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Birth of Stars
Discovery of Planets
Outside the Solar System
3 August 2005
AST 2010: Chapter 20
Questions about Star Formation
Are new stars still forming, or did star
formation cease a long time ago?
If new stars are still being created,
where is this occurring?
Are planets a natural result of star
formation, or is our solar system unique
in the universe?
If there are planets around distant stars,
how can we observe them?
3 August 2005
AST 2010: Chapter 20
Known Basics about Stars
Stable (main-sequence) stars, such as the Sun, maintain
equilibrium by producing energy through nuclear fusion in
their cores
The ability to generate energy by fusion defines a star
Each second in the Sun, about 600 million tons of hydrogen
undergo fusion into helium, with about 4 million tons
turning to energy in the process
This rate of hydrogen use implies that eventually the Sun (and
all other stars) will run out of central fuel
Stellar masses range from 1/12 MSun to ~200 MSun
Low-mass stars are far more common than high-mass ones
For main-sequence stars, the most massive (spectral type
O) are also the most luminous and have the highest
surface-temperature, whereas the least massive (spectral
type M or L) are the least luminous and the coolest
A galaxy of stars, such as the Milky Way, contains huge
amounts of gas and dust, enough to make billions of stars
like the Sun
3 August 2005
AST 2010: Chapter 20
Giant Molecular Clouds
Vast clouds of gas (and dust) dot
the Milky Way Galaxy
A giant molecular cloud is
an enormous, dense cloud of gas
so cold (10 to 20 K) that atoms
are bound into molecules
The masses of giant molecular clouds range from
about 1,000 MSun to about 3 million MSun
Within the clouds are lumps, regions of high density
and low temperature
These conditions are believed to be just what is required
to make new stars
The combination of high density and low temperature
makes it possible for gravity to overcome pressure
3 August 2005
AST 2010: Chapter 20
Pillars of highdensity, cool dust
and gas in the
central regions of
the Eagle nebula,
or M16
The colors in this
image (taken by
the Hubble Space
Telescope) have
been reassigned to
enhance the level
of detail visible in
the image
Go to website
Dense Globules in Eagle Nebula
One of the pillars in M16 appears to have dense, round
pockets at the tips of finger-like features protruding from it
These pockets have
been termed
evaporating gas
globules (e.g.g.s)
They may harbor
embryonic stars
Video: zooming in to
e.g.g.s in M16
3 August 2005
AST 2010: Chapter 20
Evaporating Gas Globules
3 August 2005
AST 2010: Chapter 20
EGGs in M16
3 August 2005
AST 2010: Chapter 20
Understanding Early Stages of Star Formation
The early stages of star formation are still shrouded in
mystery because they are almost impossible to observe
directly for three reasons:
The dust-shrouded interiors of molecular clouds where stellar
births are thought to take place cannot be observed with
visible light, but only with infrared and radio telescopes
The timescale for the initial collapse is estimated to be very
short astronomically (thousands of years), implying that stars
undergoing the collapse process are relatively few
The collapse of a new star occurs in a region so small that in
most cases it cannot be observed with sufficient resolution
using existing techniques
The current understanding of how star formation occurs is
the result of theoretical calculations combined with the
limited observations available
This implies that the present picture of star formation may be
changed, or even contradicted, by future observations
3 August 2005
AST 2010: Chapter 20
Stellar Birth
The first step in the process of creating a star is the
formation of a dense core within a clump of gas and dust
The process of core formation is not yet fully understood, but
gravity can be expected to play an important role
Gravity causes the core to accumulate additional matter
from the surrounding cloud material
The turbulence created inside a clump tends to cause the
core and its surrounding material to spin
When sufficiently massive material has accumulated,
gravity causes the core to collapse rapidly, and its density
increases greatly as a result
During the time a dense core is contracting to become a
true star — but before the fusion of protons to produce
helium begins — the object is called a protostar
When the protostar is still accreting material from the
surrounding cloud, dust and gas envelope the protostar,
making it observable only in the infrared
3 August 2005
AST 2010: Chapter 20
Observation of Protostars
Infrared detectors
enable observation of
possible protostars
Many stars appear to
be forming in the
Orion Nebula above
and to the right of
the Trapezium stars
They can only be
seen in the infrared
3 August 2005
AST 2010: Chapter 20
Images from the Hubble Space Telescope
Winds & Jets
Once almost all of the available material has been
accreted and the protostar has reached nearly its final
mass, it is called a T Tauri star
after one of the best studied members of this class of
Upon reaching this stage of its development, the
protostar starts producing a powerful stellar wind,
