Stars, Galaxies, and the Universe Chapter 30 Earth and Space Science
Analyzing Starlight • Nuclear fusion is the combination of light atomic nuclei to form heavier atomic nuclei • Astronomers learn about stars by analyzing the light that the stars emit. • Starlight passing through a spectrograph produces a display of colors and lines called a spectrum.
Analyzing Starlight • All stars have dark-line spectra. • A star’s dark-line spectrum reveals the star’s composition and temperature. • Stars are made up of different elements in the form of gases. • Scientists can determine the elements that make up a star by studying its spectrum.
The Compositions of Stars • Scientists have learned that stars are made up of the same elements that compose Earth. • The most common element in stars is hydrogen. • Helium is the second most common element in star. • Small quantities of carbon, oxygen, and nitrogen are also found in stars.
The Temperatures of Stars • The temperature of most stars ranges from 2,800˚C to 24,000˚C. • Blue stars have average surface temperatures of 35,000˚C. • Yellow stars, such as the sun, have surface temperatures of between 5,000˚C and 6,000˚C. • Red stars have average surface temperatures of 3,000˚C.
The Sizes and Masses of Stars • Stars vary in size and mass. • Stars such as the sun are considered medium-sized stars. • Most stars visible from Earth are medium-sized stars.
Stellar Motion • Two kinds of motion • Actual Motion • Apparent Motion
Apparent Motion of Stars • The apparent motion of stars is the motion visible to the unaided eye. • Apparent motion is caused by the movement of Earth. • The rotation of Earth causes the apparent motion of stars sees as though the stars are moving counter-clockwise around the North Star. • Earth’s revolution around the sun causes the stars to appear to shift slightly to the west every night.
Circumpolar Stars • Some stars are always visible in the night sky. These stars never pass below the horizon. • In the Northern Hemisphere, the movement of these stars makes them appear to circle the North Star. • These circling stars are called circumpolar
Circumpolar Stars • The stars of the little dipper are circumpolar for most observers in the Northern Hemisphere. • At the pole all visible stars are circumpolar. • As you move off the pole fewer and fewer circumpolar stars exist.
Actual Motion of Stars • Most stars have several types of actual motion. • Stars rotate on an axis. • Some stars may revolve around another star. • Stars either move away from or toward our solar system.
Actual Motion of Stars • The spectrum of a star that is moving toward or away from Earth appears to shift, due to the Doppler effect. • Stars moving toward Earth are shifted slightly toward blue, which is called blue shift. • Stars moving away from Earth are shifted slightly toward red, which is called red shift.
Actual Motion of StarsDoppler Effect • The spectrum of a star that is moving toward or away from Earth appears to shift, as shown in the diagram below.
Distances to Stars • Distances between the stars and Earth are measured in light-years. • light-year the distance that light travels in one year. • about 9.5 trillion kilometers (5.8 trillion miles).
Distance to Stars How big is the universe? • Proxima Centauri is about 4.3 light-years from the earth. • The light produced by Proxima Centauri takes about 4.3 years to reach earth. • Light from the sun reaches the earth in about 8 minutes. • This fact suggests that the universe is incomprehensibly large.
Measuring Distances to the Stars • Stellar parallax, the extremely slight back-and-forth shifting in a nearby star's position due to the orbital motion of Earth. • The farther away a star is, the less its parallax. • Parallax angles are very small.
Stellar Brightness • Three factors control the brightness of a star as seen from Earth: • size (how big), • temperature (how hot), • distance from Earth (how far away).
Stellar Brightness • Magnitude is the measure of a star's brightness. • Apparent magnitude is how bright a star appears when viewed from Earth. • Absolute magnitude is the "true" brightness if a star were at a standard distance of about 32.6 light-years. • The difference between the two magnitudes is directly related to a star's distance.
Apparent magnitude • The lower the number of the star on the scale shown on the diagram below, the brighter the star appears to observers. • The sun has an apparent magnitude of –26.8 • All other objects are dimmer.
End of Section 1 • Answer Questions 1-6 on page 780.
Classifying Stars • One way scientists classify stars is by plotting the surface temperatures of stars against their luminosity. • The H-R diagram is the graph that illustrates the resulting pattern. • Astronomers use the H-R diagram to describe the life cycles of stars. • Most stars fall within a band that runs diagonally through the middle of the H-R diagram. • These stars are main sequence stars.
H-R Diagram - History • A useful astronomical tool which plots stellar temperature (color) against luminosity. • Independently invented by Henry Russell in 1913 & Ejnar Hertzsprung in 1905 through the study of true brightness and temperature of stars. • Useful for studying properties & life cycles of stars: • Mass, Luminosity, Surface Temperature, Age
Don’t bother copying… • Stellar temperature/color also gives rise to “Spectral Classes.” • O (> 30,000 K). • B (10,000 – 30,000 K). • A (7,000 – 10,000 K). • F (6,000 – 7,000 K). • G (5,000 – 6,000 K) – the sun! • K (4,000 – 5,000 K). • M (< 4,000 K).
