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ScienceWeek
ASTRONOMY: ON THE MILKY WAY GAMMA-RAY GLOW
The following points are made by Nicholas White (Nature 2004 428:264):
1) The Milky Way glows brightly at gamma-ray wavelengths. Although this glow was discovered more than three decades ago, its origin has been a mystery. Now the Milky Way has been observed at these wavelengths with new clarity by astronomers using the European Space Agency (ESA) INTEGRAL observatory. F. Lebrun et al(1) report that the soft gamma-ray glow is the sum of radiation from previously unresolved point sources, many of them black holes and neutron stars, buried in gas and dust. This discovery points to a future in which soft gamma-rays will become a powerful tool for finding black holes in otherwise obscured regions.
2) Radiation such as gamma-rays and their less energetic cousins, X-rays, is used in everyday applications to see inside objects --for example, in medical scanners and for security screening at airports. This same penetrating power makes them a useful astronomical probe for finding and studying objects that may be buried deep in obscuring clouds of dust and gas. By the same token, the capability of gamma-rays to penetrate rather than be reflected makes it challenging to design gamma-ray telescopes, but until recently these telescopes have been little more than crude light buckets, with poor angular resolution (a few degrees, equivalent to many times the diameter of the Moon).
3) Despite their penetrating power, gamma-rays are eventually stopped and absorbed in the extreme depth of Earth's atmosphere -- which is fortunate for life on Earth. Consequently, the gamma-ray window is closed to Earth-bound telescopes; only with the start of the space age was it opened up for astronomical exploration. The first gamma-ray telescopes, launched in the 1970s, detected many bright cosmic sources, including the gamma-ray glow of the Milky Way.
4) The origin of these soft gamma-rays was puzzling. If it is truly a diffuse glow, then the energy being released is huge and would have ionized the molecules in the interstellar medium. The most promising explanation was that this emission originates from an unknown population of point sources, which is spread out on the sky but separated by less than the angular resolution of the gamma-ray telescopes available at the time. Confirming this theory required a new large telescope with much sharper imaging.
References:
1. F. Lebrun et al. Nature 428, 293-296; 2004
Nature http://www.nature.com/nature
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THE FORMATION AND EVOLUTION OF THE MILKY WAY GALAXY
The current consensus is that our Galaxy (the Milky Way) has four major components:
a) The Central Bulge, which consists of a dense spherical agglomeration of stars surrounding an apparent central massive *black hole of some millions of solar-masses (see related background material below).
b) A thin disk rotating around the Central Bulge, the disk with a mass of approximately 6 x 10^(10) solar-masses and consisting of relatively young stars, loose clusters of stars (open clusters), and gas and dust (interstellar material), with loose concentrations of the young stars and interstellar material into spiral arms. The thin Galactic disk is approximately 1000 light-years thick, compared with a diameter of over 100,000 light-years.
c) A faint roughly spherical halo with an estimated mass of 15 to 30 percent of the mass of the disk, the halo composed of old stars, some of which are in globular clusters, plus small amounts of hot gas, and all of it merging into the more conspicuous bulge of stars at the center of the Galaxy. The diameter of the halo is approximately the diameter of the disk.
d) An unseen halo of non-radiative matter (dark matter) with a total mass of at least 4 x 10^(11) solar-masses.
It is estimated there are in total approximately 2 x 10^(11) stars in our Galaxy, most of these with a mass less than the mass of the Sun. The thin disk is estimated to be 10 billion years old, and the globular clusters and most of the halo stars are estimated to be 12 to 14 billion years old. The Sun lies approximately 26,000 light-years from the center of the Galaxy, in one of the spiral arms.
The following points are made by Roland Buser (Science 2000 287:69):
1) In recent years it has become evident that in addition to the flattened thin disk there are one or more diffuse thicker disks superimposing the thin disk in the same plane, and a similar disk structure has been observed in several other galaxies seen edge-on.
2) Recent observations indicate that our Galaxy may have formed by aggregation of gas and stars from a reservoir of preexisting small galaxies in the local Universe. The process probably began more than 12 billion years ago with material of different original *angular momenta following two separate evolutionary lines, one into the slowly rotating halo and central bulge, and the other into the rapidly rotating disk.
3) The author suggests that the existence of distinct thick and thin disks in the Galaxy indicates that continuing mergers of satellite galaxies probably also determined the early evolution of the main structural component of the luminous Galaxy.
