The Earth and the Universe Beyond our Solar System

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

Parallax is larger for closer objects Image from: http://csep10.phys.utk.edu/astr161/lect/retrograde/copernican.html

Properties of stars Distance 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)

Properties of stars Stellar brightness Controlled by three factors Size Temperature Distance

Properties of stars Stellar brightness Magnitude - Measure of a star's brightness 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 - Measure of a star's brightness Two types of measurement 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

Properties of stars Color and temperature Hot star Temperature above 30,000 K Emits short-wavelength light Appears blue 0°C = 273 K

Properties of stars Color and temperature Cool star Temperature less than 3000 K Emits longer-wavelength light Appears red

Properties of stars Color and temperature Between 5000 and 6000 K Stars appear yellow e.g., Sun

Properties of stars 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 More than 50% of the stars in the universe are binary stars

Binary stars orbit each other around their common center of mass

Properties of stars Binary stars and 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

Hertzsprung-Russell diagram Shows the relation between stellar Brightness (absolute magnitude) and Temperature Diagram is made by plotting (graphing) each star's Luminosity (brightness) and Temperature

Variable stars Stars that fluctuate in brightness Types of variable stars Pulsating variables Fluctuate regularly in brightness Expand and contract in size Eruptive variables Explosive event Sudden brightening Called a nova

Interstellar matter Between the stars is "the vacuum of space" Nebula Cloud of dust and gases Two major types of nebulae Bright nebula Glows if it close to a very hot star Two types of bright nebulae Emission nebula Reflection nebula

The Orion Nebula is a well-known emission nebula

A faint blue reflection nebula in the Pleiades star cluster

Interstellar matter Nebula Two major types of nebulae Dark nebula Not close to any bright star Appear dark Contains the material that forms stars and planets

Stellar evolution Stars exist because of gravity Two opposing forces in a star are Gravity – contracts Thermal nuclear energy – expands

Stellar evolution Stages Birth In dark, cool, interstellar clouds Gravity contracts the cloud Temperature rises Radiates long-wavelength (red) light Becomes a protostar

Stellar evolution Stages Protostar Gravitational contraction of gaseous cloud continues Core reaches 10 million K Hydrogen nuclei fuse Become helium nuclei Process is called hydrogen burning Energy is released Outward pressure increases Outward pressure balanced by gravity pulling in Star becomes a stable main-sequence star

Stellar evolution Stages Main-sequence stage Stars age at different rates Massive stars use fuel faster and exist for only a few million year Small stars use fuel slowly and exist for perhaps hundreds of billions of years 90% of a star's life is in the main-sequence

Stellar evolution Stages Red giant stage Hydrogen burning migrates outward Star's outer envelope expands Surface cools Surface becomes red Core is collapsing as helium is converted to carbon Eventually all nuclear fuel is used Gravity squeezes the star

Stellar evolution Stages Burnout and death Final stage depends on mass Possibilities:

H-R diagram showing stellar evolution

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 star 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 star Pea size sample Strong magnetic field 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)

Crab Nebula in the constellation Taurus

Stellar remnants Black hole More dense than a neutron star 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

Galaxies Milky Way galaxy Structure Determined by using radio telescopes Large spiral galaxy About 100,000 light-years wide Thickness at the galactic nucleus is about 10,000 light-years Three spiral arms of stars Sun is 30,000 light-years from the center

Face-on view of the Milk Way Galaxy

Edge-on view of the Milk Way Galaxy

Galaxies 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

Galaxies Milky Way galaxy Halo surrounds the galactic disk Spherical Very tenuous gas Numerous globular clusters

Galaxies Other galaxies Existence was first proposed in mid-1700s by Immanuel Kant Four basic types of galaxies Spiral galaxy Arms extending from nucleus About 30% of all galaxies Large diameter of 20,000 to 125,000 light years Contains both young and old stars e.g., Milky Way

Great Galaxy, a spiral galaxy, in the constellation Andromeda

Galaxies Other galaxies Four basic types of galaxies Barred spiral galaxy Stars arranged in the shape of a bar Generally quite large About 10% of all galaxies

A barred spiral galaxy

Galaxies Other galaxies Four basic types of galaxies Elliptical galaxy Ellipsoidal shape About 60% of all galaxies Most are smaller than spiral galaxies; however, they are also the largest known galaxies

Galaxies Other galaxies Four basic types of galaxies Irregular galaxy Lacks symmetry About 10% of all galaxies Contains mostly young stars e.g., Magellanic Clouds

Galaxies Galactic cluster Group of galaxies Some contain thousands of galaxies Local Group Our own group of galaxies Contains at least 28 galaxies Supercluster Huge swarm of galaxies May be the largest entity in the universe

Red shifts Doppler effect Change in the wavelength of light emitted by an object due to its motion Movement away stretches the wavelength Longer wavelength Light appears redder Movement toward “squeezes” the wavelength Shorter wavelength Light shifted toward the blue

Red shifts 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

Raisin bread analogy of an expanding universe

Red shifts Expanding universe Most galaxies exhibit a red Doppler shift Far galaxies Exhibit the greatest shift Greater velocity Discovered in 1929 by Edwin Hubble Hubble's Law – the recessional speed of galaxies is proportional to their distance Accounts for red shifts

Big Bang theory Accounts for galaxies moving away from us Universe was once confined to a "ball" that was Supermassive Dense Hot

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 Universe will last forever Outward expansion sill stop and gravitational; contraction will follow

Big Bang theory Fate of the universe Final fate depends on the average density of the universe If the density is more than the critical density, then the universe would contract Current estimates point to less then the critical density and predict an ever-expanding, or open, universe