THE STARS
• The stars in the sky have been known since prehistoric times, as
they are visible on every cloud-free night (more than a thousand
are accessible with the unaided eye).
• The stars appear fixed, relative to each other, on the sky, and
form random patterns which the ancients named after imaginary
objects, animals, or humans (the constellations).
• Like the Sun and Moon, the stars appear to move around the sky
due to Earth’s rotation on its axis.
• In addition, the constellations appear to move, toward the west (at
a rate, at the celestial equator, of about 1 degree per day) relative
to the times of sunset, midnight, and sunrise (due to Earth’s
orbital motion around the Sun).
• Beginning with Galileo’s first use of the telescope, it was found
that vastly larger numbers of stars populate the sky than can be
seen with the unaided eye.
• He also discovered that the band of faint light, known as the Milky
Way, that appears in the night sky, is in fact a vast collection of
stars too faint to be seen individually by the naked eye.
THE STARS
• We now realize that our Sun is also a star, and only an
average one (in size, mass, and brightness) at that.
• The nearest stars other than the Sun, the Alpha Centauri
system, are about 270,000 times our Sun’s distance from Earth!
• Therefore, a star like our Sun at that distance would appear to
be only 1/(270,000)2 (or 1.37 x 10-11) the brightness of our Sun!
• The (absolute) brightnesses of stars are determined by both
their sizes and their temperatures.
• The brightest stars known may be as bright as 1,000,000 times
that of our Sun (with all wavelengths of electromagnetic
radiation included) whereas the faintest true stars may be about
10,000 times fainter than our Sun.
• The energy sources of most normal stars, like that of our Sun, is
thermonuclear fusion of hydrogen to produce helium, but other
processes may contribute during the very early and the very late
stages of stellar evolution.
• We now estimate that our Galaxy contains of the order of 100
billion stars or more.
The Constellation of Orion (Visible Light)
ASTRONOMICAL COORDINATE SYSTEMS
• Maps of the celestial sphere (the interior surface of a very large
imaginary sphere on which all celestial objects appear
superimposed) use a coordinate system similar to that used by
geographers in mapping locations on Earth’s surface.
• The latitude, or declination, of a celestial object is measured in
degrees north or south of the celestial equator (the projection
of the terrestrial equator on the celestial sphere).
• Likewise, the celestial north pole and south pole are directly
overhead of Earth’s north and south poles.
• However, the longitude in the celestial coordinate system is
measured as right ascension, in units of hours, minutes, and
seconds.
• To an observer looking directly south, stars on the celestial
equator appear to move (due to Earth’s rotation) at an angular
rate of 15 degrees per hour (15 arc minutes per minute of time,
and 15 arc seconds per second of time).
The Celestial Coordinate System
A Typical Star Chart
A portion of the Southern Milky Way, Including (at left) stars Alpha and
Beta Centauri, and (upper center) the Southern Cross constellation
THE NATURE AND BRIGHTNESS OF STARS
• The apparent (and absolute) brightnesses of stars are
often expressed in the logarithmic magnitude scale.
• The magnitude scale is such that larger magnitudes
correspond to fainter stars.
• A brightness difference of 5 magnitudes corresponds to a
factor of 100 difference in brightness, and 1 magnitude
corresponds to a factor of 2.512 in brightness.
• The faintest stars visible to the unaided eye are about 6th
magnitude, and the brightest stars in the sky appear about
0 magnitude.
• Each factor of 10 increase in distance increases the
apparent magnitude of a star by 5 magnitudes (decreases
the apparent brightness by a factor of 100).
THE STELLAR MAGNITUDE SCALE
• Since prior to the invention of the telescope, the brightnesses of
stars have been expressed in magnitudes (a logarithmic, rather
than linear, scale).
• By definition, the brightest stars observed with the naked eye are
assigned zero or negative magnitudes, with fainter stars assigned
increasingly positive magnitudes.
