We concentrate on this page on the
inception, evolution, and demise of individual stars. A helpful Web Site that
supplements the content of this page has been put online by Prof. Nick Strobel of
Bakersfield College. Also recommended are the University of Oregon site cited
in the Preface (especially relevant are Lectures 15-18, and 20) and a site
prepared in outline form by astronomers at University of
Georgia.
Before reading the next two pages,
it may be profitable for you to get an overview of Star formation
by reading a specific page from the above-cited Oregon lectures.
The number of stars in the Universe
must be incredibly huge - a good guess is 100 billion galaxies each containing
on average 100 billion stars or (1011 times 1011, which
calculates as 1022. More recently, an independent estimate using
other means states the value as 7 x 1022 stars; that survey made by
Simon Driver of the Australian National University is said by him to still be a
conservative underestimate. To make his point, here is a view of stars near us
that indicates the densities just within a part of the Milky Way.
Yet on a very clear night in dry
air, one sees at any one location, without using a telescope or binoculars, only
about 2500 "star" points within the Milky Way in the northern hemisphere and a
similar number in the southern hemisphere. (With a very few exceptions,
individual stars outside the M.W. cannot be seen by eye). When viewed from all
terrestrial vantage points that number approaches 8400. Some of the of these are
distant galaxies, so far away that the unaided eye can discern only an apparent
single light source. Many of the brightest stars are generally those closest to
the Sun (around 1 to 100 light years away). Only when powerful telescopes are
used does the astronomer realize by estimate or extrapolation that billions of
galaxies exist; by inference we deduce that these probably contain stars in
numbers similar to those that can be roughly counted in the Milky Way (in many
tens of billions). We begin this concentration on
stars with something that is itself not truly a scientific topic but which
remains useful to astronomers as a convenient "Sky Map". Such maps contain the
Constellations - patterns of certain visible stars (a few were actually
galaxies but this was not known at the time) that the ancients imaginatively
discerned in looking at individual stars within narrow patches of the celestial
hemisphere which seemed to be distinctive and readily recognized. These
arrangements were given fanciful names, of gods, animals, and other descriptors
from their everyday experiences. This began with the Babylonians in Mesopotamia,
and the system was expanded to 88 named constellations by the Greeks and Romans
some 2000 years ago. (Psychics and Fortune Tellers have used constellations as
"signs" and for horoscopes for several millenia.) This next pair of
illustrations (source StarNames) shows
some of the major constellations in the northern hemisphere, plotted in two half
circle fields:
To the ancients, the stars were all
equidistant on the "celestial sphere" and varied only in brightness. Modern
astronomers now know that individual stars/galaxies in the light points that
make up the constellation pattern are actually located at various distances from
Earth, which together with diifferences in size account for their different
brightnesses. Most of the defining stars in a constellation are located in our
galaxy, the Milky Way. Astronomers often cite individual
constellations as a reference framework in specifying the location in the
celestial sphere of some stellar or galactic feature or phenomenon on which they
are reporting. However, they need to give some specific hemisphere coordinates,
either in terms of azimuth, altitude (along a meridian) and hour time, or in
another system, the declination and right azimuth. The position of any star as
seen on some specific night will vary with the hour, time of year, and
geographic location of the observer. Thus, the movement of the Earth around its
axis causes the stars during a night's observation to follow arcuate (circles)
paths around a part of the sky where the celestial pole is located (close to the
North Star pointed to by two stars in the Big Dipper [Ursa Major]). The groups
of constellations also shift with the seasons and with the place on Earth where
the star-gazer is positioned (different constellations are seen by those in the
Southern Hemisphere of the Earth than those in the Northern Hemisphere [lookers
at the Equator will see some of the constellations visible in each Hemisphere]).
More about the constellations can be found at the Star Charts and Star
Map Internet sites. For the second site, after it appears scroll down until
you see in a sentence an underlined phrase called "northern hemisphere
constellations", an example of the usual format of such maps (this one is valid
for December). This next map, also from StarNames, shows the same look direction
(to the North), as the first map above, but is a Winter view (compare and locate
equivalent constellations; but now several have disappeared and new ones have
appeared).
Interesting, but back to Science.
The standard model for a star is, of course, our Sun. The Sun is typical of most
stars; as we shall note shortly, these stellar bodies vary from about 0.1 to 100
times the mass contained in the Sun. Without a telescope, under exceptional
viewing conditions (using binoculars), about 9000 individual stars can be seen
in the wide celestial band that is the central disc of the Milky Way (M.W.)
galaxy. Others elsewhere in the celestial hemisphere make up about 2000 points
of stellar light can be seen (in clear air, away from urban light contamination)
by the naked eye. Some are nearby within our galaxy and are not particularly
large, while others are mostly stars of the Giant/Supergiant types in the halo
(see below) around the Milky Way. Still others are galaxies that lie in
intergalactic space beyond the Milky Way but mostly within a billion light years
from Earth. Telescopes can resolve countless more stars in the M.W., can
recognize millions of galaxies, and can pick out some individual stars in nearby
galaxies.
