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ASTRONOMY
MILKY WAY GALAXY
MURCHISON
CHAPTER 15
THE MILKY WAY GALAXY
1) We have already discussed the stars, which are important parts of any galaxy, how they are born, how
they live and how they die.
2) Next we will discuss the gas and dust that accompany the stars from which stars are born.
3) These clouds of dust and gas are called nebulae. Nebula is Latin for “fog” or “mist”.
4) We will also discuss the overall structure of the Milky Way Galaxy and how, from our location inside it,
we detect this structure.
The Milky Way
1) On the clearest of nights, far away from city lights, it is possible to see a hazy band of light stretching
across the sky.
2) This band is the Milky Way – the dust, gas, and stars that make up the galaxy in which our Sun is located.
All this matter is our celestial neighborhood.
3) The nearest star is about 4 light-years away. If we look a few thousand light years outward, away from
the direction of the Milky Way, we see out of our galaxy.
4) Then it is much farther to the other galaxies and beyond.
5) Terminology can cause confusion – the Milky Way is the band of light we can see from Earth, and the
Milky Way Galaxy is the whole galaxy in which we live.
6) Like other galaxies, the Milky Way is composed of perhaps a few billion stars plus many different types
of gas, dust, and planets.
7) In the directions in which we see the Milky Way in the sky, we are looking through the relatively thin,
pancake-like disk of matter that forms a major part of our Milky Way Galaxy.
a) This disk is about 90,000 light years across, an enormous, gravitationally bound system of stars.
b) The Milky Way appears very irregular when we see it stretched across the sky – there are spurs
of luminous material that stick out in one direction or another, and there are dark lanes or
patches in which much less can be seen.
c) This patchiness is due to the splotchy distribution of gas, dust and stars.
d) Here on Earth, we are inside our galaxy together with all of the matter we see as the Milky Way.
Because of our position inside our galaxy, we see a lot of our own galaxy’s matter when we look
along the plane of our galaxy.
e) On the other hand, when we look “upward” or “downward” out of this plane, our view is not
obscured by matter, and we can see past the confines of our galaxy.
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1) The gas in our galaxy is more or less transparent to visible light, but the small solid particles that we call
“dust” are opaque.
a) So the distance we can see through our galaxy depends mainly on the amount of dust that is
present.
b) This is not surprising: We cannot always see across a smoke-filled room. Similarly, the dust
between the stars in our galaxy dims the starlight by absorbing it or by scattering it in different
directions.
2) The dust in the plane of our galaxy prevents us from seeing very far toward its center.
3) With visible light, we can see only one-tenth of the way in, about 2000 light-years.
4) Because of widespread dust, with the unaided eye and small telescopes we can see (in visible light) just
in about the same distance in any direction we look in the plane of the Milky Way.
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The Illusion that we are at the Center
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a) These direct optical observations fooled astronomers at the turn of the century into thinking
that the Earth was near the center of the Universe.
5) In 1917 American astronomer Harlow Shapley realized that the Sun was not in the center of the Milky
Way.
6) In the 20th century, astronomers began to use wavelengths other than optical ones to study the Milky
Way Galaxy.
7) Using radio astronomy, infrared, and satellites enable us to get pinpoint views of what was formerly
hidden from us.
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1) The original definition of nebula was a cloud of gas and dust that we see in visible light, though we now
detect nebulae in a variety of ways.
2) When we see the gas actually glowing in the visible part of the spectrum, we call it an emission nebula.
a) Gas is ionized by ultraviolet light from very hot stars within the nebula; it then glows at optical
(and other) wavelengths when electrons recombine with ions and cascade down to lower
energy levels, releasing photons.
b) Additionally, free electrons can collide with atoms (neutral and ionized) and lose some of their
energy of motion, kicking the bound electrons to jump to higher energy levels.
c) Photons are emitted when the excited bound electrons jump down to the lower energy levels,
so the gas slows down even more.
d) The spectrum of an emission nebula therefore consists of emission lines.
e) Emission nebula often look red because the red light of hydrogen is strongest in them. Other
types of emission nebulae can photograph as green, because of green light from oxygen ions;
other colors are possible as well.
f) The pretty false-color images that are often seen in posters and photographs can be misleading.
In them color is assigned to some specific type of radiation and need not correspond to colors
that the eye would see when viewing objects. Sometimes a cloud of dust obscures our vision in
some direction in the sky.
