Chapter 21 Notes:

Unit I: Stars and Their Characteristics

• A Constellation is a group of stars that appear to form a pattern in the sky
• We can see 88 constellations from Earth!

• Ursa Major

- contains the Big Dipper

• Ursa Minor

• contains the Little Dipper; Polaris

• Cassiopeia
• Lyra (the harp)
• Aquila (the eagle)
• Cygnus (the swan)
• Orion (the hunter)
• Taurus (the bull)
• Many, many others

• These are examples of circumpolar constellations = seen at all times from the northern hemisphere.
• These constellations all seem to revolve around Polaris (the North Star)…
• Because Polaris is above the north axis, it appears to never move in the sky.
• Stars revolve around Polaris in a counterclockwise direction.
• Some constellations can be seen only during certain seasons.
• Lyra = visible in summer / Orion = visible in winter

Distances to Stars

• Distance - Sun to earth is 150 mil. Km
• This distance is defined as an: Astronomical Unit (AU)
• If earth were 1cm from the sun, our nearest star would be 1.5 miles away!
• Alpha Centauri = 40 trillion Km away
• "Light Year" was developed to express such extreme distances…the distance light travels in one earth year > about 9.5 trillion Km per year!!!
• Alpha Centauri is about 4.3 Ly away!

• If Alpha Centauri exploded today…we would not see the explosion for 4.3 years!

Physical Properties of Stars

• Our Sun is average in every way
• Sun's diameter = 1,380,000 km across
• Sun's mass = 300,000x more than earth

Other stars:

• Some smaller than earth!
• Some 2000x bigger than our sun!
• Betelgeuse is 1/ten millionth as dense
• Sirius is so dense that 1 teaspoonful of material would weigh a ton on earth!
• Most stars are similar to our sun in mass
• Super hot stars are blue; cooler stars are red….our sun is yellow (medium in temp.)
• Red star = 3000 C; Blue star = 30000 C
• Our sun is about 5500 C

• Our sun is about 70% hydrogen and 28% helium; remainder is Ca, Na, N, C, O

Star's Brightness

• Apparent Magnitude = How bright the star appears to us from here on earth.
• Brightest are 1^st magnitude stars and the faintest is a 6^th magnitude star
• 1^st mag. star is 2.5x brighter than a 2^nd
• 1^st mag. star is 100x brighter than 6^th
• Luminosity = the actual or true brightness of a star.
• Luminosity depends on size and temperature of each star.
• Absolute Magnitude = expressing the luminosity of stars as if they were seen from the same distance.

Unit II: Kinds of Stars

• Giants
• Supergiants (100's times more luminous)

- Red supergiants are the largest of all!!!

• Dwarfs (usually faint stars; very small)

• White dwarf is about the size of earth!
• White dwarf is 100,000x more dense.

Variable Stars – stars that vary in brightness over a few days or weeks

• Cepheid variables – stars that contract and expand, thus changing brightness. Their bright-dim-bright period may be from 1 to 50 days!
• Eclipsing binary – two stars of unequal brightness continually exchange positions in alignment with earth; giving the view that a single star is blinking.

Pulsars: stars that give off both light and radio waves (invisible light).

• This may be the remains or core of a dying star
• Have been known to pulsate (blink) up to 642 times per second!

Unit III: Formation of Stars

Nebulae = clouds of gas and dust, which may be the "left-over" from an exploding star.

• Diffuse nebula – a cloud of gas and dust illuminated by a nearby star. Appears as a blurry mass in space.
• Dark nebula – a cloud of gas and dust NOT illuminated, but outlined by distant stars as a dark patch in space.

Formation of Red Giants:

> Stars in a stable state begin to swell and fusion increases causing the star to heat up

and therefore become intensely bright. It may even become a super giant.

Formation of Dwarf Stars:

• When the star can no longer "burn" and it just collapses on itself forming a dwarf star.
• Often no larger than Earth!
• May glow faintly for a billion years!
• A dwarf may flare brightly for a short period (few months) but then fade back to normal (called a Nova).

• Our own sun is about 5 billion years old!

• Is a Yellow Giant
• will remain for another 5 billion
• then will swell to a red giant
• finally collapse to a white dwarf

Supernova

• same as a nova but with much more intensity (may out shine an entire galaxy!)
• One of the largest ever witnessed was in 1987.

Neutron Stars

• The remains of an exploded star.
• May only be about 10 km in diameter!
• But would be trillions of times more dense than our sun!!!

Black Holes

• thought to form also from the collapse of the biggest stars in the universe.
• Gravity is so great that even light cannot escape.
• Appear as "black" holes in space illuminated only by distant stars around it.

Unit IV: Galaxies

Galaxies are often seen from earth as small cluster clouds in space.

• Our sun belongs to the Milky Way galaxy
• 100 billion stars in the Milky Way
• Every star in our view is in the Milky Way!
• Shaped like a large magnifying lens
• 140000 Ly in diameter
• 20000 Ly thick
• Our sun is about 23000 Ly from the center

Our galaxy belongs to a "cluster" of galaxies called the Local Group.

• Our nearest neighboring galaxy is about 2 million Ly away! (the Andromeda Galaxy)
• The Andromeda is about 2x larger

Types of Galaxies

• Spiral galaxies – spiral shaped (like swirling arms)
• Elliptical galaxies – lens-shaped (like magnifying lens)
• Irregular galaxies – smaller, fainter, and less common (no pattern to their appearance)

Quasars are objects in space that emit radio waves. They appear faint because they are so far away…they are actually thought to be the most luminous objects in space!

• May be the start of a new galaxy???