Chapter 16 Lecture Outline Beyond Our Solar System
The figure shows a region about 52 feet across occupied by a human being, a sidewalk, and a few trees all objects whose size you can understand.
Each successive picture in the chapter will show you a region of the universe that is 100 times wider than the preceding picture. That is, each step will widen your field of view the region you can see in the image by a factor of 100.
In this figure, your field of view widens by a factor of 100, and you can see an area 1 mile in diameter. The arrow points to the scene shown in the preceding photo. People, trees, and sidewalks have vanished, but now you can see a college campus and the surrounding streets and houses. The dimensions of houses and streets are familiar. This is the world you know, and you can relate such objects to the scale of your body.
You started your adventure using feet and miles, but you should use the metric system of units. Not only is it used by all scientists around the world, but it makes calculations much easier.
The photo in the figure is 1 mile in diameter. A mile equals 1.609 kilometers. So, you can see in the photo that a kilometer is a bit over two-thirds of a mile a short walk across a neighborhood.
The view in this figure spans 160 kilometers. In this infrared photo, the green foliage shows up as various shades of red. The college campus is now invisible. The patches of gray are small cities, with Wilmington, Delaware, visible at the lower right.
At this scale, you see the natural features of Earth s surface. The Allegheny Mountains of southern Pennsylvania cross the image in the upper left. The Susquehanna River flows southeast into Chesapeake Bay. What look like white bumps are a few puffs of clouds.
Notice the red color. This is an infrared photograph in which healthy green leaves and crops show up as red. Human eyes are sensitive to only a narrow range of colors. As you explore the universe, you will learn to use a wide range of colors from X rays to radio waves to reveal sights invisible to unaided human eyes.
At the next step in your journey, you will see your entire planet which is 12,756 km in diameter. Earth rotates on its axis once a day, exposing half of its surface to daylight at any particular moment. The photo shows most of the daylight side of the planet. The blurriness at the extreme right is the sunset line.
The rotation of Earth carries you eastward. As you cross the sunset line into darkness, you say the sun has set. It is the rotation of the planet that causes the cycle of day and night.
Enlarge your field of view by a factor of 100, and you will see a region 1,600,000 km wide. Earth is the small blue dot in the center. The moon whose diameter is only one-fourth that of Earth is an even smaller dot along its orbit 380,000 km from Earth. These numbers are so large that it is inconvenient to write them out.
This is nothing more than a simple way to write numbers without writing lots of zeros. In scientific notation, you would write 380,000 as 3.8 x 10 5. The universe is too big to discuss without using scientific notation.
When you once again enlarge your field of view by a factor of 100, Earth, the moon, and the moon s orbit all lie in the small red box at lower left. Now, however, you can see the sun and two other planets that are part of our solar system. Our solar system consists of the sun, its family of planets, and some smaller bodies such as moons and comets.
Like Earth, Venus and Mercury are planets small, nonluminous bodies that shine by reflected light. Venus is about the size of Earth and Mercury is a bit larger than Earth s moon. On this diagram, they are both too small to be seen as anything but tiny dots.
The sun is a star a self-luminous ball of hot gas that generates its own energy. The sun is 109 times larger in diameter than Earth, but it too is nothing more than a dot in the diagram.
This diagram has a diameter of 1.6 x 10 8 km. The average distance from Earth to the sun is a unit of distance called the astronomical unit (AU), a distance of 1.5 x 10 11 m.
Using this unit, you can say that the average distance from Venus to the sun is about 0.7 AU. The average distance from Mercury to the sun is about 0.39 AU.
The orbits of the planets are not perfect circles, and this is particularly apparent for Mercury. Its orbit carries it as close to the sun as 0.307 AU and as far away as 0.467 AU. You can see this variation in the distance from Mercury to the sun in the figure. Earth s orbit is more circular, and its distance from the sun varies by only a few percent.
Enlarge your field of view again, and you can see the entire solar system. The details of the preceding figure are now lost in the red square at the center of the diagram. You see only the brighter, more widely separated objects.
The sun, Mercury, Venus, and Earth lie so close together that you cannot separate them at this scale. Mars, the next outward planet, lies only 1.5 AU from the sun.
In contrast, Jupiter, Saturn, Uranus, Neptune, and Pluto are so far from the sun that they are easy to place in the diagram. These are cold worlds far from the sun s warmth. Light from the sun reaches Earth in only 8 minutes, but it takes over 4 hours to reach Neptune.