3) When we see the dust appear as a dark silhouette, we call it a dark nebula (or often an absorption
nebula, since it absorbs light from stars behind it)
a) The Horsehead Nebula is an example of an object that is simultaneously an emission and an
absorption nebula.
b) The reddish emission from a glowing hydrogen gas spreads across the sky near the leftmost
(eastern) star in Orion’s belt.
c) A bit of absorbing dust intrudes onto the emitting gas, outlining the shape of a horse’s head.
d) In pictures of the Horsehead Nebula, the horse head is a continuation of a dark area in which
very few stars are visible.
e) In this region, dust is obscuring the stars that lie beyond.
4) Clouds of dust surrounding hot stars, like some of the stars in the star cluster known as the Pleiades, are
examples of reflection nebulae.
a) They merely reflect the starlight toward us without emitting visible radiation of their own.
b) Reflection nebulae usually look bluish for two reasons:
1) They reflect the light from hot stars which are bluish, and
2) dust reflects blue light more efficiently than it does red light.
c) Whereas an emission nebula has its own spectrum, a reflection nebula shows the spectral lines
of the star or stars whose light is being reflected.
5) Dust tends to be associated more with young, hot stars than with older stars, since the older stars would
have had a chance to wander away from their birthplaces.
6) Nebula are closely associated with both stellar birth and stellar death.
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Nebulae
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7) The chemically enriched gas blown off by unstable or exploding stars at the end of their lives becomes
the raw material from which new stars and planets are born.
The Parts of Our Galaxy
Our galaxy has several parts:
1) The Nuclear Bulge – Our galaxy has the general shape of a pancake with a bulge at its center that contains
millions of stars, primarily old ones. This nuclear bulge has the galactic nucleus at its center. The nucleus itself if
only about 10 light-years across.
2) The Disk – The part of the pancake outside the bulge is called the galactic disk. It extends 45,000 light-years or
so out from the center of the galaxy. The Sun is located about one-half to two-thirds of the way out. This disk is
very this – 2 per cent of its width – like a phonograph record. It contains all the young stars and interstellar gas
and dust as well as some old stars. The disk is slightly warped at its ends, perhaps by an interaction with our
satellite galaxies, the Magellanic Clouds. Our galaxy looks like a hat with a turned down brim.
1) It is very difficult for us to tell how the material is arranged in our galaxy’s disk.
2) Other galaxies have properties similar to our own, and their disks are filled with great spiral arms, regions of
dust, gas, and stars in the shape of a pinwheel.
3) So we assume the disk of our galaxy has spiral arms also.
4) Though the direct evidence is ambiguous in the visible part of the spectrum, radio observations have better
traced the spiral arms.
5) The disk looks different when viewed in different parts of the spectrum. Infrared and radio waves penetrate
the dust that blocks our view in visible light, while gamma rays and x-rays show the hot objects best.
3) The Halo – Old stars (including the globular clusters) and very dilute interstellar matter from a roughly
spherical galactic halo around the disk. The inner part of the halo is at least as large across as the disk, perhaps
60,000 light years in radius. The gas in the inner halo is hot, 100,000 K, though it contains only about 2 percent
of the mass of gas in the disk. As will be discussed later, the outer part of the halo extends much farther, out to
perhaps 200,000 to 300,000 light-years. This galactic outer halo apparently contains 5 or 10 times as much mass
as the nucleus, disk, and inner halo together – but we don’t know what it consists of. We shall see later that
“dark matter” – invisible, and only detectable through its gravitational properties – is a very important
constituent of the Universe.
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1) We cannot see the center of our galaxy in the visible part of the spectrum because our view is blocked
by interstellar dust.
2) Radio waves and infrared are able to penetrate the dust.
a) The Hubble Space Telescope has seen isolated stars when before we saw them as just a blur.
3) One of the brightest infrared sources in our sky is the nucleus of our galaxy, only about 10 light years
across. 4) This makes it a very small source for the prodigious amount of energy it emits: as much energy
as radiated by 80 million Suns.
4) It also has a radio source and a variable x-ray source. High-resolution radio maps of our galactic center
show a small bright spot that could well be gas surrounding a central black hole.
5) The appearance of a spiral is an optical illusion: the arms are only apparently superimposed on each
other.
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The Center of our Galaxy and Infrared Studies
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6) Extending somewhat farther out, a giant Arc of parallel filaments stretches perpendicular to the plane of
the galaxy.
7) The very rapid motions of stars measured near the galactic center strongly implies the presence of a
supermassive black hole, 2.6 million times the Sun’s mass.
All-Sky Maps of our Galaxy
1) The study of our galaxy provides us with a wide range of types of sources. Many of these have been
known for decades from optical studies.
2) The infrared sky looks quite different with its appearance depending on wavelength. The radio sky
provides still a different picture.