A degree of luminosity of an object
in the sky (galaxy; star; glowing clouds; planet) can be represented by its
apparent magnitude - a measure of how bright it actually appears
as seen by the telescope or other measuring device. This magnitude is a function
of 1) the intrinsic brightness which varies as a function of size, mass, and
spectral type (related to star's surface temperature)and 2) its distance from
Earth. (Magnitude as applied to a galaxy, which seldom shows many individual
stars unless they are close [generally less than a billion years away], is an
integrated value for the unresolved composite of glowing stars and gases within
it.) The brightness of a star can be measured photometrically (at some arbitrary
wavelength range) and assigned a luminosity L (radiant flux). For two stars (a
and b) whose luminosities have been determined, this relationship holds:
from which can be derived:
To establish a numerical scale,
some reference star(s) must be assigned an arbitrary value. Initially, the star
chosen, Polaris, was rated at +2.0 but when it was later found to be a variable
star, others were selected to be the 0 reference value for m. The magnitude
scale ranges from -m (very bright) to +m (increasingly faint) values. The more
positive the number, the fainter is the object (planet; star; galaxy); very
distant galaxies, even though these may be extremely luminous, could have large
positive apparent magnitudes because of the 1/r2 decrease in
brightness with increasing distance. The Sun has the value - 26.5; the full Moon
is -12.5; Venus is -4.4; the naked eye can see stars brighter than + 7; Pluto
has a magnitude of +15; Earth-based telescopes can pick out stars visually with
magnitudes down to ~+ 20 (faintest) and with CCD integrators to about +28, and
the HST to about +30. Thus, the trend in these values is from decreasing
negativity to increasing positivity as the objects get ever less luminous as
observed through a telescope. Each change in magnitude by 1 unit represents an
increase/decrease in apparent brightness of 2.512; a jump of 3 units towards
decreasing luminosity, say from magnitude +4 to +7, results in a
(2.512)3 = 15.87 decrease in brightness (the formula for this is
derivable from the above equations, such that the ratio of luminosities is given
by this expression: 10(0.4)(mb -
ma). Below is a simple linear graph that shows various
astronomical objects plotted on the apparent magnitude scale:
Absolute magnitude (M) is the
apparent magnitude (m) a star would have if it were relocated to a standard
distance from Earth. Apparent magnitude can be converted to absolute magnitude
by calculating what the star's or galaxy's luminosity would appear to be if it
were conceived as being moved to a reference distance of 10 parsecs (10 x 3.26
light years) from Earth. The formula for this is:
where r is the actual
distance (in parsecs) of the star from Earth. Both positive and negative values
for M are possible. The procedure envisions all stars of varying intrinsic
brightnesses and at varying distances from Earth throughout the Cosmos as having
been arbitrarily relocated at a single common distance away from the Earth. Both luminosity and magnitude are
related to a star's mass (which is best determined by applying Newton's Laws of
motion to binary stars [a pair; see below for a discussion of binaries]). The
graph below, made from astrometric data in which mass is determined by
gravitational effects, expresses this relationship; in the plot both mass and
luminosity are referenced to the Sun (note that the numbers are plotted in
logarithmic units on both axes):
There is a relationship between
absolute magnitude (here given by L for luminosity) and mass (given by the
conventional letter M; which accounts for replacing the absolute magnitude M
with L). Here is one expression:
In the above, both L and M for a
given star are ratioed to the values determined for the Sun. Note the two
different power exponents. It seems that some stars obey a fourth power, others
a 3, and a few are just the square of the mass. The most general expression in
use is given as L = M3.5. There are relatively few stars with mass
greater 50 times the Sun. Very rarely, we can find a star approaching 100s solar
mass, but these are so short-lived that nearly all created before the last
million years have exploded, with their mass being highly dispersed, and thus
ceasing to send detectable radiation. If the Sun were envisioned as
displaced outward to a distance of 32.6 l.y., its apparent magnitude as seen
from Earth would be -26.5; its absolute magnitude would be changed to +4.85. A
quasar, which is commonly brighter than a galaxy, has an absolute brightness of
- 27 (note that in the absolute scale increasingly negative values denote
increasing intrinsic brightness). The illustration below gives the absolute
magnitudes (vertical axis) as a function of temperature (horizontal axis) for a
number of stars with popular names; note the similarity of the color bars (which
express the visual colors of the stars as seen through a telescope) to the
brightness range - this is essentially a preview version of the standard H-D
diagram, shown and discussed on this page beginning twelve figures below this,
which serves as a plot of the different types of stars and an inferred history
of a star of given size (mass):
One classification of stars is that
of setting up categories of star types (see below) in a series of decreasing
sizes and luminosities. These are the 7 Luminosity/Type Classes: Ia, Ib:
Extreme Supergiants; II: Supergiants (Betelgeuse); III:
Giants (Antares); IV: Subgiants; V: Dwarfs (Sun): VI:
Subdwarfs (metal poor); VII: White Dwarfs (burned out stars). The
oddity in this classification is the omission of a category of "Normal"; a star
is either a Giant or a Dwarf. Another classification is based on density.
