Interstellar Matter

1. Clouds of interstellar gas and dust which can be seen throughout a galaxy are broadly
termed _______________. There are four basic varieties: reflection, diffuse or emission,
dark, and planetary.

2. The densities of gaseous nebulae are very low. Even the brightest contain only about 100 /
10,000 / 1,000,000 (circle one) atoms or ions per cubic centimeter. Under standard
conditions, air has a particle density of 2.7 x 1019 molecules per cubic centimeter.

3. _______________ nebulae are normally found in OB-associations where ultraviolet light
    from nearby luminous stars (T > 25,000 K) causes the gases in the cloud to emit light in their
    own colors. The Orion nebula, located in the center of the sword of Orion, the hunter is
    probably the best example of this type of nebulosity. These types of nebulae are associated
    with stellar BIRTH/MATURITY/DEATH (Circle One). They almost always have cold
    regions of _______________ embedded in the hot (approx. 10,000 K) gas implying a lack
    of thermodynamic equilibrium. These types of nebulosities produce an
    EMISSION/CONTINUOUS/ABSORPTION (circle one) spectrum.

4. ______________ nebulae can be seen because of the light which they scatter from nearby
    stars. These clouds are composed of very small grains of _______________ which are
    nearly the same size as the light being emitted by the stars which are causing them to shine.
    This light is therefore SCATTERED/REFLECTED (circle one). The Pleiades, a star cluster
    located on the "shoulder" of Taurus, the bull contains one of the best examples of this type of
    nebulosity.

5. _______________ nebulae are formed when low mass stars, near the end of their lives,
    eject a shell of gaseous matter which is made to glow from energy escaping the degenerate,
    hot core of a ____________________ star. The ring nebula in Lyra highlights such an
    object. These types of nebulosities produce an
    EMISSION/CONTINUOUS/ABSORPTION (circle one) spectrum.

6. _______________ nebulae reveal themselves when large interstellar clouds of dust absorb
    the radiation from stars which lie behind them. These clouds which also contain hydrogen
    and helium gas, are associated with stellar BIRTH/MATURITY/DEATH (circle one). The
    Horsehead nebula, near the southern end of the belt of Orion is a prime example of this type
    of structure.

7. In interstellar nebulae there ore often compact clouds of dust having moderately high
    densities. These can shield the local environment from high energy radiation (particularly the
    short ultraviolet) allowing for the formation of molecules which can be detected through their
    radio emissions. The detection of interstellar molecules by radio astronomers serves as a
    tracer for the location of regions where stellar _______________ may be occurring.

8. Cold neutral hydrogen atoms neither absorb nor emit light at optical wavelengths, but they
    do emit radiation at a wavelength of 21 cm in the _______________ region of the
    electromagnetic spectrum. This results when the ground state electron of a hydrogen atom
    spontaneously changes its spin from the same direction as the proton to the opposite
    direction of the proton.

9. Clouds of interstellar neutral hydrogen are referred to as _______________ regions while
clouds of ionized hydrogen, found in diffuse and planetary nebulae are called
_______________ regions.

10. Absorption by interstellar dust is more prevalent NEAR THE PLANE OF THE
    GALAXY/NEAR HIGH GALACTIC LATITUDES (circle one). All stars at substantial
    distances from us appear BRIGHTER/FAINTER (circle one) and ________________
    (color) due to the scattering effects of dust in the galaxy and throughout intergalactic space.
    This tells astronomers something about the average size of the dust particles, which can be
    no more than about 0.1 micron or 10-5 cm in diameter. Typically this dust is composed of
    ________________, ________________, ________________, and
    ________________.

11. The vast majority (about 99 percent) of interstellar matter is found in the form of
DUST/GAS (circle one) which DOES/DOES NOT (circle one) absorb starlight.

THE MILKY WAY GALAXY

12. The sun is located in a huge stellar system known as the ____________________ galaxy.
The word galaxy is derived from "galas," the Greek word for _______________.

13. From the top down, our galaxy appears similar to a _______________ with huge
_______________ arms emanating from a central _______________ where the density of
stars is the greatest.

14. Viewing our galaxy from its equatorial plane, it appears as a _______________ (shape)
    structure with a central _______________. Mapping the galaxy has proven to be a very
    complex matter. We cannot observe the center of the galaxy, because there is an enormous
    amount of interstellar matter between us and the galactic nucleus. This fog results from the
    GAS/DUST (circle one) component of interstellar matter found within the Milky Way.

