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Science 3210 001 : Introduction to Astronomy
Lecture 8 : Star Formation
Robert Fisher
Items
 Reading/Homework set 7 has been posted to the website.
 Solution sets 3-4 have been posted to the website.
 March 23 (next week!) of spring break -- no class!
Review of Lecture Two Weeks Ago
 The Outer Planets
Review of Last Week
 Extrasolar planets
 51b Peg
 HD209458b
Today -- Star Formation
 Star Formation
 Interstellar Chemistry, or Where to Stop For Alcohol on Your Next
Interstellar Road Trip.
 Formation of Protostars
 Stellar Binaries
 Stellar Clusters
 Characterizing Stellar Properties
Interstellar Medium
 The interstellar medium plays a crucial role as the ultimate
reservoir of material from which stars form.
 The material released from stars over their lifetimes goes back
into the interstellar medium to replenish it.
ISM
Stars
Winds, SNe
Interstellar Medium
 The space between the stars is not empty, but contains a
complex though highly dilute mixture including
 Gas
 Dust
 Starlight
 Cosmic Rays
 Magnetic Fields
Determining the Composition of the Interstellar
Medium
 How can one go about determining the composition of the
interstellar medium, which about one hydrogen atom per cubic
centimeter on average -- about a million billion billion times less
dense that water?!
Absorption Lines
 By measuring the absorption spectra along lines of sight to
distant stars, astronomers can infer the amount of material in the
ISM to those stars.
 Piecing together the overall 3D structure of the interstellar
medium is then like assembling a massively complex jigsaw
puzzle.
Interstellar Clouds
 Many decades of painstaking observations reveal a fascinatingly
complex structure to the ISM.
 Most of the gas is concentrated in “clouds” of various types, and ranges
from tens to millions of degrees in temperature.
 Hydrogen clouds -- an “average” interstellar cloud containing mostly
atomic hydrogen at hundreds to thousands of degrees, with roughly a
few atoms per cubic centimeter.
 Giant Molecular clouds -- very dense and cold (by ISM standards) -hundreds of thousands of molecules per cubic centimeter, tens of
degrees. Location of all known star formation.
 Coronal phase -- very hot (millions of degrees), heated by powerful
blasts from supernovae. Diffuse, not concentrated in clouds.
Gas Pressure
 A gaseous system exerts a force on its surroundings.
 On a small scale, this is due to the random thermal collisions of
atoms and molecules, like balls in a bingo machine.
Pressure
 When a gaseous atom or molecule collides with walls of the
container, it bounces, which results in a change in its velocity.
 This change in velocity is an acceleration, which is also a force by
Newton’s second law.
 Pressure is defined to be force, per unit area.
Force
Gas Laws
 From the late 17th through early 19th centuries, scientists studied
how gas pressure behaved, and found it followed simple
mathematical laws.
 The hotter the gas, the greater the pressure.
 The more confined (and more dense) the gas, the greater the
pressure.
 The more gas, the greater the greater the pressure.
Why Stars Form -- A Competition Between
Gravity and Pressure
 All phases of stellar evolution -- from its formation through the
end of its life -- are determined by the competition between
gravity and pressure.
 Gravity binds a star (or cloud of gas) together. The more massive
the star, the smaller its radius, the stronger the effect of gravity.
 Gas pressure pushes outward and acts against gravity. The hotter
the gas, the greater the pressure.
The Competition Between Gravity and Pressure
 If gas pressure exceeds gravity, the gas is blown outwards.
 As the gas expands, it cools.
Gravity
Gas Pressure
The Competition Between Gravity and Pressure
 Conversely, if gravity exceeds gas pressure, the region collapses.
 As the gas contracts, it is heated.
Gravity
Gas Pressure
The Competition Between Gravity and Pressure
 If gravity balances gas pressure, the star remains in hydrostatic
equilibrium.
Gravity
Gas Pressure
Question
 If a cloud of gas collapses and compresses, does it cool down or
heat up?
Gravity
Gas Pressure
Question
 What might stop the collapse?
Gravity
Gas Pressure
Question
 If the cloud cooled by radiating away energy, so that its
temperature remained constant, would it stop collapsing?
Gravity
Gas Pressure
Dust from Space -- Brownlee Particles
 Measuring the properties of interstellar dust particles may seem
like an impossible task.