• For two stars of brightnesses b1 and b2, the relationship of their
magnitudes m1 and m2 is given by:
b1
 100
b2
m2  m1
5
• The inverse relationship is the expression of the difference in the
magnitudes of two stars, (m2 - m1), in terms of their brightnesses,
with 5 magnitudes = factor of 100 brightness ratio:
m2  m1  2.5log
b1
b2
• Hence, a difference of one magnitude is a factor of 2.512 in
absolute brightness.
THE STELLAR MAGNITUDE SCALE
• Stars with visible-light magnitude +6.0 are about the faintest
observable with the unaided eye.
• The star Sirius ( Canis Majoris) is the brightest star in the sky,
with a visual magnitude of -1.45.
• For quantitative measurements and comparisons of star
brightnesses, it is usually necessary to define stellar magnitudes
in specific spectral ranges (such as red, green, blue) or even
at specific wavelengths, rather than the total accessible (or
visible) range.
• Colors of stars are often defined by the differences between
their magnitudes at two different wavelengths (such as [UV-B]).
• The magnitude scale can also be extended to wavelength
ranges not accessible to ground-based observation, such as the
far ultraviolet or the far infrared.
THE STARS AND OUR SUN
• All stars are formed from pre-existing interstellar material (gas
and dust in the space between stars).
• Stars have a wide range of ages, ranging from less than a
million years old to nearly the age of the Universe (more than 10
billion years old).
• Stars come in a wide range of masses, sizes, temperatures, and
brightnesses (luminosities).
• Our Sun is a star, one of about 100 billion (1011) which make up
our Galaxy.
• Our Sun is about average in the range of normal stars in our
Galaxy, in mass and luminosity, and has an age (as does our
Earth) of about 4.6 billion years.
• Our Sun appears much brighter than other stars, only because it
is much closer (about 270,000 times closer!) than even the
next nearest star.
FORMATION OF STARS
• Stars are created from gas and dust distributed
throughout space, the interstellar medium.
• Our Sun and its family of planets, including our Earth,
were created about 4.6 billion years ago.
• We have evidence that some stars are much older than
our Sun, perhaps more than 10 billion years old.
• We also have direct observational evidence that new star
(and probably planet) formation is occurring at the
present time in our galaxy and others.
• Regions of space where we observe high concentrations
of interstellar material are also the regions where newly
forming or very young stars are observed.
COMPOSITION OF THE SUN AND STARS
• The composition of the Sun and most stars are nearly the same:
about 90% hydrogen, 9% helium, and 1% of all other elements, by
number of atoms.
• It is noteworthy that the oldest stars are the ones having the
smallest percentage of the “heavy elements”(in their visible surface
regions), which shows that the original material making up the
Universe was probably nearly pure hydrogen, with a much smaller
percentage (than at present) of helium, and virtually no heavier
elements.
• The abundances of the elements heavier than helium, however,
are NOT a continuously decreasing function of atomic number, but
show major up and down variations which are due to the different
efficiencies in which the various processes of nucleosynthesis can
occur.
• For example, the light elements lithium, beryllium, and boron are
lower in abundance than carbon, nitrogen, oxygen, and the
moderately heavy element, iron.
• The reason for this, is that these light nuclei are readily broken
down by the same collisions with protons and neutrons that build up
heavier elements (i.e., they have relatively low “binding energies”).
THE NATURE AND PROPERTIES OF STARS
• The energy source of most newly formed stars is the same as
that of our Sun: the thermonuclear fusion of hydrogen to
form helium.
• The observed differences between stars are primarily due to
differences in the total masses of the stars when first created,
and only secondarily due to differences in material composition.
• Astronomers have observed that there are both lower and upper
limits to the initial mass of a “normal” star.
• Objects formed with less than about 8% of the Sun’s mass (or
80 times the mass of Jupiter) never get hot enough to initiate
thermonuclear fusion of hydrogen in their cores.
• On the other hand, objects formed with masses more than about
50 times that of the Sun are unstable and tend to blow
themselves apart at a very young age!
