Download General Comments on Nebulae

General Comments on Nebulae
Originally, the word "nebula" referred to almost any extended astronomical object (other than planets
and comets). The etymological root of "nebula" means "cloud". As is usual in astronomy, the old
terminology survives in modern usage in sometimes confusing ways. We sometimes use the word
"nebula" to refer to galaxies, various types of star clusters and various kinds of interstellar dust/gas
clouds. More strictly speaking, the word "nebula" should be reserved for gas and dust clouds and not
for groups of stars. The gas and dust makes up between 10% to 15% of the visible mass of the Milky
Way.
Galaxies (called nebulae around before the 1920’s, not any more)
Early in this century, there was a great debate as to the nature of the nebulae like this one which at that
time could not be resolved into individual stars. Thanks in large part to the work of Edwin Hubble
whose famous paper "The Realm of the Nebulae" finally put the issue to rest, we now know that these
are really vast conglomerates billions of stars which are very much more distant from the Earth than
other nebulae. Our own Milky Way galaxy is just one of the billions of galaxies now known to exist. A
typical galaxy is 100,000 light-years in diameter.
Globular Clusters (called nebulae around before the 1920’s, not any more)
Globular clusters are gravitationally bound groups of many thousands (sometimes as many as a million)
of stars. They consist primarily of very old stars. Globular clusters are not concentrated in the plane of
the galaxy but rather are randomly distributed throughout the halo. There are several hundred globular
clusters associated with our galaxy. A typical globular cluster is a few hundred light-years across.
Open Clusters (called nebulae around before the 1920’s, not any more)
Open clusters are loose aggregations of dozens or hundreds of young stars. They are generally not
gravitationally bound and will disperse in a relatively short period of time, astronomically speaking. They
are often associated with more diffuse nebulosity, as well. Also called "galactic clusters" because they
are usually found in the plane of the galaxy. A typical open cluster is less than 50 light-years across.
Emission Nebulae
Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet
light from a nearby star and emit radiation as they fall back into lower energy states (in much the same
way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen
happens to be red (other colors are produced by other atoms, but hydrogen is by far the most
abundant). Emission nebulae are usually the sites of recent and ongoing star formation.
Reflection Nebulae
Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars.
Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is
more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are
sometimes both referred to as diffuse nebulae.
Dark Nebulae
Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are
physically very similar to reflection nebulae; they look different only because of the geometry of the light
source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and
emission nebulae. A typical diffuse nebula is a few hundred light-years across.
Planetary Nebulae (to be covered later in the course)
Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our Sun will
probably produce a planetary nebula in about 5 billion years. They have nothing at all to do with planets;
the terminology was invented because they often look a little like planets in small telescopes. A typical
planetary nebula is less than one light-year across.
Supernova Remnants (to be covered later in the course)
Supernovae occur when a massive star ends its life in an amazing blaze of glory. For a few days a
supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is
blown into space as a supernova remnant. A typical supernova remnant is at most few light-years
across.
Dark, Blue and Pink Nebulae
(the “Examples” are taken from http://www.aao.gov.au/images.html )
1) Dust and Reflection Nebulae
The dust is made of thin, highly flattened flakes or needles of graphite (carbon) and silicates (rock-like minerals)
coated with water ice. Each dust flake is roughly the size of the wavelength of blue light or smaller. The dust is
probably formed in the cool outer layers of red giant stars and dispersed in the red giant winds and planetary
nebulae.
Extinction Starlight passing through a dust cloud can be affected in a couple of ways. The light can be totally
blocked if the dust is thick enough or it can be partially scattered by an amount that depends on the color of the
light and the thickness of the dust cloud. All wavelengths of light passing through a dust cloud will be dimmed
somewhat. This effect is called extinction.