consisting mainly of protons and electrons streaming
away from its surface at speeds of a few hundred
kilometers per second
The wind tends to emerge more easily in the direction
of the protostar’s poles
The disk of material around its equator blocks the wind
in this direction
Consequently, two jets of outflowing material appear
in opposite directions from the protostar poles
3 August 2005
AST 2010: Chapter 20
Protostar Jets
The jets can collide with the material
around the protostar and produce
regions that emit light
These glowing regions are called
Herbig-Haro (HH) objects
They allow us to estimate the location
of the hidden protostar
True Star Being Born
Eventually, the stellar wind sweeps away the
obscuring envelope of gas and dust, leaving behind
the protostar and its surrounding disk
The protostar still continues to undergo gravitational
This generates heat inside it and slowly increases its
interior temperature
3 August 2005
AST 2010: Chapter 20
Birth of True Star
If the protostar is sufficiently massive, its
central temperature will continue to increase
to about 10 million K when nuclear fusion of
hydrogen into helium begins inside its core
At this stage, the (proto)star is said to have
reached the main sequence
It is now more or less in (hydrostatic)
equilibrium and generates energy mainly
through nuclear fusion inside its core
Thus astronomers say that a (true) star is
born when it can sustain itself through nuclear
Stars devote an average of 90% of their lives
on the main sequence
3 August 2005
AST 2010: Chapter 20
Time to Reach Main-Sequence Stage
The development of
contracting protostars
can be tracked on the
H-R diagram
The time to reach the
main sequence is
short for high-mass
as low as 10,000
long for low-mass
up to 100 million
3 August 2005
AST 2010: Chapter 20
H-R Diagram: Analogy to Weight versus Height for People
3 August 2005
AST 2010: Chapter 20
Weight and Height Change as Age Increases (Marlon Brando)
3 August 2005
AST 2010: Chapter 20
Different Paths for Different Body Types (Woody Allen)
3 August 2005
AST 2010: Chapter 20
Evidence that Planets Form around Other Stars
It is very hard to see a planet orbiting
another star
Planets around other stars may be
detected indirectly
One way is to look for disks of material
from which planets might be condensing
A big disk is more visible than a small planet
Look for the evolution of disks, evidence for
clumping into planets
3 August 2005
AST 2010: Chapter 20
Disks around Protostars
Four disks
around stars
in the Orion
The red glow
at the center
of each disk
is believed to
be a young
star, no more
than a million
years old
3 August 2005
AST 2010: Chapter 20
Dust Ring around a Young Star
A debris disk has
been found around a
star called HR 4796A
The star has been
estimated to be
young, about 10
million years old
If there are newly
formed planets
around the star, they
will concentrate the
dust particles in the
disk into clumps and
3 August 2005
AST 2010: Chapter 20
Disk around Epsilon Erdani
Evidence for a clumpy disk has been found around a
nearby star named Epsilon Eridani
The star is surrounded by a
donut-shaped ring of dust
that contains some bright
The bright spots might be
warmer dust trapped
around a planet that formed
inside the donut
Alternatively, the spots could
be a concentration of dust
brought together by the gravitational influence of a
planet orbiting just inside the ring
3 August 2005
AST 2010: Chapter 20
Planets Beyond the Solar System: Search & Discovery
If we can’t directly observe planets, can we
indirectly observe them?
Kepler’s and Newton’s laws apply
In a star-planet system, both the planet and
the star orbit a common center of mass
The planet’s motion has an effect on the star’s
As a result, the star wobbles a bit
From the observed motion and period of the
wobble, the mass of the unseen planet can be
deduced using Kepler’s laws
It is a planet if its mass is less than 1/100 the
Sun’s mass (or about 10 times Jupiter’s mass)
3 August 2005
AST 2010: Chapter 20
Doppler Method for Detecting Planets
The star slightly wobbles due to the
motion of the unseen companion planet
3 August 2005
AST 2010: Chapter 20
Discovered Planets
To date, more
than 150 “planets”
have been found
in other star
Systems of 2, 3,
and possibly more
planets have been
The masses of the
planets are
measured in
3 August 2005
AST 2010: Chapter 20
Some Properties of First 101 Extrasolar Planets Found
3 August 2005
AST 2010: Chapter 20
Explaining the Planets Seen
Now that we have a large sample of “planetary” systems,
astronomers need to refine, perhaps significantly, their
current models of planetary formation
Most of the extrasolar “planets” found are not at all like the
ones in our own solar system
Many of the extrasolar planets are similar to Jupiter in mass,
or more massive, and have highly eccentric orbits close to
their stars
This is a big surprise and is difficult for the early models to
The very massive planets orbiting close to their stars are
sometimes called hot Jupiters
There are other surprises …
The formation of planetary systems is more complex and
chaotic than we thought
Intensive search continues …
3 August 2005
AST 2010: Chapter 20