week #11 notes: the milky way: our home in the universe

Week #11 Notes:

The Milky Way: Our Home in the

Universe

Introduction

In this chapter, we describe the gas and dust (small particles of matter) that are present to some extent throughout a galaxy.

Substantial clouds of this gas and dust are called nebulae (pronounced “neb´yu-lee” or “neb´yu-lay”; singular: nebula); “nebula” is Latin for “fog” or “mist.”

New stars are born from such nebulae.

We also discuss the overall structure of the Milky Way Galaxy and how, from our location inside it, we detect this structure.

Our Galaxy: The Milky Way

On the clearest moonless nights, when we are far from city lights, we can see a hazy band of light stretching across the sky (see figure).

This band is the Milky Way —the gas, dust, nebulae, and stars that make up the Galaxy in which our Sun is located.

All this matter is our celestial neighborhood, typically within a few hundred or a thousand light-years from us.

If we look a few thousand light-years in a direction away from that of the Milky Way, we see out of our Galaxy.

But it is much, much farther to the other galaxies and beyond.

Our Galaxy: The Milky Way Don’t be confused by the terminology: The

Milky Way itself is the band of light that we can see from the Earth, and the Milky Way Galaxy is the whole galaxy in which we live.

Like other large galaxies, our Milky Way Galaxy is composed of perhaps a few hundred billion stars plus many different types of gas, dust, planets, and so on.

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.

This disk is about 90,000 light-years across, an enormous, gravitationally bound system of stars.

Our Galaxy: The Milky Way

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.

This patchiness is due to the splotchy distribution of nebulae and stars.

Here on Earth, we are inside our Galaxy together with all of the matter we see as the Milky Way (see figure).

Because of our position, we see a lot of our own Galaxy’s matter when we look along the plane of our Galaxy.

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.

The Illusion That We Areat the Center

The gas in our Galaxy is more or less transparent to visible light, but the small solid particles that we call “dust” are opaque.

So the distance we can see through our Galaxy depends mainly on the amount of dust that is present.

This is not surprising: We can’t always see far on a foggy day.

Similarly, the dust between the stars in our Galaxy dims the starlight by absorbing it or by scattering (reflecting) it in different directions.


The dust in the plane of our Galaxy prevents us from seeing very far toward its center with the unaided eye and small telescopes.

With visible light, on average we can see only one tenth of the way in (about 2000 light-years), regardless of the direction we look in the plane of the Milky Way.

These direct optical observations fooled astronomers at the beginning of the 20th century into thinking that the Earth was near the center of the Universe (see figure).


American astronomer Harlow Shapley (pronounced to rhyme with “map´lee,” as in “road map”) realized in 1917 that our Sun is not in the center of the Milky Way.

This fundamental idea took humanity one step further away from thinking that we are at the center of the Universe.

Copernicus, in 1543, had already made the first step in removing the Earth from the center of the Universe.


In the 20th century, astronomers began to use wavelengths other than optical ones to study the Milky Way Galaxy.

In the 1950s and 1960s especially, radio astronomy gave us a new picture of our Galaxy.

In the 1980s and 1990s, we began to benefit from space infrared observations at wavelengths too long to pass through the Earth’s atmosphere.

The latest infrared telescope, launched by NASA in 2003, is the Spitzer Space Telescope.

Infrared and radio radiation can pass through the Galaxy’s dust and allow us to see our Galactic center and beyond.

Nebulae: Interstellar Clouds

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.

When we see the gas actually glowing in the visible part of the spectrum, we call it an emission nebula (see figure).

Gas is ionized by ultraviolet light from very hot stars within the nebula; it then glows at optical (and other) wavelengths


Other types of emission nebulae can appear green in photographs, because of green light from doubly ionized oxygen atoms.

Additional colors occur as well. Red colors come from Hydrogen

Don’t be misled by the pretty, false-color images that you often see in the news.

In them, color is assigned to some specific type of radiation and need not correspond to colors that the eye would see when viewing the objects through telescopes.

Sometimes a cloud of dust obscures our vision in some direction in the sky.

When we see the dust appear as a dark silhouette (see figure), we call it a dark nebula (or, often, an absorption nebula, since it absorbs visible light from stars behind it).


The Horsehead Nebula (see figure) is an example of an object that is simultaneously an emission and an absorption nebula.

The reddish emission from glowing hydrogen gas spreads across the sky near the leftmost (eastern) star in Orion’s belt.

A bit of absorbing dust intrudes onto the emitting gas, outlining the shape of a horse’s head.

We can see in the picture that the horsehead is a continuation of a dark area in which very few stars are visible.

In this region, dust is obscuring the stars that lie beyond.


Clouds of dust surrounding relatively hot stars, like some of the stars in the star cluster known as the Pleiades (see figure), are examples of reflection nebulae.