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Notes by ScienceWeek:
black hole: If the terminal stages of star death leave a remnant star mass greater than 3 solar-masses, the ultimate gravitational collapse will produce a black hole, a relativistic singularity. A black hole is a localized region of space from which neither matter nor radiation can escape. The "trapping" occurs because the requisite escape velocity, which can be calculated from the relevant equations, exceeds the velocity of light and is therefore unattainable. (Concerning the apparent black hole at the center of our Galaxy, see related background material below.)
angular momenta: The term "angular momentum" refers to the momentum possessed by a body by virtue of rotation -- rotation about another body and/or rotation about its own axis.
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ASTRONOMY: ON DUST AND GAS IN THE MILKY WAY
We reside in a star system (the Milky Way Galaxy) over 100,000 light years in diameter and containing over 100 billion stars. Almost every celestial object visible to the naked eye is part of this Galaxy. Exceptions include the Magellanic clouds, which are small irregular galaxies located in the southern sky, and which are apparently satellites of our Galaxy but not part of it. Another exception is the Andromeda galaxy, just visible to the naked eye as a faint patch of light in the constellation Andromeda. The Sun lies approximately 26,000 light years from the center of the Galaxy.
The Galaxy is apparently a spiral galaxy, but attempts to measure the dimensions of individual spiral arms and other aspects of the Galaxy are hampered by obscuring dust in the Galactic disk and by the difficulty of estimating distances within the Galaxy. The Galaxy is believed to be a "*barred spiral", since there is some evidence for a bar-like structure in the central regions. The age of the Galaxy is still uncertain, but the disk is at least 10 billion years old, while the globular clusters of stars and isolated stars (halo stars) in the periphery are apparently 12 to 14 billion years old.
The disk is thin, approximately 1000 light years, compared with a diameter of over 100,000 light years. In general, astronomers know more about distant galaxies than they do about our own Milky Way Galaxy, and the major reason for this is that other stars, the gas, and especially all the dust in the disk obscure the full extent of the structure of the Galaxy as observed from within it.
The following points are made by Henry Freudenreich (American Scientist 1999 87:418):
1) The tenuous matter between the stars in our Galaxy is approximately 90 percent hydrogen and 9 percent helium. The remaining 1 percent consists of heavier elements collected into fine particles called "dust". Most of these particles are less than 1 micron in diameter and are rich in carbon and silicates.
2) Although there is a diffuse distribution of dust throughout the Galaxy, most of the dust is collected into ragged clouds of various sizes and densities. The interiors of the more massive clouds, which are shielded from starlight, are relatively cool and dense. In these interiors, hydrogen atoms are able to combine to form hydrogen molecules [H(sub2)], and for this reason these dust agglomerations are called "molecular clouds". Such dense molecular clouds are the birthplaces of stars.
3) Dust particles have complex dynamic interactions. They collide with each other and also with both neutral and ionized atoms of gas, so that dust particles are subjected to hydrodynamic and magnetic forces in addition to gravity. Dust is also pushed around by *stellar winds, which are mostly protons and electrons. As a result of these various forces, Galactic dust exhibits a variety of large-scale features -- huge bubbles, tendrils, and wavy sheets -- that are unique among the components of the Galaxy. Some of the wispy dust formations are reminiscent of cirrus clouds on Earth, and these formations are called "Galactic cirrus".
4) Radio-astronomers have used the radio emissions of atomic hydrogen atoms and carbon monoxide molecules to map the distribution of gas over most of the Galaxy. But such maps do not necessarily correspond to the distribution of stars in the Galaxy, and radio surveys provide only a partial picture of the global structure of the Galaxy.
5) The distribution of gas is also not useful for mapping the inner part of the Galaxy (within approximately 15,000 light years of the center). We do know that the density of gas and dust increases as we move inward, peaking at a distance of approximately 12,000 light years from the center, where it forms a collection of clouds known as the "molecular ring". The density is again lower inside the ring, where the apparent motions of the gas are peculiar and poorly understood.
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Notes by ScienceWeek:
barred spiral: In general, a "barred spiral galaxy" is a type of galaxy with spiral arms extending from an almost rectangular or cigar-shaped bar of stars across its central region.
stellar winds: In general, the term "stellar wind" refers to the outflow of gas from the surface of a star. The Sun, for example, loses approximately 10(-14) of its mass each year via such a wind ("solar wind").
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