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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

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characteristics of 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
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
slide5
VIF

The apparent motion of stars is due to the Earth’s daily rotation on its axis.

The stars don’t move – WE DO!!!!

slide8
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
slide9

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
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 stars1
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
slide16
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
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
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
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
slide23
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!
slide24
So…

One Astronomical Unit (AU) = 150,000,000 km

And, one light year (LY)= 9.5 trillion km(9,500,000,000,000 km)

slide26

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

slide28
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
slide29
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
supernovas
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
slide33

Etna Carinae

Supernova remnant

Supernova rings

slide35

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
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…

slide37

ORION NEBULA

VEIL NEBULA

HELIX NEBULA

KEYHOLE NEBULA

slide39

CARINA NEBULA

8000 LY FROM EARTH – 200 LY ACROSS

slide40

ESKIMO NEBULA

5000 LY FROM EARTH - 10,000 YRS OLD

slide41

EAGLE NEBULA

7,000 LY AWAY FROM EARTH

slide42

STINGRAY NEBULA

YOUNGEST KNOWN NEBULA – 130 SOLAR SYSTEMS ACROSS – 18,000 LY AWAY FROM EARTH

a star is born
A STAR IS BORN…

I WANT MY MOMMY!!

life cycle of stars
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
slide49

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

slide50

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

giants supergiants
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)
slide52

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
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
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
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!!!!!!!
black holes
BLACK HOLES
  • A supermassive star that undergoes supernova and the core is when the star is “swallowed” by its own gravity
  • readily attracts any matter and energy that comes near it - including light…
slide60

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
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
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
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 shapedgalaxy 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
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
slide71

Ursa Major’s galaxy (a galaxy that we can see within the constellation) –

50 million LY from earth!

slide75

Hoag’s Galaxy – 120,000 LY wide – perfect ring of blue stars surrounding older nucleus of yellow stars