1. Great Ideas in Science: Lecture 8 – Stars & Galaxies
Professor Robert Hazen
PROV 301
Great Idea:
The Sun and other stars use nuclear fusion reactions to convert mass into energy. Eventually, when a star’s nuclear fuel is depleted, the star must burn out.

2. Key Ideas Stars have a history – a beginning and an end
1. Stars (and planets) begin as clouds of dust and gas, called nebulae.
2. Stars radiate heat and light, which come from the energy of nuclear fusion reactions.
3. Planets form like stars, but they are too small to begin nuclear fusion reactions.

3. Definitions
Astronomy is the study of photons arriving from space.

4. Definitions Astronomy is the study of photons arriving from space.
Astrophysics is the study of the origin, evolution, and fate of stars and clusters of stars.
Cosmology is the study of the origin evolution and fate of large-scale structures of the universe.

5. What do we see from Earth?
Very close (a few light seconds)
Moon
Meteors
Satellites

6. What do we see from Earth?
The Solar System (a few light minutes to hours)
Planets
Asteroids
Comets
Other objects

7. What Do We See From Earth?
Milky Way Galaxy (to about 200,000 ly)
Other stars
Nebulae
Hydrogen halo
Central dust concentration

8. What Do We See From Earth?
Beyond our galaxy (more than 1,000,000 ly)
Other galaxies
Clusters of galaxies
Quasars

9. Almost all astronomical data come from Photons (Electromagnetic Waves)
Position in sky
Wavelength (radio to gamma ray)
Intensity (brightness)
Variation of 1-3 with time
Polarization

10. Observing Stars: What do we want to know?
Distance:
parallax (to 300 ly)
standard candles

11. Observing Stars: What do we want to know?
Distance (parallax; standard candles)
Composition (from line spectra)

12. Observing Stars: What do we want to know?
Distance (parallax; standard candles)
Composition (from line spectra)
Motion
absolute motion

13. Observing Stars: What do we want to know?
Distance (parallax; standard candles)
Composition (from line spectra)
Motion
absolute motion
red shift

14. Observing Stars: What do we want to know?
Distance (parallax; standard candles)
Composition (from line spectra)
Motion (absolute motion; red shift)
Temperature (from color)
Brightness (apparent vs. absolute)
Mass (from dynamics and theory)

15. Telescopes are Photon Collectors
Earth-based or satellite
Various detectors
Eye
Film
Electronic

16. Telescopes are Photon Collectors

17. Telescopes are Photon Collectors GMU Observatory
Telescopes are Photon Collectors GMU Observatory! Monday observing nights

18. Orbiting Observatories
Great Observatories Program
Hubble Space Telescope
Spitzer Infrared Telescope
Chandra X-Ray Observatory

19. Orbiting Observatories
James Webb Space Telescope

20. The Structure of the Sun
Stellar core
Convection zone
Photosphere
Corona

21. The Structure of the Sun
Solar Wind
Stream of particles
Northern lights

22. The Sun’s Energy Source: Fusion
3-steps of hydrogen burning
P + P  D + e+ + neutrino + energy
D + P  3He + photon + energy
3He + 3He 4He + 2 protons + photon + energy

23. The Variety of Stars Differences among stars Life Cycle; depends on:
Color (= temperature)
Apparent Brightness ( a distance effect)
Absolute brightness (Based on total energy output)
Life Cycle; depends on:
Total mass
Age

24. Observing Life Cycles of Stars
Measure many different stars and look for patterns, especially in brightness vs. temperature

25. A Hierarchy of Scientific Ideas
Fact (a confirmed observation)
Hypothesis (an educated guess)
Law (a predictive mathematical description of nature)
Theory (a well established explanation of nature)

26. The Birth of Stars The Nebular Hypothesis

27. Terrestrial (Inner) Planets
Mercury, Venus, Earth, Mars
Rocky and relatively small
Mercury and Venus too hot for life
Mars may have had life long ago

28. Gas Giant (Outer) Planets
Jupiter, Saturn, Uranus, Neptune

29. Gas Giant (Outer) Planets
Jupiter, Saturn, Uranus, Neptune
Layered structure
No solid surface

30. The Main Sequence and the Death of Stars
Stars much less massive than the sun
Brown dwarf
Glows 100 billion years
No change in size, temperature, energy output