15. Our galaxy's diameter is approximately ______________ light years. While most of the
    matter appears to reside near the galactic plane, a huge halo of _______________ clusters,
    spherically symmetrical compact groupings of stars, each containing from about 10,000 to
    one million members occupies the regions above and below the plane of the Milky Way.

16. The galactic equator runs through the middle of the band of light we refer to as the
    ___________________. It is inclined 62 degrees to the plane of the celestial equator and
    extends 360 degrees around the sky. This implies that we are not at the _______________
    of this system. Current estimates place the solar system at a distance of approximately
    _______________ light years from the galactic center.

17. There appears to be two different populations of stars in the Milky Way. These can be
    described as _______________ rich or poor stars. These two broad categories of stars
    occupy specific regions in spiral galaxies like our Milky Way. Before a star is considered
    metal rich, at least _______________ percent of its mass must be contained in elements
    heavier than hydrogen and helium.

18. Population _______________ stars can be found in the galactic center, as well as the
halo, which defines the realm of the globular clusters. Population _______________ stars
are found in the spiral arms of the galaxy.

19. Identify the two populations of stars found in the Milky Way with respect to:

Population I Population II

Color
Age
Metal Content
Interstellar Gas and Dust

20. The mass of the Milky Way is now believed to be about _______________ solar masses.
    This is higher than earlier estimates, because it is now realized that the galactic
    _______________ is much more massive than previously assumed. Not all of the mass in
    the galaxy appears to be observable in stars and interstellar matter. In the center of the
    galaxy, where densities are HIGHEST/LOWEST (circle one), a super massive
    ____________________ may reside.

21. As the galaxy rotates, stars in the galactic plane travel in _______________ (shape) orbits
around the nucleus. The orbital speed of the sun is about 150 miles/sec (250 km/sec) and it
takes about 200 million years to complete one orbit.

GALAXIES IN GENERAL

22. The basic way that matter congregates in the universe is in the form of ____________.

23. The Milky Way Galaxy is one of several THOUSANDS/MILLIONS/BILLIONS (circle
    one) of island universes which are known to inhabit the universe in which we live. Only three
    other systems are easily visible to the unaided eye. The largest and most distant of these is
    the great spiral galaxy called _________________, which lies at a distance of 2.1 million
    light years from the sun. It is slightly more massive than the Milky Way. The other two are
    the Large and the Small _________________________, satellite systems of our own
    Milky Way which can only be observed from the Southern Hemisphere.

24. Galaxies were classified by the astronomer Edwin _______________ in the 1920's
according to their appearance and structure. These fall into three broad categories: spirals,
ellipticals, and irregulars.

25. _______________ galaxies tend to be large to very large as galaxies go and have
population II stars in their nuclei and halos, plus population I stars with plenty of interstellar
gas and dust in their arms.

26. _______________ range from very small, for galaxies, to extremely large. They seem to
consist of population II stars only, and they have little gas or dust.

27. _______________ tend to be smallish, for galaxies, and have many population I stars
along with substantial quantities of gas and dust.

28. The Milky Way and Andromeda galaxies are the two largest members of a cluster of about
    _______________ (number) galaxies called the ____________________. At least one
    other spiral (Messier 33) and several irregular and dwarf elliptical galaxies coexist in this
    assemblage. Most galaxies are FOUND/NOT FOUND (circle one) in clusters.

29. The nearest giant cluster of galaxies is the _______________ cluster which contains
several DOZENS/THOUSANDS (circle one) of galaxies. The largest galaxy of this cluster
is Messier 87, a giant elliptical, which is among one of the most massive galaxies known.

30. Most typical galaxies produce ABSORPTION/EMISSION (circle one) spectra which
    arise as a composite signature of the different types of stars which are contained in them. A
    group of peculiar galaxies, known as _______________ galaxies, however, have emission
    lines and very bright nuclei.

31. There also exists another type of peculiar galaxy which emits far more radio noise than
    normal. These galaxies are known as _______________ galaxies. Some of the radio
    radiation arises from the cores of these structures, but most of these galaxies also display
    lobes filled with high-energy electrons which are located outside the optical limits of the
    object.

32. The two types of peculiar galaxies mentioned in the last several problems are also referred
    to as _______________ galaxies. They apparently have violent processes taking place in
    their cores which may be the result of rapidly rotating disks of matter plunging into large,
    super massive ____________________.

