each item following is a characteristic of a one-solar-mass star either during its protostar phase or during its main-sequence phase. match the items to the appropriate phase.
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Protostar
Protostar
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For other uses, see Protostar (disambiguation).
Star formation
Object classes Interstellar medium Molecular cloud Bok globule Dark nebula
Young stellar object
Protostar
Pre-main-sequence star
T Tauri star Herbig Ae/Be star Herbig–Haro object
Theoretical concepts
Accretion
Initial mass function
Jeans instability
Kelvin–Helmholtz mechanism
Nebular hypothesis Planetary migration vte
A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution.[1] For a low-mass star (i.e. that of the Sun or lower), it lasts about 500,000 years.[2] The phase begins when a molecular cloud fragment first collapses under the force of self-gravity and an opaque, pressure supported core forms inside the collapsing fragment. It ends when the infalling gas is depleted, leaving a pre-main-sequence star, which contracts to later become a main-sequence star at the onset of hydrogen fusion producing helium.
Contents
1 History
2 Protostellar evolution
3 Observed classes of young stars
4 Gallery 5 See also 6 Notes 7 References 8 External links
History[edit]
The modern picture of protostars, summarized above, was first suggested by Chushiro Hayashi in 1966.[3] In the first models, the size of protostars was greatly overestimated. Subsequent numerical calculations[4][5][6] clarified the issue, and showed that protostars are only modestly larger than main-sequence stars of the same mass. This basic theoretical result has been confirmed by observations, which find that the largest pre-main-sequence stars are also of modest size.
Protostellar evolution[edit]
Infant star CARMA-7 and its jets are located approximately 1400 light-years from Earth within the Serpens South star cluster.[7]
Main article: Star formation
Star formation begins in relatively small molecular clouds called dense cores.[8] Each dense core is initially in balance between self-gravity, which tends to compress the object, and both gas pressure and magnetic pressure, which tend to inflate it. As the dense core accrues mass from its larger, surrounding cloud, self-gravity begins to overwhelm pressure, and collapse begins. Theoretical modeling of an idealized spherical cloud initially supported only by gas pressure indicates that the collapse process spreads from the inside toward the outside.[9] Spectroscopic observations of dense cores that do not yet contain stars indicate that contraction indeed occurs. So far, however, the predicted outward spread of the collapse region has not been observed.[10]
The gas that collapses toward the center of the dense core first builds up a low-mass protostar, and then a protoplanetary disk orbiting the object. As the collapse continues, an increasing amount of gas impacts the disk rather than the star, a consequence of angular momentum conservation. Exactly how material in the disk spirals inward onto the protostar is not yet understood, despite a great deal of theoretical effort. This problem is illustrative of the larger issue of accretion disk theory, which plays a role in much of astrophysics.
HBC 1 is a young pre-main-sequence star.[11]
Regardless of the details, the outer surface of a protostar consists at least partially of shocked gas that has fallen from the inner edge of the disk. The surface is thus very different from the relatively quiescent photosphere of a pre-main sequence or main-sequence star. Within its deep interior, the protostar has lower temperature than an ordinary star. At its center, hydrogen-1 is not yet fusing with itself. Theory predicts, however, that the hydrogen isotope deuterium (hydrogen-2) fuses with hydrogen-1, creating helium-3. The heat from this fusion reaction tends to inflate the protostar, and thereby helps determine the size of the youngest observed pre-main-sequence stars.[12]
The energy generated from ordinary stars comes from the nuclear fusion occurring at their centers. Protostars also generate energy, but it comes from the radiation liberated at the shocks on its surface and on the surface of its surrounding disk. The radiation thus created must traverse the interstellar dust in the surrounding dense core. The dust absorbs all impinging photons and reradiates them at longer wavelengths. Consequently, a protostar is not detectable at optical wavelengths, and cannot be placed in the Hertzsprung–Russell diagram, unlike the more evolved pre-main-sequence stars.
The actual radiation emanating from a protostar is predicted to be in the infrared and millimeter regimes. Point-like sources of such long-wavelength radiation are commonly seen in regions that are obscured by molecular clouds. It is commonly believed that those conventionally labeled as Class 0 or Class I sources are protostars.[13][14] However, there is still no definitive evidence for this identification.
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The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the order in which they occur, from first to last.
Click card to see definition 👆
First to last to occur:
-molecular-cloud fragment
-contracting cloud trapping infrared light
-protostar with jets
-main-sequence star
Click again to see term 👆
The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the central temperature, from highest to lowest.
Click card to see definition 👆
Highest to lowest temp:
-main-sequence star
-protostar with jets
-contracting cloud trapping infrared light
-molecular-cloud fragment
Click again to see term 👆
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Terms in this set (32)
The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the order in which they occur, from first to last.
First to last to occur:
-molecular-cloud fragment
-contracting cloud trapping infrared light
-protostar with jets
-main-sequence star
The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the central temperature, from highest to lowest.
Highest to lowest temp:
-main-sequence star
-protostar with jets
-contracting cloud trapping infrared light
-molecular-cloud fragment
The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on their rotation rate, from fastest to slowest.
Fastest to slowest rotation:
-main-sequence star
-protostar with jets
-contracting cloud trapping with infrared light
-molecular-cloud fragment
The following figures show the spectral types of four main-sequence stars. Rank them based on the time each takes, from longest to shortest, to go from a protostar to a main-sequence star during the formation process.