31. The Main Sequence and the Death of Stars
Stars about the mass of the sun
Hydrogen burning at faster rate
Red giant (Move off main sequence)
Helium burning
Begin collapse
White dwarf

32. The Main Sequence and the Death of Stars
Very Large Stars
Go through successive collapses and burnings
Ultimately an iron core forms
The star then collapses catastrophically into a supernova

33. Supernova

34. Neutron Stars and Pulsars
Very dense and small
Has a high rotation rate
Almost no visible light
Pulsar
One kind of neutron star
Produces intense radio electromagnetic radiation
This is the end state of some supernovas

35. Black Holes Result of the collapse of the largest stars
Nothing can escape from its surface
We cannot see them directly, but:
We see gravitational impact on other stars
We can detect x-ray and gamma rays

36. Summary: Fates of Stars
1/10th Sun  Brown dwarf
Sun  Main sequence  Red giant  white dwarf
8 suns  Supernova  Neutron Star (Fe)
They last 100 million years
Heavy elements are made in supernovas
20 suns  Supernova  Black Holes
Black holes are points of pure mass

37. Cosmology
Great Idea: The universe began billions of years ago in the big bang and it has been expanding ever since.

38. The Nebula Debate Nebulae are cloud-like objects
Are they clouds of dust and gas?
Or huge collections of stars?
Before 1920s no instruments could answer this question

39. Edwin Hubble and the Discovery of Galaxies
Edwin Hubble in 1919
New Mount Wilson 100” telescope
He used cepheid variable stars to measure distance to nebula
Galaxies
Hubble discovered the universe is made of billions of galaxies.
These findings opened up the field of Cosmology

40. Galaxies (Andromeda)

41. Kinds of Galaxies
Spiral
Elliptical
Irregular & Dwarf

42. TYPES OF GALAXIES

43. DEEP FIELD IMAGE

44. The Large-Scale Structure of the Universe
The Local Group
Milky way, Andromeda galaxy, and ~50 others
Groups, clusters, superclusters
Voids

45. The Large-Scale Structure of the Universe

46. The Astronomical Distance Scale How Far Away Are Galaxies?
For close stars use “parallax”
For distant stars use “standard candles”:
Cepheid variable stars
Large galaxies
Type 1 supernovae

47. The Big Bang
Distant galaxies are moving away from us – the farther away they are, the faster they’re moving.
The early universe was hotter and denser than today.
These studies also hint at how the universe will end.

48. Evidence for the Big Bang
Universal expansion
Abundance of light elements, especially D/H
Cosmic microwave background radiation at ~ 2.7 Kelvin

49. The Redshift and Hubble’s Law
Galactic redshift

50. The Redshift and Hubble’s Law
Galactic redshift
Hubble’s Law
The farther away a galaxy, the faster it recedes
V = H x d

51. The Redshift and Hubble’s Law
Galactic redshift
Hubble’s Law
The farther a galaxy, the faster it recedes
V = H x d

52. The Redshift and Hubble’s Law
Galactic redshift
Hubble’s Law
The farther a galaxy, the faster it recedes
V = H x d

53. The Redshift and Hubble’s Law
Galactic redshift
Hubble’s Law
The farther a galaxy, the faster it recedes
V = H x d

54. The Redshift and Hubble’s Law
Galactic redshift
Hubble’s Law
The farther a galaxy, the faster it recedes
V = H x d

55. Expanding Balloon Analogy
Some Useful Analogies
Raisin-Bread Dough Analogy
Expanding Balloon Analogy

56. Some General Characteristics of an Expanding Universe
All matter heats when compressed and cools when it expands.
Hence, universal “freezings”

57. What We Don’t Know: Dark Matter and Ripples at the Beginning of Time

58. What We Don’t Know: Dark Matter and Ripples at the Beginning of Time

59. The End of the Universe Is the universe open, closed, or flat?
Current data now suggests:
The total mass suggests an open universe
Type Ia supernovas reveal the expansion rate
And the universal expansion is speeding up!!!
Ascribed to “Dark energy”
70% of the universe’s mass is is something mysterious!!!