They merely reflect the starlight toward us without emitting visible radiation of their own.

Reflection nebulae usually look bluish for two reasons: (1) They reflect the light from relatively hot stars, which are bluish, and (2) dust reflects blue light more efficiently than it does red light. (Similar scattering of sunlight in the Earth’s atmosphere makes the sky blue.

Whereas an emission nebula has its own spectrum, as does a neon sign on Earth, a reflection nebula shows the spectral lines of the star or stars whose light is being reflected.


The Great Nebula in Orion (see figure, right) is an emission nebula.

In the winter sky, we can readily observe it through even a small telescope or binoculars, and sometimes it has a tinge of color.

We need long photographic exposures or large telescopes to study its structure in detail.

Deep inside the Orion Nebula and the gas and dust alongside it, we see stars being born this very minute; many telescopes are able to observe in the infrared, which penetrates the dust.

An example in a different region of the sky is shown in the figure (left).


They include planetary nebulae (see figure) and supernova remnants.

Thus, nebulae are closely associated with both stellar birth and stellar death.

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

It was not until 1917 that the American astronomer Harlow Shapley realized that we are not in the center of our Milky Way Galaxy.

He was studying the distribution of globular clusters and noticed that, as seen from Earth, they are all in the same general area of the sky.

They mostly appear above or below the Galactic plane and thus are not heavily obscured by the dust.

When he plotted their distances and directions, he noticed that they formed a spherical halo around a point thousands of light-years away from us (see figure).

The Parts of Our Galaxy Shapley’s touch of genius was to realize that this point is likely to

be the center of our Galaxy. After all, if we are at a party and discover that everyone we see is off

to our left, we soon figure out that we aren’t at the party’s center. Other spiral galaxies are also shown (see figures) for comparison

and to show something of what our Galaxy must look like when seen from high above it.

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 is 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 our Galaxy. The Sun is located about one half to two thirds of the way out. The disk is very thin—2 per cent of its width—like a phonograph record, CD, or DVD. 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 interaction with our satellite galaxies, the Magellanic Clouds. Our Galaxy looks a bit like a hat with a turned-down brim.

15.4 The Parts of Our Galaxy

It is very difficult for us to tell how the material in our Galaxy’s disk is arranged. Still, other galaxies have similar properties to our own, and their disks are filled with great spiral arms —regions of dust, gas, and stars in the shape of a pinwheel (see figure).

So, we assume the disk of our Galaxy has spiral arms, too. Though the direct evidence is ambiguous in the visible part of

the spectrum, radio observations have better traced the spiral arms.

The Parts of Our Galaxy The disk looks different when viewed in different parts of the

spectrum (see figure). Infrared and radio waves penetrate the dust that blocks our

view in visible light, while x-rays show the hot objects best.

The Parts of Our Galaxy 3. The halo. Old stars (including the globular

clusters) and very dilute interstellar matter form 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 per cent of the mass of the gas in the disk. The outer part of the halo extends much farther, out to perhaps 200,000 or 300,000 light-years. Believe it or not, 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!

The Center of Our Galaxy We cannot see the center of our Galaxy in the

visible part of the spectrum because our view is blocked by interstellar dust.

Radio waves and infrared, on the other hand, penetratethe dust.

The Hubble Space Telescope, with its superior resolution, has seen isolated stars where before we saw only a blur (see figure, right).

In 2003, NASA launched an 0.85-m infrared telescope, the Spitzer Space Telescope (Section 3.8c, also see figure, left).

Its infrared detectors are more sensitive than those on earlier infrared telescopes.

Spitzer completes NASA’s series of Great Observatories, including the Compton Gamma Ray Observatory (now defunct), the Chandra X-ray Observatory, and the Hubble Space Telescope.

The Center of Our Galaxy One of the brightest infrared

sources in our sky is the nucleus of our Galaxy, only about 10 lightyears across.

This makes it a very small source for the prodigious amount of energy it emits: as much energy as radiated by 80 million Suns.

It is also a radio source and a variable x-ray source.

High-resolution radio maps of our Galactic center (see figure) show a small bright spot, known as Sgr A* (pronounced “Saj A-star”), in the middle of the bright radio source Sgr A.

The radio radiation could well be from gas surrounding a central giant black hole.

The Center of Our Galaxy Extending somewhat farther out, a giant Arc

of parallel filaments stretches perpendicularly to the plane of the Galaxy (see figure, right).

The orbits measured show the presence of a supermassive black hole that is about 3.7 million times the Sun’s mass.

One of the stars comes within an astonishing 17 light-hours of Sgr A*.

