Looking Up at the Sky

Imagine the Earth in its place in the Milky Way. You are standing on the Earth looking out into space. The night sky arcs over the planet. You are in awe of the vastness of space. You try to identify the constellations, and then you wonder, "Where are all those stars located? How far are they from me?"

The Milky Way

Suppose you are looking directly at the Milky Way galaxy. You are looking at the galactic plane. Imagine a flying saucer. It looks like a disk with a bulge in the middle. This is basically the shape of the Milky Way galaxy. The galactic plane is the part of the Milky Way which is in the "saucer" part of the shape. Much of what you see in the night sky is on the galactic plane of the Milky Way. This is because these stars are closer to us than stars in other galaxies; therefore, they seem brighter.

The Need For Maps

Do you ever wonder, when you are looking at a twinkle in the night sky, if the light you see is a nearby and relatively faint star or if it is very far away and very very big and bright? It is impossible to tell just by looking. We must use maps and guides to find out exactly where the stars are located. Lets find out how these maps work.

Look at the black and white map of the universe below. This map is very confusing, whether you realize it or not. It represents the entire universe on a flat screen. How is that possible? The universe is not flat!

Understanding the Map

Here is how to understand the map: Imagine yourself standing on the Earth. You look into the galactic plane and see a milky band of stars. This is our galaxy, the Milky Way Then you look along the sides of the Milky Way and see other celestial objects. Can you identify the Milky Way in this picture of the sky?

On the black and white map, the Milky Way is in the middle; it stretches from left to right. It is where most of the stars we see in the sky are clustered. Most of the objects that are not in the Milky Way are either above or below the cluster of stars on the map, just as they were in the picture of the sky. However, an object is not necessarily in the Milky Way if it looks like it is in the central cluster of stars. Furthermore, an object is not necessarily outside the Milky Way if it is not in the central cluster. How do we explain this?

Lets begin by looking at some diagrams which will help explain the way this map really works.

Measuring Latitude

Do you remember learning about how to measure latitude and longitude? On the Earth, latitude measures how far north or south a place is from the Equator. For example, France is at about 45 degrees ( ^o) north. It is halfway between the Equator and the North Pole. The Equator is at 0o (neither north nor south), and the North Pole is at 90^o north. At what latitude is your state or country?

Look at the Degrees Latitude diagram. This diagram illustrates how degrees of latitude are measured in the galactic coordinate system. The galactic plane is like the Equator. It is at 0^o latitude. The Earth is on the galactic plane. It is also at 0^o latitude.

Look at the diagram and find the celestial objects. These may be stars, faraway galaxies, supernova remnants, or any other kind of object in the universe. Look at objects 1 and 2. Notice that object 1 is at -10^o latitude and object 2 is at +10^o latitude. In galactic coordinates, we say plus and minus instead of north and south.

Latitude & Distance

On Earth, a place that is 10^o north (example: the island of Tobago in the Caribbean Sea) is the same distance from the Equator as a place that is 10^o south (example: Cape York, Australia). Each place is about 1110 km (or about 690 miles) from the Equator--the same exact distance. This is because these places are on the surface of the Earth. They are not hovering up in the sky or buried deep in the ground.

Degrees Latitude

Look at the Degrees Latitude diagram again. See the large pink circle. If all objects in outer space were exactly on the circle*, then an object that was at +10^o would always be the same distance from the galactic plane as would an object that was at -10^o. However, this is not the case. None of the objects in the diagram are exactly on the circle. Some are nearer, and some are farther.

(*Actually, the yellow circle is just imaginary. It could be much bigger or much smaller. The circle is there to help the diagram show the degrees of latitude in galactic coordinates.)

The objects at +10^o and -10^o are at different distances from the galactic plane; object 2 is closer to the galactic plane than object 1 is because it is at the same latitude and is closer to the Earth. Try to figure out where objects 1, 2, 3 and 4 would be on the black and white map.

Measuring Longitude

Now let's figure out how longitude works in galactic coordinates. Think of the Earth again. How do you measure longitude on the Earth?

Lines of longitude on the Earth are oriented north and south. They all pass through the North and South Poles. 0^o longitude runs through Greenwich, England. 0^o is the Prime Meridian. If you travel eastward from England 60^o of longitude, you arive at Orsk, Russia. Orsk is at 60^o east. If you travel westward 60^o from England, you arrive in Goose Bay, Newfoundland (Canada). Goose Bay is at 60^o west.

In galactic coordinates, the Prime Meridian, or 0^o is at the galactic center.

Degrees Longitude

Look at the Degrees Longitude diagram. See the bright galactic center and the 0^o line that runs through it. Instead of going from 0^o to 180^o east and 180^o west, galactic coordinates simply go from 0o to 360^o. There is no east/west or plus/minus in galactic longitude coordinates. Remember that 360^o is the same as 2 x 180^o, or 180^o is half of 360^o. Notice on the diagram that directly opposite of the 0^o line is the 180^o line.

Longitude & SCO X-1

Lets compare the location of Sco X-1 on the black and white map to the location of Sco X-1 on this diagram. Where is it on the black and white map? It is in the center, above the galactic plane. Does this mean that it is not in the Milky Way? Not necessarily.

Look at the location of Sco X-1 in the Degrees Longitude diagram. It is nearly at 0^o. It is in the direction of the galactic center; similarly, it is near the center of the black and white map.

Do you see now why it seems to be so far above the galactic plane? It is very close to the Earth. The latitude of Sco X-1 is about 24^o. Where would you draw it on the Degrees Latitude diagram?

Longitude & the Crab Nebula

Now lets compare the locations of the Crab Nebula. On the black and white map, it is the bright orange source all the way to the right.

On the Degrees Longitude diagram, it is near 180^o.

Now imagine yourself standing on Earth and looking at the galactic center. Suppose you want to then look at the Crab Nebula. You would have to either go to the other side of the Earth to see it (because you can't see through the Earth!), or you would have to wait for the Earth to spin around so that you are facing the opposite direction. Why do you think you won't be able to see the Crab Nebula if you are looking toward the galactic center?

(hint: compare the location of the Crab on each diagram: Degrees Longitude, Degrees Latitude diagram and Black and White map)

Wrapping It Up

Imagine the night sky as a piece of black paper that is wrapped around the Earth. The seam of the paper (where the edges are taped together) is at 180^o. The Crab Nebula would be close to the seam since it is close to 180^o. Imagine someone pulling the tape off and laying the paper flat on a table. Now where would the Crab be? It would be near the right side of the paper, just as it is on the black and white map!

The black and white map tries to show all parts of the universe on a flat screen.

View the Chandra Sky Map in our Photo Album!