The Proton-Proton Thermonuclear Fusion Reaction
• The following steps are the main source of energy in stars similar
to our Sun.
• The net result is the reaction of 4 hydrogen nuclei (protons) to
form one helium nucleus (also known as an alpha particle).
2 H + e+ + 
1. 1H1 + 1H1
1
where e+ is a positron, 2H1 is deuterium, and  is a neutrino.
3He + radiation
2. 2H1 + 1H1
2
4He + 1H + 1H
3. 3He2 + 3He2
2
1
1
• There are also a number of other, more complex routes by which
the hydrogen atoms can be converted to helium (and heavier
elements, such as beryllium and boron).
• Note, some of the energy generated in these reactions is lost in
the form of the neutrinos, most of which can escape from the star.
• Only recently, have means for detection of neutrinos (when they
arrive at Earth) verified that these reactions occur.
THE NATURE AND PROPERTIES OF STARS
• The rate of thermonuclear fusion energy generation increases
very rapidly with mass (about M3.5).
• The lifetime of a star decreases with mass, as M-2.5 because
the increased fuel supply does not compensate for the much
more rapid consumption with increasing initial mass.
• Therefore, the very hot and luminous stars we see in the sky are
much younger than our Sun, and indicate that star formation is a
continuing process in our Galaxy.
• On the other hand, “red dwarf” stars, much less massive than
our Sun, have very long lifetimes- greater than the current age
of our Universe (estimated to be about 13.6 billion years old).
• The expected lifetime of our Sun is about 10 billion years; hence
it has not yet reached the half-way point.
THE NATURE AND PROPERTIES OF STARS
• The apparent brightnesses of stars depend on both their absolute
brightnesses and their distances.
• The apparent brightness of a star varies as the inverse square of
its distance (D-2).
• The absolute brightness of a star increases in proportion to the
star’s surface area, or the square of its diameter (d2).
• The absolute brightness of a star also increases very rapidly with
surface effective temperature (as Te4), which is defined as the
temperature of a “black body” radiator giving off the same amount of
radiant energy per unit area as does the star’s surface.
• The absolute (intrinsic) brightnesses of stars range from about 10-5
that of our Sun to more than 105 times that of the Sun.
• If the distance to a star can be measured, its absolute brightness
can be calculated from its apparent brightness and the inverse
square law of apparent brightness vs. distance, usually referred to a
standard distance of 10 parsecs (1 parsec = 3.26 light years).
White Dwarf
Planet
Jupiter
.001 Solar Mass
Sirius B
1 Solar Mass
(Earth Size)
.
THE NATURE AND PROPERTIES OF STARS
• The bolometric luminosity of a star (absolute brightness inclusive
of all wavelengths of electromagnetic radiation) increases as the
fourth power of its effective temperature (proportional to d2Te4).
• The temperatures of “normal” stars range from less than 3000 K
to more than 50,000 K.
• The colors of stars depend on their temperature.
• Stars much hotter than our Sun appear blue in color, whereas
stars much cooler than our sun appear red.
• The spectral distribution (variation of the intensity of
electromagnetic radiation with wavelength) can be roughly
approximated by black body radiation curves of intensity (flux)
vs. wavelength, for which the wavelength of peak intensity varies
as 1/T.
• However, the spectra of stars do not normally resemble the
spectra of black-body radiators when examined in detail.
THE NATURE AND PROPERTIES OF STARS
• Very hot stars radiate mostly in the ultraviolet part of the
spectrum, whereas very cool stars radiate mostly infrared
radiation.
• Our Sun, with an effective temperature of about 5800 K, has its
peak radiation in the visible region of the spectrum.
• Accurate measurements of the temperatures and the bolometric
luminosities of stars require spectroscopic measurements over
a very broad range of wavelengths (including both the
ultraviolet and infrared regions of the spectrum).
• In practice, this requires measurements from a space base of
operations, because Earth’s atmosphere absorbs much of the
ultraviolet and infrared regions of the spectrum.
