68 terms

ASTRO CHAPTER 18

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accretion
The gradual accumulation of matter in one location, typically due to the action of gravity.
Barnard object
One of a class of dark nebulae discovered by E. E. Barnard.
bipolar outflow
Oppositely directed jets of gas expelled from a young star.
Bok globule
A small, roundish, dark nebula.
circumstellar accretion disk
An accretion disk that surrounds a protostar.
cluster of stars
A group of stars that formed together and that have remained together due to their mutual gravitational attraction
cocoon nebula
The nebulosity surrounding a protostar.
dust grain
A microscopic bit of solid matter found in interplanetary or interstellar space.
emission nebula
A glowing gaseous nebula whose spectrum has bright emission lines.
evolutionary track
The path on an H-R diagram followed by a star as it evolves
fluorescence
A process in which high-energy ultraviolet photons are absorbed and the absorbed energy is radiated as lower-energy photons of visible light.
giant molecular cloud
A large cloud of interstellar gas and dust in which temperatures are low enough and densities high enough for atoms to form into molecules.
H II region
A region of ionized hydrogen in interstellar space.
Herbig-Haro object
A small, luminous nebula associated with the end point of a jet emanating from a young star.
interstellar extinction
The dimming of starlight as it passes through the interstellar medium.
interstellar medium
Gas and dust in interstellar space.
interstellar reddening
The reddening of starlight passing through the interstellar medium as a result of blue light being scattered more than red.
Nebula
A cloud of interstellar gas and dust.
nebulosity
Nebula; a cloud of interstellar gas and dust.
OB association
A grouping of hot, young, massive stars, predominantly of spectral types O and B.
open cluster
A loose association of young stars in the disk of our Galaxy; a galactic cluster.
protoplanetary disk (proplyd)
A disk of material encircling a protostar or a newborn star.
prostar
A star in its earliest stages of formation.
recombination
The process in which an electron combines with a positively charged ion.
reflection nebula
A comparatively dense cloud of dust in interstellar space that is illuminated by a star.
stationary absorption line
An absorption line in the spectrum of a binary star that does not show the same Doppler shift as other lines, indicating that it originates in the interstellar medium.
stellar association
A loose grouping of young stars.
stellar evolution
The changes in size, luminosity, temperature, and so forth that occur as a star ages.
supernova remnant
The gases elected by a supernova.
supersonic
Faster than the speed of sound.
T Tauri stars
Young variable stars associated with interstellar matter that show erratic changes in luminosity.
We see an emission nebula predominantly in
the Balmer Hα red line, from recombination of electrons with nuclei in ionized hydrogen
An H II region is a region of
ionized hydrogen around one or more O and B stars.
The distinctive color of a reflection nebula is
blue, caused by the preferential scattering of starlight by very small dust grains.
Star clusters of the same type and structure appear to become fainter than expected, on the
basis of the inverse square law alone, as distance from the Sun increases. This is because
some of the light is scattered and absorbed by interstellar dust and gas between distant clusters and Earth.
A cocoon nebula is
a cloud of dust hiding a young protostar from sight
How does the temperature of an interstellar cloud affect its ability to form stars?
Higher temperatures inhibit star formation.
The main factor which determines the rate at which a protostar evolves is its
initial mass, larger masses evolving faster.
How long does it take for a 1-solar-mass star to pass through the pre-main-sequence phase?
20,000,000 years
At what point in its evolution will a protostar stop shrinking and stabilize into a star?
when nuclear processes generate enough energy and internal pressure to resist gravitational contraction
At what stage of its evolutionary life is the Sun?
main-sequence—middle age
A Herbig-Haro object is
glowing interstellar gas, heated by a high-velocity jet of matter from an evolving star.
The stars in an open cluster are useful for studying the early stages of stellar evolution because all the stars in a cluster have the same
age
If a protostar were able to contract (get smaller) without any change to its surface temperature, what would happen to its luminosity?
It would decrease, due to the smaller surface area of the protostar.
Which range of electromagnetic radiation is useful for observing new-born protostars within their gas and dust nebulas?
infrared
Giant molecular clouds of H2 and CO gas are found in which regions of our galaxy?
along the spiral arms
At what wavelengths have astronomers mapped and studied the distribution of the giant molecular clouds in space?
millimeter wavelengths, using radio telescopes
Which of the following easily observed molecular species is used as a tracer for the fundamental but difficult-to-observe H2 molecules in giant molecular clouds?
CO
If astronomers have only been observing stars for a very small fraction of the lifetime of a typical star (10-8 for a Sun-like star), how can they possibly have enough information to decide upon a star's evolutionary developments throughout its life?
By observing stars at all ages and stages of development and fitting these observations to a model developed from laws of physics.
Yes. A careful classification of the properties of all manner of stars has led to models of development that explain most observed stellar features. Section 18-1
A region of interstellar space shining with a reddish hue is a
region of hot, ionized gas
Yes. The region is ionized because ultraviolet light from hot stars has ejected the electrons from hydrogen and other atoms. When the electrons recombine with protons to form neutral hydrogen atoms again, they emit photons of Hα photons of red light. Section 18-2
What kinds of stars are responsible for the ionization of the gas in an H II emission nebula, which, when the electrons recombine with the ions, causes the nebula to emit Balmer Hα and other atomic spectral lines?
Hot O and B stars
Correct. The predominant emission from these hot stars is UV radiation which ionizes hydrogen and other atoms. Recombination of electrons with the nuclei and ions produces Hα and other atomic line emissions. Section 18-2
Many young stars in new clusters appear to be surrounded by a blue, nebulous haze. The physical process that produces this blue nebulosity is
The preferential scattering of blue starlight by very fine dust grains in the interstellar medium.
