19-Phys1403-Sim.Pdf

Foundations of Astronomy | 13e Seeds

Phys1403 – Stars and Galaxies Instructor: Dr. Goderya Chapter 10

Interstellar Medium

Exam 2 Topics for Chapter 10 • Wednesday April 18th During class time • 40 multiple choice questions and 1 short answer question. I. Interstellar Medium • Chapters A. What is Interstellar Medium – Light and Telescope B. Three Kinds of Nebulae – Atoms and Starlight – The Energy production in Sun C. Extinction and Reddening – The Family of Stars D. Long and Short wavelength observations – The Interstellar Medium – The Formation and Structure of Stars II. Components of Interstellar Medium • Sample Questions on class webpage after Thursday 12th April. A. Cool Clouds • Expect to see numerical questions: You can use one US Letter size B. The Intercloud Medium cheat sheet with equations, constants and conversion factors. • Bring a calculator, green scantron and pencil. C. Molecular Clouds D. Coronal Gas

Topics for Chapter 10 Recap:Interstellar Medium (ISM) I. The Gas-Star-Gas Cycle A. Gas and Dust from Aging Stars • A region of gas and dust between stars and B. A Preview of Star Formation galaxies. • The gas is found in atomic, molecular and ionic form. • The dust in space is called Interstellar Dust. • Average density in ISM is 1 particle per cubic centimeter. • It is the birthplace of stars. • Has dark clouds that absorb and alter light.

1 Recap: Kirchhoff’s Laws of Radiation (1) Recap: Kirchhoff’s Laws of Radiation (2)

1. A solid, liquid, or dense gas excited to emit 2. A low-density gas excited to emit light will light will radiate at all wavelengths and thus do so at specific wavelengths and thus produce a continuous spectrum. produce an emission spectrum.

Light excites electrons in atoms to higher energy states

Transition back to lower states emits light at specific frequencies

Recap: Kirchhoff’s Laws of Radiation (3) Types of Nebula 3. If light comprising a continuous spectrum Emission Planetary Diffuse Nebula passes through a cool, low-density gas, Nebula Nebula Ionized gas or plasma Explosion of low mass the result will be an absorption spectrum. Extended regions and stars contain no well Hydrogen atoms are defined boundaries excited Light excites electrons in composed mostly of H II Region atoms to higher energy states Hydrogen and dust Reflection Nebula Photons form young massive stars Dust reflects light from producing ionization nearby stars

Explosion of high mass stars Dark Nebula Frequencies corresponding to the Supernova Absorb and blocks transition energies are absorbed Remnant light from stars from the continuous spectrum.

Emission Nebulae Scattering by Dust

Emission nebulae Dust particles do not scatter red wavelengths2) Reflection well. Nebulae • Hot stars (25,000 K Spectral type B1) excite gas to produce emission spectrum They do scatter Blue wavelengths well. • Pink color produced by the blending of the red, blue, and violet Balmer lines. • Emission nebulae are also called H II regions, following the convention of using Roman numerals to indicate an atom’s ionization state. Thus, H I means neutral hydrogen; H II is ionized.

Same phenomenon makes the day sky appear blue (if it’s not cloudy).

2 Reflection Nebulae Dark Nebulae

• A reflection nebula is produced when starlight scatters • Dark nebulae are from the dust in a nebula. clouds of gas and • The spectrum consist of absorption lines. dust dense enough to • Gas is surely present in a reflection nebula, but it is not obstruct the view of excited enough to emit photons (emission lines). more distant stars. • Their shapes, as shown, are generally irregular, suggesting that, in addition to the effects of nearby stars, there are breezes and currents pushing through the ISM.

Two Effects of Dust on Stars Extinction versus Wavelength ISM can be studied at Infrared wavelengths Extinction • Dust makes distance stars appear fainter than they would be if the space was perfectly transparent. This is called interstellar extinction • Dust makes up about 1 percent of ISM • The reduction in apparent brightness is about 2 magnitude per 1000 pc. The Interstellar Medium absorbs light more strongly at shorter wavelengths.

Two Effects of Dust on Stars Example of Reddening • Nebulae that appear as dark nebulae in the optical, can shine brightly in the infrared due to blackbody radiation from the warm dust • (a) At visual wavelengths, the Horsehead Nebula is a dark peninsula Reddening extending away from a larger dark cloud. • (b) At near-infrared wavelengths the dark nebulae are nearly • Another way dust reveals itself is in the transparent and stars are visible through them. • (c) At longer mid-infrared wavelengths, the clouds glow with blackbody way it affects the color of stars. This is radiation emitted by dust grains. called Reddening. • Interstellar clouds make background stars appear redder

3 Spectral Study of ISM

• We look for: – Absorption Lines – Emission Lines • Infrared Radiation • 21 –cm Radio •X Rays • Ultraviolet

The 21-cm Line of Neutral Hydrogen Interstellar Absorption Lines Transitions from the higher-energy to the lower- • Look for narrow Interstellar absorption lines. energy spin state produce a characteristic 21-cm • In this plot a spectral line of singly ionized calcium radio emission line. produced in the atmosphere of a star is so magnified that it is seen as a broad dip in the plot. • In contrast, the absorption lines produced by two clouds in the foreground ISM are much narrower. => Neutral hydrogen (HI) can be traced by observing this radio emission.

