Our universe is comprised of 200 billion galaxies which ...



Galaxies and their classifications.

About 200 billion galaxies populate our observed universe and contain almost all of its matter. Since galaxies are the universe’s building blocks, understanding galaxy formation explains the universe’s origin.

Large empty gaps or voids between galaxies account for over 90% of space. Distances in outer space are so enormous that it often takes light billions of years to travel between them, which allow astronomers to see how they existed at much earlier times.

Astronomers observe small, young galaxies over 13 billion light years away. Since their light has traveled such great distances to reach us, it was emitted over 13 billion years ago, near the beginning of time. This distant light is a window in time revealing the early universe.

Observing these earliest galaxies is difficult for several reasons: First, the light and other radiation emitted by distant galaxies spreads out in all directions, growing weaker as the distance increases to become faint and diffuse. Second, there is no frame of reference as all galaxies are traveling at different velocities (speed and direction), complicating exact measurements. Third, their great distance exceeds the Hubble telescope’s range limits.

Rather than wait years to use the longer range James Webb telescope, astronomers needed a method of concentrating ancient light.

Aware that incredible gravity could bend light, astronomers select a supercluster of massive galaxies at a great distance from us. The supercluster’s huge gravity curves and concentrates light traveling around itself from the far side by focusing light like a lens.

Using “gravitational lensing”, astronomers search converged light from the opposite side of the supercluster to identify and study the most distant galaxies.

Not only are astronomers able to examine galaxies across great distances, but they also make very detailed and precise observations. Astronomers measure the composition, brightness (“luminosity”), rate of rotation, and level of organization of galaxies.

Galaxies are comprised of stars, dust, and gas with the composition and percentages varying by galaxy type. Astronomers can even identify separate elements by the wavelengths of their light to distinguish between generations of stars. For example, within distant, early galaxies, astronomers discern the radiation signature of isolated hydrogen and helium (without other elements) to confirm the existence of first generation stars which burn hot and discharge blue light.

Astronomers use the radiation signature of heavier elements to identify mature galaxies which lack the gas needed to form new stars. Mature galaxies occasionally have merged by gravity with other galaxies to gain new gas and temporarily revitalize their star production.

Star production by galaxies has declined significantly across the last 13 billion years. Early galaxies made hundreds or thousands of stars per year. Currently, there are many galaxies which have not made any new stars in billions of years. Even our more active Milky Way only produces a few stars each year.

Astronomers use powerful telescopes to categorize billions of galaxies into the three major types based on their shape: spiral, lenticular and elliptical (spheroid). Irregular galaxies will not be discussed in this book as they provide less insight because of inconsistency and alteration by external forces.

We also identify distinct, regular features to further classify galaxies. For example, most spiral galaxies have a bar of stars spanning their central bulge, but about one-third (1/3) do not possess this trait. Therefore, astronomers classify spiral galaxies as either barred or without a bar.

These features, however, exhibit slight variations. For example, the arms of older and larger spiral galaxies are more tightly wound than the arms of other spiral galaxies. An interesting tool for understanding galaxy formation is the constant relationship between these variations.

The stars of galaxies are more densely packed toward their galactic centers which usually surround inner black holes. This level of concentration varies by individual galaxy and also by type. For example, spiral galaxies are more expansive and have stars more evenly spread out across a greater area than more dense elliptical galaxies.

Galaxy shapes are determined by their outer stars. Spiral and lenticular galaxies have flatter discs of outer stars compared to spherical shape of elliptical galaxies. The cores of disc shaped galaxies are called “bulges” because they protrude above the plane.

Spiral Galaxies

Spiral galaxies, such as our Milky Way, are the most common, non-dwarf type of galaxy found in the universe and exhibit features which clearly distinguish them from other types. These galaxies are flat discs of mostly younger stars, which revolve around a dense, tidy central bulge of older stars. Outside of the bulge, the large majority of stars are located near the galaxy’s flat plane with minimal older stars forming a spherical halo. We know that these halo stars drifted away from the active plane long ago because they lack heavy metals found in later generation stars.

