The Universe

1
The Universe

• Stars
• Classification

• Cosmology
• Distance Measurements
• Hubble Expansion
• The Big Bang
• Fate of the Universe

• Life History
• End Stages

• Galaxies
• Fundamentals
• Classification
• Structures

2
Classification Methods

• Stars can be classified in a number of different
  ways. These include
• Composition
• Spectral Category
• H-R Category

3
Composition

• Population I
• Atmospheres contain significant amounts of
  elements other than hydrogen and helium.
• This group includes examples in all sizes and
  spectral categories.
• Our own sun is a member.

• Population II
• Atmospheres contain no significant quanties of
  elements other than hydrogen and helium.
• This group does not include any large, hot stars.

4
Spectral Category

• The spectral category of a star is determined by
  its color.
• The color is determined by surface temperature.
• Either of two phrases can be used to remember the
  sequence.

5
H-R Diagram

• Stars can also be classified by position in an
  H-R diagram.
• This is a plot of luminosity (brightness) vs.
  surface temperature (color index).

6
H-R Categories

• Main Sequence
• Red Giants and Supergiants
• White Dwarfs

7
Main Sequence

• Most stars fall in a band that runs diagonally
  from the lower right to the upper left.
• The stars on the lower right are cool and dim.
• The stars in the upper left are hot and bright.
• A star's mass sets its place in the main sequence.

8
Red Giants and Supergiants

• Red giant and supergiant stars are brighter than
  main sequence stars with the same surface
  temperature.
• They are much larger than main sequence stars in
  physical dimensions (but not mass

9
White Dwarfs

• White dwarf stars are very hot, but still very
  dim.
• They are dim because they are physically much
  smaller than main sequence stars.
• None has a mass of more than 1.44 times the sun's
  mass.

10
Life History

• The life history of a star is controlled by its
  mass.
• A star the size of our Sun ends as a white dwarf.
  
• Larger stars end as neutron stars.
• The largest stars become black holes.

11
Formation

• Stars are formed from the collapse of a nebula.
• Once the center is sufficiently hot and dense for
  hydrogen fusion, a main sequence star is formed.

12
Main Sequence Stage

• A main sequence star derives its energy by fusing
  hydrogen in its core to form helium.
• The process forms one helium nucleus from four
  hydrogen-1 nuclei in a multi-step process.

13
Main Sequence Lifetimes

• The lifetime of a main sequence star depends on
  its mass.
• Larger stars must fuse hydrogen at a faster rate
  than small ones.
• Spectral category M stars may survive for 50-100
  billion years.
• O-type stars will remain on the main sequence
  form only a million years (or less).
• A G-type star (like the sun) will remain on the
  main sequence for 10 billion years.

14
End of the Main Sequence Stage

• The main sequence stage ends when the star's core
  can no longer fuse hydrogen.
• The core now begins to contract and heat up.
  Hydrogen in the shell outside the core begins
  fusing.
• The increased heat causes the outer layers to
  expand.

15
The Red Giant Stage

• Eventually, the core of the star becomes hot
  enough to fuse helium nuclei to make carbon and
  oxygen.
• This period of a star's existence is only about
  10 as long as its main sequence stage.

16
White Dwarf Stage

• Stars with masses less than 8 times the sun's
  cannot fuse carbon and oxygen into heavier
  elements.
• These stars collapse into white dwarfs.
• At the final stage, the outer layer's of the star
  are blown off to form a planetary nebula

17
More White Dwarf Properties

• White dwarf stars have no internal source of
  energy the light they emit comes from stored
  heat.
• Because they are so small, the cooling process is
  very gradual. A white dwarf can exist for about
  50 trillion years before becoming to cool to emit
  light.
• White dwarfs don't collapse further because of a
  quantum effect called electron degeneracy
  pressure. This process cannot support a
  structure whose mass exceeds 1.44 times the sun's
  mass.

18
Novas and Supernovas

• When a white dwarf has a close companion star, it
  can pull matter from it onto itself.
• Eventually, it will accumulate enough on its
  surface to ignite a brief burst of fusion
• The result is nova star.

• If enough matter accumulates to exceed the 1.44
  solar masses the star collapses, inducing a huge
  fusion explosion.
• This result is a Type Ia Supernova.
• The star is blown completely apart.

