Nucleus (bulge) of Milky Way

The Milky Way Galaxy:

Reading: Chapter 23, The Milky Way Galaxy, A Spiral in Space

GALAXIES

Galaxies are clusters of stars, gas, dust, and Dark Matter (??) which serve as the primary indicators for our study of the structure of the Universe. We will spend the next few lectures talking about the properties of normal galaxies (with particular emphasis on our own galaxy, the Milky Way galaxy) and Active Galactic Nuclei (AGNs). We will concentrate on the aspects of galaxies which we will use in our discussions of the evolution of the Universe.

There are several kinds of normal galaxies among which are included those known as elliptical galaxies (for example, see M87), Irregular galaxies (for example, see N4449), and S0 galaxies. These classes of galaxies can be seen in the Hubble Deep Field image to left. An even better view is shown in the Hubble Ultra Deep Field view of the Universe.

Overall, typical, large galaxies were classified by Hubble using the morphological scheme which bears his name,

and have approximate properties,

      Mass:            10⁹ to 10¹² Solar masses
      Diameter:        10⁵ to 10⁶ light years
      Number of Stars: 10⁸ to 10¹² stars
      Luminosity:      10⁸ to 10¹¹ Luminosity of the Sun

MILKY WAY GALAXY: OUR HOME

The Milky Way galaxy is generally classified as a four-armed SBb barred spiral galaxy . Examples of nice spiral galaxies and a barred spiral galaxy are,

M83

M104

M51

The Milky Way may be similar in appearance to the barred spiral galaxy, NGC 1300

although this cannot be conclusively argued because it is difficult to deduce our appearance from the inside.

Artist's rendition of the Milky Way

Vital Statistics of the Milky Way

   Mass            ~ 2x10¹¹ M(Sun) (visible disk)
                   ~ 6x10¹¹ M(Sun) (total?)
   Disk            ~ 100,000 light years    (visible)
                   ~ 300,000 light years    (total?)
                   ~ 1,000 light years      (thickness at the Sun's orbit) 
   Luminosity      ~ 2 x 10¹⁰ L(Sun)
   Number of stars ~ 4 x 10¹¹ stars
   Stellar density ~ 1 star per 125 cubic light years
   Gas density     ~ 1 atom per cubic centimeter (a sugar cube)

Note--the above stellar density implies that stars are separated by roughly 5 light years (1 light year = distance light travels in 1 year ~ 6 trillion miles ~ 9.5 trillion kilometers). The Sun's nearest neighbor is Proxima Centauri at a distance of ~ 4.3 light years.

Note--There are 200-400 billion stars in our Galaxy and the mass of the Galaxy is at least 600 billion times the mass of the Sun. The average mass of a star in our Galaxy is ~0.3 times the mass of the Sun. This is consistent with the suggestion that most of the material of our Galaxy is in some nonluminous form, the so-called dark matter .

The Structure of the Milky Way: The Disk, Bulge (Nucleus), and Halo of the Milky Way

The Milky Way (as do all other spirals and the other disk galaxies known as S0s) shows a luminous disk of stars, gas, dust, a central bulge, and a larger more spherical halo. Schematically, the visible portion of the Milky Way Galaxy looks like:

The Sun is in the disk of the Milky Way galaxy in a spiral arm, about 8 kpc (~26,000 l.y.), roughly 1/2 of the way from the Bulge of the Galaxy to the edge of the visible disk. The stars, gas,and dust, in general, orbit around the center of the disk of our Galaxy. Some stars, however, (e.g., globular clusters) orbit around the center of our Galaxy in the more spherical halo. Below, we give further details of each region and highlight important aspects of each, when warranted.

The disk contains youngish stars, as evidenced by the existence of some disk stars with large masses and the relatively large amounts of elements heavier than hydrogen and helium (the metals) in their envelopes and atmospheres (Population I stars), gas, and dust which orbit about the center of the Galaxy. The star orbits about the center of the Galaxy once every 225 million years, moving at a speed of 225 kilometers per second. The plentiful gas and dust in the disk result in continuing formation of stars. The visible part of the disk is roughly 100,000 light years in diameter and around 1,000 light years in thickness. As found from the detection of HI (neutral hydrogen) gas, the actual disk is traced to large radii and the true diameter of the disk is closer to 300,000 light years.
The halo is composed of old stars, as evidenced by the small amount of elements heavier than hydrogen and helium in their envelopes and atmospheres and their distinctly redder colors compared to the disk stars. Halo stars are considered Population II stars. Objects in the halo orbit the center of our galaxy in randomly inclined orbits. The halo contains little or no gas and dust and so does exhibit recent star formation. The halo contains the majority of the mass (in the form of Dark Matter) of our Galaxy and extends to large radii with diameter around 300,000 light years.
The Bulge contains gas, dust, and stars , again distinctly redder than the stars in the disk of the Galaxy, similar to those in the halo of our Galaxy. The bulge is a little flattened with polar and equatorial diameters of 30,000 light years and 40,000 light years, respectively. There is also activity near the center of the Bulge thought to be driven by the presence of a Black Hole of mass 3.7 million times that of the Sun.

Interesting Things (from a Cosmological standpoint) We Know About Our Galaxy

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