Beyond Our Solar System Paula Messina 1. Star Types 2. Stellar Evolution 3. Galaxies 4. The Universe 1. Star Types 1

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1 Beyond Our Solar System Paula Messina 1. Star Types 2. Stellar Evolution 3. Galaxies 4. The Universe 1. Star Types 1

2 Stellar Distances The Sun is 93 million miles (1 Astronomical Unit) from Earth Traveling at 186,000 miles per second (300,000 km/sec) solar energy takes 8 minutes to arrive at Earth 93 million miles This distance can also be called 8 light-minutes What is a light-year? A light-year is a measure of distance (not a measure of time!) A light-year is the distance light can travel in a year Traveling at 186,000 miles per sec. (300,000 km/s), light can travel 5.8 trillion miles (9.5 trillion kilometers) in one year Proxima Centauri The closest star to the Sun is Proxima Centauri Proxima Centauri is part of a system that contains three stars: Proxima Centauri Alpha Centauri A (Rigil Kentaurus) Alpha Centauri B The closest star system, Alpha Centauri 2

3 Proxima Centauri We are about 4.2 light-years from the Alpha Centauri star system That s 24,360,000,000,000 miles! 1 light -year 1 light -year 1 light -year 1 light -year 0.2 light -year 5.8 trillion miles 5.8 trillion miles 5.8 trillion miles 5.8 trillion miles 1.2 trillion miles Distance and Brightness Of the three stars in the Alpha Centauri system, Proxima Centauri is the dimmest A star s brightness, as viewed from Earth, is a function of: Its distance to the observer Its absolute energy-output This is known as its absolute magnitude Apparent Magnitude Apparent magnitude is a measure of how bright a star appears to an observer on Earth Stars with great absolute magnitudes may appear brighter to us, even though they may be farther away than other stars. A star s perceived brightness increases as apparent magnitude quantity decreases (see next slide). 3

4 Apparent Magnitude of the Brightest Stars, as Viewed from Earth Rank Name Apparent Absolute Distance Magnitude Magnitude (light-year) 0 Sun Sirius Canopus Rigil Kentaurus Arcturus Vega Capella Rigel Procyon Achernar Hadar Our Closest Stellar Neighbors Name Distance Apparent Absolute (light-years) Magnitude Magnitude Sun Proxima Centauri 4.2? 15.5 Rigil Kentaurus Alpha Centauri- B Barnard's Star Wolf Lalande Sirius A Sirius B Luyten 726-8A How Do We Know a Star s Distance? Stellar Parallax: A very slight apparent shift in a star s position, as viewed from different vantage points Angle B Baseline Angle A 4

5 Stellar Color and Temperature Higher-temperature stars generally emit shorter wavelengths of light To review: Wavelength Violet Hot stars nanometers Blue nanometers Green nanometers Yellow The Sun nanometers Orange nanometers Red Cool stars nanometers Sagittarius Star Cloud Notice the variation of stellar colors, as imaged by the Hubble Space Telescope Spectral class H-R Diagram Absolute magnitude Main Sequence White dwarfs Supergiants You are here Giants Luminosity (Sun =1) The Sun is a mediumsized, midtemperature main sequence star Ho-hum Surface temperature 5

6 Odd-ball Stars Variable stars emit energy of variable intensity Some pulsate in regular, or irregular patterns Others are eruptive variables, that seem to brighten abruptly Pulsating Variables Example: Cepheid variables Eruptive Variables Hubble Space Telescope image of Cygnus Nova,

7 Interstellar Matter: Nebulae Two types of nebulae: 1. Bright nebula: Interstellar matter close to a hot, blue star A. Emission nebula B. Reflection nebula 2. Dark Nebula: Dark clouds silhouetted against a bright (starry) background Bright (Emission) Nebulae Example: The Orion Nebula Clouds of gases, mostly hydrogen They absorb ultraviolet light from a nearby blue star They re-radiate this energy as visible light Bright (Reflection) Nebulae 5 light-years Composed of particles of interstellar dust These clouds reflect light of nearby stars Example: M78 Nebula 7

8 Dark Nebulae Examples: Horsehead Nebula Molecular Cloud Barnard 68 Interstellar material, too far from a star to emit light, or to be illuminated 2. Stellar Evolution How Do Stars Change Over Time? Stellar Birth Protostar Stage Main-Sequence Stage Red Giant Stage Burnout and Death 8

9 Stellar Birth Gases within nebulae concentrate; Temperature rises to the point that (long wavelength) red light may be emitted; This contraction takes about 1 million years Protostar Stage Gravitational contractions continue; Jets of gases stream from the protostar s north and south poles, at about miles per second; The protostar burns hydrogen ; In 100,000 3,000,000 years, the jets turn off Main Sequence Stage Internal pressure = gravitational force; Large, hot, blue stars deplete their hydrogen supply in just a few million years; cooler, smaller, red stars may exist for hundreds of billions of years; Yellow stars exist for about 10 billion years; Some main-sequence stars, and all small (red) stars eventually run out of hydrogen, and die. But other stars may burn other fuels, to become (next slide) 9

