The Milky Way Galaxy

The Milky Way Galaxy is our home.  We live on a planet that orbits a star that, ultimately, orbits our galaxy's center of mass.  There is a black hole residing in the center of our galaxy that is approximately 4 million solar masses.  If you think the Sun is big, imagine 4 million of them packed into the volume of space inside the orbit of Mercury.

What is a galaxy?

According to the Oxford Dictionary, a galaxy is a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction.

The etymology of the word comes from the Greek word for 'Milky'.  This is a direct reference to the 'milky'-like appearance we can see in the sky from the arms of our own galaxy.

The image below is a map of our own galaxy.  Our Solar System resides on the inside edge of the Orion Spur.  When we look toward the galactic center we are seeing stars from our own spur, stars from the Sagittarius Arm and stars from the Scutum-Centaurus Arm.

Milky Way Map (Image Credit: NASA)

The Orion Spur is quite a minor arm in our galaxy.  It is only 3,500 light years wide and 10,000 long.  When we peer out towards the constellation Sagittarius we are looking toward the galactic center.  If you are in a dark sight you will see this brilliant 'milky' glow of billions of stars.  Also, when we peer out towards the constellation Perseus we will see the stars from that arm.

Below are some quick facts about The Milky Way:

Characteristic	Value
Type	Barred Spiral
Diameter	100,000-180,000 Light Years
Mass	800 Billion - 1.5 Trillion Solar Masses
Number of Stars	100-400 Billion
Cosmic Year	240 Million Years

Our galaxy has a mass of more than a trillion Suns.  This is incredible.  Our Sun revolves around the Milky Way's black hole every 240 million years.

It would take light more than 100,000 years to traverse the entire diameter of our galaxy.  Living on Earth with most distances measured in kilometers or miles, it is difficult to conceive the enormity of our galaxy.  This would translate into nearly a quintillion kilometers!

There are interesting things to view in each of the arms in the region near us.  Our home Orion Spur contains amazing sights such as the Orion Nebula, Dumbbell Nebula, The Pleiades and the Beehive Cluster.

If we were to examine objects in the Sagittarius Arm we would see such things as the Eagle Nebula, Omega (Swan) Nebula, and the Sagittarius Star Cloud.

Looking toward the anti-galactic center in the Perseus Arm we could see the Crab Nebula and many Open Clusters such as M36, M37 and M38 within Auriga.

The image below shows an inverted map of the galaxy with the constellations listed.  As you can see, relative to Earth, Sagittarius is directly in line with the Galactic Centre while Auriga is toward the anti-center.

Milky Way Constellations

The Milky Way is just one of billions (possibly more than a trillion) galaxies found throughout the Universe.  We can see other galaxies in our night sky with sometimes no more than our own eyes.  The Magellanic Clouds are satellite galaxies to our own and can be seen in the southern hemisphere skies.  For those living in Northern climes, the Andromeda Galaxy can be seen outside of the urban shine.  At 2.5 million light years away it is the closest major galaxy to our own.  At some point in the far distant future (~4 billion years) it will collide with our own causing a galactic merger.

All of the stars you can see in our night sky call the Milky Way their home.  If you can make your way to a dark site be sure to enjoy the view of the milky arms that form our galaxy.