galaxies

A galaxy is a huge collection of stars, star clusters, interstellar gas and dust, and dark matter which are all held together by gravity. The word 'galaxy' is derived from the Greek word "galaxias" which means "milky", it is a reference to our own galaxy the Milky Way.

Galaxies appear in the sky as huge clouds of light, thousands of light-years across. There are potentially more than 170 billion galaxies in the observable universe. Some, called dwarf galaxies, are very small with about 10 million stars, while others are huge containing an estimated 100 trillion stars. Gravity binds the stars together, so they do not wander freely through space. The light that we see from each of these galaxies comes from the stars inside it.

We live on a planet called Earth that is part of our solar system. Our own solar system is located within a galaxy. Our Sun is only one of over 100 billion stars in a galaxy called the Milky Way. All of the stars we see in the night sky are part of the Milky Way. And just like our solar system, our galaxy is in motion. The stars within the Milky Way revolve around the central core. The Milky Way itself is moving as well. In fact, all of the galaxies in the universe seem to be moving away from each other at tremendous speeds.

 

Types of Galaxies

Galaxies are labeled according to their shape. Some galaxies are called “spiral” because they look like giant pinwheels in the sky. The galaxy we live in, the Milky Way, is a spiral galaxy. Some galaxies are called ‘elliptical” because they look like flat balls. A galaxy may be called “irregular” if it doesn’t really have a shape.

Hubble’s Classification Scheme

Each galaxy is assigned a letter - E=elliptical, S=spiral, Irr=irregular

• Ellipticals are given a number to describe their roundness (0=spherical)
• Spirals are given a capital B if they are barred, and a letter a-c

• a = tight spiral arms, large bulge, little dust in the disk
• c = loose spiral arms, small bulge, lots of dust in the disk

Let us look at the characteristics of four main groups of galaxies:

1. Spiral Galaxy

• Have a distinctly flattened spiral disk with a bright center called the nucleus.
• Represented by the letter S and are divided into four subgroups – 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. 
• The arms of Sc galaxies are wound much more loosely.

Spiral galaxies are believed to be younger than elliptical galaxies, as spiral galaxies burn through their gas and dust star formation slows, they lose their spiral shape and slowly evolve into elliptical galaxies.

S0 galaxies are also known as Lenticular galaxies. 

2. 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 arranged into three subgroups according to the openness of the arms.
• These subgroups are labeled as 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.

3. Elliptical Galaxy

• Elliptical galaxy varies 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.

4. 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.

5. Starburst galaxy

• A new type of galaxy was discovered recently, called a "starburst" galaxy.
• In this type of galaxy, new stars just seem to 'burst out' very quickly.

Formation and evolution of galaxies

There are two leading theories to explain how the first galaxies formed. 

One says that galaxies were born when vast clouds of gas and dust collapsed under their own gravitational pull, allowing stars to form.

The other says the young universe contained many small "lumps" of matter, which clumped together to form galaxies. Hubble Space Telescope has photographed many such lumps, which may be the precursors to modern galaxies. According to this theory, most of the early large galaxies were spirals. But over time, many spirals merged to form elliptical. 

The galaxy-formation process has not stopped. Our universe continues to evolve. Small galaxies are frequently gobbled up by larger ones. The Milky Way may contain the remains of several smaller galaxies that it has swallowed during its long lifetime. The Milky Way is digesting at least two small galaxies even now and may pull in others over the next few billion years.

What are colliding galaxies?

When two or more galaxies are close enough to each other, gravitational forces will pull the galaxies toward each other. This gravitational attraction increases as the galaxies travel toward each other. The galaxies may pass by each other or collide.

The Antennae galaxies are an example of two spirals that are in the process of colliding. We will not see the end result during our lifetimes because this process takes hundreds of millions of years.

Sometimes, smaller galaxies plunge into larger galaxies. This type of collision produces a ripple effect, like a rock thrown into a pond. The Cartwheel galaxy is an example of this type of collision. The outer ring of blue stars in this galaxy indicates a ripple of star formation resulting from the collision.

The Milky Way and Andromeda are examples of two spiral galaxies that may eventually collide (about 5 billion years in the future). 

Galaxy mergers can take anywhere from a few hundred million to a few billion years to complete. They can trigger intense bursts of new star formation and even create gigantic black holes.

