Lec 11/12 Flashcards
(28 cards)
interstellar medium
space is not completely empty, as we find
at least some gas and dust everywhere we look. We refer to
the gas and dust found in the spaces between stars as the
interstellar medium
spectroscopy
use spectroscopy to measure the abundances of the
new elements that stars have added to the interstellar
medium.
The most straightforward technique is to observe
the spectrum of a star whose light has passed through an
intervening cloud of interstellar gas.
The cloud absorbs
some of the star’s light, leaving absorption lines in the star’s spectrum
molecular clouds
Stars are born in interstellar clouds
that are particularly cold and dense. These clouds are usually called molecular clouds because they are cold enough
and dense enough to allow atoms to combine together into
molecules
interstellar dust
Not all of the material in a molecular
cloud is gaseous. Elements such as carbon, silicon, oxygen,
and iron are often found in tiny solid grains of interstellar
dust
interstellar reddening
when stars seen near the edges of a molecular cloud appear redder than similar stars outside the cloud
-occurs for much the same reason
that our Sun appears redder when viewed through smoke or
smog:
Dust grains block shorter-wavelength (bluer) photons
of visible light more easily than longer-wavelength (redder)
photons.
Near the edges of a molecular cloud, where stars are only partially obscured, the blocking of blue light causes
stars to appear redder than their true colors.
gravity vs pressure
Gravity can create stars only if it
can overcome the outward push of the pressure within a
gas cloud, which depends on both the density and the temperature of the cloud.
thermal pressure
temperature-dependent pressure in ordinary gas clouds
why do molecular clouds avoid gravitational contraction?
Molecular clouds avoid this fate because they quickly
rid themselves of any thermal energy that builds up.
Collisions between gas molecules in the cloud transform
the thermal energy into photons by exciting the rotational and vibrational energy levels of those molecules,
which then produce emission lines in the infrared and
radio portions of the spectrum
As long as
the photons produced by these colliding molecules can
escape the cloud, the cloud’s temperature can remain
low.
why do stars tend to form in clusters?
Stars tend to form in clusters because gravity is stronger in a high mass gas cloud, making it easier for gravity to overcome the
outward force due to thermal pressure.
Precise calculations
show that at the temperatures and densities of typical molecular clouds, gravity can begin to overcome thermal pressure
only in clouds with masses greater than a few hundred times
the mass of the Sun
how can stars form in a cloud?
1) Stars can form
in such a cloud only if gravity is strong enough to overcome
the turbulent gas motion, and overcoming that motion can
require considerably more mass than is needed to overcome thermal pressure alone.
2) Magnetic fields can help the cloud resist gravity.
Observations tell us that magnetic fields are important in molecular clouds: Light from stars usually travels
through space with its electric and magnetic fields vibrating in random directions, but starlight that has passed
through a molecular cloud often has its electric and magnetic fields aligned in particular directions.
why was it difficult for first gen molecular clouds to overcome pressure
The relatively high temperatures of these first-generation
molecular clouds would have made it more difficult for gravity to overcome pressure, requiring stars to form in relatively
massive cloud fragments that would have made predominantly high-mass stars.
Such massive stars have very short
lifetimes, which would explain why we don’t
find any first-generation stars in today’s universe: They would
all have died off long ago
how does a contracting cloud become a star that shines with energy released by nuclear fusion?
Once again, the answer lies in the ongoing battle
between the inward pull of gravity and the outward push of
thermal pressure.
As long as the interior of the cloud can
continue to radiate away its thermal energy, the cloud remains cool and the pressure remains too weak to slow the
crush of gravity.
However, as the cloud becomes smaller
and denser, it becomes more difficult for radiation to escape
from the interior.
The thermal energy becomes trapped,
raising the interior temperature enough that thermal pressure begins to slow the cloud’s collapse
what slows the contraction of a star-forming cloud?
A cloud fragment will continue to collapse on itself as long
as it remains cold, and it can remain cold as long as photons emitted by molecules within the cloud can carry away
the energy released by gravitational contraction.