Starting with the least dense and progressing to the most dense (massive), this
is the sequence: Supergiant; Red Giant; Main Sequence;
Brown Dwarf; White Dwarf; Neutron Star; and Black
Hole. Each of the above bold-faced types are described in some detail on
this two-part page.
The brightest star in the northern
hemisphere of the sky is Sirius, an A type star (see the H-R plots below and
accompanying paragraphs which explain the letter designation of stars) of
apparent magnitude -1.47 that lies 8.7 light years away. Here is how it appears
through a telescope:
Closest to the Sun is an M type
star (faint), Proxima Centauri, being 4.2 light years away. Just slightly
farther away is Alpha Centauri, a G type star that is the third brightest in the
heavens (visible in southern hemisphere). Here is a telescope view of Alpha
Centauri:
The map below is a plot of the
distances from Earth (circle is 13.1 light years in radius) of the 25 nearest
individual or binary stars or local clusters in our region of the Milky Way
Galaxy:
Information Bonus: Just beyond this
limit is the star Vega (27 light years away). It has two claims to fame: 1) it
alternates with Polaris as the North Star used in navigation; the Earth's
precession brings Vega into this position every 11000 years, and 2) It was the
nearby star used as the host for an extraterrestrial civiliation in Carl Sagan's
extraordinary science fiction novel "Contact" (later made into the movie of the
same named "starring" Jodie Foster); contact was made with a planet near Vega as
a signal picked up by the Socorro, NM radio telescope array - as initially
interpreted that signal consisted of a string of prime numbers (those divisible
only by themselves and 1).
Referring to the above, the
following is extracted verbatim from the caption accompanying this image
that was displayed on the Astronomy Picture of the Day Website for February
17, 2002: What surrounds the Sun in this neck of the Milky Way Galaxy? Our
current best guess is depicted in the above map of the surrounding 1500 light
years constructed from various observations and deductions. Currently, the Sun
is passing through a Local Interstellar Cloud (LIC), shown in violet, which is
flowing away from the Scorpius-Centaurus Association of young stars. The LIC
resides in a low-density hole in the interstellar medium (ISM) called the Local
Bubble, shown in black. Nearby, high-density molecular clouds including the
Aquila Rift surround star forming regions, each shown in orange. The Gum Nebula,
shown in green, is a region of hot ionized hydrogen gas. Inside the Gum Nebula
is the Vela Supernova Remnant, shown in pink, which is expanding to create
fragmented shells of material like the LIC. Future observations should help
astronomers discern more about the local Galactic Neighborhood and how it might
have affected Earth's past climate.
The largest star so far measured in
the Milky Way is Mu Cephi (in the galactic cloud IC1396), seen as the orange
disc (also called Herschel's Garnet star) near top center of this HST image.
Located about 1800 light years from Earth, it is almost 2500 times the diameter
of the Sun.
This is an example of a rare type
of star known as a hypergiant (see next page). Another even bigger star (2800
times the solar diameter; 2.4 billion miles) is Epsilon Aurigae (in the
constellation Auriga, the Charioteer), but residing in the Milky Way about 3300
light years from Earth. This star, also known as Al Maaz (Arabic for he-goat)
and visible to the naked eye) is considered by many astronomers to be the
"strangest" star in the firmament. Every 27 years this star (magnitude 3.2)
undergoes a diminishing of brightness (about 60000 times greater than the Sun)
lasting about 2 years. The last such event was in 1983; the next in 2010. It is
thus one of a class called "eclipsing stars". The cause of this regular pattern
of luminosity change is still uncertain; some astronomers think it is caused by
the passage of a second massive star across Epsilon Aurigae's face but that
binary is so far undetected, leading to the hypothesis that the drop in
luminosity occurs when a cloud of dark material (dust) orbiting the star as a
clump obscures Epsilon Aurigae each time it moves through the line of sight to
the Earth. Most stars bigger than the Sun are
not as huge as Mu Cephi or Epsilon Aurigae. The majority are no larger than
about 100x the diameter of the Sun. This diagram illustrates the relative size
of some common stars (setting the Sun's diameter as 1), which establishes our
star as rather ordinary in the size scheme within the Milky Way:
More than half of the stars
in a galaxy are also tied locally to a second star as a companion (binary), such
that each of the pair or group orbits around a common center in space determined
by their mass-dependent mutual gravitational attraction. This arrangement is
exemplified by the image made by the HST Faint Object Camera (FOC) of the Mira
star (Omicron Ceti) in the Constellation Cetus.
This star is a Red Giant (see
below) which appears to be periodically brightening (it is credited as the first
known variable star, having been discovered in 1546 A.D.). The HST has resolved
it into a binary in which the star on the left has a White Dwarf (see below) at
its core and now receives mass from the Red Giant that accumulates until the
hydrogen burns (see first illustration at the top of page 20-6). An ultraviolet
(UV) image made by HST's FOC actually shows mass being drawn off the Red
Giant. Some stars are grouped into more
than one companion; ternary groupings (three stars orbiting about a common
center of gravity) are fairly common. Here is an image of four stars orbiting as
a unit about a gravity center in the galaxy M73.
