Transcript nebula - Harding University

An Introduction to Astronomy
Part XI: The Birth and Death
of Stars
Lambert E. Murray, Ph.D.
Professor of Physics
Interstellar Gas and Dust
 In
the late 1700’s Henry Herschell
discovered “holes in the heavens” where
there appeared to be fewer stars than
normal.
 In the late 1800’s Edward Barnard’s
photographs of these regions lead some to
believe that they were clouds of material
blocking out the starlight.
Barnard 86 – a Dark Nebula
Barnard 86 is a good
example of one of
Herschell’s “holes in
the heavens”.
The Constellation
Orion
Region of
Horsehead
Nebula
The Horsehead
Nebula:
A Dark Nebula
Close-up of the Horsehead
Nebula
Evidence of Dark Nebula
The Horsehead nebula is clearly a case where dark
material is obscuring the brighter emission nebula
behind it.
 The close-up actually reveals dim stars that are
behind the dark nebula.
 This seems to be clear evidence for dark material
in interstellar regions which can obscure the light
from stars.
 Indeed, the reddish emission nebula behind the
Horsehead is direct evidence of interstellar gas.

CO Radar Mapping (2.6 mm)
of Orion – Monoceros Region
The following image is a radar map of the Orion –
Monoceros region of the sky taken at a
wavelength of 2.6 mm.
 The 2.6 mm wavelength radar image maps CO
concentrations. The concentrations of hydrogen
are typically four orders of magnitude greater than
CO in interstellar space, but this gives a measure
of the amount of gas in interstellar space regions.
 You can tell that there are large concentrations of
gas in the regions of the horsehead and Orion
nebulae. These are believed to be rich star
formation regions.

M
More Dark Nebula
Emission and Reflection Nebula
The term nebula is used to denote a cloud of
interstellar gas and dust.
 An emission nebula is one that glows because of the
emission of specific spectral lines arising from the
excitation of atoms within the interstellar gas cloud.
 A reflection nebula is one that glows because of
scattered light from a star – this scattered light is
typically bluish in color (like the blue in our
atmosphere).
 As white light passed through a dust cloud (much
like smoke) blue light is scattered and, if the cloud is
not too thick, the spectrum of the light passing
through the cloud becomes more reddish (interstellar
reddening).

An Example of an Emission and
Reflection Nebula
Emission Nebula
IC 1283-4
Reflection Nebula
NGC 6589
NGC 6590
Extinction and Reddening
Due to the interstellar nebula, light from distant
stars does not appear as bright as they would – the
process is called interstellar extinction.
 Similarly, the interstellar nebula cause the light
from distance stars to appear more reddish –
interstellar reddening.
 Measurements of interstellar extinction and
reddening in various directions indicates that most
of the interstellar nebula are confined to the
regions of the Milky Way – the faint band of hazy
myriad stars which stretches across the night sky.

The Milky Way is a Spiral Galaxy
We are Near the Outer Edge
Looking Toward the Nucleus of Our
Galaxy– Through Dark Nebula
The Formation of Protostars
Astronomers believe that stars are born from the
gravitational collapse of large, cold regions of
dark nebular material. This collapse may be
triggered by the explosion of nearby stars creating
compression waves in the nebula.
 The picture on the next slide is a region of space
where astronomers believe this process may be
occurring.

The Knobs on the Gas Clouds
May be Regions of Concentrated
The Orion Nebula
Protostar Region?
Pre-Main-Sequence Evolution
 As
large regions of gas collapse under the
influence of gravity, they heat up.
 Initially this heat is in the form of infrared
radiation.
 Eventually the gas heats up enough to radiate
in the visible. As the gas cloud shrinks
greater internal energy is released, causing
the temperature to rise more.
A Stellar Nursery
H II region of the Swan in
visible wavelengths
Same region in infrared
wavelengths.
Notice the large number of “cool” stars, or protostars, on the
right-hand-side. These are visible in the infrared because that
wavelength can penetrate the dust and gas.
These Two Images are Lined up
These Two Images are Lined up
Pre-Main-Sequence Evolution
Tracks of Protostars
Notice that this model
gives results similar to
the mass-luminosity
data plot.
Variations in Luminosity with
Stellar Mass
 The
rising temperature coupled with the
decreasing size causes protostars of mass
greater than about 5 solar masses to
maintain a relatively constant luminosity.
 Protostars less massive decrease in size
more rapidly than the increase in surface
temperature, and the luminosity decreases.
A Star is Born
Once the protostar heats up to the point where
thermonuclear fusion occurs, the radiation
pressure will counteract the gravitational pressure
and the star will become stable as a main-sequence
star.
 Stars arising from larger mass clouds become very
luminous stars, while stars arising from less
massive clouds become less luminous.
 Gas clouds with a mass of less than about 0.08
solar masses can never heat up sufficiently for
nuclear fusion to occur, and the failed star
becomes a hydrogen-rich brown dwarf (something
like Jupiter).

