Lecture 9: The ISM- the Galaxy has Gas

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Date: February 21, 1995
Reading Assignment: pp. 424-432

Description : absorption nebula, dust clouds, 21-cm radiation, interstellar molecules

Objectives

be able to describe forbidden lines and why they occur

be able to explain why spectra from interstellar clouds narrower than those from stars

be able to describe dark dust clouds and how they appear

be able to explain why such low densities can cause background stars to be obscured

be able to describe what causes 21-cm radiation

be able to explain why 21-cm radiation is important for studying the Galaxy

be able to describe molecular clouds

be able to distinguish between molecular excitation and normal atomic excitation

be able to list some of the common molecules found in space

Lecture Outline

Slide # 1: The Interstellar Medium

The Galaxy has Gas

Slide # 2: The ISM

dust revisited

21 cm emission and HI gas

molecular emission

molecular clouds

emission nebula revisited

a quick review

Slide # 3: Interstellar Absorption (GRAPHICS)

some light is absorbed by dust

Slide # 4: The Southern Milky Way (GRAPHICS)

dark nebula

Slide # 5: Interstellar Reddening (GRAPHICS)

Blue light is absorbed more than Red light

Slide # 6: A Reflection Nebula (GRAPHICS)

the Pleaides - reflection nebula

Slide # 7: Dust Grains Polarize Light

ISM magnetic fields align dust particles

aligned particles polarize light

only part of EM radiation passes through

polarization measurements determine

particle sizes

shapes

orientations

Slide # 8: Effects Interstellar Dust

stars appear redder

appear dimmer

star light is polarized

Slide # 9: Ionization States of Hydrogen

molecular hydrogen, not ionized = H2

two atoms in the molecule

cool, dense gas

atomic hydrogen, not ionized = HI

electrons still bound to the atom

most of the ISM

ionized hydrogen = HII

electron removed from atom

found around hot, bright stars

Slide # 10: HI Gas

most interstellar gas is HI gas

atomic hydrogen gas

not ionized

HI emits a radio signal with a wavelength of 21cm

Slide # 11: 21 cm Radiation

line emission

a forbidden line

energy is MUCH lower than optical emission

Slide # 12: 21 cm Radiation (GRAPHICS)

hydrogen atoms have low energy collision

Slide # 13: 21 cm Radiation (GRAPHICS)

electrons flip their spin

Slide # 14: 21 cm Line Radiation (GRAPHICS)

electron spin flips

Slide # 15: Spin Flips

very small energy different

caused by difference in magnetic fields

cannot occur on Earth

density is too high

Slide # 16: 21 cm Radiation Excitation (GRAPHICS)

energy change from collision

Slide # 17: 21 cm Emission (GRAPHICS)

spontaneous de-excitation

Slide # 18: 21 cm Radiation

collisional excitation

a long time later, the electrons flip back

average time = 10,000 years

21 cm radiation is emitted

only occurs in low density regions

used to map the gas in our and other galaxies

Slide # 19: Size of Emission Nebula

depends on the type of star at the center

hot, bright stars emit more UV photons

does not depend on

density of ISM

composition of ISM

Slide # 20: Size of Emission Nebula (GRAPHICS)

determined using simple trigonometry

Slide # 21: The ISM (GRAPHICS)

emission nebula

Slide # 22: The HR Diagram (GRAPHICS)

HR diagram

Slide # 23: Molecular Clouds

temperature = 20K

ISM is normally 100K

density = 106 atoms per cubic cm

ISM has a density of 1 atom per cubic cm

most gas is molecular hydrogen (H2)

ISM is mostly HI gas

Slide # 24: Molecular Excitation (GRAPHICS)

rotation

higher rotation speeds have higher energies

Slide # 25: Emission and Excitation

transition between orbitals

produces most visible line emission

spin-flip of electron

HI gas- 21cm line emission (radio)

rotational excitation

molecular emission - radio

Slide # 26: Molecular Cloud Composition

almost all gas is molecular Hydrogen (H2)

very little HI

interstellar dust grains

1 part in 1012

one part in a million or billion is other molecules

60 detected in interstellar space

Slide # 27: Interstellar Molecules Detected

Carbon Monoxide (CO)