Yes. These are known as reflection nebulae, the dust grains (about the same size as cigarette smoke particles) scattering blue light preferentially. Section 18-2
What is the typical size of the particle that is the source of the blue light by which we see a reflection nebula?
Small dust grains about 500 nm in diameter, which scatter starlight.
Yes. Dust grains of this size preferentially scatter the blue light more efficiently than the red light from hot stars within the nebula. Section 18-2
Observation of spectral lines in the spectrum of a binary star system reveals several spectral absorption lines, which remain stationary in wavelength, interspersed among lines that move back and forth periodically in response to the motion of the two stars. What is the origin of these stationary lines?
Absorption by atoms in the interstellar gas between the star system and Earth.
Yes. Absorption lines from atomic species on the star surfaces will show Doppler shift because of the star's orbital motions, while fixed lines must be caused by stationary material in the line of sight, in this case, in the interstellar medium. Section 18-2
New stars are most likely to form in
old, dense clouds of gas and dust.
Yes. With the low temperature, the atoms and molecules are moving slowly enough that gravity can pull them together and cause them to collapse into stars. Section 18-3
A protostar, formed in the center of a condensing mass of dust and gas, begins to glow as it slowly warms. What process generates this heat?
The compression of the dust and gas by gravity.
Yes. Compressing a gas heats it. In protostars, the source of the energy is gravitational potential energy, which is released as the protostar contracts; inward motion of contraction is converted to random kinetic energy of atoms in the hot gas. Section 18-3
Which component of dense interstellar clouds obscures most of the visible light from new-born stars?
Dust
Yes. Fine particles of dust scatter and absorb visible light, preventing most of the visible light from reaching us. Section 18-3
Which wavelength range in the electromagnetic spectrum has proven to be the most useful in investigating star birth in dense molecular clouds?
Infrared radiation.
Yes. The peak emission for dense warm objects such as protostars and their surrounding dust clouds will be at infrared wavelengths, and this radiation can also penetrate the dust. Section 18-3
What physical process generates a force inside a pre-main-sequence star to offset the force of gravity and stop the star from slowly condensing and shrinking, thus producing a stable, non-shrinking main-sequence star?
Nuclear fusion begins as the temperature rises and this generates additional heat that produces an increase in internal gas pressure
Yes. When the core temperature reaches 107 K, fusion begins to generate extra energy that heats the gas and produces sufficient pressure to oppose the force of gravity. Section 18-4
Which physical parameter uniquely fixes a star's location on the main sequence of the Hertzsprung-Russell diagram when it reaches this stage of its evolution?
Its mass.
Yes. In the main sequence, the most massive stars are the most luminous, while less massive stars are distributed down the main sequence. Section 18-4
Which of the following objects are not associated with star formation?
Red giants.
Yes. A red giant is a star in a later phase of its evolution, after the main sequence has ended, whereas protostars occur before the main sequence and are associated with Bok globules, Herbig-Haro objects, and other phenomena. Sections 18-4 and 18-5
Which of the following characteristics describes a T Tauri star?
Variable light output, ejecting matter into space, showing emission lines in its spectrum.
Yes. T Tauri stars are relatively unstable stars as they develop from protostars to main sequence stars, partly by ejecting matter into space. Section 18-5
Which of the following objects are directly related to the bipolar outflow of gas from young stars?
Herbig-Haro objects.
Yes. These are the bright regions at either end of the bipolar flows, where the impact of the ejected gas is exciting the gas of the interstellar medium or the gas within a nebula. Section 18-5
What characteristic of star clusters makes them useful for understanding how protostars evolve onto the main sequence?
All stars in the cluster have approximately the same age.
Yes. The stars in the cluster formed while the cluster itself was forming, so at each later time, all stars in the cluster have the same age. Consequently, since the stars' ages are the same, those that are further through their lives must have evolved faster, allowing us to see stellar evolution in action even though we can't see any individual star changing or evolving. Section 18-6
When an astronomer plots the measured luminosities and temperatures of a large number of stars of a particular star cluster on a Hertzsprung-Russell diagram, she finds that the more luminous stars are on the main sequence but the stars of lower luminosity are to the right of the main sequence. What does this mean?
This is a young star cluster.
Yes. Stars that are not on the main sequence are either evolving toward it (young, pre-main-sequence stars) or have already reached the main sequence and have left it again. But more luminous stars evolve faster, so if they are still on the main sequence then the less luminous ones must still be evolving toward it. Section 18-6
Molecular hydrogen, H2, is thought to be the most abundant molecule in interstellar space, but is difficult to detect. Which easily detected molecule occurs in space along with hydrogen, the measurement of which allows us to estimate the amounts of molecular hydrogen?
CO, carbon monoxide.
Yes. CO emits strongly at 2.6 mm in the microwave region of the electromagnetic spectrum, and there appears to be a well-defined relationship between the abundance of CO and the abundance of H2. Section 18-7
What are giant molecular clouds?
Star formation regions up to about 100 parsecs across and containing about a million solar masses of material
Yes. The famous Orion nebula is part of a giant molecular cloud. The visible nebula has been ionized by the hot, young stars inside it, but behind the visible nebula, star formation is continuing in hidden parts of the cloud. Section 18-7
If supernovae are stars blowing up (self-destructing), how can a supernova be part of a star formation process?
The explosion of the star creates a shock wave that compresses nearby interstellar clouds.
Yes. When the material from the explosion encounters an interstellar cloud, it creates a shock wave in the cloud that compresses the cloud very suddenly. This sudden compression can initiate star formation in the cloud. Section 18-8