Binary star systems are recognized
by three means: 1) visual, through a telescope (as in the above two images); 2)
by periodic drops in brightness caused by passage of one star across another
(eclipse; an uncommon observation condition); and 3) by measuring spectral
characteristics in which both a Doppler shift towards the red and the blue occur
as one star moves away and the other towards Earth (and the reverse) along
pathways of their mutual orbits.
To demonstrate the second means,
examine this diagram which shows the brightness levels (and magnitude
variations) for the binary star Algol:
The larger star (in blue) is
mutually orbiting a smaller star (red) which has a notably smaller output in its
own luminosity. When the latter star passes in front of the larger star, there
is a notable drop in the combined luminosities, as the partial eclipse cuts out
some light from the larger star. Then, when the red star passes behind the blue
star, there is a small drop in luminosity since the smaller, now occulted star
does not send any of its light to the observing telescope. Spectral line shifts are used to
study the motions of binary stars. We will treat stellar spectroscopy in detail
on page 20-7 As a preview,
the spectral method can be illustrated by looking at a pair of spectral strips
for two similar stars that are mutually orbiting:
Bright lines for hydrogen appear in
the top and bottom (dark background) strips. This fixes a reference location for
excited hydrogen in the rest state. The two center spectral strips include the
same hydrogen lines, the first strip acquired from one and the second the other
star. Note that the lines in one have moved to the left and the other to the
right of the reference lines position. The spectrum on the bottom center has
been blueshifted (see page
20-9) towards shorter wavelengths; the spectrum at the top center has been
redshifted towards longer wavelengths. This is explained thusly: The bottom star
is in motion towards the observing system on Earth whereas the top star is
moving away from the telescope. This would occur when the two stars are aligned
sideways to the line of sight and are moving in opposite directions around a
common center of gravity. For some visual binaries, movements
over time can be observed and plotted, such as illustrated here for the star
Mizar (in the Ursa Major constellation), which is resolvable into Mizar A and
Mizar B.
The Chandra X-ray Observatory has
imaged a close binary pair in the M15 Galaxy. Prior to obtaining this image, the
object was thought to be a single star, but at x-ray wavelengths, it is now
resolved into a faint blue star and a nearby companion believed to be a neutron
star giving off high energy radiation. Thus:
Turning now to stellar evolution,
to preview what will be examined in some detail (shown in chart form later on
this page), the pattern of a star's history follows a pathway that, depending on
its total mass, eventually splits into one of two branches (/> or \>), as
it leaves what is known as the Main Sequence. This is: Development of a large
cloud of denser gas made up of predominantly molecular hydrogen (H2)
+ dust --> Protostar --> T-Tauri Phase --> Main Sequence /> (if mass
less than 8 solar masses)--> Red Giant --> Planetary Nebula --> White
Dwarf; OR \> (if mass greater than 8 solar masses) --> Supernova -->
Neutron Star and/or Black Hole (depending on mass [size]).
Both star classification and
evolution can be summarized in a graphlike chart that consists of a plot of
luminosity (vertical axis) versus star surface temperature which is expressed
also by (correlated with) the star's visual color (note also the Spectral Type
designations at the top). This is known as the Hertzsprung-Russell (H-R)
Diagram. Mass densities are shown as numbers on the the central line that
defines the Main Sequence (M.S.) of stars. Most known stars lie along this line;
they describe a stage in which a star reaches some fixed size and mass and
commences burning of most of its hydrogen before changing to some other star
type off the sequence. Star types, which are defined on the basis of stellar
surface temperatures page 20-7), are shown by the letters (O, B,...etc.)
assigned to each group and evolutionary pathways for some are indicated. This
particular plot also shows along the right ordinate the total time that Main
Sequence stars of different masses spend on that sequence before evolving along
the several principal pathways (see below); as far as we now know, stars do not
completely vanish, but survive as dwarfs or Black Holes ( but the latter in
principle can disappear by evaporation as Hawking radiation).
La/Lb = (2.512)mb -
ma
mb - ma = 2.5
log(La/Lb)
From Nick Strobel's Astronomy Notes.
