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Matrix Software > Learng Astrology > Astrophysical Directions > External Galaxies > Local Triplet

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Back to External Galaxies   |   Back to Astrophysical Direction 

Local Triplet

The nearest neighbors to the Milky Way are the Magellanic Clouds in the southern hemisphere, where they appear as large nebulous naked-eye objects. The Large Magellanic Cloud (LMC) or Nubecula Major (the 'Greater Little Cloud') and the Small Magellanic Cloud (SMC) or Nubecula Minor, the 'Lesser Little Cloud') appear to the naked-eye as detached portions of the Milky Way and they are, in fact, satellites of our galaxy. The Magellanic Clouds are a close binary pair of Irregular dwarf galaxies that, along with our galaxy, form a loose triplet sharing a common barycenter and located about 5 kiloparsecs from the center of our galaxy in the direction of the LMC.

The Magellanic Clouds

The Magellanic Clouds are located at less than one tenth the distance to the Andromeda galaxy (M.31) and are near enough that it is possible to obtain the color-magnitude diagrams of star clusters. A 20 inch telescope working on the clouds is therefore equivalent to the 200 inch telescope working on the more distant Andromeda galaxy. In the Magellanic Clouds, we do get the kind of view or perspective that we need to estimate the nature of our own Milky Way and other external galaxies. Here is a summary of data on these members of the Local Group as of 1968: (Other data is given in list of members of the Local Group of galaxies)

Geometric Center (1950) 81° = R.A. 12°= R.A.
-69°48 = Declination -73°06 = Declination
Apparent diameter 11.8° 4.2°
Inclination 65° 30°
Neutral H Mass ( Sun=1) 5.4x10 4.8x10
Total Mass (Sun=1) 6x10 1.5x10
Radial Velocity (km.sec) +270 +168
Note: Inclination is defined as the angle between the line of sight and the line perpendicular to the fundamental plane of a galaxy.

The Magellanic Clouds are dwarf Irregular galaxies, and the LMC has a Hubble type and luminosity classification Ir or SBc III-IV, and the SMC is classified Ir IV or Ir IV-V. The stellar content of the clouds is similar to our own galaxy with the following possible differences:

  • Some of the globular clusters in the Clouds differ significantly from their galactic counterparts. The very red colors of stars near the tips of the cluster color-magnitude relations are very striking.

  • Galactic Cepheids seem to differ from those in the Clouds in color and in the mean relation between period and pulsation amplitude.

  • There is less convincing evidence for differences between novae, giants with Mv = 0 and the giant branches of young clusters in the clouds and in the galaxy. (nine novae have been observed in the Clouds; four in the SMC, and five in the LMC)

  • The interstellar gas in the SMC, and perhaps in the LMC as well, is not as dusty as that in the Galaxy.

  • The ratio of gas mass to total mass is much higher in the Clouds than it is in the Galaxy. This shows that the Magellanic Clouds are less evolved than the Galaxy.

  • The observed differences between the clouds and the galaxy occur among both old and young stellar populations. Possibly small differences in the helium and/or heavy element abundance are responsible for the observed differences between stars in the Galaxy and the Magellanic Clouds.

Copyright (c) 1997-99 Michael Erlewine

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