Galaxy by counting stars (assume all are same luminosity and no absorption)

Shape of the Milky Way To be surrounded by a band of stars in the sky implies that most stars are in one plane (and we are in it ourselves). Because it is brighter in one direction, that implies we are not at the center.

Models of the Galaxy The distribution of stars was used to infer the shape. Another issue was: how big is the Galaxy. You need to be able to know the luminosity of stars – even very distant stars… Or perhaps you can make use of globular clusters. Then there is the question of whether there is any absorption between the stars.

Spiral Nebulae and the Zone of Avoidance Another strange distribution was found for the spiral nebulae. It was unclear what these were or how far away they were. They could be forming solar systems, or other galaxies (of course, we weren’t sure how big ours was). It was also found that some of them had rather large radial velocities (and maybe proper motions).

Variable Stars – A Standard Candle

The Shapley- Curtis Debate In 1920, 2 astronomers debated the nature of the Galaxy before the National Academy of Science. They were from N. and S. California (Lick and Mt. Wilson). They also wrote papers. Here are their arguments which are a good example of how science actually works in the process of discovery. Shapley Curtis

Basic Structure of the Galaxy