19
Supergiants

• Star with mass gt 8 times the sun's mass become
  supergiants.
• The process begins in the same way as for red
  giant stars.
• The core of a supergiant becomes so hot and dense
  that fusion continues beyond helium

20
Fusion in Supergiants

• Eventually, a supergiant star develops an
  onion-like structure, each successive layer
  consists of heavier elements.
• The inner core consists of iron.
• Fusion stops with iron, because fusing iron
  absorbs energy.

21
Type II Supernova

• When the iron core of a giant star attains a mass
  of 1.44 times the sun's mass, it undergoes a
  catastrophic collapse.
• The outer layers of the star begin falling
  inward.
• The first material to reach the core bounces and
  combines with the incoming layers to create a
  tremendous shock wave.
• The result is a tremendous outburst of fusion
  that produces a supernova.

22
Properties of Supernova

• During the explosive phase, a supernova produces
  at least a billion times as much energy as normal
  stars.
• Elements with atomic numbers higher than iron are
  also produced.
• Much of the star's mass is dispersed into space.

23
Neutron stars

• If the mass of a supernova remnant is less than 3
  solar masses, a neutron star is formed.
• This object is a neutron ball between 10 and 20
  miles in diameter!
• The structure doesn't collapse further because of
  neutron degeneracy pressure.

24
Pulsars

• Most neutron stars cannot be detected optically.
• Most are detected because they emit radio waves
  in short bursts whose period matches their
  rotation rate.
• These objects are called pulsars.

25
Black Holes

• Stars with masses greater than 3 solar masses
  collapse into black holes.
• The gravity of a black hole is so strong that its
  escape velocity exceeds the velocity of light.
• The boundary of a black hole is called the event
  horizon.

26
Detecting Black Holes

• Some black holes are absorbing significant
  amounts of matter from outside.
• These objects can be detected by the X-rays
  emitted just before the matter crosses the event
  horizon.

27
Galaxies

• A galaxy is a collection of stars bound by
  gravitational attraction.
• A small galaxy may contain only a few million
  stars.
• A large galaxy (such as ours) may contain 100
  billion stars.

28
Galaxy classes

• Astronomers classify galaxies by shape. There are
  three main categories
• Spiral
• Elliptical
• Irregular

29
Spiral Galaxies

• These galaxies have a spherical core from which
  the arms emerge and wrap around in a spiral
  pattern.
• The arms contain large amounts of dust and gas
  and many Population I stars.
• Star formation is still ongoing in the arms.

30
Elliptical Galaxies

• Elliptical galaxies are oval shaped.
• They contain only Population II stars and are
  usually dimmer than spirals.
• They contain little or no dust and gas.
• They are the most common type of galaxy.

31
Irregular Galaxies

• Irregular galaxies have no definite shape.
• The best known are the Greater and Lesser
  Magellanic Clouds.
• These two are satellites of our own galaxy.
• They are visible from the Southern Hemisphere.

32
The Milky Way

• Our sun is located in a spiral galaxy called the
  Milky Way
• It is about 100,000 light-years across and
  contains about 100 billion stars.

33
Clusters of Galaxies

• Galaxies are not evenly distributed in the
  universe.
• Most are clumped in groups called clusters.
• The clusters in turn are grouped it clumps called
  superclusters.
• Even larger groupings may exist.

34
Dark Matter

• Astronomers believe that most of the mass of the
  universe is in a nonluminous form.
• There are two observations that support this
  notion
• 1. Orbital velocities of stars within a galaxy
  suggest that the galaxy's mass is much larger and
  extends out far beyond the area where stars are
  found.
• 2. Velocities of galaxies within clusters are so
  high that they would quickly disperse unless
  their masses are much higher than that of the
  visible objects.

35
Distance Measurements

• Astronomers use a number of techniques to measure
  distance.
• The most important ones are
• Parallax
• Cepheid Variables
• Type IA supernovae

36
Parallax Method

• Parallax refers to the change

37

Cosmology

• Cosmology is the study of the universe as a
  whole.
• The aims are twofold
• Determine the present overall structure.
• Understand its past and present evolution.

38

Beginnings

• From the time of Newton to that of Einstein,
  scientist tacitly assumed that the universe was
  uniform, infinite, eternal, and unchanging.
• The energy source of stars was unknown at that
  time.
• The only hint that something was wrong with this
  picture was found in the nineteenth century. It
  is now called Olber's paradox.

