The English astronomer William Herschel was the first to build and use a large reflecting telescope.
In a reflecting telescope, light is reflected from a concave mirror instead of being refracted through a lens.
All modern astronomical telescopes are reflectors.
The latest reflecting telescopes use multiple mirrors instead of single large mirrors.
Large telescopes have greater light-gathering power and better ability to resolve (distinguish) small details than do smaller telescopes.
The light collected by a telescope is directed to a photographic plate or electronic sensor which can detect objects too faint for the eye to pick up.
17-2. The Spectrometer
Telescopes are combined with spectrometers to collect more information about stars.
The spectrometer breaks light up into its separate wavelengths. The resulting spectrum is recorded on a photographic plate or electronic medium.
17-3. Spectrum Analysis
An absorption spectrum (a spectrum of dark lines on a continuous colored background) reveals that a star's structure consists of a hot interior surrounded by a cooler atmosphere. Most stars have absorption spectra.
The surface temperature of a star can be determined by spectrum analysis. The point of maximum wavelength intensity in the spectrum is a measure of its temperature.
The hottest stars are blue-white.
Stars of intermediate temperature are orange-yellow.
The coolest visible stars are red.
The composition of a star can be found from its spectrum because each element in a star has a spectrum consisting of lines with characteristic wavelengths.
Identification of spectral lines can reveal something about the physical conditions in which the elements exist.
The Zeeman effect permits the detection of magnetic fields of stars.
The motion of a star toward or away from the earth is shown by doppler shifts in its spectral lines.
17-4. Properties of the Sun
The mass of the sun is 1.99 × 1030 kg.
The sun's radius is 6.96 × 108 m.
The surface temperature of the sun is 6000 K.
The glowing gas surface of the sun is called the photosphere.
The sun consists mainly of hydrogen and helium.
The elements in the sun are present as individual atoms or ions.
Solar prominences are flamelike protuberances that project from the sun's atmosphere into space. They are associated with sunspots and seem to have magnetic fields associated with them.
The sun's corona consists of ions and electrons and is visible during solar eclipses.
The solar wind is the outward flow of ions and electrons from the sun's corona.
17-5. The Aurora
An aurora is a luminous atmospheric display produced by the excitation of atmospheric gases by streams of fast protons and electrons from the sun.
The aurora borealis is the name given to this phenomenon in the northern hemisphere, and aurora australis in the southern.
Auroras are most common in the far north and far south.
Airglow is the faint glow in the night sky due to less concentrated streams of solar particles interacting with the upper atmosphere. The degree of brightness varies with solar activity.
17-6. Sunspots
Sunspots are cooler areas on the solar surface that appear dark only by comparison with the brighter solar surface around them.
Sunspots change continually in form and have lifetimes of from 2 to 3 days to more than a month.
Galileo associated the movement of sunspots across the sun's disk with solar rotation.
Sunspots generally appear in groups.
Strong magnetic fields are associated with sunspots.
The sunspot cycle is about 11 years long.
A number of effects observable on earth are associated with the sunspot cycle. These include:
Solar storms (disturbances in the terrestrial magnetic field)
Shortwave radio fadeouts
Changes in cosmic-ray intensity
Unusual auroral activity
Some aspects of weather and climate
17-7. Solar Energy
The sun's temperature and pressure at its center are estimated to be 14 million K and 1 billion atm, respectively.
Under these conditions, matter in the sun's interior consists of free electrons and positive nuclei surrounded by a few electrons or none at all.
These atomic fragments move so rapidly that two atomic nuclei, despite their repulsive electric force, can fuse to form a single large nucleus.
When this occurs, the new nucleus has a little less mass than the combined masses of the reacting nuclei. This missing mass is converted to energy.
Most solar energy comes from the conversion of hydrogen into helium in nuclear fusion reactions. This takes place directly by collisions of hydrogen nuclei (proton-proton cycle) and indirectly by a series of steps in which carbon nuclei absorb a succession of hydrogen nuclei (carbon cycle).
The sun converts more than 4 billion kg of matter into energy every second and has emitted energy at a steady rate for a long time.
The heavier elements are formed from hydrogen and helium as raw materials subjected to extreme conditions of high temperatures and pressures.
17-8. Stellar Distances
In 1838, the German astronomer Friedrick Bessel used the shift in the relative position of a star to make direct measurements of distances to the nearer stars.
A parallax is the apparent shift in a star's position.