The Center of Our Galaxy Observations of the Galactic

center with the Chandra X-ray Observatory and the European Space Agency’s INTEGRAL gamma-ray spacecraft (see figures) reveal the presence of hot, x-ray luminous gas and stars there.

All-Sky Maps of Our Galaxy

The study of our Galaxy provides us with a wide range of types of sources to study.

Many of these have been known for decades from optical studies (see figure on next slide, and the figure at top).

The infrared sky looks quite different (see figure, middle), with its appearance depending strongly on wavelength.

The radio sky provides still different pictures, depending on the wavelength used (see figure, below).


Maps of our Galaxy in the x-ray region of the spectrum (see figure, above) show the hottest individual sources (such as x-ray binary stars) and diffuse gas that was heated to temperatures of a million degrees by supernova explosions.

The Compton Gamma Ray Observatory produced maps of the steady gamma rays (see figure, below), most of which come from collisions between cosmic rays and atomic nuclei in clouds of gas.

All-Sky Maps of Our Galaxy A different instrument on the Compton Gamma Ray Observatory

detected bursts of gamma rays that last only a few seconds or minutes (see figure).

These gamma-ray bursts, which were seen at random places in the sky roughly once per day, are especially intriguing.

NASA’s Swift satellite, mentioned in Sections 3.7a and 14.10a, was sent aloft in 2004 specifically to study them in detail.

Our Pinwheel Galaxy It is always difficult to tell the shape of

a system from a position inside it. Think, for example, of being

somewhere inside a maze of tall hedges; we would find it difficult to trace out the pattern.

If we could fly overhead in a helicopter, though, the pattern would become very easy to see (see figure).

Similarly, we have difficulty tracing out the spiral pattern in our own Galaxy, even though the pattern would presumably be apparent from outside the Galaxy.

Still, by noting the distances and directions to objects of various types, we can determine the Milky Way’s spiral structure.

Our Pinwheel Galaxy Young open clusters are good objects to use for

this purpose, for they are always located in spiral arms.

We think that they formed there and that they have not yet had time to move away (see figure).

We know their ages from the length of their main sequences on the temperature-luminosity diagram (Chapter 11).

Also useful are main-sequence O and B stars; the lives of such stars are so short we know they can’t be old.

But since our methods of determining the distances to open clusters, as well as to O and B stars, from their optical spectra and apparent brightnesses are uncertain to 10 per cent, they give a fuzzy picture of the distant parts of our Galaxy.

Parallaxes measured from the Hipparcos spacecraft do not go far enough out into space to help in mapping our Galaxy.

We need new astrometric satellites.

Why Does Our GalaxyHave Spiral Arms?

The Sun revolves around the center of our Galaxy at a speed of approximately 200 kilometers per second.

At this rate, it takes the Sun about 250 million years to travel once around the center, only 2 per cent of the Galaxy’s current age.

But stars at different distances from the center of our Galaxy revolve around its center in different lengths of time.

For example, stars closer to the center revolve much more quickly than does the Sun.

Thus the question arises: Why haven’t the arms wound up very tightly, like the cream in a cup of coffee swirling as you stir it?


The leading current solution to this conundrum says, in effect, that the spiral arms we now see do not consist of the same stars that would previously have been visible in those arms.

The spiral-arm pattern is caused by a spiral density wave, a wave of increased density that moves through the gas in the Galaxy.

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.

Stars are born at those locations and appear to form a spiral pattern (see figure), but the stars then move away from the compression wave.

15.8 Why Does Our GalaxyHave Spiral Arms?

Think of the analogy of a crew of workers fixing potholes in two lanes of a four-lane highway.

A bottleneck occurs at the location of the workers; if we were in a traffic helicopter, we would see an increase in the number of cars at that place.

As the workers continue slowly down the road, fixing potholes in new sections, we would see what seemed to be the bottleneck moving slowly down the road.

Cars merging from four lanes into the two open lanes need not slow down if the 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.


Similarly, in our Galaxy, we might be viewing only some galactic bottleneck at the spiral arms.

The new, massive stars would heat the interstellar gas so that it becomes visible.

In fact, we do see young, hot stars and glowing gas outlining the spiral arms, providing a check of this prediction of the density-wave theory.

This mechanism may work especially well in galaxies with a companion that gravitationally perturbs them.

Matter Between the Stars The gas and dust between the stars is known as the

interstellar medium or “interstellar matter.” The nebulae represent regions of the interstellar

medium in which the density of gas and dust is higher than average.

For many purposes, we may consider interstellar space as being filled with hydrogen at an average density of about 1 atom per cubic centimeter. (Individual regions may have densities departing greatly from this average.)

Regions of higher density in which the atoms of hydrogen are predominantly neutral are called H I regions (pronounced “H one regions”; the Roman numeral “I” refers to the neutral, basic state).