Young Stellar Disks and Jets
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From the discussion of our solar system, we
postulated the formation of a solar system from the
gravitational collapse of a dust and gas cloud, and the
development of a disk of material rotating rapidly
around the young star.
Similar features have been recently observe with the
Hubble Space Telescope.
In the slide that follows, the knobby jets of material
appears to be emitted along the polar axes of the star
in what is called bi-polar outflow. These small nebula
are known as Herbig-Haro objects.
Such young, gas-ejecting stars are known as T Tauri
stars, the first example having been discovered in the
constellation Taurus.
Young Star Jets and Disks
Stellar Disks in the Orion Nebula
The following images were taken by the HST of
the Orion Nebula.
 In these pictures you will see evidence of stellar
formation and of the presence of disk-like
structures surrounding these new stars.
 The four massive stars that dominate this region
are emitting radiation and gasses which are
interacting with the smaller young stars being
formed. These stellar winds may prevent the
formation of possible solar systems.

Main Sequence Stars
Once a star reaches the main sequence, it will
remain on the main sequence for about 90% of its
lifetime. For our Sun (a moderately sized star), this
means approximately 10 billion years.
 Larger, more luminous stars burn up their fuel much
more rapidly, and remain on the main sequence for a
shorter period of time.
 As we will see later, when a star leaves the main
sequence, it expands and moves toward the region
of giant and supergiant stars.

Star Clusters Provide Evidence for
our Model
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High-mass stars evolve more rapidly than low-mass
stars.
An association of hot, massive stars (an OB
association) will emit vast quantities of uv radiation
into the nebula from which it was born.
This high-energy uv radiation actually ionizes the
hydrogen gas of the nebula. These free protons then
combine with free electrons and emit the characteristic
red color associated with these emission nebula (socalled H II regions).
We want to examine the HR diagram for the young
stars associated with an H II region (NGC 2264).
NGC 2264
in
Monoceros
HR Diagram of the Young Star
Cluster NGC 2264
Note that the more massive,
luminous stars in the cluster
(at the upper left end) have
already reached the main
sequence, while the less
massive protostars (at the
lower end) have not yet
joined the main sequence.
The Pleiades Star Cluster
In contrast to the young star cluster NGC 2264, an
HR diagram of the Pleiades star cluster shows that
these stars have already reached the main
sequence, and the older stars are actually
beginning to leave.
 The Pleiades star cluster is a cluster of very bright,
blue (hot) stars.
 Both star clusters we have examined are known as
open clusters or galactic clusters, possessing
barely enough mass to hold themselves together in
a cluster.

The Pleiades and Their HR Diagram
Other Stellar Nurseries
The radiation pressure from very bright stars may
create compression waves in a surrounding nebula,
as in the Rosette Nebula.
 Likewise, when a star dies, it may generate a
massive explosion which can send large
compression waves out into the interstellar medium,
compressing the gasses that are there.
 This compression of interstellar gasses may be the
breeding places for new stars.
 The following images illustrate some of these
massive compressional waves generated by
radiation pressure and by exploding stars.

The Rosette Nebula: Compression
by Radiation Pressure
An
Exploding
Star
Another Exploding Star
Yet
Another
Exploding
Star
The Sun Expands in Old Age
Once a star like our Sun becomes a main sequence
star, it remains stable for about 10 billion years.
 At the end of that time, the hydrogen fuel in the
center of the Sun will become depleted; there is
too much helium to efficiently continue the
thermonuclear fusion process at the core.
 When that happens, the radiation pressure from
the center of the Sun will be reduced and the core
will collapse toward the center due to gravity.
 The region just outsider the core will heat up and
begin to “burn” hydrogen and this will cause the
Sun to expand. This process continues as the Sun
expands out to the size of the Earth’s orbit,
creating a red giant.