3) Maps of our galaxy in the x-ray and gamma-ray regions of the spectrum show the hottest sources.
a) Different experiments on the Compton Gamma-Ray Observatory produced maps of the steady
gamma-rays and those bursts of gamma rays that last only minutes.
b) The gamma ray bursts, which were seen at random places in the sky, happened roughly once
per day.
c) Though some models suggested that they are produced within our galaxy – either very close to
us, or in very extended halo – more recent observations have conclusively shown that they are
actually in galaxies billions of light-years away.
d) They may be produced when extremely massive stars collapse to form black holes, or when a
neutron star merges with another neutron star of with a black hole.
e) The Chandra X-ray Observatory is producing more detailed images of x-ray sources than had
ever been available.
f) Studies of the highest-energy electromagnetic radiation like x-rays and gamma rays and of
rapidly moving cosmic-ray particles are part of the field of high-energy astrophysics.
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1) Because of our observational position in the galaxy, it is difficult to see exactly the shape of our galaxy.
2) This makes it difficult to trace the spiral pattern in our own galaxy, even though the pattern would
presumably be apparent from outside the galaxy.
3) By noting the distances and directions to objects of various types, we can determine the Milky Way’s
spiral structure.
4) Young open clusters are good objects to use for this purpose because they are always located in spiral
arms.
5) We think they formed there and that they have not yet had time to move away.
a) We know their ages from the length of their main sequence on the temperature-luminosity
diagram.
b) Also useful are O and B stars (or loose groups of them) because the lives of such stars are so
short we know they can’t be old.
c) Other signs of young stars are the presence of emission nebula.
d) We know from studies of other galaxies that emission nebula are preferentially located in spiral
arms. 6) In mapping the locations emission nebula, we are really again studying the location of
the O stars and the hottest of the B stars, since radiation from these hot stars provides the
energy for the nebula to grow.
6) When the positions of open clusters, O stars and B stars, and the H II regions of known distance are
studied (by plotting their distances and directions as seen from Earth) they appear to trace out bits of
three spiral arms, which are relatively nearby.
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The Spiral Structure of our Galaxy
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7) Interstellar dust prevents us from using this technique to study parts of our galaxy farther away from the
Sun.
8) Another valuable method of mapping the spiral structure in our galaxy involves spectral lines of
hydrogen and of carbon monoxide in the radio part of the spectrum.
a) Radio waves penetrate the interstellar dust, allowing us to study the matter throughout our
galaxy, though getting the third dimension that allows us to trace out spiral arms remains
difficult.
Why Does our Galaxy Have Spiral Arms
1) The Sun revolves around the center of our galaxy as a speed of approximately 200 kilometers per
second.
2) At this rate, it takes the Sun about 250 million years to travel once around the center, only 2 percent of
the galaxy’s current age.
3) Stars at different distances from the center of our galaxy revolve around the galaxy’s center in different
lengths of time.
4) Stars closer to the center revolve much more quickly than does our Sun.
a) Therefore the question arises: Why haven’t the spiral arms wound up very tightly?
b) The leading current theory to this question is that the spiral arms we see now do not consist of
the same stars that would previously been visible in those arms.
5) The spiral-arm pattern is caused by a spiral density wave, a wave of increased density that moves
through the gas in the galaxy.
6) This density wave is a wave of compression, not of matter being transported. It rotates more slowly than
the actual material and causes the density of passing material to build up.
7) ) Stars form at those locations, and give the optical illusion of a spiral, but the stars then move away
from the compression wave.
a) An analogy is a crew of workers fixing potholes in two lanes of traffic of a 4 lane highway. A
bottleneck occurs at the location of the workers; if you could observe the traffic from a
helicopter, you would see an increase in the number of cars at that place. As the workers
continued slowly down the road, fixing potholes in new sections, we would seem to see the
bottleneck move slowly down the road. Cars merging from four lanes into the two open lanes
need not slow down traffic is light, but they are compressed more than in other fully open
sections of the highway. Thus, the speed with which the bottleneck advances is much smaller
than that of individual cars.
b) Similarly in our galaxy we might be viewing only some galactic bottleneck at the spiral arms.
c) The new massive stars heat the interstellar dust so that it becomes visible.
d) In fact, we do see young, hot stars and glowing gas outlining the spiral arms, which are checks of
this prediction of the density-wave theory.
e) This mechanism may work especially well in galaxies with a companion that gravitationally
perturbs them.
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1) The gas and dust between the stars is known as the interstellar medium.
2) The nebulae represent region of the interstellar medium in which the density of gas and dust is higher
than average.
3) For many purposes we may consider interstellar space as being filled with hydrogen at an average
distance of about 1 atom per cubic centimeter (individual regions may have densities departing greatly
from this average).
