exam 2 pt 2 Flashcards
pass exam 1
Star Formation
In order to form new stars we need the raw
material to make them. Our Galaxy contains vast quantities of this
“raw material”, atoms or molecules of gas
and tiny solid dust particles found between
the stars.
Interstellar Matter
The interstellar medium consists of gas and dust.Gas is atoms and small molecules:
* 90% hydrogen
* 9% helium
* 1% heavier elements
This image shows distinct
reddening of stars near the edge of
the dust cloud (Barnard 68).
Dust absorbs light, and
reddens light that gets
through. Dust is larger clumps of particles - about 10-7 m in
size (wavelength of visible light) - silicates, carbon,
iron, some dirty-ice
Star Formation
The air we breathe contains about
10^19
atoms per cubic centimeter. By comparison, interstellar space is incredibly sparse, with only about one atom per cubic centimeter if the gas in the Galaxy were spread evenly. Dust particles are even rarer, with just a few hundred to a few thousand grains in an entire cubic kilometer of space.
Interstellar gas is composed primarily by:
90% hydrogen, 9% helium, and 1% heavier elements. The composition of interstellar gas
mirrors that of the Sun, stars, and the Jovian planets.
Interstellar Hydrogen
Ionized hydrogen (H*) - gas is found in Emission
nebulae: hot, glowing area associated with the
formation of large stars.
Molecular hydrogen (H2) - Found in cold dark
dust clouds.
Atomic Hydrogen (H) - Found in cold regions
without stars between dust clouds.
Imaging Excited Hydrogen
Emission nebulae are composed of atomic
hydrogen gas that is ionized by near-by stars.
These regions are called HII regions and this
visible red light has wavelength of 625-750 nm. Orbital Excitation - electron absorbs
energy and moves to higher energy orbit.
Emits visible and UV photons when it deexcites.
Imaging Atomic Hydrogen
“21-cm” radiation (radio wave) is emitted by non-ionized atomic hydrogen
Spin-flip changes energy without changing
atomic orbital. Use the signal to study regions that
contain atomic hydrogen and no
visible stars. Because of its long wavelength, this
radiation from atomic hydrogen reaches
Earth unaffected by interstellar debris
Imaging Molecular Hydrogen
CO emissions reveal huge molecular cloud complexes.
This image of the Milky Way covers about 1/4 of the sky. Bright regions are large CO signals.
Cold, dense clouds contain
H2 - molecular hydrogen - which
does not emit any radio signal.
Also contain trace amounts of
CO - carbon monoxide, HCN -
hydrogen cyanide, NH2 -
ammonia, H20 - water, H2CO -
formaldehyde
These molecules emit
radio waves when they
de-excite.
“Seeing” Interstellar Matter
Dust clouds absorb blue light preferentially; spectral lines do not shift.
Amount of dimming gives
estimate on amount of dust.
Dark spot is gas cloud, not absence of stars.
Nebulae
Nebula - region of space that is clearly distinguishable through a telescope
(dark or bright) , but is not sharply defined like a planet or a star
Emission Nebula - glowing cloud of hot interstellar medium.
Milky Way Galaxy
Bright areas are fields of stars, dark areas are obscured by dust.
Nebular Structure
Visible nebular glowing part of much larger interstellar cloud. Glow red from the UV excitation of Hα line. Sometimes called HII regions.
Reflection Nebula glows blue because blue light scattered to observer more than red
Radio-Astronomy
The study of stars and galaxies involves analyzing hydrogen signals. Red and white contours show formaldehyde signals at two different frequencies, combining visible and radio signals. Emission nebulae are part of larger dark dust clouds, which start glowing when new stars form and ignite. The colder, denser areas of these clouds are only visible in radio waves, and Doppler analysis shows the cloud is contracting!
Nebular Size
These nebulae are very large and have very low density;
their HUGE size means that their total masses are
large despite the low density.
Emission Nebulae
Emission nebulae are star forming regions - “lit” by bright young stars.
Close-up of M20
12 pc across - huge!
