The crammed centre of Messier 22This is the center of the globular cluster Messier 22 as seen by the Hubble Space Telescope. Globular clusters are more-or-less spherical collections of densely packed stars; they are relics of the early years of the Universe with ages of in the range of 12 to 13 billion years. The Universe is only 13.8 billion years old.

M22 is one of about 150 globular clusters in the Milky Way, and it is also one of the closest to Earth, only about 10,000 light-years away. The cluster has a diameter of about 70 light-years and appears to take up a patch of sky the size of the full Moon. However, the light from the stars in the cluster is not very bright because it is dimmed by dust and gas between the solar system and the cluster.

M22 in particular has some fascinating features: six planet-sized objects that are not orbiting a star have been detected in the cluster, and it contains two black holes.

Image Credit: ESA/NASA

Cosmic Yardsticks

A cosmological measuring tapeThis is the spiral galaxy NGC 3021 which lies about 100 million light-years away in the constellation of Leo Minor (The Little Lion).

This galaxy contains several Cepheid variable stars which can be used work out the distance to the galaxy. These stars pulsate at a rate that is closely related to their intrinsic brightness. Measurements of their rate of pulsation and their observed brightness provide information used to calculate the distance to them and, thus, the galaxy itself.

NGC 3021’s Cepheids were also used to calibrate another even brighter distance marker that can be used over greater distances: a Type Ia supernovae. One of these bright explosions was observed in NGC 3021 in 1995.

Image Credit: ESA

Arp 273

Apr 273Arp 273 is a group of interacting galaxies 300 million light years away in the constellation Andromeda. The larger of the spiral galaxies, known as UGC 1810, is about five times more massive than the smaller companion UGC 183. UGC 1810 is tidally distorted into a rose-like shape by the gravitational pull of the smaller galaxy which is thought to have actually passed through the larger one.

Image Credit: NASA

A Dwarf Galaxy

The mysteries of UGC 8201The galaxy UGC 8201 is classified as a dwarf irregular galaxy because of its small size and chaotic structure. It’s a bit less than15 million light-years away in the constellation of Draco (the Dragon). As with most dwarf galaxies, it is a member of a larger group of galaxies, in this case, the M81 galaxy group. This group is one of the nearby neighbors to the Local Group of galaxies which contains our galaxy, the Milky Way.

Image Credit: ESA / NASA