Observations of the 21-cm Line (1) Observations of the 21-cm Line (2) Radio observations reveal many clouds of neutral hydrogen orbiting the center of our galaxy HI clouds moving towards Earth HI clouds moving away from Earth

Individual HI clouds with different radial G a l a c t i c p l a n e velocities resolved

All-sky map of emission in the 21-cm line (from redshift/blueshift of line)

4 Interstellar Emission Lines Interstellar Emission Lines • Infrared observations reveal emission from complex • These images of our galaxy molecules display the entire sky spread into an oval.

(a) At a distance of 23 million • The infrared image ( top ) shows light-years, the galaxy clouds of cold gas and dust. M51, also known as the • In the visual-wavelength image ( Whirlpool Galaxy, is similar to our Milky Way middle ), dense, opaque dusty Galaxy. clouds in the ISM are silhouetted (b) Infrared observations against distant stars. reveal emission from complex molecules called • The X-ray image ( bottom ) PAHs (red). Many of shows the location of very hot those molecules are part gas produced in most cases by of dust particles in the galaxy’s ISM. the supernova explosions of dying stars.

Infrared Emission Lines Components of the ISM

• The Spitzer Space Telescope imaged the center of the The ISM occurs in two main types of clouds: Milky Way Galaxy at a wavelength of 8 microns (8000 nm) • The brightest regions are clouds heated by young, massive stars. • HI clouds: • Individual stars can be seen as bright spots. Cold (T ~ 100 K) clouds of neutral hydrogen (HI); • Some of the clouds are so large, dense, and cold that they moderate density (n ~ 10 – a few hundred atoms/cm3); are opaque and dark even at this infrared wavelength. size: ~ 100 pc (1pc = 3.26 Ly)

• Hot intercloud medium: Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm3); gas can remain ionized because of very low density.

Molecules in Space, Part 1 Molecules in Space, Part 2 • In addition to atoms and ions, the • The most easily observed molecule in interstellar medium also contains space molecules – CO = Carbon Monoxide → Radio emission • Molecules store specific energies in their: – OH = Hydroxyl → Radio emission – Rotation • The most common molecule in space

–Vibration –H2 = Molecular Hydrogen → Ultraviolet • Transitions between different rotational / absorption and emission vibrational energy levels lead to emission – Where there’s H2, there’s also CO – typically at radio wavelengths – So – Use CO as a tracer for H2 in the ISM

5 Molecular Clouds Giant Molecular Clouds, Part 1 • Molecules are easily destroyed • Where stars are born? (“dissociated”) by ultraviolet photons from – Higher density (up to 100,000s particles/cm3) hot stars – Size: up to 100 pc; Total mass: up to 1 million – Can only survive within dense, dusty clouds, solar masses where UV radiation is completely absorbed – Deep inside, gravity can pull the matter – UV emission from nearby stars destroys inward and create new stars molecules in the outer parts of the cloud • Formation of Stars in molecular clouds – High density (n ~ 100s-1,000s particles/cm3) depend on Temperature, Mass and – Size: ~ 15-60 pc; Total mass: ~ few 100 solar Radius masses

Giant Molecular Clouds, Part 2 Coronal Gas • The dark lower and right- • On the right side of this hand margins of this combined X-ray and visual image show part of a wavelength image, as many as Giant Molecular Cloud 200 hot, luminous, newly formed named N90. O and B stars are inflating • Distance 200,000 light- bubbles of coronal gas that emit years (ly). X-rays, represented here as • The cloud material is blue. dense and dusty. • On the left are clouds of • In the center of the image expanding coronal gas is an H II region and produced by other hot stars plus reflection nebula several supernova explosions. containing a cluster of Intense star formation within this young stars that cloud is blowing it apart. condensed from the cloud material.

The Four Components of the The Gas-Stars-Gas Cycle, Part 1 Interstellar Medium All stars are constantly blowing gas out into space (recall the solar wind) Component Temperature Density Main – The more luminous the star is, the stronger is its [K] [atoms/cm3] Constituents stellar wind HI Clouds 50 – 150 1 – 1000 Neutral The red supergiant star Betelgeuse hydrogen; other is expelling a powerful wind of gas atoms ionized and dust. That wind forms a bow 3 4 Intercloud Medium 10 -10 0.01 Partially ionized shock where it collides with the (HII) H; other atoms fully ionized ISM. That bow shock is not Coronal Gas 105 -106 10-4 –10-3 All atoms highly centered on the star because it is ionized H speeding through the ISM in the direction shown by the arrow in the Molecular Clouds 20 - 50 103 -105 Neutral gas; dust and figure. molecules

6 The Gas-Stars-Gas Cycle, Part 2 Matter cycles from the ISM to form stars and back into the ISM. Our galaxy is slowly but steadily using up its star- making supplies and exhausting its reserve of the hydrogen fuel that powers newborn stars.

Acknowledgment

• The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only • Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class.