M = m + 5 - 5log10r,
From J. Silk, The Big Bang, 2nd
Ed., © 1989. Reproduced by permission of W.H. Freeman Co., New York
A star on the Main Sequence will
follow some pathway during its subsequent history. To illustrate this
progression, look first at this evolution diagram for a star the mass of the
Sun:
The key steps in the progression
are 1) exhaustion of the main nuclear fuel; 2) change to a Red Giant; 3)
explosion to the Planetary Nebula phase; 4) survival of a central core as a
White Dwarf star. In the version below of the H-R
diagram, the various major star types or states for stars that are not on the
Main Sequence are shown (the spread of those types as a function of
temperature-luminosity variations is plotted). Among the off-M.S. evolved star
groups are four types of Giants (Sub; Red; Bright; Super), T Tauri; and the two
major types of cepheids. These are discussed again on this page or elsewhere in
this Section. Not shown is the recent designation of LT for Brown Dwarfs. Note
that the letters at the bottom include some like B0 and B5 or K0-K5; this
denotes subdivision of each class into temperature subclasses (0 being hottest
and 5 coolest in a class). Temperature ranges (in °K) are: O class = greater
than 30000; B = 11000 - 30000; A = 7500 - 11000; F = 6000 - 7500; G = 5000 -
6000; K = 3500 - 5000; M = less than 2500. Colorwise, the first three are all
"blue-white" stars, F is bluish to white; G is white to yellow; K is yellow
orange; and M is red.
This next diagram is another H-R
variant in which some well-known stars with specific names (visible to the naked
eye or through a telescope) have been plotted. The Red Dwarfs and Blue Giants
are specified in this version. On the right is the size of a star (in terms of
radius) relative to the Sun taken as 1.:
This diagram extends the history of
a G star by showing the sequence of star stages from its very inception as a
nebular mass that grows into a protostar, then to the M.S., and on to a final
dwarf state.
The pathways of protostars to the
Main Sequence, as shown on this modified H-R diagram, depend on their mass (in
multiples of a solar mass) at the stage when they commence proceeding to the M.S
and initiate hydrogen fusion. The times involved in this transition will vary
systematically with mass; thus, a 15 solar mass protostar takes only about 10000
years to reach the M.S. whereas a 2 solar mass star may require up to 10,000,000
years for the process to begin fusion:
This next diagram shows the
evolutionary history of three stars of differing mass at the upper, central, and
lower ends of the Main Sequence after they leave the M.S.:
These pathways are somewhat
generalized. When the details are plotted, the path of a star of 5 solar mass
size from the M.S. to a Red Giant can be more complex, as shown in this
multi-step example:
The largest number of individual
stars in galaxies fall between just under 1 solar mass to about 10 solar masses.
As these burn their hydrogen fuel into helium, they begin to burn that helium
and further brighten, cast off some of the outer hydrogen, and become luminous
(for stars under a solar mass of 2.3, there is a short-lived large increase in
luminosity known as the helium flash phase). Then, as the helium burns to carbon
(which organizes into a core of degenerate C and some O; see page 20-7), such stars follow
what is known as the asymptotic giant branch (AGB) pathway which begins with a
second Red Giant state (steps 10-15 above.
This shedding enroute to the
planetary nebula phase (further described below) can appear in a spectacular
manner, as shown by this HST image of the Helix Nebula, in which the red ring is
excited hydrogen: A star's precise position along the
Main Sequence depends on the total mass of H fuel that collects during the
formative phase into the gas ball. Some stars (e.g., Type M) have masses as low
as 1/20th of the Sun (1 solar mass is the standard of reference as is the
luminosity of the Sun, also set at 1), whereas others fall within a range of
greater masses that may exceed 50 solar masses (Type O). The high mass stars on
the Main Sequence are brighter and bluer whereas those at the lower end of the
M.S. tend to be yellow to orange. The initial quantity of mass in a star is the
prime determinant of its life expectancy, which also depends on its evolutionary
history and final fate. As a general rule, small stars may take more than 50
billion years to burn out completely, stars in the size range of the Sun live on
the order of 5 to 15 billion years, and much bigger stars carry their cycle to
completion in a billion or less years. Stars whose masses are similar to the
Sun's actually will burn about 90% of their hydrogen during their stay on the
Main Sequence. Stars with greater than 50 solar masses may complete their M.S.
burning in just 20-30 million years.
The lifetime spent on the Main
Sequence is approximately proportional to the inverse cube of the star's mass
(this is true for most stars, especially massive ones; stars less than a solar
mass have lifetimes closer to the inverse 4th power). The relation between size
(mass) and age is shown in this next diagram: The fate of stars (at the end of
their history) of all sizes (and different masses) can be conveniently
summarized in this Evolution diagram:
From B.C. Chaboyer, p. 53, Scientific American, May 2001
A variation of this diagram (which unfortunately does not produce well when reprocessed after being taken off the Internet) is included here because of its pictorial wealth of information:
Of special interest are the end products of each evolutionary path. After burnout or explosion, small stars end up as White Dwarfs; intermediate stars as Neutron Stars; and the largest stars as Black Holes.
Now to a more detailed discussion
of the history of stars as expressed in the above diagrams.
Stars develop within galaxies in
nebulae (also called Giant Molecular Clouds [GMC] composed mostly of
H2) by progressive sub-fragmentation, aggregation and contraction of
gas and dust into centers of higher density. These nebulae represent localized
concentration of gases brought about by several processes such as the driving
force of shock waves from supernova explosions and intergalactic magnetic
fields. The clouds turn very slowly but this helps to develop "seed" locations -
internal denser regions that bring the gases toward them because of greater
gravitational attraction. The H-He atoms in these denser local regions assemble
into gas balls and dust clouds by collisions and gravitational forces at
initially low temperatures (100's of ºK) in a turbulent process of condensation,
generating heat (in large part dissipated as thermal radiation). Thus, molecular
hydrogen clouds are the regions of gas where most new stars are born.