These spiral galaxies have rotating pairs of arms extending from the central bulge, usually connected by a bar of stars. It is crucial to recognize that the spiral arms are found only in pairs; we never observe a spiral galaxy with an odd number of arms. The duality of the interwoven pairs is not incidental, but rather a major, but overlooked, clue to solving the mystery of galaxy formation.

The spiral arms, which range from clearly defined to vaguely fuzzy, tend to contain large amounts of gas and dust which is used to make new stars. In the arms, we find young stars.

All spiral galaxies have a central bulge which protrudes above the flat plane. These stars within the bulge also have orderly movement but in a spherical shape. The bulge contains little gas or dust, and thus mostly older stars.

We observe a direct relationship between the size of the bulge and the age of the stars found in the bulge. A spiral galaxy with a larger bulge tends to have older red stars, while smaller bulges contain more new stars.

We also observe that galaxies with larger bulges have more tightly wound arms than galaxies with smaller bulges. Furthermore, galaxies which are observed later in time also have more tightly wound arms than earlier galaxies.

In the center of the bulge, we often observe a black hole, which typically contains less than five percent (5%) of the galaxy’s total mass. Many people incorrectly think that the black hole sucks in any star that approaches. Instead, these stars probably orbit the black hole similar to how comets move around our sun.

The majority of modern spiral galaxies have a bar of stars which span the central bulge to connect the inner most ends of the two spiral arms. Astronomers categorize spiral galaxies into “barred” or “without bar”. Another crucial clue is that the percentage of spiral galaxies with the bar feature has risen dramatically across time.

In summary, spiral galaxies are disc shaped with a central bulge, protruding above their flat plane. Their stars, both inside the bulge and in the outer arms, move in an orderly fashion. The bulge stars are older, while the stars in the arms are younger and in the presence of gas and dust (future star material). Most modern spiral galaxies have a central bar with that percentage on the rise.

Lenticular Galaxies

Named for their “lens” shape, lenticular galaxies occupy an intermediate position between chaotic elliptical and orderly spiral galaxies.

This galaxy type consists primarily of older stars and show reduced amounts of dust, gas and other star material. The central bulge of these galaxies is chaotic with disorderly star motion.

The outer disk of these galaxies rotates around the bulge which sometimes has a central bar. They galaxies lack the defined arms and orderly structure of spiral galaxies. Sometimes, they are referred to as “armless spirals.”

If they are oriented so that we view their edge, these galaxies resemble flat spiral galaxies. However, if they are view top down or perpendicular to their plane, they resemble elliptical galaxies.

Initially, these galaxies were incorrectly thought to be the middle position in the progression from elliptical to spiral types. Now, we realize that no such transformation occurred. Modern astronomers cannot agree on how this galaxy type formed.

Elliptical Galaxies

Elliptical galaxies are spheroid in shape. Many elliptical galaxies are oblong, which is why they were originally named “ellipticals.” This elongated shape is the result of merger with other galaxies or distortion from gravity, as they are found primarily in areas of high concentrations of galaxies, which are called “walls” or “filaments”.

Their large, round cores show disorderly star movement around the center. Technically, astronomers refer to this trait as “central velocity dispersion”. This terminology basically means that the core stars move in random directions at various speeds without noticeable or predictable configurations.

These elliptical galaxies have very little dust or gas with which to make new, young stars. Their stars are old red stars, with very few young, blue stars. Many of these elliptical galaxies stopped producing stars about 10 billion years ago. Astronomers refer to this event as “early quenching” and this galaxy type as “Red and Dead”.

Some of these elliptical galaxies were re-invigorated via merger with other galaxies from which they obtained new star material.

The most massive elliptical galaxies possess huge amounts of matter and the largest central black holes. They have the greatest star densities in their surrounding central regions, but the density declines dramatically away from the center.

Dwarf Elliptical and Lenticular Galaxies.

The dwarf galaxies are the most common galaxy type in our universe. They are characterized by their small size and their reduced number of stars. Their traits are similar to their larger cousins.

These dwarf galaxies are often be found near other larger galaxies. Our Milky Way galaxy has several dwarf galaxies in rotation around it.

Ultra-Diffuse Galaxies

A new type of galaxy has recently been discovered: the ultra-diffuse galaxy which is spread out across a large area with very low density.

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