Pluto s orbit is so elliptical that it can come closer to the sun than Neptune does as Pluto did between 1979 and 1999.
When you again enlarge your field of view by a factor of 100, the solar system vanishes. The sun is only a point of light, and all the planets and their orbits are now crowded into the small red square at the center. The planets are too small and reflect too little light to be visible so near the brilliance of the sun.
Nor are any stars visible except for the sun. The sun is a fairly typical star, and it seems to be located in a fairly average neighborhood in the universe. Although there are many billions of stars like the sun, none is close enough to be visible in the diagram which shows an area only 11,000 AU in diameter.
The stars are typically separated by distances about 10 times larger than the diameter of the diagram. Except for the sun at the center, this diagram is empty.
Now, your field of view has expanded to a diameter a bit over 1 million AU. The sun is at the center, and you can see a few of the nearest stars. These stars are so distant that it is not reasonable to give their distances in astronomical units.
To express distances so large, define a new unit of distance the light-year. One light-year (ly) is the distance that light travels in one year roughly 10 13 km or 63,000 AU. It is a common misconception that a light-year is a time. Have you heard people say, It will take me light-years to finish my term paper? Next time, you can tell them that a light-year is a distance, not a time.
The diameter of your field of view in the figure is 17 ly. The nearest star to the sun, Alpha Centauri, is 4.2 ly from Earth. In other words, light from Alpha Centauri takes 4.2 years to reach Earth.
In the figure, the sizes of the dots represent not the sizes of the stars but their brightness. This is the custom in astronomical diagrams, and it is also how star images are recorded on photos. Bright stars make larger spots on a photo than faint stars. The size of a star image in a photo informs you not how big the star is but only how bright it looks.
Now, you expand your field of view by another factor of 100, and the sun and its neighboring stars vanish into the background of thousands of other stars. The field of view is 1,700 ly in diameter.
Of course, no one has ever journeyed thousands of light-years to photograph the solar neighborhood. So, this is a representative photo of the sky. The sun is a relatively faint star that would not be easily located in a photo at this scale.
If you expand your field of view by a factor of 100, you see our galaxy a disk of stars about 75,000 ly in diameter. A galaxy is a great cloud of stars, gas, and dust bound together by the combined gravity of all the matter. Galaxies range from 1,500 to over 300,000 ly in diameter and can contain over 100 billion stars.
As you expand your field of view by another factor of 100, our galaxy appears as a tiny luminous speck surrounded by other specks. The diagram includes a region 17 million ly in diameter, and each of the dots represents a galaxy. Notice that our galaxy is part of a cluster of a few dozen galaxies.
If you again expand your field of view, you see that the clusters of galaxies are connected in a vast network. Clusters are grouped into superclusters clusters of clusters. The superclusters are linked to form long filaments and walls outlining voids that seem nearly empty of galaxies. These appear to be the largest structures in the universe.
The Universe Cosmology Study of the Universe I. Kant mid 1700 s Island Universes - fuzzy patches Edwin Hubble 1919 Cephid variables Compare absolute magitude to observed brightness Determined the fuzzy patches were indeed out of our galaxy
Light-year Distance light travels in one year Slightly less than 10 trillion km
Properties of Stars Distance Measuring a star's distance can be very difficult Stellar parallax Used for measuring distance to a star Apparent shift in a star's position due to the orbital motion of Earth Measured as an angle Near stars have the largest parallax Largest parallax is less than one second of arc
Distance Distances to the stars are very large Units of measurement Kilometers or astronomical units are too cumbersome to use Light-year is used most often Distance that light travels in 1 year One light-year is 9.5 trillion km (5.8 trillion miles) Other methods for measuring distance are also used
Properties of Stars Stellar brightness Controlled by three factors Size Temperature Distance Magnitude Measure of a star's brightness
Properties of Stars Stellar brightness Magnitude Two types of measurement Apparent magnitude Brightness when a star is viewed from Earth Decreases with distance Numbers are used to designate magnitudes Dim stars have large numbers and negative numbers are also used