Contain newly formed, hot O- or Btype stars that produce lots of UV light
which in turn ionizes the gas.
Dust Lanes
Part of the nebula containing regions of
star-forming activity. Revealed with IR
wavelength observation.
Cold, Dark Dust Clouds
Average temperature of dark dust clouds (called absorption nebulae) is a few tens of kelvins.
Note - “empty” space has temperature of ~ 100 K.
A thousand times denser (1000 atoms/cm3) than surrounding space
summary
The interstellar medium consists of gas and dust.
Nebula - any region of space that is clearly distinguishable through a telescope
(dark or bright) , but is not sharply defined like a planet or a star. Emission Nebula - glowing cloud of hot interstellar gas - indication of a star forming region.
Dark Dust Cloud (absorption nebula) - dense, cold, interstellar regions of space - difficult to
“see” because blocks light behind it.
Molecular Cloud - cold neutral gas molecules (H2…) - part of huge molecular cloud complexes
The reddish color of emission nebulae indicates that:
. hydrogen gas is present.
Cosmic Rays
In addition to gas and dust, a third class of particles is found in
interstellar space: cosmic rays
The term “cosmic ray” is
misleading: they are particles
and have nearly the same
composition as ordinary
interstellar gas.
Cosmic rays are mostly high-speed atomic nuclei and electrons.
Speeds equal to 90% of the speed of light are typical.
Almost 90% of the cosmic rays are hydrogen nuclei (protons)
stripped of their accompanying electron.
Cosmic Rays
Serious problem in identifying the source of
cosmic rays: light travels in straight lines, while
cosmic rays are charged particles, and their
direction of motion can be changed by
magnetic fields. The paths of cosmic rays are
curved both by magnetic fields in interstellar
space and by Earth’s own field.
The best candidates for a source of cosmic rays are the supernova explosions,
which mark the violent deaths of some stars
Interstellar Matter - summary
Study of the stars and galaxies through hydrogen signals.
Red/white contours - formaldehyde
signal at two different frequencies.
overlay of visible
and radio signals
Doppler analysis of signal indicates cloud is contracting! - Stage 1
Molecular hydrogen (H2) - Found in cold
dark dust clouds. Observe via radio signal from
molecular vibrational transitions. This region is
where star formation occurs.
Ionized hydrogen (H*) - gas is found in
Emission nebulae: hot, glowing area associated with
the formation of large stars. Observe via red light
from hydrogen electron atomic energy level
transition.
Atomic Hydrogen (H) - Found in cold regions
without stars between dust clouds. Observe via
radio signal from hydrogen electron “spin-flip”
Star Formation
Star formation happens when part of a large dust cloud begins to contract
under its own gravitational force; as it collapses, the center becomes hotter and
hotter until nuclear fusion begins in the core.
When looking at just a few atoms, the gravitational force is nowhere near strong
enough to overcome the random thermal motion.
For cool cloud (~10-100 K), need 10^57 atoms.
This happens when some shock to the cloud compresses it.
Star Formation - stage I
Stage 1: Interstellar cloud starts to contract, probably triggered by shock or
pressure wave from nearby star. As it contracts, the cloud fragments into
smaller pieces. Takes a couple million years.
T=10 K, size ~50 ly, 109 p/m3
If an interstellar cloud contracts to become a star, it is due to which force?
Gravitational . Nebular contraction -
Cloud of gas and dust
contracts due to gravity
Condensation theory -
Interstellar dust grains help
cool cloud, and act as
condensation nuclei.Star formation happens when part of a dust cloud begins to contract
under its own gravitational force.
Star Formation - Stage 2 & 3
Stage 2
Individual cloud fragments begin to collapse.
T~100 K, size ~ 100X our solar system, 1012 p/m3
Once the density is high enough, there is no further fragmentation.
Note - contraction produces energy, but radiation escapes thin cloud.
Stage 3
Spherical gas ball about the size of our solar system.
Density has increased 1018 p/m3. Radiation can not escape anymore.
The interior of the fragment has begun heating, and is about 10,000 K.
