In 1929 Edwin Hubble discovered that the universe is expanding. Ever since, we have been striving to fully comprehend the implications of his discovery. Our understanding of the universe and our place in it has evolved from an anthropocentric, static, earth-centered model to a dynamic, evolving cosmos where galaxies are flung across time and space, where the cosmic horizon is quickly receding and the discoveries that await us are limited only by our imagination. Based on Edwin Hubble’s discovery that the universe is expanding, a study was begun in 1998 to determine the expansion rate of the universe at great distances. Culminating with the 2011 Nobel Prize in Physics being awarded to 2 Americans and an Australian, it was determined that the expansion rate of the universe is not decreasing but increasing at great distances, a finding that was quite unexpected and had far-reaching implications for our cosmological models and understanding of the expanding universe. In this presentation, I discuss this discovery in detail and how a specific type of exploding star (supernova) was used to make this discovery.

1. Exploding Stars As Beacons At the Edge of
the Universe
The Universe is Speeding Up!

2. The 2011 Nobel Prize in Physics was awarded to
Saul Pearlmutter, University of California, Berkeley
Adam Riess, Johns Hopkins University and the
Space Telescope Science Institute
Brian Schmidt, Australian National University and Mt.
Stromlo Observatory
For “the discovery of the accelerating expansion
of the Universe through observations of distant
supernovae"

3. How do we know how distant an object
is and, hence, it’s fundamental or
“intrinsic” brightness?

4. One method to determine distances in the universe is
based on intrinsic or absolute brightness
By comparing all objects to each other at a standard
distance, we can make valid comparisons between
them. The brightness of an object at this standard
distance is known as its Absolute Magnitude
If we know how intrinsically bright an object is and,
because light behaves according to an Inverse Square
Law (a light source that is twice as distant will be ¼ as
bright), we can easily determine its distance

7. Objects that allow us to determine distances
are known as
Standard Candles

8. Brief History
1908: Henrietta Swan Leavitt, after observing
variations in the brightness of certain variable
stars, known as Cepheid Variables, and noting
that the brightness depends on that variation,
codifies the “Period-Luminosity” relation

9. Brief History
1912: Vesto M. Slipher measures the radial
velocities of the Spiral Nebulae, most notably,
the great Spiral Galaxy in Andromeda, M-31.

10. Brief History
Radial velocity is the speed at which an object is
moving along our light of sight. If the object is
moving relative to us as observers, its light will
be subject to the Doppler Effect: if it is receding,
the light would be “Red Shifted”; if is
approaching, the light would be “Blue Shifted”

11. Brief History
1919: Edwin Hubble is appointed to the Mt.
Wilson Observatory by Carnegie Astronomer,
founder and visionary, George Ellery Hale

12. Brief History
1922 - 1923: through his observations of the
Spiral Nebulae with the 2.5 meter (100”) Hooker
reflector on Mt. Wilson, Edwin Hubble
concludes that the “Spiral Nebulae” are
separate galaxies outside the Milky Way

13. Brief History
1929: along with Milton Humason, Hubble
formally codifies and publishes his Distance/
Velocity Law. Known as the Hubble Metric, it
relates distance with recessional velocity: the
further away an object is, the faster it is
receding. Not only is this observation
consistent with a uniformly expanding space, it
is consistent with Einstein’s General Theory of
Relativity

16. Hertzprung-Russell Diagram
This diagram is a plot of luminosity
(absolute magnitude) against
the color of the stars ranging
from the high-temperature
blue-white stars on the left
side of the diagram to the
low temperature red stars on
the right side.
This diagram below is a plot of 22000 stars
from the Hipparcos Catalogue
together with 1000 low-luminosity
stars (red and white dwarfs). The
ordinary hydrogen-burning dwarf
stars like the Sun are found in a
band running from top-left to
bottom-right called the Main
Sequence. Giant stars form their
own clump on the upper-right side
of the diagram. Above them lie the
much rarer bright giants and
supergiants. At the lower-left is the
band of white dwarfs - these are the
dead cores of old stars which have
no internal energy source and over
billions of years slowly cool down
towards the bottom-right of the
diagram.