Properties of Stars Stellar brightness Magnitude Absolute magnitude "True" or intrinsic brightness of a star Brightness at a standard distance of 32.6 light-years Most stars' absolute magnitudes are between -5 and +15
Color and temperature Properties of Stars Hot star Temperature above 30,000 K Emits short-wavelength light Appears blue Cool star Temperature less than 3000 K Emits longer-wavelength light Appears red
Color and temperature Properties of Stars Between 5000 and 6000 K Stars appear yellow Binary stars and stellar mass Binary stars Two stars orbiting one another Stars are held together by mutual gravitation Both orbit around a common center of mass
Binary stars Properties and stellar of Stars mass Binary stars Visual binaries are resolved telescopically More than 50% of the stars in the universe are binary stars Used to determine stellar mass Stellar mass Determined using binary stars The center of mass is closest to the most massive star Mass of most stars is between one-tenth and fifty times the mass of the Sun
Binary Stars Orbit Each Other Around Their Common Center of Mass
The Universe
The Universe
The Universe
Interstellar Matter: Nursery of the Stars Interstellar matter Strings and clumps of matter Nebulae (clouds) 2 TYPES: Bright nebulae Glows if it close to a very hot star Two types of bright nebulae Emission nebula Reflection nebula
Interstellar Matter: Nursery of the Stars Bright nebulae Emission nebulae Active, star-forming regions UV light emitted under low pressure Appear red
Interstellar Matter: Nursery of the Stars Bright nebulae Reflection nebulae Reflect light of nearby stars Appear blue
Interstellar Matter: Nursery of the Stars Bright nebulae Planetary nebulae Originate from dying stars Resemble giant planets
Interstellar Matter: Nursery of the Stars Dark nebulae Made of hydrogen (90%), helium, and interstellar dust Too distant from stars to be illuminated Appear as: Opaque objects against bright backgrounds Starless regions in space Contains the material that forms stars and planets
Classifying Stars: H-R Diagrams Hertzsprung-Russell Diagram Shows the relation between stellar brightness (absolute magnitude) and temperature Diagram is made by plotting each star s: Luminosity (brightness) and Temperature
Classifying Stars: H-R Diagrams Main-sequence stars 90% of all stars Band through the center of the H-R diagram Sun is in the main-sequence Giants (or red giants) Large and very luminous Upper-right on the H-R diagram Very large giants are called supergiants Only a few percent of all stars White dwarfs Fainter than main-sequence stars Small (approximate the size of Earth) Lower-central area on the H-R diagram Not all are white
Classifying Stars: H-R Diagrams
Stellar Evolution Stars exist because of gravity Two opposing forces in a star are: 1. Gravity: contracts 2. Thermal nuclear energy: expands Stages Birth In dark, cool, interstellar clouds Gravity contracts the cloud Temperature rises Becomes a protostar
Stellar Evolution Protostar Gravitational contraction of gasses continues Core reaches 10 million K Hydrogen nuclei fuse Become helium nuclei Process is called hydrogen fusion Energy is released Outward pressure balanced by gravity Star becomes a stable main-sequence star
Stellar Evolution Main-sequence stage Stars age at different rates Massive stars Use fuel faster Exist for only a few million years Small stars Use fuel slowly Exist for perhaps hundreds of billions of years 90% of a star s life is in the main-sequence
Stellar Evolution Red giant stage Hydrogen burning migrates outward Star s outer envelope expands Surface cools Surface becomes red Core collapses as helium converts to carbon Eventually all nuclear fuel is used Gravity squeezes the star
Stellar Evolution
Stellar Evolution Burnout and death Final stage depends on mass Low-mass star 0.5 solar mass Red giant collapses and becomes a white dwarf
Medium-mass star Between 0.5 and 3 solar masses Red giant collapses, planetary nebula forms, then becomes a white dwarf
Stellar Evolution Massive star Over 3 solar masses Terminates in a supernova Interior condenses and may produce a hot, dense object that is either a neutron star or a black hole
Stellar Remnants White dwarf Small (some no larger than Earth) Dense Can be more massive than the Sun Spoonful weighs several tons Atoms take up less space Electrons displaced inward Called degenerate matter Hot surface Cools to become a black dwarf
Stellar Remnants Neutron stars Forms from a more massive star Star has more gravity Squeezes itself smaller Remnant of a supernova Gravitational force collapses atoms Electrons combine with protons to produce neutrons Small size
