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On a clear night far from city lights, you look up and see this. What do you think you are looking at?
On a clear night far from city lights, you look up and see this. What do you think you are looking at?
On a clear night far from city lights, you look up and see this. What do you think you are looking at?
Stretching across the dark night sky, not easily visible when the Moon is in the sky, is a faint irregular glowing strip of light. For thousands of years peoples of various cultures tried to explain what they saw, sometimes using stories. Here are some examples:
The seventh Princess of Heaven fell in love with a poor herdsman and ran away to marry him. When her mother sent soldiers to bring her home, the herdsman chased them away. Seeing her daughter’s husband running, the mother dropped a silver pin to make a silver stream to separate the lovers forever. Eventually, her father allowed her to have an annual reunion with her husband — black birds escorted her across the stream. The Milky Way is that silver stream. The young lovers are the stars Vega and Altair on either side of it.
When the world was created, the people gathered around Black God to place stars in the sky. Coyote was frustrated by how long it was taking. He threw the bag of stars over his head, forming the Milky Way.
The goddess Isis spread large quantities of wheat across the sky. We see this bounty as the Milky Way.
The Milky Way is made of the ashes of campfires.
The Milky Way is a long, blue, cloud-eating shark.
The Milky Way is along the circular path where the Sun once moved across the sky. It looks different than the rest of the sky because the Sun scorched it.
The Milky Way is a typical barred spiral galaxy with the following parts:
Galactic bulge: Dense, spherical region of older stars in the center of the Milky Way with a supermassive black hole at its center
Disc: Flattened plane containing most of the galaxy's younger stars, gas, and dust
Halo: Sparsely populated spherical region surrounding the disc containing older stars, globular clusters, and dark matter
Globular cluster: Small group of stars bound together by gravity
Spiral arm: Concentrated regions of stars, dust, and gas extending from the galactic bulge
Bar: Central bar-shaped structure across the galactic bulge
Use the descriptions above to help you match the labels below onto the image.
At the galactic center of almost every galaxy is a black hole. A black hole is a region of space packed with so much mass that its own gravity prevents anything from escaping—even light.
This is Sagittarius A*, the black hole in the center of the Milky Way Galaxy
How did scientists determine there was a black hole in the center of the Milky Way galaxy?
Use the website Black Holes: Sagittarius A* to help answer this questions. Use the timeline scroll bar to see the changes.
Hubble's Law
In the 1920s, a scientist named Edwin Hubble was studying galaxies. He looked at light from these galaxies and noticed something interesting called redshifts. This helped him figure out how fast the galaxies were moving. Hubble also discovered a type of star called Cepheid variables in these galaxies. Cepheid variables are special because they change in brightness over time.
Hubble found out that bigger, brighter Cepheid stars have longer periods of brightness changes, while smaller, dimmer Cepheids have shorter periods. By watching how fast these stars pulsed, he could estimate how bright they were and how far away the galaxies were.
When Hubble made a graph showing the speed of the galaxies compared to their distance, he noticed two important things:
Most galaxies (except a few close ones) are moving away from us. This means the universe is getting bigger.
The farther away a galaxy is, the faster it is moving away. This idea is known as Hubble's Law.
It's important to note that when we say galaxies are moving away, we really mean that space itself is expanding. This expansion creates more space between us and distant galaxies, pushing them farther away than closer ones.
If the embedded website below doesn't work for you, click here to open it in a new tab.
Zoom out until you can see a bunch of galaxies. The galaxy with the red circle around it represents your location (the Milky Way). The red arrows represent the galaxies' velocities.
Can you find a galaxy that is moving toward you?
The arrow lengths convey how fast galaxies are moving.
Describe the velocities (length of arrows) of galaxies nearby your location versus the velocities (length of arrows) of galaxies that are far away.
Click on a different galaxy to make it the observation point.
Does Hubble's Law still apply?
Summarize Hubble's Law:
The farther away a galaxy is, the it moves away from Earth.
Use the website Interacting Galaxies: Future of the Milky Way to help answer these questions. Use the timeline scroll bar to see the changes.
Which galaxy is moving towards the Milky Way?
What will be formed once the 2 galaxies merge?