17. The Standard Solar Model indicates that for
a 1 Solar Mass Star 100% of the star’s
luminosity is achieved at 0.30 R, using
0.70 M
Energy transport is radiative for inner 0.70R
With increasing altitude, the solar interior
cools and energy transport transitions
from radiative to convective

18. All stable stars on the Main Sequence are said to
be in a state of
Hydrostatic Equilibrium
Where outward gas pressure is balanced by the
inward pull of Gravity.

22. After all but 12% of the initial hydrogen abundance has
been consumed in the star’s core, energy production
will transition to a hydrogen burning shell
surrounding a non-fusing and degenerate helium
core

23. The sun will shine for a total of 12 billion years
and will use the helium that was produced during
that time to make carbon and oxygen

24. Stellar Evolution
• Solar Mass Stars
–Lifespan 10s of billions of years
–End of life
• Red Giant
• Helium Burning;
– He burning is very temperature sensitive: Triple-alpha fusion
rate ~ T40!
– Consequences:
» Small changes in T lead to Large changes in fusion
energy output
• Carbon-oxygen core;
• Carbon-oxygen White Dwarf

25. Cessation of core hydrogen fusion reactions results in
core contraction and gravitational heating
Due to core contraction and the resulting gravitational
heating, hydrogen fusion reactions resume in a shell
surrounding the core
Core helium fusion reactions begin via the Triple-Alpha
process once temperatures reach 100,000,000 K in
the core
Star moves up and to the right, ascending the RGB of
the HR Diagram, leaving the Main Sequence

27. Solar-mass stars
the process stops at helium fusion with a carbon-oxygen white
dwarf that cools over billions of years

28. Electron degeneracy and Degenerate Matter
• Quantum Theory
– Specific electron energy levels
– Pauli Exclusion principle
• No two electrons can occupy the same state
– When the triple-alpha process in a red giant star is complete, those
evolving from stars less than 4 solar masses do not have enough energy
to initiate the carbon fusion process. They collapse until their collapse is
halted by the pressure arising from electron degeneracy. An interesting
example of a white dwarf is Sirius-B.
– For stellar masses less than about 1.44 solar masses, the energy from
the gravitational collapse is not sufficient to produce the neutrons of a
neutron star, so the collapse is halted by electron degeneracy to form
white dwarfs. This maximum mass for a white dwarf is called the
Chandrasekhar limit.
White Dwarfs

29. Discovered by Subrahmanyan Chandrasekhar when he
was a graduate student at Cambridge, while in transit
to England, the Chandrasekhar Limit is 1.44
M(sol). It is the maximum nonrotating mass which can
be supported against gravitational collapse by electron
degeneracy pressure.
Born October 19, 1910, Lahore,
British India, now Pakistan
Died August 21, 1995, Chicago,
Illinois
Nobel Prize in Physics (1983)

30. The CHANDRA Orbiting X-Ray Observatory: An
Example of an X-Ray Telescope and observing
platform: http://chandra.harvard.edu

31. Sirius
Is a binary star system 9 light years away that consists of Sirius-A, a main sequence star
and Sirius-B, a 1 solar mass white dwarf

32. Sirius
Is a binary star system 9 light years away that consists of Sirius-A, a main sequence star
and Sirius-B, a 1 solar mass white dwarf

33. Sirius
Is a binary star system 9 light years away that consists of Sirius-A, a main sequence star
and Sirius-B, a 1 solar mass white dwarf

34. Sirius
Is a binary star system 9 light years away that consists of Sirius-A, a main sequence star
and Sirius-B, a 1 solar mass white dwarf

