Earth image Our Solar System in the Milky Way Galaxy and in the Universe

Dr. Pamela Gore
Georgia Perimeter College


  1. Explain what constitutes a galaxy.
  2. Discuss the classification of galaxies.
  3. Describe the position of the solar system in the Milky Way galaxy.
  4. Explain how we know that the galaxies are moving apart.
  5. Explain what can the spectrum can tell us about stars and galaxies.
  6. Explain how the Doppler Effect relates to movement.
  7. Discuss the scientifically determined age of the universe.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E1. Students will explore current scientific views of the universe and how those views evolved.
  • S6E1b. Describe the position of the solar system in the Milky Way galaxy and the universe.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • The sun is a medium-sized star located near the edge of a disk-shaped galaxy of stars, part of which can be seen as a glowing band of light that spans the sky on a very clear night. The universe contains many billions of galaxies, and each galaxy contains many billions of stars. To the naked eye, even the closest of these galaxies is no more than a dim, fuzzy spot.
  • The sun is many thousands of times closer to the earth than any other star. Light from the sun takes a few minutes to reach the earth, but light from the next nearest star takes a few years to arrive. The trip to that star would take the fastest rocket thousands of years. Some distant galaxies are so far away that their light takes several billion years to reach the earth. People on earth, therefore, see them as they were that long ago in the past.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Light interacts with matter by transmission (including refraction), absorption, or scattering (including reflection). To see an object, light from that object--emitted by or scattered from it--must enter the eye.
  • The sun is a major source of energy for changes on the earth's surface. The sun loses energy by emitting light. A tiny fraction of that light reaches the earth, transferring energy from the sun to the earth. The sun's energy arrives as light with a range of wavelengths, consisting of visible light, infrared, and ultraviolet radiation.

Galaxies - Introduction

A galaxy is an organized system of hundreds of millions to thousands of billions of stars, sometimes mixed with interstellar gas and dust.

Our sun and solar system are part of the Milky Way galaxy.

Galaxies can be seen in every direction in space, each with billions of stars. Galaxies often appear to be distinct but fuzzy patches of light.

Charles Messier (1730-1817) cataloged more than 100 fuzzy celestial objects, sometimes called Messier objects, and named M1 to M110.

Dreyer compiled the New General Catalog of nearly 8000 objects around 1900. Most of these fuzzy objects are planetary nebulae and star clusters that are part of our galaxy, but extragalactic objects (or galaxies) were also included.

Nearest neighboring large galaxy = Andromeda Galaxy (M31). The relatively "nearby" Andromeda Galaxy (M31) is about 2.2 million light years away.

The Local Group is a group of our nearest galaxy neighbors, held together by their mutual gravitational attraction. About 20 galaxies are in this area.

Classification of Galaxies

In the 1920's, Hubble devised a classification of galaxies:
  1. Spiral galaxies (30%)
  2. Elliptical galaxies (most common - 60%)
  3. Lenticular galaxies (transitional orms between sprial and elliptical galaxies)
  4. Irregular galaxies (10%)

Spiral galaxies are flat disks with a nuclear bulge, a halo of old stars, and spiral arms with young stars. Some have a bar-shaped concentration of stars in the center (barred spirals). Arms emerge fromt he ends of the bar. Dust is readily visible as dark streaks. The Milky Way galaxy is a spiral galaxy.

Globular clusters encircle spiral galaxies. Elliptical galaxies are spheroidal in shape (elliptical in two dimensions). Old stars are dominant. There is no prominent internal structure. They are circled by a halo of globular clusters. Little or no gas and dust are present. Almost all has been converted into stars.

Irregular galaxies are ot disk-like or spheroidal and have no nucleus. They have a chaotic, irregular appearance. Some have bars, but no arms. Sites of active star formation with young stars and luminous gas clouds. Some very old stars are present in globular clusters.

Examples of irregular galaxies are the Small Magellanic Cloud and the Large Magellanic Cloud.

