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Our Galaxy - Our Satellite Galaxies

Our Milky Way galaxy is a part of a larger group, but there are several smaller galaxies that are close enough to us to be considered "satellite galaxies." There are 12 known satellites of our galaxy:
 
Name: Distance (Light-Years): Diameter (Light-Years):
Sagittarius Dwarf (Sgr) 50,000 10,000
Large Magellanic Cloud 160,000 20,000
Small Magellanic Cloud 180,000 15,000
Ursa Minor Dwarf Galaxy (UMi) 220,000 1000
Sculptor Dwarf Galaxy (Scl) 260,000 1000
Draco Dwarf Galaxy (Dra) 270,000 500
Sextans Dwarf Galaxy (Sex) 290,000 3000
Carina Dwarf Galaxy (Car) 330,000 500
Fornax Dwarf Galaxy (For) 450,000 3000
Leo II Dwarf Galaxy (LeoII) 670,000 500
Leo I Dwarf Galaxy (LeoI) 830,000 1000
Pegasus Dwarf Galaxy 2,600,000, 2000

The last eight dwarfs (small galaxies) are in the direction of the named constellation, but the stars of the constellations are in our own galaxy.

With the exception of the Large and Small Magellanic Clouds, all of our satellite galaxies are of the Dwarf Spheroidal class. These galaxies were once thought of as globular clusters, but these dwarf galaxies are different by two major differences:

  • Globular clusters contain old stars of the same population - the Dwarf Galaxies also contain old stars, but of as more expanded variety (i.e. they did not form from the same, single molecular cloud)
  • The Mass to Luminosity ratios are much higher in Dwarf Galaxies than in Globular Clusters - this means there is significant levels of Dark Matter in Dwarf's

In addition, dwarf galaxies seem to have a higher ratio of Iron versus globulars. What that means is not entirely clear, but indicates that some newer stars must also have existed within the dwarf in the past.

We do not have many images of our dwarf neighbors, but here is an image of the Sagittarius Dwarf Galaxy:

Another dwarf galaxy, the farthest satellite galaxy, is the Leo I dwarf galaxy:

The newest member of our satellite galaxies is the Pegasus Dwarf Galaxy. This galaxy was discovered by the Keck telescope (it was hidden until then by the glare from M31 - the Andromeda Galaxy). This is a larger dwarf that contains blue stars:

These dwarf galaxies are beyond the view of the unaided eye, and though a telescope might be confused as a globular clusters. But there are two satellites that are unmistakable: The Large and Small Magellanic Clouds.

These two irregular galaxies reside in the Southern skies and therefore hidden from view to those of us in the United States, Canada and Europe.

The Large Magellanic Cloud:

This galaxy is classified as an irregular galaxy that may have tiny arms. Its distorted shape is though to be the result of the gravity from our galaxy.

The Large Magellanic Cloud is host to the Tarantula Nebula and the very important supernova remnant of SN1987a - the supernova that proved a flood of neutrinos does emanate from the dying core.

The Tarantula Nebula:

A nebula like this, containing a large amount of dust and hydrogen, is though to be a nursery for new stars to be born.

SN1987a:

The Small Magellanic Cloud has its own share of notoriety. In 1912, Henrietta Leavitt discovered a period/luminosity relation of a newly discovered variable called a Cepheid variable from within the Small Magellanic Cloud. Her discovery would allow the accurate measure of long distances by using the Cepheid period/luminosity as a ruler.

The Small Magellanic Cloud (above) is also considered an irregular galaxy, but does not have the near distinct central bar or arms of the Large Magellanic Clouds.

 

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