The Universe

1
The Universe

• is space and everything in it.

2
Galaxies

• A galaxy is a cluster of stars, gas, and dust
  that are held together by gravity.
• There are three main types of galaxies
• Irregular
• Elliptical
• Spiral

3
Types of Galaxies Irregular

• Some galaxies do not have definable, regular
  shapes and are known as irregular galaxies.
• They contain young stars, dust, and gas.

4
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.

5
Types of Galaxies Spiral

• 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.
• Contains middle aged stars

6
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.
• Our Sun is located at the edge of the Milky Way
  galaxy.

7
Nebulae (plural for Nebula)

• Stellar Nebula- A gigantic cloud of gas and dust
  from which stars are made sometimes called a
  Stellar Nursery
• Planetary Nebula- When a star runs out of fuel to
  burn, its outer layers of gas are blown outward
  in the shape of a ring.

8
Stars

• A star contains hydrogen gas that forms helium
  through a process known as nuclear fusion. Stars
  form when gravity pulls together gas and dust
  from stellar nebula.
• It releases enormous amounts of energy which
  causes the star to become very hot and glow.
• Vary in mass, size, and temperature
• Held together by gravity.

9
A Star is Born

• A star is made up of a large amount of gas, in a
  relatively small volume.
• A stellar nebula, on the other hand, is a
• large amount of gas and dust,
• spread out in an immense
• volume.
• All stars begin their lives as parts of stellar
  nebulas.

10
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.

11
Lifetimes of Stars

• Before they can tell how old a star is,
  astronomers must determine its mass.
• Medium mass stars, such as the Sun, exist for
  about 10 billion years.
• These are known as
• main sequence stars.

12
Lifetimes of Stars

• Stars with more mass have shorter lives than
  those with less mass.
• Small stars use up their fuel more slowly than
  large stars, so they have much longer lives,
  about 200 billion years.

13
Lifetimes of Stars
Stars form in a stellar nebula, from collapsing
clouds of interstellar gas and dust. This is
called a protostar.
The rest of the life cycle depends on the mass of
the star.
When a star runs out of fuel, it will become a
white dwarf, neutron star, or black hole.
14
Main Sequence Stars

• Main Sequence is the stage in which stars spend
  the majority of their lifetime.
• Small and medium stars first
• become red giants.
• Their outer layers expand during the red giant
  phase.
• Eventually, the outer parts grow bigger still and
  drift out into space.

15
Main Sequence Stars...

• The blue, white, hot core is left
• behind causing a white dwarf.
• When there is no more energy, it becomes a black
  dwarf. It is the remaining burnt-out cinder left,
  as the star goes out.

16

• A dying giant, or supergiant star, can suddenly
  explode. Within hours, the star blazes millions
  of times brighter.
• The explosion is called a supernova.

17
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.

18
Black Holes

• The most massive stars may have more than 40
  times the mass of the Sun. One might have more
  than five 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.