 However, it is possible to capture cometary dust in high-flying
aircraft. It is believed these dust grains are quite similar to those
in the interstellar medium.
Formation of Molecules in Space
 When radio and millimeter technology first became available in
the 1960s, theorists predicted that it would be impossible for
atoms to combine to form molecules in the vastness of space.
Molecules in Space
 Instead, astronomers found that molecules were very common in
interstellar space -- particularly CO and NH3.
 The most common molecule in interstellar space -- H2 -- doesn’t
emit this type of radio emission and so is nearly invisible.
CO Map of Horsehead Nebula (BIMA/M. Pound)
Molecules in Space
 As time progressed, astronomers have discovered increasingly
complex molecules like formaldehyde (H2CO) and ethyl alcohol
(C2H6O) in molecular clouds.
 These complex molecules would require not only two atoms
colliding and sticking, but many such collisions.
 Such a gaseous phase process is indeed impossible at the low
densities of interstellar space, and even more so once the
disruptive effects of cosmic rays and UV radiation from starlight
are taken into account.
 How then can molecules form in interstellar space?
Molecule Formation on Dust Grains
 It turns out that dust grains, although comprising only about 1%
of the mass of a giant molecular cloud by mass, play an essential
role in catalyzing the formation of molecules :
 1)Individual atoms impact the surface of the grain and adhere to it.
 2) Over very long times -- thousands of years -- the atoms bounce
around over the surface of the grain due to random thermal motion.
 3) Eventually two atoms encounter each other on the surface of the
grain and combine to form a molecule, which then leaves the surface
after releasing its heat of formation.
Cooling by Dust Grains
 Besides playing an essential role in the formation of molecules,
dust grains also play a key role in the energy budget of a giant
molecular cloud.
 Energy budget is key to determining when stars form -- cooling
leads to loss of pressure support, which enhances star formation.
 The gas is heated by a variety of sources -- starlight, cosmic rays,
and gravitational contraction.
 At the same time, the gas radiates away light energy in the radio
portion of the spectrum, which cools the gas.
Cooling by Dust Grains
 At low densities, the radiation from molecules can escape freely,
but at higher densities the radiation is absorbed.
 At these higher densities, dust grains continue to cool the gas
indirectly, through collisions.
 An atom or molecule collides with the dust grain, which then
radiates away the energy in the infrared.
Infrared Radiation
Cooling by Dust Grains
 Eventually the gas becomes so dense that it absorbs the radiated
infrared radiation as well.
 At this point the gas is “optically thick,” and will heat up.
 From the standpoint of star formation, heating the gas is a crucial
step towards arresting the collapse of the gas and beginning to
form the “first core” which will form a star.
 From the standpoint of astronomical observers, the dust provides
a sensitive view of even the densest regions of a giant molecular
cloud, even when gaseous lines become optically thick.
Other Supporting Forces
 In addition to gas pressure, other pressure support from different
physical processes can act to help support the star -- these
include light pressure, magnetic pressure, and turbulent pressure.
 Each of these mechanisms shapes the process of star formation
in different ways.
 The detailed picture resulting from all of these effects is still being
actively researched and so remains at the forefront.
Light Pressure
 The light from the most massive stars is so intense that it exert a
significant influence over its surroundings, as in this Spitzer
image of the Eta Carina nebula.
The Influence of Turbulence
 In the 1990s, astrophysicists first began to seriously consider the
influence of turbulence on star formation.
 Earlier models had assumed that star formation occurred in
spherically-shaped giant molecular clouds, even though the
majority of stars form in highly-complex clouds that bear no
resemblance to a sphere whatsoever.
Turbulence in the Interstellar Medium
 The complex structure seen in real giant molecular clouds eluded
the explanation of simple spherical models.
Simulation of Star Formation in a Turbulent Gas
Cloud
QuickTime™ and a
Planar RGB decompressor
are needed to see this picture.
Star Formation
 Imagine an idealized model of star formation, from a spherical
parent gas cloud (sometimes called a “molecular cloud core”)
without turbulence or magnetic fields.
 Initially, before the star has formed, the parent gas cloud is a state
of hydrostatic balance.
~ 1 Solar Mass
10,000 AU
Star Formation
 The loss of pressure support (possibly through cooling) leads to
gravitational collapse of the cloud.
 As the cloud collapses, the effect of rotation becomes more
significant. The cloud flattens into a collapsing, rotating thin disk.
