Category Science

Galaxies come in all shapes and sizes, but astronomers divide them into just a few main types. More than half of all galaxies are elliptical, named because of their egg-like shape. They are made up of large numbers of old, red stars and have very little gas and dust with which to make new ones. Around a third of all galaxies are spiral in shape, like the Milky Way. Old stars are packed tightly in their centres, while new stars are continually being born from the large amounts of gas and dust in their spiral arms. Some spiral galaxies are classed separately because their nucleus is elongated into a bar. This bar is made up of stars in motion, and the spiral arms extend from the ends of the bar. Some galaxies cannot be classed as either spirals or ellipticals because they have no recognizable shape. These galaxies are called irregulars, and are full of gas in which new stars are forming.

In our universe are four identified types of galaxies. These galaxies are composed of stars, gas and dust held together with gravitational forces. Each galaxy type is named for its shape, making it easy to identify which type of galaxy you are looking at. You may already be familiar with the galaxies: spiral, barred spiral, elliptical and irregular. These fascinating galaxies were each formed from a unique mixture of gases and matter that came together, painting our magnificent galaxies, each one a unique work of artistic beauty.

Spiral Galaxy – Spiral galaxies are characterized by a distinct flattened spiral disk with a bright center called the nucleus. Our own Milky Way is a spiral galaxy. Spiral galaxies are represented by the letter S and are divided into four subgroups. These are S0, Sa, Sb, and Sc. S0 galaxies have a bright nucleus but have no spiral arms. Sa galaxies have spiral arms that are wound tightly around the nucleus while the arms of Sc galaxies are wound much more loosely.

Barred Spiral Galaxy – A barred spiral galaxy is very similar to a spiral with one important difference. The arms spiral out from a straight bar of stars instead of from the center. About one third of all spiral galaxies are barred spiral in shape. Barred spiral galaxies are represented by the letters SB and are arranges into three subgroups according to the openness of the arms. These subgroups are labeled SBa, SBb, and SBc. SBa galaxies have a short bar of stars extending from the center while SBc galaxies have a long, well-defined bar. Our own Milky Way galaxy is now believed to be a barred spiral galaxy.

Elliptical Galaxy – Elliptical galaxies vary in shape from completely round to extremely elongated ovals. Unlike spiral galaxies, they have no bright nucleus at their center. Elliptical galaxies are represented by the letter E and are divided into seven subgroups according to their shape. These subgroups are labeled E0 to E7. E0 galaxies nearly circular in shape while E7 galaxies are extremely elongated or stretched out.

Irregular Galaxy – A fourth type of galaxy is known as the irregular galaxy. These galaxies have no discernible shape or structure. Irregular galaxies are divided into two classes, Im and IO. Im class galaxies are the most common and show just a hint of structure. Sometimes the faint remnants of spiral arms can be seen. IO class galaxies are completely chaotic in form. The large and small Magellanic Clouds are examples of Im class irregular galaxies.

Astronomers cannot see what lies at the centre of the Milky Way because of the large amounts of dust in the way. Radio waves are not affected by dust, however, so scientists have been able to make detailed maps of the galaxy. They have found that at the centre of the Milky Way lies an enormous black hole —as large as Jupiter’s orbit around the Sun — which is sucking in matter and pouring out energy with the strength of almost 100 million Suns.

If you look up on a dark, clear night, away from city lights, you may see a wide band of faint light stretching above you, stiller than a cloud and glittering with densely packed stars. Translated from the Ancient Greek as “Milky Way” for resembling spilled milk on the sky, that band of light is the center of our galaxy. At its center, surrounded by 200-400 billion stars and undetectable to the human eye and by direct measurements, lies a supermassive black hole called Sagittarius A, or Sgr A* for short.

The Milky Way has the shape of a spiral and rotates around its center, with long curling arms surrounding a slightly bulging disk. It’s on one of these arms close to the center that the sun and Earth are located. Scientists estimate that the galactic center and Sgr A* are around 25,000 to 28,000 light-years away from us. The entire galaxy is around 100,000 light-years across.

A supermassive black hole is the largest type of black hole in a galaxy, with a mass millions of times that of our sun. Though it can’t be imaged directly because black holes pull in all light, scientists have inferred its presence by looking at the speed and motion of stars and matter close to the galactic center. They have inferred that the movements are influenced by the gravitational pull of a black hole.

No one knows how the black holes at the center of galaxies form, but some suspect that they may begin as a cluster of smaller black holes that merge, or as a smaller black hole that consumes enough matter to become a supermassive one.