Stellar Remnants Neutron stars Pea size sample Weighs 100 million tons Same density as an atomic nucleus Strong magnetic field First one discovered in early 1970s Pulsar (pulsating radio source) Found in the Crab Nebula (remnant of an A.D. 1054 supernova)
Stellar Remnants
Stellar Remnants Black holes More dense than neutron stars Intense surface gravity lets no light escape As matter is pulled into it Becomes very hot Emits x-rays Likely candidate is Cygnus X-1, a strong x- ray source
Stellar Remnants
Galaxies and Galactic Clusters Three basic types of galaxies 1. Spiral galaxy Arms extending from nucleus About 30 percent of all galaxies Large diameter of 20,000 to 125,000 light years Contains both young and old stars e.g., Milky Way
Milky Way galaxy Rotation Around the galactic nucleus Outermost stars move the slowest Sun rotates around the galactic nucleus once about every 200 million years Halo surrounds the galactic disk Spherical Very tenuous gas Numerous globular clusters
Galaxies and Galactic Clusters
Galaxies and Galactic Clusters
Galaxies and Galactic Clusters 2. Elliptical galaxy Ellipsoidal shape About 60% of all galaxies Most are smaller than spiral galaxies; however, they are also the largest known galaxies
Galaxies and Galactic Clusters 3.Irregular galaxy Lacks symmetry About 25% of all galaxies Contains mostly young stars e.g., Magellanic Clouds
Galaxies and Galactic Clusters Galactic cluster Group of galaxies Some contain thousands of galaxies Local Group Our own group of galaxies Contains > 40 galaxies Supercluster Huge swarm of galaxies May be the largest entity in the universe
Galaxies and Galactic Clusters
Galaxies and Galactic Clusters Galactic Interactions Collisions between galaxies Driven by one galaxy s gravity disturbing another A large galaxy may engulf a dwarf satellite galaxy Two dwarf satellite galaxies are currently merging with the Milky Way Two galaxies of similar size may pass through one another without merging Interstellar matter will likely interact Triggers an intense period of star formation In 2 to 4 billion yrs, 50% probability that Milky Way and Andromeda Galaxies will collide and merge
Galaxies and Galactic Clusters
The Big Bang Theory Big Bang Theory Describes the birth, evolution, and fate of the universe Universe was once confined to a ball that was: Supermassive Dense Hot About 13.8 billion years ago, universe began expanding rapidly in all directions
The Big Bang Theory Doppler effect Change in wavelength due to motion Movement away stretches the wavelength Longer wavelength Light appears redder Movement toward squeezes the wavelength Shorter wavelength Light shifted toward the blue
The Big Bang Theory
The Big Bang Theory Doppler effect Amount of the Doppler shift indicates the rate of movement Large Doppler shift indicates a high velocity Small Doppler shift indicates a lower velocity
The Big Bang Theory Most galaxies exhibit a red Doppler shift Far galaxies Exhibit the greatest shift Greater velocity Discovered in 1929 by Edwin Hubble Hubble s Law Recessional speed of galaxies is proportional to their distance Accounts for red shifts
The Big Bang Theory
The Big Bang Theory Big Bang Theory Accounts for galaxies moving away from us Universe was once confined to a ball that was: Supermassive Dense Hot
Predictions of the Big Bang Theory If the universe is unimaginably hot, then researchers should be able to detect the remnant of that heat Continued expansion of the universe would stretch the waves so by now they are detectable as long-wavelength radio waves Cosmic background radiation Detected in 1965
The Big Bang Theory Big Bang marks the inception of the universe Occurred about 15 billion years ago All matter and space was created Matter is moving outward Fate of the universe?
Two possibilities: Big chill Stars slowly burn out Replaced by invisible degenerate matter and black holes» Travel outwards through an endless, dark, cold universe Big crunch Outward flight of galaxies slows and eventually stops Gravitational contraction causes all matter to collide and coalesce into high-energy, high-density state, from which the universe began
Two other constituents complicate the fate of the universe: Dark matter One quarter of the universe Produces no detectable light energy Exerts a force much like gravity Dark energy Exerts a force that pushes matter outward Though to be the dominant force in behind the fate of the universe» Predicts universe will expand forever
The Big Bang Theory Fate of the universe Final fate depends on density of the universe If density is more than the critical density, universe will contract So, What is Dark Matter and Dark Energy Current estimates are less than critical density Predicts ever-expanding,or open, universe But Absence of evidence is not evidence of absence - Carl Sagan