• Even observations from space must take into account the
extinction, and its variation with wavelength, due to interstellar
dust particles, which is highly variable with view direction.
THE NATURE AND BRIGHTNESS OF STARS
• Absolute magnitudes are defined as equal to the apparent
magnitude of a star if it were viewed at the standard
distance of 10 parsecs (32.6 light years).
• Our Sun would appear as a magnitude 4.8 star at the
standard distance of 10 parsecs, and so has an absolute
magnitude of 4.8.
• Magnitudes must be specified according to the wavelength,
or range of wavelengths, covered in the measurement.
• The magnitude wavelength bands longest established in
astronomy are the U, B, V (ultraviolet, blue, visual) bands
which are centered near 350, 450, and 550 nanometers,
respectively.
• Other magnitude bands can be specified for wavelengths in
the infrared or farther into the ultraviolet.
PROPERTIES OF STARS
• The observable properties of stars (at least in principle) include
their apparent brightnesses, their distances, and the spectral
distributions of their emitted radiation (over the entire range of
the electromagnetic spectrum).
• From these can also be derived other properties, such as the
temperatures, diameters, chemical compositions, and ages.
• In some cases, we can determine a star’s speed of rotation, its
velocity of motion through space, and whether or not it has close
companions (stars or planets) which cannot be observed directly.
• One of the most useful methods of comparing stars of different
types is to plot each of them on a graph of the star’s absolute
visual or bolometric luminosity versus its temperature (which is
related to the star’s spectral type or color).
• A plot of this type is known as a Hertzsprung-Russell diagram
(or “HR diagram” for short).
PROPERTIES OF STARS
• Most stars fall in a relatively narrow band extending from the lower right
(low temperature and low luminosity) to the upper left (high temperature
and luminosity) known as the main sequence.
• The main sequence, where most stars lie, corresponds to the
hydrogen-burning phase of stellar evolution.
• The location of a star on the HR diagram is determined primarily by the
initial mass of the star.
• The trend of the main sequence indicates that hotter stars are not only
brighter due to their higher temperatures, but are larger in diameter as
well.
• Stars spend most of their lifetimes at relatively fixed locations on the
main sequence (i.e. they do not move along the main sequence during
the main part of their lifetimes).
• Stars that have nearly depleted their supplies of hydrogen and are
entering their final stages of stellar evolution, follow separate tracks on
the HR diagram, as yellow or red giants or supergiants, followed by
tracks on the opposite side of the main sequence, as white dwarfs.
• As will be discussed later, the most massive stars may end their lives as
even more exotic objects than white dwarfs (that is, as neutron stars or
black holes).
Color and Size Variations in the Hertzsprung-Russell Diagram
Blue Giants/ Supergiants
Supergiants
Red Giants
Main Sequence
White Dwarfs
PROPERTIES OF STARS
• Giant and Supergiant stars are in late stages of their evolution,
having depleted hydrogen in their cores and now sustaining
themselves by the fusion of helium and heavier elements.
• In the final phases of their evolution, most stars collapse to form
white dwarf stars, which no longer have any internal source of
energy, other than stored heat.
• This process includes the ejection of the outer layers of the star,
which form a relatively short-lived planetary nebula, which is
made to glow by the far-ultraviolet radiation from the small but
extremely hot central core of the star.
• A white dwarf star has a mass comparable to that of the Sun,
but a size only about that of our Earth!
• Very massive stars can end their lives more suddenly and
spectacularly in supernova explosions, ejecting their outer
layers in a much more sudden and violent manner, and leaving
behind their cores as even more compact neutron stars or
black holes.
SPECTRAL CLASSIFICATION OF STARS
• The spectral type of a star is obtained from detailed
measurements of the star’s spectrum (not from color alone),
and includes details of the star’s absorption line (and, in rare
cases, emission line) spectra, as well as its continuous
spectrum.