It’s easy for photons emitted by molecules to escape from the cloud
early in the process of contraction, while the cloud’s density is still fairly low, because the photons are unlikely to
run into any other molecules after they are emitted.
This situation begins to change, however, as continued contraction packs the cloud’s molecules closer together
trapping thermal energy
Cloud contraction makes it increasingly difficult for emitted photons to escape, because
the growing density of the cloud increases the chances that a photon on its way out will be absorbed, leaving the
absorbing molecule in an excited state.
Collisions between
the excited molecule and other molecules can then change the absorbed photon’s energy back into thermal energy, which prevents that energy from escaping the cloud
what marks the first stage of star formation?
The central region of the cloud fragment eventually grows dense enough to trap almost all the radiation inside
it.
When that happens, the inner regions of the contracting cloud can no longer radiate away their heat.
The central temperature and pressure begin to rise dramatically, and
the rising pressure pushes back against the crush of gravity,
slowing the contraction.
This change marks the first stage of star formation.
protostar
The dense center of the cloud fragment is now a protostar (sometimes referred to as a pre-main-sequence star)—a clump of gas that will become a new star.
Protostars look starlike, with surface temperatures and luminosities similar to those of true stars.
However, a protostar is not yet a true star because its core is not
yet hot enough for nuclear fusion
growth of a protostar by gas infall
A protostar’s mass
grows with time because a molecular cloud fragment contracts in an “inside-out” fashion.
Gravity is strongest
near the protostar, where the gas density is greatest.
The gas in the outer part of the cloud fragment feels a weaker gravitational pull, so it initially remains behind as the protostar forms
protostellar disk
A protostellar disk is a flattened, rotating disk of gas and dust that surrounds a newly forming star, or protostar, during the earliest stages of star formation. It plays a critical role in both the birth of stars and the formation of planetary systems.
Formation and Structure:
A protostellar disk forms when a cloud of gas and dust collapses under its own gravity.
As the cloud collapses, conservation of angular momentum causes it to spin faster and flatten into a disk.
At the center of this disk, material accumulates to form the protostar, while the surrounding disk feeds additional matter onto it.
Role in Star Formation:
Over time, as the protostar heats up and reaches a high enough core temperature, nuclear fusion begins — marking the birth of a true star.
protostellar wind
The strong magnetic field may also help to generate a strong protostellar wind—an outward flow of particles similar
to the solar wind
close binary
In some cases, gravitational interactions between the binary pair of protostars and other protostars and gas clumps in their vicinity can remove angular momentum
from the binary system.
If that happens, then their orbit
gets smaller, and the two stars can end up quite close to each other.
The resulting pair is called a close binary.
how does nuclear fusion begin in a newborn star?
Once a protostar has accreted a significant amount of mass, its interior grows quite hot.
Ultimately, the temperature at the protostar’s center becomes high enough for nuclear
fusion.
However, a protostar with a mass similar to the Sun’s must wait millions of years for fusion to begin, because the process of gravitational contraction slows once the star starts trapping its thermal energy inside.
key factor in allowing the central temp to rise
radiation of energy from the protostar’s
surface into space.
The energy going into space comes from
thermal energy inside the star. Without this loss of thermal energy, the interior pressure would hold gravity at bay, so the protostar would stop contracting and its central temperature would remain fixed.
The radiation from the surface allows the star to lose enough thermal energy for
gravitational contraction to continue.
Calculations show that a contracting protostar retains half the thermal energy released by gravitational contraction, which explains the
rising temperature in the core
when does a protostar become a true star?
A protostar becomes a true star when its core temperature exceeds 10 million K, making it hot enough for hydrogen fusion to operate efficiently through the proton–proton
chain
The ignition of fusion halts the protostar’s gravitational contraction and marks what we consider the birth of a star.
The newborn star’s interior structure stabilizes because the energy produced in the center
matches the amount radiated from its surface.
The star is now a hydrogen-fusing main-sequence star
life track
(also called an evolutionary track) for a single star in
relation to the standard main sequence. Each point along a
star’s life track represents its surface temperature and luminosity at some point during its life