33. Even more peculiar than the active galaxies are _______________, which are only seen at
    very great distances. They appear starlike and are often variable in light. These objects
    generate huge amounts of energy across the entire electromagnetic spectrum from a very
    small volume of space. They are probably the cores of very young and extremely active
    galaxies.

THE UNIVERSE

34. All matter-energy everywhere is a good definition for the __________________.

35. The study of the large-scale structure of the universe is known as __________________.
It is by far the most speculative branch of present-day astronomical research.

36. In 1823 Heinrich _________________ published a paper on why the sky is dark at night.
    If the universe were infinite in extent, there would be an infinite number of stars in the sky and
    every available space in the heavens would be occupied by a luminary. The night sky should
    therefore have the same brightness as the surface brightness of a typical star. Why then is the
    nighttime sky dark? Explain the riddle of ________________ Paradox using the following
    clues as a guide.
    a. Is the universe infinite?
    ___________________________________________________
    _________________________________________________________________
    b. The expanding universe:
    __________________________________________________
    ______________________________________________________________________
    ______________________________________________________________________
    ______________________________________________________________________
    c. The red shift:
    ___________________________________________________________
    ______________________________________________________________________
    d. Dust:
    _________________________________________________________________

37. In 1916 Albert Einstein demonstrated that the universe could not be static; it must either
    expand or contract. During the next two decades, Georges Lemaitre and George Gamow
    proposed that the entire visible universe had been formed from a single primordial event,
    known as the ___________________.

38. In 1924 Edwin _______________ was able to conclusively prove that the so-called spiral
    and elliptical nebulae were in fact galactic systems in their own right. In 1929 he determined
    that all of the more distant galaxies were _______________ from us at rates which were
    proportional to their distances. Remember, that the determination of the distances to galaxies
    must be obtained without knowing their red shifts or there is no way one can independently
    correlate expansion rates (red shifts) with distances.

39. The Hubble constant, H0, is a determination of the rate at which the universe is
    _______________. It is measured in kilometers/second-- megaparsec. The best
    determination of H0 at present is _______________ km/sec-- Mpc. One megaparsec equal
    one million parsecs or 3.26 million light years.

40. Current estimates of the Hubble constant range from 45-75 km/sec-- Mpc. It is an
    extremely important number to clarify, for it gives astronomers the ability to determine the
    _______________ of the universe. The current value of H0 implies a 20 billion year old
    universe, or the maximum time since the big bang. A lower constant suggests an (a)
    OLDER/YOUNGER (circle one) universe because if would take objects longer to reach
    their observed distances. If H0 = 30 km/sec-- Mpc, then the universe would be
    approximately 30 billion years of age. A higher constant implies just the opposite. In fact if
    H0 = 75 km/sec-- Mpc, then the universe can only be about 13 billion years of age. The
    reciprocal of the Hubble constant is known as the Hubble time (T0 = 1/H0).

41. The oldest stars in the Milky Way galaxy are about _______________ billion years of age.
    Since these luminaries could only have been formed well after the big bang, it is
    EASY/DIFFICULT (circle one) to reconcile their ages with a Hubble constant as large as 75
    km/sec-- Mpc.

42. When we look out into space we are also looking back into ______________. The farther
out we see, the EARLIER/LATER (circle one) into the history of the universe we peer.

43. Quasars have provided astronomers with evidence for an evolutionary universe, that is, a
    universe which changes with time. How?_______________________________________
    ________________________________________________________________________
    The 3 K microwave background radiation represents the first visible artifact of the big bang
    itself, where the universe had thinned and cooled to the point that energy could escape. It also
    gives credence to an evolutionary universe since the big bang was a once and done event which
    happened at the creation of space and time.

44. One variant of the big bang hypothesis is a universe that will have a sufficiently high enough
    mean _______________ so that gravity will halt its expansion and force it to collapse upon
    itself to perhaps rekindle yet another big bang event sometime in the future. This type of
    accordion universe is called an _______________ universe.

45. If the mean density of the universe is less than the closure density, the universe will
____________________. Such a universe is termed an _______________ universe. Present
observations indicate that the universe is probably _______________.

46. Over the years there have been other cosmological models which do not invoke a primordial
    event. One of these hypotheses, first proposed in 1948 by Gold, Hoyle, and Bondi suggested a
    universe that was infinite in space and time, expanding, and homogeneous. Hydrogen was being
    created in the voids formed by the expanding galaxies. This hypothesis, no longer considered
    valid, is called the ____________________ theory.