The Interstellar MediumThe Interstellar MediumThe Interstellar MediumThe Interstellar Medium (ISM)

Material found in space between stars

- space almost a perfect vacuum

- large amounts of material are present

Two Components

Interstellar DustInterstellar DustInterstellar DustInterstellar Dust

Interstellar GasInterstellar GasInterstellar GasInterstellar Gas

Dust in the ISMDust in the ISMDust in the ISMDust in the ISM - solid grains of heavier elements -silicon, carbon, ice

- 2% of ISM

Dust absorbs (blocks) light and heats up

� Interstellar ExtinctionInterstellar ExtinctionInterstellar ExtinctionInterstellar Extinction

� visible in InfraredInfraredInfraredInfrared

Sombrero Galaxy, visible

Sombrero Galaxy, infrared

Dust can also scatter light - blue scatters more than red

���� Reflection NebulaReflection NebulaReflection NebulaReflection Nebula (blue)

Gas in the ISMGas in the ISMGas in the ISMGas in the ISM

- atoms, molecules of H, He

- 98% of ISM (mostly H)

- well mixed with dust

Hydrogen gas can exist in three states

- depending on conditions

1. Atomic Hydrogen (H(H(H(HI)))) - most common form of Hydrogen

- in most regions (except hottest/coldest)

- low density: 1 atom/cm3 (air = 3 x 1019)

- gives off energy at λ = 21 cm (radio)

HI map of Milky Way galaxy

HI map of galaxies M81 and NGC 3077

2. Ionized Hydrogen (H(H(H(HII)))) - occur in the hottest regions in

space (T > 10,000 K)

- surrounding very hot stars (O, B)

- gives off H emission lines (red)

� Emission NebulaEmission NebulaEmission NebulaEmission Nebula

Rosette Nebula

Orion Nebula ↑

Horsehead Nebula (Orion) ↓

North American Nebula

IC 5067, The Pelican Nebula

3. Molecular Hydrogen (H(H(H(H2222)))) - occur only in coldest, densest

regions (T < 50 K)

- completely invisible

- can only find with “tracers”

- other molecules in same region

that are visible (CO, H2O, etc)

- estimated 105 H2 for every 1 CO

CO map of Orion H2O in Orion Nebula

a little bit of everything:

Rho Ophiuchus

Keyhole Nebula

Thackeray’s Globules

Trifid Nebula

Life of a StarLife of a StarLife of a StarLife of a Star

The “Battle”: The “Battle”: The “Battle”: The “Battle”: GravityGravityGravityGravity vs. vs. vs. vs. Gas PressureGas PressureGas PressureGas Pressure

Gravity depends on mass and distance Pressure depends on temperature

Gas Pressure Gravity If Gravity < Pressure,

- will expandexpandexpandexpand (an HII region)

If Gravity = Pressure

- Equilibrium/stableEquilibrium/stableEquilibrium/stableEquilibrium/stable (a star)

If Gravity > Pressure

- will contractcontractcontractcontract (molecular cloud)

To form stars and planets: - best locations are Giant Molecular Clouds

- cold and dense

���� Two Important PrinciplesTwo Important PrinciplesTwo Important PrinciplesTwo Important Principles ����

Gravitational ContractionGravitational ContractionGravitational ContractionGravitational Contraction::::

1. Release of grav. potential energy

- contraction � rising temperature

2. Contraction of an “Ideal” (normal) Gas

- will continue indefinitely

- until alternate energy source is

available

- or until it is no longer an ideal gas

Forming stars:Forming stars:Forming stars:Forming stars:

Giant Molecular Cloud - starts collapse and fragments into smaller and smaller regions

- one large cloud can form up to 1 million stars

http://www.astro.ex.ac.uk/people/mbate/Cluster/cluster3d.html

PROTOSTAR:PROTOSTAR:PROTOSTAR:PROTOSTAR:

- Smallest fragment of a cool, dense gas cloud

What happens:

• Protostar shrinks due to gravity

• Internal temperature increasing

Why?

• Gravity >>>> Gas Pressure

• Grav. Pot. Energy converted to heat

>> Hydrogen Fusion begins <<>> Hydrogen Fusion begins <<>> Hydrogen Fusion begins <<>> Hydrogen Fusion begins <<

What happens:

• H Fusion starts at center of protostar

Why?

• Temperature at center reaches 10 million K

MAIN SEQUENCE STARMAIN SEQUENCE STARMAIN SEQUENCE STARMAIN SEQUENCE STAR::::

What happens:

• Becomes stable (normal) star, H fusion in

core

Why?

• Core and envelope are 75% Hydrogen

• Energy source stops collapse

• Establishes Hydrostatic & Thermal Equil.

Questions & Answers:Questions & Answers:Questions & Answers:Questions & Answers: Q: Why are many stars (~50%) in binary or multiple star systems? A: Fragmentation of Cloud. - Many stars are formed together in the same region Q: Why are 90% of all stars on the Main Sequence? A: Main Sequence = H fusion. - Every star starts with H fusion - Stars undergo H fusion for most of their life Q: Why are there many more low mass stars (M) than high mass stars (O)? A: Fragmentation of Cloud - much more likely to have many small “lumps” or fragments - much less likely to have few massive “lumps” or fragments

Q. Why is there a maximum mass for a star and a minimum mass?

A: Heat from Gravitational Collapse - Too much mass: so much heat that the gas pressure overwhelms gravity (> 150 M�)

- Too little mass: not enough heat to start H fusion. (< 0.08 M�)

- Object becomes Brown Dwarf

Main Sequence Lifetimes: - determined by amount of fuel - rate of fuel consumption

TM.S. = Mass / Luminosity