H-R Diagram cont. • Stars located in the upper-right position of an H-R diagram are called giants, luminous stars of large radius. • Supergiants are very large. • Very small white dwarf stars are located in the lower-central portion of an H-R diagram. • Ninety percent of all stars, called main-sequencestars, are in a band that runs from the upper-left corner to the lower-right corner of an H-R diagram.
Points of Note • Stars spend 90% of their lives on Main Sequence • Main Sequence stars are burning only Hydrogen • High mass stars live fast, die young: • 20 Solar Mass Star - 10 Million Years • Sun - 10 Billion Years • Red Dwarf - >100 Billion Years
Differences Between High Mass and Low Mass Stars • Stars that are more massive than the Sun have stronger gravitational forces. • These forces need to be balanced by higher internal pressures. • These higher pressures result in higher temperatures which drive a higher rate of fusion reactions. • The Hydrogen within the core of a high mass star therefore gets used up much faster than in the Sun and “ages” faster. • Low mass stars “age” slower.
Star Formation • A star brings in a nebula. • As gravity pulls particles of the nebula closer together, the gravitational pull of the particles on each other increases. • As more particles come together, regions of dense matter begin to build up within the cloud.
Nebula • New stars are born out of enormous accumulations of dust and gases, called nebula, that are scattered between existing stars.Nebula comes from the Latin for “cloud”. The Orion Star Forming Complex
Dark Nebula • When a nebula is not close enough to a bright star to be illuminated, it is referred to as a dark nebula. • Horsehead Nebula is a dark nebula.
Bright Nebula • A bright nebula glows because the matter is close to a very hot (blue) star. • Emission nebulae: derive their visible light from the fluorescence of the ultraviolet light from a star in or near the nebula.
Bright Nebulae • Reflection nebulae: relatively dense dust clouds in interstellar space that are illuminated by reflecting the light of nearby stars.
Stellar Lifecycles • The process by which stars are formed and use up their fuel. • What exactly happens to a star as it uses up its fuel is strongly dependent on the star’s mass. The Orion Nebula - Birthplace of stars
Protostars • Gravity within a nebula compacts it to form a flattened disk.The disk has a central concentration of matter called a protostar. • The protostar continues to contract and increase in temperature for several million years and becomes plasma.
The Birth of a Star • A protostar’s temperature continually increases until it reaches about 10,000,000°C. • At this temperature, nuclear fusion begins. • The process releases enormous amounts of energy. • The onset of nuclear fusion marks the birth of a star. Once this process begins, it can continue for billions of years.
A Delicate Balancing Act • As gravity increases the pressure on the matter within the star, the rate of fusion increase. • In turn, the energy radiated from fusion reactions heats the gas inside the star. • The outward pressures of the radiation and the hot gas resist the inward pull of gravity. • This equilibrium makes the star stable in size.
The Main-Sequence Stage • Energy continues to be generated in the core of the star as hydrogen fuses into helium. • A star that has a mass about the same as the sun’s mass stays on the main sequence for about 10 billion years. • Scientists estimate that over a period of almost 5 billion years, the sun has converted only 5% of its original hydrogen nuclei into helium nuclei.
Leaving the Main Sequence • When almost all of the hydrogen atoms within its core have fused into helium atoms the core of the star contracts because of gravity. • As the temperature rises the last of the hydrogen atoms fuse and send energy into the outer shell.
Giant Stars • A star enters its third stage when almost all of the hydrogen atoms within its core have fused into helium atoms. • A star’s shell of gases grows cooler as it expands. As the gases in the outer shell become cooler, they begin to glow with a reddish color. These stars are known as giants.
Supergiants • Main-sequence stars that are more massive than the sun will become larger than giants in their third stage. • These highly luminous stars are called supergiants. • These stars appear along the top of the H-R diagram. • Despite the high luminosity these stars are relatively cool.
The Final Stages of a Sunlike Star • When all the helium has been used up, the fusion will stop. • With no energy available the star will enter its last stages.
Planetary Nebulas • As the star’s outer gases drift away, the remaining core heats these expanding gases. • The gases appear as a planetary nebula, a cloud of gas that forms around a sunlike star that is dying.
When it runs out of Helium fuel it begins to contract and heat up. The Sun increases its luminosity. The outer layers of the Sun expand, cool and redden again. The outer layers of the Sun start streaming away from the core. This material forms a nebula surrounding the Sun. The Sun’s Planetary Nebula
White Dwarfs • As a planetary nebula disperses, gravity causes the remaining matter in the star to collapse inward. • A hot, extremely dense core of matter - a white dwarf - is left. • White dwarfs shine for billions of years before they cool completely.
Novas and Super novas • When a star explosively brightens, it is called a nova (new star). Excessively large explosions are called supernovas. • During the outburst, the outer layer of the star is ejected at high speed. • After reaching maximum brightness in a few days, the nova slowly returns in a year or so to its original brightness.