Longest to shortest time:
-M6 -G2 -A5 -O9
Provided following are the spectral types of four different main-sequence stars. Rank the stars based on the strength of the radiation pressure that pushes outward as they are forming, from highest pressure to lowest pressure.
Highest to lowest radiation pressure:
-O9 -A5 -G2 -M6
Provided following are four different ranges of stellar masses. Rank the stellar mass ranges based on how many stars in each range you would expect to be born in a star cluster, from highest number to lowest number.
Highest to lowest number:
-less than 1 solar mass
-between 1 and 10 solar masses
-between 10 and 30 solar masses
-between 30 and 60 solar masses
Each item following is a characteristic of a one-solar-mass star either during its protostar phase or during its main-sequence phase. Match the items to the appropriate phase.
Protostar phase:
-energy generated by gravitational contraction
-radius much larger than the Sun
-pressure and gravity are Not precisely balanced
-luminosity much greater than the Sun
Main-sequence phase:
-energy generated by nuclear fusion
-lasts about 10 billion years
-surface radiated energy at same rate that core generates energy
As a clump of interstellar gas contracts to become a main-sequence star, its changing position on the H-R diagram tells us __________.
how its outward appearance is changing
Watch the red dot representing the protostar in the video. After it reaches its highest point on the diagram, how do the protostar's surface temperature and luminosity change as it approaches the main sequence?
Its surface temperature increases, but its luminosity decreases.
When does a newly forming star have the greatest luminosity?
when it is a shrinking protostar with no internal fusion
When a newly forming star is at its greatest luminosity, what is its energy source?
gravitational contraction
The five colored curves on the diagram have arrows pointing to the left. Each of these five curves represents a star of a different __________.
mass
The arrows on each protostar's curve on the diagram's indicate that __________.
protostars change in surface temperature and luminosity as they develop
Which protostars maintain nearly the same luminosity throughout the time that they are protostars?
protostars with masses about 10 or more times that of the Sun
Based on the protostar tracks on the diagram, which statement must be true about the Sun?
The Sun was much more luminous when it was a protostar than it is today.
Suppose two protostars form at the same time, one with a mass of 0.5MSun and the other with a mass of 15MSun. Which of the following statements are true?
The 15MSun star will end its main-sequence life before the 0.5MSun star even completes its protostar stage.
The life tracks shown on the diagram for different mass protostars are based on computer models. Observationally, how can astronomers test whether these models are correct?
By observing and comparing protostars and stars of different masses within a single star cluster.
Stellar Life Cycle
Stellar Life Cycle
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. The table shows the lifetimes of stars as a function of their masses.[1] All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.Nuclear fusion powers a star for most of its life. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. Later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase. Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, whereas more massive stars can fuse heavier elements along a series of concentric shells. Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole. Although the universe is not old enough for any of the smallest red dwarfs to have reached the end of their lives, stellar models suggest they will slowly become brighter and hotter before running out of hydrogen fuel and becoming low-mass white dwarfs.[2]
Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models.
Representative lifetimes of stars as a function of their masses
The life cycle of a Sun-like star.
Artist’s depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right.
Birth of a star
Schematic of stellar evolution.
Protostar
Stellar evolution begins with the gravitational collapse of a giant molecular cloud. Typical giant molecular clouds are roughly 100 light-years (9.5×1014 km) across and contain up to 6,000,000 solar masses (1.2×1037 kg). As it collapses, a giant molecular cloud breaks into smaller and smaller pieces. In each of these fragments, the collapsing gas releases gravitational potential energy as heat. As its temperature and pressure increase, a fragment condenses into a rotating sphere of superhot gas known as a protostar.[3]
The further development heavily depends on the mass of the evolving protostar; in the following, The protostar mass is compared to the mass of the Sun: 1.0 M☉ (2.0×1030 kg) means 1 solar mass.
Protostars are encompassed in dust, and are thus more readily visible at infrared wavelengths. Observations from the Wide-field Infrared Survey Explorer (WISE) have been especially important for unveiling numerous Galactic protostars and their parent star clusters.[4][5]
Brown dwarfs and sub-stellar objects
Protostars with masses less than roughly 0.08 M☉ (1.6×1029 kg) never reach temperatures high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs. The International Astronomical Union defines brown dwarfs as stars massive enough to fuse deuterium at some point in their lives (13 Jupiter masses, 2.5 × 1028 kg, or 0.0125 solar masses). Objects smaller than 13 Jupiter masses are classified as sub-brown dwarfs (but if they orbit around another stellar object they are classified as planets).[6] Both types, deuterium-burning and not, shine dimly and die away slowly, cooling gradually over hundreds of millions of years.
Hydrogen fusion
A dense starfield in Sagittarius
For a more massive protostar, the core temperature will eventually reach 10 million kelvin, initiating the proton-proton chain reaction and allowing hydrogen to fuse, first to deuterium and then to helium. In stars of slightly over 1 M☉ (2.0×1030 kg), the carbon–nitrogen–oxygen fusion reaction (CNO cycle) contributes a large portion of the energy generation. The onset of nuclear fusion leads relatively quickly to a hydrostatic equilibrium in which energy released by the core exerts a “radiation pressure” balancing the weight of the star’s matter, preventing further gravitational collapse. The star thus evolves rapidly to a stable state, beginning the main-sequence phase of its evolution.
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