1. Stars, Galaxies, and the Universe

2. The Scale of the Universe
Astronomers often use parallax to measure the distances to nearby stars.
Parallax is the apparent change in position of an object when you look at it from different places.
Astronomers define the universe as all of space and everything in it. The universe is enormous, almost beyond imagination.

3. The Scale of the Universe
Since the numbers astronomers use are often very large or very small, they frequently use scientific notation to describe sizes and distances in the universe.
Astronomers use a unit called a light year to measure distances between stars. A light year is the distance light travels in one year, about 9,460,000,000,000,000 meters.

4. Characteristics of Stars
Stars are made of gases, which produce their own light and heat.
Earth’s nearest star is the sun.
All stars live and die. These changes are very slow and happen over billions of years.
Today, our sun is about halfway through its life. It has enough hydrogen in its core to keep shining for another five billion years.
After that time, there will be no hydrogen left to make energy to keep the sun going.

5. Characteristics of Stars
Astronomers classify stars according to color, temperature, size, composition, and brightness.
A star’s color reveals its surface temperature. Blue stars are the hottest and red stars are the coolest.
The chemical composition of most stars is about 73% hydrogen,25% helium and 2% other elements. But each star contains different amounts of various elements. Astronomers use a spectrograph to determine the elements found in a particular star.
The brightness of a star depends upon both its size and temperature. A stars apparent brightness is the brightness as seen on Earth. Its absolute brightness is the brightness the star would have if it were at a standard distance from Earth.

6. What is an H-R Diagram?
A Hertzsprung-Russell diagram is a graph of stars’ surface temperature versus their absolute brightness.
Astronomers use the H-R diagram to classify stars and to understand how stars change over time.
Most stars fall in a diagonal area of the diagram called the main sequence. The brightest stars are located near the top of an H-R diagram while the dimmest stars are located at the bottom.

7. The Lives of Stars All stars begin their lives as parts of nebulas.
A nebula is a large cloud of gas and dust spread out in an immense volume.
A star is made up of large amount of gas in a small volume.
In the densest part of the nebula, gravity pulls gas and dust together. A contracting cloud of gas and dust with enough mass to form a star called protostar.
A star is born when the contracting gas and dust from the nebula become so dense and hot that nuclear fusion starts.

8. Star Ages
Star ages are related to their starting masses. In general, stars with the smallest starting mass-such as white dwarfs-last for the longest time, up to 100 billion years. Stars that start with average masses-such as the sun –last about 10 billion years or a little more than twice the age of the sun today. Stars with the largest masses may last only a few billion years.

9. Will the sun last forever?
Thousands of years after its hydrogen fuel has run out, the Sun’s outer layers will expand outward like a giant balloon. This will happen because of extra heat coming out of the core. At this point, the sun will become a red giant star. It will swell to about a hundred time its size it is today and turn red. The sun will be so large that it will swallow up the planet Mercury and perhaps Venus.

10. What happens to a star when it runs out of fuel?
A low- mass and medium-mass stars begin to run out of fuel, their outer layers expand and they become red giants. Eventually, their outer layers drift off into space, forming a glowing cloud of gas called a planetary nebula. The blue-white core of the star is left behind cools and becomes a white dwarf. White dwarfs have about the mass of the sun but are only the size of Earth.

11. What happens to a star when it runs out of fuel?
As high-mass stars begin to run out of fuel they become red supergiant. When a supergiant runs out of fuel, it can explode suddenly. The explosion, a supernova, blazes millions of times brighter. The remains of the high mass star may form a neutron star. Rapidly spinning neutrons are known as pulsars. If the original star was extremely massive, what remains after a supernova may be a black hole. A black hole is an object with gravity so strong that nothing, not even light can escape.

12. Will the sun last forever?
The outer layers of the sun will then move far away from the star. A huge shell of gas will be shed by the sun. This shell of gas will contain almost half of the material that makes up the sun. The material will form a giant cloud, that will grow much larger than the whole solar system. This cloud is called nebula. When a star forms a nebula, it is a sign that the star’s death is very near.

13. Death of Stars
After a star runs out of fuel, it becomes a white dwarf, a neutron star or a black hole.
What determines which of these three stars become is it’s mass.
When a high-mass star runs out of fuel, it explodes. A low-mass star becomes a red giant, forms a planetary nebula, and then becomes a white dwarf. Stars that are most massive becomes a black whole.

14. Galaxies
If you look into a clear sky at night, away from a city or highway light, you will see a thick band of stars across the sky. What you are really seeing is the side view of a family of stars called a galaxy.
Our solar system is located in a spiral galaxy called the Milky Way. The Milky Way Galaxy is a group of about 200 billion stars that includes the sun and almost all other stars you can see in the night sky.
Space contains at least a billion galaxies beyond our own Milky Way Galaxy. The shapes of these galaxies fall into four major classes; spiral galaxies, elliptical galaxies, irregular galaxies and quasars.

15. Galaxies
Spiral galaxy looks like a pinwheel. Its arms are full of young stars, and new ones form there.
Elliptical galaxies look like round flattened balls. They contain billions of stars but have little gas or dust between the stars. Stars are no longer forming inside them, so they contain only old stars.
Irregular galaxies do not have a regular shapes. They are smaller than spiral and elliptical galaxies. They contain young, bright stars and include a lot of gas and dust to form new ones.
Quasars are active, young galaxies with black holes at their center. Gas spins around the black hole, heats up, and glows.

16. The Expanding Universe
According to the big bang theory, the universe formed in an instant, billions of years ago, in an enormous explosion. New observation lead many astronomers to conclude that the universe will likely expand forever.
Since the big bang, the universe has been expanding. Astronomer Edwin Hubble discovered that almost all galaxies are moving away from us and from each other. Hubble’s law states that the farther away a galaxy is the faster it is moving away from us.