Download The Interstellar Medium and Star Formation

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Class website http://www.astro.ufl.edu/~jt/teaching/ast1002/
Optional Discussion sections: this week - only Thursday from 12.30pm ~1.30pm start in Pugh 170, then Bryant 3
No homework this week - focus on your Observing Project.
Reading this week: Ch. 0, 1, 2.1-2.7, 4.1-4.3, 5-10, 11, 12
Midterm 2: results via canvas e-learning later this week
Observing project deadline: Thursday Oct. 27th 2016, however, you
are strongly advised to complete observing by Fri. Oct. 7th.
Email me Astro-news, jokes, tunes, images: [email protected]
Printed class notes? Name tags?
Key Concepts: Lecture 24: The Formation of Stars & Planets
The Interstellar Medium - 3 Phases of Hydrogen
Giant Molecular Clouds: isolated & clustered star formation
Different types of Nebulae
Stages of Star Formation
- Collapse of gas cores - the rotation problem
- Disks around forming stars - the generation of outflows
- Disk around forming stars - the birth of planets
Star formation in clusters
Mass limits of stars
The end of star formation
The Interstellar Medium and Star Formation
• Massive stars have short life
times (<10 million years)
– This is much shorter
than the age of the
universe or of the Earth
– Massive stars are still
seen today
• Thus Star Formation must
be occurring in the present
epoch of the universe!
• Indeed, we see examples of
forming massive and lowmass stars.
• Locations of massive
stars: often see gas clouds
nearby.
The Interstellar Medium (ISM)
• Space is not a perfect vacuum: it is filled with a
tenuous gas of mostly Hydrogen [H] (74% of the
mass) and Helium [He] (24% of the mass) mixed
together. This is similar to the composition of the Sun.
• The heavier elements make up just 2% of the mass of
the interstellar medium, including elements in small
dust grains, mixed together with the H and He.
Gas Clouds in the Milky Way Galaxy
The Phases
of the ISM
• The hydrogen gas can exist in
these 3 phases:
– - atoms (H)
– - molecules (H2)
– - ions (H+)
• Much of the volume of space in
our Galaxy is filled with atomic
and ionized H gas.
• About 60% of the hydrogen is
atomic, only a small amount
(<10%) is ionized, and the rest
is molecular.
• The atomic H gas emits radio
photons with wavelength =
21cm
Giant Molecular Clouds
• Most stars form from large clouds of cold gas
called Giant Molecular Clouds
• Mostly molecular hydrogen H2
– contain other more complex molecules
– contain dust which makes them opaque to optical
and UV light
• Most massive objects in our
Galaxy 105-107 solar masses
• Coldest objects in the
universe 10-30 K
Initial conditions for star formation:
• Massive, dusty,
molecular gas
clouds, known as
“Giant Molecular
Clouds” (GMCs).
Dust blocks visible
light, so need
infrared, radio and
X-ray observations to
see the interior.
• The GMC fragments
into smaller clumps
and cores of gas that
can form star clusters
and individual stars
Orion Giant Molecular Cloud in CO molecule line emission
Dark Nebula, Reflection Nebula, Emission Nebula
Example of a dark cloud core
Recall Kirchoff’s Laws
Horsehead Nebula in Orion
Stage 1:
Interstellar cloud starts to contract, perhaps triggered by shock or
pressure wave from nearby star. As it contracts, the cloud fragments into
smaller pieces.
Stage 2:
Individual cloud fragments begin to collapse. Once the density is high
enough, there is no further fragmentation.
Stage 3:
The interior of the fragment has begun heating, and is about 10,000 K.
Stage 4:
A Protostar forms and grows by accreting gas via an “accretion disk”.
Bipolar outflows are launched from each side of the disk.
Formation of Stars like the Sun
Starting from a cold, low-density gas cloud core, gravity
causes compression, increasing the density and temperature.
The luminosity of the object rises, first powered by
gravitational energy, then later by nuclear fusion.
