The Interstellar Medium

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

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

Red, White, and Blue Nebulae

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 nebula “HII region” Reflection nebula Balmer emission Ionizing radiation extinction & reddening

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

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.

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

The Milky Way A “whole sky view on a dark clear night shows a band of light running across the sky. It has some kind of structure running through the middle of it.

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.

Variable Stars – A Standard Candle How can we get the scale of the Galaxy? Parallaxes won’t work.

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

Mapping the Galaxy : Radio Astronomy We can only see our local neighborhood because of interstellar dust. To penetrate this, we can use radio wavelengths (much longer than the size of dust particles). Of course, something has to be producing radio emission…

Sources of Radio Emission -1 Thermal emission from cold interstellar clouds At a few 10s of K, blackbody emission will be in the radio, or somewhat hotter clouds have a long wavelength tail

Sources of Radio Emission -2 2) In a strong magnetic field, spiraling electrons will produce non-thermal “synchrotron” radiation. This can happen near stars or compact objects, or from cosmic rays in the galactic field.

Sources of Radio Emission – 21 cm radiation Neutral hydrogen has a very weak radio spectral transition. So the Galaxy is transparent to it. On the other hand, there’s a lot of neutral hydrogen. So we can see it everywhere. There are also molecular lines from CO and other molecules. The transition occurs because electrons and protons have “spin”. Having the spins aligned is a higher energy state. So in about 10 million years it will decay to the ground state (anti-aligned). Or a 21-cm photon can be absorbed and align the spins. Because the Galaxy is transparent, it is hard to tell where the emission is coming from along the line-of-sight. But because we know its precise wavelength, Doppler shifts in this line can tell us how the gas is moving.

Optical and Radio Sky The large features out of the plane in the radio map are just very close (one is a local supernova-blown bubble).

Spiral Arms in Galaxies Since inner orbits are faster than outer orbits, you might think that is why one sees spiral arms. But these would rapidly wind tightly; galaxies have had ~100 rotations since they formed. Instead, the spiral arms are “density waves”: apparent patterns where stars are denser due to slowing down from mutual gravity.

Density Waves Traffic jams are good examples of density waves. Certain parts of the freeway may have a high density of cars, yet individual cars do not stay with the pattern, but flow through it. They move slowly when at high density, and move quickly when at low density. The site of an accident might produce a stationary density wave (but again, cars are always moving through it). Thus, the spiral arms of a galaxy are just a pattern that may rotate slowly or not at all; individual stars will be passing through it all the time.

Tracers of Spiral Arms In addition to radio maps, you can use HII regions or O&B stars to try to locate spiral arms. The Sun is near the Orion-Cygnus arm, but that is a recent” occurrence. It’s been around about 18 times. 21-cm radio map

Spiral Arms and Star Formation When the ISM passes through it, it gets compressed, and star formation is enhanced. This makes bright hot young stars, and the pattern stands out.

Spiral Tracers from Outside In other galaxies, the arms are easy to see because their ISM does not hide optical diagnostics from us. There are always only a few arms (often 2), and they are never too tightly wound. O & B stars HII regions 21-cm radiation

The Galactic Center Infrared all-sky image Central region (X-rays) Only infrared light can penetrate the dust and show us the heart of our Galaxy. Even though we are in it, we are far enough from the center that it looks like other edge-on spirals. Central region (X-rays)

The Heart of the Galaxy To see to the center, we must use infrared or radio telescopes. A strange mini-spiral swirls, casting off a ring of molecular gas. Magnetic fields are produced, and pervade the Galaxy. AO infrared image of true center…

At the Core Lurks A Monster … Recent adaptive optics pictures in the infrared at the Galactic Center show stars orbiting a central invisible mass. Kepler’s Laws yield a mass inside one light year of 2.7 million solar masses! It has to be a black hole (but apparently it is napping at the moment…)

Stellar Populations Stellar Population Location Star motions Ages of stars Brightest stars Supernovae Star clusters Association with gas and dust? Active star formation? Abundance of heavy elements (mass) Population I Disk and spiral arms Circular, low velocity Some < 100 million years Blue giants Core collapse (Type II) Open (e.g., Pleiades) Yes 2% Population II Bulge and halo Random, high velocity Only > 10 billion years Red giants White dwarf explosions (Type I) Globular (e.g., M3) No 0.1 - 1% Population II stars are old and metal poor, found in large orbits in a random spherical distribution. Population I stars are young and metal rich (including hot stars), all orbiting in the disk in the same direction.

Galactic Structure Disk (Pop I) (stars, ISM, open clusters) Bulge (II) & Halo : Pop II (stars, globular clusters) Dark Matter Halo

Formation of the Galaxy