Name: Maheen Mansoor
Seat No: H18154048
Course: SST-612
(Steller Structure & Evolution)
Class: M.Sc. (Final year)
Assignment-1:
Protostar
Introduction:
Stars are the most widely recognized astronomical objects, and represent the most fundamental
building blocks of galaxies. The age, distribution, and composition of the stars in a galaxy trace
the history, dynamics, and evolution of that galaxy. Stars go through a natural cycle, much like
any living beings. This cycle begins with birth, expands through a lifespan characterized by
change and growth, and ultimately leads to death. The time frame in the life cycle of stars is
entirely different from the life cycle of a living being.
What is Protostar?
Protostars are young stars within the earliest phase of stellar evolution. Protostars are not
yet main-sequence stars as they have not absorbed all the surrounding interstellar gas and have
yet to ignite through the fusion of hydrogen into helium. The formation of stars begins with the
collapse and fragmentation of molecular clouds into very dense clumps. These clumps initially
contain ~0.01 solar masses of material, but increase in mass as surrounding material is
accumulated through accretion. The temperature of the material also increases while
the area over which it is spread decreases as gravitational contraction continues, forming a more
stellar-like object in the process. During this time, and up until hydrogen burning begins and it
joins the main sequence, the object is known as a protostar.
How is a Protostar Formed?
The formation of stars can be encouraged or sped up by disturbances in the gas clouds that
compress the gas such as other nearby stars or supernovae.
As the cloud collapses, it begins to spin and by the time a protostar is formed, the cloud flattens
and there is a protostellar disk spinning around the protostar. These disks probably slow the
rotation of the protostar, and sometimes coalesce into planetary systems.
As the protostar rotates, it generates a strong magnetic field. The magnetic field also generates a
strong protostellar wind, which is an outward flow of particles into space. Many protostars also
send out high-speed streams or jets of gas into space. Usually there are two jets flowing out
along the rotation axis of the protostar. Eventually the wind and the jets clear away the extra gas
around the protostar.
Protostar physics:
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During the initial collapse of a nebula into a protostar, the temperature of the collapsing
material is low around 30 K. During this stage, the protostar may occupy a sphere with a
radius of around 1000 astronomical units. The protostar remains shrouded in a cloud of
dust and gas and only radio emissions are detectable from this phase of its development.
As the protostar collapses, it develops a magnetic field which increases in intensity as the
star’s size decreases. Observations from satellite X-ray observatories have detected
unexpectedly intense X-ray radiation emanating from cold protostars. It is speculated that
the magnetic fields of protostars accelerate matter to high velocities up to 2000 km/s.
When the accelerated material collides with the protostar, X-rays are emitted. The
process that generates the magnetic fields is still the subject of speculation – one
explanation is a dynamo-effect-like process.
Protostars can fire out jets of material: Herschel Space Observatory showed evidence of a
protostar, dubbed the ‘stellar sprinkler’, emitting jets of hydrogen and oxygen at
velocities of 200,000 km/hr.
Astrophysicists have recently succeeded in imaging a dusty accretion disc that surrounds
a very young protostar. They describe the disc as having a ‘hamburger-shaped’
appearance. HH 212, a very young protostar around 40,000 years old in the constellation
of Orion, has in addition to an equatorial disc, two powerful jets of materials emanating
from its poles.
Fig: structure of protostar in the main accretion phase
(If all the conditions of temperature, pressure and mass does not meet and the star is not massive enough
to sustain nuclear fusion, it is a failed star which is called brown dwarf)
Fig: Understanding star formation includes understanding how planets form, including planets like Earth
Deuterium Burning:
Even though protostars can't fuse normal hydrogen atoms, they can fuse deuterium. Deuterium is
a rare form of hydrogen that has both a proton and a neutron in its nucleus. Deuterium is much
easier to fuse than normal hydrogen, so both protostars and some brown dwarfs are able to fuse it
into helium.
Deuterium burning is important in protostars because the reaction keeps the temperature inside at
a constant 1 million degrees, like a stellar thermostat. Since normal hydrogen fusion doesn't
occur until 10 million degrees, it will remain a protostar longer, accreting more matter and
becoming a more massive star.
Brown dwarf:
Brown dwarfs are sub stellar objects that are not massive enough to sustain nuclear fusion of
ordinary hydrogen (1H) into helium in their cores. The core temperatures of brown dwarfs must be
below about 3 million degrees, as at this temperature fusion becomes sustainable. Surface
temperatures are 1,000 degrees Kelvin or less. Because of their low temperatures and small sizes, brown
dwarfs have extremely low luminosities (about 1/100,000th of the solar luminosity). This makes them
extremely difficult to observe.
References:
1. http://sunshine.chpc.utah.edu/Labs/StarLife/protostars.html
2. https://www.space.com/42790-brown-dwarfs-coolest-stars-hottest-planets.html
3. https://study.com/academy/lesson/protostar-definition-formationfacts.html#:~:text=Protostars%20are%20young%20stars%20within,fusion%20of%20hyd
rogen%20into%20helium.
4. https://lco.global/spacebook/stars/protostar/
5. https://en.wikipedia.org/wiki/Stellar_evolution