Interstellar gas is composed of
90% hydrogen, 9% helium by weight.
What effect does even thin clouds of dust have on light passing through them?
It dims and reddens the light of all more distant stars.
Some regions along the plane of the Milky Way appear dark because
stars in that region are hidden by dark dust particles
Due to absorption of shorter wavelengths by interstellar dust clouds, distant stars appear
redder
The spectra of interstellar gas clouds show that they have the same basic composition as
stars
in the Milky Way galaxy, gas and dust are found
everywhere
Emission nebulae like M42 occur only near stars that emit large amounts of
ultraviolet radiation
What is the primary visible color of an emission nebula?
red because of reddening by dust
In which wavelengths have the births of new stars been best mapped recently?
infrared
What two things are needed to create an emission nebulae?
hot stars and interstellar gas, particularly hydrogen
A large gas cloud in the interstellar medium that contains several type O and B stars would appear to us as
an emission nebula
Which of these is not a consequence of dust in the interstellar medium?
red light from the emission nebulae
A "fuzzy" dark or light patch in the sky is called a
a nebula
A reflection nebula is caused by
starlight scattered by dust particles.
interstellar dust clouds are best observed at what wavelength?
radio and infrared
What information does 21 cm radiation provide about the gas clouds?
their motion; their distribution; their density; their temperature
Neutral hydrogen is most obvious in the electromagnetic spectrum at:
21 cm in the radio region.
What makes the subject of star formation so difficult and complex?
Stars live too long to be observed from birth to death.
What is the force that keeps a main sequence star from blowing apart?
gravitation
Which event marks the birth of a star?
fusion of hydrogen atoms into helium atoms
What happens when an interstellar cloud fragment shrinks?
It first becomes opaque; Density rises; Temperature rises; Pressure rises.
During a protostar's T Tauri phase, it
may develop very strong winds
A newly formed protostar will radiate primarily at which wavelength?
infrared
As a star forms, the photosphere first appears:
when the protostar forms
At what stage of evolution do T Tauri stars occur?
when a protostar is on the verge of becoming a main sequence star
What is characteristic of a main sequence star?
The rate of nuclear energy generated in the hydrogen to helium fusing core equals the rate radiated from the surface.
On an H-R diagram, a protostar would be
above and to the right of the main sequence.
What is the force that keeps a main sequence star from collapsing on itself?
radiation pressure
Most stars, probably all stars, formed
in a cluster of stars
From stage 4 to stage 7 of star formation, the object plotted on the H-R diagram moves so that
its luminosity decreases, while its temperature increases.
Stage 4 of star formation is when the object can exhibit violent surface activity producing extremely strong protostellar winds. This phase is also called the
T Tauri phase
What is the key factor that determines the temperature, density, radius, luminosity, and pace of evolution of a protostellar object?
mass of the nebula
A cloud fragment too small to form a star becomes:
a brown dwarf
During stage 3 of star formation, the dense, opaque region at the center of the cloud is called a
protostar
Which is characteristic of globular star clusters?
old age and hundreds of thousands of stars, only about 30 ly wide
The single most important determinant of the temperature, density, radius, luminosity, and pace of evolution of a protostar is its
mass
Higher mass protostars enter the main sequence
faster and at a higher luminosity and temperature.
Which of these is NOT a source of the shock waves that lead to protostars?
expanding Herbig-Haro objects
Why are star clusters almost ideal "laboratories" for stellar studies?
Stars in clusters have the same age, similar composition, and are at the same distance away.
When a typical open cluster forms, which type of stars are formed most often?
low mass M type dwarfs
What are the characteristics of an open cluster of stars?
a few hundred, mainly main sequence stars