Stellar object 07427-2400 is a
young forming massive star about 100000 year old located 20000 light years from
Earth. It has a huge protostellar disc (GMC) of accreting molecular hydrogen
that is spiraling into its massive central star (now about 100 times the
luminosity of the Sun). In the process, shock waves are produced that move
against the disk, making it luminous also by exciting the hydrogen and ionized
iron. The IRAS Observatory has produced this image
One way to study GMCs is to plot
the distribution of excited carbon monoxide (CO) dispersed within the molecular
hydrogen. In this state CO produces two prominent emission lines at 1.3 and 2.6
mm in the near radio wave segment of the EM spectrum. (H2 does not
emit strong signals in the radio region.) Here is the CO pattern that occurs in
the Orion Nebula (a GMC which also contains strong HII (ionized H)
regions (see below)
Outside the clouds, H and He also
are dispersed, at much lower densities, as the principal elements distributed in
interstellar space; the density of free H (mostly neutral) in that space is
estimated to be between 3 and 8 atoms per cubic meter. This atomic hydrogen when
excited but not ionized is detectable by its signature at a 21 cm wavelength as
determined through radio telescopy, representing photon radiation given off when
excited hydrogen reverts to its lowest energy state. But, in spiral galaxies
most atomic hydrogen gas has been rearranged in long streamers between arms of
existing stars, as seen in this 21-cm radio telescope image of the Milky Way.
When GMCs heat up to temperatures
above about 5000° K, the hydrogen can be ionized (see Page 20-7 for a discussion of
the different ionized states of hydrogen and their characteristic spectral
lines). This gives rise to strongly emitting clouds that are referred to as
HII Regions (Atomic hydrogen is denoted by HI;
alternate forms of this symbol are H II or HII). One prominent line used to
image and study HII regions is Hα, whose line lies at 0.656 µm - the
N3 --N2 transition in Balmer series. These clouds are photogenic and deserve
several examples here. First, an emission nebula as imaged by a telescope used
in the 2Mass project (inventory of stellar objects in the Visible-Near IR):
We follow this with an image of
part of M16 (Eagle Nebula) that has heated to the HII temperature
range, with colors chosen to indicate that HII clouds also are bright
in the Near-IR: And lastly, an image which contains
an emission cloud (pink) and two smaller reflection clouds (molecular hydrogen)
(blue): Before organizing into an galaxy or
after a galaxy has formed, the initial nebulae will have irregular shapes. Some
nebulae appear dominated by dark dust, mixed with hydrogen. These may have
elongated shapes, some of which are described as "pillars". Part of the Eagle
nebula contains such dark dust concentrations, as seen here:
A close view of one of these
pillars (said by many as the most fascinating image yet obtained by the HST) is
shown on page 20-11.
Another type of dark dust-rich clot, with sharp boundaries, of star-forming
material is called a "Bok Globule" (see several examples on Page 20-4), which commonly
produces a large number of massive O-type stars, the brightest on the Main
Sequence, which have short life times. Here is a typical grouping of dark
patches that belong to the Bok Globule category:
A pair of Bok Globules in IC 2944
appear to be merging in this HST close-up:
Now look at part of the Keyhole
nebula, some 8000 light years from our own galaxy. Its size is about 200 l.y. in
diameter. It is classed as a dark nebula, but in this rendition computer
processing brings out its rich colors. (Note: the term nebula, derived
from the Latin for "cloud", has multiple meanings. In the early 20th century,
the word was applied to bright objects in the sky that Hubble and others showed
to be galaxies; now, the term is restricted to any collection of hydrogen gas
and dust that may occur outside of a galaxy, as intragalactic material, or as
remnants of exploding stars.) A good review of the types of nebulae is found at
The Web Nebula.
A typical gas and dust cloud (as
shown below, a subsection within a developing spiral nebula, NGC253) consists of
a number of bright, bluish new stars midst swirls of hot hydrogen-rich gas, and
some older stars.
This next pair of images confirms
once more the value of HST in providing details about astronomical features. The
first image of the Swan Nebula, M18, some 5000 light years from Earth, was taken
from the Anglo-Australian ground telescope. The box indicates the area imaged by
the HST Wide Field Camera, showing an almost 3-D like view of the clouds in this
small part of the nebula. Red colors indicated excited sulphur; green in this
rendition is hydrogen gas (in other renditions using different color filters H
is often shown in red), and blue associates with oxygen.
One of the largest nebulae is the
Carinae nebula, seen only from Earth's southern hemisphere. It is a bright
nebula (and contains the supernova Eta Carina star) that lies just beyond the
above Keystone nebula.