• Dense cores in molecular clouds collapse
– Gravity pulls them together
– They have low pressure
• Pressure in center increases
– Temperature rises due to contraction
– Density rises
• More cold material falls in
• Temperature becomes high enough for fusion
• A new star is born! Visible once dust is gone
Star Formation on the HR diagram
Disks Around Young Stars
The last stages can be followed on the H-R
diagram:
Stage 5:
The protostar’s luminosity decreases even
as its temperature rises because it is
becoming more compact.
Stage 6:
The core reaches ~10 million K, and
nuclear fusion begins. The star continues
to contract and increase in temperature.
Stage 7:
The star has reached the Main Sequence
and will remain there as long as it has
hydrogen to fuse in its core.
Collapse of Gas Cores:
The Rotation Problem
• Since our Galaxy rotates all GMCs rotate, at least a little
• When a cloud collapses to a star it contracts by a factor of
nearly 107
– Thus it will be rotating 107 times faster than the GMC
– Rotating material naturally collapses into a disk
– Rotation will prevent disk from collapsing
• Most main sequence stars rotate fairly slowly: how did they
lose their spin?
Zoom to Orion
The Rotation Problem
• Possible solutions
– Formation of planets in the disk
– Formation of a second (binary) star in the
disk
– Magnetic fields, spinning with the disk and
star, fling gas away: Outflows
• When astronomers looked at gas around
young, forming stars they found
spectacular outflows.
• An outflow phase is associated with the
formation of all stars and may help them
lose their spin.
The Formation of Planets
• Many planets have been detected around other stars
(we will learn how in the final section of the class).
Planetary systems are common!
• Many of the planetary systems are very different from
our own Solar System, e.g., with massive planets,
including gas giants very close to their star.
• We think planets form from the disks around their host
star, shortly after the star forms. As in our Solar System,
this probably involves coagulation of small dust grains into
pebbles, then rocks, then boulders, then planetesimals, then
planets. Some planets become massive enough to also
accumulate Hydrogen and Helium gas.
• However, during and after formation, it seems that
some planets are able to migrate in their disks, drifting
inwards to settle close to the star. We do not know why
this did not happen so much in our own Solar System!
Jets and Outflows from forming stars are fast:
• ~100 km/s, so we
can see the evolution
over just a few years.
Stars are usually born in star clusters
Stars often form in clusters,
with several hundred or more
members. The most massive
clusters have millions of
stars.
Orion Nebula Cluster in the
Sword of Orion: About 1000
stars forming in a parsec size
region over a period a few
million years. Most massive
star is about 40 solar masses.
Animation taken with Hubble telescope over a few years
Large cluster in Large Magellanic Cloud (LMC) Galaxy
• About 10,000
stars. Older
than Orion formed about
50 Myr ago.
Gas has been
blown away
by winds
from the stars.
Stars in a cluster
• Have about the same age
• Have about the same composition (they
formed from the same cloud)
• Are about the same distance from us, so we
can tell the relative luminosities of the stars
by measuring their relative fluxes:
Luminosity= Flux x 4π x distance2
The Mass Limits of Stars
• M<0.1Msun - Brown
dwarfs
– Central temperature in
core never hot enough
for fusion
– Contracts and cools
• M>150Msun
– Very rapid fusion in
core
– So luminous the
radiation blows back gas
and prevents more stars
more massive than this
from forming
Eta Carina,
thought to be
one of the most
massive stars in
our Galaxy with
about 150Msun
Note it is
blowing out lots
of gas
The End of a Star-forming Cloud
• Only about 1% of the
mass of a GMC goes
into forming stars. Why?
• Energy from the stars
heats and dissipates the
cloud
– Outflows and winds
– Radiation (photons)
– Supernova explosions
Summary of Star Formation
• Gravity should collapse cold
gas clouds to form stars
• Giant molecular clouds are
the sites of most star
formation
• Young stars have disks and
outflows
• Planets are likely to form in
the disk
• Stars tend to form in clusters
• Radiation and winds from
young, massive stars destroy
the remains of the gas
clouds.