Presentation on theme: "Chapter 21 STARS. Characteristics of Stars Groups of stars that form patterns in the sky are called constellations –Example: Ursa Major (Big Bear), Ursa."— Presentation transcript:
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!!!!
Here is a time-lapse photo of circumpolar star movement…
The positions of the constellations as viewed from Earth changes from season 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
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 Physical Properties of Stars
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)
Apparent Magnitude 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 not tell 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 fission takes 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
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…
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 STELLAR NURSERY STARS FORM IN A NEBULA OF GAS & DUST SUN-LIKE STARS (UP TO 1.5 X MASS OF OUR SUN) RED GIANTWHITE DWARF BLACK DWARF MASSIVE STARS (1.5 – 3 X OUR SUN) SUPERMASSIVE STARS > 3 X OUR SUN RED SUPERGIANT SUPERNOVA NEUTRON STAR BLACKHOLE PLANETARY NEBULA (NOVA)
DEATH OF A SUN-LIKE STAR SUN-LIKE STAR RED GIANTNEBULAWHITE DWARF BLACK DWARF LONGEST, MOST STABLE PERIOD OF A STAR’S LIFE – CONVERTS HYDROGEN TO HELIUM, RADIATING HEAT & LIGHT NUCLEAR FUEL DEPLETES, CORE CONTRACTS, SHELL EXPANDS OUTER LAYERS DRIFT OFF INTO SPACE IN SPHERE- LIKE PATTERN STAR COOLS ARE SHRINKS BECOMING ONLY A FEW THOUSAND MILES ACROSS! NO NUCLEAR REACTION STAR LOSES ALL HEAT TO SPACE AND BECOMES COLD AND DARK CARBON BALL
GIANTS/SUPERGIANTS the brightest & largest kind of star luminosities of 10,000 to 100,000 radii of 20 to several hundred solar radii (they are about the size of Jupiter's orbit!!!!) two types are red supergiants (Betelgeuse and Antares) and blue supergiants (Rigel)
Betelgeuse a red supergiant, with about 20 times the mass and 800 times the radius of the Sun, so huge that it could easily contain the orbits of Mercury, Venus, Earth, Mars & Jupiter. It will probably explode as a supernova at some point within the next 100,000 years. Even at its relatively remote distance, it normally ranks as the tenth brightest star in the sky. Rigel, a blue supergiant, has a diameter of about 100 million kilometers, some seventy times that of the Sun. Within a few million years, it will probably evolve to become a red supergiant like its neighbor in Orion (though not in physical space), Betelgeuse.
Dwarf Stars A term used, oddly enough, to describe any star that is of normal size for its mass The Sun, for example, is classified as a yellow dwarf In general, dwarf stars lie on the main sequence and are in the process of converting hydrogen to helium by nuclear fusion in their cores
White Dwarfs A medium sized star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size Typically part of a planetary nebula Eventually cools into a BLACK dwarf (lump of carbon) –This takes BILLIONS of years! –This is the fate of OUR SUN!
Neutron Star The imploded core of a massive star produced by a supernova explosion The most dense known objects in the universe! –A sugar cube size of neutron star material weighs 100 million tons!!!!!!!
3,700 LY wide dust-disk encircling a 300 million solar mass blackhole in the center of an elliptical galaxy. The disk is a remnant of an ancient galaxy collision and could be “swallowed” up by the blackhole in a few billion years.
Big Bang Theory The Big Bang Theory is the dominant scientific theory about the origin of the universe According to the big bang, the universe was created sometime between 10 billion and 20 billion years ago from a cosmic explosion that hurled matter and in all directions
Galaxy Formation The formation of all the galaxies is explained by the Big Bang Theory Simply put, it states that the universe was a big ball of hydrogen gas that exploded outward The expanding cloud had areas that condensed into galaxies that are still expanding out from the center (the universe is getting larger) –We can see this via RED SHIFT!
Galaxies system containing millions to billions of stars –Ex. the Milky Way galaxy contains over 500,000 million stars Milky Way galaxy is a spiral shaped galaxy with a large central cluster of stars, and thinner “arms” radiating out from the center –The solar system is located on one of the arms of the Milky Way galaxy about 2/3 away from the center
Origin of the Milky Way Formed 10-12 billion years ago Possibly collided with smaller galaxies Globular star clusters formed Stars and solar systems formed roughly 5 billion years ago