CHAPTER 18 AST104

The effect of interstellar dust on starlight is

TO DIM AND REDDEN DISTANT STARS BY PREFERENTIALLY SCATTERING THEIR BLUE LIGHT

Which of the following is not a characteristic of T Tauri stars?

NUCLEAR REACTIONS IN THE CORE

There are several mechanisms that can trigger star formation in a cold, dark nebula. In each of these the key to star formation is

TO COMPRESS THE GAS AND DUST SO THAT GRAVITATION WILL OVERCOME THE GAS PRESSURE

An object that is formed by the gravitational collapse of an interstellar cloud, and is slowly contracting ad heating up to become a stark, is called

PROTOSTAR

What is the characteristic color of an emission nebula?

RED

The main factor which determines the rate at which a protostar evolves is its

INITIAL MASS, LARGER MASSES EVOLVING FASTER

An H Ⅱ region is a region of

IONIZED HYDROGEN AROUND ONE OR MORE O AND B STARS

The space between stars is now known to contain

GAS, MADE UP OF ATOMS, MOLECULES AND DUST PARTICLES

What is the lowest mass that an object can have and still be a star?

0.08 SOLAR MASSES

Where would you look to find a newly forming star?

AN OPEN CLUSTER

Interstellar Medium

the matter between stars, composed of two components, gas and dust, intermixed throughout all of space

Dust Grain

An interstellar dust particle, roughly 10^-7 m in size, comparable to the wavelength of visible light

Extinction

the dimming of a starlight as it passes through the interstellar medium

Reddening

Dimming of starlight by interstellar matter, which tends to scatter higher-frequency (blue) components of the radiation more efficiently than the lower-frequency (red) components.

Polarized

the alignment of the electric fields of emitted photos, which are generally emitted with random orientations

Emission Nebulae (Definition)

produced when a hot star excites the gas near it to produce an emission spectrum

Dust Lanes

a dark lane of obscuring interstellar dust in an emission nebula or galaxy

Reflection Nebula

produced when starlight scatters from a dusty nebula, consequently the spectrum of a reflection nebula is just reflected absorption spectrum of starlight

HI Regions

Region of space containing primarily neutral hydrogen.

Dark Dust Clouds

a large cloud, often many parsecs across that contain gas and dust in a ratio of about 1012 gas atoms for every dust particle. Typical densities are few tens or hundreds of millions of particles per cubic meter

Molecular Cloud Complexes

Collection of molecular clouds that spans as much as 50 parsecs and may contain enough material to make millions of Sun-size stars.

what does the interstellar medium consist of?

gas and dust.

What is the dust like in interstellar mediums?

is more like soot or smoke; larger clumps of particles

What does Dust do in Interstellar mediums?

absorbs light, and reddens light that gets through

Interstellar dust grains

are complex in shape (left); on the right is the result of computer modeling of how a dust grain might grow.

nebula is a term

used for fuzzy objects in the sky

Dust clouds:

block light from more distant stars.

The matter among the stars is collectively called:

the interstellar medium, and is made up of gas and dust.

The gas is in the interstellar medium is:

made up of atoms and molecules.

The interstellar dust contains:

thicker clumps of atoms and molecules; more complex than the gas.

Light can only be absorbed or scattered by a particle with a size that:

is comparable in size or larger than the wavelength of the light.

Dust blocks light with shorter wavelengths:

more easily.

Size of a dust grain is:

10-7 m - about the size of visible light wavelength.

So, what type of light is interstellar dust easily blocking?

visible light and uv light.

The dimming of starlight (by partially blocking it) by interstellar matter is called:

extinction.

The interstellar medium blocks much more of stars' blue light, which has shorter wavelengths, than red light. This effect is called:

reddening.

Can anyone think of a "everyday real life" example of reddening?

Red sunsets.

Even though stars appear dimmer and redder than they normally would from the interstellar medium, the patterns of their spectral lines:

do not change.

Since the patters of their spectral lines do not change:

we can determine their spectral class, giving us their true luminosity and color.

We can then learn about how much dust there is by:

studying the effects that the dust had on the starlight.

Interstellar Gas Density:

~10^6 atoms/m3 (1 atom/cm^3)

Interstellar Dust Density:

~10^-6 dust particles/m3, (1000 per cubic kilometer)

With densities so low, how is starlight blocked?

Space is VAST, and particle sizes are about the same size as visible wavelengths (which is the type of light many stars primarily emit.)

Interstellar Gas Composition:

90% hydrogen, 9% helium, and 1% heavier elements

Interstellar Dust Estimated Composition:

silicates? graphites? iron? (Might be formed from interstellar gas.)

Dust probably also contains:

"dirty ice" and trace amounts of methane, ammonia, and other chemical compounds

Interstellar gas atoms are:

roughly spherical

Interstellar dust is believed to be:

rod-like

Dust is believed to be rod-shaped because:

starlight is both dimmed and polarized, or aligned.

