The Magellanic Clouds

Two small irregular galaxies called the "Magellanic Clouds" are relatively near the Milky Way. The Large Magellanic Cloud is at about 160,000 light years and the Small Magellanic Cloud is at about 200,000 light years from us. This compares with about 100,000 light years across our galaxy and about three million light years to the Andromeda galaxy, the nearest large galaxy.

These smaller galaxies apparently looked somewhat like clouds to Ferdinand Magellan who with his men were the first Europeans to view them in the southern hemisphere. Magellan's five ships and 240 men were the first from Europe to venture here, and the remnant of one ship and 17 men who returned to Spain had the distinction of being the first to sail around the world.

The Large Magellanic Cloud was the location of Supernova 1987A, which provided us with valuable information about neutrinos. It is also the location of one of our best candidates for a black hole in a binary star system.

Star Clusters

Within our Milky Way galaxy are recognizable clusters of stars. Containing from dozens to millions of stars in a region a few parsecs across, they have been useful for stellar studies. Presumed to have formed about the same time because of their association, they serve as laboratories for the study of star formation and evolution.

A familiar example of what is called an "open cluster" is the Pleiades, or Seven Sisters which can be seen with the naked eye. When a Hertzprung-Russell diagram (H-R diagram) is done for this cluster, stars of all colors (spectral classes) are seen. This implies that the cluster is relatively young, since the O and B type stars are short-lived. An age of 25 million years or less is estimated. Containing about 3000 stars, the Pleiades cluster is about 120 pc from the Sun and spans about 4 pc.

A very different story is told by Omega Centauri, a dense globular cluster of stars. Omega Centauri is the largest known globular cluster in our galaxy. It is a very dense collection of stars about 5000 pc from the Sun and spans about 40 pc. Globular clusters are roughly spherical and can contain hundreds of thousands to millions of stars. The population of Omega Centauri is estimated to be about 10 million. The H-R diagram of Omega Centauri reveals an absence of O and B type stars, the hot blue stars that have short lifetimes. This tells us that such clusters are composed of old stars. Globular clusters consist of red and yellow (low and intermediate mass) stars less than about 0.8 solar masses.

These are images of the Omega Centauri cluster credited to NASA and UIT. The left image is in the visible while the right image is an ultraviolet image taken by the Ultraviolet Imaging Telescope. Most of the stars are in the red range, as explained above, but some stars are in later stages of their evolution that cause them to exhibit higher temperatures for a relatively brief period on their way to their final white dwarf stages.

Omega Centauri can be seen with the unaided eye, and was included in an early star catalog as a star. It is one of about 150 known globular clusters in the Milky Way.

Shown at left is the globular cluster G1 in galaxy M31 (The Andromeda Galaxy). It was imaged by the Hubble Space Telescope. It is the largest known globular cluster in M31 or the local cluster of galaxies, and it is thought to contain about 300,000 stars. 图中左侧显示的是位于银河系M31（仙女座星系）中的球状星团G1。该星团由哈勃太空望远镜拍摄。它是M31或本地星系团中已知最大的球状星团，被认为包含约30万颗恒星。