The Sun as a Red Giant
Helium Core Burning
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Once a star becomes a red giant, it will remain a red
giant as its outer regions continue to burn the available
hydrogen.
During this time, helium ash continues to accumulate
in the center of the star as gravity pulls this heavier
material to the center, heating up the core.
Eventually, the star’s core will re-ignite when the
temperature of the core gets hot enough for helium to
“burn”.
The ash of helium burning is both oxygen and carbon.
For low-mass stars (less than 3 solar masses) the onset
of helium burning produces an explosive “helium
flash”.
After the helium flash the star settles down to burning
helium and becomes a smaller, hotter star.
Post-Main-Sequence Evolution
Core helium burning begins
where the evolutionary tracks
make a sharp downward turn
in the red giant region of the
diagram.
HR Diagrams for Globular
Clusters
Many globular clusters are associated with our
galaxy (and with others).
 These star clusters can be easily analyzed using an
HR diagram, because all the stars are essentially
the same distance from us. We need plot only
their apparent magnitude vs. their temperature.
 An HR diagram of M13 is shown on the next
slide.
 This diagram indicates a group of stars in which
the hottest stars have already moved off the main
sequence. This star cluster, therefore, must be
relatively old.

HR Diagram
for M13
The Horizontal Branch
 The
horizontal branch stars in this last HR
diagram are believed to be stars that have
already experienced the helium flash.
 Note the “gap” in the horizontal branch.
This is the region occupied by the variable
stars.
Composite HR Diagrams for
Several Star Clusters
The turn-off point for each
cluster gives us an estimate
of the age of each star
cluster. Notice the “gap” in
the central part of this
diagram.
The Instability Strip (or Gap)
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The apparent gap in the previous
HR diagram for various star clusters
seems to correspond to the region
where we often find “variable”
intensity stars.
Stars moving across the upper
region of this strip correspond to
Cepheid variables, while stars
moving across the lower part of the
strip correspond to RR Lyrae
variables.
Variable Stars
Variable stars are stars that periodically vary in
brightness.
 There are two types of variable stars:

– RR Lyrae variables correspond to low-mass, post
helium-flash stars. Their periods are all shorter than
one day.
– Cepheid variables correspond to high-mass stars and
appear to pass back and forth through the instability
strip. These stars are particularly important because
astronomers have found that their period is directly
related to their average luminosity.
Mira (omicron Ceti),the First Variable
This pulsating variable was discovered in 1595 by
a Dutch minister and amateur astronomer David
Fabricius.
 He noted the Omicron Ceti varied in apparent
brightness – sometimes being bright enough to see
with the naked eye, and sometimes fading
completely from view.
 By 1660, astronomers realized that the star’s
brightness varied with a period of 332 days.
 Mira is an example of long period variables – cool
red giants that vary in brightness by a factor of
100 or more over a period of months or years.

Mira: A Long-Period Variable
RR Lyra Variables
 These
are typically low-mass, metal poor
stars – often associated with globular
clusters.
 They all have periods of less than a day.
Cepheid Variables
Cepheids have a characteristic light curve showing
an abrupt brightening, followed by a slower
dimming.
 They are large mass, highly luminous stars which
can be seen over great distances.
 Delta-Cephei, the first discovered Cepheid
variable was discovered in 1784, and was found to
vary in brightness by a factor of 2.3 with a period
of about 5.4 days.

The Period-Luminosity Plot for
Cepheid Variable Stars
Cepheid Variables and Distance
Variations in the brightness of Cepheid variables
corresponds to variations in the size of the star (as
determined from Doppler shift measurements).
 Type I Cepheid variables are metal-rich, and are
brighter than type II Cepheid variables.
 Cepheid variables can act as “standard candles” to
determine the distance to stars, since the period
luminosity curve provides a means of calibration
of the Cepheids.
 Knowing the luminosity, and the apparent
magnitude of a Cepheid variable enables
astronomers to determine the distance to the star.