39

Olber's Paradox

• If the universe is infinite and eternal, why is
  the night sky dark?
• If the universe is infinite in extent, then their
  should be an infinite number of stars in any
  direction we look.
• If the universe is eternal, we should be
  receiving some light from all of them.
• The total amount of energy falling on Earth from
  each point in the sky should therefore be
  infinite!

40

The Hubble Expansion

• In the first half of the twentieth century, two
  telescopes were built (at Mts. Wilson and
  Palomar) that could produce spectra for other
  galaxies.
• Edwin Hubble used the Doppler shifts in these to
  determine the radial velocities of other galaxies.

41

Hubble's Law

• Hubble found that most galaxies are receeding
  from us.
• Their recession velocity is proportional to their
  distance from us.
• This effect is consistent with Einstein's general
  relativity. It happens because the space between
  galaxies is growing.

42

Value of the Hubble Constant

• Current measurements indicate that the Hubble
  constant has the value.
• The reciprocal of the Hubble constant is roughly
  the age of the universe.
• This age is about 13.7 billion years.

H0 slope of graph
43

The Big Bang

• The Hubble expansion shows that the universe was
  smaller in the past.
• The Big Bang theory postulates that it began as a
  very hot and dense system that quick expanded and
  cooled.

44

A (Very Brief) History of Time

• During the 1st second, protons, neutrons, and
  electrons form.
• After about 3 minutes, temperatures are cool
  enough to fuse nuclei without immediately
  breaking them up. Some hydrogen is fused to
  helium during this phase.
• About 300,000 years after the beginning, the
  universe cooled to the point that electrons could
  combine with nuclei to form atoms. At this
  point, the universe becomes transparent.

45
History Continued

• Once atoms formed, density variations within the
  universe began condensing regions due to the
  action of gravity.
• These clumps are called protogalaxies.
• Stars formed inside the protogalaxies by the same
  process, making them into galaxies.
• Star formation in some galaxies is still ongoing.

46

Evidence

• The theory predicts that about 25 of the mass of
  the universe will be helium.
• When atoms form, the radiation in the universe
  ceased to interact strongly with matter. This
  radiation would now behave like blackbody
  radiation of an object whose temperature is about
  3 K. This radiation has been detected.
• The theory is consistent with the idea that the
  universe looked different in the past than it
  does now.

47

Quasars

• Quasars are objects that have very large red
  shifts.
• They are therefore thought to be very distant
  from us. The most distant is about 15 billion
  light years away.
• Quasars very in brightness too rapidly to be
  very large.

48
What are Quasars?

• To be visible at such great distances, they must
  be extremely bright
• Currently, quasars are thought to be black holes
  at galactic centers that are actively swallowing
  entire stars.

49

Cosmological Significance

• The fact that quasars are not seen nearby
  indicates that none currently exist.
• This implies that the universe has changed with
  time.

50
The Ultimate Fate of the Universe

• The basic issue is Will the universe continue
  expanding forever, or will it eventually stop and
  begin contracting?
• If the latter, then it may eventually return to
  the same state as when it began (the Big Crunch).
• Another issue is the current overall structure.

51
The Options

• Space is curved like a saddle (Open universe).
• Space is Euclidean (flat universe).
• Space is curved like a sphere (closed universe).

52
Structure Options (cont.)?

• A low matter density implies that space has open
  curvature (infinite).
• There is one density for which space is flat
  (also infinite).
• A high matter density leads to a space with
  closed curvature (finite, but unbounded).

53
Possible Fates

• Since gravity is acting to slow the expansion
  rate, it was originally thought that there were
  only three possibilities.
• The expansion would gradually slow down,
  approaching a constant positive value.
• The expansion would slow down, approaching but
  never reaching zero.
• The expansion rate would go to zero, then become
  negative (contraction).

54
Testing Fates

• We can in principle choose between these fates by
  measuring the rate at which the Hubble constant
  changes with time.
• Type 1A supernovae provide a means to do this.
• The method involves measuring the Hubble constant
  for distant objects only and comparing to values
  measured for nearby objects.

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The Answer?

• The results seem to show that the Hubble constant
  is increasing with time (the expansion is
  accelerating).
• This means that the universe will almost
  certainly continue expanding.