Just as the sun’s gravity holds the objects of the Solar System in their orbits, the gravitational pull of the Milky Way keeps _two smaller galaxies in tow. The two Magellanic Clouds consist of thousands of star clusters that orbit the Milky Way every 1.5 billion years. The Large Magellanic Cloud (right) is made of the same mix of gas and stars as the Milky Way, but is less than one-twentieth as large. The Small Magellanic Cloud is slowly being pulled apart by the gravity of our galaxy.

Our sun is part of a massive collection of stars in the Milky Way galaxy. These hundreds of billions of stars orbit the galaxy’s center. But did you know that there are things that are even bigger orbiting the Milky Way’s center? Other galaxies orbit it too! These less massive galaxies have their own impressive collection of stars, which all orbit their own center; but the galaxies and everything in them orbit our galaxy too. It’s as if our galaxy is the sun and those other galaxies are planets. Astronomers call them “satellite galaxies.”

The Milky Way has a number of satellite galaxies, but the biggest one is the Large Magellanic Cloud. It is about 163,000 light-years away and around 1/100th the size of the Milky Way. Unlike our spiral galaxy, this one lacks a clean spiral shape. Some scientists think that is because the Milky Way and other galaxies are pulling and warping it.

In terms of distance, there are two contenders for closest satellite galaxy. One group of stars is small enough that astronomers consider it a “dwarf galaxy.” The other group is so close that they still debate whether or not it is part of our galaxy or its own dwarf galaxy. Astronomers have named the one that everyone agrees on the Sagittarius Dwarf Spheroidal Galaxy. It’s about 50,000 light-years away from the Milky Way center. It orbits over the top and down below the disk of our galaxy, like a ring over a spinning top.

But there is something even closer to our Milky Way—a cluster of stars named by some to be the Canis Major Dwarf Galaxy. Scientists estimate that it contains around a billion stars. It is so close to the edge of the Milky Way that it is closer to our solar system than to our galaxy’s center. It’s about 25,000 light-years away from us.

Some scientists don’t think the Canis Major cluster of stars is actually its own galaxy or dwarf galaxy. Instead they think it is just a dense area of faraway stars that are still part of the Milky Way. Either way, it is clear that this bunch of stars has been pulled very close to our Milky Way by our galaxy’s massive gravity. Over time, this could be the fate of other satellite galaxies in the area. They could all one day merge into an even larger Milky Way galaxy!

Our solar system lays roughly two thirds of the way from the centre of the galaxy, on the inner edge of a spiral arm called the Orion Arm, or the Local Arm. From Earth, the Milky Way appears as a river of milk stretching across the night sky. This is because we are viewing it from inside. The infrared image above gives a clearer view of the Milky Way as it stretches across space.

The Milky Way is a grand spiral galaxy, which astronomers think has four major spiral arms: Perseus, Cygnus, Scutum-Crux, Sagittarius. Some astronomers think we might actually just have two arms, Perseus and Sagittarius. The Sun is located in the inner rim of the Orion Arm, which is thought to be an offshoot of the Sagittarius Arm. The Sun is located about 26,000 light-years away from the center of the galaxy.

Before telescopes, the Milky Way just looked like a bright area in the sky, but when Galileo first turned his telescope on the region in 1610, he realized that it was actually made up of faint stars. The astronomer Immanuel Kant correctly guessed that this might be a cloud of stars held together by gravity, like the Solar System.

The famous astronomer William Herschel attempted to map out the stars in the Milky Way to get a sense of the galaxy’s size and shape, and determine the Sun’s position in it. From Herschel’s first map, it appeared the Sun was at the center of the Milky Way. It was only later on that astronomers realized that gas and dust was obscuring our view to distant parts of the galaxy, and that we were actually in the outer region of the Milky Way.

The astronomer Harlow Shapley accurately determined where the Sun is in the MIlky Way in the early 20th century by noticing that globular clusters were uniformly located above and below the Milky Way, but they were concentrated in the sky towards the constellation Sagittarius. Shapely realized that many globular clusters must be blocked by the galactic core. He created one of the most accurate maps of the Milky Way.

It wasn’t until the 20th century, with the development of larger and more powerful telescopes that astronomers could see the shape of other spiral galaxies, located millions of light-years away. In 1936, Edwin Hubble used cepheid variables as yardsticks to measure the distances to many galaxies, and prove conclusively that the Universe was filled with galaxies, each with as many stars as our own Milky Way.