• To allow accurate placement of the star on the HR Diagram,
spectral classification criteria must include factors dependent on
both temperature and luminosity.
• The main spectral types of stars, in order of decreasing
temperature, are designated by letters of the alphabet, in the
sequence: O, B, A, F, G, K, M.
• Within each letter classification, temperature decreases in the
numerical sequence 0, 1, ….9.
• Our Sun, with an effective temperature of about 5800 K, is of
spectral type G2.
• The spectral types of stars also provide information on their
absolute luminosities and their elemental abundances
(compositions).
PROPERTIES OF STARS
• The hottest main-sequence stars we know are designated spectral
type O3 (about 50,000 K), and the coolest stars are designated
type M9 (about 3000 K).
• The very hot stars on the upper main sequence are very rare, but
are also very bright and so can be seen at very large distances
from Earth.
• The cool lower-main sequence M stars, or “red dwarfs”, are very
numerous, but are (relatively) very faint, and so can be seen and
studied only at relatively close distances.
• The lower limit of mass and luminosity of M stars is that
corresponding to the lowest temperature and pressure in the core
of the star at which fusion of hydrogen to helium can occur.
• This lower limit of mass is about .08 solar masses, or about 80
times the mass of Jupiter.
BROWN DWARFS
• Recently, a class of objects too low in mass to fuse hydrogen to
helium (less than about .08 solar masses) but more massive than
planets, called “brown dwarfs”, cooler and less massive than M
dwarf stars, has been verified to exist by telescopic observations.
• Brown dwarf stars create their energy by fusion of deuterium (the
isotope of hydrogen having both a proton and a neutron in its
nucleus, or “heavy hydrogen”) which does not require as high a
combination of temperature and pressure as does fusion of ordinary
hydrogen.
• Brown dwarfs have temperatures between 3000 and about 1000 K.
• The lower mass limit of brown dwarfs is about 15 times the mass of
Jupiter; hence this is considered to be the dividing line between
brown dwarfs and planets.
SPECTRAL CLASSIFICATION OF STARS
• The main luminosity spectral classes, in order of
decreasing luminosity, are designated by capital Roman
numerals in the sequence: I (supergiants), II (bright giants),
III (giants), IV (subgiants), V (dwarfs, or main sequence),
and VI (subdwarfs).
• Our Sun, a main-sequence star, has a spectral classification
of G2 V. However, the giant star  Aurigae (Capella), actually
a binary system, has a classification G8 III. It is slightly
cooler, but much brighter than, our Sun.
• The range of luminosity on the HR Diagram, including effects
of both size and temperature, is extremely large (extending
from less than 10-5 to more than 106 that of the Sun).
• Even the range of luminosities of Main Sequence stars, from
the hottest to the coolest, is very large (a factor of 109)!
Effective Temperature and Absolute Luminosities
of Main Sequence Stars
Spectral Type
O3
O5
O7
O9
B0
B2
B5
B7
B9
A0
A2
A5
A8
F0
Teff (K)
52,500
44,500
38,000
33,000
30,000
22,000
15,400
13,000
10,500
9,520
8,970
8,200
7,580
7,200
MV (mag)
-6.0
-5.7
-5.2
-4.5
-4.0
-2.4
-1.2
-0.6
+0.2
+0.6
+1.3
+1.9
+2.4
+2.7
Mbol (mag)
-10.7
-10.1
-8.9
-7.8
-7.1
-4.7
-2.7
-1.6
-0.3
+0.3
+1.1
+1.7
+2.3
+2.6
L/Lsun
1,400,000
790,000
260,000
97,000
52,000
5700
830
320
95
54
26
14
8.6
6.5
Effective Temperature and Absolute Luminosities
of Main Sequence Stars
Spectral Type
F5
F8
G0
G2
G5
G8
K0
K2
K4
K7
M0
M2
M5
M8
Teff (K)
6440
6200
6030
5860
5770
5570
5250
4900
4590
4060
3850
3580
3240
2640
MV (mag)
+3.5
+4.0
+4.4
+4.7
+5.1
+5.5
+5.9
+6.4
+7.0
+8.1
+8.8
+9.9
+12.3
+16.0
Mbol (mag)
+3.4
+3.8
+4.2
+4.5
+4.9
+5.1
+5.6
+6.0
+6.4
+7.1
+7.4
+8.0
+9.6
+11.90.0012
L/Lsun
3.2
2.1
1.5
1.1
0.79
0.66
0.42
0.29
0.19
0.10
0.077
0.045
0.011
SPECTRAL CLASSIFICATION OF STARS
• The recent verification of a class of objects called brown
dwarfs, which are below the mass limit needed for energy
generation by thermonuclear fusion of hydrogen, but still
able to generate energy by thermonuclear fusion of
deuterium, has further extended the lower limit of the HR
Diagram.
