Our Own Galaxy - Structure
Galaxy derives from the Greek word galaktikos which means "milky white". The word "Galaxy" (capitalized) is used to refer to our own galaxy, the Milky Way. Today we know this band of light is composed of countless numbers of individual stars and that we are among them. We're far out from the center of a thin disk. In the early part of the 20th century, Kapteyn (~1920) counted stars to discern the shape of the Galaxy. He used spectral types to get absolute magnitudes and the distance modulus to derive the locations of the stars (the stellar distribution). Doing this it appears that we are near the center of an elliptical distribution of stars! But we know that the Galaxy is a thin disk and we are near the edge. So a model with us at the center is not correct. Then what was wrong? There are dust particles in interstellar space (~0.1 micron in size) which Kapteyn did not consider at that time. Dust particles absorb and scatter light strongly in the UV and visible. Stars appear dimmer and hence appear further away than they really are. We can only see stars to about 1 to 2 kpc away in the visible. Then what's the real picture of the Galaxy? Our own galaxy consists of about 200 billion stars, with our own Sun being a fairly typical specimen. It is a fairly large spiral galaxy and it has three main components: a disk, in which the solar system resides, a central bulge at the core, and an all encompassing halo.
Disk: The disk of the Milky Way has four spiral arms and it is approximately 300pc thick and 30kpc in diameter. It is made up predominantly of Population I stars which tend to be blue and are reasonably young, spanning an age range between a million and ten billion years. Bulge: The bulge, at the centre of the galaxy, is a flattened spheroid of dimension 1kpc by 6kpc. This is a high density region where Population II stars predominate---stars which tend toward red and are very old, about 10 billion years. There is growing evidence for a very massive black hole at its centre. Halo: The halo, which is a diffuse spherical region, surrounds the disk. It has a low density of old stars mainly in globular clusters (these consist of between 10,000 - 1,000,000 stars).The halo is believed to be composed mainly of dark matter which may extend well beyond the edge of the disk.

Milky Way in Multiwavelengths

Unlike the visible spectrum, infrared and radio wavelengths allow us to see through the dust. We can directly see the shape of the Milky Way. Besides infrared and radio, we can get the idea what other events are going on within Our Galaxy by probing it in other wavelengths. Click each picture to study the Galaxy in multiwavelengths.

	Radio Continuum 0.4 GHz
	Atomic Hydrogen
	Radio Continuum 2.7 GHz
	Molecular Hydrogen
	Infrared
	Mid Infrared
	Near Infrared
	Optical
	X-Ray
	Gamma Ray

Radio Continuum 0.4 GHz Intensity of radio continuum emission from high-energy charged particles in the Milky Way, from surveys with ground-based radio telescopes (Jodrell Bank Mark I and Mark IA, Bonn 100-meter, and Parkes 64-meter). At this frequency, most of the emission is from electrons moving through the interstellar magnetic field at nearly the speed of light. Shock waves from supernova explosions accelerate electrons to such high speeds, producing especially intense radiation near these sources. Emission from the supernova remnant Cas A near 110¡Ælongitude is so intense that the diffraction pattern of the support legs for the radio receiver on the telescope is visible as a cross shape. [Go back to diagram]

Atomic Hydrogen Column density of atomic hydrogen,derived on the assumption of optically thin emission, from radio surveys of the 21-cm transition of hydrogen. The 21-cm emission traces the "cold and warm" interstellar medium, which on a large scale is organized into diffuse clouds of gas and dust that have sizes of up to hundreds of light-years. Most of the image is based on the Leiden-Dwingeloo Survey of Galactic Neutral Hydrogen using the Dwingeloo 25-m radio telescope; the data were corrected for sidelobe contamination in collaboration with the University of Bonn. [Go back to diagram]

Radio Continuum 2.7 GHz Intensity of radio continuum emission from hot, ionized gas and high-energy electrons in the Milky Way, from surveys with both the Bonn 100-meter, and Parkes 64-meter radio telescopes. Unlike most other views of our Galaxy presented here, these data extend to latitudes of only 5¡Æ from the Galactic plane. The majority of the bright emission seen in the image is from hot, ionized regions, or is produced by energetic electrons moving in magnetic fields. The higher resolution of this image, relative to the 408 MHz picture above, shows Galactic objects in more detail. Note that the bright "ridge" of Galactic radio emission, appearing prominently in the 408 MHz image, has been subtracted here in order to show Galactic features and objects more clearly. [Go back to diagram]

