The Interstellar Medium

and

Star Formation Interstellar Matter

Nebulae

Absorption

Emission

Neutral

Molecular

H II Regions

Reflection

Star Formation

Star Forming Regions

Stages of Formation

Examples The Fox Fur Nebula

The space between the stars is not

completely empty, but filled with a very

dilute gas and dust. It produces some of

the most beautiful objects in the sky.

We are interested in the interstellar

medium because

a) dense interstellar clouds are the birth

place of stars

b) clouds alter and absorb the light from

stars behind them

The Interstellar Medium (ISM)

Interstellar Matter The interstellar medium consists of gas and dust.

Atoms, mostly hydrogen

and helium, and small

molecules make up the

gas.

The dust is more like

clumps of soot or

smoke (and ice?).

Dust absorbs light and

reddens light that gets

through by scattering.

This image shows

distinct reddening of

stars near the edge of

the dust cloud

Interstellar Reddening

Visible Infrared

Barnard 68

Blue light is strongly scattered

and absorbed by interstellar

clouds

Red light can more easily

penetrate the cloud, but is still

absorbed to some extent

Infrared

radiation is

hardly

scattered at all

Foreground

interstellar

clouds make

the

background

stars appear

redder

Interstellar Matter

Reddening

can interfere

with

blackbody

temperature

measurement,

but spectral

lines do not

shift

Interstellar Matter Interstellar dust grains are complex in shape (left); on the

right is the result of computer modeling of how a dust

grain might grow.

Interstellar Matter

Dust grains are known to

be elongated, rather than

spherical, because they

polarize light passing

through them.

They also may be

slightly conductive

because they polarize

and rotate radio waves.

Nebulae

“Nebula,” Latin for

cloud, is a general

term used for fuzzy

objects in the sky.

A dark or

absorption nebula

is a dust cloud.

An emission

nebula glows by

excitation from hot

stars.

Some nebulae are

not clouds. They

will come up later. M20: The Trifid Nebula

Dark or Absorption Nebulae

Barnard 86

Dense clouds

of gas and

dust absorb

the light from

the stars

behind;

They appear

dark in front

of a bright

background Horsehead Nebula

Dark or Absorption Nebulae

Light from distant stars may pass through more than

one nebula; it is often possible to sort out the spectra

of the star and the nebulae.

Structure of the ISM

HI clouds: Cold (T ~100 K) clouds of neutral hydrogen (HI)

Moderate number density (n ~10 – a few hundred

atoms/cm3)

Size: ~100 pc—they can be detected at radio

frequencies

Hot intercloud medium (HII regions): Hot (T ~ a few 1000 K)

Ionized hydrogen (HII)

Low density (n ~0.1 atom/cm3)

Gas remains ionized because of the very low

density.

The ISM contains two main types of emission nebulae:

Detecting HI clouds

21-Centimeter Radiation Interstellar gas emits low-energy radiation by means of

the spin-flip transition in the hydrogen atom.

21-Centimeter Radiation

The emitted photon has a

wavelength of 21 centimeters,

which is in the radio portion of

the electromagnetic spectrum.

Actual 21-cm spectra are

complex because the lines are

Doppler-shifted and

broadened.

The Doppler shift is caused by

the radial velocity of the

cloud. It is used to measure

the radial velocity.

Molecular Clouds The densest gas clouds are also very cold, around 20

K. These clouds tend to contain more molecules than

atoms.

Transitions between rotation states of a molecule emit

radio-frequency photons unique to the molecule.

Molecular Clouds

Fortunately, radio waves are not strongly

absorbed, so molecular gas clouds can be

detected even though there may be other gas

and dust clouds in the way.

These clouds consist mostly of molecular

hydrogen, which unfortunately does not emit

in the radio portion of the spectrum.

Other molecules present are CO, HCN, NH3,

H2O, CH3OH, H2CO, and more than a hundred

others.

Molecular Clouds

Here are some

formaldehyde (H2CO)

emission spectra from

different parts of the Trifid

Nebula (M20).