The HST Wide Field Camera has
recently imaged a small cluster of stars in an early stage of their
organization. This is in the Small Magellanic Cloud, about 200,000 light years
away. This "cloud" (almost 10 light years wide) consists of glowing hydrogen gas
within which numerous stars are embedded. At least 50 of those that can be
resolved appear to be young, massive stars. As time continues, these stars will
enlarge as gravity pulls in the surrounding nebular material. Because of their
large size, their destiny is to rapidly burn up their hydrogen fuel, and
eventually explode as supernovae (see below), many ending as neutron stars.
As a large number of stars develop
from a nebula, and become luminous as hydrogen-burning ensues, processes
including radiation pressure from starlight will allow the stars to be seen
through the diminishing dust and gas. The nebula may continue to produce more
new stars if it draws more hydrogen from beyond its boundaries, but generally
nebulae tend to use up available H2 and may deactivate. Stars may
then form elsewhere as new clouds develop and reach conditions favoring stellar
generation. Individual stars develop along
fairly well known blueprints. A central clot of mainly gas organizes and is
surrounded by an envelope usually enriched in dust. As the protostar heats up,
some of its material is ejected by magnetic forces as jets, such as in these two
examples:
The expulsion of these high speed
gases and charged particles can cause parts of the surrounding nebular masses to
be excited and glow in luminescent patches. This phenomenon is known as
Herbig-Haro (HH) Objects. Here is one example:
The emergence of these objects at
two opposing sides (bipolar) of the protostar is typical. The next HST view
shows this HH effect in a glowing "cloud" which is located near the end of a jet
(bright hemisphere, to the right) passing through it.
In the nascent star phase, the dust
and gases form a very large volume of organizing material called a "globule" (at
least some of these are Bok Globules; see above). A globule in the inner Milky
Way, designated DC303.8-14.2, shown below was first detected by ESA's IRAS
satellite. In this trio of images, the left image of the globule, obtained
during the Digital Sky Survey observations, shows the extent of the nebular mass
seen in visible red light. The center image made by Kimmo Lehntinen's team using
VLT ANTU telescope at ESO's Observatory in Chile, is the inset of the left image
shown here in color from several infrared band images on this telescope, It
shows a distinct ring of gases and dust that emits strongly in the infrared. The
right image (of the inset of the center image) indicates several jets of the
Herbig-Haro type involved in the early stages of formation of the eventual star.
From the above discussion we
conclude that the dominant behavior during the pre-Main Sequence history of a
protostar is marked by light gases continuing to inflow and build up the star's
mass and size. Much of the dust remains as a thick disk outside the star, such
as this example: As will be further explained on page 20-11, disks like this
are the potential conditions that lead to planet formation. Meanwhile, the star
approaches pressure-temperature levels capable of initiating hydrogen fusion, as
described in the next paragraph.
As more matter accrues within a
growing nebula, its internal gravity continues to increase and draw in still
more gases. Gravitationally-driven collapse into forming stars induces
compression and further heat rise. The protostar phase is reached as
temperatures rise to 2000 - 3000° K. At ~10,000° K, the H begins to ionize
(electrons stripped away) and, in the process, loses some heat energy by
radiation which tends to slow or counter the compression. Over time, the cloud
eventually reaches a density that requires it to then undergo local clumping of
gases into clots that grow into still denser concentrations to become stars
(these smaller clots can exist for much of the galaxy's life but are the sites
of further star formation). Here is a Hubble Space Telescope (abbreviated as HST
and described on page
20-3) view of the Orion nebula, which appears to be in an early stage of
organization into stars (hence, a younger nebula).
Here is a gas cloud in the Orion
nebula, which lies within the Milky Way at a distance of 1500 l.y., as seen by
HST's Wide Camera; but on the right is the same area imaged in the IR in which a
bright small star "shines through" as a protostar.
Seen close-up is the central part
of Orion, in which the reds are related to the excitation of hydrogen gas (using
a red filter on the HST image).
The message given by this
example is that anyone - including those who are not professional scientists -
can participate in the exploration of the Cosmos. In July, 2003 a report was released
stating that the Orion nebula contains the hottest stars yet discovered in the
Universe. Temperatures were obtained using Chandra X-ray data. The 3 hottest
were supermassive stars shown in the right panel below:
The single hottest of these stars
reaches a surface temperature of 60 million degrees Centigrade (108,000,000° F),
more than double the value of the previous record holder. More recently, a new image made
through ESA's telescope in Chile has disclosed in the Lynx nebula a cluster of
hot blue-white stars that is a million times moe luminous than the Orion
nebula:
As the early stages of star
formation proceeds, the cloud tends to gather around the star in a more isolated
manner, removed from neighboring gas and dust nebula. It may then enter the T
Tauri phase at which the growing star starts to generate strong stellar winds.
The cloud disk still can exceed 150 A.U. in dimension. This telescope image
shows the glowing cloud (rendered here in blue, but actually of a different
color) around the incipient, still poorly organized central star (a binary
pair).