• Brown dwarfs occupy the interval between the lowestmass M stars capable of fusing hydrogen to form helium
in their cores (about 0.08 solar masses) to the lowest
mass objects capable of fusing deuterium in their cores
(about 0.015 solar masses or 15 Jupiter masses).
• Most brown dwarfs detected to date have been
companions of nearby, ordinary stars, as brown dwarfs
are extremely faint in visible light, and hence hard to
detect except very close to our solar system.
PROPERTIES OF STARS
• In addition to the main sequence on the HR diagram, we
also have stars which are plotted in the upper right and
lower left regions of the diagram.
• Stars in the upper right portion of the HR diagram have
relatively low temperatures but high luminosities, which
implies that these stars must have very large diameters.
These stars are, therefore, called red giants or red
supergiants.
• On the other hand, stars in the lower left region of the
diagram have high temperatures, but low luminosities,
which implies that they must have very small diameters.
These stars, therefore, are called white dwarf (or blue
dwarf) stars.
PROPERTIES OF STARS
• As we will discuss later, the stars which fall in the Giant
and Supergiant regions of the HR diagram, and especially
those which are in the White Dwarf region, are in the late
stages of their lifetimes.
• These are stars which have used up their supplies of
hydrogen in their central cores, and so no longer
produce energy by thermonuclear fusion of hydrogen
in their cores.
• This, in combination with observations of new star
formation in our Galaxy at present, indicate that not all
stars were formed at the same time - their ages, as well
as their initial masses, span a wide range.
Comparison of Sizes of our Sun with Giant and Supergiant Stars
Ground-Based (Visible Light) Spectra of Stars
Spectra of Stars of Various Spectral Types
(Ground-Based Spectral Range)
Space UV
Ground Accessible
Bellatrix
Sirius
Regulus
Altair
Comparison of visible-near UV and “space UV” (below 3000 A) spectra of stars, vs. spectral type. Note
that the UV flux is much greater than the ground-observable visible and near-UV flux for the hotter stars.
APPLICATIONS OF SPECTROSCOPY TO THE
STUDY OF STARS
• In addition to determinations of temperature and composition,
spectroscopy of stars can be used to determine a number of
other properties as well.
• The luminosity of a star of a particular spectral type can be
determined from the apparent width of spectral lines
(independent of Doppler broadening due to temperature).
– Stars of high luminosity (giants and supergiants) have spectral lines
which are narrower than those of main-sequence stars of the same
surface temperature, because of the much lower pressure in the
outer layers of these stars.
– Likewise, the low-luminosity white dwarf stars have broader
spectral lines than do main-sequence stars of the same surface
temperature, because of the much higher pressure (due to high
surface gravity) conditions in these stars.
• Stars in binary systems can be distinguished from each other,
even if not separable in direct imagery, due to the oppositelydirected Doppler shifts of their spectral lines as they orbit around
their common center of gravity.
ENERGY SOURCES OF THE SUN AND STARS
INTERNAL STRUCTURE OF STARS
HOW STARS DIE
• All stars eventually deplete the supplies of hydrogen in their
central cores, where thermonuclear fusion of the hydrogen to
form helium produces their energy outputs.
