Galaxies
The Neighbors Looking outward from the Sun s location in the Milky Way, we can see a variety of other galaxies: Small Magellanic Cloud (Digital Sky Survey) Large Magellanic Cloud (credit: Eckhard Slawik)
The Neighbors Both of the Magellanic Clouds are classified as irregular galaxies: No clear disk structure No pattern to star formation Usually contains a lot of gas, but not in a disk Irregular galaxies tend to be interacting with another galaxy. They may be small or quite large. They are the grab-bag for all galaxies that don t fit into the other categories.
The Neighbors In addition to the Magellanic Clouds, the Milky Way has many other companions: dwarf galaxies. They have less mass and fewer stars than the Milky Way. Some are elliptical, others irregular: Leo I dwarf elliptical IC 1603 dwarf irregular
The Neighbors Andromeda Galaxy (M31): The largest neighbor to the Milky Way galaxy About 2.5 million light-years away
The Neighbors M110 Andromeda has two elliptical galaxies which are thought to be the bulges of small spiral galaxies that M31 tore apart long ago. M32
The Neighbors Andromeda and the Milky Way are the dominant galaxies in a cluster of galaxies known as the Local Group. A third small spiral, the Triangulum Galaxy or M33, is the other spiral in the neighborhood. About 3 million lightyears away. All three spirals have smaller companions.
The Local Group With Andromeda and the Milky Way, there are about 50 other galaxies spread over a region 10 million light-years across: the Local Group of galaxies.
Spiral Classification: bulges M104, large bulge M74, small bulge
Spiral Classification: arms Andromeda Galaxy tightly wound arms Triangulum Galaxy loosely-wound arms
Spiral Classification: bars The bulges of some spiral galaxies are barred. That is, the bulge is elongated in one direction. NGC 1300, a barred spiral About 69 million light-years away
Spiral Classification: bars Here s a few more examples
Classification of the Milky Way As we ve discussed, it s tricky to map the structure of the Milky Way since we view it from the inside. However, studies of the Galaxy s bulge shows that it is elongated. Current measurements suggest the Milky Way might look like M109 (image at right). You could be here?
Elliptical Classification Spiral galaxies contain a large fraction of gas and dust and continue to form stars. Many galaxies, especially in cluster environments, show very little gas and dust and contain a larger fraction of ancient stars (and globular clusters). In crowded environments, collisions would be more common. These collisions would trigger star formation, using up the gas and dust. Supernovas would blow off the rest little gas, no recent star formation old population of stars
Elliptical Classification These older-looking galaxies are classified as elliptical ( round ) or lenticular ( lens-shaped ). Ellipticals tend to be rounder and lenticulars have a shape similar to the disk of a spiral. The orbits of the stars are more random than in a spiral galaxy: like a very large bulge of a spiral. Ellipticals come in a very wide range of sizes: very small ( dwarf ellipticals ) up to the largest galaxies known ( giant ellipticals ).
Elliptical Classification Recall the dwarf elliptical companions of the Andromeda Galaxy. M32 (left) and M110 (right) contain less than 1% of the mass of the Milky Way. Notice neither has dark lanes that suggest much dust and gas.
Elliptical Classification Some of the largest galaxies known are giant ellipticals. M87 sits at the core of the Virgo cluster, some 52 million light-years away. About same size as Milky Way, but 20x more mass!
Irregular Classification The Magellanic Clouds are just two examples of irregular galaxies. Other examples include pairs of interacting or colliding galaxies. The Mice : a pair of merging spiral galaxies The streams of stars are flung outward from each galaxy.
Irregular Classification There are also large and complex irregulars: over-active, ring, & just-plain-odd galaxies. Centaurus A (NGC 5128) is a giant elliptical with a warped dust lane. AM0644 is a ring galaxy.
Irregular Classification This is M82, a galaxy undergoing a burst of star formation and supernovas. The galaxy is literally blowing the gas out of the galaxy with supernova explosions (bright dots in the x-ray image). optical x-ray
Irregular Classification Quasars and radio galaxies are another type of active galaxy. Radio observations show jets of particles that shoot out 100,000 s of light-years. Centaurus A is the closest galaxy with big jets.
Active Galaxies Other examples of radio galaxies:
A Monster in the Middle These galaxies are powered by accretion onto supermassive black holes at their centers. Matter falling toward black hole is heated and some escapes as jets moving near lightspeed. Artist rendering of disk, jets, and black hole.
A Sampling of Galaxies The following images are of various kinds of galaxies from different telescopes. Can you classify each galaxy?
M101 27 million l.y. A. spiral B. barred spiral C. elliptical D. irregular
How can you tell this galaxy has lots of star formation? A. it has a yellow-ish bulge B. it has lots of blue stars C. it is fairly round in shape
M81 12 million l.y. big brother to M82 A. spiral B. barred spiral C. elliptical D. irregular
NGC 4013 55 million l.y. A. spiral B. barred spiral C. elliptical D. irregular
If this is a spiral galaxy, where is its disk? A. we are seeing it edgeon, so it appears flat B. it contains few stars so it is difficult to see C. this galaxy contains only a bar, not a disk
NGC 4365 68 million l.y. star-like dots are globular clusters in this galaxy s outskirts A. spiral B. barred spiral C. elliptical D. irregular