Looking at the Interstellar Medium A Star Is Born Looking at the Interstellar Medium

The Stuff in Space Material exists between the stars and planets “Building Blocks” for more stars and planets Interstellar Medium Composed of Gas and Dust

Interstellar Medium Seen in telescopes Large clouds of gas and dust are stellar “nurseries” ISM is clumpy! Obscures objects beyond it

Interstellar Gas Gas is mostly individual atoms Individual neutral atoms and ions Molecules Electrons Hydrogen is main component Very tenuous, doesn’t block light Some parts cool, some parts hot

Interstellar Dust More complex composition 1% of ISM Clumps of atoms and molecules Much larger than gas particles so… Can block light Composition is not well known

Interstellar Dust Light passes through dust cloud Extinction-dimming of light Longer Wavelengths pass through Short Wavelengths get absorbed Reddening- scattering of blue light Red light makes it through Changes star’s apparent color

Dust and Gas Dust is believed to come from mass loss winds in stars (like solar wind) Gas of wind can cool and solidify Dust grains provide coagulation seeds for molecules

Evidence of Gas and Dust Observed Nebulae (clouds) Dust and Gas form different types of nebulae 4 basic types Emission (Bright) nebula Dark nebula Reflection nebula Molecular Clouds

Emission Nebula Example: Orion Nebula Spectra has emission lines (hot gas) Does not shine under it’s own light Powered by hot stars H II region (ionized hydrogen)

Emission Nebula OB associations Form H II region hot spot on molecular cloud Drives new star formation Reddish hue is from Hydrogen H II regions are star “nurseries” Very bright!

Emission Nebula Heart and Soul Nebulae Astronomy Picture of the Day

Emission Nebula The Eagle Nebula and a close up of a star forming region Astronomy Picture of the Day

Molecular Clouds Composed mostly of H2 (hard to detect) Use other molecules as tracers 80 known ISM molecules Many are organic molecules Associated with H II regions Majority of ISM is here

Molecular Clouds 10’s of ly across (6 trillion miles= 1ly) Cool clouds (Dark) Occur in huge complexes Contain enough gas to make millions of stars 1000 + complexes in our Galaxy

Molecular Cloud Barnard 68 APOD

Molecular Cloud Horsehead Nebula Note: there are several types of nebulae in this panorama APOD

Dark Nebula Example: Snake Nebula Contain gas and dust Dust blocks light Cool (10’s K) Larger than our Solar System Also, dust lanes w/ H II regions

Dark Nebula Snake Nebula APOD

Reflection Nebula Gas and Dust Absorption line spectra (stars) Doesn’t generate own light Scatters blue light from starlight passing through Nebula appears blue (like sky) Example: Pleiades Nebula

Reflection Nebula Witch Head Nebula APOD

A Panorama in Orion Can you ID the different types of nebula here? APOD

Neutral Hydrogen Presence was suspected H II regions come from it Not observed until 1951 Need to observed from its own radiation Low-energy Radio emissions from the gas itself

21-cm Radiation From single electron orbiting nucleus Not from excitation, from spin of electron 2 possible configurations Parallel Anti-parallel Lower energy one preferred Spin flipping emits 21-cm photon

Coronal Interstellar Gas Very hot gas Highly Ionized Very low density Exists between clouds Why?

Star Birth Star’s life is a dance between gravity and radiation pressure All stars have a similar origin Cold, dark molecular clouds Collapse to form stars

Cloud Collapse Giant molecular cloud Something to trigger collapse (shockwave) Cloud begins to collapse under it’s own weight Jean’s Instability Cloud will begin to heat up as it collapses

Cloud Collapse Heating causes outward pressure Balances out gravity pushing inward Collapse is “lumpy”, fragmentation Pockets collapse faster (denser) Centrally dense region is where star will eventually form

Cloud Collapse Fragmentation occurs in several ways Dozens of Massive Stars Hundred or Thousands of Sun-like Stars No evidence for single star formation Single stars must escape after formation Collapse can occur with or without rotation

Cloud Fragmentation Sun sized star comes from 2 solar mass fragment 100 times radius of Solar System Less than 100K Fragmentation ceases as density of each fragment increases Interior becomes opaque Radiation is trapped

Fragmentation to Protostar 10,000 + years passed Central part 10,000K Outer part cool Dense, opaque central region=Protostar Still contracting, material raining down on it

Protostar (Sun predecessor) 1,000,000 years have passed Not hot enough for P-P chain Still about size of Mercury’s orbit More luminous than Sun (bigger) Surrounded by a dusty shroud Vaporizes nearby dust

Protostar Protostar Dust Free Zone IR photon Outer Envelope of Gas and Dust

Protostar to Star Protostar first appears on HR diagram Protostar loses shroud Vaporizes Falls onto Star Blown away by wind Contracts, Luminosity , Temp  Hayashi track on HR Diagram Violent Surface Wind (T Tauri Star)

T-Tauri Stars Exhibit strong winds Bipolar flows Clear gas and dust away from young star so it is at last visible Where the outrushing gas impacts stationary gas in the ISM a bright hot spot appears Herbig-Haro object

T-Tauri Stars Also appear to vary in brightness Likely associated with star’s magnetic field, much like the active Sun Have “star spots” like sun spots Are still collapsing to final size Larger in size and therefore brighter than they will be as main sequence stars

T-Tauri Star A false color image of the T-Tauri system Note jet APOD

T Tauri Tantrum

HH object APOD Hubble Heritage

Pre-Main Sequence Star 10,000,000 Years Now a “True Star” P-P Chain has begun Larger and Cooler than Main Sequence Star Still slowly contracting

At last, the Sun! Contraction continues Central Temp=15,000,000K Outward pressure balances inward gravity Hydrostatic Equilibrium (HSE) Contraction Stops, Balance Reached Main Sequence Star! All main sequence stars are in HSE

Massive Stars Steps occur faster Collapse of Cloud occurs in similar way Central part still collapses faster Fragments are larger No T Tauri phase Chain Reaction from OB association >100 M Gravity can’t hold together

Failed Stars Some cloud fragments are too small Don’t get hot enough H-fusion never occurs Warm due to collapsing Brown dwarfs Cooling objects <0.08M (Jupiter)

Collapse with Rotation Rotation during collapse leads to orbiting clumps around protostar These clumps can attract more material via gravity and form planetismals These could be a future solar system

Summary All stars have a “cloudy” beginning Different types of nebula Stars collapse from clouds