• As we have seen, this process is faster for higher initial masses
of the star.
• When the supply of hydrogen in the core is depleted, the star’s
gravity compresses the core to a state of higher density and
temperature, at which fusion of helium and heavier elements can
occur (although with less energy yield per gram than with
hydrogen fusion).
• What happens to a star when its thermonuclear fuel is depleted,
also depends on the initial mass of the star.
• Stars similar to or somewhat greater than our Sun in mass,
become Red Giant stars. These stars produce energy by fusion
of hydrogen in shells surrounding the core, and by fusion of
helium in the core to produce carbon.
HOW STARS DIE
• The next stage of evolution, beyond the relatively short-lived
Red Giant stage, is for the star to gradually blow off its outer
layers while its central regions collapse to a smaller volume.
• During the period when the outer layers are being blown away,
ultraviolet radiation from the very hot central core of the star
causes this gas to glow, giving rise to what is known as a
Planetary Nebula.
• Following this, only the core of the star is left behind, which
becomes a White Dwarf star.
• A white dwarf star has a mass comparable to that of our Sun,
but is about the size of our Earth!
• The material of a white dwarf star has a typical density of about
1500 kilograms (1.5 metric tons!) per cubic centimeter.
• A white dwarf star has no source of energy other than its stored
internal heat; it eventually cools off to form a “black dwarf”.
• The white dwarf stars populate the lower left portion of the
Hertzsprung-Russell Diagram.
Planetary Nebula NGC 7293
Sirius Binary Star System
Ground Based – Visible Light
Chandra – X-Ray Image
The binary star system of Sirius consist of a spectral type A1 main
sequence star, Sirius A (top in each photo), the brightest star in the sky in
visible light, and a white dwarf star, Sirius B, which is much smaller (about
the size of our Earth) but much hotter than Sirius A (hence is the brighter
of the two in the Chandra X-ray telescope image, at the right).
Hubble Space Telescope View of Sirius B and Sirius A
Sirius B
STELLAR EVOLUTION
STELLAR EVOLUTION
HOW STARS DIE
• A star much more massive than the Sun, may actually explode at the
end of its life. For a short period of time, the exploding star or
supernova may appear billions as times as bright as our Sun!
• A supernova explosion is believed to be triggered by collapse of the
core of the star, which results from the depletion of its resources of
thermonuclear energy.
• This also results in ejection of the outer layers of the star, at high
velocities, into interstellar space.
• A supernova may, for a short period of time, be brighter than the entire
galaxy of stars within which it resides, and hence can be seen at very
great distances.
• The most recent naked-eye-visible supernova known to have occurred
in our galaxy, is the one in the year 1054 A.D. (that, for a short period of
time, appeared brighter than the planet Venus!) which created the Crab
Nebula, in the constellation Taurus .
• Supernovas may leave behind even more dense cores than do stars like
our Sun. For example, neutron stars or black holes may have masses
much larger than that of our Sun, but concentrated in objects only about
20 kilometers in diameter!
SUPERNOVAS
• Other supernova events observed during the human historical
record since the 1054 AD event, included a supernova in 1604,
in the constellation Cassiopeia, observed by Johannes Kepler.
• Recent observations of the Cassiopeia supernova remnant have
been made with the Hubble Space Telescope, the Chandra Xray observatory, and the Spitzer infrared space telescope.
• The most recent supernova that was visible to the unaided eye,
was the one in 1987 in our satellite galaxy, the Large Magellanic
Cloud.
• Because of its relative closeness, as well as developments in
astronomical instrumentation, SN 1987A was the first one to be
observed in the far-ultraviolet spectral range (by the
International Ultraviolet Explorer (IUE) satellite), and the first for
which the star that exploded could be identified in images taken
before and after the explosion.
• This supernova was also the first one in which the (previously
predicted) burst of neutrinos accompanying the event was
detected, by underground-based instruments on Earth.