35. Sirius-B
White Dwarfs
Represent the end-state for sun-like stars
and stars less than 5 solar masses

36. Planetary Nebulae
Image credit: Thomas Madigan, 3/14/2010
Image acquired with 0.61m R-C

37. Image credit: Thomas Madigan, 3/14/2010
Image acquired with 0.61m R-C
Planetary Nebulae

38. Supernovae
A Star’s Spectacular End
Type Ia
the type used as a Standard Candle to determine
the expansion rate of the universe
Type II
Core Collapse

39. Type Ia Supernovae
A Consequence of Gravity

40. Type Ia Supernovae
A Consequence of Gravity
Stellar Dynamics
The Equivalent of 70% of the Sun’s mass or
1.4 x 10^30 Kg is compressed into a
sphere the size of the earth! This results
in the Intense gravity of a White Dwarf
being equal to 230,000 times the gravity of
the earth!

41. Type Ia Supernovae
A Consequence of Gravity
As a member of a parasitic binary
star system, the white dwarf draws
material from a victim star
The accreting material, mostly
hydrogen, builds up on the surface
of the White Dwarf

42. Type Ia Supernovae
A Consequence of Gravity
When a critical mass has
accumulated on the surface of the
White Dwarf, known as the
Chandrasekhar Limit, conditions
become similar to those present in
the cores of massive stars, stars
that end as Type II supernovae

43. Type Ia Supernovae
A Consequence of Gravity
When this happens, a runaway
thermonuclear fusion reaction
engulfs the White Dwarf,
obliterating the star, resulting in a
Type Ia supernova, one of the
brightest, most powerful events in
the universe

44. Type Ia Supernovae
A Consequence of Gravity
Could the sun end up as a Type Ia
Supernova?

45. Type Ia Supernovae
A Consequence of Gravity
Could the sun end up as a Type Ia
Supernova?
Yes!

46. Type Ia Supernovae
A Consequence of Gravity
Key factors as an accurate Standard Candle
High Intrinsic Luminosity: visible at tremendous distances, across
the universe! For a moment in time, the supernova blazes with
the light of a billion suns!

47. Type Ia Supernovae
A Consequence of Gravity
Key factors as an accurate Standard Candle
High Intrinsic Luminosity: visible at tremendous distances, across
the universe! For a moment in time, the supernova blazes with
the light of a billion suns!

48. Type Ia Supernovae
A Consequence of Gravity
Key factors as an accurate Standard Candle
High Intrinsic Luminosity: visible at tremendous distances, across
the universe! For a moment in time, the supernova blazes with
the light of a billion suns!
Predictable, consistent luminosity, tightly constrained by the
precise nature of the Chandrasekhar Mass limit

49. The Expanding Universe
And Dark Energy!
Because of the reliability of Type Ia supernovae as accurate
standard candles, Pearlmutter, Riess and Schmidt observe a
departure from Hubble’s Law in the expansion rates of galaxies
at extreme distances; the Type Ia supernovae were under-
luminous based on the distances implied by their observed Red
Shifts

50. The Expanding Universe
And Dark Energy!
Because of the reliability of Type Ia supernovae as accurate
standard candles, Pearlmutter, Riess and Schmidt observe a
departure from Hubble’s Law in the expansion rates of galaxies
at extreme distances; the Type Ia supernovae were under-
luminous based on the distances implied by their observed Red
Shifts
How can this happen?

51. The Expanding Universe
And Dark Energy!
Because of the reliability of Type Ia supernovae as accurate
standard candles, Pearlmutter, Riess and Schmidt observe a
departure from Hubble’s Law in the expansion rates of galaxies
at extreme distances; the Type Ia supernovae were under-
luminous based on the distances implied by their observed Red
Shifts
How can this happen?
They are farther away, a result only possible if they are receding
at a greater velocity

52. The Expanding Universe
And Dark Energy!

53. Where do we go from here?
Continued observations of very deep objects,
objects whose visible light has been red shifted
deep into the InfraRed, objects that are beyond
the reach of even the mighty Hubble Space
Telescope

54. James Webb Space Telescope
A Next Generation Space-borne Telescope