A light-year is the distance light travels in a year and is equal to 9.46 × 1012 km.
The apparent brightness of a star is its brightness as seen from the earth; its intrinsic brightness is the star's true brightness.
The apparent brightness of a star depends on its intrinsic brightness and its distance from the earth.
If both the apparent and the intrinsic brightness of a star are known, its distance from the earth can be calculated.
The American astronomer Walter Adams discovered a way to find the intrinsic brightness of a star by examining its spectrum.
17-9. Variable Stars
A variable star is one whose brightness varies continually.
Most variable stars repeat a fairly definite cycle of change; others show irregular fluctuations.
Cepheid variables are a special class of variable stars that are useful to astronomers in determining the distance to certain star groups.
Cepheid variables are very bright yellowish stars 5 to 10 times as heavy as the sun.
The American astronomer Henrietta Leavitt discovered that the intrinsic brightness of a Cepheid variable is related to its period of fluctuation.
Comparing the intrinsic brightness with the Cepheid variable's apparent brightness gives its distance.
17-10. Stellar Motions
The stars are not fixed in space; most stars are moving at speeds of several kilometers per second relative to the earth.
The sun and the planets are moving toward the constellation Cygnus at a speed of 200 to 300 km/s.
17-11. Stellar Properties
Only the masses of binary stars can be determined, but such stars are common.
Stellar masses range from 1/40 to 100 times that of the sun.
The majority of stars have surface temperatures between 3000 and 12,000 K.
The size of a star can be found from its temperature and its intrinsic brightness.
The diameters of the smallest stars are about 20 km across; the largest have diameters 500 or more times that of the sun.
17-12. H-R Diagram
A Hertzsprung-Russell (or H-R) diagram is a plot of intrinsic brightness versus temperature for stars.
Each point on the diagram represents a particular star.
About 90 percent of all stars belong to the main sequence.
The most abundant stars in the main sequence are red dwarfs.
Outside of the main sequence, most of the remaining stars belong to the red giant class and to the white dwarf class.
The position of a star on the H-R diagram is related to its physical properties.
The sun occupies a middle position on the H-R diagram. Such stars have moderate temperatures, densities, and masses, rather small diameters, and spectra in which lines of metallic elements are prominent.
Red giants have low densities, large diameters, low surface temperatures, and hot cores.
White dwarfs have small diameters (they are comparable in size to the earth), high densities, and high surface temperatures.
The density of a white dwarf is about 106 g/cm3.
To attain densities this high, the matter in white dwarfs must consist of collapsed atoms whose electrons and nuclei are packed closely together.
Although the universe contains numerous white dwarfs, only a few thousand are known, because they are very faint and only the nearer ones can be seen.
17-13. Stellar Evolution
Stars originate in gas clouds in space. These clouds are largely hydrogen.
Gravitational contraction results in a heated, glowing clump of matter.
Some thousands or millions of years later the star's temperature rises to the point where nuclear reactions begin.
The greater the mass of an H-R main sequence star, the higher its temperature.
When the hydrogen supply runs low in a star like the sun, several events take place.
Gravitational contraction increases core temperature and new nuclear reactions become possible.
The star becomes a red giant.
The new nuclear reactions run out of fuel and the star shrinks to form a white dwarf.
At this stage, a bubblelike shell of gas from the outer part of the star, called a planetary nebula, is released into space.
Ultimately the star ceases to radiate at all and becomes a black dwarf.
17-14. Supernovas
A star having over 8 times the sun's mass at the upper end of the H-R main sequence collapses when its fuel runs out. It then explodes violently, creating a supernova.
A supernova is billions of times brighter than the original star.
Nuclear reactions during the explosion create the heaviest elements and send them into space.
After the supernova explosion, the remaining matter is compressed into a dwarf star of extraordinary density called a neutron star.
17-15. Pulsars
A pulsar is a rapidly spinning neutron star that emits bursts of light and radio waves.
The power output of a pulsar is about 1026 W.
17-16. Black Holes
A black hole is a "dead" star whose matter is packed so densely that its gravitational field is strong enough for the escape speed to be greater than the speed of light.
Only extremely heavy stars become black holes; lighter stars eventually become white dwarfs or neutron stars.
If a black hole is a member of a double-star system, its presence can be revealed by its gravitational pull on the other star and by the emission of x-rays as its gravitational field pulls matter away from the other star.
Enormous black holes are thought to be at the centers of galaxies.
