Gas and dust collect around the black hole to form an accretion disk. ![]() The Milky Way is not currently an active galaxy, although it likely experienced a burst of activity in the past few million years.Īstronomers think this excess energy comes from areas near the galaxies’ central supermassive black holes, which range from hundreds of thousands to billions of times the mass of our Sun. They can be spiral, elliptical, or irregular. Active GalaxiesĪround 10% of known galaxies are active, which means their centers appear more than 100 times brighter than the combined light of their stars. Irregular galaxies may also hold significant amounts of gas and dust – essential ingredients for making new stars. Irregular galaxies born from galaxy interactions or collisions typically host a mix of older and younger stars, depending on the characteristics and composition of the original galaxies. Some scientists theorize that some large irregular galaxies could represent an intermediate step between spiral and elliptical galaxies. Or perhaps when galaxies collide, they create a larger, oddly shaped mashup. Some, like gas-rich dwarf galaxies, may be new, formed by material pulled from such encounters. For example, one spiral galaxy passing another with a stronger gravitational pull could lose some of its material, become distorted, and morph into a new shape. They range from dwarf irregular galaxies with 100 million times the Sun’s mass to large ones weighing 10 billion solar masses.Īstronomers think these galaxies’ odd shapes are sometimes the result of interactions with others. Irregular galaxies have unusual shapes, like toothpicks, rings, or even little groupings of stars. The Hubble Space Telescope captured this image of NGC 5264, an irregular dwarf galaxy. Scientists think the presence of a bar indicates that a galaxy has reached full maturity. Barred spirals sport ribbons of stars, gas, and dust that cut across their centers. The youngest stars form in gas-rich arms, while older stars can be found throughout the disk and within the bulge and halo.īoth the Milky Way and the Andromeda galaxies belong to a subtype known as barred spirals, which make up two-thirds of the group. Spiral galaxies are surrounded by halos, mixtures of old stars, star clusters, and dark matter – invisible material that does not emit or reflect light but still has a gravitational pull on other matter. The spiral arms can be wound tightly or loosely, and some cannot be seen from Earth because we view the galaxy from the side, edge on. These galaxies resemble giant rotating pinwheels with a pancake-like disk of stars and a central bulge or tight concentration of stars. Our Milky Way is one example of a broad class of galaxies defined by the presence of spiral arms. Chu (University of Illinois, Urbana) and STScI CFHT Image: Canada-France-Hawaii Telescope/J.-C. This stunning view of spiral galaxy M101, also known as the Pinwheel galaxy, was captured by the Hubble Space Telescope.
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