Notes on Stars and Galaxies
I. Stars & Their Characteristics
A. People often notice groups of stars that seem to form patterns in the night sky. These patterns are called constellations.i.e. The Big Dipper (Ursa Major), The Little Dipper (Ursa minor), Cassiopeia, Orion
B. The constellations rotate about a fixed point in the Northern Hemisphere as the Earth rotates on its axis. This is Polaris (The North Star).
C. The constellations change as the seasons progress because the Earth is in a different part of its orbit around the Sun.
D. Distances to stars are immense! The distance to the nearest star, our Sun, is considered 1 Astronomical Unit (1 AU). (93 million miles)
1. The distance light travels in 1 year is considered 1 light year.
- light travels 300,000,000 meters/second!
- 1 light year = 95,000,000,000,000 kilometers!
- The next nearest star, Alpha Centauri, is 4.3 light years
away!
E. Properties of Stars:
1. The Sun
- Diameter = 1,380,00 km
- Mass = 300,000 times earth’s mass
- Density = 1.4 g/cm3
2. Stars can be smaller than Earth or 2000 times as large as
our Sun!
3. Colors of stars are indicative of their temperatures.
Red = cooler stars
Yellow = medium temp
Blue/White = very hot stars
4. Stars are made mostly of hydrogen & helium
F. The brightness of stars is measured by astronomers as well.
1. Apparent magnitude refers to how bright the star appears
to us on Earth.
Bright stars = 1st magnitude
Dimmer stars = 6th magnitude
2. Luminosity refers to the actual brightness of a star. It is
also called absolute magnitude.
II. Kinds of Stars
A. Red giants are HUGE stars that are cooler and redder in color.
B. Super-bright red giants are called supergiants. These are the
largest of all stars and are very short-lived.
C. Dwarf stars are as large as the Earth, but are very dense. They
are 100,000 times as dense as earth!
D. Variable Stars vary in brightness over regular intervals. Cepheid
Variables pulsate over 1-50 day intervals.
E. Pulsars give off POWERFUL bursts of radio waves once a second.
Astronomers feel this is the core of a star that blew up.
III. Life Cycles of Stars
A. Stars form from huge clouds of gas and dust in space called
nebulae.
1. The gravity of the matter in this cloud causes the particles
to condense into a large sphere of matter.
2. Friction and pressure heats up the matter in the cloud. and
it begins to glow as a protostar.
3. When 1,000,000,000°C is reached, nuclear fusion takes
place and the star “ignites.”
Nuclear fusion – 2 hydrogen atoms smack together hard
enough to stick together and form a
helium atom. this gives off A LOT of
energy!
4. When the force of gravity is in balance with the force of
the exploding hydrogen, the star is said to be in the stable
state.
5. When the hydrogen in the core is used up, (this takes billions
of years to do) the star becomes unstable and expands and
cools to form a huge red giant.
6. As even more of the core is used up, it can no longer support
the heavy outer layers and collapses to form a white dwarf.
- These are the size of the earth
- Eventually fade and die
- Large white dwarfs can flare up brilliantly and explode
as a supernova. These can outshine entire galaxies!
7. The stuff left over after a supernova is called a neutron
star.
- Atoms’ electrons are crushed into their nuclei
- 10 km dia. and trillions of times more dense than the
sun! Most dense objects in the universe
8. As neutron stars collapse further onto themselves, their
gravity becomes so great that not even light can escape it.
It now becomes a black hole.
IV. Galaxies & The Universe
A. Galaxies are systems of stars containing billions of stars!
B. There are billions of galaxies in the universe!
C. The nearest 17 galaxies make up The Local Group.
D. Our galaxy is called the Milky Way. It is a spiral galaxy.
E. Types of galaxies:
1. Elliptical –
2. Irregular -
3. Spiral -
F. Origin of the universe – The Big Bang Theory
1. 15 billion years ago all matter in the universe was
concentrated into a mass the size of the sun.
2. This mass exploded to form all of the stuff we see in
the universe today.
3. Evidence -
a. Background radiation (an echo) of the explosion
still exists in deep space.
b. All galaxies seem to be moving apart.
STARS ********************************************************************************************************************************