Cloud Fragmentation
As an interstellar cloud contracts, gravitational instabilities cause it to fragment into smaller pieces. The pieces themselves continue to fall inward and fragment, eventually forming may 10s or 100s of individual stars.
Rotation
Competes with gravity's inward pull.
The more rapid the rotation, the greater the tendency for the gas to escape and the greater the gravitational force needed to retain it.
Magnetism
Can hinder a cloud's contraction. It prevents the charged particles in the cloud from moving in certain directions and can slow or even halt the gravitational collapse of a molecule cloud.
Protostellar Evolution of a Solar Type Star
Stage 1
Interstellar Cloud: dense, dark, cold dust cloud that is extremely massive.
Star formation is triggered when come external event squeezes a cloud beyond the point where pressure can resist gravity's inward pull.
Best way to study: radio emission
Stage 2 (Solar Type)
A collapsing cloud fragment.
Temperature begins to rise at the centre to approx. 100K. Process of continued fragmentation is eventually stopped by increasing density within the shrinking cloud.
Best way to study: radio emission
Stage 3 (Solar type)
Fragmentation ceases
10,000 of years have gone by and density is higher. Core heats up considerably to 10,000 K.
Dense opaque region forming at the centre of the cloud is a protostar.
PHOTOSPHERE becomes evident.
Stage 4** (Solar type)
PROTOSTAR
Protostar evolves, shrinks, density grows and temperature rises.
- Centre is 1,000,000K
- Several 1000x the luminosity of the sun
- Luminosity is due to release of gravitational energy
- Can be plotted on H-R diagram (appears at top right-> big
Hayashi Track
Star moves from stage 4 to 6
- Also called T Tauri phase
- A time of violent surface activity including stellar winds
Stage 5
Protostar approaching main sequence.
Stage 6*
A Newborn Star
- 10 million years old
- At 10 million K, nuclear burning begins in the core
- Protons begin fusing helium nuclei in the core
Stage 7*
Main sequence Arrival
- Temp reaches 15 million K, Surface temp 6000K
- Hydrostatic equilibrium is reached
Zero Age Main Sequence
A star larger than the sun reaches Main Sequence when hydrogen burning begins in the core.
Minimum mass of gas needed to generate core temperatures high enough to begin nuclear fusion
0.08 solar masses
- This is the lower limi on the masses of all stars
Brown Dwarfs
A sub-stellar object that formed through the contraction and fragmentation of an interstellar cloud but failed to reach critical mass (0.08 solar masses) needed to start hydrogen fusion
Planets
Anything smaller than 0.08 solar masses
Wien's Law
Higher temperatures mean shorter wavelengths
Protostellar Wind
Protostars often exhibit strong winds.
Bipolar flow
Strong heating within the turbulent disk and a powerful protostellar wind combine to produce a bipolar flow, expelling 2 jets of matter in the directions perpendicular to the disk.
Shock Wave
A wave of matter, which may be generated by a newborn star or supernova, that pushes material outward into the surrounding molecular cloud. The material tends to pile up, forming a rapidly moving shell of dense gas.
Emission nebula
A glowing cloud of hot interstellar gas. The gas glows as a result of one or more nearby young stars which ionize the gas. Since the gas is mostly hydrogen, the emitted radiation falls predominantly in the red region of the spectrum because of the hydroge
Open cluster
Loosely bound collection of 10s to 100s of stars a few parsecs across, generally found in the plane of the Milky Way.
- Most stars are on the Main Sequence
Globular clusters
Tightly bound, roughly spherical collection of 100s of 1000s of stars spanning about 50 pc.
- Found in the halo regions of galaxies
- Contains the most old stars
- Most stars have moved off the main sequence (right of diagram)
Associations
Less massive but more extended clusters. Contain only a few 100 bright stars, span 10s of parsecs.
- Rich in very young stars.
What basic competitive process controls star formation?
The competing effects of GRAVITY which tends to make an interstellar cloud collapse, and heat (PRESSURE) which opposes that collapse.
What distinguishes a collapsing cloud from a protostar and a protostar from a star?
Collapsing cloud/protostar) The existence of a photosphere, meaning that the inner part of the cloud becomes opaque to its own radiation, signalling the slowing of the collapse phase
Protostar/star) Nuclear fusion in the core and equilibrium between press
Do stars evolve along the main sequence?
No - different parts of the main sequence correspond to stars of different masses. A typical star stays at roughly the same location on the main sequence most of its lifetime.
How can a "snapshot" of the universe today test our theories of the evolution of individual objects?
We assume that we observe objects at many different evolutionary stages, and that the snapshot therefore provides a representative sample of the evolutionary stages that stars go through.
Why might we expect multiple episodes of star formation to occur in some locations?
Star formation may be triggered by some external event, which might cause several interstellar clouds to start contracting at once. Alternatively, the shock wave produced when an emission nebula forms may be sufficient to send another nearby part of the s
If stars in a cluster all form at the same time, how can some influence the formation of others?
Because stars form at different rates - high-mass stars reach the main sequence and start disrupting the parent cloud long before lower-mass stars have finished forming.
