A Universe of Galaxies

Ian Dell’Antonio

Brown University

The night sky

Looking up at night (from a dark site!), you can see many stars, the Moon, up to five planets.

The moon and planets are part of our own neighborhood—the Solar System.

Our neighborhoodSpace is big. The Sun is 93 million miles away!

Zooming out to see all the planets.

But keep in mind that the nearest star (after the Sun!) is about 50,000 times further away than Pluto!

We see many patterns in the stars. Most of them are not real…

For example, this is Orion:

And this is what it looks like from other directions!

In addition to the stars, we see a few fuzzy patches. With the naked eye, most are very unimpressive. The most prominent of these is a whitish band across the sky (most visible in summer from the northern hemisphere): the Milky Way.

Other Nebulae

In addition to the Milky Way, other objects were seen in antiquity.

In 964, the persian astronomer

Abd-al-Rhaman Al-Sufi made the first known drawing of one:

The Milky Way extends across the sky, to the south. To see it best, you have to travel to the southern hemisphere.

The telescope:

To see more detail, you need a special tool:

Or, more appropriately, a big telescope!

With the telescope, the fuzzy patches could be resolved. Some were clouds of glowing gas:

The Milky Way resolved into billions of stars:

And that little nebula in Andromeda– a ``spiral nebula”

What are the “spiral nebulae”?

Until the 1920s, this was not known. There were two competing camps. One camp believed that they were clumps of stars within the Milky Way. The other, that they were systems of stars like the Milky Way.

The key to the debate was to measure the distances (and thus the sizes) of the nebulae.

Measuring distancesThere are many ways to measure distances. The most basic is to use triangulation (or, in astronomical terms, the parallax)

We have an astronomical analogue of the moving car: the Earth as it orbits around the Sun!

The stars are very far away, so the parallax angle is tiny– the nearest stars shift in position by less than 1 second of arc (equivalent to the apparent size of a quarter as seen from almost 1000 feet away!)

This angle is so small that the effect wasn’t observed until 1838. Today, however, we can measure the parallax for over a million stars!

This is less than 1 percent of the stars in the galaxy. To measure distances to the far side of the Milky Way, you need another method.

Pulsating stars

There is a class of stars, called Cepheid variables, which pulsate, with a period of pulsation proportional to their luminosity.

Distances from luminosities

You are all familiar with the idea that the further away a light source is, the fainter it appears. You can turn this concept around—if you know how bright a light source is, measuring how bright it appears tells you the distance!

Mapping the Milky Way

Face-on view:

The other nebulae are far!

Measuring Cepheid variables in Andromeda revealed that it was about 600,000 parsecs away—and at that distance, it was as big (actually slightly bigger) than the Milky Way.

The Nebulae are galaxies in their own right!

Our Neighbors: The Magellanic Clouds:

Andromeda

A Bestiary of Galaxies.Elliptical galaxy:

Spiral Galaxies—just like our own: (Edge on first)

Some are small:

Some are colliding!

Billions of Galaxies!

What makes galaxies different?

ROTATION!

Also, the type of stars– Elliptical galaxies have very few blue stars. That’s why they look red. Blue stars are new stars—so Ellipticals are only made of old stars.

The galaxies are redshifted!

Hubble’s Diagram

The expanding Universe