A galaxy is a collection of stars, all held together by mutual gravitation.
The Milky Way is that portion of our galaxy visible to us in the night sky.
Our galaxy is shaped like a disk and is about 100,000 light-years in diameter and 10,000 light-years thick near the center.
Our galaxy is composed of about 100 billion stars, almost all of which revolve about its center.
The sun is located about 28,000 light-years from the center of our galaxy and makes a complete circuit of the galaxy once every 240 million years.
The center of our galaxy is obscured by clouds of gas and dust; a black hole likely exists at the galaxy's center.
18-2. Stellar Populations
The stars in our galaxy fall into two categories.
Population I stars are in the galactic disk and are of all ages.
Population II stars are in globular clusters above and below the galactic disk.
The stars in globular clusters:
Range in number from about 10,000 to more than 50,000 stars
Are mostly very old
Are richer in hydrogen and helium and poorer in heavy elements than Population I stars
18-3. Radio Astronomy
A radio telescope is a directional antenna connected to a sensitive radio receiver.
Sources of radio waves from space include:
The thermal motion of ions and electrons in a very hot gas
High-speed electrons that move in a magnetic field
Hydrogen atoms and molecules of various kinds
Molecules in space can be identified by their radio emissions.
18-4. Galaxies
Most galaxies appear as flat spirals with two curving arms that radiate from a bright nucleus.
Galaxies are not evenly distributed in space, but are concentrated in groups of up to a few hundred.
Our galaxy and about two dozen others are members of the Local Group.
The Milky Way and Andromeda galaxies will collide in 2 or 3 billion years.
About seven-eights of the matter in the universe isnon-luminous dark matter, the nature of which is unknown..
18-5. Cosmic Rays
The Austrian physicist Victor Hess suggested that ionizing radiation from outside the earth is continually bombarding the atmosphere. This radiation was later called cosmic radiation.
Primary cosmic rays, which are the rays as they travel through space before they reach the earth, are atomic nuclei. Most were probably shot into space during supernova explosions in our galaxy.
Secondary cosmic rays are formed when primary cosmic waves enter the earth's atmosphere and disrupt atoms in their path to produce showers of secondary particles.
18-6. Red Shifts
The spectra of galaxies show red shifts, implying that all galaxies (except a few in the Local Group) are receding from the earth.
The amount of shift increases with the distance of the galaxy from the earth.
The recession speeds of galaxies can be computed from the extent of their red shifts.
Hubble's law relates the red shift of a galaxy to its distance from the earth.
The greater the distance, the faster the galaxy is traveling.
The speed increases by about 21 km/s per million light-years.
All galaxies appear to be moving away from each other; therefore, the universe seems to be expanding.
18-7. Quasars
Quasars are strong emitters of light and/or radio waves.
Thousands of quasars have been found and there seem to be many more.
Quasar red shifts are usually very large indicating that they are very distant objects.
Quasars radiate many times more energy than do ordinary galaxies, yet are far smaller in size.
Many astronomers believe that a massive black hole is possibly at the heart of every quasar, and that quasars are the cores of newly formed galaxies.
18-8. Dating the Universe
According to the big bang theory, the universe began in a violent burst in which space, time, matter, and energy came into existence.
Analysis of light emitted by globular clusters and white dwarf stars sets a lower limit to the age of the universe at 12-13 billion years.
Using Hubble's law gives a maximum age of the universe at 14 billion years.
The Hubble's law calculation ignores the braking effect of gravity on the expanding matter of the universe. Accounting for the effect of gravity, the age of the universe must be younger than 14 billion years.
Recent studies using cosmic background radiation set the age of the big bang as 13.7 billion years ago.
18-9. After the Big Bang
Immediately after the big bang, the universe was a compact, intensely hot mixture of matter and energy. Photons had enough energy to materialize into particle-antiparticle pairs which annihilated each other.
A few seconds after the big bang, the universe cooled to the point that photons no longer had enough energy to form particle-antiparticle pairs.
Annihilation of particles and antiparticles continued.
Because there were slightly more particles than antiparticles, there were only particles when the annihilation was finished.
When the universe was about a minute old, nuclear reactions began to form helium.
After 300,000 years, the universe cooled down to about 3000 K, cool enough for electrons and nuclei to combine into atoms.
Radiation that originated early in the history of the universe fills today's universe and can be detected in the range of radio waves.
Three observations support the big bang theory:
The uniform expansion of the universe
The relative abundances of hydrogen and helium
The cosmic background radiation
The universe may be undergoing a continuous series of expansions and contractions.
Before the discovery that the expansion is currently speeding up, debate existed if the universe is open or closed.
In an open universe, expansion will continue forever.
In a closed universe, gravitational attraction will cause expansion to stop and all matter and energy will come together in a big crunch followed by another cycle of expansion and contraction.
Recent studies, based on the discovery of dark energy, favor an open universe .
18-10. Origin of the Solar System
The sun and the planets formed together from a swirling cloud of gas and grains of ice and rock.
As the sun shrank in the process of forming a star, the resulting protosun left behind a spinning disk of gas and dust.
Bits of matter in the disk collided and stuck together to eventually form planetesimals that ultimately joined to become the planets.
Once the protosun became a true star, its intense solar wind swept the solar system free of remaining gas and dust.
Gravitational compression and radioactivity caused the earth to melt and separate into a dense iron core and a lighter mantle.
18-11. Extrasolar Planets
Many of the stars in the Milky Way are similar to the sun and likely have planetary systems.
Extrasolar planets are hard to find because:
They are so far away
Planets are very dim compared to stars
Planets are small in size
More than 100 extrasolar planets have been detected.
Life likely exists elsewhere in the universe, although it has yet to be discovered.
18-12. Interstellar Travel
Interstellar travel seems impossible with existing technology because of the great distances between the stars.
Because of the distances involved, it is unlikely that earth has been visited by beings from another world.
18-13. Interstellar Communication
Existing radiotelescopes can be used to send messages to or detect radio signals from interstellar civilizations.
The SETI (Search for Extraterrestrial Intelligence) program uses radio telescopes to search for radio signals from extraterrestrial intelligent life.
So far, no contact has been made with other worlds but the hope remains.