Molecular Hydrogen Column density of molecular hydrogen inferred from the intensity of the J =1-0 spectral line of carbon monoxide, a standard tracer of the cold, dense parts of the interstellar medium. Such gas is concentrated in the spiral arms in discrete "molecular clouds." Most molecular clouds are sites of star formation. The molecular gas is pre-dominantly H2, but H2 is difficult to detect directly at interstellar conditions and CO, the second most abundant molecule, is observed as a surrogate. The column densities were derived on the assumption of a constant proportionality between the column density of H2 and the intensity of the CO emission. [Go back to diagram]

Infrared Composite mid-and far-infrared intensity observed by the Infrared Astronomical Satellite (IRAS) in 12, 60, and 100 micron wavelength bands. The images are encoded in the blue, green, and red color ranges, respectively. Most of the emission is thermal, from interstellar dust warmed by absorbed starlight, including star-forming regions embedded in interstellar clouds. The display here is a mosaic of IRAS Sky Survey Atlas images. Emission from interplanetary dust in the solar system, the "zodiacal emission," was modeled and subtracted in the production of the Atlas. [Go back to diagram]

Mid Infrared Mid-infrared emission observed by the SPIRIT III instrument on the Midcourse Space Experiment (MSX) satellite. Most of the diffuse emission in this wavelength band is believed to come from complex molecules called polycyclic aromatic hydrocarbons, which are commonly found both in coal and interstellar gas clouds. Red giant stars, planetary nebulae, and massive stars so young that they remain deeply embedded in their parental molecular gas clouds produce the multitude of small bright spots seen here. Unlike most of the other maps, this map extends only to 5¡Æ above and below the Galactic plane. [Go back to diagram]

Near Infrared Composite near-infrared intensity observed by the Diffuse Infrared Background Experiment (DIRBE) instrument on the Cosmic Background Explorer (COBE) in the 1.25, 2.2, and 3.5 micron wavelength bands. The images are encoded in the blue, green, and red color ranges, respectively. Most of the emission at these wavelengths is from relatively cool giant K stars in the disk and bulge of the Milky Way. Interstellar dust does not strongly obscure emission at these wavelengths; the maps trace emission all the way through the Galaxy, although absorption in the 1.25 micron band is evident toward the Galactic center region. [Go back to diagram]

Optical Intensity of visible (0.4 - 0.6 micron) light from a photographic survey. Due to the strong obscuring effect of interstellar dust, the light is primarily from stars within a few thousand light-years of the Sun, nearby on the scale of the Milky Way. The widespread bright red regions are produced by glowing, low-density gas. Dark patches are due to absorbing clouds of gas and dust, which are evident in the Molecular hydrogen and Infrared maps as emission regions. Stars differ from one another in color, as well as mass, size and luminosity. Interstellar dust scatters blue light preferentially, reddening the starlight somewhat relative to its true color and producing a diffuse bluish glow. This scattering, as well as absorption of some of the light by dust, also leaves the light diminished in brightness. The panorama was assembled from sixteen wide-angle photographs taken by Dr. Axel Mellinger using a standard 35-mm camera and color negative film. The exposures were made between July 1997 and January 1999 at sites in the United States, South Africa, and Germany. Image courtesy of A. Mellinger. [Go back to diagram]

X-Ray Composite X-ray intensity observed by the Position-Sensitive Proportional Counter (PSPC) instrument on the Rontgen Satellite (ROSAT). Images in three broad, soft X-ray bands centered at 0.25 , 0.75, and 1.5 keV are encoded in the red, green, and blue color ranges, respectively. In the Milky Way, extended soft X-ray emission is detected from hot, shocked gas. At the lower energies especially, the interstellar medium strongly absorbs X-rays, and cold clouds of interstellar gas are seen as shadows against background X-ray emission. Color variations indicate variations of absorption or of the temperatures of the emitting regions. The black regions indicate gaps in the ROSAT survey. [Go back to diagram]

Gamma Ray Intensity of high-energy gamma-ray emission observed by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory (CGRO). The image includes all photons with energies greater than 300 MeV. At these extreme energies, most of the celestial gamma rays originate in collisions of cosmic rays with hydrogen nuclei in interstellar clouds. The bright, compact sources near Galactic longitudes 185¡Æ, 195¡Æ, and 265¡Æ indicate high-energy phenomena associated with the Crab, Geminga, and Vela pulsars, respectively. [Go back to diagram]