Hydrogen Cyanide (HCN)

Ammonia (NH3)

Water (H20)

Methal Alcohol (CH3OH)

Formaldehyde (H2CO)

Ethyl Alchol (CH3CH2OH)

Slide # 28: The ISM (GRAPHICS)

molecular clouds

Slide # 29: Constellation Corner (GRAPHICS)

Constellation De Jour

Slide # 30: Canis Minor (GRAPHICS)

Feb 15 - 9pm - S - 4.0

Slide # 31: Canis Minor (GRAPHICS)

Feb 15 - 9pm - S - 4.0

Slide # 32: Canis Minor (GRAPHICS)

Feb 15 - 9pm - S - 5.0

Slide # 33: Cepheius (GRAPHICS)

Fairfax - Feb 1 - 10pm - N - 4.0

Slide # 34: Cepheius (GRAPHICS)

Fairfax - Feb 1 - 10pm - N - 4.0

Slide # 35: Types of Nebula

reflection nebula

reflect starlight and appear blue

emission nebula

emit line radiation and usually appear red

some appear greenish

dark nebula or dust lanes

absorb light and appear black

Slide # 36: Emission Nebula (GRAPHICS)

M42 - the Orion Nebula - an HII region

Slide # 37: Emission Nebula (GRAPHICS)

the Vela Nebula - Supernova Remnant

Slide # 38: Nebula (GRAPHICS)

M20 - the Trifid Nebula

emission, absorption, and reflection

Slide # 39: Emission Nebula (GRAPHICS)

M57 - the Ring Nebula - a planetary nebula

Slide # 40: Emission Nebula Power Sources

high luminosity stars

lots of photons to ionize atoms

hot stars

much more blue light (Wien's law)

more ultraviolet photons

lots of ultraviolet photons needed

photons must ionize the gas

ultraviolet photons have lots of energy (photoelectric effect)

Slide # 41: The HR Diagram (GRAPHICS)

HR diagram

Slide # 42: Forbidden Lines

we can't observe in laboratories

collisions happen before for the atoms de-excites

the collisions change the spectral lines

lines like this are called "forbidden"

must have LOW DENSITY to exist

Slide # 43: Forbidden Line Examples

OIII - doubly ionized oxygen

green glow in some nebula

HI -21cm emission

radio emission from neutral hydrogen

Slide # 44: Interstellar Medium

dust

grains

extinction

absorption

gas

emission lines

forbidden lines

HI/HII

molecules

molecular hydrogen

molecular clouds

Slide # 45: Ionization States of Hydrogen

molecular hydrogen, not ionized = H2

two atoms in the molecule

cool, dense gas

atomic hydrogen, not ionized = HI

electrons still bound to the atom

most of the ISM

ionized hydrogen = HII

electron removed from atom

found around hot, bright stars

Slide # 46: Measuring Stars

distance

luminosity and brightness

motion- radial and transverse velocity

diameters

color index and Spectral Types

HR Diagram

stellar masses

Slide # 47: Stellar Magnitudes

close stars appear brighter than far stars

inverse square laws

large stars appear brighter than small stars

luminosity - size relationship

hot stars appear brighter than cool stars

Stephan's law

Slide # 48: Stellar Masses

determined in some binary star systems

Kepler's law

all stars on the MS with the same spectral type have the same mass

hot stars on the MS have higher masses than cool stars

Slide # 49: Stellar Mass

mass and luminosity are related for MS stars

massive stars are more luminous

stellar lifetime and mass are related

massive stars live shorter lives

Slide # 50: The Sun

atmosphere and photosphere

layers and temperatures

magnetic field - sunspots & cycles

activity - prominences and flares

interior

physical laws

nuclear energy

probing the core and convective zone