 Clusters are important for our distance ladder, so let's point out a few things about them. There are two kinds of clusters. Open clusters, usually a few hundred stars lightly bound by gravity, and globular clusters, hundreds of thousands of stars tightly bound by gravity. The Pleiades, or Seven Sisters, is an open cluster of extremely luminous blue stars, and it is one of the nearest star clusters to Earth. It is one of only a few open clusters whose distance can be measured by a parallax. This image is a close-up view of the jewel box cluster, taken by Hubble. Several very bright pale blue supergiant stars, a solitary ruby red supergiant, and a variety of other brilliantly colored stars are visible in the image. The huge variety in brightness exists because the brighter stars are 15 to 20 times the mass of the Sun. While the dimmer stars are less than half the mass of the Sun. This image shows a pair of colossal stars inside the Trumpler-16 embedded in the Carina Nebula, an immense nebula of gas and dust that lies approximately 7,500 light years from Earth. WR-25 is the brightest, situated near the center of the image. The neighboring TR-16-244 is the third brightest. Just to the upper left of WR-25. Second brightest to the left of WR-25 is not in the cluster. It's a low mass star located much closer to the Earth, and just happens to be in the line of sight to the Trumpler-16 cluster. The small open star cluster, Primus-24, lies at the core of the large emission nebula, NGC-6357. We'll cover emission nebula in another segment on the Milky Way. The brightest object in the picture is designated Prismus-24-1. It was once thought to weigh as much as 200 times the mass of the Sun. However, the high resolution Hubble Space Telescope images of the star show that it is really two stars orbiting one another. Here, we're zooming in to a glittering open star cluster. It contains a collection of some of the brightest stars seen in our Milky Way galaxy. Called Trumpler-14, it is located 8,000 light years away in the Karina Nebula. Because the cluster is only 500,000 years old, it has one of the brightest concentrations of massive luminous stars in the entire Milky Way. These blue-white stars are burning their hydrogen fuel so ferociously that they will explode as supernova in just a few million years. The combination of outflowing stellar winds and ultimately supernova blast waves will carve out caverns in nearby clouds of gas and dust. These fireworks will kick-start the beginning of a new generation of stars in an ongoing cycle of star birth and death. NGC 6791 is one of the oldest and largest open clusters known. It is 10 times larger than most open clusters and contains roughly 10,000 stars. Penetrating 25,000 light years of obstructing dust and stars, Hubble uses infrared to provide the clearest view yet of a pair of the largest young clusters of stars in our Milky Way. They are located less than 100 light years from the very center of the galaxy. ARCH's cluster is so dense over 100,000 of its stars would fill a spherical region that only contains 5 stars in our local neighborhood.