Chapter 11The Interstellar Medium

Chapter 11The Interstellar Medium

The Eagle Nebula

Interstellar MediumInterstellar Medium

• Gas - atoms and small molecules

• Dust - clumps of atoms and molecules

Figure 11.1Milky Way Mosaic

DustDust

• Typical dust grains are size of wavelength of visible light

• Opaque to short wavelengths: optical, UV and X-ray

• Transparent to long wavelengths: radio and infrared radiation

• Shorter wavelength reduced - “reddening”

Figure 11.2Reddening

Figure 11.3Reddening in Earth’s Atmosphere

Composition of Interstellar Medium

Composition of Interstellar Medium

• 90% of gas is atomic or molecular H

• 9% is He

• 1% is heavier elements

• Dust composition not well known

Density ofInterstellar Medium

Density ofInterstellar Medium

• 106 atoms per cubic meter

• Best laboratory vacuum is 109 atoms per cubic meter

• About 1 dust particle for every trillion or so atoms

• Vast distances cause absorption

Figure 11.4Milky Way in Sagittarius

Star-forming regionsStar-forming regions

• Emission nebula (nebulae plural)

• Glowing clouds of hot interstellar matter

• Messier objects (18th century system)

• Contain newly formed hot O- or B-type star

• UV light from star ionizes gas

• Electrons recombine, causing glow

Figure 11.5Galactic Plane

Figure 11.6 - M20 top and M8 bottom

Figure 11.7Trifid Nebula

Figure 11.8Nebular Structure

Figure 11.9Emission Nebulae

Figure 11.10Nebular Spectrum

Dark Dust CloudsDark Dust Clouds

• More than 99% of space is devoid of emission nebulae and stars

• Such dark regions are 100 K temperature

• Dark dust clouds found in dark regions

• Cooler and more dense than surroundings

• Made of dust and primarily gas

Figure 11.11Obscuration and Emission

Figure 11.12Dark Dust Cloud

Figure 11.13Horsehead Nebula

Hydrogen 21-cm emissionHydrogen 21-cm emission

• Much of interstellar gas is atomic H

• H has proton and electron

• Two possible quantum states:

• Spins same direction (higher energy)

• Spins opposite direction (lower energy)

• Energy difference between states small

• Emits photon of 21 cm wavelength

Figure 11.14Hydrogen 21-cm Emission

21-centimeter line21-centimeter line

• Radio wavelength• Much larger than dust particles• Passes right through dust clouds

Molecular CloudsMolecular Clouds

• Cold regions - 20 K

• Gas is molecules, not atoms

• Molecular Hydrogen, H2, is common but

doesn’t emit radio wavelengths

• Use tracer molecules, CO, HCN, NH3,

H2O, H2CO

Figure 11.15Molecules near M20

Figure 11.16Molecular Cloud Complexes - CO emission

Formation of sun-like starsFormation of sun-like stars

• Seven stages of formation• Fight between gravity and heat• Gravity pulls particles inward• Heat (via pressure) pushes particles

outward

Figure 11.17Atomic Motions

Triggering a clumpTriggering a clump

• Possibilities:• O and B stars form a shock wave• Supernova forms a shock wave• Interstellar cloud cools and collapses• For a 100 K cloud, need 1057 atoms to

make clump permanent• Roughly mass of sun

Table 11.2Seven Stages of Prestellar Evolution of a Sun-like Star

Stage 1 - Interstellar cloudStage 1 - Interstellar cloud

• Up to 10’s of parsecs across• 10 K• 109 particles/m3

• Thousands of times mass of sun• Collapse and fragmentation

Figure 11.18Cloud Fragmentation

Stage 2 - Cloud fragmentStage 2 - Cloud fragment

• Fragment contains about 1 to 2 M

• About 100X size of solar system• 1012 particles/m3 at center• Radiation escapes except at center• Center might be 100 K

Stage 3 - Fragment/protostarStage 3 - Fragment/protostar

• About size of solar system• 1018 particles/m3 at center• Radiation escapes except at center• Center might be 10,000 K• Center is a protostar

Figure 11.19Orion Nebula, Up

Close

Stage 4 - ProtostarStage 4 - Protostar

• About size of Mercury’s orbit• 1000X luminosity of sun (large area)• Center around 1,000,000 K• Surface is 3000 K• Can track on H-R diagram

Figure 11.20Protostar on the H-R Diagram

Stage 5 - Evolving ProtostarStage 5 - Evolving Protostar

• About 10X size of sun• 10X luminosity of sun (large area)• Center around 5,000,000 K• Surface is about 4000 K• Violent activity with bipolar flow • T-Tauri phase

Figure 11.21Interstellar Cloud Evolution

Figure 11.23Protostellar Outflow

Figure 11.24Protostars

Figure 11.22Newborn Star on the H-R Diagram

Stage 6 - Newborn starStage 6 - Newborn star

• About 1,000,000 km radius• Surface is about 4500 K• Center around 10,000,000 K• Hot enough to ignite nuclear burning

Stage 7 - Main sequenceStage 7 - Main sequence

• Reaches size & luminosity of sun

• Center around 15,000,000 K

• Surface is about 6000 K

• Pressure and gravity now balance

• Reaches Main sequence

• 40 - 50 million years from stage 1 - 7

• On Main sequence 10 billion years

Figure 11.25Prestellar Evolutionary Tracks

Zero-age main sequenceZero-age main sequence

• The most massive fragments contract into O type stars in a million years

• M type stars contract in a billion years• Start “clock” when star reaches main

sequence

Failed starsFailed stars

• Some cloud fragments too small to become stars (< 0.08 M)

• Form brown dwarfs (no fusion)

• Hard to observe

• Perhaps as many brown dwarfs as stars in our galaxy

Figure 11.26Brown Dwarfs

Star clustersStar clusters

• Cloud collapse forms a group of stars• Star clusters useful to study• Formed at same time out of same

cloud

Open clusters & AssociationsOpen clusters & Associations

• Open cluster• Several 100 to several 1000 stars

• Association (more extended)• Few 100 stars

Figure 11.27Newborn Cluster

Figure 11.28Pleiades Open Cluster

Globular clustersGlobular clusters

• Roughly spherical• 100,000s to millions of stars• Generally found away from plane of Milky

Way galaxy• Lack upper main sequence stars• At least 10 billion years old• 150 globulars around Milky Way survivors

of much larger original population

Figure 11.29Globular Cluster

Figure 11.30Young Stars in Orion

Figure 11.31Protostellar Collisions - Supercomputer simulation