The Interstellar Medium

Slides:



Advertisements
Similar presentations
Star Formation Why is the sunset red? The stuff between the stars
Advertisements

Chapter 21: The Milky Way. William Herschel’s map of the Milky Way based on star counts In the early 1800’s William Herschel, the man who discovered the.
Introduction to Astrophysics Lecture 13: The Milky Way Galaxy.
Chapter 19: Between the Stars: Gas and Dust in Space.
Chapter 14 Our Galaxy The Milky Way Revealed Our Goals for Learning What does our galaxy look like? How do stars orbit in our galaxy?
Galaxies-I. By the 1700’s the old notion that the Earth was the center of the Universe was overthrown by the success of Newton’s theory of universal gravitation,
Lecture 19 The Interstellar Medium The Stuff Between The Stars.
Chapter 15 The Milky Way Galaxy.
CLUES TO THE FORMATION AND EVOLUTION OF THE MILKY WAY
14.2 Galactic Recycling Our Goals for Learning How does our galaxy recycle gas into stars? Where do stars tend to form in our galaxy?
The Milky Way Galaxy Chapter 15. The Milky Way Almost everything we see in the night sky belongs to the Milky Way We see most of the Milky Way as a faint.
Lecture Outline Chapter 15: Our Galaxy © 2015 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc. Clicker Questions Chapter 11 The Interstellar Medium.
The Interstellar Medium Astronomy 315 Professor Lee Carkner Lecture 19.
The Interstellar Medium ( 星際物質 、星際介質 ) Chapter 10.
The Interstellar Medium Astronomy 315 Professor Lee Carkner Lecture 19.
The Mass of the Galaxy We can use the orbital velocity to deduce the mass of the Galaxy (interior to our orbit): v orb 2 =GM/R. This comes out about 10.
The Milky Way Galaxy 19 April 2005 AST 2010: Chapter 24.
The Interstellar Medium Astronomy 315 Professor Lee Carkner Lecture 18.
Stars Earlier in the course, I told you stellar spectra are black bodies Why are there all these features?
The Milky Way Center, Shape Globular cluster system
The Milky Way. The Milky Way: Our Home Galaxy What are the different components of the Milky Way? How do we see those components? What does a map of each.
The Milky Way Galaxy. The Milky Way We see a band of faint light running around the entire sky. Galileo discovered it was composed of many stars. With.
Levels of organization: Stellar Systems Stellar Clusters Galaxies Galaxy Clusters Galaxy Superclusters The Universe Everyone should know where they live:
The Milky Way Our Galaxy Please press “1” to test your transmitter.
The Milky Way Galaxy Chapter 12:. The Milky Way Almost everything we see in the night sky belongs to the Milky Way. We see most of the Milky Way as a.
© 2010 Pearson Education, Inc. Chapter 19 Our Galaxy.
End of Ch. 13 III. Cycle of Birth and Death of Stars: Interstellar Medium Ch. 14.
The Interstellar Medium. I. Visible-Wavelength Observations A. Nebulae B. Extinction and Reddening C. Interstellar Absorption Lines II. Long- and Short-Wavelength.
The Milky Way Galaxy.
The Milky Way Galaxy Our home in the Universe. Overview Galaxies = groupings of matter within empty Universe –contain stars, dust, gas –formed in early.
Ch. 14. The Milky Way Ch. 14. Ch. 14 OUTLINE Shorter than book 14.1 The Milky Way Revealed 14.2 Galactic Recycling (closely related to Ch. 13) 14.3 The.
A short course in The Milky Way and the ISM Dr. Maura McLaughlin West Virginia University July Pulsar Search Collaboratory.
Copyright © 2010 Pearson Education, Inc. Life Cycle of the Stars.
Susan CartwrightOur Evolving Universe1 The Milky Way n From a dark site the Milky Way can be seen as a broad band across the sky l l What is it?   telescopes.
Note that the following lectures include animations and PowerPoint effects such as fly-ins and transitions that require you to be in PowerPoint's Slide.
The Milky Way II AST 112. Interstellar Medium The space between stars is not empty! – Filled with the Interstellar Medium (ISM) Star formation is not.
Review for Quiz 2. Outline of Part 2 Properties of Stars  Distances, luminosities, spectral types, temperatures, sizes  Binary stars, methods of estimating.
Radio Astronomy Emission Mechanisms. NRAO/AUI/NSF3 Omega nebula.
Copyright © 2012 Pearson Education, Inc. Chapter 14 Our Galaxy.
ASTR112 The Galaxy Lecture 7 Prof. John Hearnshaw 11. The galactic nucleus and central bulge 11.1 Infrared observations (cont.) 11.2 Radio observations.
Lecture 30: The Milky Way. topics: structure of our Galaxy structure of our Galaxy components of our Galaxy (stars and gas) components of our Galaxy (stars.
Star Formation Why is the sunset red? The stuff between the stars
Chapter 19 Our Galaxy.
The Interstellar Medium. Red, White, and Blue : Nebulae.
UNIT 1 The Milky Way Galaxy.
Chapter 11 The Interstellar Medium
AST101 Lecture 20 The Parts of the Galaxy. Shape of the Galaxy.
Harry Kroto ifa.hawaii.edu Harry Kroto 2004
Copyright © 2010 Pearson Education, Inc. Clicker Questions Chapter 14 The Milky Way Galaxy.
Our Milky Way Galaxy. The Milky Way Almost everything we see in the night sky belongs to the Milky Way. We see most of the Milky Way as a faint band of.
Chapter 11 The Interstellar Medium
Copyright © 2010 Pearson Education, Inc. Chapter 14 The Milky Way Galaxy Lecture Outline.
Galaxies: Our Galaxy: the Milky Way. . The Structure of the Milky Way Galactic Plane Galactic Center The actual structure of our Milky Way is very hard.
Milky Way: Galactic Structure and Dynamics Milky Way has spiral structure Galactic Bulge surrounds the Center Powerful radio source Sagittarius A at Center.
AST101 Lecture 20 Our Galaxy Dissected. Shape of the Galaxy.
Star Formation The stuff between the stars Nebulae Giant molecular clouds Collapse of clouds Protostars Reading
The Milky Way. The Milky Way: Our Home Galaxy What are the different components of the Milky Way? How do we see those components? What does a map of each.
The Milky Way Announcements Assigned reading: Chapter 15.1 Assigned reading: Chapter 15.1 Please, follow this final part of the course with great care.
© 2017 Pearson Education, Inc.
Chapter 19 Our Galaxy.
III. Cycle of Birth and Death of Stars: Interstellar Medium
The Milky Way Galaxy.
Our Milky Way Galaxy.
Chapter 11 The Interstellar Medium
The ISM and Stellar Birth
The Interstellar Medium
Chapter 19 Our Galaxy All-Sky View.
Dust Dust cloud The disk Lots of dust in spiral galaxies The bulge
Presentation transcript:

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