100 AU
Molecular Core
 The collapse of the disk is arrested at the center once the gas
begins to heat up and can support itself under its own weight.
 At this point, the central “core” is entirely molecular in
composition, is a few hundreds of degrees at its surface, and has
a radius of a few AU (comparable to the orbit of Jupiter).
 This object is sometimes called the first, or molecular core. It is
still far too cold to ignite nuclear reactions.
100 AU
A Protostar is Born
 The first molecular core continues to radiate energy, and must
obtain this energy from some source.
 Its only reservoir is gravitational energy, so it is forced to contract,
which causes it to heat up further.
 Eventually the temperature becomes high enough to dissociate
molecules inside the first core, which leads to overall collapse.
 This process is arrested once more, producing a protostar which
begins to ignite nuclear reactions in its core to power it.
Observational Evidence for Young Protostars
 Astronomers have seen some nearby young stellar systems
which are roughly consistent with this timeline for the formation of
young protostars.
Winds, Jets, and Outflows
 Once they begin to burn hydrogen, protostars power spectacular
outflows moving at speeds of hundreds of thousands of miles per
hour back into the interstellar medium.
XZ Tauri
 This time-lapse movie for the binary XZ Tauri was taken over
three years on the Hubble Space Telescope. The image covers
roughly 1000 AU.
 The outflow appears to be highly sporadic, but it remains unclear
how it is being powered or even which binary member is
powering it.
HH 30
 Perhaps the most famous outflow system. This set of images
taken over several years by Hubble reveal the powerful jet
moving at hundreds of thousands of miles per hour is precessing
like a top over time, and is also highly episodic.
Binary Stars
Binary Stars
 The majority of stars (unlike our sun) exist in bound systems of
two stars orbiting about one another.
Artist’s Conception of a Red Giant Orbiting a Black Hole
Binary Stars
 Binary stars are significant because they allow the masses,
periods, and separations of each star to be accurately
determined.
 The orbits remain relatively fixed over time, so knowing the
amount of angular momentum in the system gives us an
additional clue about how the stars formed.
Center-of-Mass
Primary
Secondary
Visual Binaries
 Some binaries have wide enough orbits that the stellar
components can be resolved in a telescopic image.
 In these cases, the binary period, orbital separation, and masses
can all be determined directly.
Sirius A
Sirius B
Hubble Image of Sirius A/B Visual Binary
Spectroscopic Binaries
 In some cases, the two binary stars are close enough that they
cannot be resolved in a telescopic image.
 The Doppler technique can be used to detect many binaries
spectroscopically that could not have been detected visually.
Double-line Spectroscopic Binary
Spectroscopic Binary
 Observations of the stellar spectra over time reveal the period of
the binary system as well as the separation, and hence the
masses.
Eclipsing Binaries
 Some binaries, like planetary transits, eclipse one another along
our line of sight to them.
 Measuring the light curve from the system gives us both stellar
radii in addition to the stellar masses and orbital separation.
Alvan Clark (1804 - 1887) and the Discovery of
Sirius B
 Alvan Clark was the foremost telescopic lens manufacturer of his
time.
 Manufactured lenses for Naval Observatory (where Pluto’s moon
Charon was discovered) and the University of Chicago Yerkes
Observatory (which just shut down research very recently).
 In 1862, when testing a new 18 inch telescope at the Dearborn
observatory at Northwestern University in Evanston, he
discovered a companion to Sirius -- Sirius B.
Sirius A/B
 Sirius B turns out to be an eclipsing binary, so that its radius can
also be determined from the eclipse measurements.
 These observations revealed a highly unusual structure -- a mass
about that of the sun, and a radius about that of the Earth.
 Sirius B became the first-known white dwarf star. How it managed
to support itself against gravity would require entirely new
physics.
Artist’s Conception of Sirius A/B
Demographics of Young Binaries
 Period distribution of binaries informs our understanding of star
formation.
Stellar Clusters
Stellar Clusters
 Stars rarely form in isolation. Most stars form in giant molecular
clouds with enough material to form tens of thousands to
hundreds of thousands of stars.
 These stellar clusters are gravitationally bound to one another.
 Two major types of stellar clusters can be distinguished on the
sky.
 These two types of clusters are thought to have very different
formation mechanisms -- in particular, globulars are known to be
ancient, dating to the formation of the galaxy, whereas open
clusters are much younger.