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Matter between the Stars
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4) Regions of higher density in which the hydrogen atoms are predominately neutral are called H I clouds
(“H one clouds; the Roman numeral I refers to the neutral, basic state).
5) Where the density of and H I region is high enough, pairs of hydrogen atoms combine to form molecules
(H2).
6) The densest part of the gas associated with the Orion Nebula might have a million or more hydrogen
molecules per cubic centimeter. So hydrogen molecules are often found in H I clouds.
7) A region of ionized hydrogen, with one electron missing, is known as an H II region (from “H two”, the
second state – neutral is the first state and once ionized is the second).
8) Since hydrogen, which makes up the overwhelming proportion of interstellar gas, contains only one
proton and one electron, a gas of ionized hydrogen contains individual protons and electrons.
9) Wherever a hot star provides enough energy to ionize hydrogen, an H II region (emission nebula) results.
a) Studying the optical and radio spectra of H II regions and planetary nebulae tells us the
abundances of several chemical elements (especially helium, nitrogen, and oxygen).
10) How these abundances vary from place to place in our galaxy and in other galaxies helps us choose
between models of element formation and of galaxy evolution.
a) Tiny grains of solid particles are given off by the outer layers of red giants. They spread through
interstellar space and dim the light from distant stars.
b) This “dust” never gets hot, so most of its radiation is in the infrared. The radiation from the dust
scattered out among the stars is faint and very difficult to detect, but the radiation from clouds
of dust surrounding newly formed stars is easily observed from the ground and from infrared
spacecraft.
c) They found infrared radiation from so many stars in our galaxy that we think about one star
forms in our galaxy each year.
d) Since the interstellar gas is “invisible” in the visible part of the spectrum different techniques are
needed to observe the gas in addition to observing the dust. Radio astronomy is the most widely
used technique.
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1) The first radio astronomy observations were of continuous radiation; no spectral lines were known.
2) If a radio spectral line is known, Doppler-shift measurements can be made, and we can tell about
motions in our galaxy.
3) A radio spectral line corresponds to a wavelength at which the radio radiation is slightly more, or slightly
less intense.
a) A radio station is an emission line on a home radio.
4) Since hydrogen is the most abundant element in the Universe the most-used radio spectral lines from
the lowest energy levels of interstellar hydrogen atoms.
a) This line has a wavelength of 21 cm.
5) A hydrogen atom is basically an electron “orbiting” a proton.
a) Both the electron and the proton have the property of spin, as if each were spinning on its own
axis.
b) The spin of the electron can be either in the same direction as the spin of a proton or in the
opposite direction.
c) The rules of quantum mechanics prohibit intermediate orientations. The energies of the two
allowed conditions are slightly different.
d) If an atom is sitting alone in space in the upper of these two energy states, with its electron and
proton spins aligned in the same direction, is has a certain small probability that the spinning
electron will spontaneously flip over to the lower energy state and emit a bundle of energy – a
photon.
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Radio Observations of our Galaxy
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e) We thus call this a spin-flip transition. The photon of hydrogen’ spin-flip transition corresponds
to radiation at a wavelength of 21 cm – the 21 cm line.
f) If the electron flips from the higher to the lower energy state, we have an emission line
g) If it absorbs energy from passing continuous radiation, it can flip to the higher energy state and
we have an absorption line.
6) If we were to watch any particular group of hydrogen atoms in the slightly higher-energy state, we
would find that it would take 11 million years before half of the electrons had undergone spin flips
a) We say that the “half-life” if 11 million years for this transition.
b) Hydrogen atoms are generally quite content to sit in the upper state
c) There are so many hydrogen atoms in space that enough 21-cm radiation is given off to be
detected.
d) The existence of the line was predicted in 1944 and discovered in 1951, marking the birth of
spectral-line radio astronomy.
Mapping Our Galaxy
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1) The 21-cm hydrogen line has proven to be a very important tool for studying our galaxy because this
radiation passes unimpeded through the dust that prevents optical observations very far into the plane
of the galaxy.
2) The 21-cm hydrogen line can even reach us from the opposite side of our galaxy, whereas light waves
penetrate the dust clouds in the galactic plane only about 10 percent of the way to the galactic center
3) Astronomers have been able to find the distances to the clouds of gas that emit 21-cm radiation.
a) They use the fact that gas closer to the center of our galaxy rotates with a shorter period than
the gas farther away from the center.
b) Though there are substantial uncertainties in interpreting the Doppler shifts in terms of distance
from the galaxy’s center, astronomers have used this to make some maps.
4) The 21-cm maps show may narrow arms but no clear pattern of a few broad spiral arms like those we
see in other galaxies.