Here are two more T Tauri stars,
the one on the left showing the nebular shield that masks the bright growing
star and the one on the right showing another T Tauri star as seen in the
infrared:
The star now rapidly contracts as
it passes through the Hayashi phase. This relies on the proton-proton
nuclear reaction which releases radiation energy that causes a notable increase
in luminosity. However, hydrostatic equilibrium (see below) is not yet reached
as the growing star continues to experience disruptive convection. This next view shows a star
after most of its accretionary disk material has been incorporated into its
mass, as it nears the stage where it will be on the Main Sequence.
When a star has finally organized
into its hydrogen-burning sphere, it may eject and dissipate its remaining
nebular material as shown in this image of what is now known as McNeil's nebula
(named after its initial discover, an amateur astronomer):
For stars of masses near that of
the Sun, it takes about 10 million years to work through the protostar phase and
another 20 million years to join the Main Sequence. More massive stars reach the
Main Sequence more rapidly. Below is a view taken through the Japanese Suburu
Telescope of S106, with mass twenty times that of the Sun, which began to burn
only about 100,000 years ago. This star, 2000 l.y. from Earth, still is showing
dust and gas flowing into the central body.
An early stage of another massive
star, AFGL2591, 10 times the size of the Sun, has been viewed in infrared light
by the newly operational Gemini North Telescope on Mauna Kea, Hawaii. Some 3000
l.y away in the Milky Way (located against the backdrop of the Constellation
Cygnus), the central region of the forming star is still disorganized. Infalling
material continues its growth but also sets off a return outflow of gas and
dust.
After a star has moved onto the
Main Sequence, the history of its life cycle there will be a continuous
(somewhat oscillating) "contest" between contractive heating during stages of
gravitational collapse and expansive cooling by thermal radiation outbursts
whenever rising temperatures increase hydrogen ionization. Generally, an
evolving star tends to seek out a balance [hydrostatic equilibrium] between
inward gravitational forces and outward radiation pressure developed from the
burning of the star's nuclear fuel. This is illustrated in this simplistic
diagram:
In its early life, the contraction
phase ultimately dominates, so that a star's deep interior temperature
eventually will be raised above 107 K (varies with star size), at
which stage a fundamental nuclear reaction within the hydrogen gas commences.
This involves thermonuclear fusion: p + p => H2 + e+ +
neutrinos (H2 or deuterium is a single proton and a neutron and
e+ is a positron [emitted]). That change of state results in thermal
energy release which contributes to continual rises in temperature. Deep within
the star, an alternate but dominant fusion process involves melding of 4 single
protons into a single helium nucleus consisting of two protons and two neutrons.
As temperatures increase further, some protons, neutrons, deuterium (and minute
amounts of tritium [H3]) combine (in a three step process) into
helium (He4 nuclei [2p, 2n]) which migrate into the star's interior
towards its core. In these reactions, some of the mass is converted to energy (E
= mc2) which radiates outward as the source of the star's luminosity
and which produces the outward pressure that counteracts inward forces owing to
gravitational contraction. Luminosity varies as the fourth power of a star's
mass (thus a star with twice the mass of the Sun shines 16 times brighter).
Helium remains stable until
temperatures approach 100 million° K, at which state it reacts with more protons
and neutrons to transmute into other elements of higher mass numbers (see
below). More massive Main Sequence stars can generate Carbon; some of this
element may be in the star initially if it is formed from previous gases and
particles that contain carbon produced in earlier star generations. This
carbon-enriched star, as its temperature rises and interior pressure increases,
can go through another fuel-burning process known as the CNO. Through a series
of steps as reactions of Carbon with Hydrogen protons take place, first
C12 is converted to isotopes of Nitrogen or O15 but
reaction with He4 will lead to C12 again plus energy
released as positrons and neutrinos.
When the H => He process reaches
a steady state, gravitational contraction no longer dominates (attains a balance
called hydrostatic equilibrium)), the star's total radiant (EM) energy
output per second (defined as its luminosity; also referred to as brightness)
becomes constant, and the star reaches a stable state on the Main Sequence
(M.S.), populated by stars that are primarily in the hydrogen-burning stages.
This equilibrium - in which inward directed gravity forces are more or less
countered by outward radiation pressure - is maintained during most of the
star's life on the Main Sequence. These stars spend up to 90% of their total
lives on the Main Sequence.
Because this page is very long and has so many illustrations that
downloading is lengthy, the second half (remainder) of the page on Stars is
continued on page 20-5a, accessed by the Next button at the bottom here and at
the top of this page.
This view of part of the
Orion Nebula is inserted here to make a special point. While Hubble, Chandra and
other space telescopes, and some of the large ones on Earth usually seem to
provide the most spectacular images, small telescopes operated by "amateur
astronomers", if used effectively, can yield their own superb views. The image
below was made through a 14 inch "backyard telescope" by Russell Croman using
filters and exposing on a tracked target for 7 hours. The color output rivals
some HST images. Red in the image highlights sulphur-rich parts of the nebula;
green show hydrogen enrichment, and blue singles out oxygen.