The Cygnus Loop is the expanding shell (plus shockexcited interstellar gas) resulting from a supernova
explosion.
The Crab Nebula in Taurus is the remnant of the 1054 A.D. supernova
explosion (shown here is an image taken by the Hubble Space Telescope).
Supernova 1987a in the Large Magellanic Cloud
Before
During
Chandra X-Ray Images of Supernova Remnants
Cassiopeia A SN 1604
SN 1006
Supernova Remnant in the Large Magellanic Cloud
NEUTRON STARS AND BLACK HOLES
• Type II supernovas may leave behind compact objects of even
higher density than white dwarf stars (which consist of carbon,
oxygen, and neon, and are in an electron-degenerate state).
– White dwarf stars cannot be more massive than about 1.4
solar masses (the Chandrasekhar Limit).
• A neutron star is the most compact object which can be directly
detected by its emission of electromagnetic radiation, and
consists of neutrons in their degenerate energy state.
• A neutron star is much smaller than a white dwarf (electron
degenerate) star, packing more mass than that of the Sun in a
volume only about 20 km in diameter.
• A star must have a mass of at least 2 times that of our Sun at
the end of its energy-generating lifetime, in order to collapse to
the size and density of a neutron star.
• The maximum mass that a neutron star can have is about 3
solar masses.
NEUTRON STARS AND BLACK HOLES
• Neutron stars are so highly compressed that the electrons and
nuclei of its constituent atoms are forced together to form neutrons.
This material, therefore, is comparable in density to that of atomic
nuclei.
• Neutron stars can also have much higher rotation speeds (many
times per second!) in comparison to white dwarf stars.
• This gives rise to the observation of radio-emitting objects known
as pulsars, which emit radio pulses at a frequency of many cycles
per second.
• The neutron star remnant of the Crab Nebula supernova event was
first detected at radio wavelengths as a pulsar, but was later also
detected by its pulsations in visible and X-ray wavelengths.
• In comparison to white dwarf stars, neutron stars are much smaller
in size, but can be of much higher temperature (of order of a million
K) when first formed.
Crab Nebula Supernova Remnant
Neutron Star
NEUTRON STARS AND BLACK HOLES
• The Chandra X-ray observatory has obtained direct X-ray
images of the Crab pulsar, as well as of the very hot nebular gas
in its vicinity.
• The Hubble Space Telescope has also imaged the Crab pulsar,
and has also observed, elsewhere, a rapidly-moving neutron star
(presumed remnant of a supernova explosion).
• Although, by definition, black holes cannot be directly imaged,
their existence is confirmed by their effects on companion stars
and/or interstellar material.
– A star may appear, from the Doppler shifts of its radiation, to be
revolving around an unseen object.
– Interstellar gas may be heated and made luminous, and may
revolve around, an unseen object.
– In extreme cases (“supermassive” black holes) the heated
interstellar gas may form “jets” which appear to come from the
central region of a galaxy (example is the galaxy known as M87).
Chandra X-Ray Image of Crab Nebula
Neutron Star
Neutron Star
BLACK HOLES
• A more massive still (about 3 times the Sun’s mass) supernova
remnant can collapse to an even higher density state than that of
a neutron star- such that its gravity is so strong that not even
light (and other forms of electromagnetic radiation) can
escape from the star!
• Such an object is known as a black hole, since nothing (not even
light) can escape its gravity.
• Black holes can be detected only by the effects of their gravity on
nearby stars or interstellar gas.
• For example, the presence (and mass) of a black hole can be
determined if a visually single companion star is observed to
travel in a circular or elliptical orbit in seemingly empty space.
• On a much larger scale, supermassive black holes can be
formed in the central regions of galaxies, by collisions between
massive stars and uptake of interstellar material by initially
smaller-mass black holes.
• These can have hundreds, or even thousands, of times the mass
of our Sun, and are evident not only by their effects on nearby
stars, but on the interstellar material as well.