Chapter 21 STARS
Characteristics of Stars • Groups of stars that form patterns in the sky are called constellations • Example: Ursa Major (Big Bear), Ursa Minor (Little Bear), and Orion • The last two stars in Ursa Major’s “dipper” are called the “Pointer Stars” and are be used to find Polaris (North Star) • Polaris is located directly above the North Pole (90º N), and is only visible in the northern hemisphere (above the Equator)
Circumpolar Constellations • Because of the Earth’s rotation, the constellations appear to move • If the constellations 1) appear to move around Polaris and 2) can be seen at all times of year and 3) can be seen at all times of night, they are called circumpolar constellations • The constellations Ursa Major and Ursa Minor are both circumpolar constellations • Using time exposure photography, the apparent motion of the stars around Polaris can be recorded as circular trails
VIF The apparent motion of stars is due to the Earth’s daily rotation on its axis. The stars don’t move – WE DO!!!!
The positions of the constellations as viewed from Earth changes fromseason to season • This is caused by the revolution of the Earth and the change in Earth’s position in its orbit around the sun • Example: Orion the Hunter is a winter constellation
Ex – when the Earth is in this position (Nov 21), the bright sun during the day blocks our view of all of the constellations toward the lower right side of the diagram
Physical Properties of Stars • Stars differ in size, density, mass, composition, and color • The color of a star is determined by it surface temperature (ESRT’s P. 15 top) • The hotter the star, the bluer the color. The cooler the star, the redder the color. (yeah, yeah, I know, it’s backwards….) • The sun is an AVERAGE SIZE, medium, yellow star
Physical Properties of Stars • Most stars are made up of mostly hydrogen and helium (approx. 98%) • The remaining 2% may be other elements • A spectral analysis (remember Ch. 20) of the star can tell us what elements a star is made of, since the radiated spectrum depends on a star’s composition and temperature
Some stars may appear to be brighter than others • The star’s brightness may be described in three ways 1. APPARENT MAGNITUDE 2. LUMINOSITY 3. ABSOLUTE MAGNITUDE (See the H-R Diagram in the ESRT’s P.15)
ApparentMagnitude • How bright a star appears (apparent) to us on Earth • The farther a star is from Earth (increasing distance), the dimmer it will look even though it may actually be a very bright star • Because of this, apparent magnitude does nottell the true brightness of a star
Luminosity • The actual (true) brightness of the star • Depends on the size and temperature of the star • Hotter stars are more luminous (brighter) than cooler stars • If the temperatures are the same, a larger star will be more luminous
Absolute Magnitude • The luminosity of the stars if they all brought to the same distance from Earth • aka – picture all the stars lined up the same distance from Earth, then compare their brightness • This is the most useful when comparing the brightness of the stars
The sun is the closest star to Earth • It is approx. 150,000,000 km (93,000,000 miles) from the Earth • This distance is called an astronomical unit (AU) • The next closest star to Earth, after the sun, is Proxima Centauri • It is 300,000 times farther away from Earth than the sun. Because of the great distances in space, larger units of measure must be used • The light-year is the distance that light travels in one year • Since light can travel 300,000 km/sec (186,000 miles/sec), light travels 9.5 trillion km/year!!! • Proxima Centuri is 4.3 light-years from Earth!
So… One Astronomical Unit (AU) = 150,000,000 km And, one light year (LY)= 9.5 trillion km(9,500,000,000,000 km)
Okay… let’s calculate the distances from Earth to each planet in Astronomical Units (AU)
Remember – 1 AU = 150,000,000 km Just divide the distance from the Sun in km by 150,000,000 km. Example: Jupiter = 778,300,000 km 150,000,000 km Jupiter is 5.19 AU from the Sun
large clouds of dust and gas in space are the basic materials needed for star formation • the majority of this gas is hydrogen • some outside force causes the cloud of gas and dust to be pushed together • as the gas and dust get closer, friction between the particles causes the temperature to increase • the attraction of gravity between the particles causes them to continue to move together, and density also increases
friction increases and temperature increases until the center becomes so hot that nuclear fusion takes place • hydrogen atoms are forced together to form helium atoms, and a tremendous amount of energy is released • In a nuclear reactor like Indian Point, nuclear fissiontakes place • This is when radioactive atoms are split apart to release energy
OK, so stars form from hydrogen gas and dust, but where does that gas & dust come from????
SUPERNOVAS • One of the most energetic explosive events occur at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy • If the star is particularly massive, then its core will collapse and in so doing will release a huge amount of energy • This will cause a blast wave that ejects the star's gas envelope into interstellar space
Etna Carinae Supernova remnant Supernova rings
SUPERNOVA 1987 – right image is the star that became the left image after going supernova – shone brighter than most galaxies for a few months!
NEBULAE • Clouds of dust & gas (supernova remnants?) • 2 Main Types: • Diffuse Nebula – nearby star illuminates the gas/dust cloud • Dark Nebula – Dark patch against more-distant stars (dust/gas is blocking the light from stars behind it) Here are some images of nebulae, courtesy of our friend Hubble…
ORION NEBULA VEIL NEBULA HELIX NEBULA KEYHOLE NEBULA
CARINA NEBULA 8000 LY FROM EARTH – 200 LY ACROSS
ESKIMO NEBULA 5000 LY FROM EARTH - 10,000 YRS OLD
EAGLE NEBULA 7,000 LY AWAY FROM EARTH
STINGRAY NEBULA YOUNGEST KNOWN NEBULA – 130 SOLAR SYSTEMS ACROSS – 18,000 LY AWAY FROM EARTH
A STAR IS BORN… I WANT MY MOMMY!!
LIFE CYCLE OF STARS • VIF!!!! - A star’s life cycle is determined by its MASS • The larger the star, the faster it burns out! • A star’s MASS is determined by the MATTER available in the nebula of formation
LIFE CYCLE OF STARS SUN-LIKE STARS (UP TO 1.5 X MASS OF OUR SUN) RED GIANT PLANETARY NEBULA(NOVA) WHITE DWARF BLACK DWARF STELLAR NURSERY MASSIVE STARS (1.5 – 3 X OUR SUN) RED SUPERGIANT SUPERNOVA NEUTRON STAR STARS FORM IN A NEBULA OF GAS & DUST SUPERMASSIVE STARS > 3 X OUR SUN RED SUPERGIANT SUPERNOVA BLACKHOLE
DEATH OF A SUN-LIKE STAR SUN-LIKE STAR RED GIANT NEBULA WHITE DWARF BLACK DWARF STAR COOLS ARE SHRINKS BECOMING ONLY A FEW THOUSAND MILES ACROSS! NO NUCLEAR REACTION LONGEST, MOST STABLE PERIOD OF A STAR’S LIFE – CONVERTS HYDROGEN TO HELIUM, RADIATING HEAT & LIGHT STAR LOSES ALL HEAT TO SPACE AND BECOMES COLD AND DARK CARBON BALL NUCLEAR FUEL DEPLETES, CORE CONTRACTS, SHELL EXPANDS OUTER LAYERS DRIFT OFF INTO SPACE IN SPHERE-LIKE PATTERN