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How Our Galaxy Was Discovered

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  • 0:02 Discovering our Galaxy
  • 0:54 Early Concepts of our Universe
  • 2:18 Modern Observations
  • 4:13 Harlow Shapley
  • 6:11 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will outline for you the basics of how the galaxy you live in, the Milky Way Galaxy, was discovered over time. You'll also learn about RR Lyrae and Cepheid variable stars.

Discovering Our Galaxy

It wasn't all that long ago that people argued our place in the world. What is our planet like? Is it flat or round? Is it at the center of the solar system or not?

These questions seem a bit silly now, but they could not have been answered without discoveries. We had to discover that the Earth was round and that Earth orbits the Sun.

Likewise, it may seem really silly to say that we discovered the Milky Way Galaxy, the galaxy we live in. I mean, we're in it. What's there to discover?

But long ago, we weren't sure where we were located in the cosmos, within what (if anything), and how big it is.

Let's take a look at the basics of how it was that scientists discovered that we live in an enormous collection of stars, the Milky Way Galaxy.

Early Concepts of Our Universe

Ever since ancient times, people have looked up at the sky to see a hazy band of stars stretching across the night sky, something that's now called the Milky Way.

This band clued later astronomers into the idea that our sun must be part of a wheel-shaped cloud of stars, and the Milky Way is simply a projection of this cloud of stars onto our night sky from our vantage point within it.

Just imagine someone taking a hula hoop and placing it right over your head. You won't be able to see the hula hoop from the outside looking in. However, part of the hoop will block part of your vision as it stretches out in front of your eyes, cluing you into the fact that it must be something circular that's blocking your view.

In the 18th century, a brother and sister team of astronomers named Sir William Herschel and Caroline Herschel began to count and map out the stars in the night sky in a methodical and three-dimensional manner. Their observations showed that the stars we see are located in a disk-like shape. Nevertheless, their other assumptions, such as our Sun being at the center of the galaxy, were later shown to be incorrect, as you'll soon find out.

Modern Observations

Some stars in the night sky, called variable stars, pulsate in brightness, going from bright, to faint, to bright again. Such a cycle is known as the star's period of pulsation.

In the early 20th century, astronomer Henrietta Leavitt used a kind of variable star called a Cepheid to realize that a relationship between a star's period and luminosity carries great importance. For simplicity's sake, luminosity is how bright a star truly is (its intrinsic brightness), as opposed to how bright it appears to us on Earth (its apparent brightness).

The Cepheid variable star Leavitt saw is a kind of variable star that has a period of pulsation between 1 and 60 days.

Cepheids lie in a special region of the H-R diagram called the Instability Strip. This is a region of the H-R diagram where pulsating variable stars are located, such as Cepheid variable and RR Lyrae stars.

RR Lyrae variable stars are just variable stars with a period of pulsation of 12 to 24 hours. The H-R diagram is a diagram that plots a star's luminosity vs. surface temperature.

In any case, what Leavitt discovered was something now called the period-luminosity relation, which is the relationship between a Cepheid variable star's period of pulsation and its intrinsic brightness.

Her discovery opened up the way for modern astronomers to finally prove we live inside a humongous collection of stars, a galaxy we call the Milky Way Galaxy.

Harlow Shapley

And so, the baton was passed on to astronomer Harlow Shapley. Armed with the wealth of knowledge of his predecessors, he could finally prove that we lived within a big wheel of stars.

Through observation and calculation, Shapley realized that the center of the galaxy could not be near the Sun, as prior astronomers had thought. This belief opened up the possibility that we live in a much larger collection of stars than we had previously imagined.

To find the distance to the true center of the Milky Way Galaxy, Shapley needed to find the distance to some faraway star cluster. But he didn't have a good way of doing so until Leavitt's discoveries.

Shapley figured out that the distances to the variable stars Leavitt observed could be ascertained only if their luminosities were known.

To figure out their luminosities, he began by observing the proper motion of the variable stars and tacked on some good old math. The proper motion of a star is the rate at which a star moves across the sky.

Once Shapley's mathematical wizardry was complete, and he knew the variable star's luminosity, he could use the period-luminosity relation to figure out the distance to any star cluster that contained variable stars.

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