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 HI T~5000K, n~0.1-1/cc

– Diffuse gas• HII regions (ionized H) T~10000K, n~0.01-0.1/cc• Hot intercloud medium T~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 NebulaeBlue light is scattered by dust more efficiently than red light, so dust seen in scattered light looks bluish.

Dark CloudsAssociated 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

Ionizingradiation

“HII region” Reflectionnebula

extinction & reddening

Balmer emission

Emissionnebula

Kirchoff’s Laws1) 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 T4 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 GalaxyDemocritus (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