Our Solar System's Location in the Milky Way Galaxy

The Milky Way Galaxy is visible as a milky band that stretches across the night sky. It is best seen in very dark skies, far from light pollution.

The Milky Way Galaxy is a spiral galaxy containing roughly 200 billion stars. The galaxy is about 80,000 to 120,000 light-years across, and less than 7,000 light-years thick. (Note that a light year is the distance that light can travel in a year.)

Almost everything that we can see in the night sky(except for other galaxies) is part of the Milky Way Galaxy.

Our solar system is located in the outer reaches of one of the spiral arms. Our sun is about 26,000 to 28,000 light-years from the center of the Milky Way Galaxy, and about 20 light years above the galaxy's equatorial plane.

It takes approximately 200-250 million years for our solar system to orbit once around the Milky Way. In this orbit, the solar system travels at about 155 miles/sec (250 km/sec).

This web link will show you a picture of the probable position of the solar system in the Milky Way galaxy.

There are no photos of the Milky Way galaxy taken from outside. That would require a space probe to be able to get beyond the galaxy, but that hasn't yet been done.

Astronomers think that the Milky Way galaxy is a spiral galaxy, like the Andromeda Galaxy or the Whirlpool Galaxy. However, recent research indicates it may be a barred spiral, like M91.

Movement of galaxies and the Big Bang Theory

  1. The galaxies are rapidly moving apart. This indicates that the galaxies were closer together in the past.
    (This was discovered in 1929 by Edwin P. Hubble and is called Hubble's Law).
  2. Observed temperature of the universe today (background microwave radiation) is 3 degrees above absolute zero.
  3. Present abundances of hydrogen and helium.


The universe is expanding. Everything began together at a point, and a big explosion occurred, causing things to move apart rapidly.

This interpretation is called the Big Bang Theory.

How do we know that the galaxies are moving apart?

In order to understand how we know the galaxies are moving apart, we must first understand something about light, and the spectrum.

I. Light

The study of electricity and magnetism (electromagnetism) led to the discovery of the wave-like nature of light.

All firms of radiation are wave phenomena:

Remarkably, radiation also exhibits properties of particles.

(A photon is a unit of eletromagentic energy with particle and wave behavior).

Everyone has a general familiarity with electricity - such as sparks or static electricity - and also with magnets.

Electricity has 2 kinds of charges: positive (+) and negative (-).
Opposite charges attract.
Similar charges repel.

Magnets also repel or attract each other and certain metal objects (iron).
A magnet has 2 poles, N and S.

There is a connection between electricity and magnetism.

In 1820, Hans Christian Oersted, a Danish scientist discovered that electric currents create a magnetic field.
In 1873, James Clerk Maxwell demonstrated the relationship mathematically in four equations known as Maxwell's Equations, which are the foundation for the theory of electromagnetism.

II. Wave Behavior

Everyone has a basic familiarity with wave phenomena.
Throw a stone into a pond and observe the waves moving outward from the central point.

Microsoft clip art.

The colors of the rainbow represent different wavelengths of electromagnetic radiation (visible light),

Microsoft clip art.

If sunlight is passed through a prism, a rainbow is produced.

Microsoft clip art.


This is due to refraction or the bending of light as it passes from one medium to another (air to glass to air).

Different wavelengths are refracted (or bent) by different amounts causing a dispersion of colors or spectrum.

The reason that the colors differ is because their wavelengths differ.

In the atmosphere, raindrops disperse light forming rainbows.


The colors of the spectrum are:


The red light has a long wavelength (0.7 µm (micrometers or microns), 700 nanometers, or about 7000 angstroms) and the violet light has a short wavelength (0.4 µm (micrometers or microns), 400 nanometers, or about 4000 angstroms)

This is why red light is on the outside of the curve of the rainbow, and violet light is on the inside of the curve.

1 angstrom = 10-8 cm or 10 -10 m
1 nanometer = 10-7 cm or 10-9 m
1 µm (micrometer or micron) = 10-4 cm or 10-6 m

We can't see radio waves but they are in the air all around us. Radio waves can be detected by an antenna, which detects oscillating electrical fields. The electrons in the antenna move back and forth, producing an oscillating current.