The Death of Stars
The manner in which a star “burns out” depends
strongly on the mass of the star.
 Low-mass stars (with masses less than about 2 – 3
solar masses) end relatively quietly, producing a
planetary nebula and a white dwarf.
 High-mass stars, however, end much more
violently, producing a supernova and either a
neutron star, or a black hole – depending upon the
original mass of the star.

The Death of a Low-Mass Star
Expansion upon
hydrogen burn-out.
Expansion upon
helium burn-out.
Planetary Nebula Arise when the Outer
Shell of the Star is Blown Away
The shapes of the planetary
nebula are quite varied,
depending upon the magnetic
fields associated with the star
and upon previous nebula
surrounding the star.
White Dwarfs
 The
remaining core of the low-mass star is
small and hot and is called a white dwarf.
 This white dwarf slowly burns out, with no
further excitement, eventually becoming a
black dwarf – unless it has a companion star
which can feed it more stellar material!
Companion Stars
White Dwarfs as Companion
Stars
Sometimes a white dwarf may be one of a binary
star system.
 As material (hydrogen) from its companion leaks
onto the surface of the white dwarf, the pressure
and temperature build up until the outer hydrogen
shell ignites and causes an explosion – producing
one class of nova.
 This type of nova may occur several times, being
somewhat periodic as material leaks from the
companion star and subsequent nova occur – these
are called recurrent nova.

Type Ia Supernova
When the companion star of a binary system
expands greatly and looses a large amount of its
outer gas shell very rapidly, the white-dwarf
companion may be compressed to the point where
the carbon-oxygen core begins to “burn”.
 This gives rise to a type Ia supernova – a very
large explosion which destroys the white dwarf.

The Death of High-Mass Stars
Unlike the low-mass stars, the death of high-mass
stars is much more dramatic.
 These stars end their existence explosively in what
is known as a type II supernova event, where the
outer layers of the star are blown away.
 During the supernova explosion, the stars
luminosity may increase as much as 100 million
time.
 The inner part of the star is compressed in the
supernova explosion and produces either a neutron
star, or a black hole.

Pulsars
 In
November of 1967 Jocelyn Bell, working
with a newly developed radio telescope at
Cambridge University detected a strange
periodic signal with an extremely regular
period of 1.3373011 seconds.
 This was initial taken as an indication of
intelligent life, until other similar objects
were detected in other parts of the sky.
 The regular pulsing radio sources soon
came to be known as pulsars.
Radio Emissions from one of the First Pulsars to be
discovered, PSR 0329+54, with a period of 0.714
seconds.
But how can it pulse so rapidly?
What is the source of these Pulsars?
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Many different explanations were initially proposed, which
were later discarded.
Some proposed that the stars surfaces pulsated this rapidly,
but pulsations this rapid would cause the star to explode.
Some astronomers proposed that the pulsating signal arose
from the rotation of a star.
Although most astronomers at that time believed that the
majority of “dead” stars were white dwarfs, the discovery
of very rapid pulsars, like the one in the Crab Nebula
(period of 0.033 seconds), indicated that a new type of star
– much smaller and much more dense must be the source
of these pulsations.
These stars must be similar to the “neutron stars” proposed
by Fritz Zwicky and Walter Baade.
Neutron Stars and Pulsars
The Crab Nebula: Remnants of a
Supernova
X-ray image
The Crab Pulsar
Shortly after the discovery of the first pulsars,
strong bursts of radio energy were observed
coming from the Crab Nebula with a frequency
almost 10 times greater than previous pulsars.
This was additional proof that the source must be
neutron stars.
 Further evidence for a rotating neutron star was
the discovery that the frequency of the Crab pulsar
was slowly decreasing as would be the case of a
source constantly emitting energy.
 Later, both an optical pulsar and an x-ray pulsar
with the same frequency were observed in the
Crab Nebula.

The Spin-Down of a Pulsar
 As
a pulsars continuously emits radiation, it
slowly decreases its rotational velocity.
This is the so-called “spin down” of a
pulsar.
 Adjustments in the surface of a neutron star
(similar to earthquakes) cause sudden jumps
in the stars angular momentum, introducing
“glitches” in a pulsar’s spin-down plot.
Long and Short Pulsars
 In
1982, scientists were surprised to find a
pulsar with a frequency of 642 Hz. Since
then many “millisecond” pulsars have been
discovered.
 The longest period pulsar has a period of
over 8 seconds.
End of Part XI