ASTR Chap 15 - Star Formation & the Interstellar Medium

At the start of the evolution of a protostar, the radius of a 60 M protostar is roughly _________ a 1 M main-sequence star.

100 times bigger than

An accretion disk forms around a collapsing protostar because infalling material must conserve:

Angular momentum

A _________ is a failed star that shines primarily because of energy derived from its gravitational collapse rather than nuclear burning.

Brown dwarf

Stars forming in molecular clouds tend to form first in the low-density periphery.

False

The lowest-density gas in the interstellar medium is also the coldest.

False

The source of energy for a contracting protostar comes from:

Gravitational Potential Energy

If you wanted to study regions where star formation is currently happening you could use:

H-alpha emission to look for O and B stars 21 cm radiation to find neutral hydrogen clouds Radio emission from carbon monoxide (CO) to find molecular cloud cores Infrared emission to identify T Tauri stars

If you could watch stars forming out of a gas cloud, which stars would form first?

High-mass stars

21-cm radiation is important because:

It allows us to study neutral hydrogen in the interstellar medium

What is the most likely explanation for the dark area in space images?

It is a region with thick dust blocking the starlight coming from behind.

What happens as a protostar contracts?

Its density rises. Its temperature rises. Its radius decreases. Its pressure rises.

The most common types of stars in our galaxy are:

Low-mass Stars

A surprising fact about a 1 M protostar is that, even though nuclear reactions have not yet started in their cores, they are _________ than the Sun.

More luminous

What critical event transforms a protostar into a normal main-sequence star?

Nuclear fusion begins in the core.

The entire process of star formation is really just an evolving balance between:

Pressure & Gravity

According to the conservation of angular momentum, if an ice skater starts spinning with her arms out wide, then slowly pulls them close to her body, this will cause her to:

Spin faster

Because angular momentum must be conserved, as a gas cloud contracts due to gravity it will also:

Spin faster

At which point in time does the collapsing cloud have the greatest angular momentum?

The cloud has the same angular momentum at each point in time.

If a collapsing interstellar cloud formed only a protostar without an accretion disk around it, what would happen?

The forming protostar would be rotating too fast to hold itself together.

What primarily makes it difficult to observe protostars?

They occur in dusty regions

In protoplanetary environments, the plane of an accretion disk is perpendicular to the axis of rotation of the interstellar cloud out of which it forms.

True

Molecular hydrogen atoms are found only inside dense clouds where they are shielded from stellar radiation.

True

Star formation in a molecular cloud can be slowed by the strength of its magnetic field and turbulence caused by supernovae and stellar winds from massive stars.

True

The average density of the interstellar medium is many times less dense than the best vacuum on Earth.

True

We observe neutral hydrogen gas using 21-cm emission.

True

When winds blow the gas away from a forming protostar it becomes visible as a T Tauri star.

True

Sitting in a 100°F hot tub feels much hotter than standing outside on a 100°F day. This analogy illustrates why:

an astronaut would feel cold standing in the 10^6 K intercloud gas

As a protostar contracts:

either the luminosity decreases or the temperature increases

A protostar's evolutionary "track" in the H-R diagram traces out:

how the protostar's luminosity, temperature, and radius change with time

Brown dwarfs are considered failed stars because:

hydrogen fusion never begins in their cores

Where have astronomers observed the existence of planets?

in our Solar System orbiting stars other than the Sun orbiting stars in binary systems traveling on their own through the Milky Way, not orbiting a star

As a protostar evolves, its temperature:

increases due to the kinetic energy of infalling material

The red emission in the photo below is due to:

ionized hydrogen (H II region)

A high-mass protostar remains roughly constant in _________ and increases in _________ as it follows its evolutionary track.

luminosity; temperature

Molecular cloud cores are places where you might find:

protostars Herbig-Haro objects molecular hydrogen (H2) carbon monoxide (CO)

Interstellar clouds are:

regions where hydrogen tends to be denser than the surrounding gas

Which of the following is responsible for heating the bulk of the very hot intercloud gas?

supernovae

Use this Hertzsprung-Russel diagram to complete the following statement. A low-mass protostar remains roughly constant in _________ and decreases in _________ as it follows its evolutionary track.

temperature; luminosity

Dust in the ISM appears dark in _________ wavelengths and bright in _________ wavelengths.

visible; infrared