Pleaides
 The most famous open cluster of stars is the Pleaides cluster.
Pleiades
Pleiades in the X-ray Band
 The brightest stars of the Pleiades are actually only the tip of the
iceberg -- many more stars are members of the cluster, as is
evident in this X-ray image.
Nebra Sky Disk
 The Bronze Age Nebra sky disk is one of the oldest known
representations of the night sky -- dating from c. 1600 BC
Germany.
 It is believed that the Pleiades is represented in the upper right of
the image.
Globular Clusters
 Globular clusters are some of the most magnificent sights in the night
sky, containing hundreds of thousands of stars in a relatively compact
space of a few tens of thousands of light years in diameter.
 The central densities of the cluster become high enough that stellar
collisions can occur.
 There is some evidence for these stellar collisions in “blue stragglers”.
 There is also long-standing speculation, and some evidence that
continued stellar collisions may lead to massive black holes of thousands
of solar masses at the center of the globular.
The Globular Cluster M80
Galactic Distribution of Globular Clusters
 Globular clusters are distributed in a sphere around the galaxy.
 Other disk galaxies have been observed to have their own
system of globular clusters surrounding them.
 Some globulars may pass through the plane of the galactic disk
from time to time, stripping away some stars in a “disk shocking”.
Stellar Associations
 Open clusters eventually become less dense over time, and form
a loosely-packed, unbound stellar association that will eventually
break apart.
 Membership in the association can only be confirmed by
inspecting the motions of the stars on the sky -- their proper
motions -- carefully.
Christmas Tree Cluster
 One example of an association is the Christmas Tree cluster.
 Like all other associations, it is unbound and will eventually move
apart.
Stellar Properties
 Astronomers have historically characterized stars by their color.
From our knowledge of the blackbody radiation emitted by all
bodies, we know this stellar color translates directly into surface
temperature.
Mnemonics
 The spectral classes can be easily remembered by
 Oh Be A Fine Girl/Guy Kiss Me
 Many other mnemonics have been created -- or make up your own!
 Oh Boy, Astronomy Final's Gonna Kill Me
 Out Back A Friend Grows Killer Marijuana
 Oven Baked Ants Fried Gently Keep Moist
 Only Boring Astronomers Find Gratification Knowing Mnemonics
Stellar Magnitudes
 Astronomers will often classify the brightness of stars by their
magnitude.
 The original classification was meant to agree loosely with an
ancient system due to Ptolemy -- magnitude 1 stars are among
the brightest on the sky, and magnitude 6 stars are among the
faintest visible to the naked eye.
 Each increment on the magnitude scale represents not a linear
shift, but a multiplicative factor of 2.5. A magnitude 2 star Is 2.5
times fainter than a magnitude 1 star, and so on.
Stellar Magnitudes
 It is important to realize that these magnitude ratings reflect the apparent
brightness of a star. Two stars of the same intrinsic brightness at two
different distances will have two different magnitudes.
 If one also knows the distance to the star (not always the case!), then
one can correct for the distance and obtain an intrinsic magnitude. By
convention this is chosen to be a distance of 10 pc, or 32 LY.
 Examples of apparent magnitudes
 Sun




-26.3 (intrinsic 4.8)
Vega
0
Uranus
5.5
Pluto
13
HST Limit 30
Charting Stars -- The HR Diagram
 Around the beginning of the 20th century, two astrophysicists -noticed something fundamental about the properties of stars.
 They compared stellar properties by displaying intrinsic stellar
luminosity along one axis and temperature along the second axis.
HR Diagram
 When displayed in this fashion, definite patterns popped out
immediately.
Question
 Which star is larger, A or B?
A
B
Classification of Stars
 This classification system helped identified the major classes of stars :
 Main Sequence. This is where all stars begin their lifespans burning
hydrogen, and where they spent most of their life. (Example : our sun.)
 Giant Branch. After stars deplete their supply of hydrogen they swell up to an
enormous radius and begin burning hellium and heavier elements on this
branch. (Example : Alderberan.)
 Supergiants. Among the brightest stars in the universe, shortly before the end
of their lifespan. (Example : Rigel.)
 White Dwarfs. Stars similar in mass to our sun will wind up as white dwarfs --
extremely dense, hot stellar remnants. (Example : Sirius B.)
Question
 Which star is more evolved, A or B?
B
A
Two Weeks from Today -- Stellar Evolution