If a newly forming star has an excess of heat, then it will likely have
A slower contraction rate
The gravitational contraction of an interstellar cloud is primarily the result of its
mass
The interstellar cloud from which our Sun formed was how big in size?
A thousand times more massive than the Sun
A protostar that will eventually turn into a star like the Sun is significantly
more luminous than the Sun
Prestellar objects in which nuclear fusion never starts are referred to as
brown dwarfs
The current theory of star formation is based upon
Amassing evidence from many different regions of our Galaxy
If an initial interstellar cloud is very massive, the result would be
the formation of more stars
One of the primary differences between the Pleiades cluster and Omega Centauri is that Pleiades cluster is much
younger
If an H-R diagram shows a young cluster, the main-sequence turn off would shift to a _________________ when compared to an older cluster.
higher temperature
A typical open cluster will dissolve in about the same amount of time since
Dinosaurs walked the earth
Briefly describe the basic chain of events leading to the formation of a star like the sun.
Stage 1, 2- Interstellar Cloud contracts, and it fragments into smaller pieces
Stage 3 - Fragmentation ceases
Stage 4, 5 - Protostar evolves
Stage 6 - New Star is Born; protons begin fusing helium nuclei in core.
Stage 7 - Star reaches main sequence where
What is the role of heat in the process of stellar birth?
A star needs a very high core temperature in order to create the pressure needed to balance out the gravitational and pressure pulls. Once this balance is created, it is possible for the nuclear fusion that create the star and keep it balanced to happen.
What is the role of rotation in the process of stellar birth?
Rotation can compete with gravity's inward pull; as the cloud contracts, it must spin faster so the "bulge" grows and material on the edge flies off into space... the more rapid the rotation, the greater the tendency for gas to escape, and the greater the
What is the role of magnetism in the process of stellar birth?
Magnetism can hinder the contraction of an interstellar gas cloud, causing it to contract in a distorted way. Charged particles pull the magnetic field toward the cloud's centre
What is an evolutionary track?
A graphical representation of a star's life as a path on the H-R diagram.
Why do stars tend to form in groups?
- The presence of young O and B type stars in the vicinity of a supernova suggests that the birth of stars is often initiated by the violent deaths of others. O and B type stars themselves, perhaps born of a passing shockwave, may in turn create new shock
Why does the evolution of a protostar slow down as the star approaches the main sequence?
Once an object begins fusing hydrogen in its core (as in Stage 7, the last stage of a star) and est. a "gravity-in, pressure-out" equilibrium, it is destined to burn steadily for a very long time, therefore, its surface temperature and luminosity will rem
In what ways do the formative stages of high-mass stars differ from those of stars like the sun?
Higher mass stars tend to place higher on the H-R diagram.
- They become O-type stars in about 1/50 of the time it took the Sun to become a star.
What are brown dwarfs?
Stellar figures, objects that formed though the contraction and fragmentation of an interstellar cloud, just as stars do, but fell short of the critical mass of about 0.08 solar mass needed to start hydrogen fusion in their cores.
What are T Tauri stars?
The first "star" or protostar to be observed in the stage of prestellar development in which there is a high rate of violent surface activity causing strong protostellar winds.
Stars live much longer than we do, so how do astronomers test the accuracy of theories of star formation?
Astronomers observe the objects in space as they appear today at different stages along their evolutionary paths.
At what evolutionary stages must astronomers use radio and infrared radiation to study prestellar objects? Why can't they use visible light?
- At stages 1 and 2, radio emission from interstellar molecules within the interstellar clouds are examined.
- Later stages can be examined with infrared radiation because their temperatures are much hotter. (e.g. Stages 4, 5, 6)
- Visible light can't be
Why has it been so difficult until recently to demonstrate that stars and protostars actually exist within star forming regions?
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What is a shock wave? Of what significance are shock waves in star formation?
- A shockwave is created by expanding waves of matter driven outward by the high temps and pressures in the nebula. As the waves crash into surrounding molecular cloud, interstellar gas tends to pile up and become compressed. Such a shell of interstellar
Explain the usefulness of the H-R diagram in studying the evolution of stars. Why can't evolutionary stages 1-3 be plotted on the diagram?
The H-R diagram is useful as it allows us to plot and predict the lives of stars.
Evolutionary stage 1-3 cannot be plotted because their surface temperatures do not meet the requirements of the diagram.
Compare the times necessary for the various stages in formation of a star like the Sun. Why are some so short and others so long?
Star formation depends on mass, therefore if a star is of a lower mass, it will develop much slower than a star of a higher mass.
What do star clusters and associations have to do with star formation?
Physical interactions - close encounters and even collisions - between protostars within a star cluster may be very important in determining the properties of stars that eventually form.
Compare and contrast observed properties of open star clusters and globular star clusters.
Open Clusters have much younger stars, all along the main sequence.
Globular Clusters have much older stars, on the right side of the H-R diagram. They exist in the halos of galaxies.
How can we tell whether a star cluster is young or old?
A young star cluster will be surrounded by emission nebulae and stars will be hotter.
An old star cluster will be located in the halo, will have much larger, cooler stars that aren't along the main sequence.
In the formation of a star cluster with a wide range of stellar masses, is it possible for some stars to die out before others have finished forming? Do you think this will have any effect on the cluster's formation?
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