Milky Way

The Milky Way Galaxy is our home galaxy in the universe. Our solar system—which includes the sun, Earth and seven other planets—is part of this galaxy, called the Milky Way. The Milky Way contains hundreds of billions of stars like our sun. All the stars and planets, you can see are part of the Milky Way galaxy. Our closest neighbor is Proxima Centauri. It is about 4.2 light-years from Earth. Earth is located about halfway between the center of the Milky Way and its outer edge.

The rotational center of the Milky Way is known as the Galactic Centre and it is located about 26,000 light-years from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius.

Milky Way is a group of about 50 galaxies called the Local Group. The largest, most massive galaxies in the Local Group are the Milky Way, Andromeda and the Triangulum Galaxy. Each of these galaxies has a collection of satellite galaxies surrounding them.

The Andromeda galaxy is the closest galaxy to the Milky Way and it’s about 2 million light-years away. The Milky Way is set to collide with the Andromeda galaxy in about 5 billion years.

Parts of Milky Way

1. Galactic disk – Most of the Milky Way’s more than 200 billion stars are located here. The galactic disk consists of the following parts:

• Nucleus is the center of the disk.
• Bulge is the area around the nucleus including the immediate areas above and below the plane of the disk
• Spiral arms extend outward from the center. Our solar system is located in one of the spiral arms of the Milky Way.

2. Globular clusters – A few hundred of these are scattered above and below the disk. The stars here are much older than those in the galactic disk.

3. Halo – A large, dim region that surrounds the entire galaxy. It is made of hot gas and possibly dark matter. Most of the mass of the galaxy lies in the outer portions of the galaxy (like the halo), where there is little light given off from stars or gases.

Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is a satellite dwarf galaxy of the Milky Way that is among the closest galaxies to Earth. It is at a distance of about 163,000 light-years from the Earth. Along with its companion dwarf galaxy the Small Magellanic Cloud, the LMC is visible as a faint cloud in Southern Hemisphere skies. It lies on the border of the constellations Dorado and Mensa. The Milky Way is consuming gas that is flowing from the Magellanic Clouds. Eventually, these two smaller galaxies might collide with the Milky Way. Both the LMC and the SMC have star-forming regions, and the LMC was the site of the spectacular 1987 supernova explosion.

Andromeda Galaxy

The Andromeda Galaxy is the closest largest galaxy to the Milky Way. This galaxy is named after the constellation of Andromeda. It is also known as Messier 31 or M31. This spiral galaxy is located at a distance of 2.5 million light-years from our galaxy. It is the largest in the local group or local cluster but not the largest galaxy overall.

It is believed that this galaxy was formed between 5 to 9 billion years ago when two smaller galaxies were collided and merged. 

Astronomers use this galaxy to understand the origin of other such galaxies because it is the nearest to our planet. It is the most distant object that can be spotted with the naked human eye.

The Andromeda Galaxy was once classed as a nebula. It has various satellite galaxies including 14 dwarf galaxies. The length of this galaxy is approximately 260,000 light years.

The Andromeda Galaxy is approaching us at a speed rate of 100 to 140 kilometers per second. The Andromeda Galaxy and the Milky Way are being drawn closer and closer over time. Astronomers believe that these two galaxies will be merged in roughly 5 billion years.

Doppler Effect and Red Shift

The Doppler Effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves.

Approaching light sources to show a blueshift and receding light sources show a redshift. 

When stars are moving away from other stars or objects at acceleration, this is a Redshift. 

When a star is moving toward Earth, its wavelengths of light are compressed. This causes the dark lines in the spectrum to shift toward the blue-violet end of the spectrum. This means that the astronomical light source (star or galaxy) is approaching Earth.

Hubble used the Doppler Effect for light to measure the speed at which stars and galaxies are approaching or receding from us. He found that all galaxies beyond the Local Group show a redshift in their spectra, this means that they must be moving away from Earth. If all galaxies outside the Local Group are moving away from Earth, then the entire universe must be expanding.

Hubble’s Law

Hubble’s Law is the statement in astronomy that galaxies move away from each other, and that the velocity with which they recede is proportional to their distance. It leads to the picture of an expanding universe and by extrapolating back in time, to the Big Bang Theory.

Big Bang Theory

The leading theory about the formation of the universe is called the big bang theory. According to this theory, approximately 13.7 billion years ago, the universe began with an enormous explosion. The entire universe began to expand everywhere at the same time.