The Milky Way is our home in the Universe. It is made up of over 200 billion stars, including the Sun, as well as large amounts of gas and dust. It looks like a giant spiral from above, but if it was viewed from the side it would appear as a flat band of stars. This is because the Milky Way is over 100,000 light years long, but only 2000 light years thick. The centre of the Milky Way is made up of a bright nucleus of old, cool stars. Emerging from the central galactic bulge are several spiral arms made up of gas, dust and young stars.

When you look up at the night sky, assuming conditions are just right, you might just catch a glimpse of a faint, white band reaching across the heavens. This band, upon closer observation, looks speckled and dusty, filled with a million tiny points of light and halos of glowing matter. What you are seeing is the Milky Way, something that astronomers and stargazers alike have been staring up at since the beginning of time.

But just what is the Milky Way? Well, simply put, it is the name of the barred spiral galaxy in which our solar system is located. The Earth orbits the Sun in the Solar System, and the Solar System is embedded within this vast galaxy of stars. It is just one of hundreds of billions of galaxies in the Universe, and ours is called the Milky Way because the disk of the galaxy appears to be spanning the night sky like a hazy band of glowing white light

Our Sun (a star) and all the planets around it are part of a galaxy known as the Milky Way Galaxy. A galaxy is a large group of stars, gas, and dust bound together by gravity. They come in a variety of shapes and sizes. The Milky Way is a large barred spiral galaxy. All the stars we see in the night sky are in our own Milky Way Galaxy. Our galaxy is called the Milky Way because it appears as a milky band of light in the sky when you see it in a really dark area.

In 1755, the philosopher Immanuel Kant claimed that some bright objects in space were giant collections of stars, and he named them island universes. The work of Edwin Hubble in the 1920s proved that these island universes were galaxies that lay beyond our own. A galaxy is an enormous collection of stars, dust and gas, held together by its own gravity. Even the smallest galaxies contain hundreds of thousands of stars, and it takes light many thousands of years to travel from one side to the other.

The visible universe is vast. It is 93 billion light years across, and contains more than 100 billion galaxies. The average galaxy contains about 100 billion stars, and untold numbers of planets. Yet a century ago there was serious doubt among many astronomers that the universe was much more than 100,000 light years across. Arguments about whether the universe was small or large became known as the Great Debate.

It is often known as the Shapley-Curtis debate, so named after Harlow Shapley and Heber Curtis, and a public debate they had in 1920. Shapley, you may remember, used observations of globular clusters to correctly show that the Sun is not the center of our galaxy. Curtis was an astronomer who studied nebulae, as well as solar eclipses.

The debate centered on the distance to certain nebulae. At the time, “nebula” referred to anything (excluding comets) that appeared “fuzzy” rather than distinct like a star or planet. So things like the Orion nebula (a stellar nursery), the Crab nebula (a supernova remnant) were considered nebulae just as they are today, but what we now call galaxies were also known as nebulae. The Andromeda galaxy, for example, was known as the Great Andromeda Nebula.

Curtis argued that Andromeda and other spiral nebulae were in fact “island universes”, similar in size to our own Milky Way “universe”. This would mean that not only were these nebulae 100,000 light years across or more, they must be millions of light years away. He based this argument on the fact that more novae were observed in Andromeda alone than were observed in the entire Milky Way. Why would that be the case if Andromeda were small and close? He also noted that some spiral nebulae had rather large redshifts, meaning that they were moving much faster than other objects in the universe.

Shapley argued that what we now call the Milky Way galaxy was the bulk of the universe. Spiral galaxies such as Andromeda must be relatively close and small. He based this view on several points. In 1917 Shapley and others observed a nova in the Andromeda nebula. For a brief time the nova outshined the central region of Andromeda. If Andromeda were a million light years away, as Curtis contended, then this nova (we now know it was a supernova) would need to be far brighter than any known mechanism could produce. There were also observations of the Pinwheel galaxy, seen above, by Adriaan van Maanen. He claimed that the Pinwheel had visibly rotated over the span of years. If the Pinwheel was rotating as van Maanen observed, then it couldn’t possibly be 100,000 light years across. For an object that large to rotate in a matter of years the stars would need to move faster than light.

After the debate the general opinion was that Shapley had won. His own observations of the shape of the Milky Way and the 1917 supernova, and van Maanen’s observations gave the small universe model solid footing. Besides, the idea that objects could be millions of light years away seemed patently absurd.