NATS 1750, Sec. A, Jan. 12, 2004 Announcements:
website for the course http://www.students.yorku.ca/~rossb/
the publishers of the textbook have a website: www.prenhall.com/tarbuck Dec. midterm make-up, Fri. Jan 16, 2:30 to 4:30 room 103, Petrie Sci. Eng. Bldg
Agenda, (chapter 23 of Earth Science, 10th edition)
Wrap up “the big picture”. Big Bang ->dark nebula, (precursor to solar system formation) • • • • • • • • properties of stars, H-R diagram, inter-stellar matter, Star evolution, Stellar remnants, Supernova (“stardust to you”) redshift, Hubble flow, fate of Universe next lecture, nebular hypothesis and members of our Solar System
properties of stars
• Distance - parsec [parallax arc second]
• Used for measuring distance to nearby stars • Apparent shift in a star's position due to the orbital motion of Earth Extremely distant objects, DIAGRAM IS assumed infinitely far away NOT TO SCALE Solar system A nearby star
Apparent motion
2 A.U.
2 times the parallax angle If the parallax angle is 1”, the distance is 1 parsec ” is the symbol for 1 second of arc, 60*60 ” = 1 degree
properties of stars
• Distance - parsec [parallax arc second]
– – – – closer objects subtend a larger parallax angle than distant ones a parsec is about 3.26 ly (Lightyear) a ly is the distance light travels in one year (3*105 km/s)*(number of seconds in a year)
properties of stars
• colour and temperature (of surface)
– hot stars are blue, 30,000 Kelvin – coolest stars are red 3,000 K, – our Sun is yellow, ~5,000 K
• most solar systems have multiple stars
– such as binary systems (2)
• more massive stars “burn faster”, die sooner than smaller mass stars
properties of stars
• Brightness – Controlled by three factors
• Size • Temperature • Distance – apparent magnitude (how bright as seen from Earth) – absolute magnitude (actual brightness - placed at a common distance from Earth, 32.6 ly)
Hertzsprung-Russell diagram
Shows the relation between stellar
• Brightness and • Temperature
Diagram is made by graphing each star's
• Luminosity (absolute magnitude) and • Temperature (of surface, or colour)
Hertzsprung-Russell diagram
Some parts of an H-R diagram • Main-sequence stars
• 90% of all stars • Band through the center of the H-R diagram • Sun is in the main-sequence
• Super Redgiants
• • • • • Very luminous Very large Upper-right on the H-R diagram “die” in supernova explosions seeds the interstellar medium for future star birth
Idealized Hertzsprung-Russell diagram
Interstellar matter
Nebula
• Cloud of dust and gases • Two major types of nebulae
• Bright nebula
• Glows - close to a very hot star
• Dark nebula
• Not close to any bright star • Appear dark • Contains the material that forms stars and planets • this material can gravitionally collapse and form a solar system
Stellar evolution
Stars exist because of gravity Two opposing balancing forces in a star are
• Gravity – contracts • Thermal nuclear energy – expands
Stages
• Birth
• • • • • In dark, cool, interstellar clouds Gravity contracts the cloud Temperature rises Radiates long-wavelength (red) light Becomes a protostar
Stellar evolution
Protostar
• Gravitational contraction of gaseous cloud continues • Core reaches 10 million Kelvin, the star “ignites” • 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
Evolutionary stages of massive stars ( >3 Solar masses)
Red Supergiant
• Pre-collapse • Onion-like
“blows up” once all “fuel” is used up
Supernovae
• One of the most energetic events in Universe • “burning” in stars is a thermonuclear reation
FUSION (light atoms “join” to make heavier atoms and there is a release of massive amounts of energy)
• this energy balances with gravity • Once all the fuel is used up (Fe collects in the core), gravity causes the star to collapse. • The massive explosion is extremely hot • so hot that heavier (than Fe) elements are generated from collisions.
Stellar remnants
White dwarfs (left over from a medium mass) • 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
• 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
Black hole
• More dense than a neutron star • Intense surface gravity lets no light escape (Event Horizon) • 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 MilkyWay Galaxy
Galaxies
Other galaxies
• Existence was first proposed in mid-1700s by Immanuel Kant • Four basic types of galaxies
• • • • Spiral galaxy Barred spiral galaxy Elliptical galaxy Irregular galaxy
• Distant galaxies are receding from us (in all directions)
Red shifts
When sources (eg. sound or light) move toward (or away) from us, the sound or light wave is compressed (or stretched) to the observer. The tone of a car appoarching us has a higher pitch than it does after passing us. After passing and moving away from us the tone is lower (the frequency is lower), this is called a redshift doppler effect.
Observer (Earth) and source (distant galaxy) are at rest, the colour you see is the colour of the source Observer is at rest, but source is moving if source is receding, colour appears redder
bluer if source is appoaching
From Astronomy Today, Chaisson and McMillan
Red shifts
Amount of the Doppler shift indicates the rate of movement
we know the true colour (of the stars and the galaxy) • Large Doppler shift indicates a high velocity • Small Doppler shift indicates a lower velocity
Expanding universe
• Most galaxies exhibit a red Doppler shift
• All distant galaxies are moving away from us
Raisin bread analogy of an expanding universe
Red shifts
Expanding universe (evidence for Big Bang)
• Most galaxies (not nearby) exhibit a red Doppler shift
• most distant galaxies • Exhibit the greatest red shift • Greater velocity • Discovered in 1929 by Edwin Hubble • Hubble's Law – the recessional speed of galaxies is proportional to their distance • trace them back to get an age of the Universe
Big Bang theory
Big Bang marks the inception of the universe
• Occurred about 15 billion years ago • All matter and space was created
Matter and space is moving outward Fate of the universe
• Two possibilities
• Universe will last forever • Outward expansion will 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 • (the gravitational force between all the mass in Universe is large enough to halt the expansion) • Current estimates point to less then the critical density and predict an ever-expanding, or open, universe
Next lecture
• Nebular hypothesis of Solar System formation • our Solar System • planets and minor members