Reddening Not all wavelengths are scattered equally. Just as our air scatters the bluer colors in sunlight more
efficiently than the redder colors, the amount of extinction by the interstellar dust depends on the wavelength. The
amount of extinction is proportional to 1/(wavelength of the light). Bluer wavelengths are scattered more than
redder wavelengths. The 1/(wavelength of the light) behavior of the scattering indicates that the dust size must be
about the wavelength of light (on the order of 5000Å or 1 ten-thousandths of a milli meter). Less blue light reaches
us, so the object appears redder than it should. This effect is called reddening, though perhaps it should be called
“de-blueing”.
Why is the sun red at sun set? You see the same effect when you observe the orange-red Sun close to the
horizon. Objects close to the horizon are seen through more atmosphere than when they are close to zenith. At
sunset the blues, greens, and some yellow are scattered out of your line of sight to the Sun and only the long waves
of the orange and red light are able to move around the air and dust particles to reach your eyes.
Why is the sky blue? Much of the blue light from the sun gets scattered away from our line of sight towards
the sun. So then we see this scattered light all over the sky, which makes the sky look blue.
Examples of Reflection Nebulae
Pleiades The Pleiades are one of the finest and nearest examples of a reflection nebula associated with a
cluster of young stars. The cluster itself is a group of many hundreds of stars about 400 light years from Earth in
the northern constellation of Taurus and has been recognised since ancient times. Seven of the brightest stars are
quite easy to see with the unaided eye and bear the names of the Seven Sisters, the daughters of Atlas. The
nebulosity seen here is light reflected from the particles in a cloud of cold gas and dust into which the cluster has
drifted. It appears blue because these tiny interstellar particles scatter blue light more efficiently than the longer
wavelengths of red light and it is streaky because the particles have been aligned by magnetic fields between the
stars.
Reflection nebula in Orion This group of nebulous stars is just half a degree north of the much brighter
Orion Nebula and has largely been ignored because of it. The group of stars here appear as a single star to the
unaided eye, the northernmost 'star' in the sword of Orion. Most of the blue nebulosity is starlight scattered by
dust, while some of the stars are sufficiently hot to excite the wisps of hydrogen that linger here and create a
distinctive red glow. To the south of the nebula (at the bottom of the picture) faint traces of the yellow reflected
light of the Orion nebula can just be detected.
Trifid nebula
Stars, the Sun included, were born within clouds of dusty gas such as the Trifid Nebula.
Measuring some forty light years across, this nebula contains enough gas to make many thousands of suns. Within
it a number of young hot stars have already formed. The hottest cause the gas, mostly hydrogen, to emit its
characteristic red light. Around the red emission nebula the gas contains many dust grains which preferentially
reflect the blue component of starlight, and to the north (top) of the nebula can be seen a bright star which
illuminates part of the dust to create a region of blue reflection. it In some parts of the nebula there are so many
dust grains that they hide the glowing gas, producing the three dark lanes which give the object its name.
2) Gas and Emission Nebulae
About 99% of the interstellar medium is gas with about 90% of it in the form of hydrogen (atomic or molecular
form), 10% helium, and traces of other elements. At visible wavelengths, however, dust has a greater effect on the
light than the gas.
The hydrogen gas is observed in a variety of states: in ionized, neutral (or atomic), and molecular
forms. The ionized hydrogen emits light in the visible band, neutral hydrogen emits light in the radio band (at 21
cm - more later) and molecular clouds emit light at other radio wavelengths. At optical wavelengths we can see
ionized hydrogen in emission (below), and molecular gas as black silhouettes (see under dark nebulae).
Emission nebulae are so named because their light is emitted from gas excited by energetic radiation from nearby
hot stars. The gas is always rich in hydrogen, the most abundant element, and it glows with a distinctive red hue.
Emission clouds are often called H II regions (the “II” of H II means that hydrogen is missing one electron).
