1. Stars & Galaxies

2. Types of Galaxies
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter.
Enormous amounts of gas revolve around a black hole at the center. The gas heats up and shines brightly.

3. Types of Galaxies – Elliptical...
Elliptical galaxies look like
flattened balls.
These galaxies contain billions
of stars, but have little gas and
dust between the stars.
Because of the lack of gas and dust, new stars cannot form in most elliptical galaxies, and so they contain only old stars.

4. Types of Galaxies – Irregular…
Some galaxies do not have definable, regular shapes and are known as irregular galaxies.

5. Types of Galaxies – Barred Spiral…
Our Milky Way galaxy has recently (in the 1990s) been confirmed to be a type of spiral galaxy known as a barred spiral galaxy.
Spiral galaxies consist of a flat, rotating disk with stars, gas, and dust and a central concentration of stars, known as the bulge. The bulge is surrounded by a much fainter halo of stars.

6. The Milky Way Galaxy
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7. A star is born…
A star is made up of a large amount of gas, in a relatively small volume.
A nebula, on the other hand, is a
large amount of gas and dust, a cloud,
spread out in an immense
volume.
All stars begin their lives as parts of stellar nebulas.

8. A star is born…
Gravity can pull some of the gas and dust in a nebula together.
The contracting cloud is
then called a protostar.
A star is born when the contracting gas and dust become so hot that nuclear fusion begins.

9. Lifetimes of Stars…
Small mass stars use up their fuel more slowly than large stars, so they have much longer lives, about 200 billion years.
Medium size - Mass stars, such as the Sun, exist for about 10 billion years.
Large - Massive stars, such as the 100 times the Sun, could exist for about 1 billion years.

10. Lifetimes of Stars…
Before they can tell how old a star is, astronomers must determine its mass.
Stars with more mass have shorter lives than those with less mass.

11. Lifetimes of Stars…
A star uses hydrogen as fuel. When it runs out of fuel, it can no longer convert hydrogen to helium for energy. The star will die out.
The rest of the life cycle depends on the mass of the star

12. Lifetimes of Stars…
When a medium or small star runs out of fuel, it will become a white dwarf.
This is called a white dwarf, because all that is left of the star is the blue, hot core.
When there is no more energy in the white dwarf, all that remains is the burnt-out cinders. As the star goes out, it becomes a black dwarf.

13. Neutron Stars… After a star explodes, some material
from the star is left behind. This
material may become part of a
planetary nebula.
The core will compress
and form a neutron star.
Neutron stars are even
smaller and more dense
than white dwarfs.

14. Black Holes…
The most massive stars may have more than 40 times the mass of the Sun. One might have more than 5 times the mass of the Sun left, after it becomes a supernova.
The gravity of this mass is so strong that the gas is pulled inward, packing it into a smaller and smaller space. These massive stars become black holes when they die.

15. Main Sequence Stars…
Small and medium stars begin as protostars in a stellar nebula. As they grow through the main sequence of their life, their outer layers expand to become red giants.
Eventually, the outer parts grow bigger drift out into space. This gas cloud is called a planetary nebula.
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16. Main Sequence Stars…
As the gases burn off the star, it keeps its blue, white hot core (like a white hot coal in a fire) is left behind called a white dwarf.
Once a white dwarf uses all of its remaining energy, it becomes a black dwarf.
This is the remaining burnt-out cinder as the star goes out. It is cold and dead.

17. Main Sequence Stars…
Massive stars begin as protostars in a stellar nebula. As they grow through the main sequence of their life, their outer layers expand to become red supergiants.
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18. Supernova…
A dying giant, or supergiant star, can suddenly explode. Within hours, the star blazes millions of times brighter.
The explosion is called a supernova.

19. Neutron Stars… After a star explodes.
The core will compress and form either a neutron star or a black hole.

20. The Milky Way Galaxy
The word galaxy is derived from the Greek galaxias, meaning "milky", a reference to the Milky Way.
Our Solar system is located toward the outer half of the Milky Way.
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21. The Universe…
Universe is space and everything in it.