Matter Between the Stars Wherever a hot star provides enough energy to ionize

hydrogen, an H II region (emission nebula) results (see figures).

Our Galaxy

The 21-cm hydrogen line has proven to be a very important tool for studying our Galaxy (see figure) because this radiation passes unimpeded through the dust that prevents optical observations very far into the plane of our Galaxy.

It 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 per cent of the way to the Galactic center, on average.

Mapping Our Galaxy Astronomers have ingeniously been able to find out

how far it is to the clouds of gas that emit the 21-cm radiation.

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.

Though there are substantial uncertainties in interpreting the Doppler shifts in terms of distance from the Galaxy’s center, astronomers have succeeded in making some maps.

These maps show many narrow arms but no clear pattern of a few broad spiral arms like those we see in other galaxies (Chapter 16).

Radio Spectral Linesfrom Molecules

Studying the spectral lines provides information about physical conditions—temperature, densities, and motion, for example—in the gas clouds that emit the lines.

Studies of molecular spectral lines have been used together with 21-cm line observations to improve the maps of the spiral structure of our Galaxy (see figure).

Observations of carbon monoxide (CO), in particular, have provided better information about the parts of our Galaxy farther out from the Galaxy’s center than our Sun.

We use the carbon monoxide as a tracer of the more abundant hydrogen molecular gas, since the carbon monoxide produces a far stronger spectral line and is much easier to observe; molecular hydrogen emits extremely little.

The Formation of Stars Astronomers have found that giant molecular

clouds are fundamental building blocks of our Galaxy.

Giant molecular clouds are 150 to 300 light-years across.

There are a few thousand of them in our Galaxy. The largest giant molecular clouds contain about

100,000 to 1,000,000 times the mass of the Sun. Since giant molecular clouds break up to form stars,

they only last 10 million to 100 million years.

The Formation of Stars We know that young stars are found in the center of

the Orion Nebula (see figures, left and middle). The Trapezium (see figure, right), a group of four hot

stars readily visible in a small telescope, is the source of ionization and energy for the Orion Nebula.

The Trapezium stars are relatively young, about 100,000 years old.

The Formation of Stars The Orion Nebula, though prominent at visible wavelengths, is

but an H II region located along the near side of the much more extensive molecular cloud (see figure).

The Formation of Stars

The Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope is able to record infrared light that had penetrated the dust, bringing us images of newly formed stars within the Orion Molecular Cloud (see figure).

At a Radio Observatory What is it like to go observing at a radio telescope? First, you decide just what you want to observe, and

why. You have probably worked in the field before, and your

reasons might tie in with other investigations underway. Then you decide with which telescope you want to

observe, usually the most suitable one accessible to you; let us say it is the Very Large Array (VLA) of the National Radio Astronomy Observatory.

You send in a written proposal describing what you want to observe and why.

Your proposal is read by a panel of scientists. If the proposal is approved, it is placed in a queue to

wait for observing time. You might be scheduled to observe for a five-day period

to begin six months after you submitted your proposal.

At a Radio Observatory At the same time, you might apply (usually

to the National Science Foundation) for financial support to carry out the research.

Your proposal possibly contains requests for some salary for yourself during the summer, and salary for a student or students to work on the project with you.

You are not charged directly for the use of the telescope itself—that cost is covered in the observatory’s overall budget.

At a Radio Observatory You carry out your observing at the VLA headquarters at

Socorro, New Mexico. A trained telescope operator runs the mechanical aspects of the

telescope. You give the telescope operator a computer program that includes

the coordinates of the points in the sky that you want to observe and how long to dwell at each location.

The telescopes (see figure) operate around the clock—one doesn’t want to waste any observing time.

At a Radio Observatory The electronics systems that are used to treat the incoming

signals collected by the radio dishes are particularly advanced. Computers combine the output from the 27 telescopes and

show you a color-coded image, with each color corresponding to a different brightness level (see figure).

Standard image-processing packages of programs are available for you to use back home, with the radio community generally using a different package from that used in the optical community.

You are expected to publish the results as soon as possible in one of the scientific journals, often after you have given a presentation about the results at a professional meeting, such as one of those held twice yearly by the American Astronomical Society.

At a Radio Observatory

Astronomy has become a very collaborative science. Many consortia of individual scientists, such as those studying

distant supernovae, have dozens of members. Telescope projects have also become so huge that collaboration

is necessary. The Atacama Large Millimeter Array

(ALMA), to be built in Chile on a high plain where it hasn’t rained in decades (see figure), will use at least 50 high-precision radio telescopes as an interferometer to examine our Galaxy and other celestial objects with high resolution.

It is a joint project of the United States’ National Science Foundation, the European Space Agency, and Chile.

week #11 notes: the milky way: our home in the universe

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