10 Red Giant Stage I Hydrogen is depleted at the core, but fusion may still take place in the star s outer surface; The core grows hotter (gravitational energy is converted to heat energy); This inner heat causes the star to expand; The surface cools (emitting cooler, red light); The core continues to collapse; (continued) Betelgeuse, a Red Supergiant Red Giant Stage II When internal temperatures reach 100,000,000 K, the star begins to convert helium to carbon; Very massive stars may generate all elements, up to Atomic Number 26 (Iron) in thermonuclear reactions. Burnout and Death Dependent on the mass of the star Next three slides: 1. The death of low mass stars 2. The death of medium-mass stars 3. The death of massive stars 10

11 The Death of a Low-Mass Star Small, cool, red stars consume their supplies of hydrogen; They collapse to form hot, white dwarfs White dwarfs are about the size of Earth The Death of a Medium- Mass Star (like the Sun) Blinking Eye Nebula Like low-mass stars, medium-mass stars collapse into white dwarfs; During their collapse from red giant to white dwarf, these stars discard their outer atmospheres; These spherical clouds, called planetary nebulae, glow from the energy emitted by the small central white dwarf. Cat s Eye Nebula G , a young supernova remnant, may be the result of a supernova event about 1600 years ago. The Death of a Massive Star The massive star runs out of fuel, producing less heat; The gravity exceeds the outward force due to gas pressure; The star implodes, causing a shock wave to move out from the star s interior; Remnants of the star s interior may condense to form neutron stars, or black holes. 11

12 Massive Star Remnants I A possible neutron star, embedded in the supernova remnant IC443 Neutron stars: About 20 km. across; Rotation increases as diameter decreases; About 20 km. across; More massive than the Sun; A pea-sized sample of this material would weigh about 100 million tons; Some of these stars emit radio beacons that radiate from the stellar poles; Such radio sources appear to pulsate as they spin. These are known as pulsars. Massive Star Remnants II Pulsars At the heart of the Crab Nebula, the remnant of a supernova that occurred in the year 1054, is a rotating neutron star, or pulsar. Pulsar PSR B Massive Star Remnants III Black holes Smaller than neutron stars More dense than neutron stars A theoretical drawing of the possible black hole, at the center of galaxy MCG Gravitational field is so great that no matter and no energy would be able to escape 12

13 Figure Tarbuck/Lutgens Stellar evolution in a nutshell 3. Galaxies Galaxies Galaxies are concentrations of stars. Types of Galaxies: 1. Spiral (like the Milky Way) 2. Barred Spiral 3. Elliptical 4. Irregular 13

14 The Milky Way: Our Spiral Galaxy The Milky Way contains about 200 billion stars, their planets, and thousands of nebulae. The Milky Way is approximately 100,000 lightyears in diameter. Our solar system is 26,000 light-years from the center of the Galaxy. All objects in the Galaxy revolve around the Galaxy's center. It takes 250 million years for our Sun to pull us through one revolution around the center of the Milky Way. Spiral Galaxies (cont.) M31, or the Andromeda Galaxy, is the most distant object visible with the naked eye. Andromeda, a spiral galaxy, is 200 million light-years from Earth. About 20% of all galaxies are regular spiral galaxies Barred Spiral Galaxies NGC1300, a barred spiral galaxy, is about 75 million light-years away; it is about 150,000 light-years across A rigid bar of stars rotates around the galactic center; About 10% of all galaxies are barred spirals. 14

15 Elliptical Galaxies Generally smaller than spiral galaxies; About 60% of all galaxies are elliptical Irregular Galaxies Irregular galaxies lack symmetry; They account for only 10% of the total number of galaxies; The Large Magellanic Cloud (LMC, left) is a companion galaxy to the Milky Way, and is visible from the Southern Hemisphere without a telescope 4. The Universe 15

16 The Universe... All stars and planets; all galaxies; every molecule of hydrogen; every atom of carbon; you; me; all nebulae; every neutron star; every black hole (if they exist); a lot of things we haven t discovered yet; and a heck of a lot of empty space. Unusual Spectra If the two most abundant elements in the universe are hydrogen and helium...??????...then why does most of the universe appear spectrally like this? Doppler Shift Light waves lengthen as they move away from an observer Light waves become compressed when they move towards an observer The resulting change in the wavelengths of light, as viewed from an advancing or retreating object, is known as the object s Doppler Shift. 16

17 Doppler Shifted Spectra Red-shifted hydrogen Red-shifted helium Most stars show evidence of redshifting. This implies that most objects in the universe are moving apart from one another If Everything is Moving Outward could this be evidence of an explosion? Did the universe begin as a result of a big bang? The Beginning of the Universe...heralds the end of this course! See you at the final! 17