Having a lot of gas and dust means that these galaxies have a lot of star formation going on within them. Elliptical galaxies, such as M87 left , have very little gas and dust. Because gas and dust are found in the clouds that are the birthplaces of stars, we should expect to see very few young stars in elliptical galaxies.
In fact, elliptical galaxies contain primarily old, red stars also known as Population II stars. Elliptical galaxies have a broader range in size than other types of galaxies.
The smallest are dwarf elliptical galaxies, which can be less than 10 percent of the size of the Milky Way. But ellipticals can also stretch to more than a million light-years across, and contain more than ten trillion stars. It is the huge mass of gas in a cluster of galaxies that can bend the light from a more distant galaxy. Furthermore, there is very little interstellar matter neither gas nor dust , which results in low rates of star formation, few open star clusters, and few young stars; rather elliptical galaxies are dominated by old stellar populations, giving them red colors.
Ellipticals are old. The stars that live within them are among the oldest in the universe. Unlike their spiral cousins, elliptical galaxies have shut down their star formation engines.
Devoid of much of the needed gas and dust, they no longer create new stars. If we lived in an elliptical galaxy, we would see the stars of our galaxy spread out all around the sky, not in a single band. Some spiral galaxies have arms that are wound tightly, while other galaxies have very loosely-wound arms. Spiral galaxies come in a wide range of sizes, from 5 to kiloparsecs across, have masses between and solar masses, and luminosities ranging from to time that of the Sun.
The majority of spiral galaxies rotate in the sense that the arms trail the direction of the spin. Galaxies at this extreme may have no clear spiral arm structure, resulting in a lens-like appearance they are sometimes referred to as lenticular galaxies.
These galaxies seem to share as many properties with elliptical galaxies as they do with spiral galaxies. At the other extreme, the central bulge is small and the arms are loosely wound. In these Sc galaxies, luminous stars and emission nebulae are very prominent. Our Galaxy and the Andromeda galaxy are both intermediate between the two extremes. Photographs of spiral galaxies, illustrating the different types, are shown in Figure 3, along with elliptical galaxies for comparison.
Figure 3: Hubble Classification of Galaxies. Elliptical galaxies are on the left. On the right, you can see the basic spiral shapes illustrated, alongside images of actual barred and unbarred spirals. The luminous parts of spiral galaxies appear to range in diameter from about 20, to more than , light-years. Recent studies have found that there is probably a large amount of galactic material that extends well beyond the apparent edge of galaxies.
This material appears to be thin, cold gas that is difficult to detect in most observations. From the observational data available, the masses of the visible portions of spiral galaxies are estimated to range from 1 billion to 1 trillion Suns 10 9 to 10 12 M Sun. The total luminosities of most spirals fall in the range of million to billion times the luminosity of our Sun 10 8 to 10 11 L Sun.
Our Galaxy and M31 are relatively large and massive, as spirals go. There is also considerable dark matter in and around the galaxies, just as there is in the Milky Way; we deduce its presence from how fast stars in the outer parts of the Galaxy are moving in their orbits.
Elliptical galaxies consist almost entirely of old stars and have shapes that are spheres or ellipsoids somewhat squashed spheres Figure 4. They contain no trace of spiral arms. In the larger nearby ellipticals, many globular clusters can be identified.
Dust and emission nebulae are not conspicuous in elliptical galaxies, but many do contain a small amount of interstellar matter. Figure 4: Elliptical Galaxies. Other elliptical galaxies can be seen around the edges of this image. Elliptical galaxies show various degrees of flattening, ranging from systems that are approximately spherical to those that approach the flatness of spirals.
The mass in a giant elliptical can be as large as 10 13 M Sun. The diameters of these large galaxies extend over several hundred thousand light-years and are considerably larger than the largest spirals.
Although individual stars orbit the center of an elliptical galaxy, the orbits are not all in the same direction, as occurs in spirals.
We find that elliptical galaxies range all the way from the giants, just described, to dwarfs, which may be the most common kind of galaxy. Dwarf ellipticals sometimes called dwarf spheroidals escaped our notice for a long time because they are very faint and difficult to see.
The luminosity of this typical dwarf is about equal to that of the brightest globular clusters. Intermediate between the giant and dwarf elliptical galaxies are systems such as M32 and M, the two companions of the Andromeda galaxy. While they are often referred to as dwarf ellipticals, these galaxies are significantly larger than galaxies such as Leo I. Figure 5: Dwarf Elliptical Galaxy. M32, a dwarf elliptical galaxy and one of the companions to the giant Andromeda galaxy M M32 is a dwarf by galactic standards, as it is only light-years across.
The Local Group and its neighbor galaxy cluster, the Virgo Cluster , both lie within the larger Virgo Supercluster , a concentration of galaxies that stretches about million light-years across.
The Virgo Supercluster, in turn, is a limb of Laniakea, an even bigger supercluster of , galaxies that astronomers defined in Galaxies in clusters often interact and even merge together in a dynamic cosmic dance of interacting gravity. When two galaxies collide and intermingle, gases can flow towards the galactic center, which can trigger phenomena like rapid star formation.
Our own Milky Way will merge with the Andromeda galaxy in about 4. Because elliptical galaxies contain older stars and less gas than spiral galaxies, it seems that the galaxy types represent part of a natural evolution: As spiral galaxies age, interact, and merge, they lose their familiar shapes and become elliptical galaxies.
But astronomers are still working out the specifics, such as why elliptical galaxies follow certain patterns in brightness, size, and chemical composition. The universe's first stars ignited some million years after the big bang, the explosive moment Gravity had sculpted the first galaxies into shape by the time the universe turned million years old , or less than 3 percent of its current age.
Astronomers now think that nearly all galaxies— with possible exceptions —are embedded in huge haloes of dark matter. Theoretical models also suggest that in the early universe, vast tendrils of dark matter provided normal matter the gravitational scaffold it needed to coalesce into the first galaxies. But there are still open questions about how galaxies form. Some believe that galaxies formed from smaller clusters of about one million stars, known as globular clusters , while others hold that galaxies formed first, and later birthed globular clusters.
It's also difficult to figure out how many of a given galaxy's stars formed in situ from its own gas , versus forming in another galaxy and joining the party later. By letting astronomers peer into the universe's farthest reaches—and earliest moments—instruments such as NASA's James Webb Space Telescope should help resolve lingering questions.
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