Transcript Astronomy Galaxies & The Universe

Astronomy (cont.)
Cosmic Measurements
Astronomical Unit (AU)
- distance from earth to the sun
~150 million kilometers
- used to express distances to other planets
Light year – distance light travels in one year at
300,000 km/sec
- which equals 9.5 x 1012 km
(If 1 ft. equals 1AU, then 120 miles equals 1 light year)
Lightyear
Therefore, a light second is 186,000 miles
(300,000 kilometers). A light year is the
distance that light can travel in a year, or:
 186,000 miles/second * 60 seconds/minute *
60 minutes/hour * 24 hours/day * 365
days/year = 5,865,696,000,000 miles/year
 A light year is 5,865,696,000,000 miles
(9,460,800,000,000 kilometers).
That's a long way!

Properties of Stars
Mass & Size → Density
- smallest are smaller than Earth
- largest are 2000 times bigger than our sun
Temperature (color)
red → cooler 5,0000 F (3,000 K)
blue → hottest 40,0000 F (30,000 K)
Properties of Stars
Composition – done by light spectrum analysis,
most stars are made of hydrogen & helium
(determined by temperature & composition
no two are alike)
Properties of Stars
Star Brightness
luminosity – depends on size & temperature
apparent magnitude – as seen from Earth:
brightest are ≤ 1 faintest are ≥ 6
absolute magnitude – expresses luminosity of
stars as if they were all seen 32.6 lightyears from Earth (our sun = 4.8)
Life of the Stars: Origin

Nebula – huge cloud of gas and dust in
space

Gravity causes the gas in the nebula to
contract to form a Protostar (– very
young star that is not hot enough to
shine by nuclear fusion H2 + H2 → 2He2 )
 this is electromagnetic radiant energy

As gravity packs matter more tightly – the
protostar’s temperature rises until it reaches
a temperature high enough for nuclear fusion
to begin
Nuclear fusion is Hydrogen fusing to form
Helium….
H2 + H2 = 2He + electromagnetic radiant
energy
When the release of
electromagnetic radiant energy
(pushing out) reaches a balance
with the gravity (pushing in) the
star stops contracting and
reaches the stable state.
 H-R Main Sequence
The original mass of the star determines its
temperature and the color. (Big – glows
blue, medium – glows yellow-orange,
small – glows red)
Red Giants / Red Super-giants form as H2
fuel is used up and gravity overpowers
released energy, center core contracts
while outer layer expands (He2 + He2 → C
fusion starts in the core)
-- outer layers expand and cool (hence it is RED)
Hertzsprung-Russell diagram
Each star is represented by a dot. One
uses data from lots of stars, so there are
lots of dots. The position of each dot on
the diagram corresponds to the star's
luminosity and its temperature
 The vertical position represents the star's
luminosity (absolute magnitude).
 The horizontal position represents the
star's surface temperature (color).

http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html
Life Cycle of Stars
Average to small stars collapse again after C fuel is
used up → white dwarf (Earth size)
Large stars (at least 7 times our sun) when fusion (of carbon)
stops, a central iron core is left, intense gravitational energy
causes further collapse, creates heavier elements
→ explosion causes loss of ½ the stars elements, a supernova
is born
Supernova fades – neutron star is left (core of pure
neutrons), eventually gravity overpowers
→ black hole is created
Constellations – a group of stars that
appears to form a pattern in the sky
circumpolar
constellations –
constellations
that appear to
never set below
the horizon:
Ursa Major, Ursa
Minor, Thesius,
Cassiopeia, and
Draco
Zodiac Constellations
Constellations
that
appear
along
the
ecliptic