Ultraviolet light from hot O & B stars ionizes the surrounding hydrogen gas (which is typically a few thousand
degrees hot). When the electrons recombine with the protons, they emit light mostly at visible wavelengths, and
primarily at 6563Å, thus giving the hydrogen emission nebulae their characteristic red color. Sometimes the
hydrogen is mixed with oxygen, which emits green light. Green and red together give yellow, which explains some
of the yellowish tints in some emission line nebulae.
Examples of Emission Nebulae
The Great Nebula in Carina, NGC 3372 Although no bright naked-eye stars are associated with the
Carina nebula now, 150 years ago there blazed forth here one of the most unusual and peculiar stars ever seen.
The star is known as Eta Carinae (more on that later) and for a few months in 1843 it was the second or third
brightest star in the sky. Since then it has faded and is today about 1000 times fainter than it was at its brightest as
the nebula it created during its outburst has cooled and become opaque. The whole region around Eta Carinae is
rich in hot stars of which Eta is an extreme example and it is their combined radiation that produces the
spectacular Carina nebula that dominates this picture. The nebula and its peculiar star are about 7000 light years
away.
The Rosette Nebula and NGC 2244 cluster The Rosette Nebula exhibits a striking circular symmetry
which gives it the appearance of a partly opened rose, an allusion further enhanced by the rich red hues seen in
this color photograph. Near the centre of the nebula is a cluster of blue stars catalogued as NGC 2244. These
stars are responsible for making the nebula visible and for creating the hollowed-out central cavity. This cluster of
stars formed from the gas which now surrounds it less than a million years ago and is thus very young on the
cosmic timescale. The gas and dust at the centre of the nebula have been forced away from the bright stars by
radiation pressure and the intense stellar wind which is often associated with very hot stars, forming a hollow
centred on the cluster. This will gradually expand and dissipate until the stars are free from nebulosity.
The Orion nebula The hydrogen is often mixed with oxygen, which provides green light. Green and red
together give yellow, while dust, which both absorbs starlight and reflects it, provides blue reflection nebulae. All
these effects are seen in the beautiful Orion nebula, alongside. The nebulae in Orion feature extensively in this
collection because, at 1500 light years distant, they are among the closest and best-studied of the Galaxy's starforming regions.
3) DARK NEBULAE AND MOLECULAR CLOUDS
Dark nebulae are usually only visible because they hide the light of something in the background that is bright.
Molecules in the interstellar medium tend to be clumped together into clouds with masses anywhere from just a
few solar masses to over a million solar masses with radii ranging from a few parsecs to over 100 parsecs.
Molecules need to have some sort of shielding from the high-energy light from stars. Otherwise, the energetic
photons would dissociate the molecules. Molecular clouds have dust in them. The dust grains may provide the
shelter for molecules to form. Compared to the size of atoms, the dust grains are enormous and have many pits
and recesses for atoms to congregate and combine. Stars form in the molecular clouds. If the molecular cloud is
cold and dense enough it can collapse under its own gravity.
Dust cloud Barnard 86 and open cluster NGC 6520 The majority of old stars in our Galaxy, as in
most others, are yellowish in color. This is because the hotter, bright blue stars have relatively short but spectacular
lives. We see these old stars in vast numbers as the brightest patches of the Milky Way and a powerful telescope
like the AAT is needed to see them as individuals. Superimposed on this distant background is a small cluster of
young blue stars, NGC 6520. In the same region and possibly associated with the cluster is a dark cloud, Barnard
86. The cluster and cloud are probably associated and the dust is visible only because it blocks out light from the
myriads of stars beyond.
A group of Bok globules in IC 2944 Between the the Southern Cross and the rich Carina region, on the
southern border of Centaurus, is a large, almost featureless emission nebula, IC 2944. It is against this uniform,
bright backdrop that we see a small group of dark clouds of the kind known as 'Bok globules'. They are named for
the Dutch- American astronomer who first drew attention to them as the possible sites of star formation. These
dark markings are discrete, opaque dust clouds, the largest containing enough material to form several stars the
mass of the sun. The globules are not some line of sight coincidence; the brightened rim of the largest clearly
shows it to be associated with the nebulosity of IC 2944.