Historically, nebula was used to describe:

any "fuzzy" patch in the sky.

Emission nebula

bright, glowing nebula

Dark nebula

no light passing through

Emission nebulae are:

stellar nurseries; where stars are "born."

Emission nebulae are regions of:

glowing, ionized gas.

Emission nebulae glow because they have:

hot O and B stars within their centers emitting plenty of energetic ultraviolet light.

UV light ionizes surrounding gas, and when electrons recombine with atoms:

visible light is emitted.

We mostly see the most common type of light emitted by hydrogen - red light - because:

emission nebulae are mostly made up of hydrogen.

Emission nebulae form only near stars that emit plenty of:

ultraviolet light (UV light)

Emission nebulae often contain dust lanes:

Obscuring dust that are a part of the nebulae.

Reflection nebulae are different from an emission nebula, are caused by, and appear:

blue due to the scattering of light due to starlight scattered by dust particles.

A real-life example of the color of reflection nebulae in the earth's atmosphere:

The blue sky.

Short wavelengths are:

easily scattered.

Structures in nebulae are formed by:

photoevaporation (heats and disposes radiation, "eats away")

Emission nebulae often called HII regions because:

they are comprised primarily of ionized hydrogen.

Spectroscopic studies of emission nebulae show that they are made up of the same stuff that make up the Sun and other stars.

This makes sense - nebulae are the site from which stars form!

The average emission nebulae temperature is:

8000 K.

Most interstellar clouds are:

Bigger than our solar system.

Emission nebulae can emit "forbidden" lines, which aren't truly forbidden, just rare.

The green light in the Orion nebula is a result of a forbidden transition of O III.

The Sun seems to reside in a heated cavity of space called the "Local Bubble."

Such bubbles likely form from stellar explosions that occurred long ago.

Dark dust clouds have temperatures of:

a few tens of kelvins (colder than surroundings)

Dark dust clouds are:

Very dense: 10^7 - 10^12 atoms/m3

Dark dust clouds:

Absorb optical light and emit radio waves

Dark dust clouds are cold, and so produce:

an absorption spectrum.

Dark dust clouds absorption spectra reveal that:

they have compositions similar to other astronomical objects, like stars.

The life history of our own star is known to us by

Directs observation of its current stage plus other stars in their stages of modeling

The interstellar medium (ISM) consists of:

Gas and Dust

In modern astronomy, a nebula (plural nebulae) is a:

cloud of a gas in space.

The dense, cold component of the interstellar medium from which stars are believed to form is made of:

molecular clouds.

The composition of molecular clouds in the interstellar medium is dominated by:

molecular hydrogen gas and helium gas.

Molecular clouds are probably about 1 per cent dust by mass.

The dust is VERY IMPORTANT to these clouds. It is HIGHLY OPAQUE to visible and ultraviolet light, and so keeps most hard electromagnetic radiation out of the inner regions of the clouds. This prevents the destruction of molecules by hard radiation. Moreover, it is probable that many molecules form on dust grains: free atoms stick onto the grains, meet there, bond, and then escape in molecular form: i.e., the grains act as catalysts. Dust tends to promote molecule formation and molecules tend to need dust. Thus, whenever you have a lot of dust, you often have molecules and vice versa.

"Let's play Jeopardy! For $100, the answer is: It happens whenever a star changes its luminosity and/or its surface temperature." What is ___________, Alex?

movement on the Hertzsprung-Russell (HR) diagram

Star formation in a dusty molecular cloud probably requires some triggering event to initiate the collapse to dense cores that will become stars. Two possible trigger mechanisms are:

SUPERNOVAE which compress molecular clouds and CLOUD-CLOUD COLLISIONS which also compress the colliding molecular clouds.

In a FREE-FALL contraction of part of molecular cloud:

the part starts fall to toward a high density point because of gravitational attraction. Pressure forces are negligible in slowing the fall because it is a free-fall contraction.

The collapsing dense regions that develop into stars and initially have temperatures of order 10 K are called:

dense cores.

A protostar is sometimes conveniently defined to be a:

dense core of gas contracting to become a star that is hot enough to radiate in the infrared, but not yet sufficiently hot for nuclear burning.

The contraction of a protostar is halted eventually by:

the heat generated by the turning on of nuclear burning which INCREASES the gas pressure inside the protostar.

Star formation in giant molecular clouds often results in the formation of OB associations: collections of hot, bright OB stars that ionize the surrounding molecular cloud and evaporate dust because of their strong ultraviolet emission. The gas region ionized by an OB associations is called a/an:

H II region.

"Let's play Jeopardy! For $100, the answer is: They are relatively thin, round objects consisting of gas and/or dust and/or particles: the material goes around some large astro-body in nearly circular orbits of varying radii in the same direction." What are __________, Alex?

disks

Disk formation is:

a common event in star formation as far as astronomers can tell.