Emission Nebulae (HII Regions)

HII regions are

created by UV

radiation from hot

stars ionizing neutral

hydrogen. This

induces a shock

wave front which

emanates from a star

and penetrates the

neutral hydrogen

cloud until the

energy of the

radiation drops

below the ionization

energy of hydrogen

Emission Nebulae (HII Regions)

A hot star

illuminates a gas

cloud

It excites and/or

ionizes the gas

(electrons kicked

into higher energy

states)

Electrons

recombine, falling

back to the

ground state to

produce Hα

emission lines. The Fox Fur Nebula NGC 2246 The Trifid Nebula

Reflection Nebulae

These images illustrate a

reflection nebula and how it

forms. Light from a reflection

nebula is usually blue coming

from scattered light.

Emission and Reflection Nebulae

Star-Forming Regions

Star formation is ongoing; star-forming regions

are seen in our galaxy as well as others.

Star-Forming Regions

Star formation happens when part of a

dust cloud begins to contract under its

own gravitational force

As it collapses, the center becomes

hotter and hotter until nuclear fusion

begins in the core.

Interstellar clouds are usually stable and

some form of shock is thought to be

necessary to begin collapse.

Star-Forming Regions

Rotation can interfere

with gravitational

collapse, as can

magnetism; clouds may

very well contract in a

distorted way.

Shock Waves and Star Formation

Shock waves from a nearby star formation can be

the trigger needed to start the collapse process in

an interstellar cloud.

Shock Waves and Star Formation

Possible triggers that cause shock

waves:

Death of a nearby Sun-like star

Supernova

Density waves in galactic spiral

arms

Galaxy collisions

Shocks Triggering Star

Formation

Henize 206

(infrared)

The Formation of Stars Like the Sun

As a star forms from an interstellar cloud, it goes

through several evolutionary stages

The Formation of Stars Like the Sun The first stage of stellar evolution

Stage 1:

The interstellar cloud starts to contract. As it

contracts, the cloud fragments into smaller

pieces.

The Formation of Stars Like the Sun

Stage 2:

Individual cloud fragments begin to collapse.

Once the density and temperature is high

enough, there is no further fragmentation.

Stage 3:

The interior of the fragment has begun to heat

from the loss of gravitational energy and the

center is about 10,000 K.

The Formation of Stars Like the Sun

Stage 4:

The core of the cloud is

now a protostar, and

makes its first

appearance on the H–R

diagram.

The Formation of Stars Like the Sun

Planetary formation has begun, but the protostar

is still not in equilibrium – all heating comes

from gravitational collapse.

The Formation of Stars Like the Sun Stages 5, 6 and 7 can be followed

on the H–R diagram:

The protostar’s luminosity

decreases even as its

temperature rises because it is

becoming more compact.

At stage 6, the core reaches

106 K, and nuclear fusion begins.

The protostar has become a star,

but it is not in equilibrium.

The star continues to contract

and increase in temperature until

it is in equilibrium. This is stage

7: The star has reached the main

sequence and will remain there

as long as it has hydrogen to

fuse.

The Contraction of a Protostar

When the star first reaches the main sequence, it will be on the lower edge

of the main sequence band. This is called zero age main sequence (ZAMS)

From Protostars to

Stars

Ignition of

fusion

processes

4 1H 4He

Star emerges

from the

enshrouding

dust cocoon

Evidence of Star Formation

Nebula around

S Monocerotis:

Contains many massive,

very young stars,

including T Tauri Stars:

strongly variable; bright

in the infrared.

Protostellar Disks and Jets

Herbig-Haro Objects

Disks of matter accreted onto the protostar (“accretion

disks”) often lead to the formation of jets (directed

outflows; bipolar outflows): Herbig-Haro objects

Globules

Bok

globules:

~10–1000

solar

masses;

Contracting

to form

protostars

Globules

Evaporating gaseous globules

(“EGGs”): Newly forming stars

exposed by the ionizing radiation

from nearby massive stars

The Orion Nebula:

An Active Star-Forming Region

Location of

the Great

Nebula in

Orion (M 42)

The Trapezium

The Orion Nebula

The 4 trapezium stars:

Brightest, very young

(less than 2 million

years old) stars in the

central region of the

Orion nebula

Infrared image: ~ 50

very young, cool, low-

mass stars X-ray image: ~ 1000

very young, hot stars

Only one of the

trapezium stars is

hot enough to

ionize hydrogen in

the Orion nebula