A dark cloud in Scorpius This curious un-named nebula is similar to the cometary globule seen in AAT 71,
but is much more massive. Its swept-back shape is governed by radiation from very luminous stars some distance
from the nebula. The direction of the radiation source can be seen from the flow pattern in the dark cloud and the
extensive bright red rims at the end of the nebula that is exposed to radiation from the hot stars. This part of the
nebula is reminiscent of the famous Horsehead nebula in Orion, which is also the consequence of starlight
destroying a dark cloud. The source of the energy here is the Scorpius OB association, a group of brilliant, very hot
stars, about 5000 light years away from us.
The Horsehead Nebula in Orion This curious dark nebula is one of the best-known images in astronomy,
probably because of its chance likeness to a recognisable form. The horse-head shape is an extension of a large
cloud of dust which fills the lower part (east) of the picture and hides the light of stars beyond. The outer surface
of the dusty gas (IC 434) runs roughly north-south and is illuminated by sigma Orionis (off the top of the picture)
which causes the hydrogen there to fluoresce, outlining the horse-head shape. Though conspicuous here, the
Horsehead is very difficult to see visualy, eben with a large telescope. A bright star is partially enveloped in the
dust cloud and its scattered light is seen as the large, irregular blue reflection nebula, NGC 2023.
4) CHALLENGE PICTURES
What types of nebulae do you see?
Antares and the Rho Ophiuchi Dark Cloud The dusty region between Ophiuchus and Scorpius contains
some of the most colorful and spectacular nebulae ever photographed. The upper part of the picture is filled with
the bluish glow of reflected light from hot stars near a huge, cool cloud of dust and gas where stars are born.
Dominating the lower half of the picture is the over-exposed image of the red supergiant star Antares, a star that it
is steadily shedding material from its distended surface as it nears the end of its life. These solid particles reflect
Antares' light and hide it in a nebula of its own making. Finally, partly surrounding Sigma Scorpii at the left of the
picture is a red emission nebula, completing the most comprehensive collection of nebular types ever seen in one
photograph.
Differences in red - why?
NGC 3576 and 3603 in Carina These two star-formation regions appear side by side in the sky and seem
to be linked as parts of an extensive nebula. However, the apparent intimacy is line-of-sight effect because these
two objects are at quite different distances from the Sun. The curious looped nebula NGC 3576 is about 7000 light
years from us while its neighbour NGC 3603 is more than twice as far away. That NGC 3603 is the more distant
is confirmed by its color, which is a ruddier red than the pinkish hue of NGC 3576. The change in color is due to
absorption of the blue-light component of the nebulosity by dust particles in the much longer line of sight. This
effect is known as interstellar reddening, though it would be better described as 'de-blueing'
Multiple colors - cause?
Cometray nebula Cometary globules are isolated, relatively small clouds of gas and dust within the Milky
Way. This example contains enough material to make several sun-sized stars. The head of the nebula is itself
opaque, but glows because it is illuminated by light from very hot stars nearby. Their energy is gradually destroying
the dusty head of the globule, sweeping away tiny particles that scatter the starlight as a faint, bluish reflection
nebula. This particular globule also shows a faint red glow, probably from excited hydrogen, and seems about to
devour an edge-on spiral galaxy, which in reality is millions of light years away.
Review Questions
(No need to hand in, they are meant as study guides only)
1. What is the interstellar medium composed of?
2. How do we know that the non-gaseous part of the ISM cannot be made of rocks but, rather, of small “dust”
3.
4.
5.
6.
7.
8.
particles?
How does dust make stars appear redder than they actually are?
How does dust cause the extinction of starlight?
Why is the sun red at sun set? Why is the sky blue?
What are H II regions and how are they produced? What is going on at the atomic level?
Why would the presence of a H II region indicate a site of star formation?
What is the importance of the discovery of organic molecules in the interstellar medium?