III. Atomic Spectra

There are three types of spectra.
  1. Continuous spectrum - produced by a glowing solid, liquid, or gas under certain conditions. The colors blend smoothly. Example: the light generated by a common tungsten light bulb. Incandescent = emits light when hot.

  2. Emission spectrum or bright-line spectrum. Seen as bright lines on a dark background. This type of spectrum is produced when a rarified gas is heated until it glows. The bright lines occur at specific wavelengths depending on the composition of the gas that produces them.

  3. Absorption spectrum or dark-line spectrum. When light passes through a cool gas, the gas absorbs part of the spectrum, producing black lines (absorption lines) on the spectrum. The lines are sometimes called Fraunhofer lines. The sun and most stars have dark-line spectra.

Each element has its own spectral signature of lines. By noting the positions of the lines, the composition of the gas can be determined.

The interior of the sun produces a continuous spectrum, but when the light passes through cooler and less dense gases of the solar atmosphere, these gases absorb light of particular wavelengths, depending on the composition of the gases. This produces dark lines.

An atom in the gas absorbs a photon, which causes electrons to become excited and move to higer orbits (they change their distance from the nucleus of the atom). Atoms can't absorb all photos - only those with certain energies (or certain wavelengths) because of the orbitals available and the amount of energy needed to move an electron to another orbit.

No two elements absorb exactly the same wavelengths of light. Therefore it is possible to determine which elements are present in the gas by analyzing the spectrum.

We use spectral lines to determine the composition of stars.

We use a device called a spectroscope or spectrograph to study the spectrum of the sun, the stars, and various other sources of light.

A diffraction grating can also be used to produce a spectrum in place of a prism.

Now that we understand something about light and the spectrum, we can apply this knowledge to how we know the galaxies are moving apare.

The spectrum of a star (or galaxy) reveals: Light reaching us from distant receding galaxies has its absorption lines shifted toward the red end of the spectrum. This is referred to as the red shift.

The red shift indicates that the galaxy is moving away from the Earth, and that the universe is expanding.

W.M. Slipher first noted the red shift in 1914 .

Doppler Effect

A similar phenomen, except related to sound, is the Doppler Effect.

If object is moving toward you, the wavelengths are compressed, which causes high pitched sound or shorter wavelengths.

If object is moving away, the wavelengths are elongated, which causes low pitch or shorter wavelengths.

Same thing works with light as sound. Both travel in waves.

Light reaching us from distant receding galaxies has shifted toward the red end of the spectrum. (look at the wavelength position of absorption lines).

Other theories for the origin of the universe:

Formation of stars, galaxies, solar systems, planets

Material begins to clump together as it moves away from the center of the Big Bang. Molecular clouds form (raw material for new star systems).

"Nebular hypothesis or Solar Nebula Hypothesis". Nebular means cloud.

How old is the Universe?

Calculations of the age of the universe depend on the calculation of the Hubble Constant, a number which refers to the rate of expansion of the universe.

Controversy arose in Fall 1994
Age of the universe has been calculated to be about 10-15 billion years; calculations show that the age must be less than 20 billion years.
BUT 1994 data from Hubble Space Telescope was interpreted to indicate a high rate of expansion, resulting in an age of only about 8 billion years.
This stirred up a lot of excitement in Astronomy. For details, see the March 6, 1995 issue of TIME Magazine, p. 76-84.

Update: April 7, 1997, front page, New York Times. National Academy of Sciences held a colloquium on the age of the universe in March 1997. Many cosmologists now think that the age of the universe is likely to be between 12 and 14 billion years. Later observations from the Hubble Space Telescope observations of Cephids (pulsating stars) have given somewhat lower expansion rates. Lower expansion rates have been suggested by other recent studies as well. This seems to mean that the age of the universe is most likely 15-20 billion years.

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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 29, 2005